Plants need photosynthesis and respiration because the two processes solve different biological problems. Photosynthesis captures light energy and stores it in carbon compounds, while cellular respiration transfers energy from those compounds to ATP that cells can use. Making glucose is not the same as supplying usable energy to every cell, so photosynthesis cannot replace respiration.
This distinction is central to IB Biology plant biology. In the current course, the molecular mechanisms are covered mainly in C1.2 Cell respiration and C1.3 Photosynthesis, while plant gas exchange and transport explain how the required substances reach cells. This article focuses specifically on why the two processes are both necessary; for broader plant content, use the topic-wide IB Biology Plant Biology HL explained guide.
The short answer: plants must capture energy and then make it usable
Photosynthesis converts light energy into chemical energy. In simplified form, plants use carbon dioxide and water to produce glucose and oxygen:
carbon dioxide + water + light energy → glucose + oxygen
Cellular respiration then oxidizes glucose or other respiratory substrates. In aerobic respiration, oxygen is used and energy is transferred to ATP:
glucose + oxygen → carbon dioxide + water + energy transferred to ATP
The essential relationship is therefore:
Photosynthesis supplies organic carbon and stored chemical energy.
Respiration releases and transfers part of that energy to ATP.
ATP powers immediate cellular work, including active transport, biosynthesis, growth and cell maintenance.
A useful analogy is that photosynthesis charges an energy store, whereas respiration converts the stored energy into a form the cell can spend. The analogy is imperfect, but it prevents the common mistake of treating glucose and ATP as interchangeable.
What photosynthesis provides to a plant
Photosynthesis is an anabolic process because it contributes to the construction of larger organic molecules from smaller inorganic substances. Carbon dioxide supplies carbon, while water supplies electrons and hydrogen. Light provides the energy needed for this energetically demanding transformation.
In plants, photosynthesis takes place mainly in chloroplast-containing cells in leaves and other green tissues. The process can be separated into two connected sets of reactions.
Light-dependent reactions
The light-dependent reactions occur in the thylakoid membranes of chloroplasts. Photosynthetic pigments absorb light, electrons become excited, and electron transport helps establish a proton gradient. Chemiosmosis through ATP synthase produces ATP, while electrons and hydrogen are used to reduce NADP.
Water undergoes photolysis, producing electrons, protons and oxygen. An important IB exam point is that the oxygen released by photosynthesis originates from water, not from carbon dioxide.
Carbon fixation and carbohydrate production
The products of the light-dependent reactions support carbon fixation in the chloroplast stroma. At HL, students study how Rubisco catalyses carbon fixation in the Calvin cycle and how ATP and reduced NADP support the production of triose phosphate.
Triose phosphate can contribute to the synthesis of:
glucose and sucrose
starch
cellulose
lipids
amino acids, when suitable mineral nutrients are available
nucleotides and other carbon compounds
Photosynthesis therefore provides more than a respiratory fuel. It supplies much of the organic material from which a plant constructs new cells, cell walls, storage compounds and reproductive structures. The IB Biology photosynthesis exam guide covers these stages in greater molecular detail without treating them as substitutes for respiration.
Why do plants respire if photosynthesis already produces ATP?
The light-dependent reactions do produce ATP, but this does not remove the need for mitochondrial respiration. Much of the ATP generated in chloroplasts is used locally to support the Calvin cycle and other chloroplast processes. It should not be imagined as a universal ATP supply that is freely distributed to every plant cell.
Mitochondrial respiration supplies ATP for cellular work across the plant. This is particularly important in cells that lack chloroplasts, such as many root cells and internal stem tissues, and whenever light-dependent ATP production cannot occur.
Respiratory ATP supports processes such as:
active transport of mineral ions across membranes
synthesis of proteins, nucleic acids and polysaccharides
cell division and growth
loading and unloading substances during transport
maintenance of ion gradients
repair and replacement of cell components
opening, closing and regulation of stomata
Research on photosynthetic cells also shows that mitochondria remain metabolically important in the light. They contribute to cellular energy and redox balance rather than simply switching off when chloroplasts become active. For an exam answer, however, the clearest core point is that photosynthesis stores energy in carbon compounds, whereas respiration transfers energy into ATP for cellular use.
How cellular respiration releases usable energy
Cellular respiration is a catabolic process involving the controlled breakdown and oxidation of organic molecules. Releasing energy through a sequence of enzyme-controlled reactions allows some of that energy to be transferred to ATP instead of being lost rapidly as heat.
In aerobic respiration, glycolysis begins in the cytoplasm. At HL, students also examine the link reaction, Krebs cycle, electron transport and chemiosmosis associated with mitochondria. Oxygen acts as the final electron acceptor in the aerobic electron transport chain, enabling continued oxidative phosphorylation.
ATP is useful because its hydrolysis can be coupled to energy-requiring reactions. Cells continuously regenerate ATP from ADP and inorganic phosphate rather than accumulating a large permanent ATP reserve. The C1.2 Cell Respiration study hub provides the syllabus-level detail needed for SL and HL revision.
