AP Biology CMR @ CHS ‘95 B.Rifepage 1 / 7
II. Unit 2: The Working of the Cell - Chapter 5
5.1 Energy Is The Capacity To Perform Work
Energy is defined as the capacity to do work.
Kinetic energy is the energy of motion.
Heat is the kinetic energy associated with randomly moving molecules.
Potential energy is the stored capacity to perform work. The most important form of potential energy in living things is the potential energy stored in the arrangement of atoms (chemical bond energy) in molecules. This is called chemical energy.
5.2 Two Laws Govern Energy Conversion
Thermodynamics is the study of energy transformations that occur in matter.
First Law of Thermodynamics (Law of energy conservation)
The total amount of energy in the universe is constant; this energy can be transferred or transformed but neither created nor destroyed.
Second Law of Thermodynamics When one from of energy is transformed into another form, some useful energy will always be lost as heat and therefore energy cannot be recycled.
Every energy change results in increased disorder (heat). The amount of disorder in a system is called entropy. In other words, the amount of disorder (entropy) is always increasing in the universe.
Living things continually lose useful energy to the environment, but a new supply comes to them in the form of organic food produced by photosynthesizing organisms.
Only 42% of solar energy gets to the earth’s surface; the rest is absorbed by or reflected into the atmosphere and becomes heat.
Of this usable portion, only 2% is eventually utilized by plants; the rest becomes heat
Of this, only 20% is eaten by herbivores; a large proportion of the remainder becomes heat. (fossil fuels)
Of this, only 30% is ever eaten be carnivores; a large proportion becomes heat. (fossil fuels)
Conclusion: Most of the available energy is never utilized by living things.
5.3 Life’s Chemical Reactions Either Store Or Release Energy
Endergonic reactions require an input of energy equal to the difference in the potential energy of the reactants and products. These reactions are synthetic ones that require energy input.
Photosynthesis (Chapter 7) is an important process that is endergonic, requiring the energy of sunlight to cause energy-poor reactants to react to form energy-rich products.
Exergonic reactions result in an output of energy equal to the difference in the potential energy of the reactants and products. These reactions are degradative ones that release energy.
Burning and cellular respiration are both exergonic processes by which the chemical energy of the reactants is released to form energy-poor products. In the case of burning, this happens all at once, with much “waste” of the chemical energy to form heat and light.
Cellular respiration (Chapter 6) is an important biological process that releases the potential energy of sugar reactants, slowly, to form some energy- poor reactants and, most important, to convert chemical energy of sugar into smaller, usable amounts of chemical energy in the form of ATP.
Cellular metabolism is the sum total of all the endergonic and exergonic reactions in cells.
5.4 ATP Shuttles Chemical Energy Within The Cell
Most endergonic cellular reactions require small amounts of energy, rather than the large amounts of energy available in food storage molecules.
Adenosine triphosphate (ATP) is the energy-rich, spendable, “small change” of cellular reactions. It transfers usable amounts of energy from exergonic, food energy-releasing reactions to the endergonic reactions where cell work is done.
The hydrolysis (Section 3.3) of ATP to release some of its chemical energy is an exergonic reaction. When ATP gives up its energy, it forms ADP and an energy shuttle, the phosphate group.
The phosphate group is one of the reactants and the energy source for an endergonic reaction. This energizing process is known as phosphorylation. The products of the reaction hold chemical energy and are ready to do work.
ATP regeneration is the reverse process. Endergonic reactions involved in cellular respiration phosphorylate (and energize) ADP in dehydration synthesis (Section 3.3).
Energy coupling - using energy released from exergonic reactions to drive essential endergonic reactions. ATP is regenerated about once each minute.
5.5 Enzymes Speed Up The Cell’s Chemical Reactions By Lowering Barriers
Enzymes are large protein molecules that function as biological catalysts.
A catalyst is a chemical that speeds up the reaction without being consumed by it.
The energy of activation is the amount of energy, an “energy barrier”, that must be put into an exergonic (energy producing) reaction before the reaction will proceed.
Enzymes speed up the cell’s chemical reactions by lowering energy barriers.
5.6 A Specific Enzyme Catalyzes Each Cellular Reaction
A specific enzyme catalyzes each cellular reaction. No reaction can occur in a cell unless its own enzyme is present and active.
