Food, energy and cellular respiration

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Lesson notes

What is Cellular Respiration?

Cellular respiration is the process of breaking down food molecules (like glucose) to release **energy** stored in **ATP**.
It uses an **inorganic electron acceptor** (usually oxygen) to transfer electrons and produce ATP.
Respiration is a **catabolic** reaction: large molecules are broken into smaller ones, releasing energy.
The overall reaction is a slow, controlled **combustion** of nutrients.

Aerobic vs. Anaerobic Respiration

**Aerobic respiration** uses oxygen (O₂) as the final electron acceptor; it produces up to **36–38 ATP** per glucose.
**Anaerobic respiration** uses other molecules (e.g., nitrate, sulfate) as electron acceptors; it yields less ATP.
**Fermentation** is anaerobic but not respiration—no external electron acceptor is used.
Aerobic metabolism is up to **15 times more efficient** than anaerobic metabolism.

The Overall Equation

The simplified equation for aerobic respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + **energy (ATP + heat)**.
The reaction is **exergonic** (negative ΔG), meaning it releases energy spontaneously.
Energy is captured in **ATP** molecules, which power cellular work.

Glycolysis (First Stage)

Glycolysis occurs in the **cytosol** of cells and does **not** require oxygen.
One glucose (6-carbon) is split into two **pyruvate** (3-carbon) molecules.
It produces a net gain of **2 ATP** and **2 NADH** per glucose.
Glycolysis is also called **"sugar splitting"** and is common to all living organisms.

The Krebs Cycle (Second Stage)

The Krebs cycle takes place in the **mitochondrial matrix** (eukaryotes) or cytoplasm (prokaryotes).
Pyruvate is converted into **acetyl-CoA**, which enters the cycle.
Each turn produces **2 CO₂**, **3 NADH**, **1 FADH₂**, and **1 ATP** (per pyruvate).
The cycle completes the oxidation of glucose to carbon dioxide.

Electron Transport Chain and Oxidative Phosphorylation

NADH and FADH₂ donate electrons to the **electron transport chain** (ETC) in the inner mitochondrial membrane.
Electrons flow through protein complexes, pumping **protons (H⁺)** across the membrane.
The proton gradient drives **ATP synthase** to produce most of the cell's ATP (≈34 ATP per glucose).
Oxygen is the **final electron acceptor**, forming water (H₂O).

ATP Yield and Efficiency

Theoretical maximum yield is **38 ATP** per glucose, but actual yield is about **29–30 ATP** due to leaks and transport costs.
Anaerobic respiration yields only **2 ATP** per glucose (from glycolysis alone).
ATP stores energy in its **phosphate bonds**; breaking the terminal phosphate releases energy for cellular work.

Importance in Living Organisms

Cellular respiration provides energy for **growth, repair, movement, and transport** across membranes.
Plants respire too—about half of the CO₂ in ecosystems comes from plant respiration.
Nutrients used include **sugars, amino acids, and fatty acids**.

Energy pyramid showing energy flow from producers to consumers; only ~10% of energy transfers between trophic levels.

Overview of aerobic cellular respiration showing glycolysis, Krebs cycle, and electron transport chain in a cell.

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Practice questions

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1.What is the main purpose of cellular respiration?
Easy
- ATo produce ATP from the breakdown of food molecules
- BTo produce carbon dioxide and water
- CTo break down glucose into pyruvate
- DTo consume oxygen
2.Which of the following is the correct overall equation for aerobic cellular respiration?
Easy
- AC6H12O6 + 6O2 → 6CO2 + 6H2O + energy
- B6CO2 + 6H2O → C6H12O6 + 6O2
- CC6H12O6 → 2C3H4O3 + 2ATP
- DC6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
3.Where does glycolysis occur in a cell?
Easy
- ACytosol
- BMitochondrial matrix
- CInner mitochondrial membrane
- DNucleus
4.What is the net ATP yield from glycolysis per molecule of glucose?
Medium
- A2 ATP
- B4 ATP
- C36 ATP
- D1 ATP
5.Which process requires oxygen to produce ATP?
Medium
- AAerobic respiration
- BAnaerobic respiration
- CFermentation
- DGlycolysis
6.In which organelle does the Krebs cycle and oxidative phosphorylation occur in eukaryotic cells?
Medium
- AMitochondrion
- BChloroplast
- CRibosome
- DNucleus
7.What is the approximate maximum number of ATP molecules that can be produced from one molecule of glucose through aerobic respiration?
Hard
- A36-38 ATP
- B2 ATP
- C12-15 ATP
- D100 ATP
8.Which of the following is NOT a product of the Krebs cycle?
Hard
- AOxygen
- BCarbon dioxide
- CNADH
- DATP

