What is the primary difference between synthesis and breakdown reactions in metabolism?

Synthesis reactions (anabolism) build larger, complex molecules from smaller ones and require an input of energy. Breakdown reactions (catabolism) simplify complex molecules into smaller units and typically release energy for the cell to use.

How do the storage forms of glucose differ between plants and animals?

Plants primarily convert excess glucose into starch for energy storage and cellulose for structural support. Animals convert excess glucose into glycogen, which is stored mainly in the liver and muscle cells.

What is the difference between the components of a lipid and a protein?

Lipids are synthesized from one glycerol molecule and three fatty acid molecules. Proteins are synthesized from chains of amino acids, which are produced by combining glucose with nitrate ions.

Why is it a misconception to say metabolism is just 'digestion'?

Digestion is merely the initial breakdown of food in the gut. Metabolism encompasses all chemical reactions within every cell, including the synthesis of new proteins, the storage of energy, and the release of energy through respiration.

What error is made when assuming plants only perform photosynthesis and not metabolism?

Plants perform extensive metabolic reactions, including using glucose from photosynthesis to build starch, cellulose, and proteins (using nitrates), as well as performing respiration to release energy for growth.

What is the common mistake regarding the fate of excess amino acids?

Students often think excess amino acids are stored; however, they cannot be stored by the body. They must be broken down in the liver to form urea, which is then excreted by the kidneys.

Metabolism is the sum total of all the chemical reactions that occur within a cell or the body of a living organism.

Urea is a nitrogenous waste product formed in the liver from the breakdown of excess amino acids. It is excreted from the body via the kidneys in urine.

What are the building blocks of a lipid?

A lipid molecule is composed of one glycerol molecule chemically bonded to three fatty acid molecules.

How is the energy from respiration used in metabolic synthesis?

Respiration is an exothermic process that releases chemical energy. This energy is 'coupled' to endothermic metabolic reactions, providing the necessary power to bond small molecules together into larger ones, such as amino acids into proteins.

Metabolism | AQA GCSE Science

Combined Science Trilogy / Biology

Bioenergetics

Metabolism

Summary

Metabolism represents the totality of chemical processes occurring within a living organism to maintain life. It involves a continuous cycle of synthesizing complex molecules from simpler building blocks and breaking down larger molecules to release energy or manage waste products.

1. Definition & Core Concepts

Metabolism is defined as the sum of all chemical reactions that take place within a cell or the entire body of an organism.
These reactions are not random; they are highly organized into metabolic pathways where the product of one reaction becomes the substrate for the next.
Most metabolic processes are enzyme-controlled, meaning biological catalysts are required to ensure reactions occur at rates sufficient to sustain life.
The energy required for these reactions is primarily supplied by cellular respiration, an exothermic process that transfers energy from glucose.

A diagram showing the cycle of metabolism, illustrating the relationship between the synthesis of complex molecules (requiring energy) and the breakdown into simple building blocks (releasing energy).

2. Synthesis of Biological Molecules

Carbohydrate Synthesis: Plants convert glucose into starch for energy storage and cellulose to strengthen cell walls. In animals, glucose is converted into glycogen for storage in the liver and muscles.
Lipid Synthesis: One molecule of glycerol is combined with three molecules of fatty acids to form lipids (fats and oils), which serve as long-term energy stores and structural components of cell membranes.
Protein Synthesis: Glucose is combined with nitrate ions (absorbed from soil in plants) to produce amino acids, which are then sequenced and bonded to form diverse proteins.

3. Breakdown and Waste Management

4. Key Distinctions

5. Exam Strategy & Tips

Identify the Building Blocks: Always remember the specific components for each macromolecule: Lipids = 1 Glycerol + 3 Fatty Acids; Proteins = Amino Acids; Carbohydrates = Simple Sugars (Glucose).
Energy Linkage: When asked about the importance of respiration in metabolism, emphasize that respiration provides the chemical energy necessary for synthesis reactions to occur.
Plant vs. Animal Storage: Distinguish between starch (plants) and glycogen (animals) as the primary storage polymers of glucose.
Nitrogen Source: Remember that plants require nitrates from the soil to turn glucose into amino acids; without nitrogen, protein synthesis cannot occur.

Metabolism

Summary

1. Definition & Core Concepts

Metabolism is defined as the sum of all chemical reactions that take place within a cell or the entire body of an organism.
These reactions are not random; they are highly organized into metabolic pathways where the product of one reaction becomes the substrate for the next.
Most metabolic processes are enzyme-controlled, meaning biological catalysts are required to ensure reactions occur at rates sufficient to sustain life.
The energy required for these reactions is primarily supplied by cellular respiration, an exothermic process that transfers energy from glucose.

2. Synthesis of Biological Molecules

Carbohydrate Synthesis: Plants convert glucose into starch for energy storage and cellulose to strengthen cell walls. In animals, glucose is converted into glycogen for storage in the liver and muscles.
Lipid Synthesis: One molecule of glycerol is combined with three molecules of fatty acids to form lipids (fats and oils), which serve as long-term energy stores and structural components of cell membranes.
Protein Synthesis: Glucose is combined with nitrate ions (absorbed from soil in plants) to produce amino acids, which are then sequenced and bonded to form diverse proteins.

3. Breakdown and Waste Management

Respiration: The most critical breakdown reaction is the oxidation of glucose to release energy. The general equation for aerobic respiration is: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$
Protein Turnover: When proteins are in excess or no longer needed, they are broken down into amino acids. In the liver, the amino group is removed from excess amino acids in a process called deamination.
Urea Formation: The toxic ammonia produced during deamination is immediately converted into urea, which is then transported to the kidneys for excretion in urine.

4. Key Distinctions

Feature	Synthesis (Anabolic)	Breakdown (Catabolic)
Energy Change	Endergonic (Consumes energy)	Exergonic (Releases energy)
Molecular Change	Small $\rightarrow$ Large	Large $\rightarrow$ Small
Primary Examples	Protein synthesis, Glycogen formation	Respiration, Protein decay
Biological Goal	Growth, repair, and storage	Energy production and waste removal

5. Exam Strategy & Tips

Identify the Building Blocks: Always remember the specific components for each macromolecule: Lipids = 1 Glycerol + 3 Fatty Acids; Proteins = Amino Acids; Carbohydrates = Simple Sugars (Glucose).
Energy Linkage: When asked about the importance of respiration in metabolism, emphasize that respiration provides the chemical energy necessary for synthesis reactions to occur.
Plant vs. Animal Storage: Distinguish between starch (plants) and glycogen (animals) as the primary storage polymers of glucose.
Nitrogen Source: Remember that plants require nitrates from the soil to turn glucose into amino acids; without nitrogen, protein synthesis cannot occur.

Feature

Synthesis (Anabolic)

Breakdown (Catabolic)

Energy Change

Endergonic (Consumes energy)

Exergonic (Releases energy)

Molecular Change

Small

\rightarrow

Large

\rightarrow

Small

Primary Examples

Protein synthesis, Glycogen formation

Respiration, Protein decay

Biological Goal

Growth, repair, and storage

Energy production and waste removal