Energy can be converted from one form to another form

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Chapter 5: Ground Rules of Metabolism – Energy Flow, Metabolic Pathways, Enzymes

Energy can be converted from one form to another form
Energy can be converted from one form to another form. Many living organisms take advantage of this. For example, plants use carbon dioxide, water, and energy from the sun to produce glucose, a molecule that can be broken down to provide energy for cellular reactions. Other organisms, such as ourselves, consume plants and other organisms, and the energy stored in some of the their molecules can be used to fuel cellular reactions in our cells. You also take advantage of energy conversion when you drive your car. Energy stored in gasoline molecules is converted to movement when you press on the accelerator. globalchange.umich.edu

What Is Energy? Energy is the capacity to do work
Work is a force acting on an object that causes the object to move

What Is Energy? Chemical energy The energy that powers life
The objects that move are electrons, which reposition during chemical reactions

Laws of Thermodynamics
2 fundamental types of energy Kinetic energy the energy of movement e.g. light, heat, electricity, moving objects Potential energy stored energy e.g. chemical energy in bonds, electrical charge in a battery, a rock at the top of a hill

FIGURE 6-1 From potential to kinetic energy
Perched atop an ice floe, the body of the penguin has potential energy, because the ice is much higher than the ocean. As it dives, the potential energy is converted to the kinetic energy of motion of the penguin's body. Finally, some of this kinetic energy transferred to the water causes the water to splash and ripple.

The Laws of Thermodynamics
describe the availability and usefulness of energy

First Law of Thermodynamics
Energy can neither be created nor destroyed Total amount of energy within a system remains constant unless energy is added or removed from system

Second Law of Thermodynamics
Amount of useful energy decreases when energy is converted from one form to another Ex. When glucose is broken down in the body to get energy, not all of the stored energy in that molecule is used. Some of the energy is lost in the form of heat, which is not a usable form of energy for the organism. As energy is converted from one form to another, Entropy (disorder) increases

FIGURE 6-2 Energy conversions result in a loss of useful energy
FIGURE 6-2 Energy conversions result in a loss of useful energy. There are 100 units of chemical energy stored in the gas can. However, when that fuel is burned, only 25 units of that energy is converted to kinetic energy so that the car can move. The other 75 units of energy is lost in the form of heat. The second law of thermodynamics states that as energy is converted from one form to another, usable energy decreases and entropy increases.

Entropy A cell works similarly to keeping your room clean. In a cell and in keeping a neat room, to keep things organized and in their place, there must be a constant input of energy to counteract the effects of entropy (disorder). A cell must have a constant input of energy in order to function properly.

Energy of Sunlight Autotrophic organisms, such as plants, acquire their energy from the sun. Heterotrophic organisms like the monkey above get energy by consuming other organisms. freefoto.com picturesindia.com

Chemical Reactions Processes that form or break chemical bonds between atoms Chemical reactions convert reactants to products Reactants Products

Chemical Reactions Reactions can be categorized as exergonic or endergonic based on energy gain or loss

Exergonic Reactions Release energy
Reactants contain more energy than products

Exergonic Reactions Ex: the burning of glucose

Activation Energy All chemical rxns require an initial energy input (activation energy) to get started Electrons of an atom repel other atoms and inhibit bond formation

FIGURE 6-6 Energy relations in exergonic reactions
An exergonic ("downhill") reaction, such as burning sugar, proceeds from high-energy reactants (here, glucose and O2) to low-energy products (CO2 and H2O ). The energy difference between the chemical bonds of the reactants and products is released as heat. To start the reaction, however, an initial input of energy—the activation energy—is required.

Endergonic Reactions Require an input of energy
Products contain more energy than reactants

Endergonic Reactions Ex: photosynthesis

Coupled Reactions Exergonic rxns drive endergonic rxns
Energy-carrier molecules used to transfer energy within cells

Energy Carrier Molecules
Energy carrier molecules are only used within cells because they are unstable newgeology.us motherearthnews.com

most common energy carrying molecule Composed of an adenosine molecule and 3 phosphates

ATP Energy stored in high-energy bond extending to last phosphate
Heat is given off when ATP breaks into ADP (adenosine diphosphate) and P (phosphate)

ATP – Coupled Reactions
Energy released when ATP is broken down into ADP + P is transferred to endergonic rxns through coupling

Electron Carriers Energy can be transferred to electrons in glucose metabolism and photosynthesis Electron carriers transport high-energy electrons 2 common e- carriers: Nicotinamide adenine dinucleotide (NAD+) Flavin adenine dinucleotide (FAD+)

FIGURE 6-12 Electron carriers
Low-energy electron-carrier molecules such as NAD+ pick up electrons generated by exergonic reactions and hold them in high-energy outer electron shells. Hydrogen ions are often picked up simultaneously. The electron is then transferred, with most of its energy, to another molecule to drive an endergonic reaction, often the synthesis of ATP.

