Cell Membranes

1. Cell Membranes Biology for Majors

2. Cell Membranes: The Fluid Mosaic Model The plasma membrane is composed of a bilayer of phospholipids, with their hydrophobic, fatty acid tails in contact with each other. The membrane is studded with proteins, some of which span the membrane. Some of these proteins serve to transport materials into or out of the cell. Carbohydrates are attached to some of the proteins and lipids on the outward-facing surface of the membrane. These form complexes that function to identify the cell to other cells.

3. Cell Membranes: The Fluid Mosaic Model The fluid nature of the membrane owes itself to the configuration of the fatty acid tails and the presence of cholesterol embedded in the membrane (in animal cells). The mosaicnature of the membrane owes itself to the proteins and protein-carbohydrate complexes found throughout the membrane. These are not firmly fixed in place. Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux.

4. The Fluid Mosaic Model

5. Phospholipids Phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell because of their hydrophilic heads and hydrophobic tails.

6. Phospholipids in Aqueous Solutions In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. 

7. Selective Permeability Plasma membranes are selectively permeable —they allow some substances to pass through, but not others. They allow: • Lipid-soluble material with a low molecular weight ie. Fat soluble vitamins or hormones  • Molecules of oxygen and carbon dioxide The substances below cannot pass through the membrane and must pass through channels: • sodium, potassium, calcium, and chloride (all charged ions) • Simple sugars and amino acids

8. Passive Transport Passive transport does not require the cell to exert any of its energy. Substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a range of concentrations of a single substance is said to have a concentration gradient. Diffusion is one example of passive transport.

9. Diffusion Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm).

10. Factors that Affect the Rate of Diffusion • Extent of the concentration gradient • Mass of the molecules diffusing • Temperature • Solvent density • Solubility • Surface area and thickness of the plasma membrane • Distance travelled

11. Facilitated Transport Ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane enter the cell through facilitated transport. Facilitated transport proteins shield these materials from the repulsive force of the membrane, by acting as channels or carriers, allowing them to diffuse into the cell.

12. Channel Proteins Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers. Some channels are open while others may need to bind to certain ions to open.

13. Carrier Proteins Carrier proteins binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior. Carrier proteins are slower than channel proteins.  

14. Osmosis Osmosis is specifically the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. 

15. Tonicity Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution’s tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. • Hypotonic - The extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. • Hypertonic - The cell has a relatively higher concentration of water, water will leave the cell. • Isotonic – The osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell

16. Tonicity: Red Blood Cell

17. Tonicity: Plant Cell

18. How Do Plants Control the Effects of Osmosis? Plants, fungi, bacteria, and some protists, have cell walls that prevent cell lysis in a hypotonic solution. The cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available, producing turgor pressure, which stiffens the cell walls of the plant. In nonwoody plants, turgor pressure supports the plant. This is why plants wilt when they are too dry.

19. Practice Question: Why would it be unwise to put fresh and saltwater fish in the same tank?

20. Active Transport Active transport requires the use of the cell’s energy. Small-molecular weight material and small molecules move in two ways: • Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.  • Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport. It does not directly require ATP.

21. Primary Active Transport. Primary active transport moves ions across a membrane, creating an electrochemical gradient.

22. Electrochemical Gradients Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients.

23. Carrier Proteins for Active Transport A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions.

24. Secondary Active Transport As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane such as amino acids and glucose. This process, secondary active transport, is shown on the next slide.

25. Secondary Active Transport Diagram

26. Active Transport: Large or Bulk Materials Some materials are simply too large to enter or exit a cell but passive or simple active transport. Large materials may be brought into a cell by endocytosis. Large materials may be expelled from a cell by exocytosis.

27. Endocytosis In endocytosis the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane. The three types are: • Phagocytosis • Pinocytosis • Receptor-mediated endocytosis

28. Phagocytosis Cells ingest large particles, including other cells, by enclosing the particles in an extension of the cell membrane and budding off a new vacuole.

29. Pinocytosis Cells take in molecules such as water from the extracellular fluid. 

30. Receptor-Mediated Endocytosis Receptor proteins in the plasma membrane ensure only targeted substances are brought into the cell.

31. Exocytosis Exocytosis is the bulk expulsion of materials. Waste material for example is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space. This is also used for secretion of proteins and neurotransmitters.

32. Methods of Transport, Energy Requirements, and Types of Material Transported

33. Practice Question Why do cells need so many different types of transport?

34. Quick Review • What is the structure and function of membranes, especially the phospholipid bilayer? • What is the difference between passive and active transport? How are substances directly transported across a membrane? • What are the primary mechanisms by which cells import and export macromolecules? • What are the main components of a signal transduction pathway?