Transport across cell membranes

Question 1

Describe the fluid mosaic model of membranes

Answer 1

Phospholipid bilayer with a flexible shape (fluid)
Varying extrinsic and intrinsic protein embedded (mosaic)

Question 2

Explain the role of cholesterol and glycolipids in the membrane

Answer 2

Cholesterol = increases stability by connecting phospholipids
Glycolipids = cell recognition

Question 3

Define osmosis

Answer 3

Diffusion of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential

Question 4

How does osmosis affect plant cells?

Answer 4

Osmosis in = protoplast swells (turgid)
Osmosis out = protoplast shrinks (flaccid)

Question 5

Define simple diffusion

Answer 5

Net movement of small, lipid-soluble molecules from an area of higher concentration to an area of lower concentration
It’s a passive process (requires no ATP)

Question 6

Define facilitated diffusion

Answer 6

Specific channel / carrier proteins transport large, polar molecules down the concentration gradient
It’s a passive process

Question 7

Name 5 factors that affect the rate of diffusion across a membrane

Answer 7

Temperature
Distance
Size of molecule
Surface area
Difference in concentration

Question 8

How are cells adapted to maximise diffusion?

Answer 8

Many channel + carrier proteins
Folded membrane (increases S.A.)

Question 9

Define active transport

Answer 9

Specific carrier protein transports molecules from an area of low concentration to an area of high concentration
It’s an active process (ATP hydrolysis releases phosphate that binds to carrier protein)

Question 10

How does co-transport work in the small intestine?

Answer 10

1. Na+ actively transported out epithelial cells into bloodstream
2. Na+ concn lower in epithelial cells than lumen of gut
3. Transport of glucose from lumen to epithelial cells is coupled to facilitated diffusion of Na+ down gradient