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Elena Aragon October 4, 2009 Membranes 1. What does selective permeability mean and why is that important to cells? Plasma membranes have selective permeability, meaning certain substances can cross it more easily than others. This process is fundamental to cell processes and is possible due to the component molecules of the membranes. Selective permeability allows beneficial molecules to get in and waste to get out. 2. What is an amphipathic molecule? An amphipathic molecule has a hydrophilic region as well as a hydrophobic region. A phospholipid is amphiapathic, as are most of the proteins of membranes. 3. How is the fluidity of cell’s membrane maintained? The cell membrane maintains its fluidity by a mosaic of proteins attached to a double layer of phospholipids. The membrane is mostly held together by hydrophobic interactions. Lipids and proteins can drift in the plane of the membrane but it is only possible to transverse the membrane when the hydrophilic part of the molecule crosses the hydrophobic part of the membrane. Phospholipids change quite quickly positions. Proteins drift as well, although much more slowly, and some are quite immobile. The membrane remains fluid until the phospholipids settle into an arrangement at a cooler temperature. The temperature depends on the type of phospholipids. 4. Label the diagram below – for each structure – briefly list it’s function: extracellular matrix – attaches to proteins that coordinate extracellular and intracellular changes carbohydrate – added to proteins in the endoplasmic reticulum, which then transform into glycoproteins Page 1 of 4 glycoprotein – proteins that acquired carbohydrates, secreted from vesicles on the outside of the plasma membrane cytoskeleton – contains microfilaments that bond to membrane proteins to stabilize location of certain membrane proteins as well as maintain cell shape cholesterol – reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, at low temperatures it disrupts packing of phospholipids and therefore solidification glycolipid – lipids that acquired carbohydrates, secreted by vesicles on the outside of the plasma membrane integral protein – penetrate the hydrophobic core of the lipid bilayer, many of which completely span the membrane peripheral protein – not embedded in the lipid bilayer, loosely bound to surface of the membrane, often to exposed parts of the integral proteins 5. List the six broad functions of membrane proteins. The six functions of membrane proteins are transport, enzymatic activity, signal transduction, cell to cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix. 6. How do glycolipids and glycoproteins help in cell to cell recognition? Glycolipids and glycoproteins aid in cell to cell recognition by being identification tags that are specifically recognized by other cells. Glycolipids and glycoproteins are lipids and proteins covalently bonded to a carbohydrate, respectively. 7. Why is membrane sidedness an important concept in cell biology? Membrane sidedness is an important concept in cell biology. This is because the two lipid layers, which differ in specific lipid composition and contain proteins with a directional orientation in the membrane, is important for endocytosis and exocytosis. When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes a part of the cytoplasmic layer of the plasma membrane. This means that molecules that start out on the inside face of the endoplasmic reticulum end up on the outside face of the plasma membrane, due the membrane sidedness. 8. What is diffusion and how does a concentration gradient relate to passive transport? Diffusion is the movement of molecules from high concentration to low concentration. A concentration gradient relates to passive transport because a substance diffuses down its concentration gradient and thus it is spontaneous and doesn’t require any ATP from the cell, thus it is passive transport. The concentration gradient itself represents potential energy. 9. Why is free water concentration the “driving” force in osmosis? Free water concentration is the driving force in osmosis because water moves from a high concentration to a low concentration thus the concentration gradient decides in which direction water will flow. 10. Why is water balance different for cells that have walls as compared to cells without walls? Water balance is different for cells with walls compared to cells without walls due to pressure. Cells without walls that are immersed in an isotonic environment, there will be no net movement of water across the plasma membrane, because water is flowing across the membrane at the same rate in both directions. Thus, in an isotonic environment, the volume of a cell without walls is stable. In a hypertonic solution, the cell will lose water to its environment and most likely die. In a hypotonic solution, water will enter the cell at a faster rate than it leaves, thus the cell will burst. Cells without walls thus must have special adaptations for osmoregulation, the control of water balance. For cells with walls, the cell can only expand so much before it exerts a pressure that opposes further uptake. In an isotonic solution, the cell becomes limp because there is no tendency for water to enter. In a hypertonic solution, however, the cell wall is no benefit because Page 2 of 4 the cell will lose all of its water either way. 11. Label the diagram below: 12. What is the relationship between ion channels, gated channels and facilitated diffusion? Facilitated diffusion is the process of polar molecules and ions diffusing passively accrose the membrane with the help of transport proteins. Ion channels, many of which are gated channels, function by a stimulus that causes them to open or close, allowing facilitated diffusion to occur. The stimulus may be either electrical or chemical, and is different than the substance being transported. 13. How is ATP specifically used in active transport? To pump a molecule across a membrane against its gradient, the cell must expend energy in the form of ATP. When the cell expends energy it is called active transport. Carrier proteins rather than channel proteins are used to enable the molecule to move against the gradient. ATIP powers active transport by transferring its terminal phosphate group directly to the transport protein. This induces the protein to change its conformation in a manner that translocates a solute bound to the protein across the membrane. For example, the sodium-potassium pump exchanges sodium for potassium across the plasma membrane. 14. Define and contrast the following terms: membrane potential, electrochemical gradient, electrogenic pump and proton pump. Membrane potential is the voltage across a membrane, acting like a battery that affects the traffic of all charged substances across the membrane. It favors the passive transport of cations into the cell and anions out of the cell because the inside of the cell is negative. Electrochemical gradient is the combination of forces acting on an ion. Electrogenic pump is a transport protein that generates voltage across a membrane, the sodium-potassium being the major one of animal cells. Proton pump is the main electrogenic pump of plants, fungi, and bacteria, which works by Page 3 of 4 actively transporting hydrogen ions out of the cell. 15. What is cotransport and why is an advantage in living systems? Cotransport is a single ATP-powered pump that transports a specific solute, indirectly driving the active transport of several other solutes. It is an advantage in living systems because a substance that has been pumped across a membrane can do work as it moves back down across the membrane by diffusion, being very efficient. 16. What is a ligand? A ligand is a general term for any molecule that binds specifically to a receptor site of another molecule. 17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated endocytosis. Phagocytosis is a type of endocytosis that involves large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendridic cells. Pinocytosis is another type of endocytosis for smaller molecules in which the cell ingests extracellular fluid and its dissolved solutes. Receptor-mediated endocytosis is the movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in, enabling a cell to acquire bulk quantities of specific substances. Page 4 of 4