Higher level Topic summary of C1.2 Cell Respiration of IB Biology of the new guide (first exams 2025). This document contains all the subtopics/understandings/guidelines that should be covered for this topic. Only Higher level content.
Cell Respiration HL
C1.2.7 Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell
respiration
Oxidation: Loss of electrons/hydrogen atoms/Gain of oxygen atoms
Reduction: Gain of electrons/hydrogen atoms/Loss of oxygen atoms
Electron carrier: Substance that accepts and loses electrons, linking these reactions → Redox
reactions
★ Electron carrier in cell respiration is NAD
➔ NAD has one positive charge (NAD+)
➔ In respiration, substances are oxidized by removing two electrons
➔ NAD+ accepts two electrons and one proton from hydrogen atoms
◆ NAD+ + 2H+ + 2e- → NADH + H+
Reduced NAD is just NAD. → NAD is a coenzyme
When a molecule is oxidized and loses electrons, NAD picks up those electrons and becomes
NADH.
C1.2.8 Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of
ATP and reduced NAD
Glycolysis has 4 steps: Phosphorylation of glucose, Lysis, Oxidation, and ATP formation.
● First part of aerobic respiration where a monosaccharide, GLUCOSE, is the substrate.
● Happens in the cytoplasm of the cell
● Glucose is converted into pyruvate in a chain reaction
● Small yield of ATP and no oxygen consumed
PHOSPHORYLATION:
● Addition of Phosphate (PO4) to a molecule
● Uses ATP
● Molecule becomes more unstable → More likely to react
○ Glucose →(ATP) Glucose-6-phosphate
● Then, glucose is converted into fructose
● Fructose: Symmetrical molecule that can be split in half
○ Glucose-6-phosphate → fructose-6-phosphate
● Second phosphorylation:
○ Fructose-6-phosphate → fructose-1,6-biphosphate
LYSIS:
● Fructose is split in half resulting two triose phosphates
○ Fructose-6-phosphate → 2 triose phosphate
OXIDATION:
● Both triose phosphates are oxidized by removing hydrogen atoms
● Hydrogen accepted by NAD, becoming reduced NAD
, ●Triose oxidation produced an organic acid: glycerate carrying 2 phosphates
●Second phosphate group attaches
○ Triose phosphate + phosphate → biphosphoglycerate
ATP FORMATION:
● ATP forms because biphosphoglycerate has two phosphate → 2 ATPs
● Glycerate is converted to pyruvate
● 1 glucose produces two biphosphoglycerate → Each biphosphoglycerate
produced 2 ATP → Four ATPs are produced per glucose.
OVERALL:
➔ One glucose containing six carbon atoms is converted into two pyruvates
➔ 2 NAD molecules are reduced → Reduced NAD
➔ Net yield = 2 ATPs
◆ Two are used in the first stages, 4 are produced in the end stage.
◆ Does not require oxygen so it is useful when oxygen supply is limited.
C1.2.9 Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell
respiration
In glycolysis, glucose, NAD and ADP must be reloaded for the process to continue.
● If ADP runs out because it converted to ATP there is no need for glycolysis
● NAD will run out unless its regenerated by oxidation of reduced NAD
NAD regeneration:
➔ Two hydrogen atoms are transferred to another molecule (Oxidizing reduced NAD)
➔ Hydrogen is transferred from reduced NAD to pyruvate → lactate
◆ Called LACTIC FERMENTATION
When there is no oxygen, pyruvate and reduced NAD cannot oxidize to be used in
glycolysis, producing NO ATP.
➔ To avoid this, regeneration of reduced NAD needs to happen (anaerobic
respiration)
● Human red blood cells do not have mitochondria, thus they need this process
● In period of high energy demand where there is more oxygen needed than oxygen
produced, muscles cells undergo lactic fermentation
● Lactobacillus (bacteria) undergo this process and is used for yogurt production,
converting lactose into lactic acid
C1.2.10 Anaerobic cell respiration in yeast and its use in brewing and baking
Ethanol formation: Pyruvate is decarboxylated into ethanal, producing carbon dioxide. Then
ethanal is reduced by oxidating reduced NAD, reforming NAD+ and ethanol
Saccharomyces cerevisiae (Yeast):
➔ Yeast respires anaerobically
◆ Produces CO2, causing air pockets in the bread, making it rise
◆ Ethanol evaporates in the heat when baking
➔ Used to brew alcohols (wine, beer), depending on the sugar source of fermentation
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