Membrane transport

• Simple diffusion

  Diffusion is the spreading out of particles in liquids and gases, and it happens because the particles are in random motion. The particles move from a high concentration to a low concentration- down the concentration gradient. It is a passive process, meaning that it does not need energy to occur. Particles pass through phospholipids in the plasma membrane, and can only happen if the phospholipid bilayer is permeable to those specific particles. Non-polar particles can diffuse through the bilayer easily, but polar particles such as ions cannot, due to the hydrophobic centre of the membrane.
• Example of simple diffusion


  Oxygen enters cells through simple diffusion, as it is non-polar and small enough to pass easily through the phospholipid bilayer.
• Facilitated diffusion


  Diffusion is the spreading out of particles in liquids and gases, and it happens because the particles are in random motion. The particles move from a high concentration to a low concentration- down the concentration gradient. polar molecules cannot easily pass through the membrane due to the hydrophobic centre of the membrane, and can enter the cell through facilitated diffusion, aided by channel proteins. they are intrinsic proteins with a channel within them. The channel is a pore that connects the cytoplasm to the aqueous solution outside of the cell. Both the diameter of the pore and its chemical properties determine that only one specific type of particle can pass through. Some channels can also be opened and closed. Small particles pass through easier than larger particles.
• Example of facilitated diffusion


  Glucose is transported through facilitated diffusion, as it is a polar molecule and is therefore unable to pass through lipid bilayer without the aid of channel proteins.
• Osmosis


  Osmosis is the net movement of water particles either in or out of a cell. water moves down the concentration gradient- from a high concentration of water (dilute solution) to a low concentration of water (concentrated solution). This is because when substances are dissolved in water, the form intermolecular bonds with it, restricting the movement of the water molecules. osmosis is a passive process, as no energy is expended to make it occur. Although the phospholipid bilayer of the plasma membrane is hydrophobic, water molecules are small enough to pass through it. Some cells (such as root hair or kidney cells) have water channels known as aquaporins, increasing membrane permeability to water.
• Example of osmosis


  Root hair cells contain aquaporins to aid osmosis. Root hair cells absorb water through osmosis that can then be carried through the transpiration stream of the plant via the xylem.
• Active transport


  Active transport is the transportation of particles through a cell membrane against the concentration gradient, using energy from cellular respiration to do so. Pump proteins are used to carry out active transport- they use energy in the form of ATP (from aerobic cell respiration) to move particles from a low concentration to a high concentration, in one direction (against the concentration gradient).
• Example of active transport


  The absorption of water and mineral ions in root hair cells via active transport is essential of the health and growth of plants
• Endocytosis


  Endocytosis is the process of transporting particles into a cell by engulfing it with its membrane. It is not a passive process, and requires ATP. It is also against the concentration gradient-materials are transported from a low concentration to a high concentration. Both pinocytosis and phagocytosis are modes of endocytosis. Pinocytosis (cell-drinking) is used to take in ECF (extracurricular fluid), and is when the cell surrounds particles and 'pinches off' their membranes, enclosing the particle within the pinocytotic vesicle, which is then usually fused with lysosomes to allow digestion. Phagocytosis (cell eating) is when phagocytes (WBCs) or single celled organisms engulf solid particles and digest them. they typically connect with lysosomes to digest the particle(s).
• Example of endocytosis


  While blood cells (phagocytes) perform phagocytosis when they engulf bacteria during defence against disease in the body
• Exocytosis


  Exocytosis is the process of transporting particles out of a cell through the fusion of vesicles with the cell membrane and expulsion of its contents. Secretory vesicles formed by the Golgi containing the secretory products fuse with the plasma membrane, which then expels the products inside of them. ATP from cellular respiration is required for this process, which is against the concentration gradient.
• Example of exocytosis


  Pancreas cells transport glucagon into the liver where it is broken down into glycogen and then glucose through exocytosis.
• Osmosis

  Osmosis is the net movement of water across a selectively permeable membrane from a hypotonic to hypertonic solution.

  The isotonic point is when there is an equal concentration of solute both inside the cell and in the solution (no net movement)

  Low water potential = hypertonic

  High water potential = hypotonic
• Osmosis

  Osmosis is a passive process, as no energy is expended to make it occur. Although the phospholipid bilayer of the plasma membrane is hydrophobic, water molecules are small enough to pass through it. Some cells (such as root hair or kidney cells) have water channels known as aquaporins, increasing membrane permeability to water. A cell is turgid when filled with water through osmosis. The cytoplasm pushes against the cell wall. When water leaves the cell through osmosis the cytoplasm pulls away from the cell wall, this process is called plasmolysis, making the cell flaccid.
• Osmolarity

  Osmolarity is defined as the measure of the concentration of solute particles in a solution- and this is done through the number of osmole of solute per litre of solution.

  An osmole is the number of particles in a solute that will split into ions when in solution.
• Receptors


  Some membrane proteins are receptors- they have bindings sites for chemical messengers such as hormones. The binding of the chemical messenger may cause the receptor to change shape in order to relay the message inside the cell.
• Receptor-mediated endocytosis

  • Specific molecules bind to receptors on the cell surface 
  • Receptor-ligand complexes form and gather in clathrin-coated pits
  • The membrane invaginates, forming vesicles that internalize the molecules