1. Homeostasis and Transport
Chapter 5 Notes
Homeostasis and Transport

2. Objectives
SC-HS-3.4.3: Students will describe cell regulation (enzyme functions, diffusion, osmosis, homeostasis); predict consequences of internal/external environmental change on cell function/regulation.

3. Passive Transport (Play video: United Streaming)
The movement of substances across the cell membrane without any input of energy
2 types: diffusion and osmosis

4. Diffusion
Movement of molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient)
Ex. People in a crowded room; air freshener
Result: concentration of molecules will be the same throughout (even distribution), reaching equilibrium
Molecules are still in motion but as one molecule moves in one direction another moves in the opposite direction resulting in no net movement

5. Diffusion Across Membranes
Cell membranes allow some molecules to pass through, but not others
2 forms: facilitated diffusion (see p. 99) and diffusion through ion channels

6. Facilitated Diffusion
For molecules that can’t freely cross the cell membrane (too large, not soluble in lipids)
Carrier proteins in the cell membrane pick up the molecules from outside the cell and transport them across the membrane into the cell, like a taxi
Movement is always down the concentration gradient (from high to low concentrations) so no additional energy is needed

7. How it works: carrier protein binds to molecule, causing the protein to change shape and protecting the molecule from the inside of the lipid bilayer. Once across the membrane, the protein releases the molecule into the cell and returns to its original shape.
Ex. Glucose – too big so must be transported
Carrier proteins are specific for a molecule. The protein that carries glucose won’t carry any other molecule than simple sugars.

8. Diffusion Through Ion Channels
Provide small passageways across the cell membrane for ions to diffuse.
Ions are insoluble in lipids so can’t freely cross the lipid bilayer of the cell membrane
Just like carrier proteins are specific for each molecule, ion channels are also specific for the ions that they allow to cross.
Ex. Na+ channels only allow Na+ to pass and will block Cl- and Ca 2+

9. Osmosis
Process by which water molecules move from an area of higher concentration to an area of lower concentration by diffusing across a cell membrane
No energy is required

10. Types of Solutions Background - Solution = solute dissolved in solvent
- Water is the universal solvent
- Ex. Sugar water; sugar (solute), water (solvent)
- Ex. Saltwater: salt (solute), water (solvent)

11. Hypertonic solution
- Concentration of solute outside the cell is higher than the concentration inside the cell
- Water will move out of the cell

12. Hypotonic solution
- Concentration of solute outside the cell is lower
than concentration of solute inside the cell
- Water diffuses into the cell

13. Isotonic solution
- Concentration of solute outside and inside the cell is equal
- No net movement of water
- Molecules move in and out of the cell at the same rate

14. If solution outside the cell is hypotonic (few solutes) to the inside, then the inside must be hypertonic (lots of solutes) to the outside
If the solution outside is hypertonic (lots of solutes) to the inside, then the inside must be hypotonic (few solutes) to the outside
Water tends to move from hypotonic (few solutes) to hypertonic solutions (lots of solutes).

15. How Cells Deal With Osmosis
Unicellular, freshwater organisms
More water outside the organism than inside, so water wants to move in
Some have contractile vacuoles that pump extra water out of the organism (requires energy)

16. Others pump solutes out of the cytoplasm to increase the amount of solutes in the environment and create an isotonic solution or at least close to an isotonic solution, stopping or slowing down osmosis into the organism (no energy required)

17. Plants
- Water enters roots and causes the cells to expand
They continue to expand until they become pressed up against the cell walls
The cell wall resists the pressure of the “full” cell membrane and osmosis stops

18. Active Transport (Play video: United Streaming)
The movement of molecules from an area of lower concentration to an area of higher concentration (against the concentration gradient)
Requires energy
Accomplished by carrier proteins, called cell membrane pumps

19. The process Carrier protein binds to molecule and changes shape
The protein protects the molecule from the lipid bilayer of the cell membrane
The molecule is released and the protein returns to its original shape

20. Example Sodium (Na+) - Potassium (K+) Pump
 Protein transports Na+ and K+ up concentration gradients
Normally: high concentration of Na+ outside the cell and high concentration of K+ inside cell
 3 Na+ bind to carrier protein inside cell and a phosphate group is taken from ATP (forming ADP) and also binds to the carrier protein
- The splitting of ATP provides energy to cause protein to change shape

21. With shape change, protein carries Na+ across membrane and releases them outside cell
Carrier protein has shape needed to bind to 2 K+ outside cell which releases the phosphate group and causes the carrier protein to change shape again
The carrier protein delivers the 2 K+ inside cell and releases them

22. The result:
 Each cycle transports 3 Na+ outside cell and 2 K+ inside cell
 More + charges outside than inside so electric gradient results (+ outside, - inside)
 Electric gradient important for conducting electrical impulses along nerve cells

23. Transporting Macromolecules
Also transporting food particles and large numbers of small particles at the same time
2 processes: endocytosis and exocytosis

24. Endocytosis
Cells ingest external fluid, macromolecules, large particles
Cell membrane folds in on itself and forms a pouch that holds the large molecules
Pouch pinches off and forms a vesicle, which fuses with lysosomes and are digested or fuse with other organelles

25. 2 types: pinocytosis and phagocytosis
Pinocytosis: transport of solutes or fluids into the cell
Phagocytosis: the movement of large particles or whole cells (often surrounds food and viruses and bacteria)

26. Exocytosis The reverse of endocytosis
Vesicles in the cytoplasm fuse with cell membrane and release contents outside cell
Used to release large molecules such as proteins
Ex. Nerve impulses