1. Cell Membrane: Structure and Function
Chapters
Objectives

2. Cell Membrane
Structure

3. A cell cannot survive if it is totally isolated from its environment.
Maintaining a consistent interior environment is crucial to the survival of the cell. Why?

4. HOMEOSTASIS
is the process by which a constant internal environment is maintained despite changes in the internal and external environment.

5. The cell membrane acts as a barrier separating the cell from its external environment.
Border control for the cell!

6. The cell maintains a consistent internal environment
by controlling the movement of substances in and out of the cell

7. It controls the ease with which substances pass into and out of the cell.
Some substances easily cross the membrane, while others cannot cross at all.

8. Find a video Cell membranes are mostly composed of;
LIPIDS, PROTEINS, and CARBOHYDRATES
PHOSPHOLIPIDS are the most abundant lipids in most membranes.

9. What if the cell membrane was only phospholipids?
hardly anything could move in or out of the cell
proteins act as channels to let bigger substances move in and out

10. Passive Transport
Diffusion and Osmosis

11. Simple Diffusion The cell membrane is said to be SELECTVELY PERMEABLE
because SOME things can pass through while others cannot.
Like Nylon or Gore-Tex

12. Which molecules can pass?
Many small, uncharged molecules can pass through the membrane freely. They simply diffuse from one side of the membrane to another.
See figure 3, page 59
Which molecules can pass?
Water
Oxygen
Carbon dioxide
Non-charged molecules
Which cannot?
Macromolecules (nucleic acids, lipids)
Glucose
Charged molecules and ions

13. Simple diffusion = the movement of particles from an area of high concentration to low concentration. (Tries to “equal out”.)
A concentration gradient is the difference in concentration between two areas. Diffusion always occurs WITH the concentration gradient.
Osmosis = the diffusion of WATER molecules from high to low concentration across a membrane

14. Three Conditions for Osmosis
A few special terms are commonly used to describe differences in solute concentration.
Isotonic = solutions have equal solute concentrations. Movement of water (osmosis) occurs equally in BOTH directions across a membrane.
Hypertonic = solutions have higher solute concentrations
Hypotonic = solutions have lower solute concentrations
*See handout questions.

15. What is hemolysis?
What happens to red blood cells in each of the three conditions above? *Pg. 63

16. Facilitated Diffusion
Some large polar substances like glucose or large ions (charged) such as sodium and chloride cannot go through the membrane by simple diffusion because they cannot pass easily through the hydrophobic middle region of the membrane.
To compensate, some proteins span the lipid bilayer and act as a pathway for large or charged materials to pass. These such proteins are called transmembrane proteins (carrier proteins).
The process by which large molecules pass through a transmembrane protein to gain entry to the cell is called facilitated diffusion.
No energy is used, but the natural diffusion (with the concentration gradient) is facilitated by the protein.

17. Facilitated Diffusion!

18. Your Task Let’s Summarize the findings of our demo. Read pages 59-64
Answer questions 1, 3, 5-8, 10