How enzymes catalyze reactions
The reactant in an enzyme-catalyzed reaction is the substrate.
One part of the enzymes binds to the substrate at the active site, holding the substrate in a specific position that facilitates the reaction.
At the end of the reaction, the substrate changes into the product, and the enzyme is released, unchanged.
Enzymes are proteins that speed up chemical reactions by lowering the energy of activation. They do this by forming an enzyme-substrate complex.
For example, the breakdown of hydrogen peroxide into water and oxygen can occur 600,000 times a second when the enzyme catalase is present.
5.7 The Cellular Environment Affects Enzyme Activity
The cellular environment affects enzyme activity. Such factors as temperature, pH, salt concentration, and the presence of cofactors often affect the way enzymes work.
Generally, an enzymes activity increases with greater substrate concentrations.
A higher temperature generally results in an increase in enzyme activity
If the temperature rises beyond a certain point, however, enzyme activity eventually levels out and then declines rapidly because the enzyme is denatured at high temperatures. During denaturation an enzyme’s shape changes so that its active site can no longer bind substrate molecules efficiently.
Each enzyme has an optimal pH which helps maintain its normal configuration. (pH affects H bonds and cause denaturation)
Many enzymes require a nonprotein cofactor (Mg++,K+,Ca++) to assist them in carrying out their functions.
Some other cofactors, called coenzymes, are organic molecules that bind to enzymes and serve as carriers for chemical groups or electrons.
5.8 Enzyme Inhibitors Block Enzyme Action
Enzyme inhibitors block enzyme action. They do this by binding with the active site (competitive inhibitors) or some other site (allosteric site) (noncompetitive inhibitors) on the enzyme, thus affecting the enzyme’s ability to bind with the substrate.
Feedback inhibition is an example of a common biological control mechanism called negative feedback. When the product is in abundance, it binds competitively with its enzymes active site; as the product is used up, inhibition is reduced and more product can be produced.
5.9 Some Pesticides And Antibiotics Inhibit Enzymes
Some pesticides and antibiotics inhibit enzymes. For example, the pesticide malathion inhibits the enzyme acetylcholinesterase, involved in nerve transmission. The antibiotic penicillin interferes with an enzyme that helps build bacterial cell walls.
5.10 Membranes Organize The Chemical Activities Of Cells
Membranes separate cells from their outside environments, including, in multicellular organisms, that environment in other cells that performs different functions..
They control the passage of molecules form one side of the membrane to the other.
In eukaryotes, they partition function into organelles.
The provide reaction surfaces, and organize enzymes and their substrates.
The plasma membrane regulates the passage of molecules into and out of the cell.
The plasma membrane is selectively permeable - it has special mechanisms to regulate the passage of most molecules into and out of the cell.
5.11 Membrane Phospholipids Form A Bilayer
Phospholipids are like fats, with two nonpolar (hydrophobic) fatty acid “tails” and one polar (hydrophilic) phosphate “head” attached to the glycerol.
In water, thousands of individual molecules form a stable bilayer, aiming their heads (hydrophilic) out and their tails (hydrophobic) in.
The hydrophobic interior of this bilayer offers an effective barrier (selectively permeable) to the flow of most hydrophilic molecules.
5.12 The Membrane Is A Fluid Mosaic Of Phospholipids And Proteins
The fluid-mosaic model of membrane structure is widely accepted at this time.
It is fluid because the individual molecules are more-or-less free to move about laterally.
Cholesterol is a common constituent of animal cell membranes and helps stabilize the fluidity at different temperatures.
Some proteins extend through both sides of the bilayer.
5.13 Proteins Make The Membrane A Mosaic Of Function
The movement of molecules through the plasma membrane is dependent upon its protein component.
Functions for plasma membrane proteins:
1. cell to cell recognition (Identification tags - particularly glycoproteins)
2. receptors for chemical messengers (trigger cell activity when a messenger molecule attaches.
3. enzymes catalyzing intracellular and extracellular reactions
4. passage of (hydrophilic) molecules across the membrane
5. Cell junctions - either attachments to other cells or the internal cytoskeleton.
5.14 Passive Transport Is Diffusion Across A Membrane
Diffusion is the tendency for particles of any kind to spread out spontaneously from an area of high concentration to an area of low concentration.
