All living cells have a cell membrane and it is probably the most important organelle of a cell. The cell membrane is composed of a phospholipid, proteins and carbohydrates. The phospholipids are arranged in a bilayer with the hydrophilic ends facing out and the hydrophobic ends facing the inside of bilayer. Membranes can contain phospholipids with different fatty acids which affects the strength and flexibility of the membrane. There are two different proteins in cell membranes.
One is a protein that spans from one side of the membrane to the other, known as an integral protein. Integral proteins are usually involved in transport of different materials across the membrane. The other type of protein is a peripheral protein which is attached on one side of the membrane. Peripheral proteins on the inside of the cell membrane usually are attached to the cytoskeleton and help maintain the shape of the cell or they may be enzymes. The peripheral proteins on the outside of the cell usually act as receptors with a binding site where specific chemicals can bind.
The carbohydrates of the cell membrane are found on the outer surface and attach to either the proteins or sometimes the phospholipids. Animal cells also contain cholesterol which stabilize the cell and act as a temperature buffer. Heating of the cell membrane can destroy it. Lipids become more fluid with heat which can make the phospholipid bilayer structure change. Also high heat can denature the proteins in the cell membrane. At low temperatures the cell membrane can solidify. The cholesterol act as a temperature buffer and helps maintain homeostasis.
At warm temperatures it makes the membrane less fluid. At low temperatures it prevents the close packing of phospholipids, preventing solidification. Many different materials enter and exit the cell constantly. There are a few main types of transport in a cell. The first is lipid diffusion which is when substances pass directly through the phospholipid bilayer. This is possible for lipid soluble substances and very small molecules. Diffusion of water is known as osmosis. Diffusion is a form of passive transport. There is also facilitated diffusion which is passive transport through a protein.
Next is active transport which is when a molecule bind to a protein on the membrane which changes shape to deliver the molecule to the other side. Because it is specific, there is a different integral protein for every molecule to be transported. This form of transport requires energy which is created from the splitting of ATP. The last form of transport across a cell is in vesicles. Vesicles are for larger molecules such as proteins or polysaccharides. Endocytosis is the transport of materials into cells while exocytosis is the transport of materials out of the cell.