Transport across membranes

• Sodium-potassium pump


  - Sodium ions are actively transported out of epithelial cells by the sodium potassium pump into the blood. In a protein-carrier molecule found in the cell surface membrane

  - This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.

  - Sodium ions diffuse into the epithelial cells down the concentration gradient through a co-transport protein in the cell surface membrane. As the sodium ions diffuse they carry either amino acid molecules or glucose molecules in the cell with them.

  - The glucose/amino acid pass into the blood plasma by facilitated diffusion using another type of carrier.
• Role of active transport in absorption

  - cotransport

  - glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium potassium pump
• Role of diffusion in absorption

  - Molecules absorbed due to the concentration gradient

  - Glucose absorbed into the cells as cells are constantly respiring

  - Maintain concentration gradient
• Active transport ATP is used in:

  - Directly move molecules.

  - Individually move molecules using a concentration gradient which has already been set up by (direct) active transport. This is known as co-transport.
• Active transport


  Movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
• Understanding water potential


  - the highest value of water potential, that of pure water, is zero, and so all other values are negative

  
  

  - the more negative the value, the lower the water potential
• Osmosis


  Passage of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.
• Facilitated diffusion (passive transport)


  Movement of molecules across a membrane via transport proteins without energy from ATP hydrolysis.
• Simple diffusion (passive transport)


  Nonpolar lipid soluble substances diffuse directly through the phospholipid bilayer. (Solids Only) high to low concentration
• Passive transport


  Exchange of substances between cells and the environment occurs that does not require metabolic energy
• Active transport

  Exchange of substances between cells and the environment occurs that require metabolic energy
• Fluid mosaic model

  Fluid - individual phospholipid molecules can move relative to one another. Flexible structure that is constantly changing

  Mosaic - proteins are embedded in the phospholipid bilayer vary in shape, size and pattern.
• Permeability of the cell surface membrane

  - controls movement in and out

  - most molecules don't freely diffuse across it because they are:

  not soluble in lipids

  too large to pass through channels

  of the same charge so are repelled

  polar

  hydrophobic tails
• Functions of glycoproteins in the membrane

  - Act as recognition sites.

  - Help cells to attach to one another and so form tissues.

  - Allows cells to recognise one another, for example lymphocytes can recognise an organism's own cells.
• Glycoproteins


  - carbohydrate chains are attached to many extrinsic proteins on the outer surface of membrane

  - act as cell surface receptors specifically to hormones and neurotransmitters
• Functions of glycolipids in the membrane


  - acts as recognition sites

  - help maintain the stability of the membrane

  - help cells to attach to one another and so form tissues
• Glycolipids


  - carbohydrate covalently bonded with lipid

  - carbohydrate portion extends from bilayer into watery environment outside cell and acts as cell surface receptor
• Functions of cholesterol in the membrane


  -reduce lateral movement of other molecules including phospholipids

  -make the membrane less fluid at high temperatures

  -prevent leakage of water and dissolved ions from the cell
• Cholesterol

  Add strength, very hydrophobic, pull together the fatty acid tails of the phospholipid molecules
• Functions of proteins in the membrane

  - provide structural support

  -act as channels to transport water-soluble substances across

  - allow active transport across the membrane through carrier proteins

  - form cell-surface receptors for identifying cell

  - help cells adhere together

  -act as receptors, e.g. for hormones
• Carrier proteins

  Bind to ions or molecules like glucose and amino acids, then change shape inorder to move these molecules across the membrane. Span the membrane. Binds with the protein, changes shape so the molecule is released to the inside of the membrane. No external energy needed. Higher concentration to lower concentration.
• Proteins channels

  Form water filled hydrophilic channels across the membrane to allow water soluble ions to diffuse across the membrane. Channels are selective to ions. The ion binds with the protein causing it to change shape so it closes to one side and open the other.
• Functions of phospholipids

  - Allow lipid soluble substances to enter and leave the cell

  - Prevent water soluble substances in and out of the cell

  - Make the membrane flexible and self-sealing
• Phosolipids

  Hydrophilic phosphate head, hydrophobic hydrocarbon (fatty acid) tails
• Xylem


  Found in plants. Used to transport water and mineral ions throughout the plant and also gives mechanical support.