Membrane Structure and Function
Ayush Noori | EduSTEM Advanced Biology
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.
Membrane Fluidity:
- 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.
Membrane Proteins:
- 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.
Membrane Permeability:
- 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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Best Regards,
Ayush Noori
EduSTEM Boston Chapter Founder
Resources:
The premier source of past and present medical literature. Most supplemental information in Extensions is available via PubMed. When searching PubMed, be sure to use the “Free full text,” and “Sort by: Best Match” filters to find relevant and accessible results.
A large database of useful 3D structures of large biological molecules, including proteins and nucleic acids. Use the search bar to find a molecule of interest, which can then be examined using the Web-based 3D viewer.
