Ch 6: A Tour of the Cell

Question 1

Explain why the cell is considered the “fundamental unit of biology.”

Answer 1

Cells are considered the fundamental unit of biology because they are an organism’s basic unit of structure and function. Many organisms exist as single-celled organisms, and larger organisms, like plants and animals, are multicellular.

Question 2

Cell fractionation is used to separate cell components based on size and density. Summarize this technique and explain why it is useful.

Answer 2

Cell fractionation separates cell components using the centrifuge, a device that spins tubes containing mixtures of disrupted cells at increasing speeds, which is a process called differential centrifuge. At each speed, the forces from the centrifuge cause subsets of cell components to settle at the bottom of the tube, which forms a pellet. Slower speeds result in a pellet with larger components, and faster speeds result in smaller components. Cell fractionation is useful because it enables researchers to study cell components in bulk, which can’t really be done with intact cells.

Question 3

Compare prokaryotic cells with eukaryotic cells.

Answer 3

Similarities:
– Have plasma membranes (aka cell membranes)
– Contain cytosol, the jelly-like substance within plasma membranes
– Contain chromosomes, which carry genes in the form of DNA
– Contain ribosomes, tiny complexes that produce proteins
Differences:
– Eukaryotic cells contain DNA in a nucleus bounded by a double membrane, while prokaryotic cells contain – DNA in a non-membrane enclosed nucleoid
(pro → pre-nucleus, referring to earlier evolution)
– The cytoplasm of eukaryotic cells contain organelles suspended in cytosol, while cytoplasms of prokaryotic cells contain no organelles, but proteins
– Eukaryotic cells are generally larger than prokaryotic cells

Question 4

Why are cells generally so small? How is this related to surface area and volume? What sets the upper limit? What sets the lower limit?

Answer 4

Cells are small because a smaller cell has a greater ratio of surface area to volume. Cells need to allow passage of oxygen, nutrients, and wastes through their plasma membrane, and surface area needs to accommodate volume, especially in cells that exchange many substances. At the lower limit, the smallest cells need to have enough DNA to program metabolism, and enough enzymes and other cellular components for a cell to sustain itself and reproduce. At the upper limit, surface area and volume need to be at a practical ratio to exchange substances.

Question 5

Describe some strategies cells use to increase their surface area to volume ratio.

Answer 5

Some cells, especially cells that exchange a lot of material in their surroundings, like intestinal cells, resort to long, thin projections from their surface called microvilli, which increase surface area without a significant increase in volume.

Question 6

A eukaryotic cell is able to divide the cell into compartments. How is this accomplished and why is it useful?

Answer 6

A eukaryotic cell contains internal organelle membranes, which have organelles that provide different local environments that support specific metabolic functions, and so that processes can occur simultaneously in the cell.

Question 7

Nucleus

Answer 7

• • Nuclear envelope: double membrane enclosing the nucleus, perforated by pores, and is continuous with the endoplasmic reticulum
• • Nucleolus: non-membranous structure that helps produce ribosomes
• • Chromatin: material made of DNA and proteins

Question 8

Mitochondria

Answer 8

• • “Powerhouse of the cell” → supplies energy for a cell to carry out its biochemical reactions
• • Organelle where cellular respiration occurs and generates ATP

Question 9

Lysosomes

Answer 9

Digestive organelle that hydrolyzes macromolecules

Question 10

Endoplasmic Reticulum

Answer 10

• • Membranous network of sacs and tubes

- - Involved in membrane synthesis and other synthetic and metabolic processes

Question 11

Golgi Apparatus

Answer 11

Active in creating, modifying, sorting, and secreting cell products

Question 12

Central Vacuole

Answer 12

• • Organelle in older plant cells
• • Stores and breaks down waste products, hydrolyzes macromolecules, and the enlargement of the vacuole is also part of plant growth

Question 13

Chloroplasts

Answer 13

• • Photosynthetic organelle in plant cells

- - Converts energy from sunlight to chemical energy stored in sugar molecules

Question 14

Nuclear Envelope

Answer 14

The nuclear envelope is a double membrane, where two membranes, each a lipid bilayer with associated proteins. The envelope has pore structures, where there are continuous inner and outer membranes. A protein structure called the pore complex lines each pore and regulates the entry of proteins, RNAs, and macromolecules. The nuclear lamina lines the nuclear side of the envelope and helps support and maintain the shape of the nucleus.

Question 15

Where in the cell do you synthesize a ribosome? What is a ribosome composed of?

Answer 15

Ribosomes are synthesized beginning from a cell’s nucleolus, a structure within the non-dividing nucleus. In the nucleolus, ribosomal RNA is synthesized from genes in the DNA. Proteins imported from the cytoplasm are assembled with rRNA into components of ribosomes. The components are exported out through the envelope pores and into the cytoplasm, where they assemble into ribosomes. A ribosome is composed of rRNA and ribosomal proteins.

