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Chapter 6

Interactions Between Cells and the Extracellular Environment


Extracellular Environment
The extracellular environment surrounding cells consists of a fluid compartment, in which molecules are dissolved, and a matrix of polysaccharides and proteins that give form to the tissues. Interactions between the intracellular and extracellular environment occur across the plasma membrane.
Body Fluids The water content of the body is divided into two compartments. Approximately 60 percent of the total body water is contained within the cells, in the interacellular compartment. The remaining 33 percent of total body water comprises the extracellular compartment. About 20 percent of this extracellular fluid is contained within the vessels of the cardiovascular system, where it comprises the fluid portion of the blood, or blood plasma.

Extracellular Matrix
The cells that comprise the organs of our body are embedded within the extracellular material of connective tissues. This material is called the extracellular matrix, and it consists of protein fibers collagen and elastin, as well as gel-like ground substance. The interstitial fluid referred to previously exists primarily in the hydrated gel of the ground substance. This is like glue! Cells will stick together.

Categories of Transport Across the Plasma Membrane
Passive transport is the net movement of molecules and ions across a membrane from higher to lower concentration (down a concentration gradient); it does not require metabolicenergy. Passive transport includes simple diffusion, osmosis, and facilitated diffusion.

Active transport is net movement across a membrane that occurs against a concentration gradient (to the region of higher concentration). Active transport requires the expenditure of metabolic energy (ATP) and involves specific carrier proteins. This kind of transport takes energy, because it is moving from a low to a high concentration.

Not all molecules can pass through the cell membrane, amino acids, DNA, nucleic acids. Others such as gas and water molecules pass through quickly.

Permeable- passable; allowing fluid to penetrate or pass through it.
Semipermeable- Permitting passage of only certain molecules. (example: Squirrel on the screen door)

Diffusion and Osmosis
  • Simple diffusion, or diffusion, is the net movement of substances from an area of higher concentration to an area of lower concentration. This movement occurs as a result of the random and constant motion characteristic of all molecules, (atoms or ions) and is independent from the motion of other molecules. Since, at any one time, some molecules may be moving against the gradient and some molecules may be moving down the gradient, although the motion is random, the word "net" is used to indicate the overall, eventual end result of the movement. To stop the molecules you would need to freeze them.
  • Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. Water can pass freely through the plasma membrane without the aid of specialized proteins.
  • Osmosis is the diffusion of water molecules through a selectively permeable membrane. When water moves into a body by osmosis, hydrostatic pressure or osmotic pressure may build up inside the body Interactions between Cells and the extracellular environment
  • Tonicity is used to describe the effect of a solution on the osmotic movement of water. For example, if an isosmotic glucose or saline solution is separated from plasma by a membrane that is permeable to water, but not glucose or NaCl, osmosis will not occur. In this case the solution is said to be isotonic to plasma. A 0.3 m glucose solution, which is 0.3sm, or 300 milliosmolal (300 mOsm), has the same osmolality and osmotic pressure as plasma. The same is true of a 0.12 m NaCl solution, which ionizes to produce a total concentration of 300 mOsm. Both of these solutions are used intravenous infusions, labeled 5% dextrose (5g of glucose per 100ml, which is 0.3m) and normal saline (0.9g of NaCl per 100 ml, which is 0.12 m). Since 5% dextrose and normal saline have the same osmolality as plasma, they are said to be isomatic to plasma.
  • Isotonic-a solution having the same total solute concentration, osmolality, and osmotic pressure as the solution with which it is compared. A solution with the same solute concentration and osmotic pressue as plasma.
  • Hypotonic- Water enters the cell, and can cause the cell to burst.
  • Hypertonic-When cells are place in hypertonic solution (such as salt water) which contains osmotically active solutes at a higher osmolality and osmotic pressure then the plasma, the cells will shrink, the water within the cell is being pulled out.
    • Sodium-Potassium pump (Na+ - K+)
    • Uses ATP to pump 3 Na+ out of the cell and 2 K+ into the cell.
    • Used to conduct signals along nerve cells.
    • Transports material into cells against the concentration gradient (active transport)
    • Coupled channels (example: Sodium wants to get back into the cell. The cells says "Okay, but you have to bring a sugar with you.")
  • Transport Proteins


