Cellular respiration is how your body breaks down glucose to...
Biology112 views·Updated 29 Aug 2026·3 pagesUnderstanding 3.3 Respiration in WJEC A-Level Biology
katie ♡@katielcnn
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1
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Glycolysis, Link Reaction & Krebs Cycle
Glycolysis kicks off in your cell's cytoplasm because glucose can't actually squeeze through mitochondrial membranes. The process starts by adding two phosphate groups to glucose using ATP, creating hexose diphosphate that splits into two triose phosphate molecules.
The link reaction happens in the mitochondrial matrix where pyruvate gets modified. It loses a carbon (forming CO₂), gets stripped of hydrogen (making reduced NAD), and combines with coenzyme A to form acetyl CoA.
The Krebs cycle is where the real action happens - acetyl CoA joins with a 4-carbon acid to make a 6-carbon compound. Through two decarboxylation steps and four dehydrogenation reactions, each cycle produces 1 ATP, 3 reduced NAD molecules, 1 reduced FAD, and 2 CO₂ molecules.
Quick Tip: Remember that glucose produces two pyruvate molecules, so the Krebs cycle actually runs twice per glucose molecule, doubling all the products!
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of 3
Electron Transport Chain & Anaerobic Respiration
The electron transport chain on the inner mitochondrial membrane is like a series of energy-harvesting stations. Reduced NAD and FAD donate electrons that travel through carrier proteins, powering three proton pumps that push H⁺ ions into the intermembrane space.
This creates a proton gradient - loads of positive charges building up outside the matrix. These protons can only get back through special channels containing ATP synthase, which harnesses their flow to synthesise ATP from ADP.
When oxygen runs out, your cells switch to anaerobic respiration. In muscle cells during intense exercise, pyruvate becomes the hydrogen acceptor and forms lactate (causing that burning sensation). Microorganisms like yeast convert pyruvate to ethanol and CO₂ through alcoholic fermentation.
Key Difference: Lactate can be broken down when oxygen returns, but ethanol accumulates and becomes toxic to cells.
3
of 3
ATP Production & Alternative Respiratory Pathways
A single glucose molecule can theoretically produce 38 ATP molecules through complete aerobic respiration - but real cells are less efficient due to energy costs of transport across membranes and proton leakage.
Lipids pack way more energy than carbohydrates. Fats get hydrolysed into glycerol and fatty acids - glycerol enters glycolysis whilst fatty acids break into 2-carbon fragments that feed directly into the Krebs cycle as acetyl CoA.
Proteins can also fuel respiration when needed. After deamination in the liver (removing the amino group as urea), the remaining carbon skeletons convert to pyruvate, acetyl CoA, or other Krebs cycle intermediates.
Energy Comparison: Fatty acids yield far more ATP per molecule than glucose because they contain more C-H bonds that can be oxidised.
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Biology112 views·Updated 29 Aug 2026·3 pagesUnderstanding 3.3 Respiration in WJEC A-Level Biology
katie ♡@katielcnn
Cellular respiration is how your body breaks down glucose to create the energy currency ATP that powers everything from muscle contractions to brain function. This complex process involves three main stages and can even work without oxygen when needed.
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Glycolysis, Link Reaction & Krebs Cycle
Glycolysis kicks off in your cell's cytoplasm because glucose can't actually squeeze through mitochondrial membranes. The process starts by adding two phosphate groups to glucose using ATP, creating hexose diphosphate that splits into two triose phosphate molecules.
The link reaction happens in the mitochondrial matrix where pyruvate gets modified. It loses a carbon (forming CO₂), gets stripped of hydrogen (making reduced NAD), and combines with coenzyme A to form acetyl CoA.
The Krebs cycle is where the real action happens - acetyl CoA joins with a 4-carbon acid to make a 6-carbon compound. Through two decarboxylation steps and four dehydrogenation reactions, each cycle produces 1 ATP, 3 reduced NAD molecules, 1 reduced FAD, and 2 CO₂ molecules.
Quick Tip: Remember that glucose produces two pyruvate molecules, so the Krebs cycle actually runs twice per glucose molecule, doubling all the products!
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Electron Transport Chain & Anaerobic Respiration
The electron transport chain on the inner mitochondrial membrane is like a series of energy-harvesting stations. Reduced NAD and FAD donate electrons that travel through carrier proteins, powering three proton pumps that push H⁺ ions into the intermembrane space.
This creates a proton gradient - loads of positive charges building up outside the matrix. These protons can only get back through special channels containing ATP synthase, which harnesses their flow to synthesise ATP from ADP.
When oxygen runs out, your cells switch to anaerobic respiration. In muscle cells during intense exercise, pyruvate becomes the hydrogen acceptor and forms lactate (causing that burning sensation). Microorganisms like yeast convert pyruvate to ethanol and CO₂ through alcoholic fermentation.
Key Difference: Lactate can be broken down when oxygen returns, but ethanol accumulates and becomes toxic to cells.
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ATP Production & Alternative Respiratory Pathways
A single glucose molecule can theoretically produce 38 ATP molecules through complete aerobic respiration - but real cells are less efficient due to energy costs of transport across membranes and proton leakage.
Lipids pack way more energy than carbohydrates. Fats get hydrolysed into glycerol and fatty acids - glycerol enters glycolysis whilst fatty acids break into 2-carbon fragments that feed directly into the Krebs cycle as acetyl CoA.
Proteins can also fuel respiration when needed. After deamination in the liver (removing the amino group as urea), the remaining carbon skeletons convert to pyruvate, acetyl CoA, or other Krebs cycle intermediates.
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