1. The Cell Membrane

2. Cell membranes are composed of two phospholipid layers.
The cell membrane, or the plasma membrane, forms a boundary between the cell and the outside environment.
It controls the passage of materials into and out of the cell.
The cell consists of a double layer of phospholipids interspersed with a variety of other molecules.

3. Phospholipids
A phospholipid is a molecule composed of three basic parts:
A charged phosphate (PO4-) head.
Glycerol
Two fatty acid chains
Together, the glycerol and the phosphate group form the “head” of a phospholipid.
The fatty acids form the tail.

4. Polarity of Phospholipids
The head of the phospholipid is polar because it bears a charge.
Water molecules are also polar.
Polar molecules love polar molecules, so water and the polar head of the phospholipid form a hydrogen bond.

5. Polarity of Phospholipids
The fatty acid tails are non-polar.
Polar molecules (like water) and non-polar molecules repel each other.
Therefore, non-polar tails are attracted to each other and repel water.

6. Sandwich Membrane
The polar (or non-polar) properties of the phospholipids causes them to arrange themselves in layers, like a sandwich.
Polar heads are like bread.
They form the outer surfaces of the membrane, where they interact with the watery environment both outside and inside a cell.
The non-polar tails are like the filling.
The are sandwiched between the layers of polar heads, where they are protected from the watery environments.

7. Molecules in the membrane.
Molecules are embedded throughout the cell membrane.
Cholesterol molecules strengthen the cell membrane.
Some proteins help materials cross through them membrane, others are components of the cytoskeleton.
Carbohydrates attached to membrane proteins serve as identification tags, enabling cells to distinguish one cell type from another.

8. Fluid Mosaic Model The cell membrane is flexible, not rigid.
The phospholipids in each layer can move from side to side and slide past each other.
This makes the cell membrane act like oil on the surface of water.
The proteins that stud the membrane act like an arrangement of colorful tiles with different textures and patterns.

9. Selective Permeability
The cell membrane allows some, but not all, materials to cross.
Selective permeability allows a cell to maintain homeostasis.
Because a cell need to maintain certain conditions to carry out its functions, it must control the import and export of certain molecules and ions.

10. Crossing the membrane. Molecules cross the membrane in several ways.
Some methods require the cell to expend energy, some do not.
How a particular molecule crosses the membrane depends on the molecules size, polarity, and concentration inside versus outside the cell.
Small non-polar molecules easily pass through the membrane.
Small polar molecules are transported via proteins.
Large molecules are moved in vesicles.
oxygen, carbon
dioxide, and other
small, nonpolar
molecules; some
water molecules
glucose and other large,
polar, water-soluble
molecules; ions (e.g.,
H+, Na+, K+, Ca++,
Cl–); water molecules

11. Chemical signals are transmitted across the cell membrane
A receptor is a protein that detects a signal molecule and performs an action in response.
It recognizes and binds to only certain molecules, which ensures that the right cell gets the right signal at the right time.
When receptors are activated (they combine with a hormone or neurotransmitter), they change shape.
This relays information to the cell.

12. Types of Proteins
Adhesion
The cell has many different types of proteins embedded inside it.
Adhesion proteins-Help the cell adhere to neighboring cells.
Communication proteins-chemical and electrical signals flow from one cell to the next.
Receptor proteins-gets a message from neighboring cells and communicates with the nucleus.
Recognition proteins-identify the cell as “self”
Passive transporters-have an open channel through which material may cross.
Active transporters-have a pump that can pull substances across against concentration gradients.
communication
receptor
recognition
passive
active

13. Intracellular Receptor
Intracellular receptors are inside the cell.
These generally react to hormones, which can pass through the cell membrane easily.
Each intracellular receptor only responds to certain chemicals, they are specific.
Intracellular receptors

14. Membrane Receptor
When a molecule cannot pass through the cell membrane, they need a membrane receptor to help them get the message through.
When the membrane receptor binds to a signal molecule, the entire membrane receptor changes shape.
As a result, it causes the molecules inside the cell to respond.
Example:
Band 3 protein binds to a membrane receptor. The receptor changes shape. This triggers processes that carry carbon dioxide from body tissues to the lungs.

15. Voyage into the Cell Membrane
Voyage inside the Cell: Membrane - YouTube