Chapter 3: The Cellular Level of Organization

Question 1

Define a cell.

Parts of a Cell

Answer 1

• Basic, living, structural and functional units of body.
• Body consists of more than 100 trillion cells.

Parts of a Cell

Question 2

What are the 3 main parts of a cell?

Parts of a Cell

Answer 2

1. Plasma membrane.
2. Cytoplasm.
3. Nucleus.

Parts of a Cell

Question 3

Define the plasma membrane of a cell.

Parts of a Cell

Answer 3

• Cell’s flexible outer surface, separates cell’s internal environment from external environment.
• Selective barrier, regulates flow of materials into/out of cell.
• Helps establish/maintain environment for normal cellular activity.
• Communication among cells and between cells and external environment.

Parts of a Cell

Question 4

Define the cytoplasm of a cell.

Parts of a Cell

Answer 4

• All cellular contents between plasma membrane and nucleus. Has 2 components: cytosol and organelles.
  1. Cytosol: Fluid portion of cytoplasm (intracellular fluid), contains water, dissolved solutes, suspended particles. Within cytosol are different types of organelles.
  2. Organelles: Characteristic shape, specific functions. Examples: cytoskeleton, ribosomes, endoplasmic reticulum, golgi comples, lysosomes, peroxisomes and mitochondria.

Parts of a Cell

Question 5

Define the nucleus of a cell.

Parts of a Cell

Answer 5

• Large organelle, houses most of cell’s DNA.
• Within nucleus, each chromosome contains thousands of hereditary units (genes) that control most of cellular structure and function.

Parts of a Cell

Question 6

What are the functions of the plasma membrane?

The Plasma Membrane

Answer 6

1. Barrier separates inside and outside of cell.
2. Controls flow of substances into/out of cell.
3. Identifies the cell to other cells (e.g. immune cells).
4. Participates in intracellular signaling.

The Plasma Membrane

Question 7

What is the fluid moasic model?

The Plasma Membrane

Answer 7

• Arrangement of lipids and proteins in plasma membrane.
• Some proteins float, others anchored at specific locations.
• Membrane lipids allow passage of lipid-soluble molecules but act as barrier to entry/exit of charged/polar molecules and ions into/out of cell.
• Other proteins act as signal receptors/as molecules that link plasma membrane to intracellular/extracellular proteins.

The Plasma Membrane

Question 8

Define the lipid bilayer.

The Plasma Membrane

Answer 8

• Framework of plasma membrane.
• 2 back-to-back layers made up of three types of lipid molecules: phospholipids, cholesterol and glycolipids.
• Bilayer arrangement occurs as lipids are amphipathic molecules (have both polar and nonpolar parts).

The Plasma Membrane

Question 9

Define phospholipids in the lipid bilayer.

The Plasma Membrane

Answer 9

• Lipids that contain phosphorus.
• About 75% of membrane lipids are phospholipids.

The Plasma Membrane

Question 10

Define cholesterol in the lipid bilayer.

The Plasma Membrane

Answer 10

• Present in smaller amounts (about 20%).
• Steroid with attached -OH (hydroxyl) group.
• Cholesterol molecules are weakly amphipathic, are interspersed among other lipids in both layers of membrane.

The Plasma Membrane

Question 11

Define glycolipids in the lipid bilayer.

The Plasma Membrane

Answer 11

• About 5% of the membrane lipids.
• Are lipids with attached carbohydrate groups.

The Plasma Membrane

Question 12

What are integral proteins?

The Plasma Membrane

Answer 12

• Membrane protein that extends into/through lipid bilayer, firmly embedded into it.
• Most are transmembrane proteins (span entire lipid bilayer, protrude into both cytosol and extracellular fluid).
• Few integral proteins are tightly attached to one side of bilayer by covalent bonding to fatty acids.
• Integral membrane proteins are amphipathic.

The Plasma Membrane

Question 13

What are glycoproteins?

The Plasma Membrane

Answer 13

• Proteins with carbohydrate groups attached to ends that protrude into extracellular fluid.
• Carbohydrates are oligosaccharides (chains with 2 to 60 monosaccharides, may be straight or branched)

The Plasma Membrane

Question 14

What is the glycocalyx?

The Plasma Membrane

Answer 14

• Sugary coat formed by carbohydrate portions of glycolipids and glycoproteins.
• Acts like molecular “signature”, enables cells to recoginize one another.
• Eenables cells to adhere to one another in some tissues, protects cells from being digested by enzymes in extracellular fluid.
• Hydrophilic properties of glycocalyx attract a film of fluid to surface of many cells.

The Plasma Membrane

Question 15

What is the function of ion channels (integral) in membrane proteins?

The Plasma Membrane

Answer 15

• Forms pore/hole which specific ion can flow across membrane.
• Most are selective; allow only single type of ion to pass through.
• Most plasma membranes include specific channels for several common ions.

The Plasma Membrane

Question 16

What is the function of carriers (integral) in membrane proteins?

The Plasma Membrane

Answer 16

• Integral proteins that act as carriers, selectively moving a polar substance/ion from one side of membrane to other.
• Carrier proteins, known as transporters.
• Example: amino acids, needed to synthesize new proteins, enter body cells via carriers.

The Plasma Membrane

Question 17

What is the function of receptors (integral) in membrane proteins?

The Plasma Membrane

Answer 17

• Integral proteins serve as cellular recoginition sites.
• Receptor recognizes and binds specific type of molecule.
• Specific molecule that binds to receptor is called a ligand of that receptor.
• Recognizes specific ligand, alters cell’s function in some way. Example: Antidiuretic hormone binds to receptors in kidneys, changes water permeability of certain plasma membranes.

The Plasma Membrane

Question 18

What is the function of enzymes (integral and peripheral) in membrane proteins?

The Plasma Membrane

Answer 18

• Integral proteins that catalyze specific chemical reactions at inside/outside surface of cell.
• Example: lactase protruding from epithelial cells lining small intestine splits disaccharide lactose in milk.

The Plasma Membrane

Question 19

What is the function of linkers (integral and peripheral) in membrane proteins?

The Plasma Membrane

Answer 19

• Anchor proteins in plasma membranes of neighboring cells to one another or to protein filaments inside/outside cell.
• Anchors filaments inside/outside plasma membrane, provide structural stability and shape for cell.
• Peripheral proteins also serve as enzymes and linkers.

The Plasma Membrane

Question 20

What is the function of cell identy markers (glycoproteins) in membrane proteins?

The Plasma Membrane

Answer 20

• Membrane glycoproteins and glycolipids often serve as cell-identity markers.

May enable cell to:
1. Recognize other cells of same kind during tissue formation.
2. Recognize/respond to potentially dangerous foreign cells. Example: ABO blood type markers.

The Plasma Membrane

Question 21

What are some functions of peripheral proteins?

The Plasma Membrane

Answer 21

• Help support plasma membrane.
• Anchor integral proteins.
• Participate in mechanical activities (ex: moving materials and organelles within cells, changing cell shape during cell division and in muscle cells, and attaching cells to one another.

The Plasma Membrane

Question 22

Describe membrane fluidity.

The Plasma Membrane

Answer 22

• Membrane lipids and proteins rotate/move sideways in their own half of bilayer.
• Neighboring lipid molecules exchange places about 10 million times per second, may wander completely around cell in minutes.
• Fluidity depends on number of double bonds in fatty acid tails of the lipids that make up bilayer, and on amount of cholesterol present.

The Plasma Membrane

Question 23

How does membrane fluidity increase?

The Plasma Membrane

Answer 23

• Each double bond puts “kink” in fatty acid tail, increases fluidity by preventing lipid molecules from packing tightly in membrane.

The Plasma Membrane

Question 24

How does cholesterol effect the membrane fluidity?

The Plasma Membrane

Answer 24

• Makes lipid bilayer stronger but less fluid at normal body temperature.
• At low temperatures, increases membrane fluidity.

The Plasma Membrane

Question 25

Describe permeable (in relation to membrane permeability).

The Plasma Membrane

Answer 25

Structure permits passage of substances through it.

The Plasma Membrane

Question 26

Describe impermeable (in relation to membrane permeability).

Answer 26

Structure does not permit passage of substances through it.

Question 27

Describe selective permeability (in relation to membrane permeability).

The Plasma Membrane

Answer 27

• Permits some substances to pass more readily than others.
• Permeability of plasma membrane to different substances varies.

The Plasma Membrane

Question 28

Describe the lipid bilayer portion of plasma membranes permeability.

