IB Biology 2023

1. C1.2 Cell Respiration
Guiding Questions:
1. What are the roles of hydrogen and oxygen in the release
of energy in cells?
2. How is energy distributed and used in cells?
Theme: C Interaction and
interdependence
Eukaryotic cells depend on
mitochondria to provide energy while
the cell helps to synthesize membrane
proteins for mitochondria. The
integration of mitochondria into the
cells gave rise to the evolution of
current eukaryotic cells.
Level of organisation:
Molecules
https://www.nature.com/scitable/topicpage/the-origin-of-
mitochondria-14232356/

2. • Yes, it is the same nucleotide that forms the ‘A’ in DNA
• ATP  Adenosine Triphosphate
• ATP is considered the “energy
currency in cells”
• Can be used immediately in
cells to release energy within
the cell
• Organic molecules (glucose) on
the other hand STORES energy
C1.2.1 ATP as the molecules that distributes
energy within cells

3. • ATP is a small molecules that can be easily
transported around
• ATP = nucleoside linked to three phosphate via
high energy bonds
• Breaking of the phosphate group (hydrolysis)
from ATP releases energy
• ATP  ADP + Pi
• ADP needs energy to get ‘recharged’ back to ATP
through respiration
C1.2.3 Energy transfer during interconversion between ATP & ADP

4. 1. For synthesizing macromolecules, eg: protein
synthesis, DNA replication (ANABOLSIM)
2. Pump (active transport) molecules or ions across
membrane
3. Move things around cells, such as chromosomes,
muscle fibers that produce contraction
4. Or movement of whole cells eg: sperm tail, flagella
• Once ATP is used and energy released & dissipate as
heat
• They need to be recharged via cell respiration
• As cells need a continuous supply of ATP to perform cell
activities
C1.2.2 Life processes within cells that ATP supplies with energy

6. *extra: Phosphorylation
• Phosphorylation  addition of a phosphate
molecules (PO4
2-) to an organic molecule
• Make phosphorylated molecule more
unstable
• More likely to react
• Phosphorylation  activate molecule
• The hydrolysis of ATP  ADP +Pi releases
energy (exogernic)
• Many rxn in. body are endorgernic (needs
energy) so need to be coupled with
hydrolysis of ATP that releases energy

7. • Cell respiration is the controlled release of
energy from organic compounds to produce
ATP =/= NOT gas exchange
• Usually from glucose
• The breaking down of chemical bonds in glucose
releases energy (hydrolysis reaction)
• But lipids/fats & proteins or other
carbon/organic compounds can also be used
• Lipids can produce twice as much energy but
requires aerobic respiration (glucose can be
broken down anaerobically to a certain extent)
• Protein digestion produces nitrogenous waste 
ammonia  urea
C1.2.4 Cell respiration as a system for producing ATP within the cell
using energy released from carbon compounds

8. Aerobic Respiration Anaerobic respiration
Normal breakdown of glucose that
release large amount of ATP (energy)
Happens when lack of oxygen or to
produce short amount of energy rapidly
Glucose + OXYGEN, lipid can be used Glucose only (no oxygen) & no lipid
Complete breakdown of glucose Partial breakdown of glucose
Large amount of energy (36-38 ATP) Small amount of ATP (2 ATP)
Carbon dioxide + water as waste
products
Pyruvate form which is fermented to
lactate or alcohol + CO2
Cytosol + mitochondria Cytosol ONLY (liquid part of cytoplasm)
C1.2.5 Differences between anaerobic and aerobic cell respiration

10. • A Respirometer could be used to
measure...
– Respiration rates of different
organisms
– Effect of temperature on
respiration
– Comparing respiration in active v.
inactive organisms
• Potassium hydroxide or soda lime
(alkali) is used to absorbed carbon
dioxide, so will not affect volume of
air
• Fluid in manometer should move
towards organism as volume of air
should reduce as oxygen is being
used up
• Temperature should be kept constant
as it will affect the volume of air
C1.2.6 Variables affecting the rate of cell
respiration (NOS)

11. REDOX reaction of Electron Carriers
• Respiration  staggered (step by step) breakdown of sugar
• This reduce the activation energy needed
• And in the process transfer energy to molecules called electron carriers via redox reaction
• Electron carriers  gain & lose electrons
as required
• Remember OIL RIG  oxidation is lose e-,
reduction is gain e-
• Linked oxidation & reduction
• Main electron carrier in respiration 
NAD (nicotinamide adenine dinucleotide)

