Biological Membranes

• - phospholipid molecules form a continuous bilayer

  - The bilayer is fluid because the phospholipids are constantly moving

  - Cholesterol molecules are present within the bilayer

  - Protein molecules are scattered through the bilayer

  - Some proteins have a polysaccharide chain attached - these are called glycoproteins

  - Some lipids also have a polysaccharide chain attached - these are called glycolipids


  Describe the 'fluid mosaic' structure
• 7nm


  How thick is the phospholipid bilayer?
• - Cholesterol is a type of lipid

  - It's present in all cell membranes (except bacterial membranes)

  - Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and more rigid


  How does cholesterol help stabilise the cell?
• - Some proteins form channels in the membrane, these allow small or charged particles through

  - Other proteins (called carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion

  - Proteins also act as receptors for molecules (eg. hormones) in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell


  How do proteins control what enters and leaves the cell?
• - Glycolipds and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules

  - They're also sites where drugs, hormones and antibodies bind

  - They act as receptors for cell signalling

  - They're also antigens

  What are the functions of glycolipids and glycoproteins?
• - Cells communicate with each other using messenger molecules

  - One cell releases a messenger molecule (eg. a hormone)

  - This molecule travels (eg. in the blood)

  - The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane


  How do cells communicate with each other?
• - Proteins in the cell membrane act as receptors for the messenger molecules. These are called 'membrane-bound receptors'

  - Receptor proteins have specific shapes, only messenger molecules with a complimentary shape can bind to them

  - Different cells have different types of receptors, they respond to different messenger molecules

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


  How do receptors in the membrane work?
• - Many drugs work by binding to receptors in cell membranes

  - They either trigger a response in the cell, or block the receptor and prevent it from working

  How do drugs work?
• - Cell damage causes the release of histamine.

  - Histamine binds to receptors on the surface of other cells and causes inflammation

  - Antihistamines work by blocking histamine receptors on cell surfaces

  - This prevents histamine from binding to the cell and stops inflammation

  How do antihistamines work?
• - Cut five equal sized pieces of beetroot and rinse them to remove any pigment released during cutting

  - Place each piece in a separate test tube with 5cm3 of water

  - Place each test tube in a water bath at different temperatures for the same length of time

  - Remove the pieces of beetroot from the tubes, leaving just the coloured liquid

  - Use a colorimeter to see how much light is absorbed by the solution, the higher the absorbance, the higher the concentration of pigment


  How could you investigate how temperature affects permeability?
• - The phospholipids don't have much energy, so they can't move very much

  - They're packed closely together and the membrane is rigid

  - But channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane

  - Ice crystals may form and pierce the membrane making it highly permeable when it thaws

  How does temperatures below 0 degrees affect membrane permeability?
• - The phospholipids can move around and aren't packed as tightly together, the membrane is partially permeable

  - As the temperature increases the phospholipids move more because they have more kinetic energy

  - This increases the permeability of the membrane

  How does temperatures between 0 and 45 degrees affect membrane permeability?
• - The phospholipid bilayer starts to melt and the membrane becomes more permeable

  - Water inside the cell expands, putting pressure on the membrane

  - Channel proteins and carrier proteins deform so they can't control what enters or leaves the cell, this increases the permeability of the membrane


  How do temperatures above 40 degrees affect membrane permeability?
• - Surrounding cells in a solvent (such as ethanol) increases the permeability of their cell membranes

  - This is because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure

  - Some solvents increase cell permeability more than others, eg. ethanol increases cell permeability more than methanol


  How does changing the solvent affect membrane permeability?
• - It's the net movement of particles from an area of higher concentration to an area of lower concentration

  - Diffusion is a passive process, no energy is needed for it to happen


  Describe diffusion
• - Small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through the spaces between phospholipids

  - Water is also small enough to fit between phospholipids, so it's able to diffuse across the membrane even though it's polar

  Which molecules are able to diffuse through cell membranes?
• - The concentration gradient: the higher it is, the faster the rate of diffusion

  - The thickness of the exchange surface: the thinner the exchange surface, the faster the rate of diffusion

  - The surface area: the larger the surface area (eg. of a cell membrane), the faster the rate of diffusion

  - The temperature: the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy


  What factors affect the rate of diffusion?
• - Phenolpthalein is a pH indicator. It's pink in alkaline solutions and colourless in acidic solutions

  - Make up some agar jelly with phenolpthalein and dilute sodium hydroxide. This forms a pink jelly

  - Then fill a beaker with some dilute hydrochloric acid. Use a scalpel to cut jelly cubes and put them into the beaker

  - As time goes on, more acid will diffuse into the jelly until the cubes eventually turn colourless

  How would you investigate diffusion in model cells?
• - Surface area: use varying sizes of cube

  - Conc. gradient: use different concentrations of acid

  - Temperature: sit beakers in different water baths


  What factors can you investigate factors affecting diffusion in agar model cells?
• - Some larger molecules (eg. amino acids and glucose), ions and polar molecules don't diffuse

  - They travel through channel proteins or carrier proteins in a process called facilitated diffusion

  - The particles move down a concentration gradient

  - It's a passive process, it doesn't use energy

  What is facilitated diffusion?
• - Carrier proteins move large molecules

  - First a large molecule attaches to a carrier protein in the membrane

  - Then the protein changes shape

  - This releases the molecule on the opposite side of the membrane

  How do carrier proteins work?
• Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient)


  How do channel proteins work?
• - Pretty similar to facilitated diffusion, a molecule (eg. Ca+) attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane

  - However, it uses ATP


  How does active transport work?
• - Some molecules are way too large to be taken into a cell by carrier proteins, eg. proteins, lipids and some carbohydrates

  - Instead a cell can surround a substance with a section of its plasma membrane

  - The membrane then pinches off to form a vesicle inside the cell containing the ingested substance - this is endocytosis

  - Whit blood cells take in larger objects like microorganisms and dead cells by endocytosis

  - This process uses ATP


  Explain how cells take in substances by endocytosis

  
• - Some substances produced by a cell (eg. digestive enzymes, hormones, lipids) need to be released

  - Vesicles containing these substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane

  - The vesicles fuse with the plasma membrane and release their contents outside the cell

  - Some substances (like membrane proteins) aren't released, they're inserted straight in the membrane

  - Uses ATP


  Explain how cells can secrete substances by exocytosis
• Osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient


  What is the definition of osmosis?
• - Hypotonic solution (higher water potential than cell): net movement of water molecules is into the cell. The cell bursts

  - Isotonic solution (same water potential as cell): water molecules pass in and out at equal rate. No change

  - Hypertonic solution (lower water potential than cell): net movement of water molecules is out of the cell. The cell shrinks


  How does water potential affect animal cells?
• - Hypotonic solution: net movement of water is into cell. The vacuole swells. The vacuole and cytoplasm push against the cell wall. The cell becomes turgid

  - Isotonic solution: water moves in and out equally. No change

  - Hypertonic solution: net movement of water is out of the cell. The cell becomes flaccid. Th cytoplasm and the membrane pull away from the cell wall. This is called plasmolysis


  How does water potential affect plant cells?