The Plasma Membrane

Answer 28

• Highly permeable to nonpolar molecules such as oxygen (O2), carbon dioxide (CO2), steroids.
• Moderately permeable to small, uncharged polar molecules such as water, urea (waste product from breakdown of amino acids).
• Impermeable to ions and large, uncharged polar molecules such as glucose.
• Permeability characteristics of plasma membrane is because lipid bilayer has nonpolar, hydrophobic interior.

The Plasma Membrane

Question 29

How do macromolecules pass the plasma membrane?

The Plasma Membrane

Answer 29

Unable to pass across plasma membrane except by endocytosis or exocytosis.

The Plasma Membrane

Question 30

What is a concentration gradient?

The Plasma Membrane

Answer 30

• Difference in concentration of a chemical from one place to another (ex: inside to outside of plasma membrane).
• Example: oxygen molecules and sodium ions (Na+) are more concentrated in extracellular fluid than in cytosol.

The Plasma Membrane

Question 31

What is an electrical gradient?

The Plasma Membrane

Answer 31

• Difference in electrical charges between two regions.
• Inner surface of plasma membrane is more negatively charged, outer surface is more positively charged.
• Occurs across plasma membrane, charge difference is called membrane potential.

The Plasma Membrane

Question 32

Why are the concentration gradient and electrical grandient important?

The Plasma Membrane

Answer 32

• Help move substances across the plasma membrane.
• Substance will move across plasma membrane down its concentration gradient.
• Positively charged substance tend to move toward negatively charged area, and vice versa.

The Plasma Membrane

Question 33

What is an electrochemical gradient?

The Plasma Membrane

Answer 33

Combined influence of concentration gradient and electrical gradient on movement of a particular ion.

The Plasma Membrane

Question 34

Define passive processes in transport across the plasma membrane.

Transport across the Plasma Membrane

Answer 34

• Substance moves down its concentration or electrical gradient to cross membrane using only its own kinetic energy (energy of motion).
• Kinetic energy is intrinsic to particles that are moving. No input of energy from cell. Example: Simple diffusion.

Transport across the Plasma Membrane

Question 35

Define active processes in transport across the plasma membrane.

Transport across the Plasma Membrane

Answer 35

• Cellular energy is used to drive substance “uphill” against its concentration or electrical gradient.
• Cellular energy used, usually form of adenosine triphosphate (ATP).
• Another way some substances may enter/leave cells is active process, tiny, spherical membrane sacs (vesicles) are used.

Transport across the Plasma Membrane

Question 36

What is diffusion?

Transport across the Plasma Membrane

Answer 36

• Passive process, random mixing of particles in solution occurs because of particles’ kinetic energy.
• Both the solutes (dissolved substances) and solvent (liquid that does dissolving), undergo diffusion.

Transport across the Plasma Membrane

Question 37

How does diffusion work?

Transport across the Plasma Membrane

Answer 37

• If particular solute is present in high concentration in one area of solution and in low concentration in another area, solute molecules will diffuse towards area of lower concentrationmove down their concentration gradient.
• Particles become evenly distributed throughout solution and solution is at equilibrium. Particles continue to move about randomly due to kinetic energy, but their concentrations do not change.

Transport across the Plasma Membrane

Question 38

What factors influence the diffusion rate of substances across plasma membranes?

Transport across the Plasma Membrane

Answer 38

1. Steepness of concentration gradient.
2. Temperature.
3. Mass of diffusing substance.
4. Surface area.
5. Diffusion distance.

Transport across the Plasma Membrane

Question 39

How does steepness of the concentration gradient effect the diffusion rate?

Transport across the Plasma Membrane

Answer 39

• Greater the difference in concentration between the 2 sides of membrane, the higher the rate of diffusion.
• When charged particles are diffusing, steepness of electrochemical gradient determines diffusion rate across membrane.

Transport across the Plasma Membrane

Question 40

How does temperature effect the diffusion rate?

Transport across the Plasma Membrane

Answer 40

Higher the temperature, the faster the rate of diffusion.

Transport across the Plasma Membrane

Question 41

How does mass of the diffusing substance effect the diffusion rate?

Transport across the Plasma Membrane

Answer 41

The larger the mass of diffusing particle, the slower the diffusion rate.

Transport across the Plasma Membrane

Question 42

How does surface area effect the diffusion rate?

Transport across the Plasma Membrane

Answer 42

• The larger the membrane surface area available for diffusion, the faster the diffusion rate.
• Example: air sacs of lungs have large surface area available for diffusion of oxygen from air into blood.

Transport across the Plasma Membrane

Question 43

How does diffusion distance effect the diffusion rate?

Transport across the Plasma Membrane

Answer 43

• The greater the distance over which diffusion must occur, the longer it takes.
• Diffusion across a plasma membrane takes fraction of a second because membrane is thin.

Transport across the Plasma Membrane

Question 43

What are the three types of diffusion?

Transport across the Plasma Membrane

Answer 43

1. Simple diffusion.
2. Facilitated diffusion.
3. Osmosis.

Transport across the Plasma Membrane

Question 44

What is simple diffusion?

Transport across the Plasma Membrane

Answer 44

• Passive process, substances move freely through lipid bilayer of plasma membranes of cells without help of membrane transport proteins.
• Nonpolar, hydrophobic molecules move across lipid bilayer through this process.
• Such molecules include: oxygen, carbon dioxide and nitrogen gases; fatty acids; steroids; and fat-soluble vitamins (A, D, E, K). Small uncharged polar molecules such as water, urea and small alcohols.

Transport across the Plasma Membrane

Question 45

What is facilitated diffusion?

Transport across the Plasma Membrane

Answer 45

• Integral membrane protein assists specific substance across membrane.
• Integral membrane protein can be either a membrane channel or carrier.
• Moves solutes that are too polar or too highly charged to move through lipid bilayer by simple diffusion.

Transport across the Plasma Membrane

Question 46

What is channel-mediated facilitated diffusion?

Transport across the Plasma Membrane

Answer 46

• Solute moves down its concentration gradient across lipid bilayer through membrane channel.
• Most membrane channels are ion channels (integral transmembrane proteins that allow passage of small, inorganic ions that are too hydrophilic to penetrate the nonpolar interior of the lipid bilayer).
• Each ion can diffuse across membrane only at certain sites.

Transport across the Plasma Membrane

Question 47

What ion channels are common in plasma membranes?

Transport across the Plasma Membrane

Answer 47

• Most numerous are selective for K+ (potassium ions) or Cl- (chloride ions).
• Fewer channels are available for Na+ (sodium ions) or Ca2+ (calcium ions).

Transport across the Plasma Membrane

Question 48

Why is diffusion of ions through channels slower?

Transport across the Plasma Membrane

Answer 48

• Slower than free diffusion throughout lipid bilayer because channels occupy smaller fraction of membrane’s total surface area than lipids.
• Facilitated diffusion through channels is still very fast process.

Transport across the Plasma Membrane

Question 49

How is a channel gated in membrane channels?

Transport across the Plasma Membrane

Answer 49

• Channel protein acts as “plug” or “gate”, change shape in one way to open pore, in another way to close it.
• Alternate between open/closed positions.
• Others regulated by chemical or electrical changes inside and outside cell.

Transport across the Plasma Membrane

Question 50

What is carrier-mediated facilitated diffusion?

Transport across the Plasma Membrane

Answer 50

• Carrier (transporter) moves solute down its concentration gradient across plasma membrane.
• Passive process, no cellular energy required.
• Solute binds to specific carrier on one side of membrane and is released on other side after carrier undergoes change in shape.

Transport across the Plasma Membrane

Question 51

What is the transport maximum?

Transport across the Plasma Membrane

Answer 51

• Number of carriers available in a plasma membrane places an upper limit on rate of which facilitated diffusion can occur.
• Once all carriers are occupied, transport maximum is reached and a further increase in concentration gradient does not increase rate of facilitated diffusion.

Transport across the Plasma Membrane

Question 52

What substances move across the plasma membrane by carrier-mediated facilitated diffusion?

Transport across the Plasma Membrane

Answer 52

Glucose, fructose, galactose and some vitamins.

Transport across the Plasma Membrane

Question 53

How does glucose cross the plasma membrane?

Transport across the Plasma Membrane

Answer 53

Enters many body cells by carrier-mediated facilitated diffusion.
1. Glucose binds to specific type of carrier protein (glucose transporter (GluT)) on outside surface of membrane.
2. As transporter undergoes change in shape, glucose passes through membrane.
3. Transporter releases glucose on other side of membrane.