12. Remember hydrogen  one proton and one electron
H+  proton
• NAD+ + 2 electrons  reduced NADH
• Actual reaction: NAD  exist as NAD+
• NAD+ + 2H+ + 2e-  NADH + H+
• NAD+ + 2H  NADH + H+
• NADH (reduced form of NAD that gains 2 electrons & 1 Hydrogen ion - proton)
• NAD+  electron & hydrogen carrier
AHL: C1.2.7 Role of NAD as a carrier of hydrogen and
oxidation by removal of hydrogen during cell respiration

13. Other electron (& hydrogen) carriers
FAD + 2H+ + 2e– → FADH2
NADP+ + 2H+ + 2e-  NADPH + H+ (only for chloroplast)
Become reduced
NAD+  oxidising agent
Become oxidised
NADH  reducing agent

14. https://www.youtube.com/watch?v=eJ9Zjc-jdys

15. Aerobic
Respiration
• Glycolysis happens first, and pyruvate will undergo a complicated
pathway to fully breakdown into CO2 and water
• Glycolysis  link reaction  Kreb (citric acid) Cycle  ETC (electron
transport chain)
• It will produce a net amount of 36 - 38 ATP
• It will produce several electron/hydrogen carriers (NADH) which will
in turn help to produce ATP from ADP through a process called
oxidative phosphorylation (in electron transport chain)
• This happens in the mitochondria

17. • Multi-step pathway  each catalysed by a
different enzyme
• Splitting of glucose (6C) to 2 pyruvate (3C)
• Without the use of oxygen, in the cytosol
1. Phosphorylation
Glucose is phosphorylate by 2 ATP  making it
less stable & reduce activation energy needed
 Fructose biphosphate
2. Lysis
Fructose biphosphate (6C) split into TWO triose
phosphate (3C)
AHL: C1.2.8 Conversion of glucose to pyruvate by
stepwise reactions in glycolysis with a net yield
of ATP and reduced NAD

18. Oxidation & Substrate level phosphorylation
3. Oxidation
• Hydrogen atoms are removed from each
of the 3C sugars (via oxidation) to reduce
NAD+ to NADH (+ H+)
• Two molecules of NADH are produced in
total (one from each 3C sugar)
4. Substate level phosphorylation
• Some energy and phosphate group are
released to synthesise ATP
• 4 ATP produced in total (2 from each
triose phosphate)

19. Summary of Glycolysis
• Happens in cytosol, Require no oxygen
• Stepwise reaction, each step by catalysed
different enzymes
• Cost 2 ATP to phosphorylate glucose
• Creates 4 ATP
• Produce NET TOTAL of 2 ATP
• Produce 2 reduced NADH + H+
• Final product pyruvate

20. • In the mitochondrial matrix  Link reaction & Kreb’s Cycle
1. Pyruvate (3C) is decarboxylated = removal of carboxyl group
• CH3 – CO – COOH  remove CO2
2. Oxidised  lose e- & H atoms
• by NAD+  NADH + H+
3. Attached with Coenzyme A
• To form Acetyl CoA (2C)
AHL: C1.2.11 Oxidation and decarboxylation of pyruvate as a link
reaction in aerobic cell respiration

21. • Acetyl – CoA (2C) joins into the Krebs Cycle
• Combine with Oxaloacetate (4C) to form
Citric Acid (6C)
• They will undergoes a series of reaction
• Involving 2 more decarboxylation  loses 2
CO2 (minus 2C)
• And 4 more oxidation – NAD+ & FAD+
• And 1 phosphorylation of ADP  ATP
• Which will produce 4C again  recycled
AHL: C1.2.12 Oxidation and decarboxylation of
acetyl groups in the Krebs cycle with yield of
ATP and reduced NAD

22. Analysis of cycles
Oxidations are also dehydrogenation reactions =
removal of 2H to form H+ & NADH

23. • One glucose molecules produces
• 2 NADH in Glycolysis + 2 ATP in glycolysis
• 2 NADH in Link reaction
• 6 NADH in Krebs Cycle + 2 ATP in Krebs Cycle
• 2 FADH2 in Krebs Cycle
• The oxidation of these electron/hydrogen carriers (reduced NADH &
FADH2) will released energy
• Energy released will be used to phosphorylate ADP+ Pi  ATP via the
electron transport chain  occurs at the mitochondrial cristae
• Cristae of mitochondria  inner mitochondrial membrane
AHL: C1.2.13 Transfer of energy by reduced NAD in the electron
transport chain in the mitochondria

