Does osmosis move dissolved substances as well as water?

No. Osmosis is the movement of water only, through a partially permeable membrane, from a dilute solution to a concentrated solution.

Why does active transport work when diffusion cannot?

Active transport moves dissolved substances from a dilute solution to a concentrated solution, against the concentration gradient. It uses energy from respiration, whereas diffusion moves substances down the concentration gradient without energy.

How do fish maintain a concentration gradient at their gills?

Blood and water flow in opposite directions across the gills. This keeps the oxygen concentration in the water higher than in the blood, so oxygen continues to diffuse into the blood.

Exchanging Substances

Diffusion, osmosis and active transport move substances across cell membranes, while surface area to volume ratio explains why organisms need exchange surfaces such as alveoli, villi, gills, leaves and roots. AQA GCSE Biology Higher, specification references 4.1.3.1, 4.1.3.2, 4.1.3.3, 1.2, 1.5, 8.2.3, 2.1 and 2.2.

GCSE
AQA
Biology
Higher
Spec 4.1.3.1 · 4.1.3.2 · 4.1.3.3 · 1.2 · 1.5 · 8.2.3 · 2.1 · 2.2
9 min read
10 revision boxes

Introduction

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1 min read

Getting substances into and out of cells

Cells take in substances they need and get rid of waste products across the cell membrane.

Three processes move substances across the membrane: diffusion, osmosis and active transport.

How easily an organism exchanges substances depends on its surface area to volume ratio. A single-celled organism has a large surface area compared with its volume, so gases and dissolved substances can diffuse directly in and out across the cell membrane, and that is enough to supply the cell.

Multicellular organisms have a smaller surface area compared with their volume, so their outside surface cannot supply everything inside. They need specialised exchange surfaces and transport systems: the lungs and small intestine in mammals, gills in fish and leaves and roots in plants.

Learning

6 boxes

Definition + 3 factors

Diffusion and what affects its rate

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. Cells use it both to take in the substances they need and to get rid of waste products across the cell membrane.

Oxygen and carbon dioxide are moved this way during gas exchange. Urea, the waste product made when proteins are broken down, diffuses out of cells into the blood plasma so the kidneys can remove it from the body. In single-celled organisms, gases and dissolved substances diffuse directly in and out across the cell membrane.

Three things change how fast diffusion happens. The difference in concentrations (the concentration gradient) matters: the bigger the difference between the two areas, the faster substances diffuse. Temperature matters. The surface area of the membrane matters too: a larger surface area means lots of a substance can diffuse at once.

3 processes compared

Osmosis and active transport

Water can move across cell membranes by osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. Only the water moves, and it moves down its own concentration gradient, so no energy is needed.

Active transport is different. It moves substances from a more dilute solution to a more concentrated solution, so against the concentration gradient. Going the 'wrong way' costs energy, and that energy comes from respiration.

This is how mineral ions get into plant root hairs. The soil solution is very dilute, so the ions have to be pulled in against the gradient. Plants need those ions for healthy growth. In the small intestine, digested food moves into the blood by both diffusion and active transport.

Process	What moves	Direction	Energy needed?
Diffusion	Any dissolved substance or gas	Down the concentration gradient (high to low)	No
Osmosis	Water only, through a partially permeable membrane	From a dilute to a concentrated solution	No
Active transport	Dissolved substances such as mineral ions	Against the concentration gradient (dilute to concentrated)	Yes, from respiration

4 steps

Calculating surface area to volume ratio

A ratio shows how big one value is compared with another. To compare organisms, you model them as simple blocks and work out their surface area to volume ratio (SA : V).

A hippo drawn inside a cuboid measuring 4 cm by 4 cm by 2 cm, next to a mouse drawn inside a cube with 1 cm edges, used to compare surface area to volume ratios.

The cube mouse's surface area is six times its volume, but the cube hippo's surface area is only twice its volume. So the larger the organism, the smaller its surface area compared with its volume.

Find the total surface area. The area of each surface is length × width, so work out each one and add them all together.
Find the volume using length × width × height.
Write the two values as a ratio, surface area : volume.
Simplify by dividing both sides of the ratio by the volume, so the volume side becomes 1.

Hippo surface area: (4 × 4) × 2 for the top and bottom + (4 × 2) × 4 for the four sides = 64 cm²

Hippo volume: 4 × 4 × 2 = 32 cm³

Ratio: 64 : 32

Divide both sides by 32: 64 ÷ 32 = 2 and 32 ÷ 32 = 1, so SA : V = 2 : 1

Mouse: surface area = 6 cm², volume = 1 cm³, so SA : V = 6 : 1

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2 min read

Why exchange surfaces are needed and what makes them effective

A single-celled organism has a large surface area compared with its volume, so gases and dissolved substances can diffuse directly in and out across the cell membrane fast enough to supply the whole cell.

