An Introduction to Metabolism AP Biology

An Introduction to Metabolism The chemistry of life is organized into metabolic pathways The totality of an organism’s chemical reactions is called metabolism Enzymes selectively accelerate each step

An Introduction to Metabolism Catabolic pathways release energy by breaking down complex molecules to simpler compounds. This energy is stored in organic molecules until it needs to do work in the cell. Anabolic pathways consume energy to build complicated molecules from simpler compounds. The energy released by catabolic pathways is used to drive anabolic pathways.

Organisms transform energy Kinetic energy is the energy of motion. Potential energy is the energy that matter possesses because of its location or structure.

Organisms transform energy Energy can be converted from one form to another. Problem: A waiter walks across the room, turns on a switch and the light comes on. How is energy converted? How is photosynthesis and respiration involved?

Are organisms opened or closed systems? Explain The energy transformations of life are subject to two laws of thermodynamics 1st Law – Energy is conserved 2nd Law – Energy transformations cause disorder Are organisms opened or closed systems? Explain

Entropy is a quantity used as a measure of disorder, or randomness. Organisms live at the expense of free energy Free energy is the portions of a system’s energy that is able to perform work when temperature is uniform throughout the system

Endergonic Reactions An endergonic reaction is one that absorbs free energy from its surroundings.  G is positive If cellular respiration releases 686 kcal, then photosynthesis, the reverse reaction, must require an equivalent investment of energy  G = + 686 kcal / mol

Exergonic Reactions An exergonic reaction proceeds with a net release of free energy and  G (Change in Free Energy) is negative For the overall reaction of cellular respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O  G = -686 kcal/mol

ATP powers cellular work ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups. 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.  

ATP/ADP Cycle ATP is a renewable resource that is continually regenerated by adding a phosphate group to ADP

Check Yourself Questions What is the difference between a catabolic reaction and anabolic reaction? How does the 1st and 2nd law of thermodynamic relate to living organisms? Why is ATP important to metabolism? Describe the ATP-ADP process. How is the 2nd law of thermodynamics related to evolution

Enzymes Enzymes are biological catalysts that speed up metabolic reactions by lowering energy barriers A catalyst is a chemical agent that changes the rate of a reaction without being consumed by the reaction

All reactions must absorb energy in order to move from reactant to product The amount of free energy absorbed is called the activation energy (EA)

Reaction with enzyme present!!

Enzymes are substrate specific Substrate – a reactant that binds to an enzyme Active Site – a pocket or groove on the surface of the enzyme in which the substrate fits Induced fit – As the substrate binds, the enzyme changes shape leading to a tighter fit

Enzyme Induced Fit Model

Mechanism used by enzymes to speed up reactions: The active site orients and binds substrates in the correct orientation for the reaction. R groups create a conducive environment Enzymes can form covalent bonds with the substrate in an intermediate step

Check Yourself Questions Define the following terms: Catalyst, Active site, substrate, induced fit What are the first and second laws of thermodynamics? What is the difference between an anabolic reaction and a catabolic reaction? What is the difference between endergonic exergonic reaction? Is  G positive or negative for each?

A cells physical and chemical environment affects enzyme activity Some conditions lead to the most active conformation and lead to optimal reaction rates. Some conditions hinder enzyme activity

Environmental factors Temperature pH Concentration of the enzyme Salt Concentration Activators and Inhibitors

Other factors that affect enzymes Coenzymes – vitamins or other organic molececule(non-protein enzyme helpers) Inhibitors – competitive and noncompetitive inhibitors block substrate binding

Enzyme Inhibition

The control of metabolism Allosteric Regulation – In many cases, the molecules that naturally regulate enzyme activity behave like reversible noncompetitive inhibitors. These molecules often bind weakly to an allosteric site, a specific receptor on the enzyme that is not the active site.

The control of metabolism One of the common methods of metabolic control is feedback inhibition in which a metabolic pathway is turned off by its end product. The end product acts as an inhibitor.

Feedback Inhibition When the product is abundant the pathway is turned off, when the product is rare it is active.

Check Yourself Questions Describe the induced fit model. What is the substrate and active site? What environmental factors help to control the reaction of enzymes? How are coenzymes different than inhibitors? Describe the different types of inhibitors.