2.1.5 Biological membranes

Question 1

State the functions of cell membranes

Answer 1

1. Partially permeable barriers between cells and their environment, organelles and the cytoplasm and even within organelles: compartmentalisation
2. Sites of chemical reactions
3. Site of cell communication (cell signalling)
4. Cell-cell communication/recognition
5. Cell-cell adhesion

Question 2

Describe the fluid-mosaic model of membrane structure

Answer 2

1. The main component of membranes is the phospholipid bilayer
2. Formed due to the hydrophilic heads interacting with aqueous environments on both sides
3. And the hydrophobic fatty acid tails facing inward, away from (repelling) the water
4. Proteins are embedded in the phospholipid bilayer with signalling, adhesion or transport functions
5. Cholesterol molecules float amongst the phospholipids, regulating its fluidity
6. Some proteins and lipids are glycosylated
7. The structure is described as fluid, because the phospholipids and proteins can freely move within the plane of the membrane
8. The structure is described as a mosaic due to the variety of proteins embedded in it

Question 3

Describe the structure and functions of intrinsic proteins

Answer 3

1. Membrane proteins are globular proteins
2. the parts that are embedded within the membrane have hydrophobic R groups on the outside (unlike water soluble proteins)
3. Intrinsic proteins span the membrane
4. usually function as channel proteins for the facilitated diffusion of small molecules and mineral ions
5. Channel proteins have hydrophilic R groups at their core providing a hydrophilic environment for their diffusion
6. Intrinsic proteins can also act as carrier proteins, which change shape to transport ions and molecules across the membrane
7. Depending on the particular protein they can carry out facilitated diffusion or active transport

Question 4

Describe the structure and functions of glycoproteins

Answer 4

1. Glycoproteins are globular proteins
2. Modified (in the Golgi apparatus) with added carbohydrate chains that face the tissue fluid
3. These can be extrinsic proteins, which do not span the entire membrane
4. But are usually intrinsic proteins with functions other than transport
5. The carbohydrate chains are adhesive and cells of tissues to stick to each other
6. Glycoproteins also function in transmitting extracellular signals to the cell interior (cell signalling) by acting as receptors for neurotransmitters and peptide hormones
7. Their shape must be complementary to the molecules they bind
8. Therefore they are also targets for toxins and medicinal drugs that manipulate cell signalling to alter physiological responses

Question 5

Describe the structure and functions of glycolipids

Answer 5

1. Glycolipids are lipids modified with carbohydrate chains added which face the exterior of the cell
2. Glycolipids are synthesised in the Golgi apparatus
3. The functions of glycolipids are mainly to do with cell-cell communication, adhesion and self/non-self recognition

Question 6

Describe the structure and function of cholesterol

Answer 6

1. Cholesterol is a lipid
2. Composed of four carbon-based rings (planar, hydrophobic)
3. And a hydroxyl group (polar)
4. Cholesterol functions to interact with phospholipids, ensuring the bilayer fluidity (movement of the phospholipids) does not get too high or too low: regulating membrane fluidity
5. If the fluidity is too high, too many gaps may increase permeability too much and cells may leak their contents
6. If the fluidity is too low, close packing of phospholipids may reduce the permeability of membranes to the diffusion of things like water, oxygen and carbon carbon dioxide
7. So yeah, cholesterol: important

Question 7

Give examples of membranes as sites of chemical reaction:

Answer 7

1. Think of examples where the enzymes that catalyse a reaction are associated or embedded within the membrane
2. Do not confuse this with electron transport chains (these are not enzymatic reactions)
3. For example, ATP synthase, in the inner mitochondrial membrane (and the thylakoid membrane of the chloroplast), converting ADP and phosphate to ATP
4. For example, Photosystem II, in the thylakoid membrane acts as enzyme to carry out the photolysis of water
5. For example NADP reductase (thylakoid membrane) reduces NADP in the light-dependent stage
6. For example, when peptide hormones bind their receptor, adenylate cyclase, an enzyme in the cell surface membrane is activated, which converts ATP to cyclic AMP, the second messenger

Question 8

Explain how temperature affects the structure and function of membranes

Answer 8

1. Temperature affects kinetic energy of phospholipids and proteins, which in turn affects membrane structure and then function
2. As temperature increases, the kinetic energy of phospholipids increases
3. This increases the fluidity of the membrane and opens more gaps in the bilayer
4. This increases the permeability if the membrane (it is less able to act as a barrier)
5. Increasing temperature causes the tertiary structure of channel and carrier proteins to change (denaturing them) so they may not be able to function.
6. Permeability to certain substances (transported by proteins) may therefore decrease as temperature rises

Question 9

Explain how lipid solvents could affect the structure and function of membranes

Answer 9

1. Lipid solvents are able to interact with the fatty acid tails of phospholipids
2. And enter into the phospholipid bilayer
3. Increasing the fluidity of the membrane
4. Increasing permeability
5. And seriously affecting cell function (dependent on controlling movement across membranes)
6. Or possibly resulting in tissue damage, if cells leak cytoplasmic contents and damage other cells of the tissue
7. High concentrations of alcohol are therefore toxic

Question 10

Describe an investigation into the effect of a factor on membrane structure

Answer 10

1. Set up a range of boiling tubes with a known volume of water with either a range of different alcohol concentrations or temperatures (waterbaths)
2. cut beetroot into equal size/surface area pieces using a cork borer
3. place one piece into each of the boiling tubes
4. for a the same length of time
5. extract the water from each tube, and measure absorbance using a colorimeter
6. keep other factors the same such as surface area of beetroot, temp (if changing alcohol), volumes of water, concentrations
7. repeat each temp/alcohol concentration
8. increasing absorbance shows more red pigment diffused out of the cells, shows more membrane permeability and a more membrane damage

