Membranes, Permeability and Particle Movement

  1. Membrane Structure
  2. Phospholipid bilayer; Fluid Mosaic Model
  3. Proteins
  4. Classified by their position:
  5. Integral
  6. Peripheral
  7. Functions:
  8. Tunnels or carriers to let water-soluble substances in or out of the cell
  1. ID badges to identify cell as belonging to self
  1. Hold adjacent cells together
  1. Enzymes
  1. Receptors: recognize important substances in the ExtraCellular Fluid and send a message to the interior of the cell (“signal transduction”). For example, a cell in your body has receptors that recognize the hormone insulin. When a receptor on the cell recognizes insulin in the ECF, it will tell the cell to take in glucose, because there is plenty in the blood.
  1. Anchors for the cytoskeleton
  1. Carbohydrates; glycocalyx

-Glycoproteins- Cell Adhesion Molecules

-Functions- cell ID, communication, and “Velcro” holding cell in place

  1. How Cells get stuff in and out (membrane permeability and particle movement
  2. Overview:
  3. Hydrophobic substances can simply cross the cell membrane. Examples: O2, CO2, alcohol, cholesterol, fat-soluble vitamins
  4. Hydrophilic substances must be transported through tunnels in the membrane. Examples: ions (Na+ etc), glucose, amino acids, water-soluble vitamins
  5. When a cell wants to move large amounts of, and/or large substances, in or out, it uses endocytosis/exocytosis. Examples: secretory products like mucin (exocytosis), food particles like a bacterium (endocytosis)
  1. Background: why stuff moves in the direction it does (diffusion and osmosis)
  2. Diffusion- describes the movement of solutes.

-because of random motion of particles, particles tend to spread out and move from areas of high concentration to areas of lower concentration, until they are, on average, approximately equally spaced (equilibrium). That is, they move down (or with) their concentration gradient. When particles move down their concentration gradient, energy is released (as if the particles slow down). [A concentration gradient can be considered a form of potential (stored) energy.]

-Each type of particle generally responds to its own gradient (example, CO2 will respond to its own gradient regardless of the gradient of O2)

-When a cell allows a solute to move in or out, following its concentration gradient, the cell does not expend energy. This is called “passive transport.”

-If a cell needs to move a solute from low concentration to high concentration (that is, if it needs to move the solute AGAINST its concentration gradient), the cell will have to expend energy by using one of its ATP batteries.

  1. Osmosis- describes the movement of water molecules

-Water, like solutes, responds to its own concentration gradient.

-Comparing 2 glasses of water: same volume. One (glass A) has only H2O, one (glass B) has H2O plus NaCl. There is more water in glass A; therefore, the concentration of WATER is higher in glass A. Water, if allowed, will spread out from A to B.

-Some terminology: hypertonic, hypotonic, isotonic. These are relative terms, you have to be comparing 2 solutions in order to use them.

-In this class, the compartments we are interested in comparing are the Extracellular Fluid (ECF) and the Intracellular Fluid (ICF)

  1. Using proteins to get hydrophilic substance across the membrane (each protein only allows a specific solute to pass)
  1. Facilitated Diffusion: passive transport using the help of proteins. (no energy required!)
  2. Channel proteins: act as a tunnel. These are used for water and certain ions.

-The water tunnels are called aquaporins, and most cells have lots of these, so water can move freely back and forth by osmosis.

-Some ion channels are always open, and the ions move in and out by diffusion. Many, however, are gated, and only open under certain circumstances (for example, if the electrical voltage changes).

  1. Carrier proteins: act as a shuttle. When they bind to the substance they are supposed to transport, they change shape and move the substance through.
  1. Active transport- similar to carrier proteins, but the products are being moved AGAINST their gradients, therefore ATP is required. (Binding of Pi causes shape change that allows solute/s through)

-primary

-secondary

  1. Coupled transport- 2 different types of solutes are moved at the same time. May be facilitated or active; often used as part of a secondary active transport system.

-Cotransport- both move in the same direction

-Countertransport- they are moved in opposite directions

-Coupled transport sometimes moves both substances against their gradients (requires ATP), sometimes moves both substances with their gradients (does not require ATP), and sometimes moves one substance with its gradient and the other one against. In this case, the energy released by the first is used to carry the other!

This is secondary active transport.

  1. Endocytosis and Exocytosis: bulk transport. Both require ATP regardless of gradients.
  2. Exocytosis- moving stuff out. Ex, mucin
  3. Endocytosis- drawing stuff in
  4. Phagocytosis- ex, bacteria
  5. Pinocytosis- ex, ECF
  6. Receptor-mediated endocytosis- ex, LDLs, folate

e. The membrane potential-Cells maintain a slightly negative charge interior to the membrane relative to the charge outside of the membrane (by putting more positive ions outside the membrane). By keeping charged particles separated, the cell maintains an electrical potential, which is a form of potential (stored) energy. One of the primary mechanisms by which animal cells do this is the sodium-potassium exchange pump, which is a carrier protein that pumps 3 sodiums out and 2 potassiums in at a time. The concentration & electrical (electrochemical) gradient for sodium is particularly high, and if it could, it would rush into cells. The volt is a unit of measure of electrical potential. Each cell type has a characteristic resting electrical potential measured in mV