Cell MembraneThe cell membrane (plasma membrane) is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape. Animal cells, plant cells, prokaryotic cells, and fungal cells have cell membranes.
Cell Membrane StructureThe cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane.
- Cell Membrane Lipids
- Phospholipids are a major component of cell membranes. They form a lipid bilayer in which their hydrophillic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.
- Cholesterol is another lipid component of cell membranes. It helps to stiffen cell membranes and is not found in the membranes of plant cells.
- Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.
- Cell Membrane Proteins
Structural proteins help to give the cell support and shape. Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters and other signaling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane.
Eukaryotic Cell StructuresThe cell membrane is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell:
- Centrioles - help to organize the assembly of microtubules.
- Chromosomes - house cellular DNA.
- Cilia and Flagella - aid in cellular locomotion.
- Endoplasmic Reticulum - synthesizes carbohydrates and lipids.
- Golgi Complex - manufactures, stores and ships certain cellular products.
- Lysosomes - digest cellular macromolecules.
- Mitochondria - provide energy for the cell.
- Nucleus - controls cell growth and reproduction.
- Peroxisomes - detoxify alcohol, form bile acid, and use oxygen to break down fats.
- Ribosomes - responsible for protein production via translation.