24. • The electron / hydrogen carriers NADH &
FADH2 will donate pair of electrons & H+ ions
(protons) (and become oxidised) to the
electron transport chain
• Which consists of several transmembrane
electron carrier proteins located in the cristae
• As the electrons pass through the chain, they
will release energy
• And the energy is used to pump H+ ions
(protons) from the matrix into the
intermembrane space of the mitochondria
• Which creates a proton (H+) gradient
AHL: C1.2.14 Generation of proton gradient by
flow of electrons along the electron transport
chain

25. • Chemiomosis  chemical osmosis
• Build up of H+ ions / protons in intermembrane space
of mitochondria
• Diffuse down concentration (electrochemical)
gradient into matrix through ATP synthase
• This releases energy needed for molecular rotation of
ATP synthase
• Which phosphorylates ADP and synthesize ATP
AHL: C1.2.15 Chemiosmosis and the
synthesis of ATP

26. • To allow electrons to continue flowing, they must
be transfer to a terminal / final electron acceptor
• which is O2  ½ O2 combines with 2 H+ to
become H2O
• Removing H+ ions from matrix also helps to
maintain hydrogen ions gradient
• Without oxygen, hydrogen carriers cannot
transfer electrons, and ATP production is halted
AHL: C1.2.16 Role of oxygen as terminal electron acceptor in
aerobic cell respiration

28. https://www.youtube.com/watch?v=LsRQ5_EmxJA

32. https://biomanbio.com/HTML5GamesandLabs/PhotoRespgam
es/respiration-interactive-page.html
https://www.youtube.com/watch?v=LQmTKxI4Wn4

33. • Anaerobic respiration is also partial
breakdown of glucose through
glycolysis
• Glucose (6C) will first break down into
pyruvate (3C)
• And in the process, releases 4 ATP
• This process also reduces NAD+ to
NADH
• This happens in the cytosol (liquid
part of cytoplasm)
AHL: C1.2.9 Conversion of pyruvate to lactate as a means of
regenerating NAD in anaerobic cell respiration

34. Cute flow chart diagrams from:
https://www.thesciencehive.co.
uk/respiration-a-level

35. • Pyruvate is then converted into
lactate in animals and alcohol + CO2 in
yeast/plants through fermentation
• This fermentation process helps to
regenerate NAD+ so that the process
of glycolysis can continuously run
• This process also uses up 2 ATP
• So anaerobic respiration only produce
a net yield of two ATP molecules per
one molecules of glucose
Fermentation of pyruvate to regenerate NAD+

38. In Animals, Fermentation produces lactic acid
• This happens in humans during strenuous activity where
large amount of ATP is needed for muscle contraction
• when amount of energy & oxygen needed is more than
amount of oxygen supplied
• So anaerobic respiration happens (only glycolysis &
fermentation) and lactate is produced
• The build up of lactate in muscles will cause muscle fatigue
• Yogurt also contain bacteria that undergo lactic acid
fermentation

39. • In dough, yeast does not have enough oxygen & thus, respire
anaerobically
• The process of anaerobic respiration to produce pyruvate is
the same as in humans
• Only the regeneration of NAD (fermentation) is different, thus
producing different products
• The carbon dioxide gas it produces is trapped in the dough,
producing bubbles of gas and causing the dough to rise
• This helps to make the bread spongy
• Any alcohol produced evaporates upon baking
AHL: C1.2.10 Anaerobic cell respiration in yeast
and its use in brewing and baking

40. *extra: BIOethanol & alcohol is also produced using yeast
• Most bioethanol is produced using sugar cane & corn
(maize) - (cleaner energy but raises food price)
• Starch and cellulose is broken down into sugars using
enzymes
• Yeast/bacteria uses sugar and respire anaerobically in
large fermenters to produce ethanol
• In wine / beer, grapes and malt are fermented with
yeast to produce alcoholic drinks
• These are then distilled to be pure and can be used
as fuel/alcoholic drinks

41. AHL: C1.2.17 Differenced between lipid and
carbohydrates as respiratory substance
• Lipids produces a higher yield of energy per gram of
lipids
• Due to less oxygen
• And more oxidizable hydrogen and carbon
• Carbohydrate  needs to pass through glycolysis & can
be used for anaerobic respiration
• Lipids  do not need glycolysis as breakdown of fatty
acids can form 2C acetyl groups and enter directly into
the pathway through link reactions via acetyl-CoA
Link B1.1 Carbohydrates & lipids

42. Glycogen is still needed as it breaks down easily and is transported
by blood. Fats in adipose tissue takes a longer time to mobilise it
Stored in Liver Stored in adipose tissue