transport across cell membranes

• phospholipid structure in membranes

  • hydrophyllic heads of both layers point to the outside of the membrane as it attracted to water on both sides
  • hydrophobic tails of both layers point to centre of membrane as repelled by water on both sides
• function of phospholipids

  • allow lipid soluble substances to enter and leave the cell
  • prevent water soluble substances entering and leaving the cell
  • make the membrane flexible and self healing
• proteins

  • some occur in the suface of the bilayer and never extend completely across it. they either give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones
  • other proteins completely span the phospholipid bilayer from one side to the other. some are protein channels, others are carrier proteins
• functions of proteins in the membrane

  • provide structual support
  • act as channels transporting water soluble substances across the membrane
  • allow active transport across the membrane through carrier proteins
  • form cell-surface receptors for identifying cells
  • help cells adhere together
  • act as receptors, for example for hormones
• cholesterol 


  occur within the phospholipid bilayer of the membrane. add strength to the membrane. they are very hydrophobic and therefore play an improtant role in preventing loss of water and dissolved ions. also pull together the fatty acid tails of the phospholipid molecules, limiting their movement without making the membrane too rigid
• functions of cholesterol

  • reduce lateral movement of other molecules including phospholipids
  • make the membranes less fluid at high temperatures
  • prevent leakage of water and dissolved ions from the cell
• glycolipids


  made up of a carbohydrate covalently bonded with a lipid. carbohydrate portion extends from phospholipid bilayer into watery environment outside cell to act as cell-surface receptor for specific chemicals
• functions of glycolipids

  • act as recognition sites
  • help cells attach to one another and so form tissues
  • help maintain stability of the membrane
• glycoproteins 


  carbohydrate chains are attached to many extrinsic proteins on outer surface of cell membrane. these glycoproteins also act as cell surface receptors, more specifically for hormones and neurotransmitters
• functions of glycoproteins

  • act as recognition sites
  • help cells attach to one another and so form tissues
  • allows cells to recognise one another eg lymphocytes recognising own cells
• permeability of cell-surface membrane


  most molecules do not freely diffuse across membrane as many are:

  • not soluble in lipids and so cannot pass through phospholipid bilayer
  • too large to pass through channels in the membrane
  • of the same charge as the charge on the protein channels so are repelled
  • electrically charged (polar) and so have difficulty passing through non-polar hydrophobic tails in phospholipid bilayer
• fluid-mosaic model of the cell surface membrane

  
• fluid-mosaic model as fluid 


  the phospholipid molecules can move relative to one another, making the membrane flexible
• fluid-mosaic model as mosaic 


  proteins embedded in phospholipid bilayer vary in shape, size and pattern in the same way as the tiles of a mosaic
• simple diffusion

  it is a passive process, meaning its energy comes from natural, inbuilt motion of particles rather than an external source like ATP.

  • all particles are constantly in motion due to their kinetic energy
  • this motion is random, with no set pattern to the way particles move around
  • particles are constantly bouncing off one another as well as off other objects e.g side of vessel
• diffusion definition


  the net movement of molecules or ions from a region of high concentration to a region of low concentration until equilibrium is reached
• facilitated diffusion 


  since large, polar molecules do not diffuse easily across the phospholipid bilayer, the movement of these molecules is made easier b protein carriers and channels. this process is called facilitated diffusion. it is also a passive process as relies on the kinetic energy of the diffusing molecules. it occurs down a concentration gradient and only occurs at specific points on the membrane
• protein channels

  • water-filled hydrophilic channels
  • allow specific water-soluble ions to pass through
  • they are selective, each openeing in the presence of a specific ion. remains closed if not present so there is control over entry and exit of ions
  • ions bind with the protein causing it to change shape in a way that closes one side of the membrane and opens the other
• carrier proteins

  • when a molecule specific to the protein is present, it binds with the protein, causing it to change shape in a way that the molecule is released on the inside of the membrane
• proteins involved in facilitated diffusion

