The Plasma Membrane:
- The plasma membrane exhibits selective permeability, allowing some substances to cross and not others, and thereby regulating the entry and exit of substances in the cell.
- Phospholipids, the most abundant lipids in membranes, as well as most membrane proteins, are amphipathic and have hydrophilic and hydrophobic regions.
- According to the fluid-mosaic model , the membrane is a mosaic of protein molecules bobbing in a fluid layer of phospholipids.
- Groups of proteins are often associated in long-lasting, specialized patches called lipid rafts, where they carry out common functions.
- A membrane is held together mostly by hydrophobic interactions, and lipids and some proteins can drift in the plane of the membrane.
- The membrane remains fluid to a lower temperature if it is rich in unsaturated phospholipids, while saturated phospholipids prevent excessive fluidity at high temperatures.
- Some organisms (such as winter wheat) can increase the unsaturated phospholipids in autumn, to keep their membranes raises from solidifying during winter.
- Cholesterol acts as a “fluidity buffer” for the membrane, restraining phospholipid movement at high temperatures, and hindering the close packing of phospholipids at low temperatures. Plants use steroid lipids more than cholesterol.
- A membrane is a collage of different proteins, often clustered together groups. There are two main kinds of membrane proteins: integral proteins and peripheral proteins .
- Integral proteins penetrate the hydrophobic interior of the bilayer (most, like integrin, are transmembrane, some only penetrate partway), and often contain 20-30 hydrophobic amino acids coiled into seven α-helices, as well as hydrophobic regions at either end.
- Peripheral proteins not embedded in the lipid bilayer, rather they are loosely bound to the surface of the membrane, often to integral proteins.
- Functions of proteins of the plasma membrane include:
- Transport – enabling diffusion across the membrane via channel or carrier proteins.
- Enzymatic activity – some membrane proteins are enzymes with active sites and these are often organized together as teams that carry out sequential steps of a metabolic pathway.
- Signal transduction – some membrane receptors have a binding site that can be stimulated by external chemical messengers, and relay the signal to the cell, often by binding to a cytoplasmic protein.
- Cell-cell recognition – some glycoproteins are identification tags that are recognized by membrane proteins of other cells via short-lived bonds.
- Cell-cell recognition is important in the sorting of cells and tissues and organs in animal embryos, and often involves membrane carbohydrates.
- Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units, covalently bonded to either lipids (forming glycolipids ) or, more often, to proteins (forming glycoproteins ).
- Intracellular joining – long-lasting bonds between membrane proteins of adjacent cells, such as gap junctions or tight junctions.
- Attachments to the cytoskeleton and ECM – microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, which in turn can bind to ECM molecules.
- For example, membrane protein CD4 helps HIV infect these cells, leading to AIDS. However, HIV infection also requires a CCR5 co-receptor, which has proved to be a safer drug target then CD4.
- Nonpolar molecules (such as hydrocarbons, CO2, and O2) and lipids are hydrophobic and can therefore diffuse across the lipid bilayer of the membrane easily.
- However, hydrophilic ions and polar molecules such as (glucose, other sugars and even water) diffuse slowly without the aid of membrane transport proteins – which include channel proteins and carrier proteins .
- Channel proteins have a channel that certain molecules (such as water via aquaporins, three billion per second) use as a tunnel through the membrane.
- Carrier proteins hold onto their substrates and change shape in a way that shuttles them across the membrane.
- Transport proteins are specific for the substance they translocate – the glucose transporter, which transports glucose molecules across the membrane 50,000 times faster than normal, rejects fructose.
- Due to their constant motion, molecules have thermal energy which allows for diffusion , the movement of particles of any substance down its concentration gradient, the region along which the density of the substance increases or decreases.
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