Cell Membrane and Transport of Materials

1. Cell Membrane and Transport of Materials

2. Lesson Aims • To study the structure and function of the plasma membrane • To examine the transport of substances across the membrane by diffusion, active transport, endocytosis, exocytosis, pinocytosis, phagocytosis,

3. Functions of Cell Membrane • Controls entry/exit of materials including water and soluble molecules • Communicates with other cells e.g. hormones binding with receptors • Important for the connections between cells

4. Danielli and Davson 1930's-40's • Studied triglyceride lipid bilayers over a water surface. • Lipids arranged themselves with the polar heads facing outward. • Formed droplets (oil in water) and the surface tension was much higher than that of cells • Added proteins, the surface tension was reduced and the membranes flattened out Danielli and Davson’s early model of the cell membrane.

5. Fluid Mosaic Model

6. Fluid mosaic model • Membrane is composed of a number of proteins (mosaic) with spaces between them being filled with fluid-like phospholipids • Proteins act as: channel receptors support carriers enzymes antigens • Phospholipids form a bilayer that acts like a barrier between the cell and the environment.

7. Phospholipid bi-layer Hydrophilic phosphate head Hyrdophobic lipid tail

8. Phospholipid Structure Phospholipid consists of: • hydrophilic head which points towards the outside environment and the cytoplasm • hydrophobic tail which repels water and point in

9. Freeze-fracture of Plasma Membrane

10. Freeze-fracture of Plasma Membrane • This electron micrograph shows the inside of a membrane and bumps, grooves, ridges.  These were later found to be proteins. 

11. Membrane composition

12. Passive Transport • molecules move down the concentration gradient • no energy is required • Diffusion - molecules moves from high concentration to low concentration e.g. minerals, oxygen and carbon dioxide • Osmosis - water molecules through a selectively-permeable cell membrane from HWC to LWC Channel-forming proteins

13. Osmosis in Red Blood Cells HWC inside and LWC outside. Water moves out of cells and cells shrink

14. Osmosis in Red Blood Cells water concentration inside = water concentration outside. No net gain or loss of water

15. Osmosis in Red Blood Cells LWC inside and HWC outside. Water moves into cells and cells burst

16. Active Transport • Molecules move against the concentration gradient • Energy (ATP) required • Proteins act as carrier molecules • Proteins are specific • Rate affected by temperature, respiratory substrate, oxygen concentration

17. Sodium/potassium pump

18. Endocytosis • Cells absorb material e.g. proteins from the outside by engulfing it with cell membrane • It is used by all cells of the body because most substances important to them are large polar molecules so cannot pass through the hydrophobic plasma membrane.

19. Phagocytosis • “Cell-eating” – large solid particles engulfed by cell • e.g. white blood cell eating fungus Candida albicans

20. Pinocytosis • “Cell-drinking” – liquid-filled vesicles formed • e.g. Amoeba showing pinocytosis

21. Exocytosis • Cell directs secretory vesicles to the cell membrane • Vesicles contain: soluble proteins to be secreted to the extracellular environment membrane proteins and lipids that are sent to become components of the cell membrane

22. The Facts You Need To Know • page 3-4 • from “membranes are composed of….” • to “as active transport depends…”