Question 11

Describe how molecules move across membranes by simple diffusion

Answer 11

1. As the membrane interior is hydrophobic, it is a barrier to polar and charged molecules/ions.
2. The close packing of the phospholipids means large molecules also cannot pass through
3. Biological membranes are freely permeable to small, non-polar molecules, such as oxygen and carbon dioxide
4. They move across membranes down concentration gradients
5. (from high to low concentrations) due to their kinetic energy
6. The rate of diffusion is affected by temperature (which affects the kinetic energy of particles), and the size of the concentration gradient
7. The rate of diffusion will also be increased by the surface area of the membrane.
8. (remember that although shorter diffusion distances can also increase the rate of diffusion, membranes are all the same width, so they are not considered a factor in this context)

Question 12

Describe an investigation into the rate of diffusion

Answer 12

1. Agar prepared with the indicator phenolpthalein (pink) is cut into blocks of different dimensions (to vary diffusion distance and surface area)
2. Agar blocks are placed in mild acid solution
3. For a specific length of time, and the distance that has become clear measured
4. OR the time taken for the entire cube to become clear is measured
5. Distance or volume is divided by time to determine the rate of diffusion

Question 13

Describe how molecules move across membranes by osmosis

Answer 13

1. Water is a polar molecule and cannot freely move across biological membranes
2. Water requires channel proteins called aquaporins
3. Water movement is driven by the concentration of solutes
4. The higher the concentration of substances dissolved in water, the lower the water potential
5. Water (net) moves from regions of high water potential (less solute) to regions of low water potential (more solute)
6. Or..down a water potential gradient

Question 14

Describe the effect of different external water potentials on animal cells (for example red blood cells)

Answer 14

1. When the water potential outside the cell is higher than inside, water will enter cells by osmosis, causing them to swell and burst due to the increased hydrostatic pressure (cytolysis)
2. When water potential outside the cell is the same as inside the cell, water is entering and leaving the cell at equal rates - there is no change in shape
3. When the water potential outside the cell is lower than inside the cell, water leaves the cell by osmosis, reducing cell volume, and causing crenation
4. Animals have homeostatic mechanisms that maintain the water potential of fluids such as tissue fluid and plasma, that prevent cytolysis and crenation

Question 15

Describe the effect of different external water potentials on plant cells

Answer 15

1. Remember that plants do not have homeostatic mechanisms to regulate water potential outside of cells. Cell structure in plants is primarily ensured by the cellulose cell wall
2. When water potential outside of plant cells is higher than inside, water enters by osmosis, increases the hydrostatic pressure, and the cell membrane exerts turgor pressure against the cell wall. Thus the cell wall prevents further entry of water and the cell is described as turgid
3. When the water potential outside the plant cell is the same as inside, the rate of water movement in both directions is the same and there is no change in shape. There is no pressure exerted against the cell wall, cells are not turgid.
4. When the water potential outside the plant cell is lower than inside the cell, water leaves the cell by osmosis, and the cell surface membrane comes away from the cell wall.

Question 16

Describe how to investigate movement across a partially membrane in model cells

Answer 16

1. Use dialysis tubing to simulate the partially permeable membrane of a cell.
2. The tubing is permeable to water and mineral ions and glucose but not starch
3. Fill dialysis tubing with a solution containing starch and glucose
4. The external solution can be varied with different water potentials, and solutes compositions, or temperature
5. The movement of substances can be monitored by measuring changes in the ‘cell’ volume or mass, or testing the solutions for the presence of biological molecules

Question 17

Describe how molecules move across membranes by facilitated diffusion

Answer 17

1. Charged, polar and relatively large molecules cannot freely move across phospholipid bilayers
2. Intrinsic proteins such as channel proteins or carrier proteins
3. Provide a hydrophilic route for their movement (channel proteins)
4. Or bind them and change shape, moving them across the membrane (carrier proteins)
5. In a direction dependent on the concentration/electrical gradient
6. At a rate dependent on the size of the gradient, number of protein molecules available and the rate at which they can work

Question 18

Describe how molecules move across membranes by active transport

Answer 18

Some carrier proteins use the energy of ATP hydrolysis to change shape

And move molecules or ions from one side of the membrane to the other

In a direction determined by the protein, and independent of the concentration/electrical gradient

Active transport is usually works to create gradients, by moving substances UP their gradients

(often, it is these gradients that allow (other) substances to move passively by diffusion)

Question 19

Summarise the ways in which bulk transport occurs across cell membranes

Answer 19

1. Relatively large volumes of liquid, or large solid particulates such bacteria enter cells by endocytosis
2. Liquids enter by pinocytosis
3. Solids enter by phagocytosis
4. Substances produced by cells are secreted by exocytosis

Question 20

Describe how exocytosis occurs

Answer 20

1. Material to released, for example, a peptide hormone, or antibodies
2. Are modified and packaged into vesicles by the Golgi apparatus
3. These vesicles move to the cell membrane along the cytoskeleton
4. The phospholipid bilayer of the vesicle then fuses with that of the cell surface membrane
5. This step requires proteins and ATP (therefore: active)
6. Once fused, the contents of the vesicle diffuse away from the cell, into the tissue fluid, and possibly into the blood plasma

Question 21

Describe how endocytosis occurs

Answer 21

1. The material to be transported nears the cell surface membrane (or the membrane is extended towards it)
2. The membrane begins to invaginate around the material in a process requiring proteins and ATP
3. The invaginated portion of the membrane then pinches off, forming a vesicle that contains external contents (liquid or solid)
4. In phagocytosis, lysosomes then fuse with the vesicle, to digest the internalised contents using hydrolytic enzymes