B 2.1 MEMBRANES & MEMBRANE TRANSPORT

• function of phospholipid bilayer

  providing a barrier to the movement of some substances into and out of the cell
• integral proteins


  amphipathic, embedded in bilayer and can be embedded across both layer and/or just one layer
• What are many integral proteins?

  transmembrane proteins - extend across both phospholipid layers
• peripheral proteins


  hydrophilic proteins that are attached to either the surface of integral proteins or to plasma membrane (via hydrocarbon chain) - can be inside or out of the cell
• What are peripheral proteins?

  bound to the surface of the membrane - reversable
• what do some peripheral membranes have

  single hydrocarbon chain attached to them (inserted into the membrane - anchoring protein to membrane)
• what do membranes have

  inner face and an outer face
• how are membrane proteins orientated

  so they can carry out their functions correctly
• why is membrane protein content variable

  due to function of membrane (active membrane = higher protein content)
• which organelles have highest protein contents

  mitochondria and chloroplasts
• membrane protein functions

  transport, receptors, cell adhesion, cell-to-cell recognition and immobilized enzymes
• what are the 2 types of transport proteins

  channel proteins and carrier proteins
• channel proteins


  form holes or pores through which molecules can travel
• carrier proteins


  change shape to transport a substance across the membrane
• Are transport proteins specific?

  yes - transport proteins are specific for the substances which they move - this allows only certain substances to pass through the cell membrane
• receptors


  binding of peptide hormones
• peptide hormones

  insulin, neurotransmitters or antibodies
• what does the binding create


  signal that triggers a series of reactions inside the cell
• immobilised enzymes

  integral proteins with the active site exposed on the surface of the membrane
• cell adhesion


  allows cells to attach to neighbouring cells within a tissue
• cell-cell recognition


  glycoproteins serve as identification tags that are specifically recognized by other cells
• What is simple diffusion?


  type of membrane transport that involves particles passing directly between the phospholipids in the plasma membrane
• Define simple diffusion

  The net movement, as a result of the random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration
• When does simple diffusion occur?

  when particles pass between phospholipids in the membrane (can only happen if phospholipid bilayer is permeable to the particles)
• what happens in the hydrophobic parts of the membrane which effect transport

  ions with charge cannot easily diffuse through (because they are polar)
• what are particles random movements caused by

  kinetic energy
• What is equilibrium?

  where particles are evenly distributed on either side of a membrane
• when would equilibrium occur in particles

  If diffusion takes place for a long enough time period
• which molecules can diffuse easily through phospholipid bilayers

  Non-polar particles such as oxygen and CO2 can diffuse through easily down their concentration gradient
• why cant ions easily pass through membranes

  centre of the membrane is hydrophobic so ions can't easily pass through
• diffusion of oxygen

  diffuse into cells from surrounding capillaries; due to respiration a concentration gradient forms
• diffusion of CO2

  carbon dioxide diffuses out of cells into the surrounding capillaries
• What factors determine diffusion

  temperature; surface area; concentration gradient; properties of molecules and ions
• Steepness of concentration gradient

  The greater the difference in concentration across a membrane, the higher the rate of diffusion
• temperature

  higher the temperature the higher the rate of diffusion as molecules have more kinetic energy
• surface area

  greater the surface area the higher the rate of diffusion
• properties of the molecules or ions

  Large molecules diffuse more slowly as they require more energy to moveUncharged molecules, e.g. oxygen, diffuse faster as they move directly across the phospholipid bilayerNon-polar molecules diffuse more quickly as they are soluble in the non-polar phospholipid bilayerAlthough polar molecules cannot easily pass through the hydrophobic part of the membrane, smaller polar molecules (e.g. urea) can diffuse at low rates
• what is osmosis

  the net movement of water molecules across a partially permeable membrane from an area of low solute concentration to an area of high solute concentration (high water potential to low water potential)
• what kind of process is osmosis

  passive process
• what is a passive process


  Process that doesn't require energy