module 2.1.5 - cell membrane

Question 1

explain the roles of the membrane

Answer 1

a partially permeable barrier between the cell and its environment
- it keeps the contents of the cell separate from its environment, and limits what molecules can enter and leave the cell
it acts as the site for certain chemical reactions and enables the cell to communicate with other cells through the process of cell signalling

Question 2

describe the fluid mosaic model of membrane structure

Answer 2

a fluid mosaic membrane consists of:
* a bilayer of phospholipid molecules
* cholesterol which regulates the fluidity of the membrane, making it more stable
* glycolipids and glycoproteins that function in cell signalling or cell attachment
* protein molecules that float in the phospholipid bilayer. some proteins are partially held on the surface of the membrane — these are called extrinsic proteins. others are embedded in the membrane — these are called intrinsic proteins. some proteins float freely in the bilayer whereas others may be bound to other components in the membrane or to structures inside the cell

Question 3

describe the role of protein in membranes

Answer 3

many functions involve moving substances across the membrane. for example, some proteins may form:
* pores that allow the movement of molecules that cannot dissolve in the phospholipid bilayer
* carrier molecules that allow facilitated diffusion
* active pumps

Question 4

explain the movement of wate molecules with water potential

Answer 4

pure water has a water potential of zero
- as solutes (sugars or salts) are added to a solution, the water potential gets lower. therefore, a salt solution has a water potential below zero, i.e. a negative potential
- water molecules will move from a solution with a higher water potential to a solution with a lower (more negative) water potential. therefore, water molecules always move down their water potential gradient

Question 5

explain the mechanism of osmosis with water potential

Answer 5

a cell placed in water has a lower (more negative) water potential than the surrounding water. there is a water potential gradient from high outside the cell to lower inside the cell
- water molecules enter the cell.

a cell placed in a strong salt solution has higher (less negative) water potential than the surrounding solution
- there is a water potential gradient from higher inside the cell to lower outside the cell, so water molecules leave the cell

Question 6

explain what will happen when you add a plant cell to water

Answer 6

water makes a plant cell turgid. the vacuole is full of watery sap and the cytoplasm pushes the plasma membrane out against the cell wall. the cell wall is strong and will stop the cell bursting

Question 7

explain what will happen when you add a plant to strong salt

Answer 7

a plant cell will lose turgidity. it will become flaccid. if the water loss continues, the cell vacuole will shrink. the cytoplasm will also shrink and the plasma membrane pulls away from the cell wall
- plasmolysis

Question 8

explain what will happen when you add an animal cell to water

Answer 8

an animal cell has no cell wall. the plasma membrane has no strength, so the animal cell will burst as waster enters the cell

Question 9

explain what will happen when you add animal cells to strong salt

Answer 9

an animal cell wall. the cytoplasm will shrivel
- appearance called crenated

Question 10

describe the role of glycoproteins and glycolipids in membranes

Answer 10

carbohydrate group on the protein or lipid molecule always has a specific shape and is used to recognise the cell — to identify it as ‘self’ or ‘foreign’. antigens on cell surfaces are usually glycolipids or glycoproteins.
- drugs and hormones can bind to these membrane-bound receptors
- medicines can be made to fit the receptors on certain cells.(eg.asthmatics take salbutamol, which fits the receptors on smooth muscle in the airways to cause relaxation)

cell signalling happens to coordinate the activities of the organism
- the shape of the glycoprotein or glycolipid may be complementary to the shape of a signalling molecule in the body. such complementary shapes can be used as binding sites to which the signalling molecules (e.g. hormones and neurotransmitter molecules in a synapse) attach. if the correct binding site is not present, the cell cannot respond to the signalling molecule
(binding sites are also used for cell attachment — the cells of a tissue bind together to hold the tissue together)

Question 11

describe the role of phospholipids in membrane

Answer 11

form a barrier that limits movement of some substances into and out of the cell, or into and out of the organelles, so the membrane is partially permeable
- small, fat-soluble substances dissolve into the phospholipid bilayer and diffuse across the membrane
- water-soluble molecules and ions cannot easily dissolve and cross the membrane. small molecules like water may diffuse across slowly, but most require special transport mechanisms