Transport across the Plasma Membrane

Question 54

What is osmosis?

Transport across the Plasma Membrane

Answer 54

• Type of diffusion, net movement of a solvent through selectively permeable membrane.
• Passive process.
• In living systems, solvent is water, which moves by osmosis across plasma membranes from area of higher water concentration to area of lower water concentration.

Transport across the Plasma Membrane

Question 55

How do water molecules pass through the plasma membrate during osmosis?

Transport across the Plasma Membrane

Answer 55

In two ways:
1. Moving between neighboring phospholipid molecules in lipid bilayer via simple diffusion.
2. Moving through aquaporins (or AQPs), which are integral membrane proteins that function as water channels.

Transport across the Plasma Membrane

Question 56

What are aquaporins (AQPs)?

Transport across the Plasma Membrane

Answer 56

• Integral membrane proteins, function as water channels.
• Play critical role in controlling water content of cells.
• Responsible for production of cerebrospinal fluid, aqueous humor, tears, sweat, saliva and concentration of urine.

Transport across the Plasma Membrane

Question 57

When does osmosis occur?

Transport across the Plasma Membrane

Answer 57

When a membrane is permeable to water but is not permeable to certain solutes.

Transport across the Plasma Membrane

Question 58

Define hydrostatic pressure.

Transport across the Plasma Membrane

Answer 58

Pressure exerted by a fluid at equilibrium at any point in time due to force of gravity.

Transport across the Plasma Membrane

Question 59

Define osmotic pressure.

Transport across the Plasma Membrane

Answer 59

• Osmotic pressure of a solution is proportional to concentration of solute particles that cannot cross membrane.
• Higher the solute concentration, the higher the solution’s osmotic pressure.

Transport across the Plasma Membrane

Question 60

Define a solution’s tonicity.

Transport across the Plasma Membrane

Answer 60

Measure of solution’s ability to change the volume of cells by altering their water content.

Transport across the Plasma Membrane

Question 61

What is an isotonic solution?

Transport across the Plasma Membrane

Answer 61

• Any solution in which a cell maintains its normal shape and volume.
• Shape is maintained as there is no net water movement into or out of cells.

Transport across the Plasma Membrane

Question 62

What is a hypotonic solution?

Transport across the Plasma Membrane

Answer 62

A solution that has a lower concentration of solutes than that inside a cell.

Transport across the Plasma Membrane

Question 63

What is a hypertonic solution?

Transport across the Plasma Membrane

Answer 63

A solution that has a higher concentration of solutes than that inside the cell.

Transport across the Plasma Membrane

Question 64

Describe what happens to RBCs in an isotonic solution.

Transport across the Plasma Membrane

Answer 64

• RBCs maintain their normal shape and volume.
• Concentrations of solutes that cannot cross the plasma membrane are the same on both sides of membrane in this solution.
• Example: 0.9% NcCl solution is isotonic for RBCs. Water molecules enter/exit at same rate.

Transport across the Plasma Membrane

Question 65

Describe what happens to RBCs in a hypotonic solution.

Transport across the Plasma Membrane

Answer 65

• Water molecules enter cell faster than they leave, causing RBCs to swell, eventually burst.
• Rupture of RBCs in this manner is called hemolysis.

Transport across the Plasma Membrane

Question 66

Describe what happens to RBCs in a hypertonic solution.

Transport across the Plasma Membrane

Answer 66

• Water molecules move out of cells faster than they enter, causing cells to shrink.
• Shrinkage of cells is called crenation.
• Example of hypertonic solution: 2% NaCl

Transport across the Plasma Membrane

Question 67

Define active transport.

Transport across the Plasma Membrane

Answer 67

Substance is moved across a plasma against its concentration gradient (“uphill”). Energy is required for carrier proteins to move solutes across membrane against a concentration gradient.
Two sources of cellular energy can be used.
1. Primary active transport: Obtained from hydrolosis of ATP.
2. Secondary active transport: From energy stored in an ionic concentration gradient.

Transport across the Plasma Membrane

Question 68

Define primary active transport.

Transport across the Plasma Membrane

Answer 68

• Energy derived from hydrolysis of ATP changes shape of carrier protein, “pumps” substance across plasma membrane against its concentration gradient.
• Body expends about 40% of ATP it generates on primary active transport.

Transport across the Plasma Membrane

Question 69

What is the sodium-potassium pump?

Transport across the Plasma Membrane

Answer 69

• Expels sodium ions (Na+) from cells, brings potassium ions (K+) into cell.
• Part of pump acts as an ATPase (enzyme that hydrolyzes ATP), often called Na+-K+ ATPase.

Transport across the Plasma Membrane

Question 70

What is the function of the sodium-potassium pump?

Transport across the Plasma Membrane

Answer 70

• Maintain low concentration of Na+ in cytosol by pumping these ions into extracellular fluid against Na+ concentration gradient.
• Pumps move K+ into cells against K+ concentration gradient.
• Maintains low concentration of Na+ and high concentration of K+ in cytosol.

Transport across the Plasma Membrane

Question 71

How does the sodium-potassium pump operate?

Transport across the Plasma Membrane

Answer 71

1. Three Na+ in cytosol bind to pump protein.
2. Binding of Na+ triggers hydrolysis of ATP into ADP, reaction attaches phosphate group to pump protein. Chemical reaction changes shape of pump protein, expelling the 3 Na+ into extracellular fluid. Shape of pump protein favors binding of 2 K+ into extracellular fluid to pump protein.
3. Binding of K+ triggers release of phosphate group from pump protein. Reaction causes shape of pump protein to change.
4. As pump protein reverts to its original shape, releases K+ into cytosol. Pump is ready again to bind 3 Na+, cycle repeats.

Transport across the Plasma Membrane

Question 72

Define secondary active transport.

Transport across the Plasma Membrane

Answer 72

• Energy stored in a Na+ or H+ concentration gradient is used to drive other substances across membrane against their own concentration gradients.
• Indirectly uses energy obtained from hydrolysis of ATP.
• Carrier protein simultaneously binds to Na+ and another substances, then changes shape so both substances cross membrane at same time.
  Symporters: Moving 2 substances in same direction.
  Antiporters: Moving 2 substances in opposite directions.

Transport across the Plasma Membrane

Question 73

Define a vesicle.

Transport across the Plasma Membrane

Answer 73

• Small, spherical sac.
• Variety of substances are transported from one structure to another within cells.
• Import materials from and release materials into extracellular fluid.

Transport across the Plasma Membrane

Question 74

Define endocytosis.

Transport across the Plasma Membrane

Answer 74

Materials move into a cell in a vesicle formed from plasma membrane. 3 types of endocytosis:
1. Receptor-mediated endocytosis
2. Phagocytosis
3. Bulk-phase endocytosis

Transport across the Plasma Membrane

Question 75

What is Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 75

• Imports materials needed by cells.
• Highly selective type of endocytosis, cells take up specific ligands (ligands are molecules that bind to specific receptors).
• Vesicle forms after receptor protein in plasma membrane recognizes and binds to a particular particle in extracellular fluid.

Transport across the Plasma Membrane

Question 76

What is the process of Receptor-medicated endocytosis?

Transport across the Plasma Membrane

Answer 76

1. Binding
2. Vesicle formation
3. Uncoating
4. Fusion within endosome
5. Recycling of receptors to plasma membrane
6. Degradation in lysosomes

Transport across the Plasma Membrane

Question 77

What occurs during binding in Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 77

• Extracellular side of plasma membrane, LDL particle that contains cholesterol binds to specific receptor in plasma membrane to form a receptor-LDL complex.
• Receptors are integral membrane proteins that are concentrated in regions of plasma membrane called clathrin-coated pits.
• Protein called clathrin attaches to membrane on cytoplasmic side. Clathrin molecules come together, forming basketlike structure around receptor-LDL complexes that causes membrane to invaginate (fold inward).

Transport across the Plasma Membrane

Question 78

What occurs during vesicle formation in Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 78

• Invaginated edges of membrane around clathrin-coated pit fuse, and a small piece of membrane pinches off.
• Resulting vesicle (clathrin-coated vesicle), contains receptor-LDL complexes.

Transport across the Plasma Membrane

Question 79

What occurs during uncoating during Receptor-mediated endocyctosis?

Transport across the Plasma Membrane

Answer 79

• Almost immediately after formed, clathrin-coated vesicle loses it clathrin coat to become an uncoated vesicle.
• Clathrin molecules either return to inner surface of plasma membrane or help forms coats on other vesicles inside cell.

