Transport across membranes

Question 1

Describe active transport (7 marks)

Answer 1

ions and molecules moving from a lower concentration to a higher concentration

carrier proteins act as pumps to move substances across the membrane

certain molecules can bind to the receptor site on carrier proteins

ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and pi

This causes the protein to change shape and open towards the inside of the membrane - molecule released on the other side of the membrane

Pi molecule released from the protein and protein reverts to its original shape

Question 2

Describe Co-transport (5 marks)

Answer 2

sodium ions are actively transported out of the epithelial cell into the blood

this reduces the sodium ion concentration in the epithelial cell

sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell

the protein the sodium ions diffuse through is a co-transported protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient

glucose then moves by facilitated diffusion from the epithelial cell to the blood