Transport of Molecules and Ions Through a Cell Surface Membrane

The Transport of Molecules and Ions through the Cell Surface Membrane

All living organisms constantly need to exchange materials with the environment in order to survive and grow. Exchange of materials involves respiratory gases, nutrients and excretory products. These materials enter and leave the organism’s system through its cell surface membrane. The physical processes involved with the exchange of molecules and ions at cellular level are; passive and active. Passive transport is transport such as diffusion and is associated with the processes of respiration and photosynthesis in the leaves of flowering plants, and the respiration across the respiratory surfaces of animals. Active transport is the uptake of materials i.e. mineral ions against concentration gradients and therefore requires metabolic energy derived from ATP.

Gas exchange occurs in flowering plants involving mainly the leaves and stems as well as parts of the root. Oxygen or carbon dioxide reaches the respiring cells via the stomata. Inside the leaf, the large intercellular air spaces in the spongy mesophyll facilitate the diffusion of gases and the cells bordering these air spaces increase the total area available for gas exchange. Respiration takes place all the time, so oxygen is continually taken up by respiring cells and carbon dioxide is released. During the hours of daylight, photosynthesis will occur in the palisade and spongy cells in the mesophyll of the leaves, involving the uptake of carbon dioxide and the release of oxygen.

Absorption of digested food occurs in the small and large intestines. Digested food is absorbed in the small intestine and water is mostly taken up from the large intestine. In the membrane of the epithelial cells of the ileum, is a glucose transporter protein, which has binding sites for both glucose molecules and sodium ions. The sodium potassium pump actively transports sodium ions out of the cells against the electrochemical gradient. Glucose molecules...