2.3 transport across cell membranes

Question 1

Fluid-mosaic model of membrane structure

Answer 1

Used to describe membrane structure

molecules within membrane can move laterally

Mixture of phospholipids, proteins, glycoproteins and glycolipids

Question 2

Structure of phospholipid bilayer

Answer 2

Phosphate heads are hydrophilic so orientate towards water

Fatty acids are hydrophobic so orientate away from water

Question 3

Structure of cell membrane

Answer 3

Phospholipid bilayer

Embedded proteins, intrinsic or extrinsic (channel and carrier proteins, intrinsic)

Glycolipids and glycoproteins

Cholesterol

Question 4

Fluid mosaic model used to explain how molecules can enter/leave a cell

Answer 4

Phospholipid bilayer:

enables movement of non-polar small/lipid-soluble molecules down a concentration gradient

Restricts the movement of larger/polar molecules

Channel proteins and carrier proteins:

Allows movement of water-soluble/polar molecules down a concentration gradient using ATP

Carried proteins:

Allows the movement of molecules against a concentration gradient using ATP (active transport)

Question 5

Features of the plasma membrane

Answer 5

Phospholipid bilayer maintains a different environment on each side of the cell

Phospholipid bilayer is fluid so can bend to take up different shapes for phagocytosis

Surface proteins used in cell recognition

Cholesterol increases stability

Question 6

Role of cholesterol

Answer 6

Makes the membrane more rigid by restricting lateral movement of molecules making up the membrane

Question 7

Simple diffusion & factors affecting rate

Answer 7

Net movement of small, non-polar molecules across a membrane down a concentration gradient

Passive process

Factors - surface area, concentration gradient, diffusion distance

Question 8

Facilitated diffusion

Answer 8

Net movement of larger/polar molecules down a concentration gradient

Through a channel/carrier protein

Passive

Factors - surface area, concentration gradients, number of channel/carrier proteins

Question 9

Role of carrier/channel proteins in facilitated diffusion

Answer 9

Carrier proteins transport large molecules, the protein changes shape when molecule attaches

Channel proteins transport charged/polar molecules through its pore

Different carrier and channel proteins facilitate the diffusion of different specific molecules

Question 10

Movement across membranes via active transport

Answer 10

Net movement of molecules/ions against a concentration gradient

Using carrier proteins

Using energy from hydrolysis of ATP to change the shape of the tertiary structure and push the substances through

Factors - pH/temperature, speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)

Question 11

Co-transport

Answer 11

Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the sodium-potassium pump, creating a concentration gradient of sodium

Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen, via a co-transporter protein

Creating a concentration gradient of glucose

Glucose moves out of cell into blood by facilitated diffusion through a protein channel

Question 12

Movement across membranes by osmosis and factors

Answer 12

Net movement of water molecules across a partially permeable membrane down a water potential gradient

Passive

Factors - surface area, water potential gradient, diffusion distance

Question 13

How might cells be adapted for transport across their internal or external membranes

Answer 13

By an increase in surface area

Increase in number of protein channels/carriers