C1.2 Cell Respiration

What is ATP, & what is it composed of?

ATP (adenosine triphosphate) is a nucleotide composed of:

• Nitrogenous base - Adenine 

• Pentose sugar - Ribose 

• 3 phosphate groups

Where is ATP stored?

• ATP is stored in the bonds between phosphate groups

  • Especially high-energy bond between 2nd & 3rd phosphate (terminal bond)

What properties make ATP ideal? (5)

Property	Why It’s Useful
Small & soluble	Easily moves around inside cells
Releases energy in small amounts	Prevents waste of energy as heat
Rapid breakdown & reformation	Can be recycled quickly & reused
Universal molecule	Used in all types of cells, across all life forms
Couples w/ many reactions	Drives both anabolic & catabolic processes

State the processes requiring ATP

1) Active transport across membranes

2) Anabolic reactions (building macromolecules)

3) Movement within or by cells

ATP-powered process: Describe ATP’s role in active transport, & give an example (3)

• ATP provides energy for carrier proteins (pumps) that move substances against their concentration gradient

• Without ATP, substances would only move passively (with the gradient)

Example: Sodium-potassium pump in nerve cells 

ATP-powered process: Describe ATP’s role in anabolic reactions, & give examples (4)

• ATP powers condensation reactions that build complex molecules from smaller ones

Examples: 

• Protein synthesis (from amino acids) 

• DNA/RNA synthesis (from nucleotides) 

• Glycogen synthesis (from glucose)

ATP-powered process: Give examples of ATP’s role in movement within/by cells (4)

Type of Movement	ATP Role
Chromosome movement	Powers spindle fibers during mitosis and meiosis
Cytoplasmic streaming	Helps move organelles through the cytoplasm
Muscle contraction	ATP is needed to detach myosin heads from actin
Flagella/cilia movement	Example: Sperm movement  ATP powers flagellum’s motor proteins like dynein

What does ADP stand for?

Adenosine diphosphate

What does Pi stand for?

Inorganic phosphate

Describe how ATP releases & stores energy (8)

1) Hydrolysis reaction: 

• ATP → ADP (adenosine diphosphate) + Pi (inorganic phosphate) + Energy 

  • A water molecule is used to break the bond between last two phosphate groups

  • Energy is released & used immediately for cellular processes 

2) Synthesis (Phosphorylation) reaction: 

• ADP + Pi + Energy → ATP

  • Energy is used to rejoin the phosphate group to ADP

  • This energy is stored in the new ATP molecule 

Define cell respiration

A controlled release of energy from organic compounds in cells, used to produce ATP

Describe the process of cell respiration (3)

1. Carbon compounds act as substrates that are broken down in a series of enzyme-catalysed steps:

• Principal substrates: Glucose & Fatty Acids

• Others (e.g. proteins)
  

2. Energy released from these reactions is used to convert ADP + Pi into ATP

3. In aerobic cell respiration, ATP is produced w/ CO₂ & H₂O as waste products

Describe the differences between cell respiration & gas exchange (4×2)

Feature	Cell Respiration	Gas Exchange
What it is	Chemical reactions that release energy	Physical process of moving gases in/out of cells
Purpose	To make ATP	To supply O₂ & remove CO₂
Location	Cytoplasm & mitochondria	Across cell membranes (e.g., lungs, leaves)
Gases used/produced	Uses O₂ (aerobic) & produces CO₂	Brings in O₂ & expels CO₂

What is the word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water + energy (ATP)

What is the word equation for anaerobic respiration?

