Cellular Respiration (HL Biology)

Glycolysis, link reaction, anaerobic/aerobic respiration, Krebs cycle

Cellular respiration

Controlled release/production of energy from broken down organic compounds in cells

Structure of ATP

Ribose, adenine, 3 phosphates in sequence

ATP cycle

1. ATP hydrolyzes and releases energy + a phosphate

2. ADP forms

3. ADP phosphorylates and absorbs energy + a phosphate

4. ATP forms

When enzymes are present, ATP takes part in [blank] reactions

Energy requiring

Preferred respiratory substrate

Carbohydrates

Which has a higher energy yield, carbs, lipids, or proteins?

Lipids

Why are carbs the preferred respiratory substrate?

They oxidize easier and can directly enter glycolysis. Lipids need to become carbs first, creating more intermediate reactions. This reduces net ATP production, because some is used in those intermediate processes. Proteins are structural so they’re a last resort.

What happens in aerobic respiration

Sugar is completely oxidized to CO₂, H₂O, and ATP (free). This occurs in the mitochondria and makes lots of ATP, where each step has a specific enzyme.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

What happens in anaerobic respiration (fermentation)

This is a different (temporary) pathway used when no oxygen is present. It occurs in the cytoplasm and doesn’t make ATP. The sole waste product is lactate.

Variables affecting respiration rate

1. Metabolic rate of cell

2. Organism size

3. Oxygen supply

4. Substrate supply

5. Temperature

6. pH

How does the metabolic rate of the cell impact respiration rate

If more energy is needed, respiration will be faster

How does organism size impact respiration rate

Small organisms’ surface area is bigger compared to their size, so the higher rate allows for this heat loss

How does oxygen supply impact respiration rate

Constant oxygen supply is needed to release maximum ATP, without it, anaerobic respiration starts

How does substrate supply impact respiration rate

This dictates the rate of respiration and quantity of products (CO₂, H₂O, ATP)

How does temperature impact respiration rate

The process is controlled by enzymes, so the optimum temperature increases respiration rate and enzyme activity

How does pH impact respiration rate

The release of CO₂ lowers pH (increases acidity) of the cell/body tissue. This affects enzyme function, decreasing their activity by moving out of the optimum pH range.

Respirometer

Measures oxygen consumption/CO₂ production by trapping a respirating organism in a sealed container with enough O₂ to be healthy. There is also a base (usually KOH) that absorbs CO₂ and a capillary tube filled with a fluid that shifts with changes in pressure caused by O₂ consumption. The movement of this fluid is divided by time to get the rate of respiration.

4 stages of cellular respiration

1. Glycolysis (glucose → pyruvate)

2. Link reaction (pyruvate → acetyl-CoA + CO₂)

3. Krebs cycle (acetyl-CoA → CO₂)

4. Electron transport chain (H’s removed in 1&3 form H₂O, series of proteins transfer e^- from NADH to make H⁺ gradient that drives the bulk of ATP synthesis)

Respiration redox reactions

Glucose is oxidized to CO₂ and O₂ is reduced to H₂O at the same time. Dehydrogenation is oxidation (exergonic), and adding a hydrogen is reduction (endergonic). This only happens when an OA and/or RA are present.

Coenzymes

H acceptors/e^- carriers (NAD & FAD)

Glycolysis stages

1. Phosphorylation

2. Lysis

3. Oxidation of Triose Phosphate

4. ATP formation

Phosphorylation

2 ATP give their phosphates to a glucose to form fructose diphosphate and 2 ADP

Lysis (splitting)

Fructose diphosphate splits into 2 triose phosphate (from here all processes are x2)

Oxidation of Triose Phosphate

The enzyme dehydrogenase oxidizes each triose phosphate, removing a phosphate and H, forming pyruvate. Each H is picked up by an NAD⁺ coenzyme, forming NADH + H⁺

ATP formation

The phosphates removed in the oxidation step combine with 2 ADP to form 2 ATP. Pyruvate then moves into the mitochondria for the rest of respiration.

NAD⁺ problem

NAD⁺ needs H to do its job, so the cell has to keep a supply of H. This is because glycolysis is the only ATP source when oxygen isn’t present, so the cell can’t afford to stop it.

Where does glycolysis take place?

Cytosol/cytoplasm

Where does the link reaction take place?

Mitochondrial matrix

Where are the enzymes for the link reaction and the Krebs cycle found?

Mitochondria

Anaerobic respiration

If oxygen isn’t present the pyruvate stays in the cytoplasm for fermentation. It accepts H from NADH (which then oxidizes back to NAD⁺) and forms lactate (in humans). In animals, the same thing happens but ethanol and CO₂ are formed instead. This process regenerates NAD⁺ and lets glycolysis continue rather than making ATP directly.

Components and function of mitochondria

1. Outer membrane (selective barrier that lets small molecules like pyruvate pass through)

2. Inner membrane (electron transport chain proteins and ATP synthase enzymes are embedded here)

3. Circular DNA

4. 70s ribosomes

5. Inter-membrane space (very small, allowing ion concentration to build up quickly and form a gradient)

6. Cristae (increase surface area)

7. Matrix

Link reaction

Pyruvate from glycolysis is actively transported through the mitochondrial membrane into the matrix (whole process x2).

1. Pyruvate decarboxylation (forms acetyl group and CO₂ waste)

2. H from acetyl reduces NAD⁺ (NADH forms and CoA combines with acetyl to form acetyl-CoA)

Common entry point (acetyl-CoA)

Carbs → glycolysis → pyruvate → link rxn → acetyl-CoA (can do aerobic/anaerobic)

Lipids → fatty acids → acetyl groups → acetyl-CoA (can only do aerobic)

Acetyl-CoA → Krebs cycle

Electron transport chain (ETC)

Series of proteins in mitochondria membrane where NADH and FADH₂ release electrons and H⁺ that gets pumped outside to generate a concentration gradient.

How do protons get pumped into the intermembrane space?

When NADH and FADH₂ release electrons they also release energy, which is then used to actively pump the protons across the inner membrane where they accumulate and form a concentration gradient.

Enzyme that brings protons back in with the concentration gradient

ATP synthase

Function of ATP synthase

Brings protons back in with the concentration gradient, this is their only way to reenter the matrix because the inner membrane doesn’t let them through.

How does ATP form in the ETC

When protons reenter the matrix with the gradient, they release energy that is then used to combine ADP and phosphates to form lots of ATP

Where do electrons go at the end of the ETC

They combine with oxygen and H⁺ to form metabolic water, oxygen acts as a terminal electron acceptor

How many ATP per NADH

2.5

How many ATP per FADH₂

1.5

Total yield of ATP per glucose molecule

32

Phosphorylation by chemiosmosis

Stored energy in a concentration gradient drives ATP synthesis

Why is oxygen needed in the ETC?

Electrons need a ‘final destination’ to be removed from the chain, this allows for the continuous flow of electrons through the system. Protons also need to be removed, and the formation of metabolic water fulfills both these purposes.