2.1.5 Biological Membranes

• What are the roles of cell surface membranes?

  Outside of the cell. Cell signalling; how the cell interacts with other cells (receptors on the outside of the membrane). Allows the creation of a concentration gradient.
• What are the roles of the cell membrane?

  Inside the organelle; around all organelles except the ribosomes. Controls what goes in and out of the cell and organelles; isolates metabolic reactions from each other; provides sites for the attachment of enzymes and ribosomes; allows the creation of concentration gradients.
• Why are cell membranes described as "fluid mosaic"?

  Phosopholipids are free to move relative to each other = fluid. The protiens in the membranes size, shape, and positions resemble a mosaic.
• What is the function of the phospholipid bilayer?

  To regulate the movement of substances in and out of the cell by allowing the cell to be selectively permeable.

  
• What is the function of intrinsic/transmembrane proteins?

  Helps transport substances that cannot diffuse across but are vital to our cells functioning.
• What are extrinsic proteins?


  Proteins that are embedded in the cell membrane; they are confined to the inner/outer surface of the membrane.
• What is the difference between carrier and channel proteins?

  Carrier proteins change shape to move substances. Channel proteins form a passageway through the membrane to diffuse polar substances, including water.
• Describe two roles of cholesterol in the cell surface membrane.

  Keeps the membranes stable at normal body temperatures; without it the cell would burst. Regulates/controls membrane fluidity (more cholesterol = less fluid; less cholesterol = more fluid.)
• Why do cholesterol molecules have a hydrophilic and hydrophobic portion?

  So they sit embedded in the hydrophobic tail and phospholipid bilayer, adding stability.
• What is the major structural similarity between glycolipids and glycoproteins?

  They are both attached to carbohydrate chains
• What is a major structural difference between glycolipids and glycoproteins?

  Lipids vs. proteins
• What important role do glycoproteins play in the cell surface membrane?


  They allow cells to be recognised as self or non self; they allow cell signalling to take place; they act as receptors for hormones; they act as binding sites for transport proteins; they enable cell adhesion and attach to water molecules to stabilise the membrane.
• What is osmosis?

  The net movement of particles from an area of less negative water potential to an area of more negative water potential across a partially permeable membrane. Substances move through the aquaporins.
• What is water potential?

  Measured in kPa; a measure of the relative tendency of water to move from one area to another. It has a maximum value of 0kPa and is never a positive value.
• The less negative the water potential…

  …the lower the tendency of water to move to the area.
• Isotonic


  The water potential inside the cell is the same as the water potential of the solution.
• Hypertonic


  The water potential of the solution is more negative than the water potential of the cell; water moves out of the cell.
• Hypotonic


  When the water potential of the solution is less negative than the water potential inside the cell; water moves into the cell.
• Plasmolysis


  The shrinking of cytoplasm away from the cell wall of a plant cell when water is lost due to osmosis.
• Cytolysis

  Cells placed in distilled water; swells and bursts as water goes in the cell.
• Crenation


  Cell shrivels up; water moves out the cell.
• What is simple diffusion?

  Requires no energy; a form of passive transport. Moves with the concentration gradient; net movement of particles from an area of high concentration to low concentration. The small, non-polar molecules just cross the bilayer, no help needed (e.g. oxygen, carbon dioxide)
• Facilitated diffusion?

  Requires a transport protein. Large, or polar, or charged particles cross the bilayer this way. It is the net movement of particles from an area of high to low concentration across a partially permeable membrane via protein channels or carrier proteins
• Active transport

  Movement of particles AGAINST a concentration gradient; from an area of low concentration to high concentration. Process requires energy from ATP and carrier proteins. Used for any type of molecule going against the concentration gradient. Involves carrier proteins.
• What is bulk transport?

  Another form of active transport; for molecules that are too large, e.g. enzymes, hormones, and whoe cells like bacteria - they are too large to move through channel or carrier proteins so move via bulk transport instead.
• What is endocytosis?

  The bulk transport of material into cells. The cell surface membrane invaginates (bends towards) when it comes into contact with the material to be transported; membrane enfolds the material until it fuses, forming a vesicle; the vesicle pinches off and moves into the cytoplasm for further processing within the cell.
• What are the two types of endocytosis?

  Phagocytosis for solids; pinocytosis for liquids.
• What is exocytosis?

  The reverse of endocytosis; vesicles formed by the golgi apparatus move towards and fuse with the cell surface membrane. The contents of the vesicle is then released outside of the cell.
• Does bulk transport require energy?

  Yes; energy in the form of ATP is required for movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials and the fusion of cell membranes as vesicles form/as they meet the cell surface membrane.
• How do low temperatures affect membrane permeability and fluidity?

  Low temperatures decrease fluidity and permeability; phospholipids have less kinetic energy so decrease their movement and become more rigid. They begin to cluster together, decreasing the fluidity and permeability, possibly affecting the entry of important molecules.
• How does high temperatures affect membrane permeability and fluidity?

  Increases permeability and fluidity; At high temperatures phospholipids have more kinetic energy so increase their movement, becoming more fluid. They being to move further apart, increasing the overall fluidity and permability.
• How does cholesterol content affect membrane fluidity and permeability?

  High cholesterol content decreases fluidity and permeability; makes the cell more rigid and less permeable. The hydrophobic portion of the cholesterol is attracted to the fatty acid tails of the phospholipid, holding them togetherm disrupting permeability as gaps in the membrane decreases.
• Affect of phospholipid tails with unsaturated fatty acids on fluidity and permeability?

  Increases fluidity and permeability; unsaturated double bonds make it harder for the fatty acids to pack close together due to the kink and bends in the otherwise straightened chain. This means gaps form in the membrane, increasing permeability.
• How do solvents affect membrane fluidity and permeability?

  Increases fluidity and permeability; non polar solvents are able to dissolve non polar substances like lipids. This allows them to move easily through the bilayer, increasing fluidity and permeability - as solvent content increases, so does permeability.
• Fick's Law


  Rate of diffusion = (S.A x Concentration Difference)/Length of diffusion distance (membrane width)
• The effect of low temperatures on a cell responsible for lots of protein synthesis and secretion:

  Vesicles secreted by the golgi containing proteins would move slowly, affecting efficiency. Membrane would also be less fluid, so vesicles would leave the cell slower - harder to be secreted because cell is less permeable.
• Effect of high temperatures on the proteins embedded in the phospholipid bilayer:

  Carrier and channel proteins will denature at higher temperatures. These proteins are also involved in transport across the membrane so as they denature, membrane permeability is affected.
• Why do bacterial cells living in high temperature environments have large amounts of cholesterol in their bilayer?

  High temperatures makes their membranes too fluid, making the bacteria unable to function. Cholesterol stabilises the membrane by holding the fatty acids together.
• Suggest and explain the content of unsaturated fatty acids in the membranes of organisms living in cold environments.

  More unsaturated fatty acids; increases the fluidity of their membranes.
• Suggest why alcohols are used in antiseptics

  Alcohols cause membranes to become permeable, allowing substances to leak in and out of the cell. If alcohol content is high enough, enough phospholipids will dissolve causing the plasma membrane to disintergrate completely, killing the cell. Hence, antiseptic.