Transport: Cell Membrane Structure and Function Biology 12 Chapter 4
FLUID-MOSAIC MODEL OF MEMBRANE STRUCTURE The cell membrane (plasma membrane) is made of two layers of phospholipid molecules (bilayer) which give it a fluid consistency. It has proteins scattered through it like a mosaic.
The phospholipid bilayer gives the cell membrane its structure. Each phospholipid has a polar (charged) head and 2 non-polar tails. The polar heads are hydrophilic and the non-polar tails are hydrophobic. When surrounded by water (e.g. tissue fluid or cytoplasm) the heads face the water and the tails face away from the water.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane carbohydrate chain Outside glycoprotein glycolipid hydrophobic hydrophilic tails phospholipid bilayer heads filaments of cytoskeleton Inside peripheral protein integral protein cholesterol
MOLECULES IN THE PLASMA MEMBRANE Glycolipids have a structure similar to phospholipids, except that the hydrophilic head is a carbohydrate chain. (Cell to cell recognition) Cholesterol helps to keep the cell membrane fluid at low temperatures and reduces the permeability of the membrane.
Proteins Peripheral proteins are on the surface of the membrane. Glycoproteins have a carbohydrate chain attached (cell recognition) Integral proteins extend right through the cell membrane.
4.1 Plasma Membrane Structure and Function Functions of Proteins Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. 2. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. e.g. Sodium Potassium pump a. b.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c. Cell Recognition Protein: With Carbohydrate chain. e.g. Rejection of heart after transplant. Receptor Protein: Shaped in such a way that a specific molecule can bind to it. e.g. hormones, Neurotransmitters. Enzymatic Protein: Catalyzes a specific Reaction. d. e.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Channel Protein Carrier Protein. Cell Recognition Protein a. b. c. Receptor Protein. Enzymatic Protein. d. e.
4.2 SELECTIVELY PERMEABLE The cell membrane selects what it will let in and out of the cell. It lets different things through in different ways, but not everything can get through. Mechanisms of transport include diffusion and osmosis, protein assisted transport, and endocytosis and exocytosis (transport by vesicle).
DIFFUSION Diffusion is the net movement of a substance from a region of higher concentration to a region of lower concentration. It requires no energy. The rate of diffusion is affected by: 1. Concentration gradient a solution consists of two parts: solvent (often water) and solute (particles dissolved in solvent). The difference in solute concentration between two areas causes diffusion. The greater the difference, the faster the diffusion. E.g. if there is more oxygen outside the cell than inside, it will diffuse to the inside.
2. Temperature increasing temp causes the particles to move faster, therefore increasing the rate of diffusion 3. Ionic/molecular size smaller substances will diffuse more rapidly because they have fewer collisions with other substances. Large molecules do not diffuse through the membrane. 4. Shape of ion/molecule may prevent it from diffusing rapidly 5. Lipid Solubility- Lipid soluble molecules can move through the lipid bilayer. Generally these molecules are other lipids. Steroid hormones like testosterone and estrogen are examples of such molecules. This easy access to cells explains the powerful and wide ranging effects of such hormones
6. Charge (+/-) Ions or molecules with a charge cannot pass through the lipid bilayer by diffusion. Water is not able to diffuse directly through the phospholipid molecules, so instead it diffuses through channels made of proteins called aquaporins. Na + and also travel through protein channels. OSMOSIS Osmosis is the diffusion of water across a differentially permeable membrane. Tonicity is the strength of a solution in relationship to osmosis. Tonicity can be described in three ways:
ISOTONIC In isotonic solutions, the concentration is the same on both sides of the membrane, therefore there is no net gain or loss of water (0.9%NaCl is isotonic to red blood cells) HYPOTONIC refers to a lower solute concentration on one side of the cell. If a cell is placed in a hypotonic solution, then the cell will gain water and may burst. For example, any concentration of salt solution less than 0.9% is hypotonic to red blood cells. This may result in hemolysis (disruption of red blood cells).
HYPERTONIC refers to a higher solute concentration. If a cell is placed in a hypertonic solution, it will lose water. A salt concentration greater than 0.9% causes shrinking of red blood cells (crenation) The shrinking of cytoplasm due to osmosis is called plasmolysis.
