Metabolism and Energy Reactions Notes

Understand the relationship between products and reactants in a metabolic reaction.
Identify the role of an enzyme in a metabolic reaction.
Summarize the roles of anaerobic and aerobic pathways in energy generation.
Illustrate the stages of the ATP cycle.

Metabolism is the sum of all chemical reactions within the body.
Reactions occur at the cellular level and often involve metabolic pathways.
Metabolic pathways are carried out by enzymes sequentially arranged in cells.
A metabolic pathway begins with a reactant (e.g., A) and ends with a final product (e.g., D).
Each step in a metabolic pathway is a chemical reaction catalyzed by an enzyme.
Substrates are converted into products, which then serve as substrates for the next enzyme-catalyzed reaction.
Reactions proceed in an organized and regulated manner.

Metabolic pathways are highly regulated by the cell.
Feedback Inhibition: An end product of a metabolic pathway interacts with an enzyme early in the pathway.
- This slows down the pathway to prevent overproduction of the product.

Enzymes are metabolic assistants that speed up chemical reactions.
Substrate (S): The reactant(s) that participate in the reaction.
Enzymes are often named for their substrates (e.g., lipids are broken down by lipase, maltose by maltase, and lactose by lactase).
Enzyme-substrate complex formation:
- $E + S \rightleftharpoons ES \rightarrow E + P$ where E is the enzyme, S is the substrate, ES is the enzyme-substrate complex, and P is the product.
Active Site: A specific region on the enzyme where substrates are brought together to react.
- The shape of the active site causes an enzyme's specificity.
After the reaction is complete, the product(s) are released, and the enzyme can be used again.
Cells require only a small amount of a particular enzyme to carry out a reaction.
Enzymes are not destroyed in a chemical reaction and can be reused.

Molecules may not react unless activated.
Energy of Activation ( $E_A$ ): The energy that must be added to cause molecules to react.
Enzymes lower the amount of energy required for activation ( $E_A$ ).
The addition of an enzyme does not change the end result of the reaction.
By lowering the energy of activation, enzymes increase the rate of the reaction.

Mitochondria are the powerhouses of the cell.
Mitochondria convert the chemical energy of glucose products into the chemical energy of ATP molecules.
Mitochondria use oxygen and give off carbon dioxide during cellular respiration.
Structure: Double membrane organelles with an outer plasma membrane and an inner membrane folded into cristae.
Cristae: Shelves projecting into the matrix, an inner space filled with a gel-like fluid.
The matrix contains enzymes for breaking down glucose products.
ATP production occurs at the cristae.
Protein complexes aid in energy conversion on the membranous shelves.
Mitochondria have their own genes and reproduce themselves.

(ATP) Adenosine triphosphate is the energy currency of the cell involved in various cell processes.
The breakdown of glucose during cellular respiration produces ATP from ADP and inorganic phosphate.
Muscle cells use ATP for contraction, and nerve cells use it for conduction of nerve impulses.
ATP breakdown releases heat, ADP, and phosphate.

Blood transports glucose and oxygen to cells, initiating cellular respiration.
Cellular respiration breaks down glucose into carbon dioxide and water.
Three pathways involved in glucose breakdown:
1. Glycolysis
2. Citric Acid Cycle (Krebs Cycle)
3. Electron Transport Chain
These pathways slowly release the energy in a glucose molecule to gradually produce ATP.

Glycolysis means sugar splitting.
Glucose (a six-carbon molecule) is split into two three-carbon molecules of pyruvate.
Glycolysis occurs in the cytoplasm and is found in most cell types.
Anaerobic Pathway: Glycolysis does not require oxygen.
Hydrogens and electrons are removed from glucose, resulting in NADH.
The breaking of bonds yields a net of two ATP molecules.

Pyruvate is a pivotal molecule in cellular respiration.
In the presence of oxygen, pyruvate enters the preparatory reaction.
Pyruvate prepares the outputs of glycolysis for the citric acid cycle in the mitochondria.
A small amount of NADH is produced per glucose.
Fermentation occurs when oxygen is not available.

Each pyruvate molecule, after modification, enters the citric acid cycle as acetyl CoA.
The citric acid cycle is a cyclical series of enzymatic reactions in the mitochondrial matrix.
Purpose: To complete the breakdown of glucose by breaking the remaining bonds.
Carbon dioxide is released, a small amount of ATP is produced, and hydrogen and electrons are carried away by NADH and FADH $_2$ .
The cellular respiration pathways can use organic molecules other than carbohydrates.
Fats and proteins can be converted to compounds that enter the citric acid cycle.

Fats break down into glycerol and fatty acids.
Glycerol can be converted to pyruvate and enter glycolysis.
Fatty acids are converted to an intermediate that enters the citric acid cycle.
An 18-carbon fatty acid results in nine acetyl CoA molecules.
Respiration of these can produce a total of 108 ATP molecules.
Proteins are less frequently used as an energy source.
The carbon skeleton of amino acids can enter glycolysis, be converted to acetyl groups, or enter the citric acid cycle.
The amino group becomes ammonia, which enters the urea cycle and becomes part of urea.
- Demination: The process where the amino group is removed from the amino acid carbon skeleton.

NADH and FADH $_2$ molecules from glycolysis and the citric acid cycle deliver electrons to the electron transport chain.
The electron transport chain members are carrier proteins grouped into complexes.
These complexes are embedded in the cristae of a mitochondrion.
Each carrier accepts two electrons and passes them on to the next carrier.

High-energy electrons enter the chain and lose energy as they are passed from carrier to carrier.
Low-energy electrons emerge from the chain.
Oxygen serves as the final acceptor of the electrons at the end of the chain, forming water.
The presence of oxygen makes the electron transport chain aerobic.
Energy released as electrons pass from carrier to carrier is used for ATP production.
The inner mitochondrial membrane contains an ATP synthase complex that combines ADP + P to produce ATP.
The ATP synthase complex produces about 32 ATP per glucose molecule.
Overall, the reactions of cellular respiration produce 36 to 38 ATP molecules.

Fermentation is an anaerobic process (does not require oxygen).
When oxygen is unavailable, the electron transport chain becomes inoperative.
Glycolysis operates as long as it is supplied with NAD $^+$ (free) to pick up hydrogens and electrons.
NADH passes its hydrogens and electrons to pyruvate molecules.
- Pyruvate + NADH → Lactate + NAD $^+$
The citric acid cycle and electron transport chain do not function as part of fermentation.
When oxygen is available again, lactate can be converted back to pyruvate.
Fermentation produces only two ATP per glucose molecule.
Fermentation results in the buildup of lactate (lactic acid).
Lactate is quickly removed by the circulatory system once aerobic conditions return.
Yeast fermentation produces alcohol and carbon dioxide instead of lactate.