Why movement matters to living things
Cells must constantly take in useful substances and remove wastes to stay alive. In living organisms, diffusion, osmosis and active transport all happen at surfaces that are thin and have a large surface area, so exchange is fast.
Movement in animals
- Gaseous exchange in the lungs: oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses out. The alveoli are thin and moist and provide a huge surface area.
- Absorption in the small intestine: the ileum wall is folded into villi. Glucose and amino acids are absorbed partly by diffusion and, when their concentration in the gut is low, by active transport into the blood.
- Amoeba: this single cell takes in oxygen and loses carbon dioxide directly by diffusion across its membrane.
Key idea
Exchange surfaces are thin, moist and have a large surface area to volume ratio, giving a short diffusion distance and a fast rate of movement.
Movement in plants
Root hair cells increase the surface area for absorption. Water enters the root hair by osmosis because the cell sap is more concentrated than the soil water. Mineral ions such as nitrate are usually more concentrated inside the root than in the soil, so they are taken in by active transport against the gradient. In the leaf, carbon dioxide diffuses into the mesophyll cells for photosynthesis, and oxygen diffuses out through the stomata. Water then travels up the plant and evaporates from the leaves, a passive process that helps pull more water in at the roots.
Example
After heavy rain the soil water is very dilute; water floods into root hair cells by osmosis, while the plant still spends ATP to pull in scarce nitrate ions by active transport.
Remember
Water always moves by osmosis; scarce ions usually move by active transport; gases move by diffusion.