Transport across the Plasma Membrane

Question 80

What occurs during fusion with endosome during Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 80

• Uncoated vesicle quickly fuses with vesicle known as an endosome.
• Within an endosome, the LDL particles seperate from their receptors.

Transport across the Plasma Membrane

Question 81

What occurs during recycling of receptors to plasma membrane of Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 81

• Receptors accumulate in enlongated protrusions of the endosome.
• These pinch off, forming transport vesicles that return the receptors to plasma membrane.
• An LDL receptor is returned to plasma membrane about 10 minutes after it enters cell.

Transport across the Plasma Membrane

Question 82

What occurs during degradation in lysosomes in Receptor-mediated endocytosis?

Transport across the Plasma Membrane

Answer 82

• Transport vesicles, which contain LDL particles, bud off the endosome and fuse with a lysosome (contain many digestive enzymes).
• Certain enzymes break down large protein and lipid molecules of LDL particle into amino acids, fatty acids, cholesterol.
• Smaller molecules then leave the lysosome.
• Cell uses cholesterol for rebuilding membranes and for synthesis of steroids (ex. estrogen).
• Fatty acids and amino acids can be used for ATP production or to build other molecules needed by the cell.

Transport across the Plasma Membrane

Question 83

What is Phagocytosis?

Transport across the Plasma Membrane

Answer 83

• Form of endocytosis in which cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses.
• Only few body cells, called phagocytes, are able to carry out phagocytosis.
• Vital defense mechanism that helps protect the body from disease.

Transport across the Plasma Membrane

Question 84

What are the two main types of phagocytes?

Transport across the Plasma Membrane

Answer 84

1. Macrophages (located in many body tissues).
2. Neutrophils (a type of white blood cell).

Transport across the Plasma Membrane

Question 85

What is the process of Phagocytosis?

Transport across the Plasma Membrane

Answer 85

• Particle binds to a plasma membrane receptor on phagocyte, causing it to extend pseudopods (projections of its plasma membrane and cytoplasm).
• Pseudopods surround particle outside cell, membranes fuse to form vesicle called a phagosome, which enters the cytoplasm.
• Phagosome fuses with one or more lysosomes, and lysosomal enzymes break down ingested material.
• Any undigested materials in the phagosome remain indefinitely in a vesicle called residual body.
• Residual bodies are then secreted by cell via exocytosis or remain stored in cell as lipofuscin granules.

Transport across the Plasma Membrane

Question 86

What is Bulk-phase endocytosis?

Transport across the Plasma Membrane

Answer 86

• Pinocytosis or “cell drinking”, form of endocytosis in which tiny droplets of extracellular fluid are taken up.
• No receptor proteins are involved; all solutes dissolved in the extracellular fluid are brought into the cell.
• Most body cells carry out bulk-phase endocytosis, especially absorptive cells in intestines and kidneys.

Transport across the Plasma Membrane

Question 87

What is the process of Bulk-phase endocytosis?

Transport across the Plasma Membrane

Answer 87

• Plasma membrane folds inward, forms a vesicle containing droplet of extracellular fluid.
• Vesicle detaches/pinches off from plasma membrane, enters cytosol.
• Within cell, vesicle fuses with a lysome, where enzymes degrade engulfed solutes.
• Resulting smaller molecules, such as amino acids and fatty acids, leave lysosome to be used elsewhere in cell.

Transport across the Plasma Membrane

Question 88

What is Exocytosis?

Transport across the Plasma Membrane

Answer 88

Releases materials from cell. Carried out by all cells. Especially important in 2 types of cells.
1. Secretory cells (liberate digestive enzymes, hormones, mucus or other secretions).
2. Nerve cells (that release substances called neurotransmitters).
In some cases, wastes are also released by exocytosis.

Transport across the Plasma Membrane

Question 89

What is the process of Exocytosis?

Transport across the Plasma Membrane

Answer 89

• Membrane-enclosed vesicles called secretory vesicles form inside cell, fuse with plasma membrane and release their contents into extracellular fluid.
• Segments of plasma membrane lost through endocytosis are recovered/recycled by exocytosis.

Transport across the Plasma Membrane

Question 90

How is the cells plasma membrane kept relatively constant?

Transport across the Plasma Membrane

Answer 90

• Balance between endocytosis and exocytosis keeps the surface area of cells plasma membrane relatively constant.
• Membrane exchange is quite extensive in certain cells.
• In pancreas, cells that secrete digestive enzymes can recycle an amount of plasma membrane equal to cell’s entire surface area in 90 minutes.

Transport across the Plasma Membrane

Question 91

What is Transcytosis?

Transport across the Plasma Membrane

Answer 91

Active process, vesicles undergo endocytosis on one side of a cell, move across the cell, and then undergo exocytosis on the opposite side.

Transport across the Plasma Membrane

Question 92

What is the process of Transcytosis?

Transport across the Plasma Membrane

Answer 92

• As vesicles fuse with plasma membrane, vesicular contents are released into extracellular fluid.
• Occurs most often across endothelial cells that line blood vessels and is means for materials to move between blood plasma and interstitial fluid.
• During pregnancy, some of Moms antibodies crosss the placenta into fetal circulation via transcytosis.

Transport across the Plasma Membrane

Question 93

What is the Cytoplasm of a cell?

Cytoplasm

Answer 93

Consists of all the cellular contents between the plasma membrane and the nucleus, and has 2 components:
1. Cytosol
2. Organelles

Cytoplasm

Question 94

What is the Cytosol?

Cytoplasm

Answer 94

• Intracellular fluid, the fluid portion of cytoplasm that surrounds organelles, constitutes about 55% of total cell volume.
• Varies in composition and consistency from one part of a cell to another.
• 75-90% water plus various dissolved and suspended components.
• Among are different type of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP and waste products.
• The site of many chemical reactions required for a cell’s existence.

Cytoplasm

Question 95

What is the Cytoskeleton?

Cytoplasm

Answer 95

A network of protein filaments that extends throughout cytosol. 3 types of filaments contrubute to cytoskeleton’s structure (as well as structure of other organelles).
1. Microfilaments
2. Intermediate filaments
3. Microtubules

Cytoplasm

Question 96

What are Microfilaments of the cytoskeleton?

Cytoplasm

Answer 96

• Thinnest elements of the cytoskeleton.
• Composed of the proteins actin and myosin and are most prevalent at edge of a cell.

Two general functions:
1. Help generate movement
2. Provide mechanial support.

Cytoplasm

Question 97

What is the function of Microfilaments?

Cytoplasm

Answer 97

• Involved in muscle contraction, cell division, cell locomotion.
• Provide much of mechanical support responsible for basic strength and shapes of cells.
• Anchor cytoskeleton to integral proteins in plasma membrane.
• Provide mechanical support for cell extensions called microvilli.

Cytoplasm

Question 98

What are Microvilli?

Cytoplasm

Answer 98

• Nonmotile, microscopic fingerlike projections of plasma membrane.
• Within each microvillus is a core of parallel microfilaments that supports it.
• Greatly increase surface area of the cell, are abundant on cells involved in absorption, such as epithelial cells that line small intestine.

Cytoplasm

Question 99

What are Intermediate filaments of the cytoskeleton?

Cytoplasm

Answer 99

• Thicker than microfilaments, thinner than microtubules.
• Are exceptionally strong, composted of several different proteins.
• Found in parts of cells subject to mechanic stress.

Cytoplasm

Question 100

What is the function of Intermediate filaments?

Cytoplasm

Answer 100

Help stabilize the position of organelles such as the nucleus and help attach cells to one another.

Cytoplasm

Question 101

What are Microtubules of the cytoskeleton?

Cytoplasm

Answer 101

• Largest of cytoskeletal components.
• Long, unbranches hollow tubes composed mainly of protein tubulin.
• Assembly begins in an organelle called the centrosome.
• Microtubules grow outward from centrosome toward periphery of cell.

Cytoplasm

Question 102

What is the function of Microtubules?

Cytoplasm

Answer 102

• Help determine cell shape.
• Function in the movement of organelles such as secretory vesicles, of chromosomes during cell division, and of specialized cell projections, such as cilia and flagella.

Cytoplasm

Question 103

What are the functions of the Cytoskeleton?

Cytoplasm

Answer 103

1. Serves as a scaffold that helps determine cell’s shape and organize cellular contents.
2. Aids movement of organelles within cell, of chromosomes during cell division, and of whole cells such as phagocytes.

