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Transcript
The Cell Membrane (a.k.a. Plasma Membrane) Fluid-Mosaic Model Membranes have certain properties. They: – act as a barrier between the cell and its environment, allowing a complex organized system to exist inside the cell. – permit the passage of selected substances into and out of the cell. – flex, bend and flow to allow the cell to change shape. Barrier between the cell and its environment All cells, from all organisms, are surrounded by a CELL MEMBRANE The cell membrane is a thin layer of lipid and protein that separates the cell's interior from its surroundings. The cell membrane is SELECTIVELY PERMEABLE. The cell membrane functions like a gate, controlling what enters and leaves the cell. The cell membrane controls the ease with which substances pass into and out of the cell-some substances easily cross the membrane, while others cannot cross at all. What can pass through the membrane is determined by – the SIZE of the particle, – the CHARGE of the particle, – whether or not it needs the help of a CARRIER MOLECULE – or if it requires the cell to spend ENERGY. Cell membranes are made mostly of PHOSPHOLIPID MOLECULES. Lipid is a simple form of fat. Phospholipids are a kind of lipid that consists of 2 fatty acids (tails) and a phosphate group (Heads). A phospholipid molecule has a polar “head" and 2 nonpolar “tails.” The phosphate head is hydrophilic meaning ”water loving". Because of its hydrophilic nature, the head of a phospholipid will orient itself so that it is as close as possible to water molecules. The lipid tails are hydrophobic meaning “water fearing.” The hydrophobic tails will tend to orient themselves away from water. Proteins in membranes • Help to move material into and out of the cell – peripheral proteins • are attached to the surface of the cell membrane, • located on both the internal and external surface. -integral proteins • Embedded in the lipid bilayer • Some extend across entire membrane, others extend to interior or exterior surface only Some integral proteins form channels or pores through which certain substances can pass. Other proteins bind to a substance on one side of the membrane and carry it to the other side of the membrane. Integral proteins exposed to the cell's external environment often have carbohydrates attached to them serve as identification badges that allow cells to recognize each other (immune function) and may act as a site where viruses or chemical messengers such as hormones can attach. Glycoproteins and Glycolipids Glycoproteins – proteins covalently attached to carbohydrates (ex. glucose, galactose, lactose) Glycolipids – Carbohydrates attached lipids. Their role is to provide energy and also serve as markers for cellular recognition. The antigens which determine blood types belong to glycoproteins and glycolipids Fluid Mosaic Model Membranes are FLUID and have the consistency of vegetable oil. The lipids and proteins of the cell membrane are always in motion. Phospholipids are able to drift across the membrane, changing places with their neighbor. Cells may vary the variety and the relative amounts of different lipids to maintain the fluidity of their membranes despite changes in temperature. Cholesterol molecules in the bilayer assist in regulating fluidity. Membrane cholesterol The following figure shows the steroid structure of cholesterol. The non-polar and polar regions are also illustrated. Membrane Cholesterol The cholesterol molecule inserts itself in the membrane with the same orientation as the phospholipid molecules. The figure shows phospholipid molecules with a cholesterol molecule in between. Note that the polar head of the cholesterol is aligned with the polar head of the phospholipids. Without cholesterol – cell membranes too fluid, not firm enough – and too permeable to some molecules. Cholesterol – adds firmness and integrity to the plasma membrane – prevents it from becoming overly fluid – helps maintain its fluidity. – helps separate the phospholipids so that the fatty acid chains can't come together and crystallize. Cholesterol helps prevent membrane extremes – too fluid, or too firm The cytoskeleton undergirds the cell membrane and provides anchoring points for integral membrane proteins Cell Junctions: connect cells together structures that help cells coordinate as part of a tissue Plant cells: – Plasmodesmata - channels between adjacent plant cells that form a circulatory and communication system Cell junctions in animal cells Tight junctions - bind cells forming leakproof sheet. Ex. Lines digestive tract Anchoring junctions - attach adjacent cells with cytoskeletal fibers but still allow materials to pass along the spaces between cells Communicating junctions allow water and small molecules to flow between cells Passive Transport Does not require the cell to use energy Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. This difference in the concentration of molecules across a space is called a concentration gradient. Diffusion Driven by the kinetic energy of motion Occurs when molecules move randomly away from each other in a liquid or gas. Rate depends on the temperature, size, and the type of molecules that are