A level-readyAll subjects · Biology · WBI11 (Unit 1)
Osmosis and transport across membranes
Substances cross membranes by diffusion, osmosis, active transport and bulk transport. The method depends on the size and charge of the molecule and the direction of the gradient.
Osmosis
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Pure water has the highest water potential (0 kPa). Adding solutes lowers it (makes it negative). So water moves into a cell placed in a more dilute solution, and out of a cell placed in a more concentrated one.
Passive transport
- Diffusion: net movement down a concentration gradient; small non-polar molecules cross the bilayer directly.
- Facilitated diffusion: polar molecules and ions cross through channel proteins (water-filled pores, often gated) or carrier proteins (bind the molecule and change shape). Still down the gradient, no ATP.
Passive means no energy from ATP is used.
Active and bulk transport
- Active transport moves substances against their concentration gradient using carrier proteins and energy from ATP. ATP is hydrolysed to ADP and phosphate, releasing energy that changes the carrier's shape.
- Endocytosis: the membrane surrounds material and pinches off to form a vesicle inside the cell.
- Exocytosis: vesicles fuse with the cell surface membrane to release contents (for example, enzymes or hormones).
Both bulk processes need ATP. Investigating tissue water potential with plant tissue in a range of solute concentrations is a recommended practical: the concentration where mass does not change has the same water potential as the tissue.
Worked example
A potato cylinder goes from 2.50 g to 2.20 g in a sugar solution. Calculate the percentage change in mass and explain it.
- % change = (2.20 - 2.50) / 2.50 x 100 = -12%
- The solution had a lower water potential than the potato cells, so water left by osmosis.
Answer: -12%: water left the tissue by osmosis.
Check yourself
- A potato cylinder changes from 3.20 g to 3.60 g in a solution. Calculate the percentage change in mass to 1 decimal place.
Show answer
12.5 %. (3.60 - 3.20) / 3.20 x 100 = 12.5%.
- A cell with water potential -400 kPa is put in a solution of -200 kPa. What happens?
- Water moves out of the cell
- No water moves
- Solute moves into the cell by osmosis
- Water moves into the cell by osmosis
Show answer
Water moves into the cell by osmosis. Water moves from higher (-200) to lower (-400) water potential.
- Which process moves ions against their concentration gradient?
- Facilitated diffusion
- Active transport
- Osmosis
- Simple diffusion
Show answer
Active transport. Only active transport works against a gradient, using ATP.
- What do facilitated diffusion and active transport have in common?
- Each uses ATP
- Each moves substances against a gradient
- Each involves membrane proteins
- Each moves only water
Show answer
Each involves membrane proteins. Both use proteins; only active transport uses ATP.
- How does a cell release digestive enzymes it has made?
- Exocytosis
- Endocytosis
- Osmosis
- Facilitated diffusion
Show answer
Exocytosis. Vesicles fuse with the cell surface membrane to release enzymes.
- How does ATP provide energy for active transport?
- It is converted into glucose
- It forms a channel in the membrane
- It is hydrolysed to ADP and phosphate, releasing energy
- It lowers water potential
Show answer
It is hydrolysed to ADP and phosphate, releasing energy. Hydrolysis of ATP releases energy that changes the carrier's shape.
- In a tissue water potential experiment, plant tissue has no change in mass in a 0.3 mol dm-3 sucrose solution. What does this show?
- The tissue has the same water potential as that solution
- The tissue has no water
- The tissue cells have burst
- Sucrose entered the cells
Show answer
The tissue has the same water potential as that solution. No net osmosis means equal water potentials.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.