Unit 10: Biological Molecules and Metabolic Pathways
B: Explore the effect of activity on respiration in humans and factors that can affect respiratory pathways
Respiration in humans
This report will cover respiratory pathways, how activity and various factors affect them in humans and details of an
investigation that carried out looking into the effect of activity on respiration in humans.
Stages in the respiratory pathway
Respiration refers to the process by which living cells release energy found in organic molecules like glucose
[SnapRevise, 2020]. The energy generated during respiration is then used to produce molecules of ATP, which can be
used as a quick source of energy [SnapRevise, 2020]. Respiration is important because it creates energy that the
body uses to function properly [BYJU's, n.d.].
ATP molecules have three components [May, P. n.d.]. Ribose serves as the basis of DNA and is found at the centre of
ATP [May, P. n.d.]. This is attached to one side by a base (such as adenine) [May, P. n.d.]. The other side of the sugar
is linked to a chain of phosphate groups [May, P. n.d.]. The bonds between the two phosphate groups on the left
side of the ATP molecule are easily broken by an enzyme [Annets, F. 2017]. This causes ATP to release energy
[Annets, F. 2017].
Figure 1: Diagram of ATP molecule [A-LEVEL NOTES, 2020]
There are two types of respiration: aerobic and anaerobic.
Aerobic respiration requires oxygen, it is represented by the following equation:
Glucose + oxygen → Carbon Dioxide + water + energy [Study Mind, n.d.].
Anaerobic respiration does not require oxygen, it is represented by the following equation in plants and fungi:
Glucose → Ethanol + Carbon dioxide + Energy [BYJU's, n.d.].
In animals and bacteria, the equation for anaerobic respiration is:
Glucose → Energy + Lactic acid [Biology Online, 2021]
Glycolysis
Glycolysis is a four-stage process that converts glucose into two molecules of pyruvate [Annets, F. 2017]. It takes
place in the cytoplasm of cells [Naifeh et al. 2021]. Enzymes catalyse the conversion of one glucose molecule into
two pyruvate molecules [Annets, F. 2017]. Glucose has six carbons, whereas pyruvate contains three carbons
[Annets, F. 2017]. This step does not require oxygen and can occur in anaerobic conditions, supplying energy to the
cell even when oxygen is not available [Annets, F. 2017].
,Unit 10: Biological Molecules and Metabolic Pathways
B: Explore the effect of activity on respiration in humans and factors that can affect respiratory pathways
1. Phosphorylation: An ATP molecule is broken down and a phosphate group is released [Annets, F. 2017]. This
phosphate group connects to carbon 6 of a glucose molecule, which produces glucose-6-phosphate [Annets,
F. 2017]. The enzyme hexokinase catalyses this reaction [Annets, F. 2017]. Glucose 6-phosphate turns to
fructose 6-phosphate [Annets, F. 2017]. The process is catalysed by the enzyme glucose phosphate
isomerase [Annets, F. 2017].
2. Splitting of fructose 1,6-bisphosphate: The second stage of glycolysis includes breaking down fructose
1,6-bisphosphate, which allows the products to create ATP [Annets, F. 2017]. Fructose 1,6-bisphosphate is
split into two molecules and the enzyme fructose diphosphate aldolase catalyses the reaction [Annets, F.
2017]. The enzyme triose phosphate isomerase creates two molecules of glyceraldehyde-3-phosphate
[Annets, F. 2017].
3. Oxidation: During this stage, glyceraldehyde 3-phosphate loses electrons [Annets, F. 2017]. Two hydrogen
atoms are removed from each glyceraldehyde 3-phosphate, producing two molecules of 1,3 bisphosphate
glycerate [Annets, F. 2017]. Glyceraldehyde phosphate dehydrogenase catalyses this reaction, but it requires
two nicotinamide adenine dinucleotide (NAD) molecules to function [Annets, F. 2017]. These molecules
function as electron acceptors [Annets, F. 2017]. They accept hydrogen atoms, resulting in the reduction of
two NAD molecules to NAD or NADH [Annets, F. 2017]. Oxidation requires two NAD coenzymes and results
in two reduced NAD (NADH) and two 1,3 bisphosphate glycerates from two glyceraldehyde 3-phosphate
[Annets, F. 2017].
