3.3Cellular Energetics

Cellular Energy — AP BIOLOGY Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 5 3 quizzes

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Lesson summary

3.3 Cellular Energy — Notes — AP Biology notes: Describe the role of energy in living organisms.Order — organisms need a constant input of energy to maintain order despite the second law of thermodynamicsUses — growth, reproduction, transport against gradients, movement, synthesi…

Inefficiency
Phosphorylation
Sequential pathways
Regulation
Same pathways
Glycolysis

Section 1 of 8Why living systems need energy

1Order

Order organisms need a constant input of energy to maintain order despite the second law of thermodynamics

2Uses

Uses growth, reproduction, transport against gradients, movement, synthesis of macromolecules and homeostasis

3Inefficiency

Inefficiency energy transfers to cell processes are inefficient, so energy is constantly lost as heat, which increases the entropy of the surroundings

4Balance

Balance energy input must exceed energy loss to maintain order and power cellular processes,
- and a significant loss of order or energy flow results in death

5First law

First law energy cannot be created or destroyed, only transformed

6Wording

Wording say that energy is released or transferred, not produced or made, and the Gibbs free energy equation is beyond the AP exam

Section 2 of 8Energy coupling and ATP

Coupling

Coupling processes that release energy are coupled with processes that need energy

Exergonic and endergonic

Breaking ATP into ADP + Pi releases about 30.5 kJ mol⁻¹, so it is exergonic (ΔG < 0).
Making sucrose from glucose and fructose requires energy, so it is endergonic (ΔG > 0).

ATP

ATP a nucleotide of adenine, ribose and three phosphates, and the immediate energy currency
The terminal phosphate bond is not unusually strong, but the negatively charged phosphates repel each other, so the molecule is unstable.
ATP is turned over continuously by rephosphorylating ADP in respiration and photosynthesis.
It is a short-term carrier, while fats and glycogen are the stores.

Phosphorylation

Phosphorylation transferring a phosphate to a reactant raises its energy so the reaction proceeds

Process	Role of ATP
Active transport	Phosphorylates the carrier protein to change its shape
Muscle contraction	Detaches and re-cocks myosin heads
Anabolic reactions	Phosphorylates substrates so synthesis proceeds
Cell signalling	Phosphate source for kinases

Section 3 of 8Energy and metabolic pathways

1Sequential pathways

Sequential pathways energy-related pathways run in steps so energy is transferred in a controlled and efficient way

2Glucose

Glucose its energy is not released in one step because that would damage the cell, but is transferred to ATP over a sequence of steps

3Link

Link the product of one reaction is typically the reactant of the next, for example pyruvate from glycolysis is a substrate of the Krebs cycle

4Types

Types catabolic pathways break molecules down and release energy, whereas anabolic pathways build molecules and need energy

5Regulation

Regulation pathways are often controlled by feedback inhibition, where an end product inhibits an early enzyme

Section 4 of 8Conserved metabolic pathways

1Same pathways

Same pathways different organisms use the same fundamental pathways and enzymes to extract energy

2Glycolysis

Glycolysis it occurs in the cytoplasm of Archaea, Bacteria and Eukarya with no specific organelle
It works without oxygen, uses widely available substrates and needs few simple cofactors, so it suits early life.

3Oxidative phosphorylation

Oxidative phosphorylation it also occurs in all three domains
It is in the plasma membrane of Bacteria and Archaea.
In eukaryotes it is on the inner mitochondrial membrane, and photophosphorylation is on thylakoid membranes.

4Endosymbiosis

Endosymbiosis the mitochondrial electron transport chain resembles that of some bacteria

Section 5 of 8Energy flow in ecosystems

1Direction

Direction energy flows one way from the Sun to producers, consumers and decomposers and leaves as heat,
- whereas matter such as carbon, nitrogen and water cycles

2Producers

Producers autotrophs capture light energy by photosynthesis, and chemoautotrophs use inorganic chemicals such as hydrogen sulfide

3Heterotrophs

Heterotrophs they obtain energy by feeding and release it by cellular respiration

4Transfer

Transfer only about 10% of energy passes to the next trophic level, so food chains are short and a pyramid of energy is never inverted

5Energy density

Energy density lipids hold about 37 kJ g⁻¹ compared with about 17 kJ g⁻¹ for carbohydrate or protein,
- and respiration transfers roughly 40% of energy to ATP with the rest lost as heat

6Metabolic rate

Metabolic rate the energy used per unit time, which is higher per gram in smaller endotherms because of their larger SA:V ratio

Section 6 of 8More from the specification

Section 7 of 8Key terms

Term	Meaning
ATP	Adenosine triphosphate, the immediate energy carrier.
Energy coupling	An exergonic reaction drives an endergonic one.
Exergonic	Releases energy (ΔG less than 0).
Endergonic	Needs energy (ΔG greater than 0).
Phosphorylation	Adding a phosphate group.
Metabolic pathway	A series of enzyme-controlled reactions.
Metabolic rate	The energy used by an organism per unit time.

Section 8 of 8Exam technique

Saying energy is recycled in ecosystems — matter cycles and energy flows through and is lost as heat.

Stating that ATP stores energy for a long time — it is a short-term carrier and fats and glycogen are stores.

Saying energy is made or produced by respiration — it is transferred from food to ATP.

Forgetting that heat is an inevitable loss.

Last updated: 18 October 2026