Chapter 11 Flashcards
What are the general functions of cellular membranes? Consider both the lipid and the protein components.
The lipid bilayer serves as a barrier to the flow, in or out of a cell, of charged and polar molecules.
Transport and channel proteins embedded in the lipid bilayer select what molecules can enter or leave the cell.
Together, the lipids and the proteins form a selectively permeable barrier that allows cells to control their internal chemistry.
Draw an amphipathic lipid?
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What parts of a structure make it amphipathic?
An amphipathic molecule has both hydrophobic and hydrophillic parts.
An amphipathic lipid has one end that is polar and is called the head group. The head group most often includes phosphate, which has a full negative charge. In the case of glycoplipids, the head group consists of sugars, which have polar hydroxyl groups.
The other end of an amphipathic lipid consists of nonpolar, hydrocarbon tails: long chains of C-C bonds and C-H bonds, but no oxygen. Some amphipathic lipids, such as cholesterol, have ring structures instead of long chains.
What properties of fatty acid tails influence the fluidity of the lipid bilayer?
The length and number of double bonds influence the fluidity.
How is a cell’s membrane asymmetric?
Membranes are bilayers. An asymmetric membrane has different lipid head groups on the surface of one monolayer relative to the other monolayer.
For example, glycolipids are in the outer monolayer of the plasma membrane, while the inner cytosolic monolayer lacks glycolipids.
The asymmetry is achieved by the covalent addition of sugars to lipid head groups that face the lumen of the endoplasmic reticulum and the lumen of the golgi apparatus, while head groups that face the cytosol are not modified by addition of sugars.
Lipids are synthesized at the endoplasmic reticulum. Enzymes in the lumen catalyze addition of sugars to lipid headgroups. As lipids move through the golgi apparatus, head groups that face the lumen are further modified. When a vesicle fuses with the plasma membrane, the head groups that were facing the lumen now face the outside of the cell.
What is the general function of a cell’s carbohydrate ‘coat’?
The sugar coat helps protect cells from drying out.
How does a cellular membrane receive and transmit information as part of their general function?
Cellular membranes also receive and transmit information: transmembrane protein receptors couple binding of external growth factors, and other input signals, to internal signal transduction pathways (such as G-proteins), and changes in membrane potential can propagate from one end of a cell to the other end (for example, neurons sending a signal from dendrite synapse to axon synapse).
How do cellular membrane conserve and convert energy as part of their general function?
Cellular membranes are also excellent for energy conversion and conservation.
Pumps can convert hydrolysis of ATP into electrochemical gradients by moving sodium or protons across a membrane.
Favorable flow of sodium or protons back into the cell can then be coupled to moving sugars, amino acids, or other ions across the membrane and against their gradients.
An electrochemical gradient of protons can also be used to generate ATP.
How does length and the amount of double bonds on a a fatty acid affect the fluidity of a lipid bilayer?
The more surface area of a fatty acid tail that is closely packed to adjacent tails, the more van der Waals attractions that can form and the less the individual lipids can move, so less fluidity. Longer fatty acid tails have more surface area than shorter tails. saturated fatty acid tails are straight and can pack closer to other saturated fatty acid tails.
In contrast, shorter fatty acid tails and unsaturated fatty acid tails, which have one or more double bonds that bend the tail, form less surface area with adjacent tails, so less van der Waals attractions and more movement of individual lipids, so more fluidity.
How does a cell’s carbohydrate ‘coat’ keep it moist?
Sugars have hydroxyl groups that form hydrogen bonds with water, keeping the cell surface moist.
What are some effects of a cell having a moist sugar coat?
The moist sugar coat provides some resistance to compression and is also slippery, so in general prevents nonselective sticking of cells.
How do cells stick together?
Cells can stick together selectively by the selective binding of proteins called lectins to specific oligosaccharides. A lectin is a transmembrane protein.
The lectins, at the plasma membrane of one cell, can selectively bind to specific oligosaccharides of the carbohydrate ‘coat’ of a second cell.
