3. Cells and Movement In and Out of Them

Question 1

What is magnification?

Answer 1

Magnification = size of image/size of object

Question 2

What is resolution?

Answer 2

⋆ Resolution: The minimum distance apart two objects can be in order for them to appear as separate items (0.2um in light microscopes)
⋆ Every microscope has a limit of resolution so magnification will sometimes just make the image more blurred

Question 3

What must we do in order to study organelles?

Answer 3

Isolate them in large numbers

Question 4

How does cell fractionation work?

Answer 4

⋆ The tissue is first placed in a cold (to reduce enzyme activity that might break down organelles), isotonic (same water potential as tissue to prevent organelles shrinking or bursting in osmosis) and buffered (to maintain a constant pH) solution
⋆ The first stage is homogenation: cells are broken up by a homogeniser to release organelles and a homogenate is produced which must be filtered to remove any complete cells or debris
⋆ The second stage is ultracentrifugation, when the fragments in the filtered homogenate are separated in an ultracentrifuge
⋆The tube of filtrate is placed in the ultracentrifuge and spun and high speed so heaviest organelles are forced to the bottom to form a thin sediment
⋆ The supernatant (fluid at the top) is removed to leave the sediment of nuclei
⋆ The supernatant is transferred to another tube and spun at a faster speed than before (mitochondria are forced to the bottom) and the process is repeated

Question 5

What are the advantages of using electron microscopes?

Answer 5

Electron beams have very short wavelengths so the microscope has a high resolving power and electrons are negatively charged so the beam can be focused using electromagnets

Question 6

What are the resolutions of a light microscope and an electron microscope?

Answer 6

Light = 0.2um
Electron = 0.1nm

Question 7

Electrons are absorbed by molecules in air. What must the microscope have around it to prevent this?

Answer 7

Electrons are absorbed by molecules in air so a near-vacuum has to be created around the microscope for it to work effectively (see diagrams p. 43)

Question 8

How does a transmission electron microscope work?

Answer 8

⋆ Beam of electrons from gun is focused onto a specimen by a condenser electromagnet so the beam passes through the specimen
⋆ Parts of it absorb electrons and appear dark whilst others allow electrons to pass through so appear bright
⋆ The image produced can be photographed to give a photomicrograph

Question 9

What are the limitations of the transmission electron microscope?

Answer 9

⋆ Resolution is 0.1nm but problems with specimen preparation mean it cannot always be achieved
⋆ Main limitations: Whole system must be in a vacuum, complex ‘staining’ process required, only black and white image, specimen must be thin, image may contain artefacts (things that result from the way the specimen is prepared), only 2D image achieved but we can slowly build a 3D image by looking at a series of photomicrographs

Question 10

How does a scanning electron microscope work?

Answer 10

⋆ A beam of electrons is directed to the specimen from above rather than below
⋆ The beam is passed back and forth in a regular pattern across the specimen
⋆ Electrons are scattered by the specimen according to the contours of the specimen surface
⋆ Computer analysis of scattered electrons and the secondary electrons produced can build up a 3D image

Question 11

What are the limitations of the scanning electron microscope?

Answer 11

⋆ Same limitations as the TEM but the specimen need not be thin because electrons do not pass through them
⋆ Lower resolving power than TEM – around 20nm

Question 12

What is a eukaryotic cell?

Answer 12

A cell with a distinct nucleus and has membrane-bound organelles

Question 13

What is the function of epithelial cells?

Answer 13

To absorb and secrete

Question 14

What is the function of the nucleus?

Answer 14

⋆ Contains hereditary material and controls cell activities
⋆ Spherical and between 10 and 20um in diameter
⋆ Control centre of cell through production of mRNA and therefore protein synthesis
⋆ Retains genetic material
⋆ Manufactures ribosomes and ribosomal RNA

Question 15

What is the structure of the nucleus?

Answer 15

⋆ Nuclear membrane: double membrane that surrounds the nucleus which controls the entry and exit of materials in and out of the nucleus and the reactions taking place within it
⋆ Nuclear pores: allows for the passage of large molecules like mRNA out of the nucleus. Around 40-100nm in diameter
⋆ Nucleoplasm: jelly-like material that makes up most of the nucleus
⋆ Chromatin: DNA found in the nucleoplasm. The diffuse form that chromosomes take when the cell is not dividing
⋆ Nucleolus: Spherical body within the nucleoplasm that manufactures ribosomal RNA and assembles ribosomes

Question 16

What is the function of mitochondria?

Answer 16

⋆ Responsible for the production of ATP from carbohydrates
⋆ Cells with a high metabolic rate and carry out lots of active transport e.g. muscle and epithelial need lots of mitochondria

Question 17

What is the structure of mitochondria?

Answer 17

⋆ Double membrane: surrounds organelle. The outer membrane controls entry and exit from the mitochondrion and the inner is folded to form extensions called cristae
⋆ Cristae: extensions of the inner membrane which provide a large SA for the attachment of enzymes involved in respiration
⋆ Matrix: semi-rigid material containing protein, lipids and traces of DNA that allows the mitochondria to control the production of their own proteins. The enzymes involved in respiration are also found here.

Question 18

What is the function of the RER?

Answer 18

⋆ Ribosomes on the outer surfaces of membranes
⋆ Provides large SA for protein and glycoprotein synthesis
⋆ Provides a pathway for the transport of materials like proteins in the cells

Question 19

What is the function of the SER?

Answer 19

⋆ No ribosomes on membrane surfaces
⋆ Synthesise, store and transport lipids and carbohydrates
⋆ Cells like liver and secretory cells e.g. epithelial cells that line the intestine that need large amounts of lipids and carbohydrates have very extensive ER

Question 20

What is the function of the golgi apparatus?

Answer 20

⋆ Add carbohydrate to proteins to form glycoproteins
⋆ Produce secretory enzymes
⋆ Secrete carbohydrates e.g. cellulose which makes up cell walls
⋆ Transport, modify and store lipids
⋆ Form lysosomes
⋆ Especially well developed in secretory cells
⋆ Proteins and lipids made by ER pass through the Golgi apparatus –> Golgi modifies proteins often adding non-protein compounds like carbohydrates –> ‘labels’ proteins, allowing them to be accurately sorted and sent to their correct destinations –> modified proteins and lipids are transported in vesicles which are regularly pinched off from the ends of the Golgi cisternae –> vesicles move to the cell surface where they fuse with the membrane and release their contents to the outside

Question 21

What is the structure of the golgi apparatus?

Answer 21

⋆ Occurs in almost all eukaryotic cells
⋆ Similar to (but more compact that) the structure of SER
⋆ Stack of membranes that make up flattened sacks (cisternae) with small rounded hollow structures called vesicles

Question 22

What is the function of lysosomes?

Answer 22

⋆ Formed when the vesicles produced by the Golgi apparatus contain enzymes like proteases
⋆ Up to 0.1um in diameter
⋆ Break down material ingested by phagocytic cells like white blood cells
⋆ Release enzymes to the outside of the cell (exocytosis) to destroy material around the cell
⋆ Digest worn out organelles so that useful chemicals can be re-used
⋆ Break down cells when they die (autolysis)
⋆ Especially abundant in phagocytic and epithelial cells

Question 23

What is the function of ribosomes?

Answer 23

⋆ Small cytoplasmic granules found in all cells
⋆ May occur in the cytoplasm or be associated with RER
⋆ Two types: 80S (eukaryotic cells, 25nm in diameter) and 70S (prokaryotic cells, slightly smaller)
⋆ Two subunits – one large and one small – each of which contains ribosomal RNA and protein
⋆ Occur in large numbers
⋆ Important in protein synthesis

Question 24

What are the properties of lipids?

Answer 24

⋆ Contain carbon, hydrogen and oxygen
⋆ Insoluble in water
⋆ Soluble in organic solvents
⋆ Main groups are triglycerides (fats and oils), phospholipids and waxes

Question 25

What are the roles of lipids?

Answer 25

⋆ Contribute to flexibility of plasma membranes and transfer lipid-soluble substances across them
⋆ Used as an energy source once oxidised
⋆ Used to waterproof because they are insoluble in water e.g. waxy cuticles
⋆ Used for insulation because fats are slow conductors of heat so we maintain body temperature
⋆ Used to protect delicate organs

Question 26

How are lipids structured?

Answer 26

⋆ Fats are solids at room temperature, oils are liquids
⋆ Triglycerides have three fatty acids combined with glycerol
⋆ Fatty acid forms bond with glycerol in a condensation reaction
⋆ All glycerol molecules are the same so differences between fats and oils occur due to variations in fatty acid structures
⋆ All fatty acids have a carboxyl (-COOH) group attached
⋆ No double carbon-carbon bonds = saturated, one double carbon-carbon bond = monounsaturated, more than one = polyunsaturated

Question 27

How are phospholipids structured?

Answer 27

⋆ One fatty acid molecule is replaced by a phosphate molecule
⋆ Fatty acids are hydrophobic but phosphate molecules are hydrophilic
⋆ Phospholipids contain a hydrophilic head and a hydrophobic tail which mixes readily with fat
⋆ The molecule is polar

Question 28

What is the test for lipids?

