Chapter 12 - Energy in Living Systems

What is cellular respiration?

Refers to the metabolic processes that produce ATP. There is two processes (one happens if there is oxygen and the other if there is no oxygen)

What are the two metabolic processes that produce ATP?

Anaerobic respiration (fermentation, only if there is no oxygen) and aerobic respiration (the most common one)

What is anaerobic respiration (fermantation)?

One of the two metabolic processes that produce ATP.

Through this process, sugars are partially degraded to produce some ATP in the absence of oxygen.

Ex. Substrate-level phosphorylation

What is substrate-level phosphorylation?

Involves the phosphorylation of ADP to create ATP. An enzyme brings a substrate with ADP and Pi to produce ATP.

• Less effective than oxidative phosphorylation.

What is aerobic respiration?

One of the metabolic processes that produces ATP. Oxygen is consumed.

This happens in the presence of oxygen as it is required.

Ex. Oxidative Phosphorylation

What is oxidative phosphorylation?

Involves the phosphorylation of ADP to create ATP. Requires oxygen and coenzymes that act as electron shuttles; involves chmiosmosis

• Coenzymes are a type of cofactors, non protein helper molecules, but are organic!

• More effective than substrate-level phosphorylation.

ADP + Pi+ energy = ATP

What are oxidation and reduction reactions?

If it’s an oxidation: loss of an electron or a hydrogen. Molecule becomes more positive.

If it’s a reduction: gain of an electron or a hydrogen. Molecules becomes less positive and more negative.

OIL RIG

What is a redox reaction?

An oxidation and a reduction reaction paired together.

• Reducing agent: what is oxidized.

• Oxidizing agent: what is reduced.

Ex. Cellular Respiration

• C₆H₁₂O₆ +6 O₂ ——>6 CO₂+ 6 H₂O + Energy

• Glucose is oxidized, oxygen is reduced.

Stages of cellular respiration

1. Glycolysis

2. Pyruvate Oxidation

3. Citric Acid Cycle (Kreb’s Cycle)

4. Oxidative Phosphorylation

Glycolysis stage

Occurs in the liquid of the cytoplasm, the cytosol (outside of the mitochondria)

Can be aerobic (oxygen requiring) or anaerobic (ana con “n” de no oxygen needed)

Glucose is oxidized to pyruvate:

• 2 ATP molecules are required. A glucose molecule (six-carbon molecule) is split into two three-carbon molecules (two pyruvate molecules), each with one phosphate.

• Both of these three-carbon molecules get oxidized (meaning they lose electrons) as a second phosphate (coming from the 2 ATP molecules) is added to them.

• The electrons released by the three-carbon molecules are used to reduce (add electrons, make less positive) the two NAD molecules into 2 NADH molecules.

• The 2 ADP molecules (used to be ATPs but gave their phosphate to the three-carbon molecules) receive a phosphate each through substrate-level phosphorylation and result in 4 ATP molecules.

By the end of glycolysis we gained: 2 Pyruvate molecues, +2 NADH and 2 ATP molecules

Pyruvate Oxidation

Aerobic reaction that requires oxygen. Pyruvate is oxidized and converted into Acetyl CoA.

The two pyruvate molecules from glycolysis are transferred into the mitochondria (matrix). Here’s the breakdown for each pyruvate molecule:

• The carboxyl group from a pyruvate molecule is removed and a CO₂ is released. This leaves us with a two-carbon compound.

• This two-carbon compound is oxidized (electrons are removed) to form acetate (CH₃COO-). The electrons released are transferred to NAD+ make NADH (NAD+ is reduced).

• A coenzyme will covalently bond to the acetate (CH₃COO-) previously formed giving us Acetyl CoA as a result.

• This Acetyl group enters the citric acid cycle.

By the end of pyruvate oxidation we end with: 2 CO2 molecules released as waste, 2 NADH molecules formed and two Acetyl CoA molecules.

Citric Acid Cycle (Kreb’s Cycle)

Takes place inside the matrix of a mitochondrion.

Completes the breakdown of glucose and begins by the entrance of the two Acetyl CoA molecules. For 1 Acetyl CoA molecule:

• The remaining glucose is oxidized (loses electrons) and two CO₂ molecules are released.

• Only 1 ATP is produced

• Lots of reduced (have lots of electrons) coenzymes (organic nonprotein helper) are obtained.

  • The electrons released by glycogen are used to reduce (add electrons to) three NAD+ molecules and obtain three NADH.

  • One FAD molecule is reduced to FADH₂

• Cycle repeats

Overall: for the two acetyl CoA molecules we obtain four CO₂ molecules, six NADH molecules, two FADH₂, and two ATP molecules.

Oxidative Phosphorylation

Aerobic respiration (requires oxygen)

The NADH and FADH₂ from all previous stages (especially from Kreb’s cycle since a lot come from there) are used to make ATP.

• The electrons of NADH and FADH₂ are passed through the electron transport chain; electrons lose energy but that energy is stored.

• At the end of the chain, the electrons reach oxygen. Oxygen combines with two electrons and two protons to form water.

• These hydrogen ions use the energy previously stored to pump and diffuse from the mitochondrial matrix into the inner membrane (against concentration gradient)

• To move from the matrix into the membrane, down their concentration gradient (from high to low), hydrogen ions diffuse through ATP synthase, a protein that recharges ADP and produces ATP (chemiosmosis).

Through the electron transport chain and ATP synthase (chemiosmosis): approximately 30-34 ATP molecules were created and H2O is released

What happens during cellular respiration if oxygen, which is required, is not present?

Glycolysis will still generate the two pyruvate molecules and the two NADH molecules, however, these would have no place to go.

The NADH molecules can be recycled to NAD+ through fermentation:

• Lactic acid: pyruvic acid + NADH = lactic acid + NAD+

• Alcohol fermentation: pyruvic acid = CO₂ + acetaldehyde + NADH = ethanol + NAD+

Fermentation is important because it allows for the production of energy in case O2 is absent. If not, then in case of O2 absence, the organism would just not produce energy and die.

How is oxidative phosphorylation (ETC) different in plants?

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