2.4: Membranes

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  1. Describe the structure of phospholipids in membranes.
    • Phospholipids form the structural basis of plasma membranes. The polar end consists of choline and phosphate and the non-polar end consists of two chains of fatty acids. The different ends allows the membrane to control the movement of substances across its structure. The hydrophobic core prevents the movement of ions and polar molecules and stops the cell dissolving in water. The hydrophilic ends of the phospholipids are in contact with aqueous solutions of either side of the membrane and can interact with their environment.
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  2. Distinguish between integral proteins, peripheral proteins and glycoproteins.
    Integral proteins contact the hydrophobic core of the phospholipid bilayer - some are transmembrane and span the entire membrane. Integral proteins have sections that are hydrophobic and the ends contacting the aqueous solution outside/inside the membrane are hydrophilic. Peripheral proteins are loosely connected to the surface of the membrane - some contact integral proteins. Glycoproteins are proteins that have a carbohydrate covalently bonded on the external side of the membrane.
  3. Summarise some of the roles of membrane proteins.
    - Channel protein
    - Carrier protein 
    - Enzymes Active Site 
    - Tight junction 
    - Glycoprotein 
    - Hormone
    • Role: Description of function
    • Passive transport: channel proteins have a hydrophilic channel that allows certain molecules and ions, e.g. aquaporins assist water movement (channel protein) 
    • Active transport: Carrier proteins change shape to take the substance across the membrane. Energy (e.g. ATP) is needed and the substance is moved against a concentration gradient, e.g. sodium-potassium pump in animal cells. (Carrier protein) 
    • Membrane-bound enzymes: Proteins that are enzymes can be embedded in the membrane in a series or system with the active sites exposed to the aqueous solution. A sequence of reactions in a metabolic pathway can occur. (Enzymes - active site) 
    • Attachment to adjacent cells: Integral proteins can bind to proteins from adjacent cells, e.g. forming tight junctions. (Tight junction) 
    • Cell identification: Some glycoproteins in plasma membranes are detected by recognition sites on proteins of other cells. The glycoproteins act as identification indicators. (Recognition site, glycoprotein) 
    • Hormone binding sites: Integral proteins have a binding site for a specific hormone. The hormone triggers a response in the cell. (Hormone).
  4. Distinguish between diffusion and osmosis.
    DIffusion is the movement of particles from an area of high concentration of particles to an area of low concentration of particles. Osmosis is a type of diffusion. It is the movement of water across a semipermeable membrane from an area of high water to an area of low water.
  5. Distinguish between simple diffusion and facilitated diffusion.
    Simple diffusion refers to the movement of particles down a concentration gradient due to the random movement of particles. Facilitated diffusion is also the passive movement of particles, does not require an input of energy, but involves the assistance of other molecules, e.g. transport proteins.
  6. Use an example to explain simple diffusion across a membrane.
    If oxygen molecules are more concentrated on the outside of a cell, the oxygen will diffuse across the plasma membrane into the cell, e.g. for aerobic cellular respiration.
  7. Use an example to explain facilitated diffusion across a membrane.
    Channel proteins that have a hydrophilic channel allow water molecules to move into/out of the cell. Water is a small molecule and can cross the membrane by simple diffusion, hoever the movement through the channel proteins (aquaporins) is faster than simple diffusion and assists osmoregulation. Facilitated diffusion is considered passive transport because the substance moves down a concentration gradient.
  8. Define active transport.
    Active transport is the movement of substances across a membrane against a concentration gradient or electrochemical gradient using energy. It can involve carrier proteins.
  9. Use an example to explain active transport across a membrane.
    Many animal cells ahve a sodium-potassium pump where sodium ion (Na+) are exchanged for potassium ions (K+). The sodium-potassium pump is involved with maintaining the electrochemical gradient, e.g. in nerve cells. When the sodium-potassium pump pumps sodium out of the cell and potassium into the cell the interior of the cell becomes negative and the exterior becomes positive, e.g. to return a nerve cell to the resting state.
  10. Define vesicle.
    A vesicle is a sac made of membrane that contains various substances.
  11. Show the sequence of events in the production of protein to its secretion from the cell.
    • Ribosomes on rough endoplasmic reticulum produce polypeptide chains. THe new protein enters the lumen of the rER and is sectioned off to form a vesicle. The transport vesicle moves to the Golgi apparatus. 
    • The transport vesicle from rER joins with the Golgi apparatus and its contents are added to the lumen. The Golgi apparatus modifies the substance. New vesicles are formed and bud off. Some of these transport vesicles contain secretions that leave the cell. 
    • Transport vesicles from the Golgi apparatus travel to the plasma membrane. The membrane fuses with the vesicle and the substance is secreted from the cell.
  12. Show how endocytosis and exocytosis influence membrane shape.
    • Endocytosis - cell takes up substances by surrounding it with membrane - during endocytosis the membrane changes shape, infolds and encloses substance to be taken into the cell, e.g. substance is highly polar or too large to enter by other means. 
    • Exocytosis - Substances are released from teh cell when vesicles fuse with the membrane - during exocytosis the membrane fuses with the vesicle, changes shape and expels the substance.
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2.4: Membranes
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