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Biology ReviewCellular Work and Related Processes
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Much of the text material is from, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon
(2004 and 2008). I don’t claim authorship. Other sources were also used and are noted.
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Outline
• Harvesting of chemical energy• Enzymes and enzyme inhibitors• Cell membrane transport
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Harvesting of Chemical Energy
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Chemical Energy
• Food, gasoline, and other fuels are forms of chemical energy, a type of potential energy.
• Carbohydrates and fats have carbon backbones that make them rich in chemical energy.
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Harvesting of Chemical Energy
• Cells and internal combustion engines use similar physical processes to perform work.
• A gasoline engine mixes oxygen with octane in an explosive chemical reaction that breaks-down the covalent bonds in the carbon backbone for rapid liberation of energy.
• The reaction moves the pistons in the cylinders, which ultimately drives the wheels.
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Harvesting of Chemical Energy (continued)
• Cells use oxygen to harvest chemical energy from food molecules in a more controlled process known as cellular respiration.
• Aerobic cellular respiration, which requires oxygen, occurs in the mito-chondria of eukaryotic cells.
• Anaerobic respiration, which does not use oxygen, occurs in the cytosol of prokaryotic and eukaryotic cells.
• In both types of respiration, the molecule ATP (adenosine triphosphate) is generated as a chemical energy source for cellular work.
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ATP Molecule
http://biology.clc.uc.edu
Adenosine triphosphate (ATP)
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Efficiencies
• Automobile engines extract about 25 percent of the chemical energy to produce kinetic energy to drive the wheels—the rest is converted to heat.
• Cells extract about 40 percent of the chemical energy from food mole-cules to perform cellular work.
• The waste, or byproducts, of internal combustion engines and cellular respiration are mostly CO2 and water.
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Cellular Work
• Cellular work consists of maintaining the body’s metabolism (sum of all its chemical processes) and powering muscle contractions.
• The remaining 60 percent of the chemical energy generates body heat to maintain the body at a constant temperature—about 98.6oF or 37oC.
• Sweating and other cooling mechanisms enable the body to lose excess heat including in hot environments and physical exercise.
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Life Requires Chemical Energy
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Calories
• Calorie is a unit of chemical energy used in the physical and biological sciences.
• A calorie (c) is the amount of energy required to raise the temperature of one gram of water by one degree Celsius (oC).
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Calories (continued)
• The caloric content of food is measured by burning it completely to ash under a container of water, and measuring the increase in the water temperature.
• A handful of peanuts has enough chemical energy to boil more than a quart of water if the peanuts could be completely converted to heat.
• A calorimeter is used by food scientists to measure the caloric con-tent of foods.
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Bomb Calorimeter
http://chemistry.umeche.maine.edu
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Kilocalories
• The use of calories (c) to express the energy content of food is not practical since a calorie is a very small unit of measurement.
• The daily recommended diet for adults would be about 2.0 x 106, or two million calories.
• Instead, kilocalories (kcal or C) are used in which one kilocalorie is equals to 1,000 calories (c).
Kilo- = 1,000
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Kilocalories (continued)
• The daily recommended diet for adults is about 2,000 kilocalories (C).
• Gender, age, basal metabolic rate, physical activity, and other factors determine recommended caloric intake.
• The calories listed on food labels are always expressed in kilocalories.
• A way to minimize confusion between c and C is to refer to kilocalories as food calories.
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Food Label
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Caloric Densities
http://www.asymptotia.com
http://www.wisegeek.com
Each plate contains about 200 food calories.
Although certain foods may have about equal food caloric content,
they can differ substantially in caloric densities.
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Caloric Accounting
• Caloric accounting considers a person’s food intake, basal metabolic rate, and physical activity.
• Caloric imbalances between food intake, and basal metabolic rate and physical activity can lead to weight gain or weight loss.
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Calorie Expenditures
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600 kcal (fast), 200 kcal (slow)
535 kcal (2 mph)
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160 kcal (3 mph)
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510 kcal (fast) �170 kcal (slow)
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Enzymes and Enzyme Inhibitors
http://www.unc.edu
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Metabolism and Enzymes
• The sum of all chemical reactions in an organism is known as its metab-olism.
