Answer to Question #271559 in Cell Biology for Baudouin

Question

Membranes are essential components of all cells. 

• Briefly describe the structure of a eukaryotic plasma membrane, identifying at least two component macromolecules and describing what their basic function is within the membrane.

 The plasma membrane protects the integrity of the interior of the cell by allowing certain substances in, while keeping other substances out.

• Summarize the manner in which small molecules are permitted to pass through the membrane ensuring you explain the difference between passive, facilitated, active, and coupled transport by comparing and contrasting them.
• Finally, explain the difference between endocytosis and exocytosis and provide an example of a molecule (remembering to state its function) that is transferred using this method.

Answer 1

Eukaryotic plasma membrane has a fluid-mosaic model and consist of a double layer of either phospholipids or, in the case of chloroplasts, glycosylglycerides, in which proteins are embedded .In most membranes, proteins make up about half of the membrane’s mass. There are integral , peripheral and anchored proteins. Also plasma membrane has glycoproteins and glycolipids.The composition of the lipid components and the properties of the proteins vary in different organisms.

So, let’s look at the structure and function of phosphatidylcholine(phospholipid) and integral protein. Phosphatidylcholine is a class of phospholipids in which two fatty acids are covalently linked to glycerol, which is covalently linked to a phosphate group. Also attached to this phosphate group is a component, called the head group, such as choline .In contrast to the fatty acids, the head groups are highly polar; consequently, phospholipid molecules display both hydrophilic and hydrophobic proper ties (they are amphipathic). The nonpolar hydrocarbon chains of the fatty acids form a region that is exclusively hydrophobic—that is, that excludes water. They provide fluid and plastic properties of cell membranes.

Most integral proteins span the entire width of the phospholipid bilayer, so one part of the protein interacts with the outside of the cell, another part interacts with the hydrophobic core of the membrane, and a third part interacts with the interiot of the cell, the cytosol. They can serve as ion channels and as certain receptors that participate in signal transduction pathways.

The main difference between active and passive is the use of energy during cell transport of materials.Passive transport on the other hand, is the movement of molecules from higher to lower concentration. Because material is moving with the gradient, energy is not required.Passive transport moves across a concentration gradient, or a gradual difference in solute concentration between two areas. Facilitated diffusion is diffusion using carrier or channel proteins in the cell membrane that assist in the movement of molecules across a concentration gradient.Passive transport doesn't require energy (ATP), active transport does require energy. Passive transport moves molecules with the concentration gradient (high to low), while active transport moves molecules against the concentration gradient (Low to High).

First of all, endocytosis and exocytosis are the types of active transport, so they require an energy of ATP to occur in a cell.The main difference is in the directions of delivery of substances. Endocytosis is the general term for processes of vesicular transport in which substances enter the cell. It plays key roles in nutrient uptake,cell signaling, and cell shape changes. Exocytosis is the general term for processes of vesicular transport in which substances leave the cell.

Some of the endocytotic processes require special proteins during vesicle formation. The best known protein is clathrin. The presence of clathrin is certainly important for endocytic vesicle formation, many vesicles are formed in a clathrin-independent manner using other proteins ( caveolins or flotillins). There are three mechanisms of endocytosis in the cell:

-Pinocytosis

-Phagocytosis

-Receptor-mediated endocytosis

Exocytosis is the process by which a vesicle moves from the cytoplasm to the plasma membrane, where it discharges its contents to the extracellular space. Presence of specific proteins on their surface (coatomers) mediate the movements of these vesicles. There are two mechanism of exocytosis:

-the constitutive

-the regulated secretory

Examples:

-acetylcholine is secreted by exocytosis into the synaptic cleft. Acetylcholine is a neurotransmitter, which provides muscle contractions.

-cholesterol travels in the blood in particles called low-density lipoproteins (LDLs), each LDLs bind to LDL receptors on plasma membranes and then enter the cells by endocytosis.Cholesterol is used for membrane synthesis and the synthesis of other steroids.