In this topic you will learn about cell activities and how they can help people.

Substances flow in and out of cells through the cell membrane all the time. Cells use substances to get energy and grow. This process, called diffusion, occurs when molecules of a substance move from an area of higher concentration to an area of lower concentration. The cell membrane is a kind of barrier. Cells get food, oxygen, and other substances from the environment. They also release wastes, such as carbon dioxide. Substances move in and out of the cell through the cell membrane. Some kinds of molecules are able to pass through a cell membrane by diffusion. Such movement through cell membranes without the use of energy is called passive transport.

The movement of water in and out of a cell is important in keeping the cell alive. The diffusion of water through a cell membrane is called osmosis. Water molecules tend to move from an area of greater concentration to an area of lesser concentration. When the concentration of water molecules is the same on each side of a cell membrane, a state of equilibrium, or balance, is reached.

When materials require energy to move through a cell membrane, it is called active transport. In active transport, molecules move from an area of lesser concentration to an area of higher concentration.

Cells need energy to carry on life processes. The energy comes from the Sun, and through food-in particular, molecules of carbohydrates and sugars. Energy from the Sun is trapped in the food-making process. Photosynthesis is the food-making process in producers using sunlight. The producers take in water from the soil and carbon dioxide from the air as raw materials and change them into two products. The products-the food-are sugar and oxygen.

Energy for a cell's activities is locked within molecules of sugar. The energy in a sugar molecule is released by a process called respiration. Usually the process requires oxygen, which living things take in from air or water. The sugar molecules are broken down, and energy and wastes are released. The wastes are carbon dioxide and water. Nitrogen is another example of a waste produced in your body. Wastes are collected by the blood, which delivers them to the kidneys for removal. The kidneys release the waste through urine. If the kidneys fail to eliminate wastes, blood can be artificially filtered through a process called hemodialysis.

Some cells carry on respiration without oxygen, a process called fermentation. One-celled organisms called yeasts carry on fermentation by feeding on sugars. They break the sugars down and produce carbon dioxide gas and an alcohol called ethanol.