Transport over membranes

Question 1

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

Function of phospholipid

Answer 2

formation of bilayer, selectively permeable boundary

Question 3

Function of cholesterol

Answer 3

strengthens membrane, maintains correct tension

Question 4

Function of glycolipid

Answer 4

cell surface recognition

Question 5

Function of glycoprotein

Answer 5

cell surface receptors e.g. insulin hormone

Question 6

Function of extrinsic protein

Answer 6

e.g. enzymes

Question 7

intrinsic channel / carrier protein

Answer 7

transport of non-lipid soluble / hydrophilic (charged) molecules e.g. ions, glucose

Question 8

Functions of the cell-surface membrane

Answer 8

Forms the boundary between the cell’s cytoplasm and its environment - allowing different conditions and reactions to occur inside and outside a cell

Controls the movement of substances in and out of a cell (helping to maintain homeostasis in the cell)

Question 9

Functions of organelle membranes

Answer 9

Controls entry and exit of materials in and out of the organelle
Separates organelles contents from the cytoplasm so that specific metabolic reactions can take place within them
Can provide an internal transport system (eg. ER)
Can isolate enzymes that may damage the rest of the cell (eg. in lysosomes)
Provides surfaces on which reactions can occur

Question 10

Functions of membrane proteins

Answer 10

Provide structural support
Act as receptors for molecules e.g. hormones
Help cells to adhere (“stick”) together
Form cell-surface receptors (cell recognition)
Act as channel and carrier proteins for the transport of large polar / charged substances
Enzymes

Question 11

Explain the permeability of membranes.

Answer 11

Most molecules do not freely diffuse across cell membranes as they are…

Not lipid soluble and therefore cannot pass through the hydrophobic layer of the phospholipid bilayer

Too large

Polar molecules and therefore require specific transport carriers / channels (with hydrophilic pores)

Question 12

Describe the formation of a phospholipid bilayer.

Answer 12

Hydrophilic, phosphate heads face outwards, attracted to water..

Both inside and outside of the cell.

Hydrophobic fatty acid tails turn inwards, repelled by water.

Question 13

Explain simple diffusion

Answer 13

Small, non-polar (lipid soluble) molecules diffuse directly over the phosoplipid bilayer
net diffusion. Down the concentration gradient from higher concentration to lower concentration. This is a passive process as it does not require energy/ no ATP required instead uses the kinetic energy of the molecules

Question 14

Factors that effect the rate of simple diffusion

Answer 14

Temperature ( more kinetic energy at higher temperatures)
Steepness of concentration gradient
size/nature of molecules

Question 15

Explain Facilitated diffusion

Answer 15

Large polar (lipid insoluble) molecules for example charged ions such as sodium cannot diffuse directly over the phosphlolipd biayer but use carrier/channel proteins with a hydrophilic pore.
Net diffusion
down the concentration gradient from a higher concentration to a lower concentration.
Passive as it does not require ATP uses the kinetic energy of the molecules.

Question 16

Channel proteins

Answer 16

Intrinsic membrane proteins with a hydrophilic pore in their centre spanning the phospholipid bilayer through which charged, hydrophilic solutes can diffuse.

Question 17

Carrier proteins

Answer 17

Operate by binding to the target molecule, undergoing a change in shape and then releasing the molecules on the other side of the membrane.

Carrier proteins alternate between two shapes (conformations) so that the molecule binding site is accessible on one side of the membrane and then the other.

Question 18

Factors that effect the rate of facilitated diffusion

Answer 18

Temperature (more kinetic energy at higher temperature)
Steepness of concertation gradient
Size/nature of molecules

Question 19

Active transport involves three essential features:

Answer 19

Ions and molecules are moved against their concentration gradients from lower concentration to a higher concentration

The process requires energy from the hydrolysis of ATP.

The process occurs via intrinsic membrane transport proteins: carriers (hydrophilic pore) / channels

Question 20

Bullet point the sequence of events in the active transport of an ion

Answer 20

• Active transport transport hydrophilic / polar (lipid insoluble) / charged molecules

-Ion combines with a specific carrier protein / binding site on one side of the membrane

-ATP is hydrolysed and a phosphate binds to the carrier

-The carrier changes shape (conformation)

-The ion is released onto the other side of the membrane

-The phosphate dissociates from the carrier

-The carrier returns to its original shape

Question 21

Co-transport

Answer 21

1. Sodium ions are being actively pumped out of the cuboidal cells by active, ATP driven Na / K exchange pumps.
2. This sets up a sodium ion concentration gradient., with a higher concentration of sodium ions on the outside.
3. 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).
4. The glucose can then passively diffuse out through other carrier proteins onto the other side / passing into the blood capillaries.

Question 22

How are the cells that line the ileum adapted for absorption?

Answer 22

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 23

Osmosis

Answer 23

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