Supercharge your learning!

Use adaptive quiz-based learning to study this topic faster and more effectively.

Metabolic processes

Metabolism refers to all the reactions that occur in living cells.

It includes:

  • Breaking down compounds to produce energy (catabolism)
  • Synthesising new cells and cell components from broken down compounds (anabolism).

Organisms use different methods to obtain organic compounds for metabolism. These methods are collectively called metabolic strategies.

Autotrophs obtain organic compounds by synthesising them from inorganic compounds.

Plants and algae absorb light energy and $$\ce{CO_2}$$ to synthesise organic compounds.

Heterotrophs need to consume complex organic compounds. They cannot synthesise organic compounds from inorganic compounds.

Animals obtain these compounds from their diets. Complex organic molecules are broken down through digestion. Fungi are also heterotrophic.

Adenosine triphosphate (ATP) is the molecule that transports and temporarily stores cellular chemical energy. All respiring organisms use ATP.

ATP provides energy for many metabolic processes. It can be described as the energy currency of cells.

ATP is formed of three phosphate groups, an adenine base and a ribose sugar (containing 5 carbon atoms). The three phosphates give ATP a highly unstable structure.

ATP. The phosphate groups are yellow and red.
ATP. The phosphate groups are yellow and red.

When a phosphate group is lost through a process called hydrolysis, a large amount of energy is released. When ATP loses a phosphate group (P$$_i$$), it becomes ADP (adenosine diphosphate).

$$$\ce{ATP -> ADP + P_{i} + energy}$$$

ATP is continually reused. A human has only 250 grams of ATP in the body at a time, but we utilise our own body weight in ATP daily.

ATP is made by adding a phosphate group P$$_i$$ to an ADP (adenosine diphosphate) molecule. This process is called phosphorylation.

Phosphorylation occurs during respiration. There are two main types.

  • Substrate-level phosphorylation takes a phosphate group from another molecule and adds it to the ADP. This occurs in glycolysis and in the Krebs cycle.
  • Oxidative phosphorylation adds a free phosphate group to ADP, rather than taking one from a different molecule.

During aerobic respiration, 38 ATP molecules are produced per molecule of glucose.

ATP. The phosphate groups are yellow and red. They are added by the process of phosphorylation.
ATP. The phosphate groups are yellow and red. They are added by the process of phosphorylation.

An electron transport chain is the transfer of electrons and hydrogen ions across a phospholipid membrane (such as in the mitochondria) to generate energy in the form of ATP.

Hydrogen ions are simply protons, and this process causes a difference in concentrations called a proton gradient. The proton gradient causes a difference in charge which provides energy for the cell to use.

Electron transport chains are the mechanism by which energy is obtained:

  • From light in photosynthesis
  • From the breakdown of organic compounds in cellular respiration

Electron transport chains allow for the extraction or storage of energy. They work by creating chains of redox (reduction and oxidation) reactions.

Oxidation reactions result in a net loss of electrons and cause energy to be released. Reduction reactions result in a net gain of electrons and require energy.

Electron carriers (also known as cofactors) are used to deliver or receive electrons in electron transport chains.

In respiration, the electron carriers are:

  • Reduced NAD (nicotinamide adenine dinucleotide), written as NADH.
  • Reduced FAD (flavin adenine dinucleotide), written FADH$$_2$$.

They are oxidised in the electron transport chain and donate electrons across phospholipid membranes, i.e. the mitochondrial membrane.

In photosynthesis, the electron carrier is nicotinamide adenine dinucleotide phosphate (NADP).

NADP is reduced in the electron transport chain. It accepts electrons at the end of the chain, allowing for energy to be stored.

NAD and FAD are coenzymes. These are molecules that work with enzymes, but are not themselves enzymes.