Question 16

Ribosomes are the structures which are the site for protein synthesis. In the cell there are two types of ribosomes: bound and free. Contrast the location and function of bound ribosomes with those of free ribosomes.

Answer 16

Ribosomes are cellular components made of ribosomal RNAs and proteins that carry out protein synthesis. Free ribosomes and structurally bound ribosomes are structurally identical, but free ribosomes are suspended in the cytosol and bound ribosomes are attached to the outside of the ER or nuclear envelope. Free ribosomes make proteins that function in the cytoplasm, whereas bound ribosomes make proteins intended to be inserted into the membrane, packaging within certain organelles, or exporting proteins out.

Question 17

What tasks does the endomembrane system carry out in eukaryotic cells?

Answer 17

The endomembrane system carries out many tasks in eukaryotic cells, and includes synthesizing proteins, transporting proteins into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification.

Question 18

Which cellular structures make up the endomembrane system? How are the membranes of these structures in contact with each other?

Answer 18

The endomembrane system is composed of organelles bound within membranes in different parts of eukaryotic cells. These organelles include the nuclear envelope, ER, Golgi apparatus, lysosomes, the plasma membrane, and various kinds of vesicles (tiny sacs made of membrane) and vacuoles. The membranes of these structures are related through direct physical continuity or by transfer as vesicles.

Question 19

Rough ER

Answer 19

• • Studded with ribosomes on the outer surface of the membrane, thus appearing “rough”
• • Produces secretory proteins
• • After forming them, the rough ER keep them separate from proteins produced by smooth ER
• • Most secretory proteins are proteins with carbohydrates covalently bonded to them, aka glycoproteins
• • Also acts as membrane factory by growing in place and adding membrane proteins and phospholipids to its own membrane
• • Can also make membrane phospholipids

Question 20

Smooth ER

Answer 20

• • Outer surface has no ribosomes
• • Varies in diverse metabolic processes that depend on cell type
• • Processes include synthesizing lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions
• • Other enzymes of smooth ER help detoxify drugs and poisons

Question 21

What types of cells would have extensive smooth ER? What types of cells would have extensive rough ER? How is this related to their function?

Answer 21

Cells with an extensive smooth ER would typically be cells that synthesize and secrete steroid hormones, like testes and ovaries. Cells that help detoxify drugs and poisons, like liver cells, also have an extensive smooth ER. Cells that need to synthesize and secrete proteins would have extensive rough ER. For example, some pancreatic cells synthesize the protein insulin in the ER and secrete the hormone into the bloodstream.

Question 22

A vesicle budded off of the ER. Explain what happens to the vesicle and its contents as it reaches, moves through and then exits the Golgi apparatus.

Answer 22

The Golgi apparatus acts as a warehouse for receiving, sorting, shipping, and manufacturing. The Golgi apparatus can modify, store, and send off products from the ER. They are extensive in cells specialized for secretion. Once vesicles from the ER move to the Golgi, they combine to form new cis Golgi cisternae, which matures in a cis-to-trans direction.

Question 23

What is the last step in Golgi apparatus protein “shipping?” Why is this important?

Answer 23

The last step in shipping is transporting proteins back to the ER, or a less mature region. This is important because they get shipped back to someplace where their functions are needed.

Question 24

What is contained in a lysosome? What is its function? Where is it made?

Answer 24

Lysosomes are membranous sacs containing hydrolytic enzymes, which are used by eukaryotic cells to digest (hydrolyze) macromolecules. The contents of a lysosome work best in acidic environments, thus not very active in the neutral pH of cytosol in the cytoplasm. Lysosomes are made by the rough ER and transferred to the Golgi apparatus.

Question 25

What is phagocytosis? How are lysosomes involved? Give an example from your body!

Answer 25

Phagocytosis is the process where amoebas and unicellular protists eat by ingesting smaller organisms or food particles. A lysosome fuses with the food vacuole, and the hydrolytic enzymes within the lysosome digests the food. Macrophages are human white blood cells that defend the body from bacteria by engulfing and destroying them.

Question 26

Food vacuole

Answer 26

Formed by phagocytosis, and fuses with lysosomes to aid the digestion of food particles the food vacuole contains.

Question 27

Contractile vacuole

Answer 27

A common vacuole in unicellular protists living in freshwater that pump excess water out of the cell to maintain suitable concentration of ions and molecules.

Question 28

Central vacuole

Answer 28

Found in mature plant cells, and developed from combining smaller vacuoles. Contains cell sap and is the plant cell’s main repository of inorganic ions, including potassium and chloride. Plays a major role in the growth of plant cells, which enlarges as the vacuole absorbs more water.

Question 29

How do vacuoles protect plants and help them grow?

Answer 29

The central vacuole enlarges as it absorbs water, which enables the cell to grow with minimal investment in a cytoplasm.

Question 30

Describe at least two common characteristics between mitochondria and chloroplasts.