    • Transport proteins in the plasma membrane transfer solutes such as small ions (Na+, K+, Cl-, H+), amino acids, and monosaccharides.
    • The proteins involved with active transport are also known as ion pumps.
    • The protein binds to a molecule of the substance to be transported on one side of the membrane, then it uses the released energy (ATP) to change its shape, and releases it on the other side.
    • The protein pumps are specific, there is a different pump for each molecule to be transported.
    • Protein pumps are catalysts in the splitting of ATP → ADP + phosphate, so they are called ATPase enzymes.
      • The sodium-potassium pump (also called the Na+/K+-ATPase enzyme) actively moves sodium out of the cell and potassium into the cell. These pumps are found in the membrane of virtually every cell, and are essential in transmission of nerve impulses and in muscular contractions.


    Carrier-Mediated Transport--analogy: an elephant carrying people and supplies
  • A carrier--with characteristics of specificity and saturation is required for this transport, which occurs from the blood into cells such as muscle, liver, and fat cells. This is passive transport because the net movement is to the region of lower concentrations and ATP is not required.
    Active transport is similar. Within the cell we have low calcium ions and on the outside we have a high concentration of calcium ions. Because we are going against the concentration. ie: going up hill you have to use energy. If you are going through a gate you can go through without using energy.
  • The Membrane Potential
    Cell Signaling

    Ch 6 Questions
    1. The rate of diffusion is dependent on which factors?
    a. temperature of the solution
    b. the permeability of the plasma membrane to the
    diffusing substance
    c. concentration differences across the two sides of the membrane
    d. a and b
    e. all of the above

    2. What statement for osmosis is NOT true?
    a. the simple diffusion of solvent (water) through a membrane
    more permeable to the solvent than to the solute
    b. osmosis depends on a difference in total solute concentration
    c. osmosis depends on the chemical nature of the solute
    d. none of the above

    3. In passive transport what type of energy is required?
    a. No energy required, passive transport is the net movement of molecules and ions across a membrane from higher to lower concentration.
    b. ATP
    c. Osmosis
    d. both b and c.

    4. What fibers have been likened to reinforcing iron bars in concrete?
    a. Collagen
    b. Integrins
    c. Elastins
    d. Collagens and Elastins

    5. A membrane which allows some molecules to pass into or out of a cell is?
    a. Carrier Mediated Transport
    b. Selectively Permeable Transport
    c. Passive Transport
    d. Facilitated Diffusion
    e. None of the Above

    6. Why are the Sodium-Potassium Pumps in cells so important?
    a. The extra sodium Na+ is used for "coupled transport" of other molecules.
    b. Sodium and Potassium are 2 of the electrolytes in the body and are essential for the nerves, and muscles.
    c. The active pumping of the sodium out of the cell prevents cell damage or from causing the cell to burst.
    d. All the above
    e. none of the above

    7. What would be an example of a semipermeable membrane
    a. a plastic bag
    b pantyhose
    c.glass
    d a plastic bucket
    e. b & c

    8. In what kind of solution will a RBC swell and possibly burst?
    a. hypertonic solution
    b. hypo-osmotic
    c. isotonic solution
    d. hypotonic solution

    9. The chemical driving force for a substance crossing a cell membrane,
    a. depends only on the concentration gradient, regardless of whether or not the substance is an ion.
    b. depends only on the concentration gradient if the substance is uncharged, but also depends on the electrical force if the substance is an ion.
    c. is the total driving force on the substance, even if it is an ion.
    d. is the force that pushes molecules across the membrane, but only if the substance is actively transported.
    e. always favors movement of a molecule into the cell.

    10. The osmotic pressure of a solution depends on
    a. the concentrations of all solute particles contained in it.
    b. the concentrations of all permeant solute particles contained in it.
    c. the concentrations of all impermeant solute particles contained in it.
    d. pressure exerted on the solution by the atmosphere.

    11. What do pumps and carriers have in common?
    a. They both transport molecules up electrochemical gradients.
    b. They both transport molecules down the electrochemical gradients.
    c. They both transport lipid-soluble substances preferentially.
    d. They both utilize ATP to transport molecules
    e. They both are specific to certain molecules.

    12. The movement of water across a plasma membrane occurs by: a. an active transport water pump.
    b. a facilitated diffusion carrier
    c. simple diffusion through membrane channels.
    d. all of these.





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