Respiration in Humans
Unit 10: Biological Molecules and Metabolic Pathways
Learning aim(s) B: Explore the effect of activity on respiration in humans
and factors that can affect respiratory pathways
B.P2: Explain the stages involved in the human respiratory pathway
,Stages involved in respiratory pathways
Metabolism refers to the myriad of chemical reactions occurring within an organism's cells,
which can be classified into two main types: catabolic processes, such as cellular respiration,
and anabolic processes, like photosynthesis, in the case of plants. In the course of a single day,
your body utilizes an amount of chemical energy equivalent to your body weight, necessitating
the ongoing production of this energy for use by cells.
- Figure 1: Structure of ATP
Cellular respiration plays a crucial role in generating this energy by producing Adenosine
Triphosphate (ATP), a molecule that serves as a storage for chemical energy needed by our
cells to perform various functions, such as muscle contractions and enzyme-mediated reactions.
The structure of ATP consists of adenine, ribose, and three phosphate groups, with the bonds
between the two terminal phosphate groups being easily cleavable by enzymes, leading to an
immediate release of energy upon their breaking.
As a result of this bond breakage, ATP gets converted into adenosine diphosphate (ADP) and
inorganic phosphate, thereby providing the necessary energy for cellular processes to occur.
Key terms
- Catabolic-reactions
- Anabolic - reactions that produce a molecule. Inorganic - does not contain carbon that
involves the breakdown of a molecule
The process of cellular respiration can be represented in a simplified word equation:
1
, glucose + oxygen → carbon dioxide + water (+ energy)
However, cellular respiration is a considerably more complex series of metabolic pathways that
involve over 30 distinct steps. A thorough understanding of these pathways is essential for
biochemists to accurately diagnose patients when they encounter abnormalities.
Cellular respiration can be divided into four stages. The first stage, glycolysis, does not require
oxygen and is thus considered anaerobic. The subsequent stages—pyruvate oxidation, the citric
acid cycle, and the electron transport chain—require oxygen and are therefore classified as
aerobic processes.
Key terms
- Anaerobic - does not require oxygen.
- Aerobic-requires oxygen.
Glycolysis
occurs within the cytoplasm of cells and is a multi-step process that causes the conversion of
glucose into two molecules of pyruvate. Enzymes facilitate the conversion of one glucose
molecule (a 6-carbon compound) into two pyruvate molecules (each containing 3 carbons). At
this stage of cellular respiration, it does not require oxygen and is able to take place under
anaerobic conditions, which then allows the cell to generate small amounts of energy when the
presence of oxygen is absent. There are multiple reactions involved in glycolysis, each
catalyzed by a distinct enzyme. Even though glycolysis yields only two ATP molecules, the
resulting pyruvate acts as a substrate for producing additional ATP during subsequent stages of
cellular respiration.
The initial phase of glycolysis, known as phosphorylation, involves the addition of a phosphate
group to activate the glucose molecule, enabling its subsequent splitting. This process entails
the following series of enzymatic reactions:
- When an ATP molecule is broken down through hydrolysis, it liberates a phosphate
group which attaches to the sixth carbon of a glucose molecule. This molecular union
subsequently results in the formation of glucose-6-phosphate. The efficiency of the
process is due to the catalytic role played by the specific enzyme known as hexokinase
- Glucose-6-phosphate is converted into fructose-6-phosphate, a reaction catalyzed by
the enzyme glucose phosphate isomerase.
- Another ATP molecule is hydrolyzed, and the released phosphate group binds to the 1st
carbon of fructose-6-phosphate, resulting in fructose-1,6-bisphosphate (with phosphate
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