1. Membrane Function
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2. Essential Idea: Membranes control the composition of cells by active and passive transport
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Membrane Transport
Understandings:
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport
The fluidity of membranes allows materials to be taken into cells by endocytosis and released by exocytosis. Vesicles move materials within cells.
Applications:
Structure and functions of sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons
Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis
Skills:
Estimate the osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions

3. I. Cell Membrane controls what enters & exits the cell A
I. Cell Membrane controls what enters & exits the cell A. Semi-permeable: only certain materials may freely cross. Large and charged particles are usually blocked. B. Selective: Membrane proteins may regulate the passage of material that cannot freely cross

4. II. Passive Transport Does not require energy in the form of ATP
Does require a concentration gradient – a difference in concentration in two adjacent areas
Depends of size and charge of the molecules

5. Discuss as a table: How and why does molecule size influence molecular motion?

6. Diffusion Can occur through air, water, across a membrane, etc.
Molecules move constantly, randomly, and away from each other
Results in a NET movement of molecules from and area of higher concentration to an area of lower concentration
Results in the elimination of the concentration gradient - equilibrium

7. 5. The rate of diffusion can be influenced by a number of factors, including:
a. Temperature- affects kinetic energy of particles in solution.
b. Molecular size- larger particles are subjected to greater resistance within a fluid medium.
c. Steepness of gradient- rate of diffusion will be greater with a higher concentration gradient

8. C. Facilitated Diffusion
1. Substances diffuse through a substance-specific carrier protein or protein channel in the cell membrane
Why is this necessary?
***Still Passive! Still moving DOWN the concentration gradient!***

9. Discuss as a table: How/why are membrane proteins substance-specific?

10. a. Protein Channels
Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other.
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli.
Channel proteins have a much faster rate of transport than carrier proteins.

11. b. Carrier Proteins
Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane.
Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction.
Carrier proteins have a much slower rate of transport than channel proteins.

12. Osmosis
Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration
Water goes through aquaporins – specialized protein channels for water

14. Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentrations to high.

16. II. Active Transport
A. Requires ATP to move molecules against their concentration gradient (from low to high)

17. Allows cell to create and maintain a concentration gradient – can be used to do work
Requires a protein pump

18. Sodium-Potassium Pump

19. III. Vesicular Transport
Allows large molecules or groups of molecules to move across membrane
Relies on membrane fluidity
Taking in = endocytosis
Putting out = exocytosis

20. Two main types of endocytosis:
Phagocytosis- The process by which solid substances are ingested (usually to be transported to the lysosome)
Pinocytosis- The process by which liquids/dissolved substances are ingested

22. Discuss with a partner: Does the size of the cell membrane increase or decrease from endocytosis?

25. As a table, come up with at least one question that you would like answered or clarified.