Revision notes

## Cellular Respiration: Energy from Food - **Cellular respiration** is the process cells use to break down food molecules (like glucose) and release energy stored in ATP. - **Overall reaction** (aerobic): Glucose + Oxygen → Carbon dioxide + Water + ATP (energy) - **Key point**: Energy is released slowly in controlled steps, not all at once like burning. ### Types of Respiration - **Aerobic respiration**: Uses oxygen (O₂) as the final electron acceptor. Produces up to ~29–30 ATP per glucose. Much more efficient than anaerobic. - **Anaerobic respiration**: Uses an inorganic molecule other than oxygen (e.g., sulfate) as electron acceptor. Still produces ATP, but less than aerobic. - **Fermentation** (not respiration): No external electron acceptor; produces only 2 ATP per glucose. ### Main Stages (Aerobic) 1. **Glycolysis** (in cytosol): Splits one glucose (6C) into two pyruvate (3C each). Produces **2 net ATP** and **2 NADH**. No oxygen needed. - Reaction: Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 ATP + 2 H⁺ + 2 H₂O 2. **Pyruvate oxidation** (in mitochondria): Each pyruvate is converted to acetyl-CoA, producing NADH and CO₂. 3. **Krebs cycle** (in mitochondria): Acetyl-CoA is fully oxidized to CO₂. Produces ATP, NADH, and FADH₂. 4. **Electron transport chain & oxidative phosphorylation** (inner mitochondrial membrane): NADH and FADH₂ donate electrons, creating a proton gradient that drives ATP synthase. Most ATP is made here. ### Key Terms - **ATP (adenosine triphosphate)**: The main energy currency of the cell. Energy is released when the third phosphate bond is broken. - **Glycolysis**: "Sugar splitting" – first step of respiration, occurs in cytosol, breaks glucose into pyruvate. - **Aerobic**: Requires oxygen. - **Anaerobic**: Does not require oxygen; uses other electron acceptors. - **Mitochondria**: Organelle where most of aerobic respiration occurs (Krebs cycle and ETC). - **NADH / FADH₂**: Electron carriers that transfer high-energy electrons to the electron transport chain. - **Oxidative phosphorylation**: Process that uses the electron transport chain and chemiosmosis to produce ATP. ### Efficiency - Aerobic respiration yields up to **29–30 ATP** per glucose. - Anaerobic (glycolysis only) yields **2 ATP** per glucose. - Aerobic is about **15 times more efficient** than anaerobic alone.

Cellular respiration: The process of oxidizing biological fuels using an inorganic electron acceptor (often oxygen) to produce ATP, which stores chemical energy for cellular activities.
ATP (adenosine triphosphate): The main energy currency of the cell; energy is released when the bond of its third phosphate group is broken, forming ADP and a free phosphate.
Glycolysis: A metabolic pathway in the cytosol that splits one glucose molecule into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH; occurs with or without oxygen.
Aerobic respiration: Cellular respiration that requires oxygen (O₂) as the final electron acceptor; produces carbon dioxide, water, and a large amount of ATP.
Anaerobic respiration: Cellular respiration that uses an inorganic molecule other than oxygen (e.g., sulfate) as the final electron acceptor; yields less ATP than aerobic respiration.
Mitochondria: Organelles in eukaryotic cells where the Krebs cycle and oxidative phosphorylation take place; the site of most ATP production in aerobic respiration.
NADH: An electron carrier molecule that transfers high-energy electrons to the electron transport chain, where they are used to produce ATP.
FADH₂: An electron carrier similar to NADH; donates electrons to the electron transport chain, contributing to ATP production.
Oxidative phosphorylation: The process in which ATP is formed using the energy released by the electron transport chain and chemiosmosis; produces most of the ATP in aerobic respiration.
Electron transport chain: A series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH₂ to oxygen, creating a proton gradient used to make ATP.

