The Cell Membrane

1. The Cell Membrane

2. Purpose of the membrane • 1) Transport raw materials into the cell. • 2) Transport manufactured products and wastes out of the cell. • 3) Prevent the entry of unwanted matter into the cell. • 4) Prevent the escape of matter needed for cellular functions.

3. Composition • The cell membrane is composed of phospholipid molecules. A Phosphate group and two fatty acids bonded to a glycerol “backbone”. The phosphate group (head) is ‘hydrophillic’ (likes water) the two fatty acid “tails” are ‘hydrophobic’ (water hating). (Pg 51 Fig 2.22)

5. Importance of their dual nature • When phospholipids are spread throughout water, a curious thing happens. The attractions and repulsions of the heads and tails cause them to form a cage-like spherical layer. (Pg 51, fig 2.23). • -The water attracting heads face in and out of the sphere. • -The non-polar tails face each other. • -This phospholipid bilayer forms the basis of the cell membrane.

6. Fluid-Mosaic Model • There are numerous proteins, carbohydrates and other lipids spread and moving throughout the membrane creating a fluid-like consistency.

8. Benefits of the Fluid Mosaic Model • Cholesterols found in animal cell membranes. These keep the membrane fluid at lower temperatures. They also prevent some other molecules from passing through. • Protein and carbohydrate arrangements in the cell membrane that allow the cell to be “recognized” by other cells. (Pg 52, fig 2.24)

9. Outside the cell • In multicellular organisms, the cells are bathed in a thin layer of extracellular fluid, which is made of water, mineral and some cell wastes that have been discarded.

10. Transport Through the Membrane Passive and Active

11. Crossing the Membrane • The membrane is a cell’s primary means for it to maintain homeostasis (the constant steady state of conditions achieved to remain healthy). • The plasma membrane is selectively permeable. It allows some materials to pass.

12. Transport • Diffusion- a passive way that molecules can move across the membrane from an area of [high] to [low] ([ ] means concentration). - materials that pass easily are: gases, small uncharged molecules, and fat soluble molecules.

13. Motion • Any atom above absolute zero (-273 oC) is in motion. The random motion within a liquid is called Brownian motion. (Think of dye spreading through a liquid). • The difference in concentrations of a substance across a membrane is called the concentration gradient.

14. OSMOSIS • The diffusion of water is called Osmosis. Where water moves from a [high] to [low]. There are different types of environments that will lead to a net direction of movement: • Isotonic condition – when the concentrations of solutes on both sides of the membrane are the same causing no net movement of water.

15. Hypotonic condition – when the [water] is higher ([lower solute]) outside the cell. Water will move into the cell to balance out concentrations, causing the cell to swell and possibly burst. • Hypertonic condition – when the [water] is higher ([lower solute]) inside the cell. Water will move out of the cell, causing the cell to shrivel. • No energy is required to diffuse water, runs on concentration gradient.

17. Facilitated Diffusion • Facilitated Diffusion – molecules that do not readily pass, due to size or charge etc.., may need assistance to cross the membrane.Specialized transport proteins help substances move in this case. • Transport proteins are selective and will only move the molecule/ion it was designed to move.

18. Carrier Proteins • Numerous carrier proteins lie within the membrane to facilitate the movement of glucose (sugar). These carriers still operate by concentration gradients and do not require energy.

19. Channel Proteins • Channel Proteins – are transport proteins that carry charged particles. They have a tunnel-like shape and have their own charge. They attract the ion like attracting a magnet. Size is a limiting factor for using these channels.

20. http://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swfhttp://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swf

21. Active transport • Certain cells require materials that may not be present in great amounts. In this case, a cell may need to pump these materials in or out of the cell against the normal concentration gradient. • This moving materials from [low] to [high] requires the cell to spend energy, and is therefore called Active transport.

22. Examples • 4 Important examples of Active Transport - kidney cells pump glucose and Amino acids out of the urine and back into the blood. - Intestinal cells pump in nutrients from the gut. - Root cells pump in nutrients from the soil - Gill cells in fish pump out sodium ions.

23. Active transport pumps are protein pumps in the membrane that grab ions and other materials and pump them across the membrane against the gradient.- one of the best understood is the Na+/K+ pump. • In some cases the unequal distribution of Na+ (or other molecule) can allow another molecule to ‘piggyback’ into the cell when regular diffusion continues.

24. http://www.northland.cc.mn.us/biology/Biology1111/animations/active1.swfhttp://www.northland.cc.mn.us/biology/Biology1111/animations/active1.swf • Once you enter this site click on MAIN at the bottom.

25. Bulk Membrane Transport

26. Bulk Transport • Material too big or material that is charged can not freely cross the cell membrane. • The cell must accommodate these substances by folding in on itself creating a vesicle to take in or expel these large molecules.

27. 2 Types of Bulk transport • Endocytosis (Endo = into; cyto = cell) – the membrane folds inward, trapping matter from the extracellular fluid and enters the cell as a vesicle.

28. 3 Types of Endocytosis • Phagocytosis (phago = eating) – the membrane folds inward, trapping solid matter from the extracellular fluid. • Pinocytosis (pino = drinking) – the membrane folds inward trapping fluids from the extracellular fluid. • Receptor-mediated Endocytosis – the membrane takes in particles after they bind to specialized receptor proteins embedded in the membrane. Cholesterol is brought in this way.

29. Exocytosis (exo= exit) – the reverse of endocytosis. A vesicle moves out towards the plasma membrane, fuses with it and opens up its’ contents into the extracellular fluid.- This is important in the body secreting manufactured products from within the cells.

30. Endocytosis and Exocytosis • http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf