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33 Cards in this Set
- Front
- Back
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The Fluid Mosaic Model |
In a biological membrane, the phospholipid bilayer serves as a lipid "lake" in which a variety of proteins "float." "Mosaic" because it is made up of many discrete components, "fluid" because they can move freely (not covalently bonded). |
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What function do carbohydrates serve in the membrane? |
They are attached to either lipids or protein molecules, and are located on the outside of the cell, where they may interact with substances in the external environment. Like some of the membrane proteins, they are crucial in recognizing specific molecules, such as those on the surfaces of adjacent cells. |
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Biological membranes can fuse during vesicle formation, phagocytosis, and related processes because of... |
...the capacity of lipids to associate with one another and maintain a bilayer organization. |
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cholesterol |
Changes membrane fluidity depending on temperature, permitting some molecules to move laterally within the plane of the membrane. High temp raises the melting point and low temp prevents stiffening. Cholesterol preferentially associates with saturated fatty acids. |
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What role does temperature play in membrane fluidity? |
Because molecules move more slowly and fluidity decreases at reduced temperatures, cellular processes that take place within the membrane may slow down or stop under cold conditions in organisms that can't keep their bodies warm. To address this, some organisms simply change the lipid compositions of their membranes, adopting unsaturated fatty acids with shorter tails. |
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peripheral membrane proteins |
Lack exposed hydrophobic groups and aren't embedded in the bilayer. Instead, they have polar or charged regions that interact with exposed parts of integral membrane proteins, or with the polar heads of phospholipid molecules. |
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integral membrane proteins |
Have hydrophobic and hydrophilic regions or domains (R groups). Some extend across the bilayer, while others are partially-embedded. |
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transmembrane protein |
An integral protein that extends all the way through the phospholipid bilayer and protrudes on both sides. In addition to one or more transmembrane domains that extend through the bilayer, such a protein may have domains with other specific functions on the inner and outer sides of the membrane.
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What role does the cytoskeleton play in membrane structure? |
The cytoskeleton may have components just below the inner face of the membrane that are attached to membrane proteins protruding into the cytoplasm, and can stabilize them and restrict their movement. |
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glycolipid |
Consists of a carb covalently bonded to a lipid and may serve as a recognition signal for interactions between cells (ex: alerting white blood cells to target cells that become cancerous). |
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glycoprotein |
One or more short carb chains covalently bonded to a protein (larger version is a proteoglycan). Often function in cell recognition and adhesion (when different branches "fit" together). |
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cell recognition |
In which one cell specifically binds to another type of cell. |
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cell adhesion |
In which the connection between the two cells is strengthened. |
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tight junctions |
Prevent substances from moving through spaces between cells, forming a quilted seal of proteins (ex: bladder lining), or maintain distinct faces of a cell within a tissue by restricting the migration of membrane proteins. |
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desmosomes |
Hold neighboring cells together, acting like spot welds or rivets, but materials can still move around in the extracellular matrix. This provides mechanical stability for tissues that receive physical stress (ex: skin). Cytoskeletal filaments composed of keratin fiber. |
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gap junctions |
Channels or connexin proteins that run between membrane pores in adjacent cells, allowing substances to pass between cells (ex: rapid spread of ion-mediated electric current in the heart). |
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integrin |
A transmembrane protein that often mediates the attachment of epithelial cells to the extracellular matrix. Has a role in maintaining cell structure via its interaction with the actin filaments of the cyoskeleton. The binding to the extracellular matrix is noncovalent and reversible ("back to front" recycling by endocytosis). |
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simple diffusion |
Small molecules pass through the phospholipid bilayer of the membrane, depending on their lipid-solubility or charge/polarity. |
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turgor pressure |
Cells with sturdy walls take up a limited amount of water, and in so doing they build up internal pressure against the cell wall, which prevents further water from entering (this is why plants stay upright and lettuce stays crisp). |
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facilitated diffusion |
Some substances must travel through proteins to cross the membrane, but the driving force is still diffusion . |
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channel proteins |
Integral membrane proteins that form channels across the membrane through which certain substances can passively diffuse. |
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carrier proteins |
Bind substances (thereby changing the shape of the protein ) and speed up their diffusion through the bilayer. Can allow for diffusion in either direction. Gradient is maintained by metabolism of the transported substance within the cell (ex: glucose). Rate of diffusion becomes constant if all carriers are saturated (occupied). |
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Why is it that a potassium channel allows only K+ and not Na+ to cross? |
Although Na+ is smaller and bears the same charge, it is not able to shed its water "shell," making it too big to pass through the funnel. |
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ion channels |
Often gated. May respond to another stimulus molecule, pH, or physical pressure. Specific for one type of ion. |
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aquaporins |
Highly specific channel proteins for water molecules. Increase membrane permeability, allowing water to diffuse more rapidly. |
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What are the three types of active transporters? |
1. Symporters (coupled transporters) 2. Antiporters (coupled transporters) 3. Uniporters |
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In what ways does active transport differ from passive transport? |
Active transport is directional, moves substances against their concentration/electrical gradient, and requires the expenditure of energy. |
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primary active transport |
Involves the direct hydrolysis of ATP, which provides the energy required for transport. |
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secondary active transport |
Does not use ATP directly. Instead, its energy is supplied by an ion concentration gradient established by primary ATP. |
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sodium-potassium pump |
An integral membrane glycoprotein. It breaks down a molecule of ATP to ADP and a free phosphate ion (Pi), and uses the energy released to bring two K+ ions in and export three Na+ (the pump protein changes shape when it is phosphorylated, and reverts when the Pi is released). It is thus an antiporter/means of primary active transport. |
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The transport of glucose into an intestinal cell is accomplished by... |
...secondary active transport, with the sodium-potassium pump providing the gradient (glucose moves in, in symport with the passive diffusion of Na+) |
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pinocytosis |
"Cellular drinking." Smaller vesicles than phagocytosis, and the process operates by bringing fluids and dissolved substances into the cell (nonspecific). Pinocytosis allows cells of the endothelium to rapidly acquire fluids and dissolved solutes from the blood. |
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receptor-mediated endocytosis |
Molecules at the cell surface recognize and trigger the uptake of certain macromolecules (ex: LDL). The receptor proteins in a coated pit (ex: clathrin coating) bind the specific molecule, which is then carried into the cell by a coated vesicle, the coating serving to strengthen and stabilize the vesicle. |
