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Transport of Materials in Living Things

14 min read

Moving substances in and out of cells

Cells need to take in oxygen, water and food and get rid of waste. There are three main ways substances cross the cell membrane.

ProcessDirectionEnergy needed?
Diffusionfrom high to low concentrationno (passive)
Osmosiswater from dilute to concentrated solution, across a semi-permeable membraneno (passive)
Active transportfrom low to high concentration, against the gradientyes, from respiration

Diffusion

Diffusion is faster when there is a bigger concentration difference, a higher temperature, smaller particles and a larger surface area. Cells are small so that substances do not have far to travel. The surface area to volume ratio shows why.

Try itWhy cells stay small

A cube-shaped cell of side has surface area and volume . As the cell grows, volume increases faster than the surface, so the ratio falls and diffusion cannot supply the middle fast enough.

Osmosis

Osmosis in an animal cellThree animal cells. In a dilute solution water enters the cell by osmosis and it swells. In a solution of the same strength there is no net movement of water. In a concentrated solution water leaves and the cell shrinks.Dilute outsidewater enters: cell swellsSame strengthno net movementConcentrated outsidewater leaves: cell shrinks
Run the osmosis experiment
Put potato strips in different sugar solutions and measure the change in mass.

Transport systems

Large organisms need transport systems because their cells are too far from the surface for diffusion alone to supply them.

Transport in humans

The heart pumps blood round the body through blood vessels. It has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump it out. The right side sends blood without oxygen to the lungs. The left side sends blood with oxygen to the rest of the body. Valves stop the blood flowing backwards.

The human heartA simplified heart with four chambers. Blood without oxygen, shown blue, comes from the body through the vena cava into the right atrium, passes to the right ventricle and is pumped to the lungs through the pulmonary artery. Blood with oxygen, shown red, returns from the lungs through the pulmonary vein into the left atrium, passes to the left ventricle and is pumped to the body through the aorta.right atriumleft atriumright ventricleleft ventriclevena cava (from body)pulmonary artery (to lungs)aorta (to body)pulmonary vein (from lungs)blood without oxygenblood with oxygen
Blood goes to the lungs to pick up oxygen, returns to the heart, and is then pumped to the body.
Blood vessels comparedCross-sections of an artery with a thick muscular wall and narrow space, a vein with a thinner wall, a wider space and valves, and a capillary whose wall is only one cell thick.Arterythick muscular wallnarrow space, high pressureVeinthin wall, wide spacevalves stop backflowCapillarywall one cell thickexchange with the cells
Part of the bloodJob
PlasmaA watery liquid that carries digested food, carbon dioxide, hormones and heat.
Red blood cellsContain haemoglobin, which carries oxygen from the lungs to the cells.
White blood cellsFight pathogens by engulfing them or making antibodies.
PlateletsHelp the blood to clot and seal a wound.

Transport in plants

Root hair cells take up water and mineral salts from the soil. Water crosses the root by osmosis into the xylem, which carries it up to the leaves. Phloem carries the sugars made in the leaves to the roots, fruits and growing parts.

A root hair cellA root hair cell with a long thin extension that increases its surface area. Water from the soil enters it by osmosis and passes across the root cortex cells to the xylem, which carries it up the plant.root hair: big surface areaxylemcortex cellswater in the soil
Transport in a plantWater and mineral salts enter through the roots and travel up the stem in xylem vessels to the leaves. Sugars made by photosynthesis in the leaves travel down and around the plant in phloem tubes.soilxylem: water and mineral salts go upphloem: sugars go down and aroundleafleafroots
Check yourself Which process needs energy from respiration?
Check yourself Water moves by osmosis from…

Key terms

Diffusion
The net movement of particles from high to low concentration.
Osmosis
The movement of water through a semi-permeable membrane from dilute to concentrated solution.
Active transport
Movement against a concentration gradient using energy.
Xylem and phloem
Plant tubes carrying water (xylem) and food (phloem).
Capillary
A tiny blood vessel with a wall one cell thick, where substances pass between the blood and the cells.

Summary

  • Diffusion and osmosis are passive; active transport uses energy.
  • Small cells have a large surface-area-to-volume ratio, helping diffusion.
  • Plants transport water in xylem and sugars in phloem; humans use blood pumped by the heart.
  • The heart has four chambers; arteries carry blood away from it at high pressure, veins bring it back, and capillaries exchange materials with the cells.
Balance of Nature Gaseous Exchange and Respiration