Metabolism Sum of all chemical rxns in a cell
Many cellular reactions are linked through metabolic pathways There are often many reactions that must occur in a metabolic pathway to get the final product.

Metabolic Pathways Endergonic rxns are coupled with exergonic rxns
2. Energy-carrier molecules capture energy and transfer it between rxns 3. Chemical rxns are regulated through enzymes

Spontaneous Reactions
Spontaneous rxns proceed too slowly to sustain life Rxn speed is generally determined by the activation energy required Rxns with low activation energies proceed rapidly at body temperature Rxns with high activation energies (e.g. sugar breakdown) move very slowly at body temperature The activation energy is the energy required to force two atoms together in a chemical bond. The electrons of each atom repel each other and must be forced together in order to react. Even spontaneous reaction have an activation energy.

Enzymes Proteins that catalyze (speed up) chemical rxns in cells

Catalysts Reduce Activation Energy
Catalysts speed up rate of a chemical rxn without themselves being used up Catalysts speed up spontaneous rxns by reducing activation energy

Catalytic Converters Catalytic converters in cars facilitate the conversion of carbon monoxide to carbon dioxide Octane + oxygen carbon dioxide + water + energy + carbon monoxide (poisonous)

Catalytic Converters Catalyst in catalytic converter speeds carbon monoxide conversion Carbon monoxide + oxygen carbon dioxide + energy

FIGURE 6-14 Catalysts such as enzymes lower activation energy
A high activation energy (black curve) means that reactant molecules must collide very forcefully in order to react. Catalysts lower the activation energy of a reaction (red curve), so a much higher proportion of molecules move fast enough to react when they collide. Therefore, the reaction proceeds much more rapidly. Enzymes are protein catalysts for biological reactions.

Enzymes Are Biological Catalysts
Enzymes orient, distort, and reconfigure molecules in process of lowering activation energy Enzymes differ from non-biological catalysts b/c: Are specific for molecules they catalyze Activity is often enhanced or suppressed by their reactants or products

Enzyme Structure Have a pocket called an active site
Reactants (substrates) bind to active site Distinctive shape of active site is complementary and specific to substrate Active site amino acids bind to substrate and distort bonds to facilitate a reaction

Enzyme Structure Three steps of an enzyme catalyst
Substrates enter active site in a specific orientation 2. Upon binding, substrates and enzyme change shape to promote a rxn 3. Products of rxn leave the active site - leave enzyme ready for another catalysis

FIGURE 6-15 The cycle of enzyme-substrate interactions
As you look at this figure, imagine the opposite type of reaction as well, in which an enzyme binds to a single molecule and causes it to dissociate into two smaller molecules.

Cells Regulate Metabolism
One enzyme usually catalyzes a single step in a chain of metabolic rxns

Control of Metabolic Pathways
Control of enzyme synthesis regulates availability Enzyme synthesized only when needed 2. Some enzymes are inactive when synthesized and must be “turned on” to be active Enzyme pepsin – found in stomach – only activated when stomach acid increases Made in the inactive form to prevent self-digesting 3. Small organic molecules can bind to enzymes and enhance/inhibit activity (allosteric regulation)

FIGURE 6-16 Some enzymes are controlled by allosteric regulation
(a) Many enzymes have an active site and an allosteric regulatory site on different parts of the molecule. (b) When enzymes are inhibited by allosteric regulation, binding by a regulator molecule alters the active site so the enzyme is less compatible with its substrate.

Control of Metabolic Pathways
Adequate amounts of formed product inhibit enzyme activity (feedback inhibition)

Drugs and Poisons Drugs and poisons often inhibit enzymes by competing w/natural substrate for active site Known as competitive inhibition

Environmental Conditions
Most enzymes function optimally only within a very narrow range of conditions 3D structure of an enzyme is sensitive to pH, salts, temperature, and presence of coenzymes

FIGURE 6-19a Enzymes function best within narrow ranges of pH and temperature. The enzyme pepsin is found in the stomach and works to break down proteins. Salivary amylase is an enzyme that is involved in the digestion of starch. Notice that pepsin functions best at a pH of 2, and salivary amylase works best around a pH of 7.

FIGURE 6-19b Enzymes function best within narrow ranges of pH and temperature.

The End

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