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Ch.5 AP Biology
Terms in this set (63)
Characteristic that the plasma membrane has that allows some substances to cross it more easily than others. This results in the cell discriminating in its chemical exchanges with its environment.
The membrane at the boundary of every cell that functions as a selective barrier for the passage of materials in and out of cells.
What as plasma membranes made of?
Lipids (mainly phospholipids) and proteins.
How are the materials of the plasma membrane arranged?
The phospholipids form a bilayer, with their hydrophilic head exposed to the water and their hydrophobic tails turned away from the water. Proteins are also amphipathic, and they are arranged in the bilayer according to their hydrophobic and hydrophilic regions.
Describe the Davson-Danielli Model of the plasma membrane.
It showed the phospholipid bilayer with proteins coating its surface. This is called the sandwich model.
Describe the Fluid Mosaic Model of the plasma membrane.
The fluid mosaic model refers to the idea that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. The molecules are not bonded together, so they are free to shift. Proteins form a collage or mosaic pattern that shifts over time.
What holds a plasma membrane together?
Primarily, hydrophobic interactions hold the membrane together.
What does the temperature at which a membrane solidifies depend on?
It depends on the type of lipids it is made of. The membrane remains fluid to a lower temperature if its phospholipids have unsaturated carbon tails, which can't pack together as closely as saturated carbon tails.
How does cholesterol affect cell membranes?
At the body temperature of humans (37 C), cholesterol makes the membrane less fluid by restraining phospholipid movement. But it also hinders the close packing of phospholipids, lowering the temperature required for the membrane to solidify. Cholesterol helps membranes resist changes in fluidity when the temperature changes.
Why do extreme environments pose a challenge for life?
When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive. Membranes that are too fluid can't support protein function either.
T or F. Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions.
Natural selection has favored organisms whose mix of membrane lipids ensures...
an appropriate level of membrane fluidity for their environment.
What determines most of the membrane's functions?
Proteins that penetrate the hydrophobic interior of the lipid bilayer. The hydrophobic regions of this protein consist of nonpolar amino acids.
Proteins that aren't embedded in the membrane at all; they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins.
What holds some membrane proteins in place?
On the cytoplasmic side, some proteins are held in place by attachment to the cytoskeleton. On the extracellular side, some proteins are attached to the fibers of the ECM.
How do integral proteins stick to the membrane?
By the solubility of their amino acids, especially their R groups.
What functions do the proteins in plasma membranes have?
1. Transport. Proteins might transport particles across the membrane. 2. Enzymatic Activity. A protein in the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. 3. Attachment to the cytoskeleton and ECM. Elements of the cytoskeleton may be bound to membrane proteins, helping to maintain cell shape and the location of certain membrane proteins. Proteins bound to ECM molecules can coordinate extracellular and intracellular changes. 4. Cell-cell recognition. Some glycoproteins serve as identification tags that are recognized by proteins of other cells. 5. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions. 6. Signal transduction. A membrane protein may have a binding site that fits the shape of a chemical messenger. This may cause the protein to change shape, allowing it to relay the message to the inside of the cell.
How do cells recognize other cells?
By binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane. The diversity of the molecules and their location on the cell's surface enable membrane carbohydrates to function as markers that distinguish one cell from another.
Describe membrane carbohydrates.
They are short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids (glycolipids) and some are covalently bonded to proteins (glycoproteins).
How does a membrane have two distinct faces?
The phospholipid composition of the two layers is different. Each protein has directional orientation. Carbohydrates are only found on the outside surface.
What crosses the lipid bilayer easily?
Nonpolar/hydrophobic materials like hydrocarbons, CO2, and oxygen.
What doesn't cross the lipid bilayer easily, and why not?
Ions/polar/hydrophilic molecules (sugar, water) don't cross easily because of the hydrophobic interior of the membrane. Large molecules also don't cross easily because they have a hard time getting through the membrane.
Proteins that allow molecules to cross the lipid bilayer without touching the lipid bilayer.
Type of transport protein that has a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane.
Channel proteins that allow the passage of water molecules through the plasma membrane. Aquaporin proteins are made of four subunits, forming a channel that allows 3 billion water molecules to pass per second.
Transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane.
Are transport proteins specific for the substance they translocate?
Yes. Each protein only allows a certain substance to cross the membrane.
What does the selective permeability of a membrane depend on?
The discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane.
Movement across membranes that doesn't require cellular energy.
The movement of particles of any substance so that they tend to spread out into the available space. Type of passive transport. A substance will diffuse from where it is more concentrated to where it is less concentrated.
The region along which the density of a substance increases or decreases. A substance will diffuse down its concentration gradient.
Is the diffusion of a molecule directional?
No, each molecule moves randomly, but diffusion of a population of molecules may be directional.
When the concentration is equal on both sides. When equilibrium is reached, there is no NET movement of materials.
Type of passive transport. Diffusion of water from an area of its high concentration to its low concentration.
What makes some water unavailable to cross a membrane?
The tight clustering of water molecules around the hydrophilic solute molecules. It is the difference in free water concentration that is important.
The ability of a surrounding solution to cause a cell to gain or lose water. The concentration of a cell's outside water relative to the cell's inside water.
If a cell is put in an isotonic environment, there will be no net movement of water across the plasma membrane.
If a cell is put is a hypertonic environment, the solute has more nonpenetrating solutes, and the cell will lose water, shrivel, and die.
If a cell is put in a hypotonic solution, water will enter the cell faster than it leaves, and the cell will swell and lyse (burst).
The control of solute concentrations and water balance.
What happens if a plant cell is placed in a hypotonic solution?
The plant cell will take in water and swell until it exerts a back pressure (turgor) on the cell that opposes further uptake. At this point, the cell is turgid, which is the healthy state of a plant cell.
What happens if a plant cell is placed in an isotonic solution?
There will be no net tendency for water to enter, and the cell will become flaccid (limp).
What happens if a plant cell is placed in a hypertonic environment?
As the plant cell is losing water and shrinking, its plasma membrane pulls away from the wall (plasmolysis). This causes the plant to wilt and can lead to plant death.
Type of passive transport where molecules cross the membrane with the help of transport proteins.
Channel protein that transports ions.
Channel proteins that open or close in response to stimulus. (like an electrical stimulus or a substance other than the one to be transported binds to the channel).
True or False. Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane, but it does not alter the direction of transport.
Movement across a membrane that does require energy. Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment.
What provides energy for most active transport?
ATP. ATP might transfer its terminal phosphate group directly to the transport protein, causing the protein to change its shape.
General term for the active transport of materials into cells against the concentration gradient.
Is the cytoplasmic side of the membrane negative or positive relative to the extracellular side?
The cytoplasmic side is negative. The inside of a cell is negative relative to the outside.
The voltage across a membrane. The membrane potential acts as an energy source that affects the traffic of all charged substances across the membrane. The membrane potential favors the passive transport of cations into the cell and anions out of the cell.
The combination of a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement) acting on an ion.
A transport protein that generates voltage across a membrane for other cell processes.
Sodium Potassium Pump
Type of an electrogenic pump. It actively transports three NA+ out of the cell and two K+ into the cell. There is a net transfer of one positive charge from the cytoplasm to the ECM.
Electrogenic pump of plants that actively transports protons (hydrogen ions H+) out of the cell.
Type of active transport. Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane.
Type of active transport that moves bulk materials out of the cell. A vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the membranes fuse and the contents of the vesicle spill to the outside of the cell.
Type of active transport that moves bulk materials into the cell. A small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell.
Endocytosis of liquids.
Endocytosis of solids (like food).
Uses receptors to catch specific kinds of molecules.
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