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Biology · Edexcel Paper 2

Exchange Surfaces and Gas Exchange

Why large animals need exchange surfaces and a transport system, how the alveoli in the lungs are adapted for gas exchange, and, for separate Biology, the factors that affect the rate of diffusion and Fick's law. Matches Edexcel Topic 8.1 to 8.5. The first two lessons are for both Combined Science and separate Biology, at Foundation and Higher; the last lesson (8.4B and 8.5B) is for separate Biology only.

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Key terms in this chapter

All 30, gathered from every lesson.

Exchange surface
A specialised surface, such as the alveoli, the gills or the villi, where substances pass into and out of an organism.
Transport system
The organs that carry substances around a large organism, such as the heart, blood vessels and blood in an animal.
Surface area
The total area of the outside of an object, across which substances can enter or leave.
Volume
The amount of space inside an object, which is the amount of living material that has to be supplied.
Surface area to volume ratio
The surface area of an organism divided by its volume.
Diffusion distance
The distance that particles have to move by diffusion, for example from the surface to a cell deep inside.
Multicellular organism
An organism made of many cells, often with specialised organs.
Urea
A waste substance made in the liver from excess amino acids and carried in the blood to the kidneys.
Mineral ions
Dissolved substances such as iron, calcium and sodium that the body needs in small amounts.
Aerobic respiration
The reaction in cells that uses oxygen to release energy from glucose, making carbon dioxide and water.
Alveolus
One of millions of tiny air sacs at the ends of the bronchioles, where oxygen and carbon dioxide are exchanged (plural alveoli).
Trachea
The windpipe, a tube held open by rings of cartilage that carries air to the bronchi.
Bronchus
One of the two tubes that branch from the trachea, one into each lung (plural bronchi).
Bronchiole
A narrow tube that branches from a bronchus and ends in a cluster of alveoli.
Gas exchange
The diffusion of oxygen into the blood and carbon dioxide out of the blood across the walls of the alveoli.
Ventilation
Breathing in and out, which keeps replacing the air in the alveoli.
Capillary
A tiny blood vessel with a wall one cell thick, where substances are exchanged with the surroundings.
Haemoglobin
The red substance in red blood cells that combines with oxygen to carry it round the body.
Concentration gradient
The difference in concentration between two places, which makes particles diffuse from the higher to the lower.
Diffusion distance
The distance particles have to travel by diffusion, which in the alveoli is the thickness of the two walls.
Rate of diffusion
How quickly particles move from a region of higher concentration to a region of lower concentration.
Fick's law
The rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the membrane.
Concentration difference
The difference in concentration between the two sides of a membrane. It makes the concentration gradient.
Surface area
The size of the area of membrane across which particles can diffuse.
Thickness of membrane
The distance that particles have to travel across the membrane, also called the diffusion distance.
Directly proportional
Changing together by the same factor, so that doubling one doubles the other.
Inversely proportional
Changing in opposite directions by the same factor, so that doubling one halves the other.
Arbitrary units
Units used to compare values without measuring an actual quantity.
Net movement
The overall movement of particles in one direction after the movement in the other direction is taken into account.
Steep concentration gradient
A large difference in concentration between two places, which gives fast diffusion.