Unit 1.1.2: Cell Membranes

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The Role of Membranes

The Role of Membranes

  • Seperating cell contents from the outside environment.
  • Separating cell components from the cytoplasm.
  • Holding the components of some metabolic pathways in place. 

Plasma membranes are partially permeable membranes - they are permeable to water and other solutes.

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The Fluid Mosaic Model

The Fluid Mosaic Model

The fluid mosaic model describes the molecular arrangement in cell membranes. Main features: 

  • A bilayer of phospholipid molecules forming the basic structure.
  • Various protein molecules floating in the phospholipid bilayer, some freely, some bound to other components within the cell.
  • Extrinsic and intrinsic proteins. Extrinsic = partially embedded in the bilayer. Intrinsic proteins = spanning the bilayer. 

-The phospholipid bilayer is the basic structural component of plasma membranes. 

-Phospholipid molecules consist of a phosphate head, that is hydrophilic (water-loving) & 2 fatty acid tails which are hydrophobic (water hating). 

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The Fluid Mosaic Model

The Fluid Mosaic Model

  • If you surround them with water, a bilayer will form. 
  • The hydrophobic tails are held away from the water molecules 
  • In this state, the phospholipid molecules can move freely within the membrane.
  • The hydrophobic region, giving the bilayer stability.
  • This creates a barrier that separates cell contents from the outside world - as many metabolic reactions take place in a water-based environment.

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Communication and Cell Signaling

Communication and Cell Signaling

Cell Signaling. Is the communication between cells to trigger a response.

  • Many molecules can act as signals. E.g. Cytokines
  • Signal Molecules fit into receptors on cell surface membranes, due to their shapes being complementary to each other. 
  • Any cell with a receptor for the hormone molecules is called a target cell.

Medicinal Drugs. Have been developed that are complementary to the shape of a type of receptor molecule. 

  • Such drugs are intended to block receptors. 
  • Viruses enter cells by binding to the receptors on the cell's plasma membrane. 
  • Some poisons also bind with receptors. 
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Passive Transport

Passive Transport. Is the processes that depend on the kinetic energy in water/gas molecules.

Diffusion. Is the movement of molecules from a region of high concentration of that molecule to a region of low concentration of that molecule down a concentration gradient.

Diffusion is affected by: 

  • Temperature: the higher, the more KE.
  • Concentration Gradient: the steeper the faster. 
  • Stirring/moving: increases movement of molecules. 
  • Surface Area: the more, the faster the rate of diffusion.
  • Distance/thickness: the more, the slower.
  • The size of Molecule: smaller molecules/ions diffuse more quickly. 
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Passive Transport

Passive Transport

  • Lipid-based molecules can simply pass through the bilayer.
  • Very small molecules/ions such as Oxygen & Carbon Dioxide molecules are small enough to pass between the phospholipid molecules. 
  • Small, charged particles such as Sodium Ions or larger molecules - Glucose, cannot pass.
  • They diffuse by facilitated diffusion, in which the membrane proteins allow the substances to diffuse through the membrane. 
  • Channel Proteins. From pores in the membrane to allow only one type of ion through. 
  • Carrier Proteins. Shaped so a specific molecule (E.g. Glucose) can fit into them at the membrane surface. They can change shape to allow the molecule through to the other side of the membrane.

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Active Transport

Active Transport. Is the movement of molecules or ions against a concentration gradient and across membranes, using energy in ATP from respiration.

  • Carrier proteins, act as pumps to allow this to happen. They are shaped in a way that is complementary to the molecule they carry.
  • They carry larger molecules & ions which cannot pass through the lipid bilayer by diffusion. 
  • They differ from the proteins in facilitated diffusion by:
    • They carry specific molecules one way across the membrane.
    • They use metabolic energy in the form of ATP.
    • They can carry molecules against the concentration gradient. 
    • They can carry molecules at a much faster rate than diffusion. 
    • Molecules can be accumulated either inside cells or organelles. 
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Osmosis. Is the movement of water molecules from high to low water potential across a partially permeable membrane.

  • Water potential. Is the measure of the tendency of water molecules to diffuse from one place to another. 

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