23 Introduction to biomolecules and energy
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Why do animals need energy?
Energy is defined as the “ability to do work”. Animals need energy to carry out all the body processes (e.g., nutrient transport, synthesis, muscle contraction) required to maintain life. Without energy, an animal is unable to move, to digest its food, to reproduce, to grow, or even to breathe. Energy requirement and balance are more important in food-producing animals with their need to synthesise nutrients (e.g., proteins, fat) for deposition into muscle, milk, and eggs. Carbohydrates are the major energy source in the diet of farm animals.
The ultimate aim of metabolic reactions is to provide energy.
Scientists use the term bioenergetics to describe the concept of energy flow through living systems, such as cells. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish their energy supplies, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism. Therefore, metabolism can be defined as “the sum total of all chemical reactions that occur in cells”.
These metabolic processes either produce larger molecules (anabolism) or break down molecules to simpler ones (catabolism) (Figure 5.1).
Example of a metabolic reaction
A +B → C +D
A and B are the reactants
C and D are the products
The arrow (→) represents reaction going forward, when in reality, it is bidirectional.
Anabolic reactions
A reaction is anabolic if it involves the production of larger products from smaller reactants. For example, synthesis of protein from amino acids is an anabolic reaction.
Catabolic reactions
A reaction is catabolic if it breaks down a larger molecule into smaller ones. For example, breakdown of a protein into amino acids is a catabolic reaction.
Example
Metabolism of sugar
This is a classic example of one of the many cellular processes that use and produce energy. Living things consume sugars as a major energy source because sugar molecules have a great deal of energy stored within their bonds. For the most part, photosynthesising organisms like plants produce these sugars. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarised as:
6CO2 + 6H2O + energy ——-> C6H12O6+ 6O2
Because this process involves synthesising an energy-storing molecule, it requires energy input to proceed. During the light reactions of photosynthesis, energy is provided by a molecule called adenosine triphosphate (ATP), which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. In contrast, energy-storage molecules such as glucose are consumed only to be broken down to use their energy. The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarised by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. The reaction is summarised as:
C6H12O6 + 6O2 ——> 6CO2 + 6H2O + energy
Both of these reactions involve many steps.
The processes of making and breaking down sugar molecules illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule and modifies it, step-by-step, through a series of metabolic intermediates, eventually yielding a final product. In the example of sugar metabolism, the first metabolic pathway synthesised sugar from smaller molecules, and the other pathway breaks sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic pathways (building polymers) and catabolic pathways (breaking down polymers into their monomers), respectively.
Metabolic reactions that occur in a cell
Hydrolysis -water reacts with reactant molecules, thereby breaking the bond between them
- General formula : A-B + H2O → A-OH + B-H
Condensation – the reverse of hydrolysis
- Combining two or more smaller molecules to form a large product
- Water is generated as product due to this reaction
- General formula : A-OH + B-H → A-B + H2O
Dive deeper
Watch American Chemical Society. (2018, November 9). The Chemistry of water | Chemistry basics [YouTube, 2:24mins]
Phosphorylation
- Phosphorylation – addition of a phosphate group (P)
- General formula : A + Pi → A-P (the free phosphate is denoted as Pi – inorganic phosphate)
Ex: ADP + Pi → ATP + H2O
Dephosphorylation
- Dephosphorylation – removal of a phosphate group (P)
- General formula : A-P → A + Pi (the free phosphate is denoted as Pi – inorganic phosphate)
- Ex: ATP + H2O → ADP + Pi
Dive deeper
Watch The Explorer’s Guide to Biology. (2019, October 1). What is phosphorylation? [YouTube, 2:01mins]
Oxidation and reduction
- Oxidation refers to loss of electrons from an atom or molecule
- Reduction refers to addition of electrons by an atom or molecule
- Oxidation and reduction reactions are always coupled
- General formula: A* + B → AB* (* represents an electron)
- So here, A is oxidised (loses an electron) and B is reduced (gains an electron)
Dive deeper
Watch CrashCourse. (2013, April 23). Redox reactions: Crash course chemistry #10 [YouTube, 11:12mins]
