Cell Membranes

1. Cell Membranes

2. Cell Membranes • Cells are surrounded by a membrane: the cell surface membrane or plasma membrane • On an EM it appears as a double line • The distance across the membrane is 7-8nm

3. The Fluid Mosaic Model What does a membrane look like?

5. Transport Across Membranes

6. There are four main methods by which substances can move across a cell membrane: • 1. Diffusion Osmosis • 2. Active Transport • 3. Endocytosis and Exocytosis Passive Transport

7. 1: Diffusion • Diffusion is the movement of molecules from a region where they are in high concentration to a region of low concentration. • The difference in concentration is the concentration gradient

8. Diffusion is a PASSIVE process • There is a net movement of molecules down the concentration gradient until equilibrium is reached • Ions move by diffusion along electrochemical gradients

9. Rate of diffusion • The rate of diffusion depends on: • Concentration gradient • Temperature • Size of molecule • Lipid solubility Extension: Look up Fick’s Law…

10. Intrinsic globular proteins within the membrane function as carriers for the transport of certain molecules concentration gradient Facilitated Diffusion Lipid Bilayer Carriers are specific for the molecules that they transport them across the membrane in the direction of the concentration gradient at a faster rate than occurs for simple passive diffusion Glucose molecules enter body cells from the bloodstream by facilitated diffusion

11. concentration gradient Lipid Bilayer

12. concentration gradient Lipid Bilayer

13. concentration gradient Lipid Bilayer

14. concentration gradient Lipid Bilayer

15. concentration gradient Lipid Bilayer

16. concentration gradient Lipid Bilayer

17. concentration gradient Lipid Bilayer

18. concentration gradient Lipid Bilayer

19. concentration gradient Lipid Bilayer

20. Rate of uptake Increasing external concentration of glucose LIMITING FACTORS AND FACILITATED DIFFUSION The rate at which molecules like glucose enter cells by facilitated diffusion is affected by factors other than the concentration gradient The availability of carriers in the membrane is the limiting factor Rate of uptake reaches a maximum at glucose concentrationA No further increase in the uptake rate despite the increasing glucose concentration Rate of uptake increases as the glucose concentration increases (i.e. the steepness of the concentration gradient increases) When glucose concentrations exceed a certain value, there are insufficient glucose carriers within the membrane for the rate of uptake to increase any further A

21. Summary of Passive Transport

22. 2: Osmosis • Osmosis is the diffusion of water molecules from an area of high concentration (of water) to an area of lower concentration across a semi-permeable membrane

23. Water Potential Osmosis can be quantified using water potential, so we can calculate which way water will move, and how fast. Water potential (ψ, the Greek letter psi, pronounced "sy") is simply the effective concentration of water. It is measured in units of pressure (Pa, or usually kPa) water always "falls" from a high to a low water potential 100% pure water has ψ = 0, which is the highest possible water potential, so all solutions have ψ < 0 you cannot get ψ > 0.

24. ψ ψ ψ

25. Cells and Osmosis. The concentration of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider: • Isotonic solution a solution of equal concentration to a cell • Hypertonic solution a solution of higher (salt) concentration than a cell • Hypotonic solution a solution of lower (salt) concentration than a cell

28. protein pump active site ATP ADP + Pi 3: Active Transport • Pumping of molecules across cell membrane using a protein pump UP a concentration Gradient

29. Most Active Transport systems are driven by metabolic energy derived from ATP • Active Transport allows cells to uptake necessary ions and molecules and remove waste products • These Mechanisms often referred to as Pumps • Most common is Sodium Pump

30. Na+ / K+ Pump • Present in all animal cells, most abundant and important of membrane pumps

31. Sodium-Potassium Pump important in controlling cell volume, reducing Na thereby reducing water uptake by osmosis • The accumulation of K ions are used in cell metabolic processes eg protein synthesis • Na-K pump can be linked to active uptake of organic molecules such as glucose and amino acids

32. 4: Endocytosis & Exocytosis • Endocytosis

33. Phagocytosis Eg. Amoeba White Blood Cells (neutrophils & monocytes) • Pinocytosis Eg. Protozoans, white blood cells, cells in embryos, liver and kidneys.