Question 12

describe the role of cholesterol in membrane

Answer 12

fits between the tails of the phospholipid molecules. It inhibits movement of the phospholipids, reducing the fluidity of the membrane
- it also holds the phospholipid tails together, for mechanical stability
- cholesterol makes the membrane less permeable to water and ions. it can also be converted into steroid hormones like testosterone, is used for waterproofing the skin, for making vitamin D, and for making bile salts

Question 13

comment on the structure of cholesterol

Answer 13

its structure is similar to that of carbohydrates in that it contains C, H, and O, has hydroxyl groups, and has six membered rings

Question 14

explain how temp affects membrane structure and permeability

Answer 14

• if temperature increases, the molecules gain kinetic energy and move about more. this increases the permeability of the membranes to certain molecules
• any molecules that diffuse through the phospholipid bilayer will diffuse more quickly because as the phospholipids move about, they leave temporary gaps between them, providing space for small molecules to enter the membrane
• if temperature increases further, the phospholipid bilayer may lose its mechanical stability (it may melt) and the membrane becomes even more permeable
• eventually, the proteins in the membrane will denature. this will further damage the structure of the membrane and it will become completely permeable

Question 15

explain how solvent affects membrane structure and permeability

Answer 15

solvents such as alcohol dissolve fatty substances as the conc of alcohol increases, the membrane is more likely to dissolve

Question 16

what is meant by the term passive transport

Answer 16

the movement of molecules that does not need metabolic energy in the form of adenosine triphosphate (ATP). it uses energy in the form of the kinetic (movement) energy. it only occurs when molecules move down a concentration gradient.

Question 17

what are the 3 types of passive transport

Answer 17

diffusion, facilitated diffusion and osmosis

Question 18

what is diffusion

Answer 18

the net movement of molecules away from a concentrated source. this may occur across a membrane if the molecules are fat-soluble or if they are small and can fit between the phospholipids in a membrane
high conc to low conc

Question 19

what is facilitated diffusion

Answer 19

diffusion across a membrane that is helped by a transport protein in the membrane. the protein could be a pore protein (which may be gated) or it could be a carrier protein

Question 20

what is osmosis

Answer 20

the net movement of water molecules across a partially permeable membrane. water molecules move down their water potential gradient (i.e. from an area of higher water potential to an area of lower water potential)

Question 21

what factors affect rate of diffusion

Answer 21

temp, conc gradient, size of molecule, thickness of membrane

Question 22

how does temp affect rate of diffusion

Answer 22

a higher temperature gives molecules more kinetic energy. at higher temperatures the molecules move faster, so the rate of diffusion increases

Question 23

how does conc gradient affect rate of diffusion

Answer 23

more molecules on one side of a membrane (or less on the other) increases the concentration gradient
- increases the rate of diffusion

Question 24

how does the size of molecules affect rate of diffusion

Answer 24

small molecules or ions can move more quickly than larger ones
- therefore, they diffuse more quickly than larger ones

Question 25

how does the thickness of membrane affect rate of diffusion

Answer 25

a thick barrier creates a longer pathway for diffusion, so diffusion is slowed down by a thick membrane

Question 26

how does surface area affect rate of diffusion

Answer 26

diffusion across membranes occurs more rapidly if there is a greater surface area

Question 27

define and explain active transport

Answer 27

the movement of molecules using metabolic energy in the form of ATP. it can move molecules against their concentration gradient and uses membrane-bound proteins that change shape to move the molecules across the membrane

Question 28

define and explain bulk transport

Answer 28

bulk transport is the movement of molecules through a membrane by the action of vesicles
- uses metabolic energy

Question 29

explain endocytosis (example of bulk transport)

Answer 29

the formation of vesicles by the plasma membrane, which moves molecules into the cell

Question 30

explain exocytosis (example of bulk transport)

Answer 30

the fusion of vesicles with the plasma membrane, which moves molecules out of the cell