BIOL 1203 Week 7: Cellular Respiration & Energy Metabolism

What is cellular respiration?

The process of nutrient breakdown with accompanying ATP synthesis

What is energy metabolism?

Overview of how energy is obtained from other nutrients

ATP (Adenosine triphosphate)

“Universal energy currency” made of an adenine group, ribose group, and three phosphate groups. Every living cell relies on ATP

What happens when a phosphate group is removed from ATP?

It breaks a bond, releasing energy to power cellular work

Reduction (redox reaction)

Process where a substance gains electrons in a reaction, becoming more negative

Oxidation (redox reaction)

Process where a substance loses electrons in a reaction, becoming more positive

NAD+

Known as an electron carrier. Grabs electrons from glucose, hydrolyzing it

NAD+ redox reaction (chemical formula)

NAD⁺ + H⁺ + 2e^- ←→ NADH

When a molecule is oxidized, it releases ________________________

Two electrons and two hydrogen ions

Why is NADH the reduced form of NAD+?

Because NAD+ gained two high-energy electrons (and an H+ proton), reducing it odidation state from positive to neutral

NAD+ redox reaction (process)

NAD+ picks up two electrons and one hydrogen ion, the remaining hydrogen ion becomes part of solvent. NAD⁺ → NADH + H⁺

FAD redox reaction (process)

FAD picks up two electrons and two hydrogen ions. FAD → FADH₂

Cellular respiration REACTANTS

• ADP, Pi = energy carriers

• NAD+, FAD = electron carriers

• O₂ = required in last step of cellular respiration

Cellular respiration PRODUCTS

CO₂, ATP, NADH, FADH₂, H₂O

Overall summary reaction for cellular respriation

C₆H₁₂O₆ + 6O₂ + 32 ADP + 32P_I → 6 CO₂ + 6 H₂O + 32 ATP

Stages of glucose metabolism

1. Glycolysis

2. Pyruvate oxidation

3. Kreb’s cycle

4. Electron transport chain reactions

Glycolysis

Multi-step process in cytoplasm that converts glucose (6 C) into two pyruvic acid (pyruvate) molecules (two 3 C)

Key events in glycolysis

1. Glucose phosphorylated twice, uses 2 ATP

2. Glucose oxidized → makes two pyruvic acid

Why is glucose phosphorylated twice in the cell?

To trap glucose into the cell. It requires two ATP.

Substrate in glycolysis

1 Glucose

Energy net totals from glycolysis

2 pyruvic acid, 2 ATP, 2 NADH + 2H⁺

Pyruvate oxidation

Oxidized pyruvate into acetyl-CoA

Byproducts of pyruvate oxidation

1 CO₂, 1 NADH

Substrate in pyruvate oxidation

2 pyruvic acid

Energy net totals from pyruvate oxidation

2 acetyl-CoA, 2 CO₂, 2 NADH + 2 H⁺

Krebs cycle/citric acid/tricarboxylic acid (TCA) cycle

A circular pathway that oxidizes nutrients to produce energy

Key events in the Krebs cycle

1. One acetyl-coA combines with oxaloacetate, forming citrate

2. Cycle continues, oxaloacetate is regenerated

3. Ultimately, acetyl-CoA converted into 2 CO₂

Substrate in Krebs cycle

2 acetyl-CoA

Energy net totals from Krebs Cycle

4 CO₂, 2 ATP, 6 NADH + 6H⁺, 2 FADH₂

Electron transport chain

A series of protein complexes and molecules in the inner mitochondrial membrane that drives aerobic respiration

Which complexes are proton pumps (Hydrogen pumps)?

Complex I, III, and IV

What provides energy for Complex I to pump hydrogen ions against its concentration gradient?

From electrons moving from a “high energy state” to a “low energy state",” releasing energy

Where does each stage of glucose catabolism occur?