4. Conversion of 1,3 bisphosphate glycerate to produce pyruvate: Four different enzymes are used to convert
1,3 bisphosphate glycerate to pyruvate [Annets, F. 2017]. This creates two ATP per molecule of 1,3
bisphosphate glycerate (total of four) by adding an inorganic phosphate to ADP during phosphorylation
[Annets, F. 2017]. The conversion of two molecules of 1,3 bisphosphate glycerate generates two pyruvate
and four ATP [Annets, F. 2017].
Figure 2: Stages of glycolysis [Carter, S. 2019]
Overall, glycolysis produces four molecules of ATP per glucose [Annets, F. 2017]. However, it uses two ATP during
phosphorylation [Annets, F. 2017]. For every glucose molecule, the following are produced: two molecules of ATP,
two molecules of NADH, two molecules of pyruvate [Annets, F. 2017].
Link reaction
The link reaction takes place in the matrix of the mitochondria [Annets, F. 2017]. It is a process that describes how
pyruvate from glycolysis is converted into acetate [Annets, F. 2017]. The link reaction is an aerobic process as it
requires oxygen.
, Unit 10: Biological Molecules and Metabolic Pathways
B: Explore the effect of activity on respiration in humans and factors that can affect respiratory pathways
- If oxygen is present, the pyruvate formed during glycolysis is converted to acetate during the link reaction
[Annets, F. 2017]. Both pyruvate molecules produced by glycolysis are decarboxylated (carbon dioxide is
removed) and dehydrogenated (two hydrogen atoms are removed); enzymes are required for this [Annets,
F. 2017].
- Carbon dioxide is a byproduct of breathing that diffuses into the bloodstream and is exhaled through the
lungs [Annets, F. 2017]. NAD accepts hydrogen atoms, resulting in reduced NAD (NADH) [Annets, F. 2017].
- A two-carbon molecule known as acetate is formed [Annets, F. 2017]. The acetate combines with coenzyme
A (coA) to create acetyl coenzyme A [Annets, F. 2017].
Figure 3: Diagram of link reaction [BioNinja, 2024]
For every two molecules of pyruvate, two NADH are made [Annets, F. 2017]. No ATP is made during this reaction,
but the two molecules of acetyl coenzyme A and the NADH are used in the Krebs cycle [Annets, F. 2017].
Krebs cycle
The Krebs cycle also occurs in the mitochondrial matrix and is classed as aerobic respiration because it requires
oxygen [Annets, F. 2017]. This stage consists of five enzyme-catalysed reactions [Annets, F. 2017]:
1. Acetyl coenzyme A from the link reaction releases acetate, which reacts with oxaloacetic acid to form citric
acid: a six-carbon molecule [Annets, F. 2017].
2. Citric acid undergoes decarboxylation (removal of CO2) and dehydrogenation. NADH, a hydrogen ion and a
5-carbon molecule are then produced [Annets, F. 2017].
3. Two 4-carbon molecules are produced in succession [Annets, F. 2017]. The 5-carbon molecule from step 2 is
decarboxylated and then dehydrogenated [Annets, F. 2017]. NADH and a hydrogen ion are formed, resulting
in a 4-carbon molecule [Annets, F. 2017]. This is converted into another 4-carbon compound [Annets, F.
2017]. One molecule of ADP is phosphorylated (with inorganic phosphate) to produce one molecule of ATP
[Annets, F. 2017].
4. The second 4-carbon molecule from step 3 is converted into another 4-carbon compound, which is
dehydrogenated [Annets, F. 2017]. A new coenzyme, flavin adenine dinucleotide (FAD), absorbs the
hydrogen atoms to form reduced FAD or FADH2 [Annets, F. 2017].
