The Cell Membrane

1. The Cell Membrane

2. Membrane structure Membranes form compartments around and within cells. For example: • Plasma membrane • Nuclear membrane • Endoplasmic reticulum • Golgi apparatus • Lysosomes • Mitochondria • Chloroplasts • Vacuoles

3. Membrane structure Basic characteristics of cell membranes • They are 8 nm thick. • They are fluid mosaics with proteins embedded or attached to a double layer of phospholipids. • They separate a living cell from its nonliving surroundings. • They control traffic into and out of the cell. • They are selectively permeable: some sub- stances pass more easily than others.

4. Membrane structure

5. Membrane structure A fluid mosaic with proteins embedded or attached to a double layer of phospholipids. Everything is free to move; not locked in place.

6. Membrane structure Fluidity: lipids and proteins are not stuck in one place; they can drift side-to-side.

7. Membrane structure Proteins and phospholipids have hydrophobic (water-hating ) and hydrophilic (water-loving ) regions. Phospholipids & proteins in a membrane Phospholipid

8. Membrane structure The hydrophobic and hydrophilic regions of phospholipids cause them to align as a bilayer when water is present. • Proteins fall into the proper region depending upon their sequence of hydrophobic and hydrophilic amino acids. ⇒

9. Membrane structure Membranes also contain cholesterol, which keeps the hydrophobic tails of the phospho-lipids from bunching up in cold temperatures. • Animals that live in cold climates have more cholesterol in their cell membranes.

10. Membrane structure Placement of membrane proteins • Integrated proteins are within the membrane. • Peripheral (external) proteins are on surface.

11. Membrane structure Functions of membrane proteins • Hormone-binding sites • Immobilized enzymes • Cell adhesion • Cell-to-cell communication • Channels for passive transport • Pumps for active transport

12. Functions of membrane proteins Hormone-binding sites • The receptor protein scoops up a hormone, such as estrogen, traveling in the blood. The receptor protein ⇒

13. Functions of membrane proteins Immobilized enzymes • Enzymes catalyze chemical reactions. • Groups of enzymes create a metabolic pathway. • Ex: enzymes for respiration in mitochondria.

14. Functions of membrane proteins Cell adhesion • Tight junction proteins bind cells together. • Adhesion belts and desmosomes glue cells together.

15. Functions of membrane proteins Cell-to-cell communication • Receptor proteins also aid cell recognition • Ex: Blood group glycoproteins (have sugar chains) • Ex: receptors for HIV let virus attach to cells

16. Functions of membrane proteins Channels for passive transport • Passive – no energy required. • Some channel proteins let large molecules (like sugars) pass through the membrane by dif- fusion. Carriers help ions pass. More real design above Channel conceptualized designs Carrier

17. Passive transport Small uncharged molecules like O2, CO2, and lipids can pass between the phospholipids of the mem-brane by simple diffusion. • Non-polar fatty acids are barriers to charged molecules • Diffusion of one thing does not affect dif- fusion of another.

18. Passive transport Click box for video.

19. Functions of membrane proteins Pumps for active transport • Cells can accum- ulate large quantities of a material if they use ATP energy. Examples: • Proton pump in mitochondria • Na+/K+ pump in nerve cells

20. Active transport In active transport, ATPenergy must be used to do work. • Work involves moving a substance against a concentration gradient – from a low to a high concentration. • Diffusion goes from high to low concentration.

21. Membrane fluidity Membrane fluidity allows it to break, change shape, and reform. • The membrane is a mosaic as a result.

22. Membrane fluidity The loose packing of phospholipids and proteins makes the membrane fluid. • It’s like a bunch of marbles rolling around together. New ones come up from below and push the old ones aside.

23. Diffusionand Osmosis

24. Diffusion and osmosis Diffusion– the passive movement of particles from a region of high concentration to a region of low concentration. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html Examples: perfume in air, smoke in air, sugar in water, oxygen through cell membranes

25. Diffusion and osmosis Diffusion– the passive movement of particles from a region of high concentration to a region of low concentration. Example: Bromine gas in a beaker separated from pure air by a glass plate.

26. Diffusion and osmosis Osmosis – the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html Hypotonic = low concentration of solute (high H2O). Water is “pulled” to the hypertonic side. Concentrations even out (same g/ml of the solute and of water).

27. Diffusion and osmosis Osmosis Ex: Two solutions with different concentrations of sugar are separated by a membrane that only lets water pass. • The hypertonic solution (right) has a lower water concentration than the hypotonic solution (left) since some water is displaced by sugar molecules.

28. Diffusion and osmosis Osmosis Two sugar solutions • Water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.

29. Diffusion and osmosis Osmosis • Osmosis requires a selectively permeable membrane. • Osmosis continues until the solutions are isotonic (=same concentration). • The direction of osmosis is determined only by a difference in total solute concentration.

30. Diffusion and osmosis Water moves across cell membranes by osmosis. The cell is swollen by distilled water. Sea water pulls water out of cell – causes dehydration. top – animal cell; bottom – plant cell

31. Passive transport Small uncharged molecules like O2, CO2, and lipids can pass between the phospholipids of the mem-brane by simple diffusion. • Non-polar fatty acids are a bar- rier to polar molecules • Diffusion of one thing does not affect dif- fusion of another.

32. Passive transport Larger molecules, with or without a charge (polar), must pass through channel proteins. • This is called facilitated diffusion. • The diffusion is aided by membrane tunnels. • No energy is used (movement is passive). • The rate of movement is determined by the number of channel or carrier proteins. • Carrier proteins are very specific. Channel protein Carrier protein

33. Passive transport

34. Active transport In active transport, ATPenergy must be used to do work. • Work involves moving a substance against a concentration gradient – from a low to a high concentration. • Diffusion goes from high to low concentration.

35. Active transport Using ATP, cells can maintain a much higher concentration of a substance than would be allowed by diffusion. • ATP is used with a carrier protein to pump materials into or out of the cell. Ex: a Na+/K+ pump in nerve cells maintains a high level of intracellular K+.

36. Active transport Concentrating ions on one side of the membrane

37. Transport within the cell Within the cell, materials may be moved from the endoplasmic reticulum to the Golgi apparatus and past the cell membrane within vesicles. • Vesicles are membrane- bound bubbles. • This is how hor- mones, etc., are secreted. • Bubble becomes part of the cell membrane. Note the sequence above!

38. Exocytosis Moving large molecules from inside to outside the cell – large proteins.

39. Endocytosis Moving large molecules from outside to inside the cell – food (bacteria into white blood cells).