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In this article, we will discuss ATP in detail. So, let us get started.
The breakdown of glucose and other respiratory substrates to create energy-carrying molecules known as ATP is referred to as cellular respiration. Let us discuss why we need energy.
The Need for Energy
We need energy for the following purposes:
- Synthesis: To create large molecules, for instance, proteins.
- Movement: To create protein fibres in muscles cells contract
- Transport: To pump molecules across cell membranes in living organisms
In the next section, we will discuss ATP in detail.
What is ATP?
ATP which stands for adenosine triphosphate refers to the energy-carrying molecule employed in cells as it has the capability to release energy quickly. ATP provides energy to the cells in a manageable way. It is a nucleotide that consists of:
- a nitrogenous base
- three phosphate groups
- a pentose ribose sugar.
ATP releases energy when the end phosphate is eliminated. Once the energy is released from ATP, it transforms into ADP (Adenosine diphosphate). ADP refers to a low-energy molecule.
By including a phosphate, ADP can be upgraded to ATP again. Energy is needed to do so.
The recycling of these molecules ensures that a consistent stream of energy-containing ATP is readily available for all metabolic passages in the cell. Nearly all the cellular processes require ATP to provide the needed energy for a reaction. ATP has the capability to transfer energy and add a phosphate (phosphorylate) to other molecules in cellular processes, for instance, active transport, DNA replication, contraction of muscles, and synthetic pathways.
Chemiosmosis in Mitochondria
ATP is synthesized by a process known as chemiosmosis. This process is defined as the movement of ions down the concentration gradient via a semipermeable membrane. Chemiosmosis is the primary source of ATP during cellular respiration in prokaryotes. This process occurs in the mitochondria of the living cells.
Chemiosmosis in Chloroplasts
Chloroplasts are found in photosynthetic autotrophs. The process occurs in the organelles during the light-dependent reactions of photosynthesis. In these reactions energy of the photoexcited electrons is utilized to create ATP for dark reactions.
Properties of ATP
The different properties of ATP are listed below:
- Small: ATP molecules are small so that they can easily move in, out, and around the cells.
- Soluble: They are soluble because the majority of the active processes occur in an aqueous environment.
- Release moderate amount of energy: ATP releases intermediate amounts of energy that is sufficient for cellular reactions. It does not release energy beyond what is needed.
- Easy regeneration: ATP is a renewable energy source.
Metabolic Pathways of Respiration – the Creation of ATP
The primary substrate within a respiratory pathway is a sugar known as glucose. The metabolic pathway in respiration can be divided into the following three elements:
- Glycolysis that occurs in the cytoplasm of the cell.
- The citric acid cycle which occurs in the mitochondria’s matrix.
- Electron transport chain that occurs in the internal membrane of the mitochondria.
In the next section of the article, we will discuss all three parts in detail.
Glycolysis
Glycolysis refers to the breakdown of glucose into two pyruvate molecules. Glycolysis is an anaerobic process which means that it does not need oxygen.
Pyruvate production from glucose includes the production of many intermediate molecules. ATP molecules in an energy investment stage are needed in glucose and intermediate molecules phosphorylation.
Electrons and hydrogen ions are eliminated from the intermediates of this cycle by dehydrogenase enzymes. They are then transferred to the coenzyme NAD, resulting in the formation of NADH. The electrons and hydrogen ions are transferred to the electron transport chain on the interior mitochondrial membrane. This not only happens in glycolysis but also in the citric acid cycle.
In an aerobic reaction where oxygen is available, the pyruvate molecules move towards a citric acid cycle. When oxygen is unavailable, then pyruvate is fermented in the cell’s cytoplasm. Fermentation is of two types:
- Lactate fermentation: Pyruvate is transformed into lactate. This reaction can be reversed and it occurs in animal cells.
- Alcoholic fermentation: The pyruvate is transformed into ethanol and carbon dioxide. It happens in plant cells and fungi, for instance, yeast cells. It is an irreversible reaction.
As a result of fermentation, much less ATP is generated as compared to aerobic respiration.
Now, let us discuss the citric acid cycle
Citric Acid Cycle
After glycolysis, the citric acid cycle occurs only in the presence of oxygen as it is an aerobic process. In this cycle, the pyruvate enters into the mitochondria matric and carbon dioxide is eliminated. The elimination of carbon results in an acetyl group that combines with coenzyme A to form acetyl coenzyme A. In this cycle, the acetyl-coenzyme A combines with a molecule known as oxaloacetate to create citrate. Due to this reason, it is referred to as a citric acid cycle.
Intermediate molecules are created when enzymes remove carbon (in carbon dioxide form) and electrons or hydrogen. As a by-product, carbon dioxide gas is released. The citric acid cycle also culminates in the formation of ATP. Gradually, these intermediate steps transform citrate back into an oxaloacetate molecule. Then, it can combine with another acetyl group.
Dehydrogenase enzymes eliminate electrons and hydrogen ions from intermediates. After that, the intermediates are moved to coenzymes NAD to create NADH. The high-energy electrons are moved to the electron transport chain.
In the next section, we will discuss the electron transport chain.
Electron Transport Chain
The final stage of the respiratory pathway is the electron transport chain. Most of the ATP molecules are produced in this stage. The electron transport chain refers to the set of proteins present on the inner membrane of the mitochondria. NADH results in the release of electrons and hydrogen ions into the transport chain.
The electrons pass their energy to the proteins in the membrane to supply energy for pumping hydrogen ions across the inner mitochondrial membrane. ATP is synthesized by a protein known as ATP synthase by a flow of ions back across the membrane. The last hydrogen ion and electron acceptor is oxygen which combines with electrons and hydrogen ions to produce water.
A total of 38 ATP molecules are created from a single glucose molecule. In the absence of glucose in the respiratory pathway, other respiratory substrates can be employed through alternative metabolic pathways. In the citric acid cycle and glycolysis, glycogen, starch, proteins (amino acids), and fats are broken down into intermediates. This results in alternative metabolic pathways to create ATP.
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