Movement across membranes/absorption

Question 1

What are the functions of a cell membrane?

Answer 1

1. Controls what substances enter/leave cell/organelles
2. Allows different conditions to be established inside/outside of cell
3. Partially permeable (only allows certain molecules through)

Question 2

Why do phospholipids form membranes?

Answer 2

• Flexibility
• Allows small, lipid-soluble substances to enter/leave cell via diffusion
• Prevents water-soluble molecules crossing

Question 3

What are intrinsic proteins?

Answer 3

• Span through WHOLE phospholipid bilayer
• Carrier/channel proteins transport water-soluble molecules

Question 4

What are extrinsic proteins?

Answer 4

Span just ONE phospholipid layer

Question 5

Why is a membrane described as being fluid-mosaic?

Answer 5

Fluid - all molecules move around within layers

Mosaic - proteins/glycoproteins/glycolipids embedded throughout

Question 6

What is the difference between a glycoprotein and a glycolipid?

Answer 6

Glycoprotein is a carbohydrate attached to a protein whilst glycolipid is a carbohydrate attached to a lipid

Question 7

What is a channel protein?

Answer 7

Protein which spans the bilayer and forms a channel which allows small, charged molecules to pass through

Question 8

What is a carrier protein?

Answer 8

Protein which carries monomers like amino acids/glucose across membrane (changes tertiary structure to do this which requires ATP so involved in active transport)

Question 9

What are receptors?

Answer 9

Proteins with a binding site complementary to specific substrate they respond to

Question 10

What are antigens?

Answer 10

Proteins which stick out of membrane/virus capsid which are “non-self” if the body doesn’t make them (triggers immune response)

Question 11

What does cholesterol do in the cell membrane?

Answer 11

Makes it more stable (restricts movement of other molecules in cell surface membrane)

Question 12

How does temperature affect membranes?

Answer 12

Affects permeability (low temps = low permeability, high temps = high permeability), if temp is too high the membrane ruptures

Question 13

Define diffusion

Answer 13

The passive net movement of molecules from an area of high to low concentration down a concentration gradient

Question 14

Which molecules can DIFFUSE across a cell membrane?

Answer 14

Small, lipid-soluble molecules

Question 15

Which molecules are transported by facilitated diffusion?

Answer 15

Larger, water-soluble molecules and ions

Question 16

Which proteins are involved in facilitated diffusion and what do they do?

Answer 16

Channel - specific, no shape change, molecules move through channel, no extra energy required

Carrier - specific, change tertiary structure to release molecules, no extra energy required

Question 17

What affects rate of diffusion?

Answer 17

• Size of concentration gradient
• Surface area it’s occurring across
• No. proteins available for facilitated diffusion

Question 18

Define active transport

Answer 18

Movement of molecules/ions from an area of low to high concentration AGAINST the concentration gradient (uses ATP and carrier proteins)

Question 19

What does ATP do in active transport?

Answer 19

1. Binds to carrier protein and is hydrolysed
2. As molecule is released, phosphate group is released and carrier protein returns to original shape

Question 20

What is the sodium-potassium pump?

Answer 20

Transports ions into cell and ions out at the same time, uses ATP to move 3 SODIUM ions OUT in return for 2 POTASSIUM ions IN

Question 21

What does water potential mean?

Answer 21

Pressure created by water molecules

Question 22

How does water potential affect diffusion?

Answer 22

Diffuses from an area of high to low water potential

Question 23

Define osmosis

Answer 23

Passive net movement of water molecules from an area of high to low water potential across a partially permeable membrane down a water potential gradient

Question 24

What is an isotonic solution?

Answer 24

Has the same water potential as the cell (molecules move in and out of cell at same rate)

Question 25

What do hypotonic and hypertonic mean?

Answer 25

Hypotonic: higher water potential than cell

Hypertonic: lower water potential than cell

Question 26

What happens to ANIMAL cells in solutions of different water potential?

Answer 26

Hypotonic - lyses (bursts)

Hypertonic - shrinks

Question 27

What happens to PLANT cells in solutions of different water potentials?

Answer 27

Hypotonic - water moves IN, turgid

Hypertonic - water moves OUT, plasmolysed

Question 28

Define co-transport

Answer 28

Transport of glucose and amino acids across a membrane along with Na+ (involves active transport and ATP)

Question 29

What are the 4 steps of co-transport?

Answer 29

1. Na+ actively transported into capillary
2. Na+ gradient created
3. Na+ and monosaccharide/amino acid co-transported into epithelial cell by facilitated diffusion
4. Monosaccharide/amino acid transported into capillary by facilitated diffusion