Only a few types of small molecules freely diffuse through the plasma membrane.
Passive transport across membranes occurs when a molecule diffuses down a concentration gradient. At equilibrium, molecules continue to diffuse back and forth, but there is no net change in concentration anywhere.
Different molecules diffuse independently of one another.
Passive transport is an extremely important way for small molecules to get into and out of cells. For example, O2 moves into red blood cells and CO2 moves out of these cells by this process in the lungs.
5.15 Osmosis Is The Passive Transport Of Water
Osmosis is the diffusion of water across a selectively permeable membrane. The presence of osmotic pressure is evident when there is an increased amount of water on t he side of the membrane that has the higher solute concentration.
When a cell is placed in an isotonic solution (same solute concentration), it neither gains nor loses water.
When a cell is placed in a hypotonic solution (less solute conc. - more water conc.) the cell gains water.
When a cell is placed in a hypertonic solution (greater solute conc. - less water conc.) the cells loses water and the cytoplasm shrinks.
5.16 Water Balance Between Cells And Their Surroundings Is Crucial To Organisms
If a plant or animal cell is isotonic with its surroundings, no osmosis occurs, and the cells do not change. however plant cells in such environments are flaccid or wilted, lacking the turgor that helps support some plant tissues.
An animal cell in a hypotonic solution will gain water and lyse (burst)
A plant cell in a hypotonic solution, the cytoplasm and central vacuoles gain water, and plasma membrane pushes against the rigid cell wall. The resulting pressure, called turgor pressure, helps give internal support to the cell.
An animal cell in a hypertonic solution will lose water and shrivel. A plant cell in a hypertonic solution will pull the plasma membrane away from the cell wall and lose turgor.
5.17 Specific Proteins Facilitate Diffusion Across Membranes
Facilitated diffusion occurs when a pored protein, spanning the membrane bilayer, allows a solute to diffuse down a concentration gradient. No energy expenditure is required in this case. The rate of facilitated diffusion depends on the number of such transport proteins, in addition to the strength of the concentration gradient.
5.18 Cells Expend Energy For Active Transport
Active transport involves the aid of a transport protein in moving a solute up a concentration gradient.
Energy expenditure in the form of ATP-mediated phosphorylation is required to help the protein change its structure and thus move the solute molecule.
Active transport proteins often couple the passage of two solutes in opposite directions across membranes. A very important example of a coupled active transport system is the Na+ - K+ pump, which functions in nerve impulse transmission (See Sec 28.5)
5.19 Exocytosis And Endocytosis Transports Large Molecules
Some molecules are too large to be transported by protein carriers; instead they are taken into the cell by vesicle formation.
In exocytosis, membrane-bounded vesicles containing large molecules fuse with the plasma membrane and release their contents outside the cell.
In endocytosis, the plasma membrane surrounds materials outside the cell, closes around the materials, and forms membrane-bounded vesicles containing the materials.
When the material taken in by endocytosis is quite large, the process is called phagocytosis (cell eating)
Vesicles may also form around large-sized molecules such as proteins; this is called pinocytosis (cell drinking). Both phagocytic and pinocytic vesicles can fuse with lysosomes whose enzymes will digest their contents.
During receptor-mediated endocytosis, a receptor binds with a specific nutrient molecule, called a ligand, and then these receptors gather at a location where endocytosis is beginning.
5.20 Faulty Membranes Can Overload The Blood With Cholesterol
Cholesterol is carried in the blood with cholesterol.
In people with normal cholesterol metabolism, excess LDL-bound cholesterol in the blood is eliminated by receptor-mediated endocytosis by liver cells.
In people with a genetic condition that results in hypercholesterolemia, fewer or no receptor sites exist, and the people accumulate LDL-bound cholesterol, perhaps leading to heart disease.
5.21 Chloroplasts And Mitochondria Make Energy Available For Cellular Work
Photosynthesis and cellular respiration are linked.
1. Solar energy is used to build energy-rich molecules in endergonic reactions in chloroplasts.
2. The energy-rich molecules release their energy to form ATP in mitochondria.
3. The chemicals involved as the reactants in chloroplasts are the products in mitochondria, and vice versa.