Flashcards

What is cellular respiration?

Cellular respiration is the process of oxidizing biological fuels (like glucose) using an inorganic electron acceptor (often oxygen) to produce ATP, which stores chemical energy for cellular activities.

What is the main purpose of cellular respiration?

The main purpose is to release chemical energy from nutrients and convert it into ATP, which cells use to power processes like biosynthesis, movement, and transport.

What does ATP stand for and why is it important?

ATP stands for adenosine triphosphate. It stores chemical energy in a form that cells can use; breaking the bond of its third phosphate group releases energy for cellular work.

What is the difference between aerobic and anaerobic cellular respiration?

Aerobic respiration uses oxygen as the final electron acceptor; anaerobic respiration uses an inorganic molecule other than oxygen (e.g., sulfate). Both produce ATP, but aerobic yields much more.

What is the overall chemical equation for aerobic cellular respiration?

Glucose + 6 O2 → 6 CO2 + 6 H2O + energy (ATP). The energy released is used to form ATP from ADP and phosphate.

Where does glycolysis occur in the cell?

Glycolysis occurs in the cytosol (cytoplasm) of cells.

What are the inputs and net outputs of glycolysis?

Inputs: glucose, 2 NAD+, 2 ADP, 2 π. Net outputs: 2 pyruvate, 2 NADH, 2 ATP (net), 2 H+, 2 H2O, and energy.

How many ATP molecules are produced per glucose during glycolysis?

Glycolysis produces 4 ATP total, but 2 are used in the preparatory phase, so the net gain is 2 ATP per glucose.

What happens to pyruvate after glycolysis in aerobic respiration?

Pyruvate is transported into the mitochondria, where it is oxidized and enters the citric acid cycle (Krebs cycle).

What are the main products of the citric acid cycle?

The citric acid cycle produces CO2, ATP (or GTP), NADH, and FADH2.

What is the role of the electron transport chain in aerobic respiration?

The electron transport chain uses electrons from NADH and FADH2 to pump protons across a membrane, creating a chemiosmotic gradient that drives ATP synthase to produce most of the ATP.

What is oxidative phosphorylation?

Oxidative phosphorylation is the process in which ATP is formed using the energy released by the electron transport chain and chemiosmosis; it produces most of the ATP in aerobic respiration.

How many ATP molecules can be produced from one glucose molecule in aerobic respiration?

The theoretical maximum is 38 ATP, but actual yields are about 29–30 ATP per glucose due to energy losses.

Why is aerobic respiration more efficient than anaerobic respiration?

Aerobic respiration yields up to 15 times more ATP per glucose (about 30 ATP) than anaerobic respiration (2 ATP).

What are the waste products of aerobic cellular respiration?

The waste products are carbon dioxide (CO2) and water (H2O).

What types of molecules can be used as fuel in cellular respiration?

Common fuels include sugars (like glucose), amino acids, and fatty acids.

What is the role of oxygen in aerobic respiration?

Oxygen acts as the final electron acceptor in the electron transport chain, allowing the production of large amounts of ATP.

What is the difference between cellular respiration and fermentation?

Cellular respiration uses an external electron acceptor (like oxygen) and produces ATP via an electron transport chain; fermentation does not use an external electron acceptor and yields only 2 ATP per glucose.

Where do the post-glycolytic reactions (Krebs cycle and oxidative phosphorylation) occur in eukaryotic cells?

They occur in the mitochondria.

What is the meaning of the term 'glycolysis'?

Glycolysis literally means 'sugar splitting' and refers to the breakdown of glucose into two pyruvate molecules.

What is the initial step of glycolysis?

Glucose is phosphorylated using one ATP to form glucose-6-phosphate, which increases its reactivity.

What is the net ATP yield from anaerobic respiration (glycolysis only)?

Anaerobic respiration (glycolysis) yields a net of 2 ATP per glucose molecule.

Do plants carry out cellular respiration?

Yes, plants respire; plant respiration accounts for about half of the CO2 generated annually by terrestrial ecosystems.

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