1. The Cell Membrane
Biology A/Cells

2. The Cell Membrane (CM) Thin, protective covering found in all
In animal cells, this is the outermost layer of the cell.
In plant cells, it is located just inside the rigid cell wall.

3. The Function of the CM
The CM acts as a barrier to certain substances, but allows others to enter and leave the cell.
When a CM allows a certain substance to pass through it, it is said to be permeable to that substance.
If a CM allows the passage of some substances, but blocks others, it is said to be semi-permeable or selectively permeable.

4. Structure of the CM
Lipid Bi-Layer: The CM is made of two layers of lipids with proteins scattered throughout.

5. Proteins in the CM
Proteins Channels: Help specific substances pass through the CM
Carbohydrate Chains: Markers to identify which cells belong to your body and which are not

6. Semi-Permeable CM’s
Certain molecules are allowed to enter and leave the cell in the following ways:
Passive Transport
Active Transport

7. Passive Transport
Passive Transport is the movement of substances across a membrane WITHOUT using energy.
Diffusion = The movement of substances across the CM from an area of high concentration to an area of low concentration.
Osmosis = Diffusion of water molecules.

8. Active Transport
Active Transport is the movement of substances across a membrane that requires ADDING ENERGY.
One example, is when the molecules of a substance are too large to easily cross the membrane
Another example, is when substances need to be moved from an area of low concentration to an area of high concentration.

9. Homeostasis
The goal of both passive and active transport is to help organisms maintain homeostasis, which is a state of balance.
As a result of the movement of substances across the CM, cell size may change: