Ch 7: Membrane Structure and Function Flashcards
(32 cards)
What is the plasma membrane and why is it selectively permeable?
The plasma membrane is the outer barrier that encloses a cell. Plasma membranes are selectively permeable because they regulate the materials that can pass through a cell, such as the passage of oxygen, nutrients, and wastes.
What are the “staple” ingredients of membranes? What other molecules might you also find involved?
Lipids and proteins are the staple ingredients of membranes, but carbohydrates are also involved. Phospholipids are especially abundant in cell membranes, due to their hydrophobic and hydrophilic components allowing them to exist as stable boundaries for cells.
Describe the “fluid mosaic model” of membranes.
The fluid mosaic model shows different proteins embedded in a fluid layer of phospholipids, cholesterol, and other components of the membrane.
Explain what influence each of the following will have on the fluidity of the molecule.
- Decreasing temperature
- Saturated v. unsaturated fatty acid tails
- Cholesterol
- Decreasing temperature causes the membrane to solidify. As temperature decreases, the phospholipids of the membrane become more closely packed. In contrast, increasing temperature causes the membrane to become more fluid.
- Saturated fatty acid tails have fewer double hydrogen bonds, thus having fewer kinks, so the hydrocarbon tails are able to pack more tightly. Unsaturated fatty acid tails have more kinks, so they cannot pack as tightly as saturated hydrocarbon tails, thus being more fluid at lower temperatures.
- The steroid cholesterol is located between phospholipid molecules in the plasma membranes of animal cells, and has different effects on membrane fluidity at different temperatures. At high temperatures, cholesterol limits the movement of phospholipids to reduce membrane fluidity. At low temperatures, it disrupts the close packing of phospholipids to prevent them from solidifying, acting like a “fluid buffer.”
Why is this fluidity important?
Fluidity is important because it affects its permeability and enzymatic processes. If a membrane is solidified, proteins at its surface will find it more difficult to move to different locations within the membrane to function, and become inactive. Additionally, a membrane that is too fluid is also not ideal.
Describe some of the variation in cell membrane lipid composition seen in various organisms, such as bacteria, fish & wheat.
Some fish that live in extremely cold environments have higher amounts of unsaturated hydrocarbon tails, which enable their membranes to remain fluid enough to survive in low temperatures. Some bacteria that live in extremely hot temperatures have membranes that contain lipids that prevent them from becoming excessively fluid. Like some plants, such as winter wheat, certain bacteria can also adjust the amount of unsaturated phospholipids depending on the temperature of the environment.
List and describe the 6 major functions of membrane proteins.
Transport
- — Can provide hydrophilic channel across membrane to transport selective solutes
- — Can change shape to transport substances across the membrane
Enzymatic activity
—- Can be a protein with its active site (where the reactant binds) exposed to substances in the adjacent solution
Signal transduction
- — A receptor protein can bind to a chemical messenger with a specific shape, and then changes its shape according to the chemical messenger
- — Then relays the message to inside the cell, usually by binding to the cytoplasm
Cell-cell recognition
—- Identification tags specifically recognized by membrane protein of other cells
Intercellular joining
—- Membrane proteins of adjacent cells can hook/bind together
Attachment to the cytoskeleton and ECM
—- Cytoskeleton can be noncovalently bound to membrane proteins that can help maintain cell shape and stabilize location of certain proteins
How are scientists using their knowledge of CD4 and CCR5 cell surface proteins to develop drug treatments for HIV?
HIV can infect cells with CCR5 on its surfaces, but cannot bind to and infect a cell lacking CCR5 on its surface. Because CD4 has so many functions in cells, it is risky to mess around with it, so scientists are aiming to develop drugs involving CCR5 instead.
What is cell-cell recognition and how are membrane carbohydrates involved?
Cell-cell recognition is a cell’s ability to distinguish one type of cell from another. Cells can recognize others by binding to molecules, usually containing carbohydrates, on the extracellular surface of the plasma membrane. Some membrane carbohydrates are covalently bonded to lipids and form glycolipids. Some are covalently bonded to proteins, and are called glycoproteins.
Carbohydrates are attached to plasma membrane proteins in the ER (see Fig.7.9). On which side of the vesicle membrane are the carbohydrates during transport to the cell surface?
The carbohydrates are on the extracellular side.
Selective permeability means that some molecules will pass through the membrane while others won’t. Molecules that can pass through will also do so at different rates.
Describe what molecules will pass EASILY through the membrane. Which molecules will pass through the membrane with difficulty?
