Unit 10- Biological Molecules and Metabolic Pathways
Assignment B - Respiration
P2. Explain the stages involved in the human respiratory pathway
Respiration is the process of which our bodies break down glucose to release energy.
Cellular or aerobic respiration is made of four stages:
- Glycolysis
- The link reaction
- The Krebs cycle
- Oxidative phosphorylation
1. Glycolysis
The first stage of aerobic respiration is glycolysis, which takes place in the cytoplasm.
Glycolysis converts glucose, a six-carbon molecule, into two smaller three-carbon molecules
called pyruvate. This stage doesn’t require oxygen, so it is an anaerobic process and is
involved in both aerobic and anaerobic respiration pathways.
, 2. The Link Reaction
The link reaction takes place in the mitochondrial matrix and converts pyruvate into a
molecule called acetyl coenzyme A (acetyl CoA). This stage does not produce any energy in
the form of ATP but does produce reduced NAD and acetyl CoA.
In the link reaction, it occurs between the glycolysis and the Krebs cycle. It connects the
glycolysis, which breaks down the glucose into a pyruvate within the cell’s cytoplasm to the
Kreb’s cycle.This reaction begins when the three-carbon pyruvate molecule, a product of
glycolysis, is actively moved into the mitochondrial matrix. Once inside, a series of chemical
changes occur: a carbon atom is detached from the pyruvate and released as carbon
dioxide. Simultaneously, the remaining two-carbon molecule undergoes oxidation, and the
hydrogen atoms released during this process convert NAD+ into NADH. Finally, this
two-carbon fragment, now referred to as an acetyl group, combines with coenzyme A (CoA)
to form acetyl-CoA.
3. The Krebs cycle
The Kreb’s cycle (also known as the citric acid cycle) is a series of reactions which generate
reduced NAD and a similar molecule called reduced FAD which are needed for oxidative
phosphorylation. Acetyl CoA from the link reaction reacts with a four-carbon molecule called
oxaloacetate. The coenzyme A portion of acetyl CoA is removed and returns to the link
reaction to be reused. A 6-carbon molecule called citrate is produced. Carbon and hydrogen
are removed from citrate, forming carbon dioxide and reduced NAD. The citrate is converted
into a 5-carbon compound. Decarboxylation and dehydrogenation occur once more, which
converts the 5-carbon compounds into the 4-carbon molecule oxaloacetate which we started
with. ATP, 2 molecules of reduced NAD, one molecule of FAD and carbon dioxide are also
formed in this step. This cycle takes place twice for each glucose molecule that is respired
aerobically.
Assignment B - Respiration
P2. Explain the stages involved in the human respiratory pathway
Respiration is the process of which our bodies break down glucose to release energy.
Cellular or aerobic respiration is made of four stages:
- Glycolysis
- The link reaction
- The Krebs cycle
- Oxidative phosphorylation
1. Glycolysis
The first stage of aerobic respiration is glycolysis, which takes place in the cytoplasm.
Glycolysis converts glucose, a six-carbon molecule, into two smaller three-carbon molecules
called pyruvate. This stage doesn’t require oxygen, so it is an anaerobic process and is
involved in both aerobic and anaerobic respiration pathways.
, 2. The Link Reaction
The link reaction takes place in the mitochondrial matrix and converts pyruvate into a
molecule called acetyl coenzyme A (acetyl CoA). This stage does not produce any energy in
the form of ATP but does produce reduced NAD and acetyl CoA.
In the link reaction, it occurs between the glycolysis and the Krebs cycle. It connects the
glycolysis, which breaks down the glucose into a pyruvate within the cell’s cytoplasm to the
Kreb’s cycle.This reaction begins when the three-carbon pyruvate molecule, a product of
glycolysis, is actively moved into the mitochondrial matrix. Once inside, a series of chemical
changes occur: a carbon atom is detached from the pyruvate and released as carbon
dioxide. Simultaneously, the remaining two-carbon molecule undergoes oxidation, and the
hydrogen atoms released during this process convert NAD+ into NADH. Finally, this
two-carbon fragment, now referred to as an acetyl group, combines with coenzyme A (CoA)
to form acetyl-CoA.
3. The Krebs cycle
The Kreb’s cycle (also known as the citric acid cycle) is a series of reactions which generate
reduced NAD and a similar molecule called reduced FAD which are needed for oxidative
phosphorylation. Acetyl CoA from the link reaction reacts with a four-carbon molecule called
oxaloacetate. The coenzyme A portion of acetyl CoA is removed and returns to the link
reaction to be reused. A 6-carbon molecule called citrate is produced. Carbon and hydrogen
are removed from citrate, forming carbon dioxide and reduced NAD. The citrate is converted
into a 5-carbon compound. Decarboxylation and dehydrogenation occur once more, which
converts the 5-carbon compounds into the 4-carbon molecule oxaloacetate which we started
with. ATP, 2 molecules of reduced NAD, one molecule of FAD and carbon dioxide are also
formed in this step. This cycle takes place twice for each glucose molecule that is respired
aerobically.
