Diffusion, osmosis & active transport

Substances move in and out of cells in three ways. The first two need no energy (they are passive); the third does.

• Diffusion — passive net movement of particles down a concentration gradient (higher to lower). Only certain substances pass through the membrane (e.g. O₂, CO₂, urea). No energy needed.
• Osmosis — diffusion of water through a selectively permeable membrane, from a region of high water (low solute) concentration to low water (high solute) concentration. No energy needed.
• Active transport — moves substances against the gradient (low → high), so it requires energy from respiration (e.g. root hairs taking up mineral ions from dilute soil).

Model it: Visking tubing acts like living material — its partially permeable membrane lets small molecules and water through but holds back larger ones. Misconception to nail: osmosis moves water only, down its concentration gradient, with no energy. Active transport is the opposite — against the gradient and energy-using.

What exactly is osmosis?

Surface area : volume & exchange surfaces

A single-celled organism can exchange everything it needs by diffusion straight across its surface. A multicellular organism cannot — so it needs exchange surfaces and transport systems. Why? Because as something gets bigger, its volume grows faster than its surface area, so the surface area to volume (SA:V) ratio falls.

Why it matters: oxygen, CO₂, water, dissolved food, mineral ions and urea all have to be transported. Big organisms evolve specialised exchange surfaces (lungs, root hairs, leaf surfaces) with huge surface areas, plus a transport system to carry substances between them.

Your turn — SA:V ratio

Which process is it?

Tap a description, then tap the transport process it belongs to.

The heart & double circulation

Humans have a double circulatory system — blood passes through the heart twice for each complete circuit of the body:

• The right side pumps blood to the lungs (to pick up oxygen) and back.
• The left side pumps blood to the rest of the body and back.

Vessels of the heart: the vena cava brings blood in; the pulmonary artery carries it to the lungs; the pulmonary vein returns it; the aorta carries oxygenated blood to the body. The left ventricle wall is thicker because it pumps blood the whole way round the body.

Right side, left side

Arteries, veins, capillaries & blood

Three types of vessel carry the blood, each adapted to its job:

Blood itself has four parts:

• Plasma — pale yellow liquid; transports water, nutrients, hormones, urea and antibodies.
• Red blood cells — carry oxygen; biconcave, no nucleus, packed with haemoglobin.
• White blood cells — part of defence against pathogens.
• Platelets — cell fragments that help the blood clot.

Required practical (SP2.2): examine an artery and a vein under a light microscope — the artery has a much thicker, more muscular wall and a smaller lumen than the vein.

Adapted for the job

Your turn — magnification

Xylem, phloem & root hairs

Plants have two separate transport tissues:

• Xylem — tubes of dead cells (xylem vessels) that carry water & dissolved minerals from the roots upwards.
• Phloem — living tubes that carry sugar from the photosynthesising leaves to the rest of the plant (for respiration, or stored as starch). This is called translocation.
• Root hair cells — increase the surface area for absorption. Water enters by osmosis; mineral salts are taken up by active transport (against the gradient, needing energy).

Watch out: xylem carries water only upwards; phloem can move sugar up or down. Minerals are absorbed by active transport, not diffusion, because the soil is more dilute than the root cells.

Xylem or phloem?

Transpiration, stomata & the potometer

Transpiration is the movement of water through the plant — up the xylem and out of the leaves. Water evaporates through tiny pores called stomata, controlled by guard cells that open and close each stoma.

Transpiration is faster when there is more light, more air movement (wind), higher temperature (and lower humidity). We measure the rate of water uptake using a potometer, timing how far an air bubble travels.

Rate: transpiration (uptake) rate = distance moved ÷ time. Increasing light, wind or temperature all speed up water loss from the leaves.

Your turn — transpiration rate

Osmosis in potato chips

The osmosis practical investigates the effect of solute concentration on plant tissue, usually potato chips (cylinders):

• Cut equal-sized potato chips; record each starting mass.
• Leave each in a different concentration of sugar (or salt) solution.
• Re-weigh and calculate the percentage change in mass.

A potato in pure water gains mass (water moves in by osmosis, since the cells have a lower water concentration). In a concentrated solution it loses mass. Using % change fairly compares chips that began at slightly different masses.

Your turn — percentage change

Your turn — SA:V again

Match each structure to its function

Tap a structure on the left, then its matching function on the right.

The big ideas to know

You've covered all three parts of Eduqas Topic 2 — transport in cells, in humans, and in plants. Press Finish to see your score.