Cell membranes

  • Created by: Hannah
  • Created on: 07-11-13 11:21

Function of cell membranes and examples

  • Membranes are partially permeable, controlling what passes through them - e.g. allows small/uncharged particles to pass through it; protein channels+transporters control the passage of larger/charged particles
  • Membranes produce different compartements inside cells -e.g. mitochondira are surrounded by 2 membranes which form cristea- isolate reactions taking place in cytoplasm
  • Membranes are important in cell signalling- e.g. a substance produced when a cell docks onto a receptor in the PM of another causing a response
  • Membranes allow electrical signals to pass along them- e.g. the membrane of the axon of a motor neurone transmits action potentials from the central nervous system to a muscle
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Structure of cell membranes: phospholipids

Phospholipid bilayer:

  • Heads are hydrophilic
  • Tails are hydrophobic:

- cytoplasm contains a lot of water+so does the fluid outside. Causes them to arrange in a bilayer(http://www.biologycorner.com/resources/lipidbilayer.gif)

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Structure of cell membranes: other components

  • Cholesterol is a lipid
  • They lie alongside the phospholipids, helping to make up bilayer
  • Helps maintain the fluidity of the membrane- preventing it becoming too stiff when temps are low and too fluid when temps are high
  • Proteins and glycoproteins e.g. channel protein and surface proteins
  • form channels through which hydrophilic substances can pass
  • act as transporters that move substances across membrane up conc. gradients with use of ATP
  • Act as receptor sites allowing specific molecules such as hormones to bind with them + cause a response in the cell
  • Act as recognition sites as they may have a complementary shape to a particular cell etc
  • Act as enzymes

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Cell signalling: pt. 1.

  • Cell signalling: the communication between cells which triggers a response inside the cell

Mechanisms: Receptor acts as an ion channel:

  • When a signal chemical attaches to the receptor which causes the channel to open and let ions into the cell, bringing a response. e.g. the signal chemical acetycholine is released at a synapse of the first neurone and diffuses to the next, binds to a receptor in the next neurone's plasma membrane- receptor is a channel through which Na+ ions can enter the cell. This sets up a nerve impulse

Mechanisms: Receptor activates a G-protein:

  • Receptor in plasma membrane interacts with a G-protein. When signal molecule attaches to the receptor, the G protein is activated, which then activates an enzyme which brings about a reaction inside a cell. e.g. adrenaline, your adrenal glands release it and is picked up by receptors in the cells of the liver- activates the G-protein, which activates an enzyme to make cyclicAMP- breaks down glycogen to glucose
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Cell signalling: pt. 2.

Mechanisms: receptor acts as an enzyme:

  • Receptor is made up of 2 parts, when signal molecule arrives it slots into both these parts, connecting them forming them into an enzyme which then brings out a reaction inside the cell. e.g. the hormone insulin which is secreted by the pancreas when blood glucose levels are too high. Insulin binds with enzyme tyrosine kinease at the plasma membranes of the liver cells- activates the enzyme which brings out a reaction inside the cell turning glucose into glycogen
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Aspirin and Vioxx


  • Reduces tendency of blood to clot+relieves pain
  • Blocks cell signalling pathways e.g. a pathway that involves the chemicals called prostaglandis- made in most cells of body after injury, they bind to receptors and activate G-proteins e.g. nerve cells respond by sending pain signals to the brain. It also causes inflammation, where blood capillaries become leaky and allow fluid + white blood cells into the damaged area
  • Aspirin inhibts an enzyme called COX-2 which produces prostiglandis from a lipid called arachidonic acid which is also used for making thromboxane- stimulates platlets to stick together + form blood clots (aspirin also inhibits this)
  • However aspirin also inhibits the enzyme COX-1 which helps produce the protective layer of mucus in the stomach- could cause damage to its walls


  • inhibits COX-2 but not COX-1 to reduce pain+ swelling however it is linked to heart disease
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Movement across cell membranes: diffusion

