Can you smell that delicious cake? Have you ever wondered why your fingers prune in water? Or how plants manage to take up nutrients from the soil? All of these things require the movement of substances by either diffusion, osmosis, or active transport. In this two-part series, we're going to discover how cells take in useful substances and remove waste using these three methods of transport. The exchange of materials occurs between cells and their environment across the cell membrane. To make this exchange as efficient as possible, some organisms have evolved specialised exchange surfaces, like the alveoli in the lungs, or root hairs in plants, or the nephrons in kidneys. Depending upon what is being exchanged and which direction along the concentration gradient things are travelling, we'll determine whether diffusion, osmosis, or active transport will be used. Let's start with diffusion. The lovely smells of the cake spread around the room by diffusion. Diffusion is the process in which particles spread out from each other. They move from high concentration, so right by the cake, to an area of low concentration, so everywhere else in the room. In diffusion, particles move down the concentration gradient until they are evenly distributed. For particles to move like this, they need to be a gas, or particles of a dissolved substance. Diffusion is seen throughout nature, and allows substances to pass into or out of cells across the cell membrane. But they must be dissolved and they must be a concentration gradient that they can move from high concentration to low concentration. Gas exchange in the lungs, so oxygen moving from the alveoli into the blood, and carbon dioxide from the blood into the alveoli, are both great examples of diffusion. Carbon dioxide moving from the air into the leaves of plants also moves by diffusion. But it doesn't have to be a gas. Our digested foods, like amino acids, move from the small intestine into the blood also moves by diffusion. You can also have facilitated diffusion, with the movement is still down the concentration gradient, but special carrier proteins are required to enable the movement. Glucos and ions need carrier proteins to move across the membrane. Osmosis is very similar to diffusion, but just for water. It is the movement of water into or out of a cell. Again, it is the movement from a dilute solution, so high concentration of water molecules, down the concentration gradient, to a more concentrated solution. So low concentration of water molecules. The water molecules move across a partially permeable membrane. Water molecules move at random, so some do go back across the membrane. But the overall net effect is that there is movement to the more concentrated side, or the side with less water, until equilibrium is reached, or until there is no net movement of water anywhere. Osmosis out of a cell can cause big changes. Animal cells shrivel up, and in plant cells the membrane and cytoplasm shrink away from the cell wall causing the cell to become flaccid. And if osmosis causes lots of movement of water into the cell, plant cells swell and become turgid. They have a strong cell wall which prevents them from bursting, whereas animal cells don't have a cell wall. And so they burst. The absorption of water by plant roots and the absorption of water in the small intestine and the colon all use osmosis. So we know all about the movement of gases and water. But what is active transport for? Watch for too to learn about active transport.