• Enzymes are specialized proteins that lower activation thresholds and speed-up many types of chemical reactions.
• The covalent bonds in molecules must be broken to initiate a chemical reaction.
• Covalent bond breakage occurs, for example, when a disaccharide (a double sugar) is hydrolyzed into two monosaccharides (single sugars).
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Enzymes
• Energy, usually in the form of heat, is needed for a chemical reaction to occur—the threshold amount of heat is called its activation energy.
• Adding substantial amounts of heat is often not possible or desirable with living cells.
• Enzymes enable metabolism to occur at lower temperatures by reduc-ing the amount of activation energy required to break molecular bonds.
• Enzymes are catalysts that lower the barriers for chemical reactions to occur.
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Induced Fit
• An enzyme is specific in the chemical reaction it catalyzes although thousands of different chemical reactions occur in the human body.
• The active site of an enzyme has a shape that fits a portion of the substrate molecule, much like the correct key readily opens a door lock.
• As an enzyme attaches to the substrate, it changes its shape slightly to enable a physical embrace between the molecules in what is called induced fit.
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Breaking of Covalent Bonds
• The enzyme places the substrate under physical or chemical stress, making it easier to break the covalent bonds and initiate the chemical reaction.
• Once the covalent bonds are broken, the enzyme molecule can bind with another substrate molecule to begin the process again.
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Enzyme Inhibitors
• Certain types of molecules can inhibit metabolic reactions by binding to enzymes and disrupting their functions.
• Enzyme inhibitors are specific to the enzymes they target.
• Some inhibitors are imposters of substrates that bind to enzymes.
• Other inhibitors bind to a different part of the enzyme and change the shape of the active site so that it can no longer bind to the substrate.
• Organisms produce enzyme inhibitors to control the overproduction of enzymes.
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Enzyme Inhibitors (continued)
• Enzymes inhibitors are manufactured for many medical and biological purposes.
• Malathion, an insecticide, inhibits an enzyme for the functioning of insect nervous systems.
• Aerial spraying of malathion has been used to control Mediterranean fruit fly infestations in southern California—it turned-out to be a controversial program.
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American Civil War
http://www.a2zcds.com
http://nmhm.washingtondc.museum
During the American Civil War, many soldiers died from bacterial infections in the treatment of their wounds—possibly as many as
died in battle.
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Antibiotics
• Antibiotics are derived from microorganisms that disable or kill bacteria.• In the 1920s, Alexander Fleming discovered penicillin when he found a
mold prevented the growth of bacteria that he was trying to cultivate in bread.
• Penicillin, the first antibiotic to be developed, inhibits an enzyme needed to form the cell walls in bacteria.
• The death rates from diseases such as bacterial pneumonia and surgi-cal infections dropped substantially once antibiotics were widely avail-able. �
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Targets of Other Antibiotics
• Ampicillin and bacitracin—bacterial cell walls.
• Erythromycin, streptomycin, and tetracycline—bacterial ribosomes.
• Ciprofloxacin—bacterial chromosomal structure.
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Cell Membrane Transport
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Cell Membrane Transport
• Cells can control the flow of materials across their plasma membranes.
• A major function of the plasma membrane, in addition to providing the cell boundary, is regulating the movement of molecules into and out of the cell.
• Three forms of transport are: diffusion, osmosis, and active transport— the first two are passive processes that do not require chemical energy.
http://upload.wikimedia.org
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Diffusion
• The heat energy of molecules causes them vibrate and this move ran-domly in what is called Brownian motion.
• A result of the motion is diffusion, the tendency of molecules to spread into the available space.
• Although each molecule moves randomly, the overall movement is in one direction, from high- to low-concentration, along its concentration gradient.
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Diffusion and Directional Movement
• The directional movement can be shown with movement of dye across a semi-permeable membrane in a container of water
• The membrane has pores large enough to pass the dye molecules but not the water.