Answer 28

⋆ Take a dry test tube –> Add sample being tested and ethanol –> Shake thoroughly to dissolve any lipid –> Add water and shake gently –> A cloudy-white colour indicates the presence of a lipid
⋆ Cloudy colour is due to the lipid being dispersed in water to form an emulsion which refracts light to make a cloudy appearance

Question 29

What is the structure of the cell-surface membrane?

Answer 29

⋆ Forms the boundary between the cytoplasm and the environment to control movement in and out of the cell
⋆ Phospholipids form a bilayer sheet
⋆ Hydrophilic head points outwards and hydrophobic tails point inwards, protected from water on both sides
⋆ Glycoprotein: branching carbohydrate portion of a protein which acts as a recognition site for chemicals like hormones
⋆ Glycolipid: acts as a recognition site e.g. for cholera toxins
⋆ Protein molecules (intrinsic and extrinsic): below
⋆ Cholesterol: adds strength

Question 30

What is the purpose of extrinsic proteins on the cell-surface membrane?

Answer 30

Occur either on the surface or are only partly embedded. They give mechanical support or act as cell receptors for molecules like hormones along with glycolipids

Question 31

What is the purpose of intrinsic proteins on the cell-surface membrane?

Answer 31

Span the phospholipid bilayer and act as carriers to transport water-soluble molecules by forming ion channels across the membrane or as enzymes

Question 32

Why is the cell-surface membrane described as fluid-mosaic?

Answer 32

⋆ Fluid – because the phospholipid molecules can move relative to one another to give the membrane a flexible structure that is always changing shape
⋆ Mosaic – because proteins vary in shape, size and pattern like the tiles of a mosaic

Question 33

How does diffusion occur?

Answer 33

⋆ All particles are always in motion due to the kinetic energy they possess
⋆ The motion is random
⋆ Particles are constantly bouncing off one another and other objects
⋆ Diffusion is the net movement of molecules or ions from a region of high concentration to a region of low concentration
⋆ Dynamic equilibrium occurs when diffusion is complete but particles keep on moving

Question 34

What affects the rate of diffusion?

Answer 34

⋆ Depends on:
Concentration gradient
Area over which diffusion takes place
Thickness of the exchange surface
⋆ Diffusion is proportional to (SA x difference in concentration) / length of diffusion path
⋆ This is not wholly applicable to cells because diffusion is also affected by:
Nature of the plasma membrane: its composition and number of pores
Size and nature of the diffusing molecule: small molecules diffuse faster than large molecules, fat soluble molecules diffusion faster than water soluble molecules and polar molecules diffuse faster than on-polar molecules

Question 35

How does facilitated diffusion work?

Answer 35

⋆ Passive process
⋆ Relies on the inbuilt motion (kinetic energy) of the diffusing molecules with no external input of energy
⋆ Occurs down a concentration gradient
⋆ Only occurs at points on the plasma membrane where there are specific protein molecules
⋆ The proteins form protein channels which allow water-soluble ions to pass through
⋆ They would otherwise diffuse very slowly through the phospholipid bilayer
⋆ Channels only open in the presence of specific ions so there is control over entry
⋆ An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane
⋆ The molecule binds with the protein which causes it to change shape in a way that the molecule is released to the inside of the membrane

Question 36

What is osmosis?

Answer 36

⋆ The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane
⋆ Water pressure is measured in kPa and is the pressure of water molecules
⋆ Negative water pressures mean a more concentrated solution
⋆ You can find the water potential of different cells and tissues by placing them in solutions of different water potentials and measuring water loss

Question 37

How does osmosis work?

Answer 37

⋆ One solution has a low concentration of solute molecules whilst the other has a high concentration
⋆ Both the solute and molecules are in random motions due to their kinetic energy
⋆ The partially permeable plasma membrane only allows water to pass through it and not other molecules
⋆ Water molecules diffuse from the area with a higher water potential
⋆ When water potentials on both sides are equal, dynamic equilibrium is reached and there is no net movement of water molecules

Question 38

How do we prevent the bursting of animal cells?

Answer 38

Animal cells are normally bathed in a solution with a similar water potential to the cells so the plasma membrane does not break down due to rapid osmosis because they usually have a very low water potential with lots of solutes dissolved in them

Question 39

How does osmosis help plant cells?

Answer 39

⋆ Central vacuole: contains a solution of water, salts, sugars and organic acids
⋆ Protoplast: consists of the outer cell-surface membrane, nucleus, cytoplasm and inner vacuole membrane
⋆ Cellulose cell wall: permeable to even large molecules
⋆ Plant cells have a much lower water potential than pure water so undergo rapid osmosis when placed in water
⋆ Unlike animal cells, they are able to control the composition of the fluid around them
⋆ Normally always bathed in pure water absorbed from by the roots
⋆ Water entering the cell by osmosis causes the protoplast to swell and press to the cell wall
⋆ The cell wall is capable of a little expansion before pressure builds up that resists the entry of any more water
⋆ The cell is kept turgid
⋆ If the cell is placed in a solution with a water potential lower than the cell itself, water leaves by osmosis
⋆ The protoplast no longer presses against the cell wall
⋆ The cell is said to be at incipient plasmolysis
⋆ The cell begins to shrink and pull away from the cell wall and the cell becomes plasmolysed

Question 40

How does direct active transport work?

