Biology 3 - GCSE

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Osmosis/Active Transport

Osmosis is a special kind of diffusion.

Osmosis is the diffusion/movement of water from a dilute to a more concentrated solution through a partially permeable membrane that allows water molecules through

Differences in the concentrations of solutions inside and outside a cell cause water to move into or out of a cell by osmosis.


Substances are sometimes absorbed against a concentration gradient by active transport.

Active transport uses energy from respiration.

Cells can absorb ions from very dilute solutions and actively absorb substances such as sugar and salt against a concentration gradient by active transport

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Exchange of gases in the lungs/ The lungs

Certain features, such as large surface area, short diffusion paths (one cell thick) and steep concentration gradients increase the effectiveness of an exchange surface.

The alveoli are the air sacs in the lungs.

The lungs are adapted to make gaseous exchange as efficient as possible. They have many alveoli, which provide a large survace area, with a good blood supply and short diffusion distances. The lungs are ventilated to maintain steep concentration gradients.


The lungs are in the thorax protected by the ribcage and separated from the abdomen by the diaphragm.

The intercostal muscles contract to move the ribs up and out and flatten the diaphragm, increasing the volume of the thorax. The pressure decreases and air move in.

The intercostal muscles relax and the ribs move down and in, and the diaphragm domes up, decreasing the volume of the thorax. The pressure increases and air is forced out.

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Exchange in the gut/ Exchange in plants

The villi in the small intestine provide a large surface area with an extensive network of blood cappillaries.

This makes villi well adapted to absorb the products of diffusion and active transport.


Plants have stomata that allow them to obtain carbon dioxide from the atmosphere.

Carbon dioxie enters the leaf through diffusion. Leaves have a flat, thin shape and internal air spaces to increase the surface area available for diffusion.

Most of the water and mineral ions needed by a plant are absorbed by the root hair cells, which increase the surface area of the roots. 

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Transpiration/ Transport systems in plants

The loss of water vapour from the surface of plant leaves is known as transpiration.

Water is lost through the stomata, which are opened and closed to let in carbon dioxide for photosynthesis.

Transpiration is more rapid in hot, dry, windy or bright conditions.


Flowering plants have seperate transport systems.

Xylem tissue transports water and mineral ions from the roots to the stems and leaves.

Phloem tissue transports dissolved sugars from the leaves to the rest of the plant including the growing regions and storage organs.

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The heart/ Keeping the blood flowing

The circulation system consists of the blood vessels, the heart and the blood. Humans have a double circulatory system.

The heart is an organ that pumps blood around the body.

The valves make sure blood flows in the right direction through the heart.

Arteries carry blood away from the heart.Veins carry blood back to the heart.


The main types of blood vessels are arteries, veins and capillaries.

Substances diffuse in and out of the blood in the capillaries

Stents can be used to keep narrowed or blocked arteries open.

Damaged heart valves can be replaced.

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Transport in the blood

The blood plasma transports dissolved food molecules, carbon dioxide, and urea and has the blood cells suspended in it.

The red blood cells carry oxygen from the lungs to the organs in the body.

Red blood cells are adapted to carry oxygen by being biconcave, giving them a bigger surface area, by containing haemoglobin and having no nucleus so more haemoglobin can fit in.

White blood cells are part of the defence system of the body.

Platelets are cell fragments involved in the clotting of the blood.

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Controlling internal conditions/ The kidneys

The internal conditions of the body have to be controlled to maintain a constant internal environment. These include body temperature, water and ion balance, and blood sugar levels.

Carbon dioxide is produced during respiration and leaves the body via the lungs when you breathe out.

Urea is produced by the liver as excess amino acids are broken down, and is removed by the kidneys in the urine.


The kidneys are important for excretion and homeostasis.

A healthy kidney produces urine by filtering the blood. It then reabsorbs all of the sugar, plus any mineral ions and water needed for the body.

Excess mineral ions and water, along with urea, are removed in the urine.

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Dialysis/ Kidney transplants

People suffering from kidney failure may be treated by regular sessions on a kidney dialysis machine or by a kidney transplant.

In a dialysis machine, the concentration of dissolved substances in the blood is restored to normal levels.

The levels of useful substances in the blood are maintained, while urea and excess mineral ions pass out from the blood into the dialysis fluid.


In a kidney transplant, diseased or damaged kidneys are replaced with a healthy kidney from a donor.

To try and prevent rejection of the donor kidney, the tissue types of the donor and recipient are matched as closely as possible. Immunosuppressant drugs are also used.

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Controlling body temperature

The body temperature is monitored and controlled by the thermoregulatory centre in the brain.

The body temperature must be kept at the level at which enzymes work best.

The body responds to cool you down and warm you up if the core body temperature changes.

The blood vessels that supply the capillaries in the skin dilate and constrict to control the blood flow to the surface.

Energy is released through the evaporation of sweat from the surface of the skin to cool the body down.

Shivering involves contraction of the muscles that produces energy from respiration to warm the body.

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Controlling blood glucose

The blood glucose concentration is monitored and controlled by the pancreas.

The pancreas produces the hormone insulin, which allows glucose to move from the blood into the cells.

In type 1 diabetes, the blood glucose may rise to fatally high levels because the pancreas does not secrete enough insulin. IT can be treated with insulin injections before meals.

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