Diffusion, water as a solvent and active transport

Last time

Before we start: last time, in Cell structures and specialised cells, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Moving in and out

Cells must take in oxygen and glucose and get rid of carbon dioxide. Most of it moves by diffusion, which needs no energy. Some must be pumped against the flow, by active transport, which does need energy.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It happens down a concentration gradient, because of the random movement of particles. The energy comes from the kinetic energy of the particles themselves, not from the cell.

Watch diffusion

Here particles start crowded on one side. They move randomly, and over time they spread out evenly. The net movement is from where there are more to where there are fewer.

Diffusion in living things

Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses out. In leaves, carbon dioxide diffuses in through the stomata for photosynthesis. Digested food, such as glucose, diffuses into the blood in the small intestine.

Four factors

A larger surface area: more particles can cross at once. A steeper concentration gradient: a bigger difference between the two sides. A higher temperature: the particles have more kinetic energy and move faster. A shorter distance: the particles have less far to travel.

Factors in a table

Here are the four factors. Each one goes the same direction: bigger area, steeper gradient, warmer, thinner. The alveoli have all four: a huge area, a steep gradient kept by breathing and blood flow, and walls one cell thick.

Water as a solvent

Many substances dissolve in water. That is why water is so important to living things. Substances must be dissolved to move in and out of cells, and to be carried in blood and in xylem and phloem. Digestion only happens in solution, and urine removes waste dissolved in water.

Active transport

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration. It goes against the concentration gradient, so it needs energy from respiration. It uses protein carriers in the membrane.

Active transport in action

The particle binds to a carrier protein in the membrane. Energy from respiration changes the carrier's shape, and the particle is released on the other side. Examples: root hair cells take in mineral ions from dilute soil water, and the gut absorbs glucose.

Diffusion or active transport?

Compare the two. The direction, the energy and the membrane proteins all differ.