1. Cell and Molecular Biology
Cell Membrane Structure and Functions
Behrouz Mahmoudi

2. Membrane Function and structure
Membranes organize the chemical activities of cells.
The outer plasma membrane
forms a boundary between a living cell and its surroundings
Exhibits selective permeability
Controls traffic of molecules in and out
•Internal membranes provide structural order for metabolism
- Form the cell's organelles
- Compartmentalize chemical reactions

3. Fluid Mosaic Model of the PM
A membrane is a mosaic
Proteins and other molecules are embedded in a framework of phospholipids
A membrane is fluid
Most protein and phospholipid molecules can move laterally

4. Macromolecules found in Membranes
Lipids
Phospholipid bilayer
Cholesterol
Proteins
Transport proteins
Receptor proteins
Recognition proteins
Carbohydrates
Oligosaccharides on glycoproteins

6. Functions of Membrane Macromolecules
Lipids
Phospholipid bilayer
Forms boundary to isolate cell contents from environment
Restricts passage of hydrophilic substances across the membrane
Cholesterol
Increases bilayer strength, flexibility
Reduces membrane fluidity
Reduces permeability to water-soluble substances

7. Functions of Membrane Macromolecules
Proteins
Transport proteins
Regulate movement of water soluble substances
Channel proteins have pores that allow passage of ions and small water-soluble molecules
Carrier proteins bind to molecules and change shape for delivery across membrane

8. Functions of Membrane Macromolecules
Proteins
Receptor proteins
Docking site for molecules outside the cell
Trigger internal cellular response
Recognition proteins
Identification tags
Oligosaccharides aid in cell-cell recognition
Cell-surface attachment sites

9. Fluid Mosaic Model of Membrane Structure
Within the phospholipid bilayer are many different types of embedded proteins and cholesterol molecules whose presence spawned the term mosaic. From scanning electron microscope images, it was observed that the embedded molecules can move sideways throughout the membrane, meaning the membrane is not solid, but more like a fluid.

10. Passive Transport
A process that does not require energy to move molecules from a HIGH to LOW concentration
Diffusion
Facilitated Diffusion
Osmosis

11. Diffusion is the movement of small particles across a selectively permeable membrane like the cell membrane until equilibrium is reached.
These particles move from an area of high concentration to an area of low concentration.
outside of cell
inside of cell

12. Semi-permeable membrane is permeable to water, but not to sugar
Osmosis is the diffusion of water through a selectively permeable membrane like the cell membrane
Water diffuses across a membrane from an area of high concentration to an area of low concentration.
Semi-permeable membrane is permeable to water, but not to sugar

13. Proteins that form channels (pores) are called protein channels
Facilitated Diffusion is the movement of larger molecules like glucose through the cell membrane – larger molecules must be “helped”
Proteins in the cell membrane form channels for large molecules to pass through
Proteins that form channels (pores) are called protein channels
outside of cell
inside of cell
Glucose molecules

14. Click

15. Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel.
Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.
Isotonic Solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.

16. Interactive Red Blood Cell
Click

17. Active Transport
Active transport is the movement of molecules from LOW to HIGH concentration.
Energy is required as molecules must be pumped against the concentration gradient.
Proteins that work as pumps are called protein pumps.
Ex: Body cells must pump carbon dioxide out into the surrounding blood vessels to be carried to the lungs for exhale. Blood vessels are high in carbon dioxide compared to the cells, so energy is required to move the carbon dioxide across the cell membrane from LOW to HIGH concentration.
outside of cell
inside of cell
Carbon Dioxide molecules

20. Endocytosis
Active process for movement of large molecules and organisms
Substance is taken in by vesicle formed from cell membrane
Phagocytosis: solid substance in vesicle
Pinocytosis: liquid droplets in vesicle; The uptake of extracellular fluids such as hormones and enzymes by the body cells
Receptor-Mediated Endocytosis: incoming substance binds to receptor
Iron Uptake, Cholesterol Uptake,

22. Exocytosis
Substance is expelled after being enclosed in a vesicle within the cell
Used to move large molecules out of the cell