Glucose → lactic acid + energy (ATP)

Describe the differences between aerobic & anaerobic cell respiration in humans (5×2)

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen required?	✅	❌
Main substrate	Glucose (can also use fatty acids, amino acids)	Glucose only
ATP yield	High (~36–38 ATP per glucose)	Low (only 2 ATP per glucose)
Waste products	Carbon dioxide (CO₂) + water (H₂O)	Lactic acid (lactate)
Where in the cell?	1. Starts in cytoplasm (glycolysis) 2. Continues & finishes in mitochondria (Krebs cycle + electron transport chain)	Entirely in cytoplasm (no mitochondria needed)

What does the rate of cell respiration refer to, & how is it measured? (6)

• How quickly a cell produces ATP by breaking down organic compounds (glucose or others)

• It is often measured by:

  • Oxygen consumption 

  • Carbon dioxide production 

  • Change in pH 

  • Heat released

Describe the methods of measuring rate of cell respiration

Method	Factor measured
Respirometer (with seeds or insects)	Volume of oxygen consumed
CO₂ probe	Rate of carbon dioxide production
pH meter in yeast/glucose solution	Drop in pH as CO₂ forms carbonic acid
Calorimeter	Heat produced by respiring organisms

Describe how rate of cell respiration varies with different variables

Variable	Effect
Temperature	Higher temps increase enzyme activity → faster rate until optimum Too hot = denaturation
pH	Each enzyme has an optimal pH. Deviations slow down or stop respiration.
Glucose concentration	More glucose = more fuel for higher rate (up to a saturation point)
Oxygen availability	Needed for aerobic respiration  Limited oxygen shifts cells to slower anaerobic respiration
Enzyme concentration	More enzymes = faster reaction, as long as substrate is available
Cell type or tissue type	Some cells (like muscles) have more mitochondria = higher respiration rates

What is NAD & its forms? (4)

• Nicotinamide Adenine Dinucleotide (NAD) is a coenzyme used in cell respiration to carry hydrogen atoms from one reaction to another 

• It exists in two forms:

  • NAD⁺ → oxidized form (ready to accept hydrogen) 

  • NADH → reduced form (carrying hydrogen)

Define redox reaction

A reaction where one substance is oxidized and another is reduced

What redox reactions are involved in respiration?

1) Oxidation	Substrate loses electrons or hydrogen (often via dehydrogenation) → this is carried out by enzymes called dehydrogenases
2) Reduction	Coenzyme gains electrons or hydrogen

Describe NAD’s role in respiration (4)

1. NAD picks up hydrogen atoms that are removed from substrates (e.g., glucose intermediates) 

2. Accepts 1 H⁺ ion and 2 electrons → becomes NADH 

3. NADH carries these electrons to the electron transport chain in mitochondria 

4. In the final step, NADH is oxidized back to NAD⁺, releasing energy to make ATP

What is glycolysis? State its metabolites

⭐ The first stage of cellular respiration

• Role: A linear pathway of enzyme-catalysed steps breaking down glucose (6-carbon) into 2 pyruvate molecules (3-carbon) 

  • There is a net gain of:

    • 2 ATP molecules

    • 2 NADH molecules (reduced NAD)

• Location: Cytoplasm

——————————————————————————————————

What’s involved (Metabolites) - 

• Starting substrate: Glucose 

• Intermediate: Pyruvate 

• Products (net gain): 2 Pyruvate, 2 ATP molecules, 2 NADH (reduced NAD⁺)

Describe the stages of glycolysis, & its purpose (4×3)

Stage	What Happens	Purpose
1. Phosphorylation	Glucose is activated by the addition of 2 phosphate groups from ATP	Makes glucose more reactive
2. Lysis	The phosphorylated glucose splits into two 3-carbon sugars	Prepares for energy extraction
3. Oxidation	Each 3-carbon molecule is oxidized — H atoms are removed and transferred to NAD⁺ → NADH	Captures energy-rich electrons
4. ATP Formation	- Phosphate groups are removed to form 4 ATP molecules are produced (2 per 3-carbon sugar) - As the final phosphate is removed, 2 pyruvates are formed	2 are used earlier → net gain = 2 ATP

