Transport Across Cell Membranes

Question 1

Function of the plasma membrane

Answer 1

Selectively permeable which means the membrane allows some materials to freely enter and others to not

Question 2

Phosholipids

Answer 2

Form a bilayer with hydrophobic tails facing inwards that repel water - fatty acid chains and hydrophilic heads face outwards - phosphate groups

Acts as a barrier to most water soluble substances so they cant leak out of the cell or enter

Question 3

Cholesterol

Answer 3

Fit in between phospholipid molecules with hydrophobic tails and hydrophilic heads but are absent in prokaryotic membranes

Increase the fluidity of the membrane because it stops the phospholipid tails packing too closely together

Question 4

Glycolipids and importance

Answer 4

Lipids with carbohydrate chains found on the outer phospholipid mono layer
Act as receptor molecules which allows them to bind with certain substances:
Signalling receptors for hormones and neurotransmitters
Receptors in cell adhesion

Question 5

Proteins and importance

Answer 5

Intrinsic proteins - transmembrane proteins e.g transport proteins
Create hydrophilic channels to allow ions and polar molecules to travel through the membrane

Question 6

When do membranes become less/more fluid

Answer 6

More: an increased proportion of saturated fatty acid chains as the chains pack together tightly so there is a high number of intermolecular forces between the chains
Less: unsaturated fatty acids chains are bent which means they are less likely to be packed closely together

Question 7

What Is diffusion

Answer 7

The net movement as a result of random motion of its molecules of a substance from a region of its higher concentration to a region of its lower concentration

Question 8

What factors control the rate of diffusion

Answer 8

Steepness of the concentration gradient: a greater difference in concentration means a greater difference in the number of molecules passing in two directions
Temperature : molecules and ions have more kinetic energy at higher temperatures
Surface area : greater number of molecules or ions that can cross at any one moment
Properties of molecules : large molecules diffuse more slowly as they require more energy to move

Question 9

What is facilitated diffusion

Answer 9

When large polar molecules and ions can’t cross the phospholipid bilayer without the help of certain proteins

Question 10

What’s a channel protein

Answer 10

Water filled pores which allow charged substances to diffuse through

Question 11

What’s a carrier protein

Answer 11

Binding site of the carrier proteins is open to one side of the membrane and then open to other side when its changes shape

Question 12

Osmosis

Answer 12

The net movement of water molecules from a region of high water potential to a region of lower water potential through a partially permeable membrane

Question 13

What is water potential

Answer 13

The tendency of water to move out of a solution so a dilute solution has a high water potential and a concentrated solution has a lower water potential

Question 14

High water potential in plants

Answer 14

As water enters the vacuole which expands the protoplasm and pressure builds up
This stops too much water entering and is described as turgid which is important for support and strength

Question 15

Low water potential in plants

Answer 15

As water leaves the vacuole the volume of the plant cell decreases - the protoplast gradually shrinks and no longer exerts pressure
The protoplast continues to shrink and pull away from the cell wall which known as plasmolysis

Question 16

What is active transport

Answer 16

Movement of molecules through a cell Membrane from a region of low concentration to a region of higher concentration using energy from respiration

Question 17

Why does active transport require energy

Answer 17

Energy is required to make carrier proteins change shape which allows it to transfer molecule

Question 18

What is active transport important in

Answer 18

Reabsorption of useful molecules in the blood after filtration
Absorption of some products of digestion

Question 19

What is co transport

Answer 19

Coupled movement of substances across a cell membrane via a carrier protein

Question 20

Absorption of glucose

Answer 20

Sodium ions are actively transported out of the ileum epithelial cells, into the blood by the sodium potassium pump which creates a concentration gradient which causes sodium ions to diffuse from lumen of the ileum into the epithelial cell by sodium-glucose co transport proteins which carries glucose with it and as a result the concentration of glucose inside the cell increases so it then diffuses out the cell into the blood

Question 21

How is a root hair cell specialised

Answer 21

Adapted for the absorption of water and mineral ions
That have a specialised shape that increases the cell’s surface area so the rate of osmosis is increased
Thinner walls so a short diffusion distance

Question 22

How is the epithelial cell of the small intestine specialised

Answer 22

• have microvilli which increases the surface area
  Each villus has a constant blood supply which continually transports the products of digestion which maintains a high concentration gradient

Question 23

How are kidney cells specialised for facilitated diffusion

Answer 23

High number of aqua porins which allow kidney cells to reabsorb water

Question 24

How are neurones and muscle cells specialised for facilitated diffusion

Answer 24

Channel proteins for sodium , potassium and calcium ions which can open and close
They play a role in the speed of electrical transmission