• An equilibrium exists once the dye is evenly diffused—the number of molecules moving across the membrane is now about the same in both directions.
• Two different dyes will diffuse along their own concentration gradients as if the other molecule did not exist.
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Concentration Gradients
Passage of time
Concentration gradient
Individual concentration gradientsRed
Green
Passage of time
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Passive Transport
• Diffusion across a semi-permeable membrane is a form of passive transport since it does not require expenditure of chemical energy.
• A cell’s plasma membrane is selectively permeable to some mole-cules.
• The membrane allows certain small ions to pass (such as Na+ and K+), but not large molecules such as proteins and phosphate groups.
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Passive Transport (continued)
• Passive transport is an important process for maintaining all living cells.
• For example, O2 enters the hemoglobin of red blood cells through passive diffusion to be transported in blood to meet the metabolic needs of the body’s tissues.
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Osmosis
• The passive transport of water across a semi-permeable membrane is called osmosis.
• Consider a membrane separating two compartments that is permeable to H2O but not to C6H12O6 (glucose), a larger molecule.
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Osmosis (continued)
• The solution with a higher concentration of solute (in this case, glucose) is hypertonic and the solution with a lower solute concentration is hypo-tonic.
• H2O will diffuse across the membrane from the hypotonic solution to the hypertonic solution until equilibrium is established.
• The solutions are isotonic once they reach equal concentrations of glu-cose.
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Osmosis (continued)
H2O molecules are small enough to pass through the semi-permeable membrane, but the glucose molecules cannot pass
because they are much larger.
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Water Regulation
• The survival of cells depends on the body’s ability to regulate water uptake and loss.
• When red blood cells are immersed in an isotonic solution, the volume of the cells remains constant.
• Hypotonic and hypertonic environments can cause a cell to expand and burst or shrivel and die.
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Red Blood Cells
http://www.ccs.k12.in.us
Distilled water = hypotonic solution.Salt water = hypertonic solution.
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Active Transport
• Active transport requires the expenditure of chemical energy to move molecules across a cell’s plasma membrane.
• A transport protein in the plasma membrane, using ATP as its energy source, pumps the solute across the membrane against its concentra-tion gradient.
• Active transport enables cells to maintain intracellular concentrations of molecules that differ from the extracellular environment.
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Active Transport (continued)
• A cell generally has a higher concentration of potassium (K+) ions and lower concentration of sodium (Na+) ions in the intracellular space.
• The concentration differences are regulated by the sodium-potassium pump of transport proteins.
• This pump is vital in enabling neurons to generate nerve impulses—or action potentials—as we will discuss in an upcoming lecture.
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Exocytosis
• Large molecules, including many proteins, are too large to fit through the plasma membrane.
• Transport vesicles carry proteins manufactured by the ribosomes, and fuse with the plasma membrane to empty their contents outside of the cell.
• The process is known as exocytosis.
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Endocytosis
• The opposite process is endocytosis—cells take in materials such as food molecules and water in vesicles that bud inward from the plasma membrane.
• Endocytosis and exocytosis both require chemical energy, and thus are active processes.
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Receptor-Mediated Endocytosis
• Another type of endocytosis is when certain external molecules bind with receptor proteins in the plasma membrane to be transported into the cell.
• This process is called receptor-mediated endocytosis.
• In a genetic disorder, plasma membranes of cells cannot take-up sufficient amounts of cholesterol bound to low density lipoproteins (LDL).
• High LDL levels result, which can lead to cardiovascular problems if left untreated or inadequately treated.
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Cell Signaling
• Many types of cells can communicate with each other across their plasma membranes.
• A signal from outside of the cell—such as a water-soluble hormone—is received by receptor proteins in the plasma membrane or cytoplasm.
• The signal triggers a chemical chain reaction inside the cell, in what is known as a signal transduction pathway.
• The signal can lead to responses such as metabolic changes in the cell or rearrangement of the cytoskeleton.
• Cell signaling is a key mechanism for many hormones of the endocrine system.
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