Cell membrane transport processes

• Vocabulary


  • Active transport
  • ATP energy
  • Blood antigen
  • Concentration gradient
  • Crenate
  • Diffusion
  • Endocytosis
  • Exocytosis
  • Facilitated transport
  • Glycolipid
  • Glycoprotein
  • Hemolysis
  • Hypertonic
  • Hypotonic
  • Isotonic
  • Na/K pump
  • Neuron
  • Osmosis
  • Osmotic pressure
  • Phagocytosis
  • Phospholipid
  • Pinocytosis
  • Polarity
  • Protein channels
  • Receptor sites
  • Selectively permeable
  • Solubility
  • Solute
  • Solvent
  • Surface area/volume ratio
  • Thyroid gland
  • Thyroxin
  • Tonicity
  • Turgor pressure
  • Villi
  • Viscosity
• Selectively permeable


  Controls what comes in and out of the cell. Does not let large, charged or polar things through without help.
• Plasma membrane


  • Fluid mosaic model: The phospholipids move, thus allowing small non-polar molecules to slip through.
  • Glycolipids and glycoproteins: Involved in cell-to-cell recognition, cell adhesion and message reception.
  • Integral proteins: Assists specific larger and charged molecules to move in and out of the cell. Can act as 'tunnels' or will change shape.
  • Cholesterol: Reduces membrane fluidity by reducing phospholipid movement. Also stops the membrane from becoming solid at room temperatures.
  • Cytoskeleton: A cytoskeleton acts as a framework that gives the cell its shape. It also serves as a monorail to transport organelles around the cell.
• Transport across the membrane

  1. Passive transport: Moves molecules from high to low in order to establish equilibrium. The molecules may or may not need to use a protein channel or carrier.

  2. Active transport: Moves molecules from low to high, against the concentration gradient and this process requires energy in the form of ATP.
• Simple diffusion


  A passive process (no energy required). Some substances will diffuse through membranes as if the membranes weren't even there. Molecules diffuse until they are evenly distributed. The molecules move from an area of high to low.
• Molecules that easily cross cell membranes by simple diffusion

  • Oxygen, carbon dioxide, alcohols, fatty acids, glycerol, and urea
• Factors that increase the rate of simple diffusion

  • Concentration: The greater the difference in concentration, the faster the diffusion.
  • Molecular size: Smaller substances diffuse more quickly. Large molecules (such as starches and proteins) simply cannot diffuse through.
  • Shape of ion/molecule: A substance's shape may prevent it from diffusing rapidly, where others may have a shape that aids their diffusion.
  • Viscosity of the medium: The higher the viscosity, the more slowly molecules can move through it.
  • Movement of the medium: Currents will aid diffusion. Cytoplasmic streaming (constant movement of the cytoplasm) will aid diffusion in the cell.
  • Solubility: Lipid-soluble molecules will dissolve through the phospholipid bilayer easily, as will gases like CO2 and O2.
  • Polarity: Water will diffuse, but because of its polarity, it will not pass through the non-polar phospholipids. Instead, water passes though specialized protein ion channels.
• Osmosis


  The diffusion of water across a selectively permeable membrane driven by a difference in the concentration of solutes on the two sides of the membrane.
• Osmosis


  • Requires no energy.
  • Is the net movement of water molecules from the area of high water to the area of low water until it is equally distributed.
  • Because membranes often restrict or prevent the movement of some molecules, particularly large ones, the water (solvent) must be the one to move.
  • To cross the membrane, water must move through a protein ion channel.
• Tonicity of a solution

  • Isotonic: The solution concentration is equal on both sides of the membrane. There is no net concentration difference across the cell membrane. Water moves back and forth, but there is no net gain or loss of water.
  • Hypertonic: The solution outside the cell is more concentrated than inside. There is more water inside the cell and the water will move out of the cell, causing the cell to shrink.
  • Hypotonic: The concentration inside the cell is more concentrated than outside. There is more water outside of the cell, and water will move into the cell, causing the cell to swell or burst.
• Facilitated transport


  Some molecules are not normally able to pass through the lipid membrane, and need channel or carrier proteins to help them move across. This does not require energy when moving from high to low (with the concentration gradient).
• Facilitated transport


  • Molecules that need help to move through the plasma membrane are either charged, polar, or too large.
  • The protein channels and carriers are very specific. Each protein channel or protein carrier will allow only one type of molecule to pass through it.
  • Many channels contain a "gate" which control the channel's permeability. When the gate is open, the channel transports, and when the gate is closed, the channel is closed.
• Active transport


  The movement of polar, large, and charged molecules moving against the concentration gradient (uphill). Requires energy in the form of ATP.
• Molecules that move by active transport

  • Ions (like Na+ and K+ in cells, and iodine) and sugars, amino acids, nucleotides
• Active transport


  • May be direct (primary) or indirect (secondary).
  • Direct (primary) active transport directly uses metabolic energy to transport molecules across a membrane (e.g. the Na/K pump).
  • Indirect (secondary) active transport uses a concentration gradient of another substance that has already been established through direct active transport.
• Examples of active transport

  • The thyroid gland accumulating iodine to manufacture the hormone thyroxin.
  • The Na/K pump in nerve membranes to restore electrical order after an impulse.
  • The proton pump in mitochondria to make ATP.
• Endocytosis


  The taking in of molecules or particles by invagination of the cell membrane forming a vesicle. Requires energy.
• Types of endocytosis


  • Pinocytosis (cell drinking): Small molecules are ingested and a vesicle is immediately formed.
  • Phagocytosis (cell eating): Large particles are invaginated into the cell (e.g. white blood cells 'eating' bacteria).
• Exocytosis


  The reverse of endocytosis. A cell releases the contents of a vesicle outside of the cell. Requires energy.
• Cell size


  • As a cell grows, its volume can increase faster than its surface area. If the cell gets too big, there will not be enough room on the plasma membrane for things to get in and out quickly enough to maintain the cell.