1. The Plasma Membrane
Biology I

2. Lesson Objectives Describe how a cell’s plasma membrane functions.
Identify the roles of proteins, carbohydrates, and cholesterol in the plasma membrane.

3. Main Idea
A cell’s plasma membrane helps maintain homeostasis.

4. A Balancing Act
All living things must maintain a balance regardless of internal and external conditions. * HOMEOSTASIS *
Survival is dependent upon this act.
Certain nutrients must be available for every cell to receive.

5. Material Control
The plasma membrane is the boundary between the cell and its environment.
It allows for a steady stream of nutrients to come into the cell and regulates the amount of nutrients within the cell.
Ex. Bouncer/security guard

6. Permeability
The key property of the plasma membrane is selective permeability.
This is the process by which the membrane allows some molecules into the cell and keeps others out.
Certain molecules, such as water, can enter at any time. Ions however must be regulated by certain channels and times when it can enter.

7. Structure of the Plasma Membrane

8. Structure of the Plasma Membrane
A phospholipid bilayer
Phospholipids have the same general structure as a lipid, with one exception.
Glycerol backbone
2 fatty acid tails
Phosphate group attached in place of the third fatty acid tail

9. Structure of the Plasma Membrane

10. Structure of the Plasma Membrane
The phosphate group greatly alters the lipid because the phosphate group is polar.
The phospholipid bilayer forms a barrier that is polar on the surface and nonpolar in the middle.
The polar group allows the cell membrane to interact with watery environments.
However, substances that can dissolve in water will not pass through the plasma membrane because they are stopped by the nonpolar middle.
Allows the environment inside the cell to be separate from the environment outside the cell

11. Structure of the Plasma Membrane
Moving with and among the phospholipids are cholesterol, proteins, and carbohydrates.
Receptor proteins transmit signals to the inside of the cell.
Proteins at the inner surface anchor the membrane to the cell’s internal support structure, giving the cell shape.

12. Structure of the Plasma Membrane
Transport proteins span the membrane and move needed substances or waste materials through the membrane.
Cholesterol is positioned among the phospholipids and helps to prevent the fatty-acid tails of the bilayer from sticking together.
Carbohydrates and identity markers define the cell’s characteristics and help cells identify chemical signals.

13. The Fluid Mosaic Model
The model of the plasma membrane is called the fluid mosaic model.
The membrane is flexible.
The phospholipids and proteins are not held in one spot; they are free to move around like a boat on water.
The components are in constant motion

14. The Fluid Mosaic Model

15. Where’s the Membrane?
Membranes are found surrounding the entire cell and the organelles within.
Holds the cytoplasm away from the internal workings of the organelles
Both prokaryotic and eukaryotic cells have plasma membranes.