Cytoplasm

Question 104

What are Organelles?

Cytoplasm

Answer 104

• Specialized structures within cell that have characteristic shapes, and perform specific functions in cellular growth, maintenance, and reproduction.
• Is little interference among chemical reactions in a cell as they are confined to different organelles.

Cytoplasm

Question 105

What are the functions of Organelles?

Cytoplasm

Answer 105

• Each type of organelle has its own set of enzymes that carry out specific reactions.
• Serves as functional compartment for specific biochemical processes.
• Cooperate to maintain homeostasis.
• The nucleus is a large organelle.

Cytoplasm

Question 106

What is a Centrosome?

Cytoplasm

Answer 106

Also called microtubule organizing center, located near the nucleus, consists of 2 components.
1. A pair of centrioles
2. The pericentriolar matrix
* During cell division, centrosomes replicate so succeeding generations of cells have capacity for cell division.

Cytoplasm

Question 107

What are the Centrioles?

Cytoplasm

Answer 107

• The 2 centrioles are cylindrical structures, each composed of nine clisters of three microtubules (triplets) arranged in a circular pattern.
• Long axis of one centriole is at a right angle to the long axis of the other.

Cytoplasm

Question 108

What is the Pericentriolar matrix?

Cytoplasm

Answer 108

• Surrounds the centrioles.
• Contains hundreds of ring-shaped complexes composed of protein tubulin.
• Tubulin complexes are organizing centers for growth of mitotic spindle, which plays critical role in cell division, and for microtubule formation in nondividing cells.

Cytoplasm

Question 109

What are the functions of the Centrosomes?

Cytoplasm

Answer 109

1. The perocentriolar matrix of centrosome contains tubulins that build microtubules into nondividing cells.
2. The pericentriolar matrix of centrosome forms the mitotic spindle during cell division.

Cytoplasm

Question 110

What are Cilia?

Cytoplasm

Answer 110

• Motile projections of the cell surface.
• Numerous, short, hairlike projections, extend from surface of the cell.
• Each cilium contains a core of 20 microtubules surrounded by plasma membrane.
• Anchored to basal body just below surface of the plasma membrane.

Cytoplasm

Question 111

How does the Cilia function?

Cytoplasm

Answer 111

• Displays oarlike pattern of beating, relatively stiff during power stroke, more flexible during recovery stroke.
• Coordinated movement of many cilia on surface of a cell causes steady movement of fluid along cell surface.
• Cells of respiratory tract have hundreds of cilia that sweep foreign particles trapped in mucus away from lungs.

Cytoplasm

Question 112

What are Flagella?

Cytoplasm

Answer 112

• Used to move an entire cell.
• Generates forward motion along its axis by rapidly wiggling in wavelike patern.
• Only example in human body is a sperm cell tail.

Cytoplasm

Question 113

What are the functions of Cilia and Flagella?

Cytoplasm

Answer 113

1. Cilia moves fluids along a cell’s surface.
2. A flagellum moves an entire cell.

Cytoplasm

Question 114

What are Ribosomes?

Cytoplasm

Answer 114

• The sites of protein synthesis.
• High content of one type of ribonucleic acid (ribosomal RNA, or rRNA), but each ribosome includes more than 50 proteins.
• Structurally, consists of 2 subunits, large and small subunits.
• Also located within mitochondria, where they synthesize mitochondrial proteins.

Cytoplasm

Question 115

What are the large and small subunits of Ribosomes?

Cytoplasm

Answer 115

• Made separately in the nucleolus, a spherical body inside the nucleus.
• Once produced, large and small subunits exit the nucleus separately, then come together in cytoplasm.

Cytoplasm

Question 116

What are the functions of Ribosomes?

Cytoplasm

Answer 116

1. Ribosomes associated with endoplasmic reticulum synthesize proteins destined for insertion in the plasma membrane or secretion from the cell.
2. Free ribosomes synthesize proteins used in the cytosol.

Cytoplasm

Question 117

What is the Endoplasmic reticulum?

Cytoplasm

Answer 117

• Network of membranes in form of flattened sacs or tubules.
• Extends from nuclear envelope, to which it is connected and projects throughout the cytoplasm.
• Extensive, constitutes more than half of membranous surfaces within cytoplasm of most cells.

Cytoplasm

Question 118

What are the two different forms of Endoplasmic reticulum?

Cytoplasm

Answer 118

1. Rough endoplasmic reticulum
2. Smooth endoplasmic reticulum

Cytoplasm

Question 119

What is Rough endoplasmic reticulum?

Cytoplasm

Answer 119

• Continuous with nuclear membrane and usually is folded into series of flattened sacs.
• Outer surface is studded with ribosomes (sites of protein synthesis).

Cytoplasm

Question 120

What is the function of Rough endoplasmic reticulum?

Cytoplasm

Answer 120

• Synthesizes glycoproteins and phospholipids.
• Produces secretory proteins, membrane proteins, and many organellar proteins.
• Proteins synthesized by ribosomes attached to rough ER enter spaces within ER for processing, sorting. Some cases, enzymes attach proteins to carbohydrates, form glycoproteins.

Cytoplasm

Question 121

What is Smooth endoplasmic reticulum?

Cytoplasm

Answer 121

• Extends from rough ER to form network of membrane tubules.
• Does not have ribosomes on outer surfaces of its membrane.
• Contains unique enzymes that make it functionally more diverse.
• Does not synthesize proteins.

Cytoplasm

Question 122

What is the function of Smooth endoplasmic reticulum?

Cytoplasm

Answer 122

• Synthesize fatty acids and steroids (ex. estrogen and testosterone).
• Liver cells: enzymes of smooth ER help release glucose into blood, inactivate/detoxify lipid-soluble drugs or harmful substances.
• Liver, kidney, intestinal cells: enzyme removes phosphate group from glucose-6 phosphate, allows free glucose to enter blood. Releases calcium ions that trigger contraction in muscle cells.

Cytoplasm

Question 123

What is the Golgi Complex?

Cytoplasm

Answer 123

• An organelle.
• Consists of 3-20 cisterns, small, flattened membranous sacs with bulging edges.
• Cisterns are often curved, giving a cuplike shape.
• Most cells have several Golgi complexes, more extensive in cells that secrete proteins.

Cytoplasm

Question 124

What are the functions of the Golgi complex?

Cytoplasm

Answer 124

1. Modifies, sorts, packages, and transports proteins received from rough ER.
2. Forms secretory vesicles that discharge processed proteins via exocytosis into extracellular fluid.
3. Forms membrane vesicles that ferry new molecules to plasma membrane.
4. Forms transport vesicles that carry molecules to other organelles, such as lysosomes.

Cytoplasm

Question 125

What are the cisterns of the Golgi complex?

Cytoplasm

Answer 125

• Cisterns at opposite ends of Golgi complex differ in size, shape, enzyme activity.
• Convex entry (cis) face is a cistern that faces rough ER.
• Concave exit (trans) face is a cistern that faces plasma membrane.
• Sacs between entry and exit faces are called medial cisterns.
• Transport vesicles from ER merge to form entry face. From entry face, cisterns are thought to mature, in turn becoming medial and then exit cisterns.

Cytoplasm

Question 126

What is the process in which proteins enter, pass through and exit the Golgi complex?

Cytoplasm

Answer 126

1. Entry face received and modifies proteins produced by rough ER.
2. Medial cisterns add carbohydrates to proteins to form glycoproteins and lipids to proteins to form lipoproteins.
3. Exit face modifies molecules further and then sorts and packages them for transport to destinations.

Cytoplasm

Question 127

Describe the process of maturation of the cisternae and exhanges that occur via transfer vesicles within the Golgi complex.

Cytoplasm

Answer 127

1. Proteins synthesized by ribosomes on rough ER are surrounded by a piece of ER membrane, buds from membrane surface to form transport vesicles.
2. Transport vesicles move toward entry face of Golgi complex.
3. Fusion of several transport vesicles creates entry face of Golgi complex, releases proteins into its lumen.
4. Proteins move from entry face into one or more medial cisterns. Enzymes in medial cisterns modify proteins to form glycoproteins, glycolipids, and lipoproteins. Transfer vesicles that bud from edges of cisterns move specific enzymes back toward the entry face and move some partially modified proteins toward exit face.
5. Products of medial cisterns move into lumen of exit face.
6. Within exit face cistern, products are further modified, sorted and packaged.
7. Some of processed proteins leave exit face, are stored in secretory vesicles. These vesicles deliver proteins to plasma membrane, where they are discharged by exocytosis into extracellular fluid.
8. Other processed proteins leave exit face in membrane vesicles, deliver contents to plasma membrane for incorporation into membrane. Golgi complex adds new segments of plasma membrane as existing segments are lost and modifies number and distribution of membrane molecules.
9. Some processed proteins leave exit face in transport vesicles that will carry proteins to another cellular destination.

