Adaptations of Exchange Surfaces in Plants and Animals

What makes an exchange surface effective?

Cells need materials from their surroundings and must remove waste. At an exchange surface, particles can move by diffusion from a higher to a lower concentration. A large surface area gives more space for exchange, while a thin barrier gives particles a shorter distance to travel. In animals, blood flowing past a surface carries absorbed substances away and brings others to it. Moving air or water over a gas-exchange surface also replaces what has been used. Together, these movements help maintain the concentration difference that drives diffusion.

Absorbing digested food in the small intestine

The small intestine absorbs digested food molecules into the blood. Its lining has many finger-like villi, and the cells covering them have even smaller projections called microvilli. Both increase the area available for absorption. Each villus has a thin covering of epithelial cells, so molecules have only a short distance to cross. Capillaries within the villus carry absorbed molecules away, helping to keep their concentration lower in the blood beside the exchange surface.

Exchanging gases in mammals and fish

In mammalian lungs, air reaches millions of tiny sacs called alveoli. Their combined surface area is large. Each alveolus has a thin wall and is surrounded by capillaries, giving oxygen a short route into the blood and carbon dioxide a short route out. Breathing moves fresh air in and used air out; blood flow and ventilation together maintain the concentration differences for gas exchange.

Fish exchange gases with water at their gills. Many gill filaments provide a large surface area. Their thin walls shorten the diffusion path, and a good blood supply carries oxygen away and brings carbon dioxide to the surface. Movement of the mouth floor draws water in and then pushes it out through the gill slits, ventilating the gills.

Taking in materials through roots

Roots absorb water and mineral ions from the soil. A branching root system carries many root hair cells, each with a long projection that increases the surface area in contact with the soil. This gives the plant more area through which materials can enter. The way water and mineral ions cross cell membranes is considered separately in osmosis and active transport; they should not both simply be described as diffusing into the root.

Exchanging gases through leaves

Leaves take in carbon dioxide for photosynthesis and release oxygen. A leaf is thin, so gases have a short distance to travel to and from its cells. Inside, loosely packed spongy mesophyll cells leave air spaces and a large internal surface for gas exchange. Gases pass between the leaf and the outside air through pores called stomata. Guard cells control whether these pores are open, allowing gas exchange while helping to limit water loss in dry conditions.

Shared adaptations

Large surface area increases the area for exchange; a thin barrier shortens the diffusion path. In animals, blood flow and ventilation help maintain concentration differences.

Animals

• Small intestine: villi and microvilli increase area; thin villus covering and capillaries aid absorption of digested food molecules.
• Lungs: many thin-walled alveoli, capillaries and ventilation support oxygen and carbon dioxide exchange.
• Fish gills: many thin-walled filaments, a good blood supply and moving water support gas exchange.

Plants

• Roots: branching roots and root hair projections provide a large area for absorbing water and mineral ions.
• Leaves: thin leaves, internal air spaces and stomata support gas exchange; guard cells control stomatal opening.