# Energy needs of living organisms

> CIE A-Level Biology · Unit 12: Energy and Respiration (AS Portion)
> Source: https://www.owlsprep.com/study/cie-9700-u12-energy-needs-of-living-organisms/

This sub-topic introduces core energy requirements of living organisms, distinguishes between anabolic and catabolic metabolism, and explains the role of ATP as the universal energy currency, forming the foundation for learning respiration.

**Prerequisites:** [Structure and function of biological molecules](https://www.owlsprep.com/study/cie-9700-u02-biological-molecules/); [Basic cell structure and organelle function](https://www.owlsprep.com/study/cie-9700-u01-cell-structure/)

## Learning objectives

- Distinguish between anabolic and catabolic metabolic reactions
- Explain why living organisms need a continuous energy supply
- Describe the role of ATP as the universal energy currency
- Identify key biological processes that require energy input

## Core Energy Requirements of Living Organisms

All living cells require a continuous input of energy to maintain order and carry out essential life processes. Following the first law of thermodynamics, energy is never created or destroyed, only converted between forms in biological systems.

**Metabolism** — The sum of all chemical reactions that occur within a living organism, divided into two broad functional classes of reactions.

- Active transport of molecules and ions across cell membranes against concentration gradients
- Anabolic synthesis of complex molecules like proteins, nucleic acids and polysaccharides
- Mechanical work such as muscle contraction and mitotic spindle movement
- Maintenance of body temperature in endotherms and nerve impulse transmission

**Worked example:** Explain why a plant still requires energy even when it is photosynthesising during the day.

1. First, identify all energy-requiring processes that occur in a plant alongside photosynthesis:
2. These include: active transport of mineral ions into root cells, synthesis of cellulose and starch from glucose, cell division for growth, and opening/closing of stomata.
3. Photosynthesis produces organic molecules that can be respired to release energy, but a continuous supply of energy is still required to sustain all these essential processes.

> **Exam tip:** When asked to list energy requirements, always give at least three named examples specific to the organism in the question.

## Anabolic vs Catabolic Reactions

All metabolic reactions fall into one of two complementary categories: anabolic and catabolic. These reactions are coupled: energy released from catabolic reactions is used to drive energy-requiring anabolic reactions.

**Anabolic Reactions** — Metabolic reactions that build larger, more complex molecules from smaller, simpler subunits. They are endergonic, meaning they require a net input of energy to proceed.

*Example:* Synthesis of proteins from amino acids, condensation of glucose to form starch

| Property | Anabolic Reactions | Catabolic Reactions |
| --- | --- | --- |
| Energy requirement | Require input (endergonic) | Release energy (exergonic) |
| Molecule size change | Build large from small | Break large into small |
| Entropy change | Decrease entropy (increase order) | Increase entropy (decrease order) |
| Example | Protein synthesis, starch synthesis | Aerobic respiration, starch digestion |

**Worked example:** Classify the following as anabolic or catabolic, with a reason for each: (a) Glycolysis, (b) Glycogen synthesis from glucose, (c) Hydrolysis of triglyceride.

1. (a) Glycolysis breaks glucose into smaller pyruvate molecules. Classification: catabolic. Reason: Large molecule broken into smaller products, energy is released.
2. (b) Glycogen synthesis joins small glucose monomers into a large polysaccharide. Classification: anabolic. Reason: Large molecule built from smaller subunits, requires net energy input.
3. (c) Hydrolysis breaks ester bonds in triglyceride to release smaller fatty acids and glycerol. Classification: catabolic. Reason: Complex molecule broken down into smaller products.

## ATP as the Universal Energy Currency

Energy released from catabolic reactions cannot be used directly for anabolic reactions and other energy-requiring processes. Instead, energy is transferred via a small, mobile molecule called adenosine triphosphate (ATP).

**ATP (Adenosine Triphosphate)** — A phosphorylated nucleotide that acts as the immediate source of energy for most cellular processes. It is called the universal energy currency because it transfers energy between energy-releasing and energy-requiring reactions in all living organisms.

*Notation:* ATP

> **info**
>
> Hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate ($P_i$) releases 30.6 kJ of energy per mole of ATP. This release is manageable enough to avoid energy waste, but sufficient to drive most cellular processes.

**Worked example:** Explain why ATP is described as the energy currency, rather than the energy store of cells.

1. First, recall the function of a currency: it transfers value (energy) between different transactions (reactions), rather than storing value long-term.
2. ATP is an unstable molecule that is rapidly hydrolysed and rapidly regenerated from ADP and $P_i$. It is not stored in large quantities in cells.
3. Long-term energy storage uses stable molecules like glycogen, starch or triglycerides. Energy from these stores is transferred to ATP when needed, which then supplies energy to processes. This matches the function of a currency, not a store.

## Common pitfalls

- **Wrong:** Stating that photosynthesis is an anabolic reaction that does not require any energy input.
  - Why it fails: Photosynthesis is overall anabolic, but early stages require energy from ATP, and all plant cells still respire to meet immediate energy needs.
  - Correct: Recognise that photosynthesis is overall anabolic (builds glucose from smaller molecules) but still requires energy input to proceed.
- **Wrong:** Describing anabolism and catabolism as independent opposite processes.
  - Why it fails: The two reaction types are tightly coupled in cells, with energy from catabolism driving anabolism.
  - Correct: Remember that metabolism relies on coupling of these two reaction types to maintain a continuous flow of energy.
- **Wrong:** Claiming ATP is the main energy store in human cells.
  - Why it fails: ATP is only used for immediate energy transfer; long-term energy storage is done with glycogen and triglycerides.
  - Correct: Recall ATP is the energy currency (immediate energy transfer) not a long-term energy store.
- **Wrong:** Forgetting to include temperature maintenance as an energy requirement for endotherms.
  - Why it fails: Endotherms require extra energy to maintain core body temperature in cold environments, so their energy demand is higher than ectotherms.
  - Correct: Always include heat generation for temperature regulation when listing energy requirements for endotherms.

## Cheatsheet

| Concept | Key Exam Points |
| --- | --- |
| Core energy needs | Active transport, synthesis, mechanical work, temperature maintenance |
| Anabolism | Builds large molecules, endergonic, requires net energy |
| Catabolism | Breaks large molecules, exergonic, releases net energy |
| ATP role | Immediate energy currency, transfers energy between reactions, not stored |

## What's next

This sub-topic establishes the foundational principles of energy flow in living organisms that underpin all remaining topics in Unit 12. Understanding the difference between anabolism and catabolism, and the role of ATP, is critical for learning the stages of aerobic and anaerobic respiration in upcoming sub-topics. You will apply these core ideas to analyse how energy is released from organic molecules via glycolysis, the link reaction, Krebs cycle and oxidative phosphorylation. These concepts also form the basis for understanding photosynthesis, the anabolic process that captures light energy to produce the organic molecules used for respiration across almost all ecosystems.

- [Glycolysis](https://www.owlsprep.com/study/cie-9700-u12-glycolysis/)
- [ATP structure and role](https://www.owlsprep.com/study/cie-9700-u12-atp-structure-and-role/)
- [Link Reaction](https://www.owlsprep.com/study/cie-9700-u12-link-reaction/)

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