L3 - transport across membranes

• How substances are transported in and out of cells across membranes:

  -diffusion

  -osmosis

  -active transport

  -secretory vesicles secrete contents outside the cell via exocytosis
• Passive processes:

  -simple diffusion

  -facilitated diffusion

  -osmosis
• Active processes:

  -active transport

  -bulk transport
• Simple diffusion:

  -net movement of molecules or ions from a region of high concentration to a region of low concentration

  -molecules diffuse down a concentration gradient

  -no metabolic energy is expended during diffusion, so it is an example of passive transport

  -the result is equilibrium (molecules or ions evenly spread out within a given space or volume)
• Diffusion through a membrane:

  -small non polar molecules in a high concentration on one side of the membrane can easily pass through the bilayer, fitting between spaces between phospholipids

  -lipid soluble molecules and very small molecules such as oxygen and carbon dioxide can diffuse directly through the lipid bilayer
• Factors that determine the rate of diffusion:

  -temperature

  -the steepness of the concentration gradient

  -stirring/moving

  -the surface area

  -distance/thickness

  -size of molecule or ion
• Temperature: higher temperature gives molecules or ions more kinetic energy. molecules move around faster, so diffusion is faster
• The steepness of the concentration gradient: the bigger the difference between two sides of the membrane, the quicker the rate of diffusion
• Stirring/moving: stirring a liquid or air currents moving gas, increases molecular movement and hence diffusion
• The surface area: the greater the surface area, the faster the diffusion can take place. this is because the more molecules or ions can cross the membrane at any one moment
• Distance/thickness: thicker membranes slow diffusion as there is a greater distance for the molecule to travel
• Size of molecule or ion: large molecules need more energy to get them to move so they tend to diffuse more slowly. non polar molecules diffuse more easily across a membrane than polar molecules because they are soluble in the non polar phospholipid tails
• Facilitated diffusion:

  -polar or large molecules pass straight through the bilayer

  -carrier proteins or protein channels allow the molecules to pass through

  -this is still a passive process as no energy is required
• Two types of protein molecule used in facilitated diffusion:

  -channel proteins

  -carrier proteins
• Channel proteins: transmembrane protein that forms a tunnel through the bilayer for ions (eg. Na+)
• Carrier proteins: change shape to help larger molecules move into and out of cells across the plasma membrane (eg. glucose)
• Osmosis:

  -diffusion of water molecules

  -the water particles move from an area of high water concentration to an area of low water concentration, down a concentration gradient

  -across a partially permeable membrane

  -passive process
• Refer to WATER POTENTIAL not water concentration
• Active transport:

  -the transport of molecules or ions across plasma membranes against a concentration gradient

  -requires energy

  -carrier proteins are involeved
• Active transport examples:

  -absorptions of mineral ions by plant roots

  -excretion of hydrogen ions and urea by kidneys

  -exchange of sodium and potassium ions in neurons and muscle cells

  -uptake of glucose and amino acids in the small intestine
• Bulk transport:

  -when extremely large substances need to be moved across a plasma membrane

  -requires energy

  -endocytosis is the bulk transport of materials into the cell

  -exocytosis is the bulk transport of materials out of the cell
• Two types of endocytosis:

  -phagocytosis for solids

  -pinocytosis for liquids
• Examples of bulk transport:

  -hormones released into bloodstream from endocrine glands

  -white blood cells engulf invading microorganisms by phagocytosis