Transport across membranes

Water potential

The pressure created by water molecules

- measured in kPa

Osmosis in plant cells

Into cell: protoplast swells

- cell becomes turgid

Out of cell: protoplast shrinks

- cell becomes flaccid

Osmosis in animal cells

Into cell: lysis

Out of cell: crenation

Define simple diffusion

Passive process

(requires no energy from ATP hydrolysis)

movement of small, lipid-soluble molecules down a concentration gradient

Define facilitated diffusion

Passive process

Movement of molecules down a concentration gradient via channel proteins

how channel proteins work

Hydrophilic channels bind to specific ions

- one side of the protein closes and the other opens

How do carrier proteins work?

Binds to complementary molecule

conformational change releases molecule on other side of membrane

Five factors that affect the rate of diffusion

1. Temperature

2. Diffusion distance

3. Surface area

4. Size of molecule

5. Difference in concentration

Ficks law

surface area x difference in concentration / diffusion distance

How are cells adapted to maximise the rate of transport across their membranes?

1. many carrier/ channel proteins

2. folded membrane increases surface area

Active transport

Active process

Movement of molecules against a concentration gradient via a carrier proteins

- using ATP

Compare and contrast active transport and facilitated diffusion

● Both may involve carrier proteins.

Active transport requires energy from ATP hydrolysis whereas facilitated diffusion is a passive process.

Facilitated diffusion may also involve channel proteins.

Features of a cell specialised for absorption

1. Large number of mitochondria

- to release energy for ATP in the form of ATP

2. Large number of channel and carrier proteins

- for facilitated diffusion

Co-transport

The movement of 2 substances using a carrier protein

How the movement of Na+ molecules out of the cell allows for the absorption of glucose into the cell

1. The movement of Na+ out of the cell maintains a concentration gradient

2. Na+ moving in by co-transport

- bringing glucose with it

Explain how co-transport is involved in the absorption of glucose / amino acids in the small intestine.

1. Na+ actively transported out of epithelial cells & into bloodstream

2. Na+ concentration lower in epithelial cells than lumen of gut

3. Transport of glucose/ amino acids from lumen to epithelial cells is 'coupled' to facilitated diffusion of Na+ down concentration gradient

How the movement of substances is affected by membrane structure

1. Phospholipid bilayer allows for the movement of non-polar/lipid-soluble substances

2. Phospholipid bilayer prevents movement of polar/lipid-insoluble substances

3. Carrier proteins allow active transport

4. Channel/carrier proteins allow facilitated diffusion/co-transport

5. Shape/charge of channel/carriers determines how much movement

6. No. of channels/carriers determines how much movement

7. Membrane SA determines how much movement

8. Cholesterol affects fluidity/permeability

Osmosis definition

Movement of water molecules down a water potential gradient