3.2.3 Transport Across Cell Membranes

Question 1

(Transport Over Membranes) Define the fluid mosaic model of membrane structure.

Answer 1

FLUID - as individual phospholipids can move freely past each other (within their particular layer).
MOSAIC - as proteins are embedded in the membrane of numerous shapes, sizes and patterns.

Question 2

(Transport Over Membranes) What are the functions of the cell surface (external) membrane?

Answer 2

• Forms the boundary between the cell’s cytoplasm and it’s environment
• Controls the movement of substances in and out of a cell

Question 3

(Transport Over Membranes) Functions of the cell surface (external) membrane: What does the boundary formed between the cell’s cytoplasm and it’s environment allow?

Answer 3

Different conditions and reactions to occur inside and outside the cell.

Question 4

(Transport Over Membranes) Functions of the cell surface (external) membrane: What does the control of the movement of substances help to maintain?

Answer 4

Homeostasis in the cell.

Question 5

(Transport Over Membranes) What are the functions of internal organelle membranes? (5)

Answer 5

• Control entry/exit of materials in/out of the organelle
• Separates organelles contents from the cytoplasm
• Provides an internal transport system
• Isolate enzymes that may damage the rest of the cell
• Provides surfaces on which reactions can occur

Question 6

(Transport Over Membranes) Functions of internal organelle membranes: What is the purpose of separating the organelles contents from the cytoplasm?

Answer 6

So that specific metabolic reactions can take place within them.

Question 7

(Transport Over Membranes) Explain the permeability of membranes.

Answer 7

Most molecules do not freely diffuse across cell membraned as they are:
- Not lipid soluble
- Too large
- Polar molecules

Question 8

(Transport Over Membranes) What is the consequence of a molecule not being lipid soluble?

Answer 8

It cannot pass through the hydrophobic layer of the phospholipid bilayer.

Question 9

(Transport Over Membranes) Why can’t molecules freely diffuse across cell membranes if they are polar?

Answer 9

They require specific transport carriers/channels.

Question 10

(Transport Over Membranes) What are the 6 functions of proteins in membranes? (6)

Answer 10

• Provide structural support
• Act as receptors for molecules
• Help cells to adhere (stick) together
• Form cell-surface receptors
• Acts as channel and carrier proteins for the transport of large polar/charged substances
• Enzymes

Question 11

(Transport Over Membranes) What are the 3 similarities between simple and facilitated diffusion?

Answer 11

• Both are passive / no energy / no ATP required
• Both use the kinetic energy of the molecules
• Molecules diffuse down a concentration gradient (high to low)

Question 12

(Transport Over Membranes) Describe the function of a phospholipid bilayer.

Answer 12

• Non-polar, hydrophobic fatty acid tails allow the diffusion of small, non-polar substances / lipid soluble molecules
• Prevents diffusion of hydrophilic, polar, charged molecules / lipid insoluble

Question 13

(Transport Over Membranes) What are the two main features of simple diffusion?

Answer 13

• Occurs directly over the phospholipid bilayer
• Molecules are either small / non-polar / lipid soluble

Question 14

(Transport Over Membranes) What are the two main features of facilitated diffusion?

Answer 14

• Uses channel/carrier proteins
• Molecules are polar/hydrophilic / charged

Question 15

(Transport Over Membranes) What are the two similarities between active transport and facilitated diffusion?

Answer 15

• Both involve channel/carrier proteins that span the membrane
• Both transport hydrophilic/polar/charged molecules

Question 16

(Transport Over Membranes) What are the three main features of facilitated diffusion?

Answer 16

• Passive/no energy required
• Utilises the concentration gradient present
• Molecules diffuse down the concentration gradient

Question 17

(Transport Over Membranes) What are the three main features of active transport?

Answer 17

• Requires energy/hydrolysis of ATP
• With AT molecules are moved against the concentration gradient

Question 18

(Transport Over Membranes) Fill in the gaps regarding the features of channel proteins: _________ membrane proteins with a ___________ ____ in their centre spanning the ____________ _______ through which ______, hydro______ solutes can _______.

Answer 18

Intrinsic
hydrophilic pore
phospholipid bilayer
charge
hydrophilic
diffuse

Question 19

(Transport Over Membranes) Fill in the gaps regarding the features of carrier proteins: Operate by _______ to the ______ molecules, undergoing a change in _____ and then releasing the molecules on the other side of the ________. Can alternate between two ______ (conformations) so that the molecules _______ ____ is accessible on one side of the membrane and then the _____.

Answer 19

binding
target
shape
membrane
shapes
binding site
other

Question 20

(Transport Over Membranes) What factors affect the rate of transport over cell surface membranes? (5)

Answer 20

• Type of molecule
• Concentration gradient
• Temperature
• Surface area/adaptation of the cell membrane
• ATP where active sites of cotransport systems are involved

Question 21

(Transport Over Membranes) Explain how the type of molecule affects the rate of transport over cell surface membranes.

Answer 21

• Non-polar/lipid soluble can diffuse directly through the membrane
• Polar (charged/hydrophilic) require a protein transport channel/protein carrier

Question 22

(Transport Over Membranes) Describe the first stage of co-transport.

Answer 22

Sodium ions are being actively pumped out of the cuboidal cells by active, ATP driven Na/K exchange pumps.

Question 23

(Transport Over Membranes) Describe the second stage of co-transport.

Answer 23

The first stage results in the set up of a sodium ion concentration gradient, with a higher concentration of sodium ions on the outside.

Question 24

(Transport Over Membranes) Describe the third stage of co-transport.

Answer 24

The co-transporter then facilitates the sodium ions diffusing in down their gradient to “pull in” glucose molecules into the cytoplasm against its gradient (maximum absorption).

Question 25

(Transport Over Membranes) Describe the final stage of co-transport.

Answer 25

The glucose can then passively diffuse out through other carrier proteins onto the other side / passing into the blood capillaries.

Question 26

(Transport Over Membranes) How are the cells that line the ileum adapted for absorption? (3)

Answer 26

• Cell surface membrane is heavily folded with microvilli to greatly increase the surface area for transport.
• Many protein transport channels / carrier proteins to increase the rate of absorption.
• Large numbers of mitochondria to produce ATP via aerobic respiration for active transport / cotransport.

Question 27

(Transport Over Membranes) Define osmosis.

Answer 27

The passive movement of water, by simple diffusion, from a region where it has a higher water potential to a region where it has a lower water potential, down a water potential gradient, through a partially permeable membrane.

Question 28

(Transport Over Membranes) Regarding osmosis, what is the effect of increasing the solute concentration?

Answer 28

Increasing the solute concentration will decrease the water potential of a solution.