  • the net movement as a result of random motion of its molecules or ions, of a substance from an area of high concentration to low concentration, down the conc. gradient (passive)
  • e.g. oxygen- is used in cells for respiration therefore conc inside cells are low. If there is more oxygen outside the cell then there is a conc. gradient for oxygen. It can pass through the plasma membrane as it is a small uncharged molecule.(http://www.daviddarling.info/images/transport_across_membranes.jpg)
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Facilitated diffusion

  • the diffusion of a substance through protein channels in a cell membrane; proteins provide hydrophilic areas that allow molecules or ions to pass through the membrane that would otherwise be less permeable to them
  • each channel made by a protein will allow only a specific ion/molecule to pass through e.g Cl-                                                                              (http://bio1151.nicerweb.com/Locked/media/ch07/07_15FacilitatedDiffusion-L.jpg)
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  • the net movement of water molecules from a region of high \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) to and area of low \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) potential, through a partially permeable membrane, down the \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) gradient
  • solutes decrease the \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) of a solution, water under pressure increases the \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png).


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Osmosis and animal cells

  • If the \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) gradient is very high into the cell, it will swell+burst (lysis)
  • If the \Psi (http://upload.wikimedia.org/math/3/9/8/398b8c3e58e961cdea0fe626771f3243.png) gradient is the opposite way around water leaves the cell by osmosis, the cell may shrink and may become crenated, the conc. of solutes in the cytoplasm increases and this may adversely affect metabloic reactions inside the cell


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Osmosis and plant cells

  • cell wall does not directly affect the movement becuse it is fully permeable to water+most solutes
  • as the cell swells the cell wall restricts the expansion of the cell, exerting pressure potential. This is called turgor
  • If the cell loses a lot of water the cell loses turgor, the cell becomes flaccid. The plasma membrane will pull away from the cell wall. The cell will become plasmolysed.(http://lgfl.skoool.co.uk/uploadedImages/Coord%2010.2_plasmloysed_cells.gif)
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Active transport

  • the movement of molecules or ions through transport proteins across a cell membrane against their conc. gradient involving ATP
  • e.g. sodium and potassium. Most cells need a higher conc. of potassium ions and a lower conc. of sodium ions than the conc outside the cell. The cell constanlty pumps out sodium ions and takes potassium ions in by active transport.
  • ATP is used to change the shape of transporter proteins- moves 3 sodium ions out of the cell and 2 potassium ions in. Called the sodium-potassium pump.(http://www.quia.com/files/quia/users/lmcgee/membranetransport/sodium-potassium-pump.gif)
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  • moving substances in bulk out of the cell is called exosytosis
  • the substance to be released is contian in a vesicle
  • vesicle is moved to the plasma membrane along microtubules
  • vesicle fuses with the membrane, emptying the contents outside the cell
  • Is an active process, requiring ATP(http://kenpitts.net/bio/images/exocytosis.gif)
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  • moving substances in bulk into a cell is called endosytosis
  • e.g. phagocyte puts fingers of cytoplasm around the bacterium, which fuse with another to form a complete ring around it, the bacterium is then enclosed in a vacuole. Enzymes can then be secreted into the vacuole to digest it.
  • cells can also move bulk liquids into the cell by endocytosis- process is the same
  • active process- rquires ATP
  • (http://click4biology.info/c4b/2/Pic2.4/endo2.gif)
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How temperature affect membrane permeability

  • as phospholipid molecules get hotter they vibrate more than they did previously, leaving temporary gaps in the membrane for pigment to escape
  • the protein molecules also vibrate more than before, creating temporary gaps if temp becomes too high the proteins will denature
  • low temps decrease permeability of the membrane- phospholipids vibrate less and pack together more tightly
  • transporter proteins may not work very well as it is difficult to make ATP at low temps
  • Human membranes can regulate their fluidity by increasing the proportion of phospholipids containing unsaturated fatty acids (have kinks in their tails) and cannot pack together as tightly and saturated fatty acids (straight tails)
  • if they cannot pack tightly they don't solidify as easily- decreases the temp. at which the membrane solidifies
  • cholesterol stops phospholipids packing together as tightly- helps keep membrane fluid at low temps.
  • Also helps at high temps, restricting the movement of the phospholipid molecules 
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