Cell membranes

• The cell membrane acts as a selective barrier, allowing certain substances to enter or leave the cell while restricting others.
• Osmosis involves the movement of water molecules through cell membranes to reach equilibrium concentration.
• Cholesterol helps maintain fluidity and stability of the cell membrane by preventing excessive packing of lipid molecules.
• Facilitated diffusion transports large or polar molecules down their concentration gradient through membrane transport proteins. This is still a passive process.
• Phospholipids are amphiphilic molecules with hydrophobic tails and hydrophilic heads that form the bilayer structure of the cell membrane.
• The cell membrane is composed of phospholipids, cholesterol, proteins, and carbohydrates.
• The cell membrane is involved in cell adhesion, allowing cells to stick together and form tissues and organs.
• Cell membranes facilitate cell communication by allowing the exchange of signals and molecules between cells.
• Cholesterol is an important component of the cell membrane that helps maintain its fluidity at different temperatures.
• Phospholipids have hydrophilic heads and hydrophobic tails that form bilayers with their polar ends facing outward and nonpolar ends facing inward.
• Cholesterol is embedded within the lipid bilayer and helps maintain its fluidity at different temperatures.
• Glycolipids and glycoproteins have carbohydrate chains attached to their structures.
• Active transport requires energy from ATP to move molecules against their concentration gradient. (Low concentration of molecules to a high concentration). It also involves carrier proteins
• Membrane proteins are involved in various processes such as active transport, signal transduction, and adhesion.
• Protein channels allow specific substances to pass through the membrane while excluding others.
• Carrier proteins bind to specific molecules on one side of the membrane and undergo conformational changes to release them on the other side.
• Protein channels allow specific substances to pass through the membrane by changing shape or opening/closing.
• Passive Transport: The movement of molecules from high concentration to low concentration without requiring energy input.
• Diffusion

  Diffusion is the net movement of molecules from a high concentration to a low concentration. It moves down the concentration gradient
• Factors affecting diffusion part 1

  • The steepness of the concentration gradient. Steeper concentration = faster diffusion
  • Temperature - increased temperature means more kinetic energy for particles meaning faster diffusion
• Factors affecting diffusion part 2

  • Surface area as bigger surface area means faster diffusion
  • The size of the molecule - bigger molecules take longer to diffuse
  • The thickness of the membrane. The thinner the membrane the quicker the molecules can diffuse (short diffusion pathway)
• Channel and carrier proteins

  Channel proteins are pores in the membrane to let a specific ion or molecule through

  Carrier proteins changes their shape once a molecule is attached and let's the molecule through the membrane
• Active transport with carrier proteins

  1. Molecules binds to carrier proteins on the side
  2. ATP binds and hydrolyses to ADP. The phosphate caused a conformational change to the carrier protein
  3. The carrier protein opens to side where the high concentration of molecules are.
• Endocytosis

  • Large molecules are taken in to cells
  • Vesicles are formed when a section of the membrane surrounds the object
  • Examples : proteins, lipids, carbohydrates, microorganisms and dead cells
• Exocytosis

  • Large molecules are released from cells
  • Vesicles formed at Golgi. They moved and then fuse with the plasma membranes and release molecules
  • Examples: Hormones, lipids and digestive enzymes
• Facilitated diffusion

  Facilitated diffusion doesn't require energy because the particles randomly move going down a concentration gradient
• Osmosis

  Osmosis is the movement of water across a partially permeable membrane down a water potential gradient this means from a high concentration of water to a low concentration of water
• Isotonic solutions have equal concentrations of solutes so they don't cause any cell damage
• Plant cells can survive hypertonic conditions by having a waxy cuticle that prevents too much water loss through transpiration
• Osmosis in animal cells

  In hypertonic solution where the solution has a high solute concentration. Animal cells become shriveled and cremated. This is because the animal cells would have a higher water potential than the solution
• Osmosis in animal cells part 2

  In hypotonic solution where the solution would have a high concentration of water so it would have a larger water potential than the animal cells. This would cause the cells to lyse (burst)
• Osmosis in plant cells

  In hypertonic solution where the plant cells would have a higher water potential than the solution. Water would move out of the plant cell and cause it to shrivel. The cell membrane becomes plasmolysed meaning it moves away from the cell wall
• Osmosis in plant cells

  In hypotonic solution, the plant cells have a lower water potential than the solution so water moves into the plant. However the plant cells doesn't burst it just becomes turgid due to its cell wall
• Factors affecting osmosis

  • Water potential gradient, the higher the water potential gradient (large difference) the faster rate of diffusion
  • Membrane thickness, the thinner the membrane the faster the rate of diffusion since it reduces the diffusion pathway
  • Surface area, the larger surface area to volume ratio the faster the rate of diffusion
• Water potential

  • It's symbol is ψ
  • It's measured in kPa
  • Pure water has a water potential of 0
  • Solution have a lower water potential than water so it would have a negative value
  • More solutes in a solution means a more negative water potential number
• Transport across cell membranes occurs through a variety of mechanisms to regulate the movement of molecules in and out of cells.
• Biological membranes

  Membranes are made of a phospholipid bilayer with hydrophobic fatty acids facing inwards and hydrophilic head out.
• Fluid - mosaic model


  The arrangement of phospholipids and proteins is known as the fluid mosaic model.

  This is because the phospholipids move relative to each other and the proteins are of various shapes and sizes.
• 6 components of the membrane

  • Glycolipids
  • Phospholipids
  • Glycoprotein
  • Cholesterol
  • Intrinsic proteins e.g. transport protein (channel and carrier proteins )
  • Extrinsic proteins
• Phospholipids functions

  • Forms the basic structure of the bilayer
  • Makes membrane flexible
  • Prevents passage of water-soluble molecules and allows the passage of a lipid-soluble molecules