Answer 40

⋆ Carrier proteins span the cell-surface membrane and accept the molecules to be transported on one side of it
⋆ Molecules bind the receptors on the channels of the carrier protein
⋆ On the inside of the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule
⋆ The protein molecule changes shape and opens to the opposite side of the membrane
⋆ The molecules are then released on the other side of the membrane
⋆ The phosphate molecule is released to the other side of the membrane
⋆ The phosphate molecule is released from the protein and recombines with the ADP to form ATP during respiration
⋆ This causes the protein to revert to its original shape ready for the process to be repeated

⋆ Occasionally one molecule is brought into the cell and another is removed at the same time e.g. the sodium-potassium pump

Question 41

Read notes in textbook/folder on sodium-potassium pump.

Answer 41

Done!

Question 42

How does active transport work?

Answer 42

⋆ Transport of molecules against a concentration gradient
⋆ Metabolic energy is required
⋆ Once inside the molecules are prevented from leaking out by the barrier of the cell-surface membrane’s bilayer
⋆ A different environment is maintained on either side of the membrane
⋆ The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using energy and carrier molecules

Question 43

Read p63-64 on absorption in the small intestine.

Answer 43

Done!

Question 44

How is the cholera bacterium structured?

Answer 44

Cell wall: physical barrier that protects against mechanical damage and excludes certain substances
Capsule: protects bacterium from other cells and helps groups of bacteria to stick together for further protection
Cell-surface membrane: acts as a differentially permeable layer which controls the entry and exit of chemicals
Flagellum: aids movement of bacterium because its rigid, corkscrew shape and rotating base help the cell spin through fluids
Circular DNA: possesses the genetic information for the replication of bacterial cells
Plasmid: gives genes that aid the survival of bacteria in adverse conditions e.g. produces enzymes which break down antibiotics

⋆ Cell wall made up of peptidoglycan which is a polymer made of polysaccharides and peptides
⋆ Contains ribosomes of the 70S type
⋆ Stores food reserves as glycogen granules and oil droplets

Question 45

How does cholera cause disease?

Answer 45

⋆ Transmitted by digestion of food or water that has been contaminated with faecal material containing the pathogen
⋆ Almost all bacteria ingested are killed by stomach acid but a few may survive
⋆ The surviving bacteria reach the small intestine and use the flagella to propel themselves through the mucus lining of the intestinal wall
⋆ Bacteria produce a toxic protein: one part of it binds to specific carbohydrate receptors on the cell-surface membrane
⋆ Only epithelial cells that make up the small intestine have these receptors which is why cholera only affects this part of the body
⋆ The other part enters the epithelial cells which causes ion channels on the cell-surface membrane to open so that bacteria usually contained within the epithelial cells can enter the lumen of the intestine
⋆ Loss of chloride ions from epithelial cells raises these water potential so water flows from cells into the lumen
⋆ Loss of ions from epithelial cells establishes concentration gradient so more ions more into epithelial cells from surrounding tissues
⋆ This establishes a water potential gradient that means water from the blood and other tissues floods into the lumen
⋆ This causes severe diarrhoea and dehydration

Question 46

How is cholera transmitted?

Answer 46

⋆ Drinking water that has not been properly purified
⋆ Untreated sewage leaking into water courses and serving food without washing hands
⋆ Organisms, like shellfish, feeding on untreated sewage released into rivers or the sea

Question 47

How does cholera cause diarrhoea?

Answer 47

By damage to epithelial cells lining the intestine, loss of microvilli due to toxins and excessive secretion of water due to toxins e.g. cholera toxin

Question 48

Why is water alone not effective in rehydration therapy after a cholera infection?

Answer 48

Water is not being absorbed from the intestine but is being lost by cells and the drinking of water does not replace ions lost from epithelial cells of the intestine

Question 49

What do oral rehydration solutions contain?

Answer 49

Water: to rehydrate the tissues
Sodium: to replace sodium ions lost from the epithelial cells of the intestine and to make optimum use of alternative sodium-glucose carrier proteins
Glucose: to stimulate the uptake of sodium ions from the intestine and to provide energy
Potassium: to replace lost potassium ions and to stimulate appetite
Other electrolytes: e.g. chloride ions to prevent electrolyte imbalance

Question 50

Read pages 69-70 to finish learning about oral rehydration therapy.

Answer 50

Done!