Chapter 9

• Both multicellular and unicellular organisms it is surrounded by a cell membrane or plasma membrane -> forming a boundary between the cells internal environment (cytoplasm) and the external environment
• Cell membrane Function

  • recognising other cell
  • transporting materials in and out of the cell
  • providing attachment sites for enzymes and hormones
  • transmitting neutral impulses in nerve cells
• The cell membranes physical and chemical properties allow it to control the exchange of materials and messages between the cell and its environment
• In organisms, plant, bacteria, fungi, and algae, a cell wall surrounds the cell membrane that provides strength and support 


  A cell wall is permeable allowing almost all materials to pass through, but it is the selectively permeable cell membrane which controls the substances move in and out of the cell
• The cell membrane controls the exchange of material between the internal and external environments of the cell
• Fluid mosaic model -- explains structure of cell membrane


  It describes the membrane as lipid bilayer made up to two layers of lipids that can flow and change shape like a fluid.

  Specialised proteins are embedded within the bilayer in various patterns, creating a mosaic like appearance

  Proteins and phospholids in the membrane control the exchange of materials between the inside and outside of the cell
• Fluid: a flexible, fluid bilayer of phospholids that enables the membrane to expand and contract + Mosaic: a mosaic of components -- including cholesterol, proteins and carbohydrates = Fluid Mosaic model of the cell membrane
• Phospholid Bilayer 


  Is made up of phospholids each having a hydrophilic head (water attracting) and hydrophobic tails (water-repelling), these phospholids form a bilayer with tails facing inward away from water and heads facing outward towards watery surroundings
• In animals, cholesterol is present in the bilayer, helping maintain membrane stability and fluidity at different temperatures
• The phospholids bilayer's lipid components makes membranes flexible and able to repair themselves, allowing cells to change shape and grow, and reseal if punctured
• Cell membrane proteins

  A variety of proteins are embedded in the phosphoplid bilayer -> it regulates the movement of substances and enables communication between cells and their external environment
• Transmembrane proteins

  These proteins extend across entire membrane -> it interacts with hydrophilic heads and hydrophobic tails of the phospholids

  Function: they act as transport proteins, controlling the entry and exist of materials and allow for cell communication
• Peripheral proteins 


  These proteins are bound to the surface of the membrane

  Function: it plays roles in cell signalling and structural support, but do not directly transport substances across the membranes
• Transport proteins: facilitate movement of ions, small molecules and macromolecules across the membrane
• Types of Transport proteins


  1. Channel Proteins (Ion Channels) Function: Channel proteins allow specific ions and simple molecules (like water) to move across the membrane without using energy in a process called facilitated diffusion
  2. Carrier proteins (Transporters) Function: Carrier proteins change shape to move molecules across the membrane, some use ATP energy to transport molecules against their concentration gradient
• Channel Proteins (Ion Channels) Example

  Calcium channels only allow calcium to pass in and out of the cell
• Carrier proteins (Transporters) Examples

  The sodium-potassium pump actively transports potassium into the cell and sodium out, maintaining the cell's electrochemical gradient
• Example of membrane proteins in multicellular organisms include:

  • Adhesion proteins -- links cells together to maintain both the three dimensional structure and the normal functioning of tissues
  • Receptor proteins -- play a crucial role in cell communication, they bind to hormones or other signalling molecules , which can trigger specific changes in cell's activities
• The cell membrane forms the boundary between a cell and its external environment. Its phospholid bilayer and embedded proteins control the movement of substances into and out of the cell (between the internal and external environment)
• Movement of materials across membranes

  Passive processes: Simple diffusion, facilitated diffusion, osmosis

  Active processes: Active transport, endocytosis, Exocytosis
• Passive Transport

  Is the movement of substances that do not require energy

  • A process where materials move down a concentration gradient, from an area of high concentration to an area of low concentration
  • Example: riding a bike downhill, no energy is needed to coast
• Many molecules move across the cell membrane passively without using energy, this movement relies on a process called diffusion
• Simple diffusion 


