BSC 2010 - Exam I Lectures and Text Pages I. Intro to Biology (2-29) II. Chemistry of Life Chemistry review (30-46) Water (47-57) Carbon (58-67) Macromolecules (68-91) III. Cells and Membranes Cell structure (92-123) Membranes (124-140) IV. Introductory Biochemistry Energy and Metabolism (141-159) Cellular Respiration (160-180) Photosynthesis (181-200) Plasma Membranes - Transport Review what moves easily across the phospholipid bilayer, and what does not. Ways of Crossing the Plasma Membrane Passive Transport vs. Active Transport Passive transport: pathways that do not involve energy expenditure from the cell (in the form of ATP) Diffusion : Molecules naturally move from an area of higher concentration to one of lower concentration until equilibrium is reached. Rate can be affected by temperature, molecule size, and charge. 1
Diffusion Is the tendency for molecules of any substance to spread out evenly into the available space (a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Net diffusion Net diffusion Equilibrium Figure 7.11 A Diffusion Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Figure 7.11 B Effects of Osmosis on Water Balance Osmosis : the diffusion of water through a semi-permeable membrane. Water moves from an area of higher water concentration to an area of lower water concentration to reach equilibrium on both sides of the membrane. 2
Osmosis Is affected by the concentration gradient of dissolved substances Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can More free water molecules (higher concentration) Figure 7.12 Osmosis Water moves from an area of higher free water concentration to an area of lower free water concentration Water molecules cluster around sugar molecules Fewer free water molecules (lower concentration) Water Balance of Cells Without Walls Tonicity Is the ability of a solution to cause a cell to gain or lose water. Has a great impact on cells without walls. Is always a comparison of relative solute concentrations between two solutions. 3 Categories of Relative Concentration (Tonicity) When comparing two solutions (A and B), solution A is * Hypotonic to solution B if solution A has a lower concentration of solutes than B. * Hypertonic to solution B if solution A has a higher concentration of solutes than B. * Isotonic to solution B if the concentration of solutes is equal. 3
3 Categories of Relative Concentration (Tonicity) Example: A cell with a concentration of sodium at 1 g/l is placed in beaker of water with a sodium concentration of 10 g/l. How do we describe this? ---We can say that the cell is hypotonic to the water in the beaker OR we can say that the water is hypertonic to the cell. Most cells are bathed in an isotonic solution, so there is no net osmosis occurring. Be sure you understand these terms and read this section of your text. Isotonicity If a solution is isotonic to the cell. The concentration of solutes is the same as it is inside the cell Therefore, the concentration of water is the same between the two solutions There will be no net movement of water Hypertonicity If a solution is hypertonic to the cell The concentration of solutes is greater in the external solution than it is inside the cell Therefore the concentration of water is greater inside the cell than outside The cell will lose water to the external solution 4
Hypotonicity If a solution is hypotonic The concentration of solutes in the external solution is less than it is inside the cell Therefore, the concentration of water is greater outside the cell. The cell will gain water from the external solution. Water balance in cells without walls Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments Must have special adaptations for osmoregulation (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Figure 7.13 Lysed Normal Shriveled Water Balance of Cells with Walls Cell walls Help maintain water balance 5
Turgidity If a plant cell is turgid It is in a hypotonic environment It is very firm, a healthy state in most plants Flaccidity If a plant cell is flaccid It is in an isotonic or hypertonic environment In a hypertonic environment, the cell may even become separated from the cell wall. Water balance in cells with walls Hyptotonic Solution Isotonic Solution Hypertonic Solution (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H 2 O H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed Figure 7.13 6
Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion Transport proteins speed the movement of molecules across the plasma membrane Facilitated diffusion: Normal diffusion occurs, but through a protein channel in the membrane, not through the phospholipid bilayer. This also takes no added energy from the cell. Facilitated Diffusion Channel proteins Provide corridors that allow a specific molecule or ion to cross the membrane EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15 Carrier proteins Carrier proteins Undergo a subtle change in shape that translocates the solute-binding site across the membrane Carrier protein Solute (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net Figure 7.15 movement being down the concentration gradient of the solute. 7
Active Transport Active transport uses energy (ATP) to move solutes against their gradients Uses carrier proteins to help molecules across membrane 1. usually pumps one thing out of cell while pumping another into the cell. e.g. Na-K pump in nerve cells - pumps Na out and K in. 2. used to keep the concentration of a certain substance higher inside the cell than outside. Very important in keeping trace element levels high enough in the cell though they may be low outside. The Need for Energy in Active Transport Active transport Moves substances against their concentration gradients Requires energy, usually in the form of ATP The sodium-potassium pump The sodium-potassium pump Is one type of active transport system EXTRACELLULAR 1 Cytoplasmic Na binds to FLUID [Na ] high the sodium-potassium pump. [K ] low Na Na [Na ] low Na CYTOPLASM [K ] high 2 Na binding stimulates phosphorylation by ATP. Na Na Na P ATP ADP Na Na Na 3 K is released and Na K 4 Phosphorylation causes the sites are receptive again; protein to change its conformation, the cycle repeats. K P expelling Na to the outside. K K 5 Loss of the phosphate restores the protein s original conformation. K K P P i 6 Extracellular K binds to the protein, triggering release of the Phosphate group. Figure 7.16 8
Review: Passive and active transport compared Review: Passive and active transport compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. Figure 7.17 Diffusion. Hydrophobic Facilitated diffusion. Many molecules and (at a slow hydrophilic substances diffuse rate) very small uncharged through membranes with the polar molecules can diffuse assistance of transport proteins, through the lipid bilayer. either channel or carrier proteins. ATP Maintenance of Membrane Potential by Ion Pumps Membrane potential Is the voltage difference across a membrane Electrochemical Gradients An electrochemical gradient Is caused by the difference in concentration of ions across a membrane 9
Electrogenic pumps An electrogenic pump Is a transport protein that generates the voltage across a membrane ATP EXTRACELLULAR FLUID H H Proton pump H H Figure 7.18 CYTOPLASM H H Cotransport: Coupled Transport by a Membrane Protein Cotransport Occurs when active transport of a specific solute indirectly drives the active transport of another solute Cotransport Cotransport: active transport driven by a concentration gradient ATP H H Proton pump H H Figure 7.19 Sucrose-H cotransporter H Diffusion of H H Sucrose H 10
Bulk Transport Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large proteins and polysaccharides Cross the membrane in vesicles Exocytosis In exocytosis Transport vesicles migrate to the plasma membrane and fuse with it, becoming part of the plasma membrane. In this way, vesicle contents are released to the outside of the cell = secretion Endocytosis In endocytosis The cell takes in macromolecules. New vesicles bud inward from the plasma membrane There are three types of endocytosis 1. Phagocytosis - (cell eating) 2. Pinocytosis - (cell drinking) 3. Receptor-mediated endocytosis - specifically transports necessary substances which are recognized by receptor molecules on the plasma membrane. 11
Three types of endocytosis Three types of endocytosis In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium Food or other particle Food vacuole Bacterium Food vacuole 1 µm Pseudopodium of amoeba In pinocytosis, the cell gulps droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. Figure 7.20 PINOCYTOSIS Plasma membrane Vesicle An amoeba engulfing a bacterium via phagocytosis (TEM). 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated Ligand pit Coat A coated pit protein and a coated vesicle formed during receptormediated endocytosis (TEMs). Plasma membrane 0.25 µm 12