Ch. 8 An Introduction to Metabolism

I. IntroductionA. The cell has thousands of chemical reactions

occurring within a microscopic space. -Example: Cellular respiration - energy from sugar is extracted.

II. Metabolism, Energy, and LifeA. Metabolic pathways

1. Metabolism is the sum of all chemicalreactions in an organism.

2. Enzymes accelerate chemical rxns.3. Catabolic pathways release energy

by breaking down complex molecules to simpler compounds.Ex. Cellular respiration

4. Anabolic pathways consume energy to build complicated molecules from simpler compounds.Ex. Synthesis of proteins from amino acids

B. Energy coupling: Energy from catabolic pathways is used for anabolic pathways. C. Bioenergetics is the study of how organisms

manage their energy resources. D. How organism transform energy

1. Energy:2. Kinetic energy:3. Potential energy:4. Chemical energy:Organisms can convert one type of energy

to another (Ex. Slide)

E. The two laws of thermodynamics (the studyof energy transformations) in regards to anopen system:1. The first law of thermodynamics: energy

can be transferred and transformed, butit cannot be created or destroyed.

2. The second law of thermodynamics: entropy increases with every energy transformation or transfer. The universeis becoming more random.

-much of the energy transformed in the universe is transformed into heat energy.

Combining the two laws, the quantity of energy is constant, but the quality is not.

F. Organisms live at the expense of free energy

1. Spontaneous processes are those that can occur without outside help. Spontaneous processes occur so that a system may become more stable.

2. The free energy (G) in a system is related to the total energy (H) and its entropy (S) by this relationship:

G = H – TS, where T is temperature in Kelvin units.

a. Free energy can also be a measure ofstability in a system. Systems high in free energy are unstable. They willlose free energy in spontaneous processes.

•delta G = G final state - G starting state

Delta G is negative in spontaneousprocesses.

3. Systems that are stable and are at equilibrium have no change in freeenergy.

Systems at equilibrium means that it can do no work.

Systems at equilibrium must receiveenergy from an outside source inorder to do work; it is nonspontaneous.

G. Exergonic and Endergonic reactions inmetabolism1. In an exergonic reaction, there is a

release of energy. -delta G is negative. It is a spontaneous reaction.

For the overall reaction of cellular respiration:

C6H12O6 + 6O2 -> 6CO2 + 6H2O

delta G = -686 kcal/mol

2. In an endergonic reaction, energy isabsorbed. -delta G is positive and free energy is stored. -it is nonspontaneous.

Photosynthesis is steeply endergonic, powered by the absorption of light energy.

•Delta G = + 686 kcal / mol.

H. Metabolic Disequilibrium: reactions in a closed system will eventually reach equilibrium and do no work.

1. Cells maintain disequilibrium becausethere is a constant flow of energy in and out of a cell.

2. Sunlight is a source of free energy for photosynthetic organisms. Nonphotosynthetic organisms depend on photosynthetic organisms for energy in the form of organic molecules.

I. ATP powers cellular work1. A cell does three main kinds of work:

•Mechanical work

•Transport work

•Chemical work2. In most cases, ATP is the immediate

source of energy.

3. ATP (adenosine triphosphate) is a type of nucleotide consisting of: -nitrogenous base adenine -ribose sugar and a chain of -three phosphate groups

4. The bonds between phosphate groups can be broken by hydrolysis.

Hydrolysis of the end phosphate group forms adenosine diphosphate [ATP ADP + Pi] and releases 7.3 kcal of energy per mole of ATP under standard conditions.

In the cell delta G is about -13 kcal/mol.

5. The bonds between the phosphate groups are referred to as high-energy.However, these bonds are weak and unstable. ADP + Pi is more stable.

ATP is more unstable than ADP becauseeach phosphate group has a negativecharge. These negative charges repelone another.

6. When ATP is hydrolyzed, the Pi bondsto another molecule. This molecule is now phosphorylated, and energized.

7. ATP is a renewable resource that is continually regenerated by adding a phosphate group to ADP.

III. Enzymes

B. Enzymes speed up metabolic reactions bylowering energy barriers.

A. Enzymes are catalysts. Catalysts changethe rate of reaction without being destroyed.

Activation energy is the amount of energy necessary to push the reactants over an energy barrier.

Enzymes work by lowering the activation energy.

C. Enzymes are substrate specific1. A substrate is a reactant that binds to the

enzyme. The enzyme catalyzes the conversion of substrate to product.

2. The substrate binds to the active site on the enzyme.

3. When the substrate binds to the activesite, the enzyme fits tighter around thesubstrate. This is called induced fit.

4. The substrate is held to the active site byweak bonds such as hydrogen and ionicbonds.

5. R-groups of amino acids in the activesite catalyze of substrate to product.

6. Once the product is made, it leaves theactive site and the enzyme is free to takeanother substrate. 7. Enzymes can catalyze reactions in both

forward and reverse directions.

D. A cell’s physical and chemical environmentcan affect enzyme activity1. Temperature

Each enzyme has an optimal temperature.

2. pH: Most enzymes have an optimal pHbetween 6-8.

However, digestive enzymes in the stomach (pepsin) have an optimal pH of 2, and intestinal enzymes (trypsin)have an optimal pH of 8.

E. Cofactors: many enzymes require non-protein helpers. These helpers are called cofactors.1. These cofactors may be attached to

the active site on enzymes, or they maybind to the substrate.2. Inorganic examples: zinc, iron, copper.3. Organic example (called a coenzyme):

vitaminsF. Enzyme Inhibitors:

1. Competitive inhibitors:2. Noncompetitive inhibitors:3. Examples: DDT, penicillin

IV. The control of MetabolismA. Molecules regulate enzyme activity:

Allosteric regulation

An activator will bind and change the shapeof the enzyme to its active form. An inhibitor will bind and cause the enzyme tomaintain its inactive form.

B. Feedback inhibition: a metabolic pathwayis turned off by its end product, which actsas an inhibitor of an enzyme within the pathway.

C. Cooperativity: a substrate binds to the enzyme, causing the enzyme to take theactive form. The enzyme’s multiple subunits are primed to accept its substrates.