Cytoplasm

Question 128

What are the 3 general destinations for proteins that leave the Golgi complex?

Cytoplasm

Answer 128

1. Lysosomes
2. Plasma membrane
3. Secretion

Cytoplasm

Question 129

What are Lysosomes?

Cytoplasm

Answer 129

• Membrane-enclosed vesicles that form from Golgi complex.
• Contain as many as 60 kinds of powerful digestive and hydrolytic enzymes, can break down variety of molecules once lysosomes fuse with vesicles formed from endocytosis.
• Work best at an acidic pH, lysosomal membrane includes active transport pumps that import hydrogen ions, thus, lysosomal interior has pH of 5.

Cytoplasm

Question 130

What are the functions of Lysosomes?

Cytoplasm

Answer 130

1. Digest substances that enter cell via endocytosis and transport final products of digestion into cytosol.
2. Carry out autophagy (digestion of worn-out organelles).
3. Implement autolysis (digestion of entire cell).
4. Accomplish extracellular digestion.

Cytoplasm

Question 131

How do Lysosomes participate in autophagy?

Cytoplasm

Answer 131

• Organelle to be digested is enclosed by membrane derived from the ER to create a vesicle called autophagosme.
• Vesicle then fuses with a lysosome. Lysosomal enzymes help recycle worn-out cell structures.
• Can engulf another organelle, digest it and return digested components to the cytosol for reuse.
• Enzymes may destroy entire cell that contains them, called autolysis.

Cytoplasm

Question 132

What are Peroxisomes?

Cytoplasm

Answer 132

• Group of organelles, smaller than lysosomes.
• Also called microbodies.
• Contain several oxidases, enzymes that can oxidize various organic substances.
• Amino acids and fatty acids are oxidized in peroxisomes as part of normal metabolism.
• Enzymes in peroxisomes oxidize toxic substances.
• Abundant in the liver.

Cytoplasm

Question 133

How do Peroxisomes protect the cell from the by-product of oxidation?

Cytoplasm

Answer 133

• Peroxisomes contain the enzyme catalase, which decomposes H2O2 (hydrogen peroxide).
• Production and degradation of H2O2 occur within the same organelle, peroxisomes protect other parts of cell from toxic effects of H2O2.
• Peroxisomes also contain enzymes that destroy superoxide.

Cytoplasm

Question 134

What are Proteasomes?

Cytoplasm

Answer 134

• Tiny barrel-shaped structures consisting of four stacked rings of proteins around a central core.
• Provide continuous destruction of unneeded, damaged or faulty proteins.
• Typical body cell contains thousands of proteasomes, in cytosol and nucleus.
• Contain myriad proteases (enzymes that cut proteins into small peptides).
• Once enzymes of proteasome have chopped up protein into smaller chunks, other enzymes break down the peptides into amino acids, which can be recycled into new proteins.

Cytoplasm

Question 135

What is the Mitochondria?

Cytoplasm

Answer 135

• “Powerhouses” of the cell.
• Generate most of the ATP through aerobic respiration.
• Cell can have hundreds or thousands of mitochondria, depending on its activity.
• Active cells that use ATP at high rates (muscles, liver, kidneys) have large number of mitochondria.
• Usually located within cell where oxygen enters the cell or where the ATP is used.

Cytoplasm

Question 136

What are the functions of Mitochondria?

Cytoplasm

Answer 136

1. Generate ATP through reactions of aerobic cellular respiration.
2. Play an important early role in apoptosis.

Cytoplasm

Question 137

Describe the structure of Mitochondria.

Cytoplasm

Answer 137

• Mitochondrion consists of external mitochondrial membrane and internal mitochondrial membrane with small fluid-filled space between.
• Both membranes are similar in structure to plasma membrane.
• Internal mitochondrial membrane contains series of folds called mitochondrial cristae.
• Central fluid-filled cavity of a mitochondrion, enclosed by internal mitochonrial membrane, the mitochondrial matrix.
• Elaborate folds of the cristae provide enormous surface afea for chemical reactions that are part of the aerobic phase of cellular respiration (the reactions that produce most of a cell’s ATP).

Cytoplasm

Question 138

What is Apoptosis?

Cytoplasm

Answer 138

Is the orderly, genetically progreammed death of a cell.

Cytoplasm

Question 139

How does Apoptosis work?

Cytoplasm

Answer 139

• Response to stimuli, certain chemicals are released from mitochondria following formation of pore in outer mitochondrial membrane.
• One of chemicals released into cytosol of cell is cytochrome c, which while inside mitochondria, is involved in aerobic cellular respiration.
• In cytosol, cytochrome c and other substances initiate a cascade of activation of protein-digesting enzymes that bring about apoptosis.

Cytoplasm

Question 140

Describe Mitochondria DNA.

Cytoplasm

Answer 140

• Mitrochondria have their own DNA, in form of multiple copies of a circular DNA molecule that contains 37 genes.
• Mitochondrial genes control synthesis of 2 ribosomal RNAs, 22 transfer RNAs, and 13 proteins that build mitochondrial components.
• Mitochondrial genes are inherited only from mother.

Cytoplasm

Question 141

What is the Nucleus?

Nucleus

Answer 141

• Spherical or oval-shaped structure, usually most prominent feature of a cell.
• Most cells have a single nucleus, although some, such as mature red blood cells, have none.
• Skeletal muscle cells and other types of cells have multiple nuclei.

Nucleus

Question 142

What is the nuclear envelope?

Nucleus

Answer 142

• Double membrane that separates nucleus from cytoplasm.
• Both layers of nuclear envelope are lipid bilayers similar to plasma membrane.
• Outer membrane of nuclear envelope is continuous with rough ER and resembles it in structure.

Nucleus

Question 143

What are nuclear pores?

Nucleus

Answer 143

• Many openings that extend through the nuclear envelope.
• Each nuclear pore consists of circular arrangement of proteins surrounding large central opening that is about 10 times wider than the pore of a channel protein in plasma membrane.

Nucleus

Question 144

What is the function of nuclear pores?

Nucleus

Answer 144

• Control the movement of substances between nucleus and cytoplasm.
• Small molecules and ions move through pores passively by diffusion.
• Most lage molecules, such as RNAs and proteins, cannot pass through nuclear pores by diffusion. Instead, involves active transport process, molecules are recognized and selectively transported through nuclear pore into or out of nucleus.

Nucleus

Question 145

What are nucleoli?

Nucleus

Answer 145

• Spherocal bodies inside the nucleus that function in producing ribosomes.
• Each nucleolus is a cluster of protein, DNA and RNA; it is not enclosed by a membrane.

Nucleus

Question 146

What is the function of nucleoli?

Nucleus

Answer 146

• Sites of synthesis of rRNA and assembly of rRNA and proteins into ribosomal subunits.
• Are prominent in cells that synthesize large amounts of protein (muscle and liver cells).
• Nucleoli disperse and disappear during cell division and reorganize once new cells are formed.

Nucleus

Question 147

What are genes?

Nucleus

Answer 147

• Cell’s hereditary units, control cellular structure and direct cellular activities.
• Arranged along chromosomes.
• Human somatic (body) cells have 46 chromosomes, 23 inherited from each parent.
• Each chromosome is a long molecule of DNA that is coiled together with several proteins.

Nucleus

Question 148

What is a genome?

Nucleus

Answer 148

The total genetic information carried in a cell or an organism.

Nucleus

Question 149

What is chromatin?

Nucleus

Answer 149

• A complex of nucleic acids (DNA or RNA) and proteins (e.g. histones).
• In cells that are not dividing, chromatin appears as a diffuse, granular mass.
• Has a beads-on-a-string structure.

Nucleus

Question 150

What is a nucleosome?

Nucleus

Answer 150

• The basic repeating subunit of chromatin packaged inside the cell’s nucleus.
• Each bead of chromatin string structure is a neuleosome that consists of double-stranded DNA wrapped twice around a core of eight proteins called histones, which help organize the coiling and folding of DNA.

Nucleus

Question 151

What are the functions of the Nucleus?

