An Introduction To Metabolism

Metabolism

• The totality of an organism’s chemical processes.

• Concerned with managing the material and energy resources of the cell.

Catabolic Pathways

• Pathways that break down complex molecules into smaller ones, releasing energy.

• Example: Respiration

Anabolic Pathways

• Pathways that consume energy, building complex molecules from smaller ones.

• Example: Photosynthesis

Energy

• Ability to do work.• The ability to rearrange a collection of matter.• Forms of energy:– Kinetic– Potential– Activation

Kinetic Energy

• Energy of action or motion.

Potential Energy

• Stored energy or the capacity to do work.

Activation Energy

• Energy needed to convert potential energy into kinetic energy.

Potential Energy

Activation Energy

Energy Transformation

• Governed by the Laws of Thermodynamics.

1st Law of Thermodynamics

• Energy can be transferred and transformed, but it cannot be created or destroyed.

• Also known as the law of Conservation of Energy.

2nd Law of Thermodynamics

• Each energy transfer or transformation increases the entropy of the universe.

Entropy

• Measure of disorder.

Free Energy

• The portion of a system's energy that can perform work.

Chemical Reactions

• Are the source of energy for living systems.

Cell - Types of Work

• Mechanical - muscle contractions• Transport - pumping across membranes• Chemical - making polymers

ATP

• Adenosine Triphosphate• Made of: - Adenine (nitrogenous base) - Ribose (pentose sugar) - 3 phosphate groups

Adenine

Ribose

Phosphates

Key to ATP

• Is in the three phosphate groups.• Negative charges repel each other and makes

the phosphates unstable.

ATP

• Works by energizing other molecules by transferring phosphate groups.

ATP vs Food

• ATP: – Renewable energy resource.– Unstable bonds

• Food:– Long term energy storage– Stable bonds

ATP Cycle

ATP in Cells

• A cell's ATP content is recycled every minute.• Humans use close to their body weight in ATP

daily.• No ATP production equals quick death.

Enzymes

• Biological catalysts made of protein.• Cause the rate of a chemical reaction to

increase.

Chemical Reaction

AB + CD AC + BD

AB and CD are “reactants”AC and BD are “products”

Enzymes

• Lower the activation energy for a chemical reaction to take place.

Enzyme Terms

• Substrate - the material and enzyme works on.• Enzyme names: Ex. Sucrase - ase name of an enzyme 1st part tells what the substrate is. (Sucrose)

Enzyme Name

• Some older known enzymes don't fit this naming pattern.

• Examples: pepsin, trypsin

Active Site

• The area of an enzyme that binds to the substrate.

• Structure is designed to fit the molecular shape of the substrate.

• Therefore, each enzyme is substrate specific.

Enzymes

• Usually specific to one substrate. • Each chemical reaction in a cell requires its

own enzyme.

Factors that Affect Enzymes

• Environment• Cofactors• Coenzymes• Inhibitors• Allosteric Sites

Environment

• Factors that change protein structure will affect an enzyme.

• Examples:– pH shifts– temperature– salt concentrations

Enzyme Inhibitors

• Competitive - mimic the substrate and bind to the active site.

• Noncompetitive - bind to some other part of the enzyme.

Control of Metabolism

• Is necessary if life is to function.• Controlled by switching enzyme activity "off"

or "on” or separating the enzymes in time or space.

Process of Cellular Respiration

Process of Cellular Respiration

• The process by which food molecules are broken down to release energy is respiration.

• Respiration that occurs in the presence of oxygen is called aerobic respiration.

• Respiration that occurs without oxygen is called anaerobic respiration.

• The energy payoff is much greater when molecules are broken down aerobically.

Glycolysis• 1st step of respiration• Glycolysis is the breakdown of glucose (6-carbon

molecule to pyruvic acid (3-carbon molecule).• Glycolysis occurs in the cytoplasm and is anaerobic.• Glycolosis produces hydrogen ions and electrons,

which combine with carrier ions called NAD+ (nicotanamide dinucleotide) to form NADH.

