AQA GCSE Biology (8461) — Homeostasis
Every time you drink a large amount of water quickly, your body has to work hard to stop your cells from swelling and bursting. This is because all cells are subject to osmosis, which is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
A dilute solution has a higher concentration of water (often called a higher water potential), while a concentrated solution has a lower concentration of water. Because animal cells lack a rigid cellulose cell wall, they are extremely vulnerable to permanent damage if their surrounding environment changes.
If an animal cell (like a red blood cell) is placed in pure water, the surrounding solution is hypotonic (more dilute) compared to the cytoplasm. Water moves into the cell by osmosis down the concentration gradient, causing the cell to swell and eventually burst in a process called lysis.
Conversely, if the cell is placed in a very salty or sugary solution, the environment is hypertonic (more concentrated). Water moves out of the cell by osmosis, causing it to shrink and shrivel, which is known as crenation. To prevent this, blood plasma must remain isotonic to the cells, meaning there is the same concentration inside and out, resulting in no net movement of water.
You can decide when to use the toilet, but you cannot stop yourself from losing water every time you breathe. The body constantly loses water, ions, and urea (a nitrogenous waste product) through various organs, and these processes are split into uncontrolled and regulated losses.
During exhalation, water leaves the lungs as water vapour. This is an unavoidable byproduct of gas exchange, and the body has no control over the volume of water lost this way.
Similarly, water, ions (such as sodium and chloride), and urea are lost from the skin through . The primary function of sweating is temperature regulation via evaporation, not balancing internal chemistry, making this another uncontrolled loss.
Because the lungs and skin cannot regulate what they lose, the kidneys must compensate. The kidneys filter the blood and provide regulated excretion, carefully controlling how much water, ions, and urea are removed via urine to maintain homeostasis.
Your body is highly efficient at storing fat and carbohydrates, but it has no way to store excess protein. Any excess amino acids consumed in the diet must be safely processed and removed from the body to prevent cellular damage.
This breakdown happens exclusively in the liver through a process called deamination. Liver enzymes remove the nitrogen-containing amino group () from the excess amino acids. The remaining parts of the amino acids (keto acids) are converted into glucose, glycogen, or fats for storage, or used in aerobic respiration.
Deamination produces ammonia (), which is highly toxic and can denature enzymes if allowed to build up. To prevent toxicity, the liver immediately reacts the ammonia with carbon dioxide () to convert it into urea ().
Urea is a less toxic substance that is safe to transport. It is released into the blood plasma, travels to the kidneys, and is safely removed from the body in urine.
Why is your urine dark yellow when you are dehydrated, but almost clear after you drink plenty of fluids? It all comes down to the microscopic filtering system inside your kidneys.
Blood enters the kidney tubules under high pressure in a process called ultrafiltration. Small molecules like water, glucose, ions, and urea are forced out of the blood and into the tubules. However, large molecules like proteins and red blood cells are too big to pass through the membrane and remain in the blood.
As the filtrate moves through the tubules, the kidneys perform selective reabsorption. They actively reabsorb 100% of the glucose back into the blood, alongside some ions and some water, depending on what the body needs. Crucially, urea is not reabsorbed; it stays in the tubule, becoming highly concentrated to form urine.
The volume of water reabsorbed by the kidneys is controlled by ADH (Anti-Diuretic Hormone) as part of a negative feedback loop to maintain osmoregulation.
Students often state that urea is produced in the kidneys. It is actually produced in the liver and only excreted by the kidneys.
In exam questions about cells in isotonic solutions, always use the specific phrase 'no net movement' of water rather than saying 'no water moves'.
When comparing animal and plant cells in pure water, you must explicitly state that animal cells lack a rigid cellulose cell wall, which is why they burst (lysis) while plant cells do not.
For marks on uncontrolled water loss, always list all three substances lost in sweat: water, ions, and urea.
Always state that ADH comes from the pituitary gland and specifically affects the 'permeability' of the kidney tubules.
Osmosis
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution
A solution with a higher concentration of water and a lower concentration of solutes.
Concentrated solution
A solution with a lower concentration of water and a higher concentration of solutes.
Partially permeable membrane
A membrane that allows small molecules (like water) to pass through but blocks larger solute molecules.
Water potential
A measure of the concentration of free water molecules; pure water has the highest water potential.
Hypotonic
A solution that is more dilute (has fewer solutes and more water) than the cell cytoplasm.
Lysis
The bursting of an animal cell due to the excessive inward movement of water by osmosis in a hypotonic solution.
Hypertonic
A solution that is more concentrated (has more solutes and less water) than the cell cytoplasm.
Crenation
The shrivelling of an animal cell due to the outward movement of water by osmosis in a hypertonic environment.
Isotonic
A solution that has the same concentration of solutes as the cytoplasm of the cell.
No net movement
The state when water moves in and out of a cell at equal rates, resulting in no overall change in volume.
Exhalation
The process of breathing out, which removes water vapour and carbon dioxide from the lungs.
Sweating
The release of liquid containing water, ions, and urea from sweat glands to the skin surface for cooling.
Urea
A less toxic nitrogenous waste product formed from ammonia in the liver.
Excretion
The removal of waste products of metabolism and substances in excess from the body.
Deamination
The process in the liver where the amino group is removed from an excess amino acid to form ammonia.
Ammonia
A highly toxic nitrogen-containing compound produced during deamination.
Ultrafiltration
The filtration of blood under high pressure in the kidneys, where small molecules pass into the tubule.
Selective reabsorption
The process where the kidneys reabsorb useful substances (like 100% of glucose) back into the blood.
ADH
Anti-Diuretic Hormone, released by the pituitary gland, which regulates water reabsorption by altering tubule permeability.
Osmoregulation
The homeostatic control of water levels and mineral salts in the blood.
Put your knowledge into practice — try past paper questions for Biology
Osmosis
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution
A solution with a higher concentration of water and a lower concentration of solutes.
Concentrated solution
A solution with a lower concentration of water and a higher concentration of solutes.
Partially permeable membrane
A membrane that allows small molecules (like water) to pass through but blocks larger solute molecules.
Water potential
A measure of the concentration of free water molecules; pure water has the highest water potential.
Hypotonic
A solution that is more dilute (has fewer solutes and more water) than the cell cytoplasm.
Lysis
The bursting of an animal cell due to the excessive inward movement of water by osmosis in a hypotonic solution.
Hypertonic
A solution that is more concentrated (has more solutes and less water) than the cell cytoplasm.
Crenation
The shrivelling of an animal cell due to the outward movement of water by osmosis in a hypertonic environment.
Isotonic
A solution that has the same concentration of solutes as the cytoplasm of the cell.
No net movement
The state when water moves in and out of a cell at equal rates, resulting in no overall change in volume.
Exhalation
The process of breathing out, which removes water vapour and carbon dioxide from the lungs.
Sweating
The release of liquid containing water, ions, and urea from sweat glands to the skin surface for cooling.
Urea
A less toxic nitrogenous waste product formed from ammonia in the liver.
Excretion
The removal of waste products of metabolism and substances in excess from the body.
Deamination
The process in the liver where the amino group is removed from an excess amino acid to form ammonia.
Ammonia
A highly toxic nitrogen-containing compound produced during deamination.
Ultrafiltration
The filtration of blood under high pressure in the kidneys, where small molecules pass into the tubule.
Selective reabsorption
The process where the kidneys reabsorb useful substances (like 100% of glucose) back into the blood.
ADH
Anti-Diuretic Hormone, released by the pituitary gland, which regulates water reabsorption by altering tubule permeability.
Osmoregulation
The homeostatic control of water levels and mineral salts in the blood.
