Biological Membranes

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  • Created by: tsungiex
  • Created on: 10-05-16 09:06

What are the functions of the Plasma Membrane?

-They are partially permeable between the cell and its environment - this mens that they do not allow all molecules to pass through.

- They allow recognition of cells.

-They allow cell signalling/communication.

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What are the functions of membranes within cells?

-They are the site of chemical reactions.

-They are also partially permeable. E.g. RNA leaves the nucleus through the nuclear membrane.

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What is the Fluid Mosaic model?

-It's a model that includes the components carbohydrates, lipids and proteins.

-It is called the "Fluid Mosaic" model because the phospholipids are constantly moving and the protein molecules are scattered like tiles in a mosaic.

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Function and features of Phospholipids

-They have hydrophillic (water loving) heads and hydrophobic (water hating) tails.

-They arrange themselves into a phospholipid bilayer - the heads stay on the outside towards the water and the tails are on the inside in the centre.

-This means that water-soluble molecules cannot pass through the centre of the bilayer such as ions.

-However fat-soluble molecules are able to pass through.

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Functions and features of Cholesterol

-Its a lipid present in any cell except for the bacterial cell.

-It fits in with the phospholipids by binding to the hydrophobic tails. This causes them to pack more closely together which keep the cell more rigid. 

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Functions and features of Proteins

-Channel Proteins - form channels to allow small or charged particles through.

-Carrier Proteins - they alllow small or charged particles to pass through the membrane by active transport or facilitated diffusion.

-Receptors - some proteins are receptors so in cell signalling, the molecule binds to the protein which triggers a chemical reaction.

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Function & features of Glycolipids & Glycoproteins

-They stabilise the membrane by forming hydrogen bonds between surrounding water molecules.

-They bind to drugs, hormones and antibodies.

-Act as receptors

-Are antigens

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What is the process of Cell Signalling?

1. One cell releases a messenger molecule.

2. This molecule travels to another cell.

3. The messenger molecule is detected by the cell because it binds to a receptor on it's cell membrane.

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What is the role of membrane bound receptors?

-Receptor proteins have specific shapes so messenger molecules with a complimentary shape bind to them.

-Different cells have different types of receptors whch respond to different messenger molecules.

-A cell that responds to a particular messenger molecule is called a target cell.

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Permeability Practical

  • Cut 5 equal sized pieces of beetroot and rinse them with water to remove any pigment released from cutting.
  • Place each of the 5 pieces in a different test tube with 5cm3 of water.
  • Place each test tube into a water bath at different temperatures but in the same intervals for the same amount of time.
  • Remove the pieces leaving the coloured water in the test tube.
  • Measure with a colorimeter the absorbance of each test tube. The higher the absorbance, the more pigment is released which means the more permeable the membrane is.
  • The same can be done for solvent and concentration.
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Factors of Diffusion

  • Concentration Gradient - the higher it is, the faster the rate of diffusion.
  • Thickness of exchange surface - the thinner it is, the faster the rate of diffusion because the distance is shorter.
  • Surface Area - the larger it is, the faster the rate of diffusion.
  • Temperature - the higher it is, the faster the rate of diffusion because particles have more kinetic energy.
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Diffusion Practical

  • Surface Area - cut the agar jelly into different sized cubes and work out the SA:Volume ratio. Time how long it takes for each cube to turn colourless when placed in the same concentration of hydrochloric acid. Usually, the cubes with the highest surface area:volume go colourless fastest.
  • Concentration Gradient - Place equal sized agar jelly cubes in test tubes containing different concentrations of hydrochloric acid. Time how long each test tube takes to go colourless. The highest concentrations usually go colourless the fastest.
  • Temperature - Prepare test tubes containing the same amount of hydrochloric acid in water baths of different temperatures. Put the equal sized cubes of agar jelly in each test tube and time how long it takes for them to go colourless. The highest temperature usually goes colourless fastest.
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Animal Cell

  • Hypotonic Solution - animal cell bursts because net movement of molecules are in the cell.
  • Isotonic Solution - Water molecules pass in and out so the cell stays the same.
  • Hypertonic Solution - animal cell is shrinked because net movement of molecules are out of the cell.
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Plant Cell

  • Hypotonic Solution - the vacuole swells and the membrane pushes against the cell wall to make it rigid.
  • Isotonic Solution - the cell stays the same.
  • Hypertonic Solution - the membrane pulls away from the cell wall so the cell becomes plasmolysed.
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Water Potential Practical

  • Prepare 6 sucrose concentrations .
  • Use a chip maker to cut the potatoes into the same sizes.
  • Divide the chips into groups of 3 and use a mass balance to measure the mass of each group.
  • Place one group in each solution.
  • Leave the chips in the solution for as long as possible at the same time.
  • Remove the chips and pat dry them gently with a paper towel.
  • Weigh each group again and record your results.
  • Calculate the % change in mass for each group.
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