Nutrient and Gas Exchange

In short: Organisms take in nutrients and gases through exchange surfaces that are large, thin, moist and close to a blood or air supply. Leaves exchange gases through stomata, roots take up water and minerals, and in humans the alveoli of the lungs and the villi of the small intestine do the job.

Starting point: what you already know

• Diffusion moves particles from high to low concentration (see The Cell Membrane and Transport).
• Photosynthesis uses $\mathrm{C}{\mathrm{O}}_2$ and releases ${\mathrm{O}}_2$. Respiration does the opposite.
• The digestive system breaks food into small molecules.

What makes a good exchange surface?

Gas exchange in plants

Leaves exchange gases through tiny pores called stomata (one is a stoma), mostly on the lower surface. Each stoma is surrounded by two guard cells.

• When guard cells take in water they swell and bend, and the stoma opens.
• When they lose water they go floppy, and the stoma closes.

During the day, $\mathrm{C}{\mathrm{O}}_2$ diffuses in and ${\mathrm{O}}_2$ diffuses out. Water vapour also escapes (transpiration), so plants close their stomata in hot, dry conditions. Inside the leaf, the spongy mesophyll has air spaces so gases reach every cell.

Roots take in water by osmosis and mineral ions (like nitrate and magnesium) by active transport through their root hairs.

Gas exchange in animals

In human lungs, air passes down the trachea, into the bronchi and bronchioles, and ends in about 480 million tiny air sacs called alveoli. Their total surface area is about the size of a tennis court. Each alveolus is one cell thick, wrapped in capillaries.

• Oxygen diffuses from the air in the alveoli into the blood.
• Carbon dioxide diffuses from the blood into the alveoli and is breathed out.

Getting nutrients: digestion

Animals are heterotrophs: they eat other organisms. Food has to be broken into small molecules before it can be absorbed.

Villi are finger-like folds in the small intestine wall, covered in even smaller microvilli. They give a huge surface area, a thin wall and a rich blood supply: a perfect exchange surface.

Quick summary

• Good exchange surfaces are large, thin, moist and well supplied with blood or air.
• Leaves exchange gases through stomata, opened and closed by guard cells.
• Roots absorb water by osmosis and mineral ions by active transport.
• Animals exchange gases through skin, tracheae, gills or lungs. Human lungs have millions of alveoli.
• Food is digested mechanically and chemically, then absorbed through villi in the small intestine.

Quiz