Transport across cell membranes

Fill in the arrows of this plasma membrane diagram

What are the 2 types of intrinsic/integral proteins?

• Carrier proteins

• Channel proteins

Define diffusion

The net movement of particles from an area of higher concentration to an area of lower concentration

How do protein channels move substances (2 steps)

1. Fills with water

2. Enabling water soluble ions to dissolve then diffuse through that channel

How do carrier proteins move substances (3 steps)

1. Carrier protein binds with molecule

2. This causes carrier protein to change shape

3. This transports the molecule to the other side of the membrane

What are 4 factors affecting rate of diffusion?

• Concentration gradient

• Thickness of exchange surface

• Surface area

• Number of carrier/channel proteins (facilitated diffusion only)

Define osmosis

The net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane

Give 3 factors that affect rate of osmosis

• The water potential gradient

• Thickness of exchange surface

• Surface area of exchange surface

Define isotonic

The water potential is the same in the solution and the cell within the solution

Define hypotonic

The water potential of solution is higher than the cell

Define hypertonic

The water potential of the solution is lower than the cell

Define active transport

The movement of particles from an area of lower concentration to an area of higher concentration using ATP and carrier proteins

Describe the process of active transport with carrier proteins (5 steps)

1. Transport is through carrier proteins spanning the cell membrane

2. Molecule binds to complementary receptor on carrier protein

3. ATP binds to the carrier protein from the inside of the cell and it is hydrolysed into ADP + Pi

4. This causes the carrier protein to change shape and release the molecule to the other side

5. The phosphate ion is then released and the protein returns to its original shape

Describe the process of cotransport of glucose and sodium ions in the ileum (4 steps)

1. Sodium ions are actively transported out of the epithelial cells into the blood

2. This creates a concentration gradient for Na⁺ to diffuse down from the lumen of the ileum into the epithelial cell

3. They do this via sodium-glucose cotransporter proteins. So glucose is transported into the epithelial cell against its concentration gradient

4. The concentration of glucose inside the cell increases. So glucose diffuses out of the cell, into the blood down its concentration gradient through a protein channel by facilitated diffusion