Biological membranes

Question 1

selectively permeable membranes

Answer 1

able to control the movement of substances into and out of cells, regulating the composition of fluid within individual cells, provides a barrier between the inside and outside of a cell, control the flow of information between cells either by recognizing signal molecules received from other cells, or by sending chemical or electrical signals to other cells, made of 3 major components: lipids, proteins and carbohydrates

Question 2

lipids, proteins and carbohydrates structure and bonds

Answer 2

have a common general structure - two layered sheets of lipid molecules which have proteins embedded in them, lipid and protein molecules can move about on the plane of the membrane, they are held together by non-covalent interactions, carbohydrates are attached by covalent bonds to some of the lipid and protein molecules

Question 3

phospholipids

Answer 3

made up of two fatty acid chains joined to a glycerol molecule and a polar phosphate containing head, the head is hydrophilic and the tails are hydrophobic, the heads are polar and tails are non-polar, makes membrane fluid and permits the passage of lipid soluble molecules

Question 4

cholesterol

Answer 4

cholesterol is a steroid, majority of molecule is hydrophobic and so is attracted to the fatty acids of the phospholipids in the membrane, OH group is hydrophilic and so orientates itself nearer the hydrophilic phosphate heads, it’s role is to regulate the fluidity of the membrane

Question 5

glycolipids

Answer 5

glycolipids have a sugar such as glucose or galactose instead of a phosphate containing head, glycolipids are found on the outer surface of the plasma membrane with their sugar exposed at the cell surface, recognition sites for substances such as insulin and act as antigens

Question 6

glycolipids and phospholipids similarities

Answer 6

both have a hydrophilic head and two hydrophobic tails, if submerged in water, these molecules will spontaneously associate to form bilayers, with the tails on the inside and the heads facing outwards

Question 7

why are bilayers formed?

Answer 7

bilayers formed because the tails have a strong tendency to stay away from water and so are squeezed together by water molecules, some bilayers will close in on themselves to form sealed compartments called liposomes which eliminates the edges where tails would be in contact with water

Question 8

membrane proteins

Answer 8

some proteins extend across the phospholipid bilayer and are often referred to as intrinsic, integral or transmembrane proteins, other proteins are only partly embedded in the membrane and are referred to as extrinsic or peripheral proteins, certain substances will need to be allowed into and out of the membrane and this is what membranes do

other functions include:
- provide channels for passive transport of materials
- act as active transport pumps to move substances
- act as hormone binding sites (hormone receptors)
- are immobilised enzymes
- help with cell adhesion forming junctions between neighbouring cells

Question 9

cell signalling

Answer 9

cells communicate with each other through chemical messengers such as hormones, these molecules will be produced by one cell and will travel to other cells in the body, these molecules will only affect some cells - will have an appropriate cell membrane receptor, the receptors will detect the messenger molecule and changes will occur in the cell, receptors in the plasma membrane that are involved in cell signalling are usually glycoproteins but glycolipids may also be involved, a cell with a receptor molecule is referred to as a target cell

Question 10

what happens when receptors are activated by a messenger molecule?

Answer 10

• may cause the release of a second messenger in the cell which brings out a series of changes
• may result in the opening of a protein channel that was previously closed allowing specific molecules to enter and exit the cell
• may activate an enzyme in the cell or the receptor may be an enzyme itself

Question 11

what is the thickness of a cell membrane?

Answer 11

7nm - 10nm

Question 12

intrinsic proteins

Answer 12

form channels or carrier proteins for water soluble molecules

Question 13

extrinsic proteins

Answer 13

act as enzymes

Question 14

glycoproteins

Answer 14

helps with cell to cell adhesion

Question 15

diffusion

Answer 15

few substances can directly diffuse throw lipid bilayer and the only molecules that can do this are lipid-soluble or non-polar molecules or very small polar molecules or uncharged molecules, it is a passive process meaning there is no energy involved and substances can only move down the concentration gradient, net movement of a substance from concentration of high to low

Question 16

facilitated diffusion

Answer 16

transport of substances across the membrane by a trans-membrane protein, the transport proteins are specific to one molecule so substances can only cross the membrane when there is an appropriate protein, also a passive process so no energy involved and substances can only move down the concentration gradient

Question 17

what are the two types of transport proteins?

Answer 17

channel proteins - form a channel in the membrane which will allow charged substances (ions) to diffuse across membranes, most channels can be gated which will allow the cell to control the entry and exit of ions

carrier proteins - used for larger molecules, have a binding site for a specific solute, constantly flips between two states so that the site is alternately open to opposite sides of the membrane, substance will bind to the side where it is at high concentration and be released where it is at low concentration

Question 18

active transport

Answer 18

the pumping of substances across a membrane by a trans-membrane carrier protein molecule which are highly specific meaning there is a different carrier for each molecule to be transported, the protein carriers are also ATPase enzymes which catalyse ATP => ADP + phosphate and the energy is released to change shape and pump the molecule across the membrane, it is an active process, it is the only transport mechanism that can transport substances up/against the concentration gradient

Question 19

endocytosis

Answer 19

process by which cells absorb molecules by engulfing them, there are two forms of endocytosis: phagocytosis and pinocytosis

it is an active process and requires ATP

Question 20

pinocytosis

Answer 20

process by which small particles and fluids are taken in, plasma membrane invaginates inwards and then the membranes fuse around the molecule to form a vesicle

Question 21

phagocytosis

Answer 21

protrusions called pseudopodia extend from the cell and wrap themselves around a large molecule (a phagocyte engulfing a pathogen)

Question 22

exocytosis

Answer 22

the process whereby substances are secreted from a cell, it is an active process and requires ATP

Question 23

osmosis

Answer 23

describes movement of water across the membrane, net movement of water molecules from an area of low concentration to an area of low concentration of a substance to an area of high concentration of a substance

Question 24

water potential

Answer 24

the tendency of water to diffuse from one area to another, used to describe the concentration of water

Question 25

aquaporins

Answer 25

special protein channels which allow water molecules through

Question 26

turgid

Answer 26

when a cell is swollen and hard

Question 27

plasmolysed

Answer 27

process in which cells lose water in a hypertonic solution