A multicellular organism has a small surface area compared with its volume. Not enough substances can diffuse in through the outside surface to supply everything inside, so these organisms need specialised exchange surfaces (such as alveoli, villi or gills) plus a transport system to carry substances to and from every cell.

Large surface area
The bigger the surface, the more of a substance can diffuse across at once.
Thin membrane
A thin membrane gives a short diffusion path, so substances only have a short distance to travel.
Efficient blood supply (animals)
Lots of blood vessels move substances into and out of the blood quickly, which keeps the concentration gradient steep.
Ventilated (gas exchange in animals)
Air moves in and out of the surface.

2 min read

Exchange surfaces in mammals: lungs and small intestine

The lungs transfer oxygen into the blood and remove waste carbon dioxide from it. That gas exchange happens in millions of tiny air sacs called alveoli (one of them is an alveolus), and their structure is built to move

O_2

and

CO_2

as fast as possible.

Left: an alveolus with a thin, moist wall beside a capillary, oxygen diffusing into the blood and carbon dioxide diffusing out. Right: a villus with a single layer of surface cells and an internal capillary network, digested food passing into the blood.

The inside of the small intestine is covered in millions and millions of tiny projections called villi. They increase the surface area enormously, so digested food is absorbed into the blood much more quickly. Digested food moves into the blood by diffusion and by active transport.

Alveoli: enormous surface area
The alveoli give a huge total area for gas exchange, roughly 75 m² in humans.
Alveoli: moist lining and thin walls
The moist lining dissolves the gases, and the walls are very thin, so there is only a short diffusion path.
Alveoli: good blood supply
Blood flowing past constantly keeps the concentration gradient of oxygen and carbon dioxide steep.
Villi: single layer of surface cells
Each villus has just one layer of cells at its surface, so digested food has very little to cross.
Villi: very good blood supply
A rich network of blood vessels carries absorbed food away, maintaining the concentration gradient and keeping absorption quick.

2 min read

Exchange surfaces in fish and plants: gills, leaves and roots

Water containing oxygen enters a fish's mouth and passes over and out through its gills. Oxygen diffuses into the blood and carbon dioxide diffuses into the water.

A fish with water entering the mouth and leaving through the gills, alongside an enlarged view of a gill showing stacked gill filaments covered in lamellae, with blood flowing through them in the opposite direction to the water flowing over them.

Carbon dioxide diffuses through stomata on a leaf's underside, into air spaces, then into cells for photosynthesis. The underside and internal cell walls are exchange surfaces. Air spaces and the flattened leaf increase the exchange surface area.

Oxygen and water vapour diffuse out through stomata. Water vapour evaporates from cells then diffuses out because there is more inside than outside. Guard cells close stomata if water is lost faster than roots replace it, preventing wilting.

Roots are adapted for exchanging materials as well.

Gill filaments
Gill filaments are thin plates that give a big surface area for gas exchange.
Lamellae
Tiny lamellae cover the filaments, increasing surface area further.
Lots of blood capillaries
Lamellae contain many blood capillaries, speeding up diffusion.
Thin surface layer of cells
A thin cell layer minimises the distance gases diffuse.
Opposite flow directions
Blood and water flow in opposite directions, maintaining a large concentration gradient. Oxygen concentration in water stays higher than in blood, so as much oxygen as possible diffuses in.

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Calculate a bacterial cell's surface area to volume ratio

A bacterial cell can be represented by a 2 μm × 2 μm × 1 μm block. Calculate the cell's surface area to volume ratio.

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Explain low blood iron in coeliac disease

Coeliac disease causes inflammation of the small intestine, which can damage the villi. Suggest why a person with coeliac disease might have low levels of iron in their blood.

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Give two adaptations of a gill for gas exchange

Give two ways in which the structure of a gill is adapted for effective gas exchange. [2 marks]

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Common questions

Does osmosis move dissolved substances as well as water?: No. Osmosis is the movement of water only, through a partially permeable membrane, from a dilute solution to a concentrated solution.
Why does active transport work when diffusion cannot?: Active transport moves dissolved substances from a dilute solution to a concentrated solution, against the concentration gradient. It uses energy from respiration, whereas diffusion moves substances down the concentration gradient without energy.
How do fish maintain a concentration gradient at their gills?: Blood and water flow in opposite directions across the gills. This keeps the oxygen concentration in the water higher than in the blood, so oxygen continues to diffuse into the blood.