  Diffusion is the net movement of particles from an area of high to low concentration

  • particles move randomly and spread out until evenly distributed
  • diffusion happens because particles are in constant random motion
  • passive process -- diffusion doesnt require extra energy
• Factors that speed up diffusion

  • increasing concentration gradient (bigger difference in concentration)
  • higher temperatures which make particles move faster
• Movement of particles down a concentration gradient from high to low concentration it is passive and doesnt require energy

  Movement of particles up a concentration gradient from low to high concentration is active and requires energy
• Facilitated diffusion

  Charged particles (like sodium and chloride) and large molecules (like glucose) cannot pass easily through the membrane

  • Proteins help these particles move in and out of the cell -- this process is called facilitated diffusion
  • carrier proteins bind to specific molecules and transport them across the membrane
  • channel proteins form passageways for small ions/ water to diffuse through the membrane

  Facilitated diffusion is a passive process -- because substances move down their concentration gradient
• Osmosis

  Is the diffusion of water through a semipermeable membrane from a region of high water concentration (low solute) to low water concentration (high solute)

  • water moves across membrane through small gaps and aquaporins
  • osmosis is passive - no energy required
• Isotonic solution

  equal solute concentration inside and outside the cell, so no net water movement
• Hypotonic solute

  lower solute concentration outside the cell, water moves into the cell
• Hypertonic solute

  higher solute concentration outside the cell, water moves out of the cell
• Solutions = solvent particles + solute particles

  High-concentration solution = low concentration of solvent + high concentration of solute

  Low-concentration solution = high concentration of solvent + low concentration of solute
• Osmosis in Animal cells

  Animals cells surrounded by a hypotonic solution may swell and burst due to water moving in by osmosis

  • Unicellular organisms - use contractile vacuoles to remove excess water and avoid bursting
  • Multicellular animals have cells bathed in isotonic fluid, preventing net water movement
• Osmosis in Plant cells

  Plant cells wont burst in hypotonic solution because of their cell walls

  • water enters the vacuole, making the cell turgid (swollen) and helping maintain the plant's structure
  • turgor pressure helps plants stay upright and supports leaves in open position
  • plasmolysis: occurs when too much water is lost, causing the cell membrane to pull away from cell wall
• Active Transport

  Moves molecules or ions against the concentration gradient (low to high concentration)

  • it requires energy from ATP
  • carrier proteins in the membrane use this energy to pump molecules or ions in one direction acting like one-way valves

  Active transport is crucial for:

  • kidney function (reabsorbing used materials)
  • muscle contraction
  • nerve impulses
• around 40% of energy while a person sleeps is used for active transport
• Endocytosis

  is the process where large particles or whole cells are moved into a cell

  • is needed for large molecules that cannot pass through the membrane by passive or active transport
  • endocytosis involves the formation of vesicles which requires energy (active process)
• Two types of endocytosis 


  Phagocytosis: the cell engulfs solids

  • white blood cells perform phagocytosis to engulf bacteria and protect body from infection

  Pinocytosis: the cell engulfs liquids

  • e.g. fat droplets in the small intestine
• ATP energy is required to move substances across the membranes by active transport and bulk transport
• Exocytosis

  Is the process by which large molecules held in vesicles within the cell are transported to outside the cell

  • in plants, cells are specialised to produce products to be relocated -> includes growth regulators, toxins to ward of predators = exocytosis is involved in these cases
• Factors that affect exchange of materials across membranes include:

  • surface area to volume ratio of the cell
  • concentration gradients (rate of diffusion increases with a higher concentration gradient)
  • physical (small molecules can easily pass, large molecules are transported via vesicles through endo/exocytosis) and chemical (uncharged molecules like alcohols can easily cross the membrane by dissolving in the phospholid bilayer, charged ions need specific transport proteins due to hydrophilic nature) nature of the materials being exchanged