  • protein channels
  • carrier proteins
• osmosis 


  the passage of water from a region of of higher water potential to a region of lower water potential through a partially permeable membrane
• water potential 


  the pressure created by water molecules

  • represented by the greek letter psi ($\Psi$)
  • measured in units of pressure, usually kiloPascals (kPa)
  • pure water has water potential of 0
• water potential

  • the addition of a solute to pure water will lower its water potential
  • the water potential of a solution must always be a negative value
  • the more solute added, the lower (more negative) the water potential
  • water will move by osmosis from a region of higher (less negative) water potential to one of lower (more negative) water potential
• osmosis explained

  • solute and water molecules are in random motion due to their kinetic energy
  • the membrane only allows water molecules across
  • water moves from higher to lower water potential (down a water potential gradient)
  • when water potential on either side of the membrane is equal, a dynamic equilibrium is established and there is no net movement of water
  • it is a passive form of transport
• osmosis and animal cells

  • red blood cell in water - absorb water by osmosis as it has lower water potential. cell surface membranes are thin and cannot stretch to any great extent, so membrane will break and burst cell. to prevent this, animal cells usually live in a liquid of the same water potential e.g blood plasma
• osmosis and plant cells

  • cell in liqiud of equal potential - no water enters or leaves, protoplast remains same size, cell condition is incipient plasmolysis
  • cell in liquid of higher water potential - water enters cell, protoplast swells, cell condition is turgid
  • water in liquid of lower potential - water leaves cell, protoplast shrinks, cell condition is plasmolysed
• active transport


  the movement of moleules or ions into or out of a cell from a region of lower concentration to one of higher concentration using ATP and carrier proteins
• ATP in active transport

  • directly moves molecules
  • co-transport - ATP individually moves molecules using a concentration gradient which has already been set up by active transport
• active transport differs from passive forms of transport as:

  • it is an active form of transport
  • metabolic energy (ATP) is needed
  • substances are moved against a concentration gradient
  • carrier protein molecules are involved
  • the process is very selective, with specific substances being transported
• active transport process:

  1. molecule or ion binds to receptor sites on carrier protein
  2. on inside of cell, ATP binds to protein, causing it to split into ADP and a phosphate molecule. causes protein to change shape and open on the opposite side of the membrane
  3. the molecule or ion is then released on the other side of the membrane
  4. phosphate molecule is released form the protein, causing protein to revert back to its original shape. phosphate recombinds with ADP to form ATP during respiration
• facts about active transport

  • more than one molecule/ion may be moved in the same/opposite direction at one time e.g the sodium-potassium pump
• microvilli

  • possessed by epithelial cells lining the ileum. about 0.6$\mu m$ in length
  • provide more surface area for the insertion of carrier proteins through which diffusion, facillitated diffusion and active transport can take place
  • increase rate of movement across membranes
• increasing movement rate across membranes

  • microvilli
  • increase number of protein channels and carrier proteins in any given area od membrane
• role of diffusion in absorbtion

  • as carbs and proteins are being digested continuosly, there is normally a greater glucose and amino acid concentration in the ileum than the blood.
  • therefore, there is a concentration gradient down which glucose moves by facilitated diffusion into the blood
  • given blood is constantly being circulated to the heart, the glucose absorbed is constantly being used by cells in respiration. this maintains gradient between ileum and blood, and rate of facililated diffusion across epithelial membranes is increased
• role of active transport in absorbtion

  • diffusion only causes equal concentration on either side of intestinal epithelium, so not all available glucose and amino acids can be absorbed in this way and some may pass out the body
  • this is prevented as glucose and amino acids are also being absorbed by active transport, so all glucose an amino acids should be absorbed into the blood
• sodium-potassium pump part 1

  1. sodium ions actively trasnported out of epithelial calls by pump into the blood. this takes place in a specific protein-carrier molecule in the membrane of the epithelial cells
  2. this maintains higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells
• sodium-potassium pump part 2

  3. sodium ions diffuse into epithelial cells down this concentration gradient gradient through a different type of protein carrier in the cell-surface membrane. as sodium ions diffuse in through this second carrier protein, they carry either amino acid or glucose molecules into the cell with them

  4. the glucose/amino acids pass into the blood olasma by facilitated diffusion using another type of carrier