• End product is 2 ATP’s

Breakdown of Pyruvic Acid

• The 2nd step that takes place in respiration is the breakdown of pyruvic acid, and aerobic process.

• Pyruvic acid (3-carbon molecule) is changed to acetic acid (2-carbon molecule). The carbon that comes off makes CO2. Acetic acid combines with a substance called coenzyme A (CoA), forming acetyl-CoA.

• This process takes place in the mitochondria.

Citric Acid Cycle

• The 3rd step of aerobic respiration is the citric acid cycle.• Acetyl-CoA combines with a 4-carbon molecule to form a 6-

carbon molecule, citric acid. Citric acid is broken down 1st to a 5-carbon molecule and then to a 4-carbon molecule, releasing CO2 at each step.

• This cycle of chemical reactions produces more ATP and releases additional electrons.

Electron Transport Chain

• The 4th part of aerobic respiration is the electron transport chain (ETC).

• The ETC is a series of molecules along which electrons are transferred, releasing energy.

• Carrier molecules bring the electrons released during glycolysis and the citric acid cycle to the ETC.

ETC (con’t)

• The molecules of the ETC are located on the inner membranes of the mitochondria.

• This is an aerobic process, because oxygen combines with two hydrogen ions to produce with water.

What happens if no oxygen is present?

If the final electron acceptor, oxygen, is used up, the chain becomes jammed. The reactions of the ETC can’t take place without oxygen.

Anaerobic Respiration

Anaerobic Respiration

• If oxygen isn’t present, there’s no electron acceptor to accept the electrons at the end of the ETC.

• If this occurs, then NADH accumulates.• Once all the NAD+ has been converted to

NADH, the Krebs cycle and glycolysis both stop (both need NAD+ to accept electrons).

• Once this happens, no new ATP is produced, and the cell soon dies. Cells have derived a method to escape dying – ANAEROBIC RESPIRATION.

• The main objective of anaerobic respiration is to replenish NAD+ so that glycolysis can proceed once again. It occurs in the cytoplasm right along with glycolysis.

• There are two forms of anaerobic respiration:– Alcoholic fermentation– Lactic acid fermentation

Alcoholic Fermentation

• Alcoholic fermentation occurs in plants, fungi (yeast), and bacteria.

• There are 2 steps to alcoholic fermentation:– The conversion of pyruvic acid to acetaldehyde

• 1 CO2 and 1 acetaldehyde is produced

– The conversion of acetaldehyde to ethanol• NADH is used to drive the reaction, releasing NAD+

• The goal of this reaction is not to produce ethanol, but it is to free the NAD+, which allows glycolysis to continue.

• The reward is 2 ATP from glycolysis for each 2 converted pyruvate. This is better than the alternative, which is 0 ATP.

Lactic Acid Fermentation

• Lactic acid can occur in some bacteria and plants, but it is mostly found in animals, including humans.

• Anytime your muscle cells require energy at a faster rate than it can be supplied by aerobic respiration, they begin to carry out lactic acid fermentation.

• There is only one step in lactic acid fermentation:

• Now, NAD+ can be used for glycolysis.• When O2 becomes available again, lactic acid

can be broken down and its store of energy can be retrieved.

• Because O2 is required to do this, lactic acid fermentation creates what is often called an oxygen debt.

Lactic Acid Fermentation

• Uses only Glycolysis.• An incomplete oxidation - energy is still left in

the products (lactic acid). • Does NOT require O2

• Produces ATP when O2 is not available.

Lactic Acid Fermentation

• Done by human muscle cells under oxygen debt.

• Lactic Acid is a toxin and causes soreness and stiffness in muscles.

Fermentation - Summary

• Way of using up NADH so Glycolysis can still run.

• Provides ATP to a cell even when O2 is absent.

Aerobic vs Anaerobic

• Aerobic - Rs with O2

• Anaerobic - Rs without O2

• Aerobic - All three Rs steps.• Anaerobic - Glycolysis only.