1. Glycolysis → cytoplasm

2. Pyruvate oxidation & Kreb’s cycle → mitochondrial matrix

3. Electron transport chain reactions → mitochondrial inner matrix

Protein Q

Shuttles electrons from Complex I → Complex III

Cytochrome C protein

Shuttles electrons from Complex III → Complex IV

What causes Complex III and IV to pump H⁺ into intermembrane space?

Energy released from protein-powered electron movement (Complex I → III → IV)

What direction does Complex I pump hydrogen ions?

Mitochondrial matrix → intermembrane space

What do electrons combine with at the end of the ETC?

Electrons combine with oxygen (final electron acceptor) to form water

ATP synthase

Enzyme that moves H⁺ in the intermembrane space down its concentration gradient (facilitated diffusion)

How does ATP synthase produce ATP?

Through chemoismosis, where the movement of a hydrogen ion gradient is used to make ATP

ATP production per NADH (electron transport chain)

2.5 ATP each

ATP production per FADH₂ (electron transport chain)

1.5 ATP each. Makes less ATP than NADH since electrons enter ETC at Complex II, skipping Complex I & releasing less energy

ATP from each stage of glucose catabolism

• ATP (direct): 4 ATP (2 Glycolysis, 2 Krebs)

• 10 NADH: 25 ATP (2 Glycolysis, 2 Pyruvate Oxidation, 6 Krebs)

• 2 FADH₂: 3 ATP (Krebs)

Why is 32 ATP not always produced by glucose?

• Because NADH transport from cytosol → mitochondrial matrix may lead to loss of 2 ATP

• ATP lost to instant energy needs and slight inefficiencies

What causes anaerobic respiration?

• Body can’t breathe oxygen fast enough

• System accumulates electrons, O₂ can’t accept electrons, ETC shuts down

• Body converts pyruvate → lactate

What happens in anaerobic respiration?

• Conversion of pyruvate → lactate regenerates NAD⁺

• Allows glycolysis to continue, but pyruvate oxidation and Krebs stops

• Only 2 ATP produced per glucose molecule

Lactic acid fermentation

Conversion of pyruvate → lactate to regenerate NAD+, allowing glycolysis to continue

When glucose is in excess, what carbohydrate metabolism processes occur?

Glycolysis (extract energy from glucose) and Glycogenesis (glucose → glycogen)

When glucose is low, what carbohydrate metabolism processes occur?

Gluconeogenesis (glucose synthesis from non-carb precursors) and glycogenolysis (breakdown of glycogen)

When is lipid metabolism generally used?

When nutrients are in excess (lipids stored as triglycerides) or glucose levels fall (triglycerides broken down)

Which organ cannot use fatty acids for energy?

The brain, which primarily uses glucose for energy. Ketone bodies can also provide some energy for the brain

Which organs preferentially use fatty acids for energy?

The liver, cardiac muscle, and resting skeletal muscle

Which molecule formed through ketogenesis provides some energy for the brain?

Ketone bodies

Lipolysis

Triglycerides → glycerol and individual fatty acids

Under extended fasting, where does oxaloacetate from the Krebs cycle go?

Towards gluconeogenesis

What are the three ketone bodies?

Acetoacetate, 3-hydroxybutyrate, acetone

Which ketone body is exhaled, responsible for the “fruity” breath smell?

Acetone

What physiological state stimulates oxaloacetate from Krebs cycle to be shunted towards gluconeogenesis?

Extended fasting, since glucose runs out and the body must perform gluconeogenesis to replenish it

Why is ketone body production important?

To minimize gluconeogenesis (hard on the liver) and save protein catabolism

Ketoacidosis

Acidic blood caused by an accumulation of ketone bodies. Can be a result of starvation, lack of carbs, uncontrolled diabetes melitus

Why is protein catabolism for energy undesirable?

Because proteins aren’t stored for energy, and instead all have functions

Sequence of use of nutrients during starvation

1. Glycogen → glucose

2. Lipids → glycerol & fatty acids, oxidized for energy

3. Protein breakdown (begins before lipid stores run out) → amino acids