Do not copy this one Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cells nucleus In an isotonic solution, there is no net movement of water. Plant cells 6.6 µm 6.6 µm 6.6 µm plasma membrane In a hypotonic solution, water enters the cell, which may burst (lysis). In a hypertonic solution, water leaves the cell, which shrivels (crenation). nucleus central vacuole chloroplast cell wall plasma membrane In an isotonic solution, there is no net movement of water. 25 µm 25 µm 40 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. (all top): David M. Phillips/Photo Researchers, Inc.; (bottom left, center): Dwight Kuhn; (bottom right): Ed Reschke/Peter Arnold
4.2 Permeability of the Cell Membrane PROTEIN ASSISTED TRANSPORT Many of the proteins that are scattered throughout the membrane provide channels for substances to pass through the cell membrane. Some require no energy and others require ATP.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inside plasma membrane carrier protein solute Outside 1. FACILITATED TRANSPORT Facilitated transport does not require energy. The molecules diffuse with the concentration gradient across the cell membrane by combining with carrier proteins. This can occur as often as 100 times per second at one site.
2. ACTIVE TRANSPORT With active transport, the molecules move against the concentration gradient, either in to or out of the cell. The molecules or ions require a carrier protein and an expenditure of energy (ATP). Some examples of this are Iodine collects in the thyroid to produce thyroxin Sodium is withdrawn from urine by kidney cells The sodium potassium pump in neurons Cells that carry out a lot of active transport have lots of mitochondria near the cell membrane to produce the energy.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. carrier protein Outside Inside 1. Carrier has a shape that allows it to take up 3 Na +. P ATP ADP 6. Change in shape results and causes carrier to release 2 inside the cell. 2. ATP is split, and phosphate group attaches to carrier. P P 5. Phosphate group is released from carrier. P 3. Change in shape results and causes carrier to release 3 Na + outside the cell. 4. Carrier has a shape that allows it to take up 2.
ENDOCYTOSIS AND EXOCYTOSIS Endocytosis and exocytosis involve transport in and out of the cell by vesicle. This requires energy and is also considered as active transport. Endocytosis Endocytosis involves movement into the cell by vesicle. Diagram:
Endocytosis is often divided in to two categories based on the size of the particle ingested: Phagocytosis (cell eating) This is common in macrophages, large white blood cells in humans which phagocytize bacteria and worn-out red blood cells. The vesicle then fuses with a lysosome and the contents are digested
Pinocytosis (cell-drinking) Vesicles form around liquid or very small particles (in blood cells and in intestinal wall) Receptor-mediated endocytosis is a type of pinocytosis in which receptor proteins bind to solutes in a coated pit, which becomes a vesicle this process is involved in bringing substances into the cell, transfer of materials between cells, and exchanges between mother and fetus.
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane paramecium pseudopod of amoeba vacuole forming vacuole a. Phagocytosis 399.9 µm vesicles forming solute vesicle b. Pinocytosis 0.5 µm receptor protein solute coated pit coated vesicle coated pit coated vesicle c. Receptor-mediated endocytosis a(right): Eric Grave/Phototake; b(right): Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher
Exocytosis In exocytosis, a vesicle fuses with the plasma membrane and its contents are secreted. Diagram:
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane Outside secretory vesicle Inside
Passage of Molecules Into and Out of the Cell
Vesicles formed by Golgi apparatus secrete cell products at cell membrane. (e.g. this is the way that insulin leaves insulin-secreting cells) CELL SIZE Cells have to be small in order to function effectively. The materials that it needs to use and the wastes that it produces must pass through its cell membrane. The surface area of the cell controls the ability of the cell to get nutrients in and wastes out. The volume of a cell controls the amount of nutrients it needs.
Cell length SA V SA/V A 1mm 6mm 1mm 6/1 = 6 B 2mm 24 mm 8mm 24/8 = 3 C 4mm 96mm 64mm 96/64 = 1.5 D 8mm 384mm 512mm 384/512 = 0.75 As cells increase in size, the surface area/volume ratio decreases. (squared function vs. cubed function)
A small cell has more surface area per unit of volume than a large cell. The surface area becomes the limiting factor in the cell s ability to survive. The cell produces more wastes than it can get rid of, or it can t consume enough nutrients for its increased volume.
Cell strategies to increase SA/V ratio 1. Size - stay small 2. Shape - get flat example: skin cells - get long and thin example: nerve cells 3. Add extensions villi and microvilli in the small intestine and the kidney
Assignment from Ch 4 p68-78 4.1 CYP p69 4.2 CYP p71 4.3 CYP p74, 76, 78 These are all found on your CYP Handout