In addition to forming the structural barrier between the internal contents of a cell and the external environment, plasma membranes contain proteins involved in the transport of molecules and other substances into and out of the cell, and they contain proteins and other molecules that are essential for receiving signals from the environment and from plant hormones that direct growth and division.
Carbohydrates associated with the plasma membrane are markers of cell type. In plants, the plasmamembrane is the site of cellulose synthesis.
Lipid molecules provide the structure for the plasma membrane, which is described by the fluid mosaic model as a dynamic ocean of lipids in which other molecules float.
Phospholipids are the most abundant lipid of plasma membranes, and they are organized in a fluid phospholipid bilayer in which sterols, proteins, and other molecules are interspersed. Phospholipids are amphipathic molecules, containing water-loving (hydrophilic) regions and water-fearing (hydrophobic) regions.
Each phospholipid consists of a three-carbon glycerol backbone; two of the carbons are attached to long-chain fatty acidmolecules, and the third carbon is attached to a phosphate-containing group. Because the fatty acids are nonpolar and hydrophobic, they tend to aggregate and exclude water.
This aggregation allows the phospholipids to form a bilayer structure that has the fatty acids of both layers in the middle and the charged, phosphate containing groups toward the outside.
This bilayer structure allows one surface of the plasma membrane bilayer to interact with the aqueous external environment, while the other interacts with the aqueous internal cellular environment.
Sterols are also found within the plasma membranes of plant cells. The major sterol found in plant cell plasma membranes is stigmasterol (as opposed to cholesterol, which is found in animal cell plasma membranes). Sterols found in plant cells are important economically as the starting material for steroid-based drugs such as birth control pills.
Membrane Proteins and Carbohydrates
Some membrane proteins span the entire length of the phospholipid bilayer and are called trans membrane proteins. Trans membrane proteins are sometimes referred to as integral membrane proteins and have varied structures and functions.
They may pass through the lipid bilayer only once, or they may be “multiple pass” trans membrane proteins, weaving into and out of the membrane many times.
The portion of a trans membrane protein that passes through the interior of the membrane often consists of amino acids that have nonpolar side chains (R-groups) and is known as the trans membrane domain.
The portion of the trans membrane protein that is on the external surface of the membrane and interacts with the aqueous environment often contains charged, or polar, amino acids in its sequence.
Membrane proteins are often important for receiving signals from the external environment as membrane receptors. For instance, protein or peptide hormones interact with trans membrane protein receptors on the plasma membrane. Membrane proteins are also involved in receiving signals such as light photons.
Membrane proteins form pores that allow ions (charged particles) to pass through the interior of the membrane. Membrane proteins called carriers are essential for bringing nutrient molecules such as simple sugars into the cell.
Not all proteins within the membrane are trans membrane proteins. Some are only loosely associated with the membrane, attached to other proteins, or anchored in the membrane by a lipid tail. These proteins,which do not span both sides of the membrane, are often called peripheral membrane proteins.
In addition to proteins, the plasma membrane contains carbohydrate molecules. Carbohydrate molecules are usually attached to membrane proteins or to lipid molecules within the bilayer. Carbohydrates provide important information about cell type and identity.
Transport Across Membranes
Transport ofmolecules into and out of the cells is an important function of the plasma membrane. Hydrophobic molecules, such as oxygen, and small, uncharged molecules, such as carbon dioxide, cross the membrane by simple diffusion.
These molecules use the potential energy of a chemical gradient to drive their movement from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side.
Diffusion works best when this concentration gradient is steep. For example, in cells that do not have the ability to carry out photosynthesis, oxygen is used almost as quickly as it enters the cell.
This maintains a sharp gradient of oxygen molecules across the membrane, so thatmolecules continually flow from the area of greater oxygen concentration outside the cell to the area of lower concentration inside the cell.
Molecules that are polar are excluded from the hydrophobic area of the bilayer. Two factors influence the transport of these kinds of molecules: the concentration gradient and the electrical gradient. Lipid bilayers separate differences in electrical charge from one side of the membrane to the other, acting as a kind of biological capacitor.
If the inside of the cell is more negative than the outside of the cell, negatively charged ions would have to move from the inside to the outside of the cell to travel with the electrical gradient. The combination of the concentration and electrical gradients is called the electrochemical gradient.
Transport of charged or polar molecules requires the assistance of proteins within the membrane, known as transporters. Channel proteins form pores within the membrane and allow small, charged molecules, usually inorganic ions, to flow across the membrane from one side to the other.
If the direction of travel of the ion is down its electrochemical gradient, the process does not require additional energy and is called passive transport.
Carrier proteins change shape to deposit a small molecule, such as a sugar, from one side of a membrane to the other. Pumps are proteins within the membrane that use energy from adenosine triphosphate (ATP) or light to transport molecules across the membrane. When energy is used in transport, the process is called active transport.
In plants, the plasma membrane is the site for the synthesis of cellulose, the most abundant biopolymer on earth. Electron microscope studies suggest that the plant cell membrane contains rosette structures that are complexes of many proteins and are the sites of cellulose synthesis.
Studies in bacteria, cotton plants, and the weed Arabidoposis thaliana have allowed scientists to isolate the gene that actually carries out the chemical reactions linking glucose molecules together into the long cellulose microfibril structure.
This gene encodes a protein called glycosyl transferase. Antibodies against the catalytic, or active, subunit of glycosyl transferase specifically label these rosette structures.
Two of these transferase molecules act simultaneously from opposite sides to add two glucoses at a time to the growing microfibril, accounting for the rotation of alternating glucoses in cellulose molecules.