1. Homeostasis and Cell Transport
Chapter 5

2. HSCE: B2.5h
Explain the role of cell membrane as a highly selective barrier (diffusion, osmosis, and active transport)

3. Passive transport systems
Diffusion-Movement of molecules from an area of higher concentration to an area of lower concentration
facilitated
Osmosis-Movement of WATER molecules across a cell membrane from an area of higher concentration to an area of low concentration.
Concentration Gradient-the difference in the concentration of molecules across a distance.
Substances naturally travel down their concentration gradient (high to low concentration).

4. Diffusion
In the absence of other influences, diffusion will eventually cause the molecules to be in EQUILIBRIUM
Cell membranes allow certain molecules to pass through, but not others. Depends on the size and type of molecule.
SIMPLE DIFFUSION- diffusion across a membrane

5. Facilitated Diffusion
Some molecules are too large or are not soluble in lipids (can’t pass the lipid bilayer). They need the help of CARRIER PROTEINS.
Does not require any extra energy to transport the molecules because they are still traveling from higher to lower concentration (passive transport).
molecule
Cell membrane
(lipid bilayer)
carrier protein

6. Facilitated Transport
Another type of passive transport carries ions across the membrane though proteins called ION CHANNELS.
Ions: sodium (Na+), potassium (K+), calcium (Ca2+), chlorine (Cl-)
Each type of ion channel is usually specific to a certain ion.
Some channels are always open, others have “gates”.
IONS
Ion channel

7. Osmosis Passive transport
In a solution, a solute (the substance being dissolved) is dissolved in a solvent (substance doing the dissolving).
Example:
SUGAR (solute) dissolves in WATER (solvent)
In cells the solutes are organic molecules and the solvent is water.
OSMOSIS: the process by which water molecules diffuse across a cell membrane from an area of higher to lower concentration.

8. Direction of Water movement across the membrane
OSMOSIS
Volume increases
Volume decreases
Direction of Water movement across the membrane

9. Hyper-, hypo-, & Isotonic
The pressure exerted by water inside the cell wall is called
TURGOR PRESSURE
When turgor pressure is gained and the cell bursts:
CYTOLYSIS
When turgor pressure is lost the cell shrinks away from the cell wall and the plant wilts:
PLASMOLYSIS
Plasmolysis
External solution is hypotonic to cytosol: water moves in…cell can burst
External solution is isotonic to cytosol: No net movement
External solution is hypertonic to cytosol: water moves out…cell shrinks

10. Active Transport systems
Active transport moves substances against the concentration gradient (from lower concentration to higher).
This requires energy most often supplied by ATP
Pumps
Sodium-potassium pump
Movement in vesicles
Move macromolecules and nutrients that are too large to pass the cell membrane or “packs” of small molecules at the same time.
Two types
Endocytosis
Exocytosis

11. Sodium-Potassium Pump
Step 1-3: 3 Na+ bind to protein from inside the cell, a phosphate is removed from ATP binding to the carrier protein and turning ATP into ADP. The protein changes shape releasing the 3 Na+ to the outside.
Steps 4-6: The protein then picks up 2 K+ from the inside, the phosphate is removed, changing the shape so that it releases the 2 K+ into the cell.

12. Sodium-potassium pump Pumps 3 Sodium ions out of the cell
Pumps 2 Potassium ions into the cell
This difference in charge is important for the conduction of electrical impulses along nerve cells.

13. Endocytosis
Cells ingest (take in) external fluid, macromolecules, and large particles.
External materials are enclosed in a “pouch” that pinches off from the cell and becomes a membrane-bound vesicle.
Two main types
Pinocytosis- transports fluids or solutes
Phagocytosis- transports large particles or whole cells.

14. Phagocytes
Many unicellular organisms feed by the process of phagocytosis.
Certain cells (Phagocytes) in animals use phagocytosis to ingest bacteria and viruses that invade the body.

15. Exocytosis
Process by which a substrate is released from the cell through a vesicle.

16. Homeostasis Cell transport Carrier Protein Is aided by which includes
Such as
Osmosis
Diffusion
Facilitated diffusion
Is aided by
Cell transport
which includes
Passive transport
Of substance
down their
Active transport
Of substances against their
Endocytosis
Exocytosis
Sodium-potassium pump
Concentration gradient
Which use a
Carrier Protein