Biology: Cell Theory

Cell Theory is used to describe cells as the basic building blocks of all living things. All organisms are composed of one or more cells and have an outer layer of a plasma membrane (or cellular membrane), which acts as a barrier that divides the cell from its surrounding environment. The plasma membrane of a cell acts as a physical barrier and regulates the exchange of materials with its surroundings. It is a permeable membrane that surrounds the cytoplasm. The contents held within the plasma membrane are called cytoplasm. In prokaryotes, or single-celled organisms, it is the only membrane. In eukaryotes, cells that contain a nucleus, the organelles are also surrounded by a plasma membrane. The membrane controls all exchanges between the inside …show more content…
Phospholipids are a class of lipids that are the major element of all cell membranes as they can form lipid bilayers. Phospholipids are amphipathic molecules. Amphipathic molecules are molecules that have one area that dissolves in water and another portion, which is unable to mix with water. The fluid mosaic model is used to describe the organization of phospholipids and proteins. The model shows that phospholipid molecules are shaped with a head and a tail region. The head section of the molecules likes water (hydrophilic) while the tail does not (hydrophobic) The tails avoid water and let the heads face the water (aqueous) areas inside and outside of the cell. The two surfaces of molecules create the lipid bilayer. Many proteins float in the lipid bilayer. Some are permanently attached while others are only attached temporarily. Some are only attached to the inner or outer layer of the membrane while the transmembrane proteins pass through the entire structure. The transmembrane proteins that cross the bilayer are very important in the transport of ions and small molecules. Membrane structure results in selective permeability. A cell must exchange molecules and ions with its surroundings, a process called selective permeability of the plasma membrane. Hydrophobic substances are soluble in lipids and pass through membranes rapidly, whereas polar molecules and ions …show more content…
The membrane allows some substances to cross more easily than others. Some molecules, such as water, can pass through the membrane easily. Other molecules pass through more slowly and some substances are blocked or inhibited from passing through the membrane. These types of passive transport do not require energy from the cell. There are several different types of this simple movement of molecules. Osmosis is the movement of water across the membrane. Diffusion is the spontaneous movement of substance down its concentration gradient. If the solution outside has a higher solute concentration (hypertonic) than the cytosol the water diffuses out through the cell membrane. If the solution has a lower solute concentration (hypotonic) then the water enters the cell. No osmosis occurs when the concentrations are equal (isotonic). The cell must balance the water intake and loss to