Explain what happens to respiration in low oxygen conditions

• In low oxygen conditions, cells switch from aerobic to anaerobic respiration

• Glycolysis still occurs but w/o oxygen, NADH can’t offload hydrogen via the electron transport chain 

  • As a solution, pyruvate is converted into lactate in the cytoplasm (lactic acid fermentation)

Describe the process of lactic acid fermentation

Step	Purpose
Pyruvate is reduced to lactate	Accepts hydrogen from NADH (via lactate dehydrogenase)
NADH is oxidised to NAD⁺	Regenerates NAD⁺ so glycolysis can continue

Compare anaerobic respiration in humans vs yeast

• Anaerobic respiration is largely similar in humans & yeast

• The difference lies in how NAD⁺ is regenerated & its final products:

Organism	Pyruvate is converted to…	Final products
Humans	Lactate	Lactate (lactic acid)
Yeast	Ethanol + CO₂	Ethanol (alcohol) + carbon dioxide

What is the link reaction? (4)

⭐ The second stage of cellular respiration

• Role: A series of enzyme-catalysed steps connecting Glycolysis to the Krebs Cycle

• Location: Mitochondrial matrix

  • Only occurs under the presence of oxygen (aerobic conditions)

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What’s involved (Metabolites) - 

• Starting substrate: Pyruvate 

• Intermediate: Acetyl group 

Products: 

• Final: Acetyl-CoA

• Byproducts: CO₂ , NADH

Describe the process of the link reaction

1. Decarboxylation

• One carbon is removed from pyruvate (3C) → releasing CO₂ 

• This leaves acetyl group (2C) to remain

2. Oxidation

• Acetyl group is oxidised (loses hydrogen atoms)

• Hydrogen is accepted by NAD⁺ & is reduced to NADH 

3. Coenzyme A (CoA)

• Acetyl group (2C) is attached to CoA → forms Acetyl-CoA

• Acetyl-CoA enters the Krebs cycle

How do carbohydrates vs lipids proceed to Krebs cycle?

• Both carbohydrates & lipids form acetyl-CoA & proceed to Krebs cycle

  • However, only carbohydrates undergo link reaction

Describe how carbohydrates undergo cell respiration (4)

1. Glucose is broken down by glycolysis in the cytoplasm to form pyruvate (3C)

2. In the link reaction, pyruvate is decarboxylated & oxidised to form an acetyl (2C) group

3. This acetyl group combines w/ coenzyme A to form acetyl‑CoA

4. This acetyl‑CoA feeds into Krebs cycle

Describe how lipids undergo cell respiration (4)

1. They are broken into fatty acids, which are transported to mitochondria

2. Fatty acids are oxidised at matrix, breaking it into multiple acetyl (2C) units

3. Each 2C acetyl unit combines w/ coenzyme A to form acetyl‑CoA

4. This acetyl‑CoA feeds into Krebs cycle

Describe the process of the Krebs cycle

1. Acetyl-CoA (2C) combines w/ oxaloacetate (4C) → forming citrate (6C) 

2. Citrate undergoes a series of reactions that: 

• Release 2 CO₂ molecules (decarboxylations) 

• Perform 4 oxidations (dehydrogenation reactions) 

• Regenerate oxaloacetate (4C) to restart the cycle

What type of reaction is oxidation

Dehydrogenation reactions = hydrogen atoms are removed from intermediates

What is the Krebs Cycle? State its metabolites

⭐ The third stage of cellular respiration

• Role: A cyclical pathway of enzyme-catalysed reactions that oxidises acetyl-CoA to release ATP

• Location: Mitochondrial matrix

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What’s involved (Metabolites) -

Starting substrate: Acetyl-CoA (2C)

Intermediates: Citrate (6C), Oxaloacetate (4C)

Products:

• Final: 6 NADH (3 per cycle), 1 FADH₂, 1 ATP  

• Byproduct: 2 CO₂

Describe the process of the Krebs Cycle (8)

1. Acetyl-CoA (2C) combines w/ oxaloacetate (4C) → forming citrate (6C) 

2. Citrate undergoes a series of reactions that: 

• Release 2 CO₂ molecules (decarboxylations) 

• Perform 4 oxidations (dehydrogenation reactions), which produce:

•  3 NADH

• 1 FADH₂

• 1 ATP

3. Oxaloacetate (4C) is regenerated for 2nd cycle

What are the stages of cell respiration?