Answer 30

Mitochondria and chloroplasts are both organelles that convert energy to forms that cells can use for work. Mitochondria are where cellular respiration occurs, which is a metabolic process that uses oxygen to generate ATP by extracting energy from sugars, fats, and other fuels. Chloroplasts are found in plants and algae, and convert solar energy to chemical energy. Chloroplasts absorb sunlight and use it to synthesize organic compounds, like sugars, from CO2 and H2O. Both organelles share similar evolutionary origins.

Question 31

The widely accepted endosymbiont theory explains the origins of mitochondria and chloroplasts in eukaryotic cells. Describe at least three pieces of evidence used to support the theory.

Answer 31

An early ancestor of eukaryotic cells engulfed an oxygen-using non photosynthetic prokaryotic cell, and the two eventually merged into a single organism: a eukaryotic cell with the oxygen- using prokaryote becoming the mitochondrion. At least one engulfed a photosynthetic prokaryote, becoming a chloroplast. Evidence includes… - - Mitochondria and chloroplasts have double membranes, which their ancestors also had. - - Mitochondria and chloroplasts contain ribosomes and DNA molecules that look like bacterial chromosomes. - - Mitochondria and chloroplasts are somewhat autonomous, like independent cells.

Question 32

Describe the structure of a mitochondrion. How many membranes? Why is the folding of the cristae important?

Answer 32

A mitochondrion is bound by double membranes, each a phospholipid bilayer with a collection of embedded proteins. The outer layer is smooth, while the inner membrane contains foldings called cristae. The inner membrane divides the mitochondrion into two spaces. The first is the narrow region between the two membranes, called the intermembrane space. The second is the mitochondrial matrix, containing enzymes that aid cellular respiration, mitochondrial DNA, and ribosomes. The cristae is important because it increases the surface area, which increases the rate of gas exchange in cellular respiration.

Question 33

The chloroplast is divided into three compartments. What is the benefit of compartmentalization?

Answer 33

A chloroplast is divided into the intermembrane space, the stroma, and the thylakoid space. The thylakoids are composed of stacks of granum. Outside the thylakoids is a fluid called stroma containing chloroplast DNA, ribosomes, and enzymes. Compartmentalization enables chloroplasts to convert light energy to chemical energy during photosynthesis.

Question 34

Explain how a peroxisome uses compartmentalization for the benefit of the cell?

Answer 34

The peroxisome is a specialized metabolic compartment bounded by a single membrane and works with chloroplasts and mitochondria in certain metabolic functions. They contain enzymes that remove hydrogen atoms and transfer them to oxygens, which produces hydrogen peroxide. Some peroxisomes use oxygen to break fatty acids down to transport to mitochondria for fuel. However, hydrogen peroxide can be harmful to other cellular components, which is why the cell’s compartment structure can keep them from damaging other cell parts.

Question 35

List and describe the major functions of the cytoskeleton.

Answer 35

The cytoskeleton is a network of fibers extending throughout the cytoplasm. Functions of the cytoskeleton: -- Give mechanical support to the cell and help maintain shape -- More dynamic and flexible than animal skeletons -- Involved in cell motility (cell movement) -- Requires interactions between the cytoskeleton with motor proteins -- Can also manipulate plasma membrane to form food vacuoles

Question 36

What role do motor proteins play in cell motility?

Answer 36

Motor proteins need to interact with the cytoplasm for cell motility. For example, the cytoskeleton and motor proteins cooperate with plasma membrane molecules to allow cells to move along fibers outside the cell.

Question 37

What is cytoplasmic streaming?

Answer 37

Cytoplasmic streaming is the circular flow of cytoplasm within cells, and helps speed the movement of organelles and the distribution of materials within the cell. (Common in large plant cells.)

Question 38

What are 3 functions of the cell wall?

Answer 38

Cell walls are structures in plant cells that help protect the plant cell, maintain its shape, and prevent excessive uptake of water.

Question 39

ALL living organisms have a plasma membrane. However, not all organisms have a cell wall. We know that plant cells have a cell wall and animal cells do not have a cell wall. What other types of organisms have a cell wall?

Answer 39

Prokaryotes, some protists, and fungi have cell walls.

Question 40

Describe the structure of a cell wall.

Answer 40

Cell walls are much thicker than plasma membranes, and are made of microfibrils made of cellulose. First there is a primary cell wall. Between primary walls is the middle lamella, which contains pectins, and helps glue adjacent cells together. Some other cells add a secondary cell wall between the plasma membrane and the primary wall, which has a strong matrix that provides protection and support.

Question 41

Animal cells are not surrounded by a cell wall, but have an elaborate extracellular matrix (ECM). Describe the structure of the ECM. What role might fibronectin play in cell function?

Answer 41

The ECM (extracellular matrix) are mainly composed of glycoproteins (proteins with covalently bonded carbohydrates). Collagen, fibronectin, and proteoglycans are such glycoproteins. Fibronectin and other ECM proteins interact and bind to cell-surface receptor proteins called integrins that are built into the plasma membrane.