Nucleus

Answer 151

1. Controls cellular structure.
2. Direct cellular activities.
3. Produces ribsomes in nucleoli.

Nucleus

Question 152

What is a Proteome?

Protein Synthesis

Answer 152

Refers to all of an organism’s proteins.

Protein Synthesis

Question 153

What is Gene expression?

Protein Synthesis

Answer 153

• Gene’s DNA is used as template for synthesis of a specific protein.
• First, information encoded in a specific region of DNA is transcribed to produce a specific mole of RNA (transcription).
• Second, RNA attaches to ribosome, where information contained in RNA is translated into corresponding sequence of amino acids to form a new protein molecule (translation).

Protein Synthesis

Question 154

What is a base triplet of DNA?

Protein Synthesis

Answer 154

• Sequence of three nucleotides in DNA, where genertic information is stored in sets of three nucleotides.
• Each DNA base triplet is transcribed as a complementary sequence of three nucleotides, called a codon.
• A given codon specifies a particular amino acid.

Protein Synthesis

Question 155

What is a Genetic code?

Protein Synthesis

Answer 155

Set of rules that relate the base triplet sequence of DNA to corresponding condons of RNA and amino acids they specify.

Protein Synthesis

Question 156

What is transcription of DNA?

Protein Synthesis

Answer 156

Occurs in nucleus, genetic information represented by the sequence of base triplets in DNA serves as template for copying information into a complementary sequence of codons. Genetic information in DNA is copied to RNA. 3 types of RNA are made from DNA template:
1. Messenger RNA (mRNA).
2. Ribosomal RNA (rRNA).
3. Transfer RNA (tRNA).

Protein Synthesis

Question 157

What is the purpose of Messenger RNA (mRNA)?

Protein Synthesis

Answer 157

Directs the synthesis of a protein.

Protein Synthesis

Question 158

What is the purpose of Ribosomal RNA (rRNA)?

Protein Synthesis

Answer 158

Joins with ribosomal proteins to make ribosomes.

Protein Synthesis

Question 159

What is the purpose Transfer RNA (tRNA)?

Protein Synthesis

Answer 159

• Binds to amino acid, holds it in place on ribosome unit, incorporated into a protein during translation.
• One end of tRNA carries a specific amino acid, and the opposide end consists of a triplet of nucleotides called anticodon.
• By pairing between complementary bases, tRNA anticodon attaches to mRNA codon.

Protein Synthesis

Question 160

What is the RNA polymerase?

Protein Synthesis

Answer 160

• Enzyme that catalyzes transcription of DNA.
• Enzyme must be instructed where to start transcription process and where to end it.
• Only one of two DNA strands serves as a template for RNA synthesis.

Protein Synthesis

Question 161

What is a promoter of DNA transcription?

Protein Synthesis

Answer 161

• Segment of DNA where transcription begins, a special nucleotide sequence.
• Promoter is located near beginning of a gene.
• This is where RNA polymerase attaches to the DNA.

Protein Synthesis

Question 162

During transcription, what bases in the DNA pair with RNA?

Protein Synthesis

Answer 162

DNA Template / RNA Strand
Adenine (A) pairs with Uracil (U)
Thymine (T) pairs with Adenine (A)
Guanine (G) pairs with Cytosine (C)
Cytosine (C) pairs with Guanine (G)

Protein Synthesis

Question 163

What is the terminator of DNA transcription?

Protein Synthesis

Answer 163

• Special nucleotide sequence, which specifies the end of the gene.
• When RNA ploymerase reaches terminator, enzyme detaches from transcribed RNA molecule and DNA strand.

Protein Synthesis

Question 164

What are introns within a gene?

Protein Synthesis

Answer 164

Regions that do not code for parts of a protein.

Protein Synthesis

Question 165

What are exons within a gene?

Protein Synthesis

Answer 165

Regions that do code for segments of a protein.

Protein Synthesis

Question 166

What is translation?

Protein Synthesis

Answer 166

• Nucleotide sequence in an mRNA molecule specifies amino acid sequence of a protein.
• Ribosomes in cytoplasm carry out translation.
• mRNA molecule binds to a ribosome. Then, mRNA nucleotide sequence specifies the amino acid sequence of a protein.

Protein Synthesis

Question 167

What is the process of translation?

Protein Synthesis

Answer 167

1. mRNA molecule binds to small ribosomal subunit at mRNA binding site. Special tRNA (initiator tRNA) binds to start codon (AUG) on mRNA where translation begins. tRNA anticodon (UAC) attaches to mRNA codon (AUG) by pairing between complementary bases. AUG is also the codon for amino acid methionine, thus methionine is always first amino acid in growing polypeptide.
2. Large ribosomal subunit attaches to small ribosomal subunit-mRNA complex, creating functional ribosome. Initiator tRNA with its amino acid(methionine), fits into P site of ribosome.
3. Anticodon of another tRNA with its attached amino acid pairs with second mRNA codon at A site of ribosome.
4. Component of large ribosomal subunit catalyzes formation of peptide bond between methionine and the amino acid carried by the tRNA at the A site.
5. Following formation of peptide bond, resulting two peptide protein becomes attached to tRNA at A site.
6. After peptide bond formation, ribosome shifts mRNA strand by one codon. tRNA in the P site enters E site and is released from ribosome. tRNA in A site bearing the 2 peptide protein shifts into P site, allowing tRNA with its amino acid to bind to newly exposed codon at A site. Steps 3-6 occur repeatedly and protein legthens progressively.
7. Protein synthesis ends when ribosome reaches stop codon at A site, causes completed protein to detach from final tRNA. Also, tRNA vacates P site and ribosome splits into its large and small subunits.

Protein Synthesis

Question 168

At what rate does protein synthesis occur?

Protein Synthesis

Answer 168

• About 15 peptide bonds per second.
• As ribosome moves along mRNA, before it completes synthesis of whole protein, another ribosome may attach behind it and begin translation of same mRNA strand.

Protein Synthesis

Question 169

What is a polyribosome?

Protein Synthesis

Answer 169

• Several ribosomes that are attached to same mRNA molecule.
• Simultaneous movement of several ribosomes along same mRNA molecule permits translation of one mRNA into several identical proteins at same time.

Protein Synthesis

Question 170

What is cell division?

Cell Division

Answer 170

The process by which cells reproduce themselbes. Two types of cell division:
1. Somatic cell division
2. Reproductive cell division

Cell Division

Question 171

What is a somatic cell?

Cell Division

Answer 171

Is any cell of the body other than a germ cell.

Cell Division

Question 172

What occurs during somatic cell division?

Cell Division

Answer 172

• Cell undergoes nuclear division called mitosis and a cytoplasmic division called cytokinesis, to produce two genetically identical cells, each with the same number and kind of chromosomes as original cell.
• Replaces dead/injured cells and adds new ones during tissue growth.

Cell Division

Question 173

What is a germ cell?

Cell Division

Answer 173

A gamete (sperm or oocyte) or any precursor cell destined to become a gamete.

Cell Division

Question 174

What occurs during reproductive cell division?

Cell Division

Answer 174

• Mechanism that produces gametes, cells needed to form next generation of sexually reproducing organisms.
• Consists of a special 2-step division called meiosis, in which the number of chromosomes in the nucleus is reduced by half.

Cell Division

Question 175

What events occur during the somatic cell cycle?

Cell Division

Answer 175

Intraphase: G1 phase, S phase, G2 phase.
Mitotic Phase: Mitosis, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis.

Cell Division

Question 176

What is the cell cycle?

Cell Division

Answer 176

• Orderly sequence of events in which a somatic cell duplicates its contents and divides into two.
• Some cells divide more than others.

Cell Division

Question 177

What are homologous chromosomes?

Cell Division

Answer 177

• Two chromosomes that makeup each pair (homologs).
• Contain similar genes arranged in the same (or almost the same) order.
• Human cells contain 23 pairs of chromosomes, for a total of 46.

Cell Division

Question 178

What are sex chromosomes?

Cell Division

Answer 178

One pair of chromosomes, designated X and Y.
Females: homologous pair of sex chromosomes consists of 2 large X chromosomes
Males: pair consists of an X and a much smaller Y chromosome.

Cell Division

Question 179

What is a diploid cell?

Cell Division

Answer 179

Cells that contain two sets of chromosomes. Also called (2n) cells.

Cell Division

Question 180

What is interphase of cell division?

Cell Division

Answer 180

• Process where cell replicates its DNA.
• Produces additional organelles and cytosolic components in anticipation of cell division.
• A state of high metabolic activity; it is during this time that cell does most of its growing.
• Consists of 3 phases: G1, S and G2.

