2.5 biological membranes

Question 1

Why is the fluid mosiac model called ‘fluid’?

Answer 1

• phospholipids and proteins can move by diffusion
• phospholipids move side to side

Question 2

Why is the fluid mosiac model called ‘mosiac’?

Answer 2

• scattered pattern produced by proteins within the phospholipid bilayer

Question 3

Structure of a phospholipid

Answer 3

• hydrophillic head
• hydrophobic fatty acid tail (non-polar)
• fatty acid tails face inwards creating a barrier against water soluable substances (polar substances)

Question 4

Role of cholesterol in the fluid mosiac model

Answer 4

regulate fluidity and stability of the membrane
* stops it being too rigid at low temperatures and too fluid at high temperatures
* stops phospholipids packing too close together

Question 5

Role of glycoproteins and glyolipids in the fluid mosiac model

Answer 5

• forms hydrogen bonds with water to stabilise membrane
• cell adhesion in tissue formation
• receptors for cell signaling
• antigens for cell recognition

Question 6

Role of extrinsic proteins in the fluid mosiac model

Answer 6

partly embedded in the bi-layer
contain mainly hydrophillic R-groups

Question 7

What are the key internal membrane functions

Answer 7

1. isolation
2. comparmentalisation
3. control what enters and leaves the cell
4. site of chemical reactions

Question 8

Definition of bulk transport

Answer 8

a form of active transport that requires ATP but doen’t require a concentration gradient

Question 9

Definition of endocytosis

Answer 9

bulk transport into cells
2 types: phagocytosis (solid) and pinocytosis (liquid)

Question 10

Function of the phospholipids in the bi-layer

Answer 10

to create a partially permeable membrane

Question 11

Defintion for exocytosis

Answer 11

bulk transport out of the cell

Question 12

Process of exocytosis

Answer 12

• transport out of the vesicles from the golgi appartus
• move towards and fuse with the cell surface membrane
• the contents are released outside of the cells

Question 13

Process of endocytosis

Answer 13

• cell membrane invaginates (bend inwards) when in contact with the substance to be transported
• membrane enfolds until the membrane fuses to make a vesicle
• this pinches off and moves to the cytoplasm for further transfer ( moved by cytoskeleton )

Question 14

Role of the cytoskeleton

Answer 14

• changes cell shape to engulf materials
• movement of secertory vesicles
• fusion with the cell membrane

Question 15

Definition of cell signaling

Answer 15

communication between cells to trigger a reaction within the target cell

Question 16

How do glycoproteins act as a receptor

Answer 16

specific shape which is complementary to the shape of the signalling molecule

Question 17

Definition of diffusion

Answer 17

net movement of a substance from a region of high concentration to a region of low concentration
- caused by natural kinetic energy

Question 18

What can pass through the PL bi-layer

Answer 18

1. oxygen/carbon dioxide (small)
2. lipid soluable molecules (alcohol, steriod hormones)
3. water

Question 19

What cannot pass through the PL bi-layer

Answer 19

1. ions (charged - so they are repelled against non-polar fatty acid tails)
2. polar/ large molecules (too large)

Question 20

Definition of faciliated diffusion

Answer 20

net movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels

Question 21

Functions of a channel protein

Answer 21

transport charged substances (ions)
acts as gates so part of the inside surface of the membrane can moves to close or open the pores

Question 22

Function of carrier proteins

Answer 22

can change between two shapes
creates a binding site to be open
the direction of movement depends of the concentrations

Question 23

Factors affecting diffusion

Answer 23

1. concentration gradient
2. temperature
3. surface area
4. distance
5. membrane
6. size of diffusing molecule

Question 24

How does the concentration gradient affect diffusion

Answer 24

the steeper the concentration gradient the faster the rate of diffusion

Question 25

How does temperature affect diffusion

Answer 25

the higher the temperature the more kinetic energy the particles will have to move around
it will occur faster

Question 26

How does surface are affect diffusion

Answer 26

the larger surface area : volume ratio of the membrane the more particles can diffuse at once
it will occur faster

Question 26

How does distance affect diffusion

Answer 26

the shorter the distance or the thinner the membrane the quicker the rate of diffusion

Question 26

How does membranes affect diffusion

Answer 26

the greater the number of protein channels or carrier proteins present the higehr the rate of diffusion

Question 27

How does the size of the diffusing molecule affect diffusion

Answer 27

the smaller the ions / molecule diffuses more rapidly than larger molecules

Question 28

Definition of active transport

Answer 28

net movement of substances against the concentration gradient ( low to high ) across a cell membrane, using ATP and carrier proteins

energy is from hydrolosis of ATP

Question 29

Defintion of osmosis

Answer 29

net movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane

Question 30

Definition of water potential

Answer 30

the pressure exerted by water molecules as they collide with the membrane

Question 31

What is the highest water potential

Answer 31

0

Question 32

Definition of isotonic

Answer 32

no overall net movement

Question 33

Definition of hypertonic

Answer 33

high solute concentration
low water concentration

Question 34

Definition of hypotonic

Answer 34

low solute concentration
high water concentration

Question 35

What happens in an animal cell if it is hypertonic

Answer 35

water leaves the cell so it crenates (shrivels)

Question 36

What happens to a plant cell if its hypertonic

Answer 36

water leaves and the cell membrane becomes detached
they are plasmolysed

Question 37

What happens to an animal cell if its isotonic

Answer 37

water goes in and out of the cell
becomes balanced (equilibrium)

Question 38

What happens in a plant cell if its isotonic

Answer 38

flaccid

Question 39

What happens in an animal cell if its hypotonic

Answer 39

it lyses (splits open) due to hydrostatic pressure (occurs in a closed system)

Question 40

What happens in a plant cell if its hypotonic

Answer 40

water goes in and the cell becomes turgid
the pressure potential prevents any further entry of water

Question 41

What is the water potential of a cell and why

Answer 41

negative water potential because it has dissolved solutes in their cytoplasm

Question 42

How does temperture affect the permeability of the PL bi-layer

Answer 42

• PL are constantly moving
• the PL gain more kinetic energy meaning the membrane becomes more fluid and gaps begin to form between the PL
• if too fluid it will loose control of the entry and exit of substances and protein channels will denature

Question 43

How does solvents affect membrane permeability of the PL bi-layer

Answer 43

organic solvents (ethanol or acetone) dissolve the membrane as they are lipid soluable
this disrupts membrane permeability