Hydrophobic molecules, or nonpolar molecules like hydrocarbons, CO2 and O2, can dissolve in the lipid bilayer and easily cross through the membrane. Polar molecules like water and glucose pass through the membrane with difficulty because their passage is impeded by the hydrophobic interior of the membrane.
What are transport proteins? Compare channel proteins with carrier proteins.
Transport proteins are proteins on the surface of plasma membranes that transport specific ions and polar molecules through the membrane while avoiding contact with the lipid bilayer. Channel proteins are proteins that can provide a hydrophilic tunnel that certain molecules can travel through. Carrier proteins hold onto their passengers and shuttles them through the plasma membrane.
What is an aquaporin? Why are they necessary for the transport of water across a membrane (think about water’s chemical bonding and chemical properties)?
Aquaporin is a transport channel protein that helps with the passage of water molecules. Most aquaporin proteins contain 4 identical polypeptide subunits, in which each form a channel water molecules use to pass through.
Define diffusion. How is dynamic equilibrium related to diffusion?
Diffusion is the movement of particles so that they randomly spread out into the available space. Dynamic equilibrium is reached when particles are diffused to equal concentrations.
Why is diffusion considered “passive” transport? What is the energy that fuels diffusion?
Diffusion is considered passive transport because it requires no energy for a substance to diffuse across a membrane. The concentration gradient, a region along which the density of a chemical substance increases or decreases, represents potential energy and is what drives diffusion. Passive transport also moves down the concentration gradient, another explanation for why it requires no energy.
Study figure 7.11 (A U tube) Explain what the water does and why it does it. Why doesn’t the sugar pass through the membrane?
The sugar molecules are too large to pass through the membrane, but water molecules are able to. The water molecules can diffuse across the membrane in random directions, but overall, the water diffuses from the side with a higher concentration of sugar to the side with lower concentration, thus achieving equal concentration. The sides have a similar concentration of sugar, but the side with more solute molecules have fewer free water molecules to offset the initially higher concentration.
Isotonic
An isotonic solution contains the same concentration of solutes as the inside of the cell. If a cell is placed in an isotonic solution, there will be no net movement of water across the membrane. Water diffuses across the membrane at the same rate in both directions.
Hypertonic
A hypertonic solution has more non penetrating solutes than the inside of the cell. If a cell is placed in a hypertonic solution, the water will diffuse out of the cell, and the cell will lose water and shrivel.
Hypotonic
A hypotonic solution has less non penetrating solutes than inside of the cell. If a cell is placed in a hypotonic solution, water will diffuse into the cell faster than it leaves, and the cell will swell and burst.
Why is it necessary to distinguish between cells without and with cell walls? What problems does a cell without cell walls face?
It is necessary to distinguish between cells with and without cell walls because cells without cell walls can’t tolerate excessive uptake or loss of water. However, this does not become a problem when the cell lives in an isotonic environment. These organisms must have other adaptations for osmoregulation, which is the control of solute concentration and water balance.
How does a single-celled organism without a cell wall, such as a paramecium, survive?
Paramecium lives in pond water, which is hypotonic to the cell. However, paramecium cells do not burst because its contractile vacuole pumps water out of the cell as it enters by osmosis.
Plant cells have cell walls. Describe the conditions that would cause a plant cell to be turgid, flaccid or plasmolyzed.
Plant cells swell as water enters, and the cell wall will expand enough until it exerts turgor pressure back onto the cell, which prevents any more water intake. The cell is turgid (very firm) when surrounded by a hypotonic solution, which helps hold up the plant. The plant cell is flaccid when its surroundings are isotonic, there isn’t water and the plant wilts. In a hypertonic solution, the plant cell loses water and also wilts.
What is facilitated diffusion?
Facilitated diffusion is when certain ions and molecules diffuse across the cell membrane with the assistance of transport proteins. This is also considered a passive transport because it moves down the concentration and requires no energy.
How does active transport compare to passive transport? Why might active transport be necessary for a cell?
Active transport is when a cell moves a solute against its concentration gradient, which requires work, and thus energy. Carrier proteins carry solutes against their concentration gradients rather than channel proteins because they pick them up rather than letting them pass through. Active transport enables a cell to maintain internal concentrations of small solutes that differ from their environment. For example, an animal cell has a high concentration of potassium ions (K+) and lower sodium ions (Na+). The plasma membrane can help maintain these by pumping Na+ out and K+ in.
