4.2.114 · Cell Membranes and Transport

Osmosis in Animals — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 18 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

0%read

Lesson summary

4.2.11 Osmosis in Animals — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how animal cells respond to solutions of different water potential and how animals regulate the water potential of body fluids.

Water movement
Consequence
Medical use
Terminology
If blood water potential is too high
Freshwater animals

Part 1 of 7Animal cells have no wall

1Structure

Structure an animal cell has only a flexible, partially permeable cell surface membrane, with no cell wall and no large central vacuole.

2Water potential

Water potential pressure potential is effectively zero, so the cell's water potential is its solute potential.

3Water movement

Water movement water passes through the phospholipid bilayer and through aquaporins, depending on the water potential of the surrounding fluid.

4Consequence

Consequence the effects of gaining or losing water are more severe than in plant cells,
so body fluids must keep a constant water potential as part of homeostasis.

Part 2 of 7Animal cells in different solutions

Surrounding solution	Net water movement	Result
Higher ψ than the cell (hypotonic, for example distilled water)	Into the cell	Cell swells and bursts, called lysis (haemolysis in red blood cells)
Same ψ (isotonic, for example 0.9% sodium chloride)	None	Normal biconcave shape retained
Lower ψ (hypertonic, for example 3% sodium chloride)	Out of the cell	Cell shrinks and the membrane crinkles, called crenation

Medical use

Medical use saline for drips and for storing organs is made isotonic with body fluids, because pure water in the blood would lyse red blood cells.

Terminology

Terminology animal cells do not plasmolyse, since plasmolysis occurs only in plant cells.

Part 3 of 7Osmoregulation in mammals

Heavy sweating or too little water intake lowers the water potential of the blood.

Osmoreceptors in the hypothalamus detect the change.

The posterior pituitary gland releases more antidiuretic hormone (ADH).

ADH increases the permeability of the collecting duct cells to water, by increasing the number of aquaporins.

More water is reabsorbed by osmosis into the blood, so a small volume of concentrated urine is produced and the blood water potential rises.

1If blood water potential is too high

If blood water potential is too high less ADH is released, the collecting duct stays impermeable and a large volume of dilute urine is produced.

2Control

Control this is negative feedback, and ADH is made in the hypothalamus and released from the posterior pituitary.

Part 4 of 7Animals in different environments

Freshwater animals

Freshwater animals the surroundings have a higher water potential than their cells, so water enters continually.
Amoeba has a contractile vacuole that collects water and expels it using ATP.
Freshwater fish produce large volumes of dilute urine and take in salts through the gills by active transport.

Marine bony fish

Marine bony fish their body fluids have a higher water potential than seawater, so they lose water.
They drink seawater, excrete salts through the gills by active transport and produce little concentrated urine.

Combined methods

Combined methods animals use passive osmosis and active transport of ions to maintain water and solute balance.

Part 5 of 7Practical observation

Red blood cells

Red blood cells suspend cells in a range of sodium chloride concentrations.
A lysed suspension goes clear and red as haemoglobin is released, and a colorimeter shows more light passing through.

Eggs without shells

Eggs without shells remove the shell with vinegar, and the egg gains mass in water and loses mass in concentrated salt or syrup,
giving a graph like that for potato cylinders.

Isotonic point

Isotonic point the concentration with no change is isotonic with the cells and matches the water potential of their cytoplasm.

Regions of the graph

Regions of the graph lower concentrations are hypotonic and higher concentrations are hypertonic (cells lose water).
- (cells gain water)

Part 6 of 7Links to other topics

1Kidney

Kidney the loop of Henle maintains a water potential gradient in the medulla.

2Oral rehydration solutions

Oral rehydration solutions they combine glucose and salts so that sodium–glucose co-transport draws water in by osmosis.

Part 7 of 7More from the specification

1Facilitated diffusion via transport proteins

Facilitated diffusion via transport proteins polar water molecules and hydrophilic solutes diffuse rapidly across selectively permeable membranes through specialized channel proteins.

2Active transport by carrier proteins

Active transport by carrier proteins carrier proteins hydrolyse ATP energy to pump ions against steep concentration gradients across animal cell surface membranes.

3Osmotic responses of animal cells

Osmotic responses of animal cells animal cells placed in hypotonic solutions swell and undergo lysis because they lack rigid plant cell walls.

4Osmosis in Animal Cells (Crenation and Lysis)

Unlike walled plant cells, animal cells placed in hypotonic solutions absorb water by osmosis until they burst (lysis),
while hypertonic solutions cause shrinkage (crenation).

5Facilitated Diffusion Mechanisms

Large polar molecules or inorganic ions pass down concentration gradients through membrane-spanning channel proteins or shape-changing carrier proteins.

6Active Transport Energetics

Moving substances against electrochemical gradients requires transmembrane carrier proteins coupled to metabolic energy derived from ATP hydrolysis.

7Membrane Permeability Investigations

Investigating cell membrane permeability quantifies leakage of pigment into surrounding solution by measuring colour intensity.
- (e.g. using beetroot tissue in varying temperatures or solvents)

8Channel protein

Hydrophilic pore for specific ions or water.

9Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
- as a result of random movement.

10Protein

A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

11Carrier

A female with one recessive sex-linked allele who is unaffected.

12Sucrose

The sugar mainly transported in phloem.

13Indicator

A chemical that changes colour depending on pH.

14Endocytosis

Bulk transport into a cell using a membrane vesicle.

15Selectively permeable

Allowing only some substances through.

16Facilitated diffusion

Passive movement through transport proteins.

Key terms

Term	Meaning
Lysis	Bursting of a cell due to excess water entering by osmosis.
Crenation	Shrinking and crinkling of a cell due to loss of water.
Isotonic	Having the same water potential as another solution or cell.
Osmoregulation	Control of the water potential of body fluids.
ADH	Antidiuretic hormone, which increases water reabsorption in the collecting duct.
Aquaporin	Channel protein that increases membrane permeability to water.
Contractile vacuole	Organelle that expels excess water from freshwater protoctists.

Avoid these mistakes

Saying animal cells burst because they have no cell wall without explaining that water enters by osmosis first.

Confusing lysis (bursting) with crenation (shrivelling).

Stating that ADH is released by the anterior pituitary or the kidney — it is made in the hypothalamus and released by the posterior pituitary.

Describing osmosis in animals as an active process — the movement of water is passive.

Last updated: 15 October 2026