Cell Structure & Membranes

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  • Created by: ernily
  • Created on: 18-04-15 19:04

Animal Cell Structure

  • Plasma Membrane: Regulates the movement of substances in and out of the cell. It has receptor molecules to respond to chemicals.
  • Nucleus: Chromatin is made from DNA. Pores allow substances to move between the nucleus and cytoplasm. The nucleolus makes ribosomes.
  • Lysosome: Contains digestive enzymes. They can be used to digest invading cells or break down worn out cell components.
  • Ribosome: The site where proteins are made.
  • Endoplasmic Reticulum: Smooth ER synthesises and processes lipids. The rough ER folds and processes proteins that are made at ribosomes.
  • Golgi Apparatus: Processes and packages new lipids and proteins. Also produces lysosomes.
  • Microvilli: Increases the surface area of the plasma membrane for absorption.
  • Mitochondria: The site of aerobic respiration. Produces ATP.
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Analysis Of Cell Components

  • Magnification: How much bigger the image is than the specimen.
    • magnification = image size / specimen size
  • Resolution: How detailed the image is; how well a microsccope distinguishes between two points that are close together.
  • Light Microscopes:
    • Use light
    • Have lower resolution (maximum 0.2 micrometres) than electron microscopes
    • Maximum of magnification of x 1500
  • Electron Microscopes:
    • Use electrons
    • Have higher resolution (maximum 0.0001 micrometres), so give more detail.
    • Maximum magnification of x 1,500,000
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Analysis Of Cell Components

  • Transmission Electron Microscopes:
    • Use electromagnets to focus an electron beam, which is then transmitted through the specimen.
    • Denser parts of the specimen absorb more electrons, so the image appears darker.
    • TEMs give high resolution images, but they have to use thin specimens.
  • Scanning Electron Microscopes:
    • SEMs scan the electron beam across the specimen.
    • This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image.
    • The image can be thick and 3D.
    • But they are lower resolution than TEMs.
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Analysis Of Cell Components

  • Cell Fractionation:
    • Homogenisation: This breaks up the plasma membrane and releases the organelles into a solution.
    • Filtration: The homogenised cell solution is filtered through a gauze to separate the large cell debris.
  • Ultracentrifugation:
    • Cell fragments are poured into a tube. This tube is put into a centrifuge and then spun at a low speed.
    • The heaviest organelles form a thick sediment, the pellet, at the bottom of the tube. The fluid above the pellet is called the supernatent.
    • The supernatent is drained off into another tube and spun at a higher speed
    • Again, the heaviest organelles form a pellet and the supernatent is drained and spun at an even higher speed.
    • This process is repeated at higher and higher speeds until all the organelles are separated out.
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Plasma Membranes

  • Plasma membranes are mostly made of lipids.
  • The phospholipid bilayer is a continuous fluid-mosaic structure.
  • Fluid: The phospholipids are constantly moving.
  • Mosaic: The protein molecules are scattered throughout the layer.
  • Triglycerides: Have hydrophilic, fatty acid tails. This makes lipids insoluble in water.
  • They are formed by condensation reactions. An ester bond is formed and a water molecule is released.
  • Saturated: Does not have any double bonds.
  • Unsaturated: Does have double bonds.
  • Phospholipids are similar to triglycerides, except one fatty acid molecule is replaced by a phosphate group.
  • Emulsion Test For Lipids:
    • Shake the test substance with ethanol, then pour the solution into water.
    • Any lipid will show up as a milky emulsion.
    • More lipids will lead to a more noticeable colour.
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Exchange Across Plasma Membranes

  • Diffusion: The net movement of particles from high concentration to low concentration.
    • Molecules will diffuse both ways until the particles are evenly distributed.
    • Particles diffuse down a concentration gradient.
    • Particles can diffuse across plasma membranes as long as they can move freely.
  • Higher concentration gradient = faster rate of diffusion.
  • Thin diffusion pathway = faster rate of diffusion.
  • Larger surface area = faster rate of diffusion
    • Microvilli increase the surface area.
    • More particles can be exchanged in the same amount of time.
    • So the rate of diffusion is faster.
  • Osmosis: Diffusion of water molecules across a partially permeable membrane from high to low water potential.
    • The plasma membrane is partially permeable; water molecules are small and can diffuse easily.
    • If two solutions have the same water potential, they are isotonic.
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Exchange Across Plasma Membranes

  • Facilitated Diffusion: Allows larger molecules to diffuse by using carrier and channel proteins.
    • Carrier proteins move large molecules.
    • A large molecule attaches to a carrier protein. The protein changes shape.
    • This releases the molecule on the opposite side of the membrane
    • Channel proteins form pores in the membrane for charged particles to diffuse through.
  • Carrier proteins are also involved in active transport, which is the same process as facilitated diffusion except energy is used to move the solute against its concentration gradient.
  • Co-transporters are a type of carrier protein.
  • They bind two molecules at a time.
  • The concentration gradient of one molecule is used to move the other molecules against its own concentration gradient.
  • Some glucose diffuses into the blood.
    • When carbohydrates first break down, there is a higher concentration of glucose in the small intestine than in the blood.
    • This establishes a concentration gradient for diffusion.
  • Some glucose uses active transport with sodium ions.
    • The sodium-potassium pump creates a concentration gradient.
    • Sodium ions diffuse into a cell via co-transporter proteins.
    • Co-transporter proteins carry glucose into the cell with sodium. Glucose enters the blood by facilitated diffusion.
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Exchange Across Plasma Membranes

  • The membrane is a good barrier against most water soluble molecules.
    • Hydrophobic tails make it difficult for water soluble substances to get through.
  • Channel and carrier proteins allow passage of water soluble substances. The membrane controls what enters and leaves.
  • Membranes contain receptor proteins, allowing cells to detect chemicals. Cell communication is vital for the body to function properly. The membrane allows cell communication.
  • The membrane allows cell recognition. Glycoproteins and glycolipids tell white blood cells that the cells are yours.
  • The membrane is fluid. The phospholipids are constantly moving. More unsaturated = more fluid. Cholesterol molecules fit between the phospholipids; more cholesterol = less fluid. Cholesteral makes the cell membrane more rigid.
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Cholera

  • Cholera bacterium is a prokaryotic organism.
    • Flagellum: Rotates to make bacterium move; so it can propel cholerae through the intestine wall.
    • Plasma Membrane: Made up of lipids and proteins. It controls the movement of substances in and out of the cell.
    • Circular DNA: Free floating.
    • Plasmids: Small loops of DNA. Contains genes.
    • Slime Capsule: Protects the bacteria from the immune system.
    • Cell Wall: Made up of peptidoglycan.
  • Cholera bacteria produce a toxin.
  • Toxin causes the chloride ion protein channels of the small intestine to open.
  • Chloride ions move into the small intestine lumen. The build up of ions lowers the water potential of the lumen.
  • Water moves out of the blood and into the lumen by osmosis.
  • The large increase of water secretion into the lumen leads to diarrhoea, causing the body to become extremely dehydrated.
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Cholera

  • Oral Rehydration Solutions are used to treat diarrhoea.
    • It is a drink containing salts (e.g. sodium/chloride ions) and sugars (e.g. glucose) dissolved in water.
    • Sodium ions increase the glucose absorption.
    • Getting the concentration right is essential.
  • Oral Rehydration Solutions can be tested on humans:
    • Ethical Issues:
      • Many trials involve children, and children can't give their consent.
      • Blind Trials: Patients don't know which treatment they are given.
      • There is a risk of the patient dying if a new Oral Rehydration Solution is wrong.
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