Membrane Structure

Membrane Structure The Dawson – Danielli model proposed in 1935 used a lipid bi-layer model Model suggest it was covered on both sides by a thin layer of globular protein

Membrane Structure Led by the Dawson – Danielli Model, Singer and Nicolson proposed that proteins are inserted into the phospholipid layer and do not form a layer on the phospholipid bilayer surfaces Believed that the proteins formed a mosaic floating in a fluid layer of phospholipids With only slight changes the Singer and Nicolson model was adopted and the Fluid Mosaic Model is used today.

Fluid Mosaic Model

Phospholipids Each phospholipid is composed of a three-carbon compound called glycerol Two of the glycerol carbons have fatty acids Non-polar and not water soluble - Hydrophobic The third carbon is attached to a highly polar organic alcohol that includes a bond to a phosphate group Organic alcohol with phosphate is highly polar and water soluble – Hydrophilic (amphipathic)

Phospholipids Have two regions, with different properties: 2 hydrocarbon tails which are Hydrophobic A phosphate head, that is negatively charged to which Hydrophilic

Phospholipids Bilayer When mixed in water they become arranged in double layers Heads face outward and tails inward This is a stable structure because of the bonds that form between phosphate heads and the surrounding water. Tails form hydrophobic interactions This is a weak intermolecular interaction Look at the combinations of bonds collectively as strong

Phospholipids Bilayer

Membrane Structure Cholesterol Is a component of animal cells Have a role in determining membrane fluidity, which changes with temperature Cholesterol restricts the movement of phospholipid molecules – reduces the fluidity Allow membranes to function effectively at a wide range of temperatures Also reduces permeability Plant cells do not have cholesterol molecules Plants depend on saturated and unsaturated fatty acids

Membrane Structure Proteins: Create diversity in membrane function Proteins of various types are embedded in the fluid matrix of the phospholipid bilayer This creates the mosaic effect referred to in the fluid mosaic model

Proteins Integral Proteins – embedded in phospholipids Show amphipathic character (both hydrophobic & hydrophilic regions) Hydrophobic areas are towards the middle Hydrophilic are on the ends close to water Peripheral Proteins – loosely attached to the surface of the membrane Often anchored to an integral proteins

Membrane Protein Functions Hormone Binding Site Insulin receptor Immobilized enzymes with the active site on the outside In the small intestine Cell Adhesion Cell to Cell communication Channels for passive transport Pumps for active transport Uses ATP

Membrane Protein Functions

Membrane Transport There are 2 types of cellular transport: Passive Transport Active Transport Passive transport Requires no energy Moves from down the concentration gradient Some molecules pass through the membrane Some molecules use channels for facilitated diffusion

Diffusion Diffusion can only occur across the membrane if the phospholipids bi-layer is permeable to the particles Hydrophobic center does not let ions with + or – charges to pass easily Polar molecules with partial + / - charges over the surface can diffuse at slow rates Small particles can pass more easily than large ones Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration

Diffusion Membranes allow some substances to diffuse through but not others. Partially Permeable Some substances move between the phospholipid molecules in the membrane Simple Diffusion

Facilitated Diffusion Channels in the membrane in which ions and other particles can pass into or out the cell Channels can be a single or group of protein molecules The diameter and properties (like charge) ensure that only one type of particle passes Involves specific carrier proteins that can change shape to accomplish this task but requires no energy.

Osmosis Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration (high water concentration) to a region of higher solute concentration (low water concentration). Water goes to the direction of the solute

Osmosis Equal concentration of solute Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

Estimating Osmolarity A concentration gradient of water allows the movement to occur as a result of solute concentration. Hypertonic – a solution with higher concentration of solute. Hypotonic – a solution with lower solute concentration Water will go from Hypotonic (hypo = below) to Hypertonic (hyper = above) Isotonic – solution in which the concentration of solute are equal. Iso = equal

Estimating Osmolarity The osmolarity of a solution is the number of moles of solute particles per unit volume of the solution. Pure water has an osmolarity of zero The greater the concentration of solutes the high the osmolarity

Osmosis in Donor Organs Osmosis can cause cells in human tissue or organs to swell up and burst. Shrink too due to net gain of loss of water by osmosis. To prevent this tissues/organs used in medical procedures such as kidney transplants must be bathed in a solution with the same osmolarity as human cytoplasm. Solution is called Isotonic Saline

Membrane Transport Active Transport Cells sometimes take in substances even when there is a higher concentration of the substance inside the cell than outside The substance is absorbed against the concentration gradient Cells can even pump cells out even though there is a larger concentration outside Energy is needed for this to occur ATP is required

Pump Proteins Protein pumps in the membrane are used for active transport. Each pump only transports particular substances This way the cell can control what is absorbed and what is expelled Pumps work in a specific direction

Sodium-Potassium Pump In axons from neurons they contain a pump protein that moves sodium ions and potassium ions across the membrane. Antiporter – sodium and potassium are pumped in opposite directions Energy comes from ATP One ATP provides enough energy to pump 2 potassium ions in and 3 sodium ions out of the cell. The concentration gradients are needed for the transmission of nerve impulses in axons

Sodium-Potassium Pump A mechanism for moving sodium (Na) and potassium (K) ions It has 5 stages: A specific protein binds to three intracellular Na ions The binding of Na ions causes phosphorylation by ATP. ATP has three attached phosphates. When it carries out phosphorylation, one phosphorylation, one phosphate is lost resulting in a two phosphate compound called ADP. The phosphorylation causes the protein to change its shape, thus expelling Na ions to the exterior. Two extra cellular K ions bind to different regions of the protein and this causes the release of the phosphate group The loss of the phosphate group restores the protein’s original shape, thus causing the release of the K ions into the intracellular space.

Sodium-Potassium Pump

Endocytosis The mass movement INTO the cell by the membrane ‘pinching’ into a vacuole

Endocytosis Endocytosis can occur in three ways Phagocytosis - solids Pinocytosis - liquids Receptor-mediated endocytosis

Exocytosis The mass movement OUT of the cell by the fusion of a vacuole and the membrane Both are possible because the of the fluid properties of the membrane (able to break and reform easily, phospholipids not attached just attracted) Example of endocytosis can be found in secretory cells (saliva)