1) Glycolysis

2) Link reaction

3) Krebs Cycle

4) Electron transport & chemiosmosis

What is the role of NADH?

• Reduced NAD (NADH) carries electrons & hydrogen down the electron transport chain 

• It is provided & accumulated from previous stages of cell respiration

Process	NADH produced
Glycolysis	2 NADH per glucose
Link Reaction	2 NADH per glucose
Krebs Cycle	6 NADH per glucose
Total from one glucose = 10 NADH

Describe the process of NADH transfer down the ETC (5)

1. NADH donates a pair of electrons to the first carrier in the ETC

2. This oxidizes NADH → converting back to NAD⁺, which can be reused

3. Electrons move along the chain, releasing energy at each step

4. This energy is used to pump H⁺ ions (protons) into the intermembrane space

5. This sets up a proton gradient needed for ATP synthesis via chemiosmosis

Define proton gradient

A difference in H⁺ (proton) concentration across a membrane

Describe the process of proton gradient formation

1. Reduced NAD (NADH) and reduced FAD (FADH₂) donate electrons to the electron transport chain (ETC)

2. As electrons flow along the ETC, they release energy

3. This energy is used to pump protons (H⁺) from the matrix into intermembrane space

4. This results in:

• High concentration of protons in the intermembrane space 

• Low concentration in the matrix

Define chemiosmosis

The movement of protons (H⁺) down their concentration gradient across a membrane through the enzyme, ATP synthase

Describe the features of chemiosmosis

⭐ Along w/ the ETC, it makes up the final stage of cell respiration

• Role: Chemiosmosis links energy released by the ETC to the phosphorylation of ADP → producing ATP

• Location: Inside the mitochondrion, specifically:

  • Across the inner mitochondrial membrane

  • Between the intermembrane space & matrix

Describe the process of ATP synthesis  (oxidative phosphorylation)

1. The ETC pumps H⁺ ions into the intermembrane space, creating a proton gradient

2. Protons flow back into the matrix through ATP synthase (a membrane protein)

3. The energy from this proton movement is used by ATP synthase to: 

• Add a phosphate group to ADP 

• Form ATP (adenosine triphosphate)

What is oxygen’s role in cell respiration?

• Oxygen is the terminal electron acceptor in cell respiration

• At the end of the ETC, it combines with:

  • Electrons coming from the ETC

  • Protons from the matrix

• This forms metabolic water & allows continuous flow of electrons along ETC 

What happens if oxygen is absent in cell respiration? (3)

1. Electrons have nowhere to go so they back up the ETC

2. The ETC stops = no more proton pumping  

3. ATP production halt

Define respiratory substrate

Molecules used to release energy in cell respiration

What are the main types of respiratory substrate?

1) Carbohydrates (like glucose) 

2) Lipids (like triglycerides → fatty acids

Compare cell respiration with carbohydrates vs lipids (6)

Feature	Carbohydrates	Lipids
Energy yield /g	Lower	Higher
Oxygen content	Higher	Lower
Hydrogen and carbon	Less oxidizable	More oxidizable
Pathway entry	Glycolysis → Pyruvate → Acetyl-CoA	Fatty acids → 2C acetyl groups → Acetyl-CoA
Used in glycolysis?	✅	❌
Used in anaerobic respiration?	✅	❌

Why do lipids have higher energy yields

1) Lipids have more C-H bonds to be oxidised during respiration

2) They are also less oxygenated → more energy released when oxygen added