Unit 5 – Metabolism: Energy and Enzymes

ENERGY

Energy – The ability to do work.

Living organisms need to acquire energy for survival

Cells require energy to:Maintain organizationCarry out reactionsDevelop, Grow and Reproduce

Forms of Energy

Kinetic Energy – energy of motion

Potential Energy – stored energy

Source of Potential EnergyFOOD – called Chemical Energy

Organisms convert chemical energy into a form of kinetic energy called Mechanical Energy.

Energy Flow

Is unidirectional in ecosystems (no cycling)

Law of thermodynamics explains this:

1st Law: Energy cannot be created or destroyed, but can be transformed.

2nd Law: Energy transformation results in a loss of usable energy.

Loss of usable energy is typically in the form of heat.

Flow of energy

Cells and Entropy

The term entropy is used to indicate the relative state of disorganization.

Cells need a constant supply of energy to maintain their internal organization.

Complex molecules tend to break apart into their building blocks.

Ex: Glucose —> Carbon Dioxide + Water

Greater Organization = less stable

The result is a loss of potential energy and an increase in entropy.

Cells and Entropy

Increased Entropy = Dispersion of Energy

Ex: Glucose —> Carbon Dioxide + Water

Decreased Entropy = Organization

Requires Energy to maintain organization.

Cells and entropy

Metabolic Reactions and Energy Transformations

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

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

A reaction will occur spontaneously if it increases entropy.

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

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

ATP: Energy for Cells

ATP (adenosine triphosphate) is the energy currency of cells.

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

Use of ATP by the cell has advantages:

It can be used in many types of reactions.

When ATP --> ADP + P, energy released is sufficient for cellular needs and little energy is wasted.

ATP is coupled to endergonic reactions in such a way that it minimizes energy loss.

The ATP cycle

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

Coupled reactions

Function of ATP

Cells make use of ATP for:

1. Chemical work – ATP supplies energy to synthesize macromolecules, and therefore the organism

2. Transport work – ATP supplies energy needed to pump substances across the plasma membrane

3. Mechanical work – ATP supplies energy for cellular movements

Metabolic Pathways and Enzymes

Cellular reactions are usually part of a metabolic pathway, a series of linked reactions, illustrated as follows:

E1 E2 E3 E4 E5 E6

A —> B —> C —> D —> E —> F —> G

A-F are reactants or substrates

B-G are the products in the various reactions

E1-E6 are enzymes.

Energy of Activation

The energy that must be added to cause molecules to react with one another is called the energy of activation (Eact).

The addition of an enzyme does not change the free energy of the reaction, rather an enzyme lowers the energy of activation.

Energy of activation (Ea)

Enzyme-Substrate Complexes

Every reaction in a cell requires a specific enzyme.

Enzymes are named for their substrates:

SUBSTRATE ENZYME

Lipid Lipase

Urea Urease

Maltose Maltase

Ribonucleic acid Ribonuclease

Enzymatic reaction

Induced fit model

Factors Affecting Enzymatic Speed

Enzymatic reactions proceed with great speed provided there is enough substrate to fill active sites most of the time.

Enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme.

Temperature and pH

As the temperature rises, enzyme activity increases because more collisions occur between enzyme and substrate.

If the temperature is too high, enzyme activity levels out and then declines rapidly because the enzyme is denatured.

Each enzyme has an optimal pH at which the rate of reaction is highest.

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

DRAW RELATIONSHIP ON BOARD

Enzyme Inhibition

Enzyme 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.

Other metabolic pathways are regulated by the end product binding to an allosteric site on the enzyme.

Feedback inhibition

Enzyme Cofactors

Presence of enzyme cofactors may be necessary for some enzymes to carry out their functions.

Inorganic metal ions, such as copper, zinc, or iron function as cofactors for certain enzymes.

Organic molecules, termed coenzymes, must be present for other enzymes to function.

Some coenzymes are vitamins.

Oxidation-Reduction and the Flow of Energy

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

Because oxidation and reduction occur simultaneously in a reaction, such a reaction is called a redox reaction.

Oxidation also refers to the loss of hydrogen atoms, and reduction refers to the gain of hydrogen atoms in covalent reactions in cells.

Photosynthesis

The overall reaction for photosynthesis can be written:

6CO2 + 6H2O + energy ---> C6H12O6 + 6O2

During photosynthesis, hydrogen atoms are transferred from water to carbon dioxide, and glucose is formed.

Water has been oxidized; carbon dioxide has been reduced.

Energy to form glucose comes from the sun.

Cellular Respiration

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

C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy

In this reaction, glucose is oxidized and oxygen is reduced to become water.

The complete oxidation of a mol of glucose releases 686 kcal of energy that is used to synthesize ATP.

Glycolysis Citric Acid Cycle

Electron Transport Chain

Organelles and the Flow of Energy

During photosynthesis, chloroplasts capture solar energy and use it to convert water and carbon dioxide into carbohydrates that provide food for other living things.

Cellular respiration, the breakdown of glucose into carbon dioxide and water, occurs in mitochondria.

It is the cycling of molecules between chloroplasts and mitochondria that allows a flow of energy from the sun through all living things.

Relationship of chloroplasts to mitochondria

Chapter Summary

Two laws of thermodynamics state that energy cannot be created or destroyed, and energy transformations result in a loss of energy, usually as heat.

As a result of these laws, we know the entropy of the universe is ever increasing, and that it takes energy to maintain the organization of living things.