Cell Division

Question 181

What is the G1 phase of cell division?

Cell Division

Answer 181

• Interval between mitotic phase and S phase.
• Cell is metabolically active; replicates most of its organelles and cytosolic componenets but not DNA.
• Replication of centrosomes begins.
• For cell with total cell cycle time of 24 hours, G1 lasts 8-10 hours, duration is variable. Very short in embroyonic cells and cancer cells.

Cell Division

Question 182

What is the G0 phase of cell division?

Cell Division

Answer 182

• Cells that remain in G1 for very long time, perhaps destined never to divide again.
• Most nerve cells are in the G0 phase.

Cell Division

Question 183

What is the S phase of cell division?

Cell Division

Answer 183

• DNA replication and centrosome replication occurs.
• Interval between G1 and G2, lasts about 8 hours.
• Once cell enters S phase, is committed to go through rest of cell cycle.
• As result of DNA replication, 2 identical cells formed during cell division later in cell cycle will have the same genetic material.

Cell Division

Question 184

What is the G2 phase of cell division?

Cell Division

Answer 184

• Interval between S phase and mitotic phase.
• Lasts 4-6 hours.
• Cell growth continues, enzymes and other proteins are synthesized in preparation for cell division, and replication of centrosomes is completed.

Cell Division

Question 185

What is the mitotic (M) phase of cell division?

Cell Division

Answer 185

• Results in formation of two identical cells.
• Consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis) to form two identical cells.
• These events are visable under microscope.

Cell Division

Question 186

What is mitosis of cell division?

Cell Division

Answer 186

The distribution of two sets of chromosomes into two separate nuclei. Process results in exact partitioning of genetic information. A continuous process; one stage merges seamlessly into the next. Divided into 4 stages:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Cell Division

Question 187

What is prophase of cell division?

Cell Division

Answer 187

• Chromatin fibers condense into paired chromatids.
• Nucleolus and nuclear envelope disappear.
• Each centrosome moves to an opposite pole of the cell.
• Process can be seen under light microscope.

Cell Division

Question 188

What is a centromere?

Cell Division

Answer 188

• Constricted region which holds chromatid pair together during cell division.
• At the outside of each centromere is protein complex known as the kinetochore.

Cell Division

Question 189

What is the mitotic spindle?

Cell Division

Answer 189

• Football-shaped assembly of microtubules that attach to the kinetochore.
• Responsible for separation of chromatids to opposite poles of the cell.

Cell Division

Question 190

What is metaphase of cell division?

Cell Division

Answer 190

Centromeres of chromatid pairs line up at metaphase plate.

Cell Division

Question 191

What is anaphase of cell division?

Cell Division

Answer 191

• Centromeres split; identical sets of chromosomes move to opposite poles of cell.
• When separated, chromatids are termed chromosomes.
• Chromosomes appear V-shaped because centromeres lead way, dragging trailing arms of chromosomes toward the pole.

Cell Division

Question 192

What is telophase of cell division?

Cell Division

Answer 192

• Nuclear envelopes and nucleoli reappear; chromosomes resume chromatin form; mitotic spindle disappears.
• Final stage of mitosis. Begins after chromosomal movement stops.
• Identical sets of chromosomes uncoil and revert to threadlike chromatin form.
• Nuclear envelope forms around each chromatin mass, nucleoli reappear in indentical nuclei and mitotic spindle breaks up.

Cell Division

Question 193

What is cytokinesis?

Cell Division

Answer 193

• Division of a cells cytoplasm and organelles into two identical cells.
• Usually beings in late anaphase with formation of a cleavage furrow (slight indentation of the plasma membrane) and is completed after telophase.
• Cleavage furrow divides cytoplasm into separate and equal portions.
• When complete, interphase begins.

Cell Division

Question 194

What are the different destinies of a cell?

Cell Division

Answer 194

A cell has 3 possible destinies:
1. Remain alive and functioning without dividing.
2. Grow and divide.
3. To die.

Homeostasis is maintained when there is balance between cell proliferation and cell death.

Cell Division

Question 195

What is apoptosis?

Cell Division

Answer 195

A normal type of cell death.

Cell Division

Question 196

What is necrosis?

Cell Division

Answer 196

• Pathological type of cell death that results from tissue injury.
• Adjacent cells swell, burst, and spill their cytoplasm into interstitial fluid.
• The cellular debris usually stimulates inflammatory response by immune system, a process that does not occur in apoptosis.

Cell Division

Question 197

What is meiosis in reproductive cell division?

Cell Division

Answer 197

The reproductive cell division that occurs in the gonads (ovaries and testes), produces gametes in which the number of chromosomes is reduced by half. Gametes contain single set of 23 chromosomes, are haploid (n) cells. Fertilization restores the dioloid number of chromosomes. Occurs in two successive stages:
1. Meiosis I
2. Meiosis II

Cell Division

Question 198

What is Meiosis I in reproductive cell division?

Cell Division

Answer 198

Begins when chromosomal replication complete. Consists of 4 phases:
1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I

Cell Division

Question 199

What is Prophase I of Meiosis I in reproductive cell division?

Cell Division

Answer 199

• Extended phase, chromosomes shorten and thicken, nuclear envelope and nucleoli disappear, mitotic spindle forms.
• Two sister chromatids of each pair of homologous chromosomes pair off (synapsis).
• Resulting 4 chromatids form structure called a tetrad.
• Parts of chromatids of 2 homologous chromosomes may be exhanged with one another.
• Exchange between parts of nonsister chromatids is crossing over, which resulting cells are genetically unlike eachother and from starting cell.

Cell Division

Question 200

What is genetic recombination in Meiosis?

Cell Division

Answer 200

• Results from crossing-over (exchange between parts of nonsister chromatids).
• Formation of new combinations of genes-and accounts for part of genetic variation among humans and other organisms that form gametes via meiosis.

Cell Division

Question 201

What is Metaphase I of Meiosis I in reproductive cell division?

Cell Division

Answer 201

Tetrads formed by homologous pairs of chromosomes line up along metaphase plate of the cell, with homologous chromosomes side by side.

Cell Division

Question 202

What is Anaphase I of Meiosis I in reproductive cell division?

Cell Division

Answer 202

• Members of each homologous pair of chromosomes separate, pulled to opposite poles of cell by microtubules attached to centromeres.
• Paired chromatids, held by centromere, remain together.

Cell Division

Question 203

What is Telophase I of Meiosis I in reproductive cell division?

Cell Division

Answer 203

Each cell has one of the replicated chromosomes from each homologoous pair of chromosomes.

Cell Division

Question 204

What is Meiosis II in reproductive cell division?

Cell Division

Answer 204

Is the second stage of meiosis and consists of 4 phases:
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II

Phases similar to those that occur during mitosis; centromeres split, and sister chromatids separate and move toward opposite poles of cell.

Cell Division

Question 205

How does anaphase I of meiosis differ from anaphase of mitosis?

Cell Division

Answer 205

During anaphase I of meiosis, the paired chromatids are held together by a centromere and do not separate. During anaphase of mitosis, the paired chromatids separate and the centromeres split.

Cell Division

Question 206

How does crossing-over affect the genetic content of the haploid gametes?

Cell Division

Answer 206

The result of crossing-over is that four haploid gametes are genetically unlike each other and genetically unlike the starting cell that produced them.

Cell Division

Question 207

When does cytokinesis begin?

Cell Division

Answer 207

Cytokinesis usually begins in late anaphase.

Cell Division

Question 208

During which phase of the cell cycle does DNA replication occur?

Cell Division

Answer 208

DNA replicates during the S phase of interphase of the cell cycle.

Cell Division

Question 209

Why must DNA replication occur before cytokinesis in somatic cell division?

Cell Division

Answer 209

DNA replication must occur before cytokinesis so that each of the new cells will have a complete genome.

Cell Division

Question 210

What is a haploid cell?

Cell Division

Answer 210

A single set of 23 chromosomes.

Cell Division

Question 211

In what unit are the size of cells measured?

Cellular Diversity

Answer 211

Measured in units called micrometers. One micrometer is equal to 1 one-millionth of a meter.

Cellular Diversity

Question 212

What are the different shapes of cells?

Cellular Diversity

Answer 212

1. Round
2. Flat
3. Oval
4. Cube-shaped
5. Column-shaped
6. Elongated
7. Star-shaped
8. Cylindrical
9. Disc-shaped

Cellular Diversity