Cell Membrane

• The cell membrane is composed of a phospholipid bilayer, which consists of two layers of phospholipid molecules.
• Phospholipids are amphiphilic molecules with hydrophobic tails and hydrophilic heads.
• Cell theory states that cells are the basic unit of structure in all organisms and also the basic unit of reproduction
• Tenets of modern cell theory

  • All known organisms are made up of one or more cells
  • All living cells arise from pre-existing cells by division
  • The cell is the basic unit of structure and function in all organisms
  • An organism’s activity depends on the activity of independent cells
  • Energy flow occurs within cells
  • Cells contain DNA which is found in the cell nucleus and cytoplasm
  • Cells of similar species have the same basic chemical composition
• Each cell is an independent unit enclosed by a cell membrane
• The cell membrane forms the boundary between the internal environment of the cell and the external environment
• The cell membrane is differentially permeable, allowing only certain molecules to pass through
• The cell membrane regulates the differing concentrations of substances in the intracellular and extracellular environments
• Structure of the cell membrane

  • The cell membrane has a structure called the fluid mosaic phospholipid bilayer model
  • Comprised of a double layer of lipids which has the ability to flow and change shape
  • Specialised protein molecules are embedded within the lipid bilayer in various patterns
  • Proteins and the lipid bilayer help to control the exchange of materials between the internal and external cellular environments
• Phospholipids


  • A type of lipid represented by a head and two tails
  • The phosphate group on the head is hydrophilic and the two fatty acid tails are hydrophobic
  • Phospholipid tails in a bilayer turn inwards to face one another, leaving the hydrophilic phosphate heads facing outwards into the watery cytoplasm
• Cholesterol


  A type of lipid found in animal cell membranes, stabilizes and provides flexibility and fluidity to membranes
• Phospholipid bilayer


  Layer turn inwards to face one another, leaving the hydrophilic phosphate heads facing outwards into the watery cytoplasm
• Lipid components


  • Provide membranes with flexibility and fluidity, allowing cells to change shape and grow
  • Lipid molecules can rearrange themselves and repair themselves
• Cholesterol


  A type of lipid found in animal cell membranes, stabilizes and strengthens the membrane, and maintains its fluidity
• Phytosterol


  A lipid compound similar to cholesterol found in plants
• Proteins in the phospholipid bilayer

  1. Embedded in the bilayer, with many penetrating from one side to the other

  2. Adhesion proteins link cells together

  3. Receptor proteins bind hormones and substances changing cell activities

  4. Recognition proteins act as markers for the immune system to determine self and non-self cells

  5. Transport proteins assist certain substances to cross the membrane
• The cell membrane is differentially permeable, meaning only certain molecules are allowed to pass through
• Passive transport is movement which does not require energy
• Diffusion is the net movement of molecules from a region of high particle concentration to a region of lower particle concentration
• Concentration gradient


  The difference in particle concentration between two regions
• Equilibrium is reached when particles are evenly distributed
• Diffusion does not require the input of energy but can be impacted by temperature
• Increasing the concentration gradient increases the rate of diffusion
• Facilitated diffusion is a form of diffusion which requires substances to be assisted across a cell membrane by transport proteins
• Carrier proteins bind to specific molecules on one side of the membrane, change shape, and release the substance on the other side
• Channel proteins form passageways for ions of a specific size and shape to move across quickly
• Facilitated diffusion uses carrier proteins to move particles down their concentration gradient
• Facilitated diffusion of ions through channel proteins down a concentration gradient
• Charged particles and relatively large molecules move into/out of cells via facilitated diffusion
• Facilitated diffusion is dependent on the concentration gradient
• Diffusion


  The net movement of molecules from a region of high particle concentration to a region of lower particle concentration
• Concentration gradient


  The difference in particle concentration between the two regions
• Factors affecting the rate of diffusion

  • Temperature
  • Concentration gradient
• Facilitated diffusion


  A form of diffusion which requires substances to be assisted across a cell membrane by transport proteins
• How carrier proteins assist substances across a cell membrane

  They bind to specific molecules on one side of the membrane, change shape, and release the substance on the other side
• How channel proteins assist substances across a cell membrane


  They form passageways through which ions of a specific size and shape can move across quickly
• Intracellular and extracellular environments at equilibrium


  Not at equilibrium. Diffusion occurs from Right to Left
• Intracellular and extracellular environments at equilibrium


  Not at equilibrium. Diffusion occurs from Top to Bottom
• Intracellular and extracellular environments at equilibrium

  At equilibrium
• Osmosis is the movement (diffusion) of water across a selectively permeable membrane from a region of high water concentration (low solute) to a region of low water (high solute) concentration. Osmosis is a type of diffusion, so requires no energy input