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Chapter 6 Metabolism: Energy and Enzymes

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Cells and the Flow of Energy

Energy is the ability to do work.

Living things need to acquire energy; this is a characteristic of life.

Cells use acquired energy to:

Maintain their organization

Carry out reactions that allow cells to develop, grow, and reproduce

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Two Basic* Forms of EnergyKinetic energy is the energy of motion.

Potential energy is stored energy (eventually transferred to kinetic energy).

Potential energy in foods is chemical energy.

Organisms: convert chemical mechanical energy for motion.

* Other forms include solar, heat, and electrical energy

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The flow of energy in ecosystems occurs in one direction; energy does not cycle.

The two laws of thermodynamics explain this phenomenon.

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Two Laws of Thermodynamics

First Law: Energy cannot be created or destroyed, but it can be changed from one form to another.

Second Law: Energy cannot be changed from one form to another without loss of usable energy.

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First law Second law

When energy transformations occur, energy is neither created nor destroyed (1st Law) but there is always loss of usable energy, usually as heat (2nd Law).

Flow of energy

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Due to the two laws of thermodynamics, living things depend on an outside source of energy.

Energy exists in several different forms and the ultimate source of energy for ecosystems is the sun.

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Cells and Entropy

Entropy is a measure of the relative state of disorganization of a system.

Low entropy = highly organized.High entropy = highly disorganized.

Systems (such as cells) tend toward disorganization and therefore cells need a constant supply of energy to maintain their internal organization.

Energy is used to “fight entropy”.

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Cells and entropy

Low entropy

High entropy

Fig. 6.2

Breakdown of glucose results in a loss of potential energy and an increase in entropy

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Metabolic Reactions and Energy Transformations

Metabolism is the sum of all the chemical reactions that occur in a cell.

A reaction will occur spontaneously if it increases entropy.

Biologists use the term “free energy” instead of entropy for cells.

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A + B C + D

Reactants Products

Chemical Reactions

Reactants are substances that participate in a reaction; products are substances that form as a result of a reaction.

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Free energy, G, is the amount of energy available to do work after a reaction has occurred.

ΔG (change in free energy) is calculated by subtracting the free energy of reactants from that of products.

A negative ΔG means the products have less free energy than the reactants, and the reaction will occur spontaneously.

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RespirationPhotosynthesis

Positive G

Negative G

Endergonic reaction - requires energyExergonic reaction - releases energy

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ATP: Energy for CellsATP (adenosine triphosphate) is the

energy currency of cells.

ATP is constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell.

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Use of ATP by the cell has advantages:

1) It can be used in many types of reactions.

2) When ATP → ADP + P, energy released is sufficient for cellular needs and little energy is wasted.

3) ATP breakdown (i.e., exergonic rxn) is coupled to endergonic reactions in such a way that it minimizes energy loss.

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The ATP cycleFig. 6.3

ATP is a nucleotide made of adenine and ribose and three phosphate groups.

ATP is called a “high-energy” compound because a phosphate group is easily removed.

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Coupled Reactions

In coupled reactions, energy released by an exergonic reaction drives an endergonic reaction.

Exergonic

Endergonic

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Coupled reactions

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Function of ATP

Cells make use of ATP for:Chemical work – ATP supplies energy to

synthesize macromolecules, and therefore the organism (ex: amino acids into proteins)

Transport work – ATP supplies energy needed to pump substances across the plasma membrane (ex: active transport)

Mechanical work – ATP supplies energy for cellular movements (ex: muscle contraction)

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Metabolic Pathways and Enzymes

Enzyme – protein molecule that acts as an organic catalyst

Catalyst – substance that increases the rate of a chemical rxn, but is unchanged by the rxn.

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An enzyme brings together particular molecules and causes them to react to produce a product.

Substrates - The reactants in an enzymatic reaction.

Enzyme + Substrate Enzyme-Substrate Complex

Enzyme + Product

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Cellular reactions are usually part of a metabolic pathway:

E1 E2 E3 E4 E5 E6

A → B → C → D → E → F → G

A-F are reactants or substratesB-G are the products E1-E6 are enzymes.

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Energy of Activation

Energy of activation (Ea) - Energy that must be added to cause molecules to react with one another.

An enzyme lowers the energy of activation.

The addition of an enzyme does not change the free energy of the reaction.

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With EnzymeWithout Enzyme

Energy of activation (Ea)

*Notice: G does not change – just energy of activation

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Enzyme-Substrate Complexes

Every reaction in a cell requires a specific enzyme.

Enzymes are named for their substrates:Substrate EnzymeLipid LipaseUrea UreaseMaltose MaltaseRibonucleic acid Ribonuclease

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-Only one small part of an enzyme, called the active site, complexes with the substrate(s).

-The enzyme is not changed by the reaction, and it is free to act again.

Fig 6.6

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Enzymes can either change, split (degradation), or form (synthesis) molecules.

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Induced fit model

The active site may undergo a slight change in shape, called induced fit, in order to accommodate the substrate(s).

Fig 6.7

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Factors Affecting Enzymatic Speed

Substrate Concentration – Speed increases with substrate concentration

Temperature – Above or below effective range, enzymes become denatured

pH – Each enzyme has an optimal pH

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Rate of an enzymatic reaction as a function of temperature and pH

Fig 6.8

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Cells regulate enzyme quantity and activity by either turning genes on or off.

Another way to control enzyme activity is to activate or deactivate the enzyme.

-Phosphorylation is one way to activate an enzyme.

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Enzyme InhibitionEnzyme inhibition occurs when an active

enzyme is prevented from combining with its substrate.

When the product of a metabolic pathway is in abundance, it binds competitively with the enzyme’s active site, a simple form of feedback inhibition.

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Feedback inhibition

Shape change

Fig 6.9

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Enzyme CofactorsPresence of enzyme cofactors may be

necessary for some enzymes to carry out their functions.

Examples:

-Inorganic metal ions, such as copper, zinc, or iron

-Some organic molecules, termed coenzymes, such as vitamins.

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Oxidation-Reduction and the Flow of Energy

Oxidation is the loss of electrons and reduction is the gain of electrons.

In covalent rxn’s, oxidation also refers to the loss of hydrogen atoms, and reduction refers to the gain of hydrogen atoms.

These types of oxidation-reduction (redox) reactions are exemplified by the overall reactions of photosynthesis and cellular respiration.

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PhotosynthesisThe overall reaction for photosynthesis

can be written:

6CO2 + 6H2O + energy → C6H12O6 + 6O2

Hydrogen is removed from water

Energy comes from the sun.

Glucose is formed

OxidizedReduced

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Cellular Respiration

The overall equation for cellular respiration is opposite that of photosynthesis:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

An energy source is now available for endergonic cellular reactions

Oxidized Reduced In the form of ATP

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Relationship of chloroplasts to mitochondria