Lecture 4 - Cell Membrane Structure and Function

4. Cell Membrane Structure and Function Introduction The cell membrane or the plasma membrane is an essential structure of cells of all the stated organisms since it acts as the barrier between the internal environment of the cell and the external world. It is involved in the management of homeostasis, signaling as well as allowing the passage of substances into and out of the cell. Composition of the cell membrane It is made up phospholipid bi-layer with proteins, cholesterol and carbohydrates suspended in it. This structure is described by what is known as the fluid mosaic model, which S. J. Singer and G. L. Nicolson produced in 1972. • Phospholipid Bilayer: o The enormously malleable fluid-mosaic membrane is said to consist of an uneven bimolecular phospholipid layer with polar phosphate head facing outwards while the non-polar fatty acid tails facing inwards to shield the polar materials. • Membrane Proteins: o Integral Proteins: These proteins are attached to the membrane and function in transport processes – they may serve as ports for molecules. o Peripheral Proteins: These are anchored on to the membrane and perform function such as signaling, provide mechanical support and determine shape of the cell. • Cholesterol: o Cholesterol molecules are embedded in the phospholipid layer to make the membrane stable yet slightly fluid depending with the type of animal cell. • Carbohydrates: o Carbohydrate molecules are usually linked to protein (glycoprotein) or lipid (glycolipid) on the exterior of the membrane involved in cell recognition and communication. Roles of the Cell Membrane The cell membrane is involved in a variety of essential cellular functions: • Selective Permeability: moves substances in and out of the cell as it pleases. Oxygen and carbon dioxide being small and non polar can easily diffuse across the membrane, while ions and other polar large molecules have to be transported in specific ways. • Transport Mechanisms: o Passive Transport: The passive transport includes diffusion and facilitated diffusion in which particles move across the concentration gradient without using energy. o Active Transport: In this process, molecules are transported across the membrane to an area with a lower concentration of a solute, thus concentration gradient is involved and it needs energy in form of ATP. Some examples include the sodium-potassium pump and proton pumps More detailed classes of transport proteins include primary active transport, secondary active transport, facilitated diffusion, and active transport. o Endocytosis and Exocytosis: These processes make it possible for the cell to take in huge structures, such as particles, (Endocytosis) or release substances (Exocytosis). • Cell Communication: o The cell membrane plays a role in cell communication, where on the outer side of the membrane are receptor molecules that bind with signaling molecules or ligands. This causes a series of events inside the cell and leads to a particular response to that stimulus. • Cell Recognition and Adhesion: o Glycoproteins and glycolipids situated on the outer side of the cell membrane support cell recognition which enables cells to know each other and even interact. This is important in immune reactions and generation of tissues. • Maintaining Homeostasis: o The cell membrane also has got the responsibility of regulating the internal environment of the cell, that is, it has to see to it that the right internal conditions for reactions are prevailed including pH, ions and temperature. The Fluid Mosaic Model The fluid mosaic model describes the cell membrane as a dynamic and flexible structure:The fluid mosaic model describes the cell membrane as a dynamic and flexible structure: • Fluidity: The phospholipids and proteins in the membrane are not embedded tightly; they are allowed to move horizontally from one place to another in the layer, and this makes the membrane to be flexible as it also corrects its self if it is damaged. • Mosaic: The membrane is composed of various proteins, lipids and carbohydrates which in sum play a functional role of the membrane. Another strength of this model is that it explains that the membrane alters its self to fit the required requirements of the cell as well as prevailing environmental conditions.