A Level subject content
12 Energy and respiration
1 outline the need for energy in living organisms, as illustrated by active transport,
movement and anabolic reactions, such as those occurring in DNA replication and protein
synthesis
• Active transport: To move molecules or ions against their concentration gradient across
cell membranes.
• Movement: To enable cellular locomotion, muscle contraction, and organelle transport.
• Anabolic reactions: Such as DNA replication and protein synthesis, where energy is
needed to build complex molecules from simpler ones.
2 describe the features of ATP that make it suitable as the universal energy currency.
• ATP contains high-energy phosphate bonds between its phosphate groups. These bonds
are relatively unstable and can be hydrolyzed (broken down) to release energy, which can
be readily used by cells.
• ATP releases energy quickly when hydrolyzed to ADP (adenosine diphosphate) and
inorganic phosphate (Pi). This rapid energy release makes ATP ideal for powering
cellular processes that require immediate energy, such as muscle contraction and active
transport.
• ATP is present in all types of cells, from prokaryotes to eukaryotes, and in every cell
compartment (cytoplasm, mitochondria, chloroplasts). This universality allows ATP to
serve as a common energy carrier throughout all biological systems.
• ATP can be regenerated from ADP and Pi through cellular respiration (in mitochondria)
or photosynthesis (in chloroplasts). This regeneration ensures a continuous supply of
ATP, maintaining cellular energy levels even during periods of high energy demand.
• ATP is water-soluble, facilitating its transport within cells and between cellular
compartments. This solubility allows ATP to diffuse easily to sites where energy is
needed without requiring special transport mechanisms.
• ATP not only serves as an energy carrier but also participates in various metabolic
pathways as a substrate or regulator. It provides energy for anabolic reactions (such as
protein synthesis and DNA replication) and is involved in signaling processes (e.g.,
phosphorylation cascades).
,3 state that ATP is synthesised by:
• transfer of phosphate in substrate-linked reactions
• chemiosmosis in membranes of mitochondria and chloroplasts
ATP is synthesized by:
• Transfer of phosphate in substrate-linked reactions.
• Chemiosmosis in membranes of mitochondria and chloroplasts.
4 explain the relative energy values of carbohydrates, lipids and proteins as respiratory
substrates.
1. Carbohydrates:
o Energy Yield: Carbohydrates yield approximately 17 kJ per gram when
metabolized.
o Availability: Glucose, the primary carbohydrate used in cellular respiration, is
readily available in cells and can be quickly broken down to produce ATP.
o Quick Energy Release: Carbohydrates can be rapidly metabolized through
glycolysis and the subsequent citric acid cycle (Krebs cycle), leading to a fast
production of ATP.
2. Lipids:
o Energy Yield: Lipids are highly energy-dense, yielding about 37 kJ per gram
when oxidized.
o Storage Advantage: Lipids are the most concentrated form of energy storage in
the body, providing a significant reservoir of energy in adipose tissue.
o Slow Oxidation: Lipid metabolism involves more complex processes compared
to carbohydrates, requiring more steps to convert fatty acids into acetyl-CoA for
the citric acid cycle. This results in a slower but sustained release of energy.
3. Proteins:
o Energy Yield: Proteins yield approximately 17 kJ per gram, similar to
carbohydrates.
o Last Resort: Proteins are not typically used as primary energy sources under
normal physiological conditions. They are primarily used for structural purposes
and enzymatic functions.
o Energy Conversion: In times of prolonged fasting or starvation, proteins can be
broken down into amino acids, which can be converted into intermediates of
glycolysis or the citric acid cycle to generate ATP. However, this process is
inefficient and can lead to loss of essential cellular components.
,5 state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon
dioxide produced to the number of molecules of oxygen taken in, as a result of respiration
The respiratory quotient (RQ) is defined as the ratio of the number of molecules of carbon
dioxide produced to the number of molecules of oxygen taken in, as a result of respiration.
6 calculate RQ values of different respiratory substrates from equations for respiration.
• Carbohydrates (Glucose):
• Glucose + 6O₂ → 6CO₂ + 6H₂O
• RQ = (6CO₂ produced) / (6O₂ consumed) = 1.0
The RQ of 1.0 indicates that for every molecule of glucose metabolized, 6 molecules of carbon
dioxide (CO₂) are produced for every 6 molecules of oxygen (O₂) consumed.
• Lipids (Triglycerides):
• Triglyceride + 23O₂ → 16CO₂ + 10H₂O
• RQ = (16CO₂ produced) / (23O₂ consumed) ≈ 0.7
Lipids yield an RQ of approximately 0.7 because their metabolism produces more carbon
dioxide relative to the oxygen consumed compared to carbohydrates.
• Proteins (General Amino Acid):
• General Amino Acid + 5O₂ → 3CO₂ + 4H₂O
• RQ = (3CO₂ produced) / (5O₂ consumed) ≈ 0.6
Proteins typically yield an RQ around 0.6 because their metabolism produces less carbon dioxide
compared to the oxygen consumed.
, 7 describe and carry out investigations, using simple respirometers, to determine the RQ of
germinating seeds or small invertebrates (e.g. blowfly larvae)
Materials Needed:
• Germinating seeds or small invertebrates (e.g., blowfly larvae)
• Simple respirometer setup (e.g., a sealed chamber with a small organism and
suitable absorbents for CO₂)
• Absorbent materials (e.g., soda lime or potassium hydroxide) to absorb CO₂
• Oxygen source (e.g., potassium chlorate or oxygen gas)
• Measuring devices (e.g., gas syringes or sensors) for recording gas volumes
• Temperature control (e.g., water bath or room temperature control)
Procedure:
1. Prepare the Respirometer:
o Set up a simple respirometer chamber that allows you to measure changes
in gas volume due to respiration. Ensure it is sealed to prevent gas exchange
with the external environment except through controlled inputs.
2. Initial Measurements:
o Measure the initial volume of gas (O₂) in the respirometer chamber using a
gas syringe or gas sensor. Record this as Vinitial .
3. Introduce the Organism:
o Introduce the germinating seeds or small invertebrates into the respirometer
chamber. Seal the chamber tightly to prevent gas leakage.
4. Allow Respiration to Occur:
o Let the organisms respire for a specific period (e.g., 30 minutes to 1 hour).
Ensure the temperature is constant to maintain consistent metabolic rates.
5. Measure Final Gas Volumes:
o After the incubation period, measure the final volumes of gas (O₂ and CO₂) in
the respirometer chamber. Record these as Vfinal, O2 for oxygen and
Vfinal, CO2 for carbon dioxide.
6. Calculations:
o Calculate the volume of O₂ consumed during respiration:
o Calculate the volume of CO₂ produced during respiration:
