3. How Is the Fluidity of Cell S Membrane Maintained?

Kate Rowe 10/4/09 Period 4

Membranes

1. What does selective permeability mean and why is that important to cells? Selective permeability is a property that all biological membranes have. Selectively permeable means a membrane that “allows some substances to cross it easier than others.” Essentially, it allows for some substances, such as waste, to leave the cell while other substances, such as organelles and larger more important molecule, do not leave. It is important because if cells did not have a selectively permeable membrane items entering and leaving the cell could not be regulated with such ease. Overall, selective permeability permits the “uptake and of nutrients and elimination of waste.” (Textbook 124) 2. What is an amphipathic molecule? An amphipathic molecule, such as phospholipids found in membranes, is a molecule that contains a hydrophilic region and a hydrophobic region. For example in a membrane the phospholipid bilayer the hydrophilic heads are pointed outwards while the hydrophobic tails point in.

3. How is the fluidity of cell’s membrane maintained? The fluidity of a cells membrane is maintained in a few different ways. First, the lateral movement of phospholipids allows for fluidity. In addition, when the phospholipids have double bonds their tails have kinks in them. This insures that the molecules do not become to closely packed which would reduce movement. Finally, Cholesterol helps to control fluidity, at moderate temperatures it reduces movement but at lower temperatures it disrupts how tightly packed the phospholipids become.

4. Label the diagram below – for each structure – briefly list its function:

of 5 Fibers of the extracellular matrix (ECM): Connects a cell to other cells. Glycoprotein: Helps cells recognize each other, cell-to-cell recognition. Carbohydrate: Can attach to proteins or lipids to form glycoproteins for cell recognition. Also, allows for other substances, such as hormones, can attach. Microfilaments of cytoskeleton: Holds the shape and structure of the cell together. Cholesterol: Helps to control the fluidity of the membrane. Peripheral protein: appendages loosely bound to the exposed, hydrophilic parts of the integral proteins, on the cytoplasmic side of the cell. Integral protein: penetrate the hydrophobic core of the lipid bilayer. Glycolipid: Created when a carbohydrate binds to a lipid, helps in cell recognition. Integral protein: proteins that cross all of the membrane, such as channel proteins. Peripheral protein: Proteins on only one side of the membrane, used for recognition and other functions.

5. List the six broad functions of membrane proteins. The six broad functions of membrane proteins are transport, enzyme activity, signal transduction, cell recognition, intercellular joining and attaching to the cytoskeleton and ECM.

of 5 6. How do glycolipids and glycoproteins help in cell-to-cell recognition? Glycolipids and glycoprotein always end with a carbohydrate chain sticking out away from the cell. These chains differ in many ways and the type and shape of the chain can be recognized by other cells. More or less, they act as ID cards describing what a cell is a where it is suppose to be.

7. Why is membrane sidedness an important concept in cell biology? Membranes always have to distinct sides, a cytoplasmic inner face and extracellular outer face. The sides contain different lipids and proteins, which affect function. This is an important concept in cell biology because it is important that the sides are correct because it affects the function of the cell. In addition, there would be no cell-to-cell recognition if the faces were reversed.

8. What is diffusion and how does a concentration gradient relate to passive transport? Diffusion is the movement of a substance from a high concentration to a low concentration. Concentration gradient is needed for passive transport to occur because passive transport is when diffusion happens across a membrane and diffusion needs a concentration gradient.

9. Why is free water concentration the “driving” force in osmosis? Free water concentration is essentially the solvent version of solute concentration. Meaning that water will flow from a high concentration of water to a low concentration. This movement is called osmosis; ergo free water concentration is the driving force of osmosis.

10. Why is water balance different for cells that have walls as compared to cells without walls? The water balance is different for cells with walls compared to without walls because a cell with walls in a hypotonic solution will be ridged and turgid. However, without a wall there is nothing to stop the cell from expanding until it bursts. A cell without a wall in an isotonic solution is balanced and won’t burst; on the other hand, the cell with a wall becomes flaccid and limp. In a hypertonic solution a cell without walls will shrivel while a cell with walls will plasmolysize.

of 5 11. Label the diagram below:

On the far left is a hypotonic solution in which a cell without a wall will burst and will be lysed, while the cell with a wall will be turgid and stable. In the middle is a isotonic solution which makes a cell without a wall the healthiest, while the cell with a wall becomes limp. Finally on the right is the hypertonic solution, where water leaves the cells making the cell without a wall shriveled (fingers getting pruney). A cell with walls in a hypertonic will go through plasmolysis (onion lab).

12. What is the relationship between ion channels, gated channels and facilitated diffusion? Facilitated diffusion is where proteins assist in moving items through the membrane. Ion channels are a type of aforementioned protein that allows ions to pass through. Gated channels are proteins that can be open or closed.

13. How is ATP specifically used in active transport? Active transport is the pumping of molecules against the concentration gradient. In order to do this the protein pumps need energy in the for of ATP. ATP does this by “transferring its terminal phosphate group directly to the transport protien” (134).

14. Define and contrast the following terms: membrane potential, electrochemical gradient, electrogenic pump and proton pump. Membrane potential is the voltage on the different sides of the membrane. Electrochemical gradient is the combination of the chemical force, of the ions concentration gradient, and the electrical force, the affect of the membrane potential. Electrogenic pump is a protein that creates voltage across a

of 5 membrane. And a proton pump is a integral protein that actively transports hydrogen ions out of the cell.

15. What is cotransport and why is an advantage in living systems? Cotransport is when a protein pump that drives a single substance out of a cell causes the active transport of several substances. For example, in order for sucrose to enter some cells it must enter with hydrogen ions, so hydrogen ions are pumped out so sucrose can enter with the hydrogen into the cell through the co transporter protein. It is important in living systems because it helps transport molecules where the system needs it most.

16. What is a ligand? A ligand is “a general term for any molecule that binds specifically to a receptor site of another molecule.”

17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated endocytosis. Phagocytosis is where a cell completely engulfs a particle forming a vesicle in the process, which is later digested when fused with a lysosome. Pinocystosis is when the cell intakes small amounts of a substance into a vesicle, it is nonspecific and it transports the substance. Receptor-mediated endocytosis is when a coated membrane buds into a vesicles. The membrane is coated with protein receptors to which ligands bind. It allows for large quantities of a specific substance to enter the cell.