diffusing. Molecules diffuse faster at higher temperatures than at lower temperatures. Diffusion always occurs down a concentration gradient (high to low). When molecules are dispersed evenly, there is no longer any diffusion because there is no longer a concentration gradient and the molecules are evenly distributed throughout the space the molecules occupy. When the concentration of the molecules of a substance is the same throughout a space, a state of equilibrium exists. Osmosis The diffusion of water across a semipermeable membrane is called osmosis. Occurs down the concentration gradient so no energy is used. Osmosis Osmosis Why does salt kill slugs? Hypertonic Solution Concentration of the solute molecules outside the cell is high than the concentration of solutes inside the cell. WATER DIFFUSES OUT OF THE CELL until equilibrium is established. Hypotonic Solution Concentration of solute molecules outside the cell is lower than the concentration of solutes inside the cell. WATER DIFFUSES INTO THE CELL until equilibrium is established. Isotonic Solutions Concentration of solutes outside and inside the cell are equal. Under these conditions, water diffuses into and out of the cell at equal rates, so there is no net movement of water. Plants •Plants in a hypotonic solution will have water diffuse into the cell until the cell membrane pushes against the cell wall. •The cell wall is strong enough to resist the pressure, the pressure that water molecules exert against the Cell Wall is called turgor pressure. Plasmolysis In a hypertonic environment, the cells shrink away from the cell wall, and turgor pressure is lost. This condition is called plasmolysis, and is the reason plants wilt. http://www.linkpublishing.com/videotransport.htm#Elodea_-_Osmosis View: osmosis, hypertonic and hypotonic videos (elodea, RBC, onion, paramecium) Animal cells placed in a hypertonic environment will have water leave the cells, making them shrink and shrivel. Placed in a hypotonic environment, water diffuses into the cells, causing them to swell and eventually burst - lyse or cytoloysis. Protozoans often live in a hypotonic solution. This presents a problem! The solution… Facilitated Diffusion Molecules move across a membrane with the help of transport proteins in the membrane. This takes place down the concentration gradient so it does not require energy. Facilitated Diffusion Carrier proteins are embedded in the cell membrane. Carrier proteins change shape when molecules attach to them. The change in shape of the carrier protein enables the molecule to cross the membrane. Carrier Proteins In order to transport solutes across the membrane, carrier proteins alternate between two conformations. – In the first conformation the protein’s binding site for the solute, located in the core of the protein, is open to one side of the membrane, – In the second confirmation the binding site is open to the other side of the membrane. Channel Proteins Channel proteins form an aqueous pore in the membrane to allow solutes to move across the membrane without coming into contact with the hydrophobic core of the lipid bilayer. A good example of facilitated diffusion is the transport of glucose into the cell. Many cells depend on glucose for much of their energy needs. Active Transport In many cases, cells must move materials up their concentrated gradient, from and area of lower concentration to an area of higher concentration. Unlike passive transport, active transport requires a cell to expend energy (ATP). Carrier-mediated active transport Carrier-mediated active transport systems use energy and membrane proteins to "pump" certain substances against a concentration gradient. This causes the substance to accumulate on one side of the plasma membrane. Ex. Na+/K+ Pump Sodium Potassium pump 3 Na+ ions from inside the cell first bind to the transport protein. A phosphate group is transferred from ATP to the transport protein causing it to change shape and release Na+ outside the cell. 2 K+ from outside the cell then bind to the transport protein and as the phosphate is removed, the protein returns to its original shape and releases the K+ inside the cell. Sodium-Potassium Pump This animation shows operation of the pump. Upper side = outside of cell; lower side = cytosol. The colored ball represents ATP; the three yellow diamonds Na+ and the two red diamonds K+. Endocytosis (3 Types) Solutes or fluids outside the cell membrane can be brought into the cytoplasm. – Phagocytosis - “Cell eating”. The cell engulfs a food particle or other cells – Pinocytosis - "Cell drinking”. – Receptor-mediated endocytosis – needs special proteins in the membrane Phagocytosis The food vesicle can then fuse with a lysosome that contains digesive enzymes. White Blood Cells (WBC, phagocytes) destroy bacteria and other unwanted cells by phagocytosis Exocytosis Process by which waste and cell products leave the cell Products made in the cell are packaged in vesicles made by the Golgi apparatus which then fuse with the cell membrane and secrete material out of the cell. Mucus and waste products are materials secreted by exocytosis. Notice the merging of the membrane… Relationship of endo- and exo- cytosis Summary of types of transport