Photosynthesis and respiration compared
Feature
Photosynthesis
Aerobic cellular respiration
Main biological role
Captures light energy and produces carbon compounds
Transfers energy from organic compounds to ATP
Metabolic type
Mainly anabolic
Mainly catabolic
Overall carbon change
Carbon dioxide is reduced and incorporated into organic molecules
Organic carbon is oxidized and carbon dioxide is released
Main location in plants
Chloroplasts of photosynthetic cells
Cytoplasm and mitochondria of living cells
Key inputs in the simplified equation
Carbon dioxide, water and light
Organic substrate and oxygen
Key outputs in the simplified equation
Carbon compounds and oxygen
Carbon dioxide, water and ATP production
Dependence on light
Requires light for the light-dependent reactions
Does not require light directly
When it occurs
When light and other required conditions are available
In light and darkness
Cells involved
Primarily chloroplast-containing cells
Living photosynthetic and non-photosynthetic cells
The overall equations appear to reverse one another, but the pathways are not literal reversals. They involve different enzymes, intermediate reactions, electron carriers, membranes and regulatory mechanisms. Stating that they are complementary is more accurate than stating that one is simply the other running backwards.
Plants respire in daylight as well as darkness. Photosynthesis only proceeds when sufficient light is available, although its rate is also influenced by carbon dioxide concentration, temperature, water availability and other factors.
In bright light
Both photosynthesis and respiration occur. If photosynthesis fixes carbon dioxide faster than respiration releases it, the plant shows net carbon dioxide uptake and usually net oxygen release. Some newly produced carbohydrate may be used immediately, while some is transported as sucrose, converted into structural materials or stored as starch.
In darkness
The light-dependent reactions stop, so photosynthetic carbon fixation cannot continue normally. Respiration continues, using stored or transported organic substrates. Leaves commonly break down starch accumulated during the day, while roots receive sugars transported from photosynthetic source tissues.
In dim light
Both processes may still occur, but photosynthesis can become slow. At the light compensation point, the rate of carbon dioxide uptake through photosynthesis equals the rate of carbon dioxide release from respiration and other relevant metabolic processes. Net carbon dioxide exchange is then zero even though both processes continue.
Above the compensation point, photosynthetic carbon gain exceeds respiratory carbon loss. Below it, the tissue consumes more organic carbon through respiration than photosynthesis replaces. This explains why a plant cannot survive indefinitely in light that is too weak, even if a small amount of photosynthesis is occurring.
Gross and net photosynthesis
Gas exchange measurements usually show the net effect of several processes rather than photosynthesis alone. A leaf in bright light may absorb carbon dioxide overall, but its cells are simultaneously producing some carbon dioxide through respiration.
The relationship can be expressed conceptually as:
net photosynthesis = gross photosynthesis − respiration and other carbon-releasing processes
Suppose a leaf fixes carbon at a gross rate of 12 arbitrary units per hour while respiration releases carbon at 3 units per hour. The measured net carbon gain is 9 units per hour. If an exam question gives gas exchange data, distinguish the underlying process from the net movement measured outside the leaf.
This also explains why saying “plants take in carbon dioxide during the day and oxygen at night” is an oversimplification. Both gases may move in both directions, but the net direction depends on the relative rates of photosynthesis, respiration and other processes.
Why roots and other non-green tissues depend on respiration
A plant is not composed entirely of photosynthetic cells. Roots are usually underground, receive little or no light and generally lack photosynthetically active chloroplasts. Many internal tissues, flowers, fruits, seeds and growing regions also depend on organic compounds supplied from elsewhere in the plant.
Sucrose is transported through the phloem from sources, such as mature photosynthesizing leaves, to sinks, such as roots, fruits and meristems. Sink cells can use these carbon compounds in respiration to produce ATP or incorporate them into new biomass.
Roots also require oxygen for aerobic respiration. Oxygen diffuses through air spaces in well-aerated soil and enters root tissues. Waterlogged soil contains less accessible oxygen because water fills spaces that would otherwise contain air, so prolonged flooding can restrict aerobic respiration and impair active mineral uptake and growth.
The relationship can be summarized as a whole-plant division of labour:
Leaves capture light and export organic compounds.
Transport tissues distribute water, minerals and sugars.
Photosynthetic and non-photosynthetic cells respire.
Roots absorb mineral ions and water but depend heavily on sugars made elsewhere.
Do plants respire continuously?
Living plant cells require a continuing supply of ATP, so respiration occurs during both day and night. Its rate is not necessarily constant. Temperature, oxygen availability, substrate supply, developmental stage and metabolic demand can all affect respiratory rate.
Germinating seeds, developing fruits and growing meristems often have high energy demands. Dormant tissues may respire much more slowly. A precise IB answer should therefore state that respiration occurs in light and darkness, not that it always proceeds at an unchanging rate.
Aerobic respiration is normally the more efficient route for ATP production when oxygen is available. Under oxygen-deficient conditions, plant cells may depend more on fermentation to regenerate NAD, but this produces far less ATP per glucose molecule. Prolonged oxygen shortage therefore threatens growth and survival.
Common IB Biology mistakes
Mistake 1: saying plants photosynthesize instead of respiring
Plants are photoautotrophs, but they still carry out cellular respiration. Autotrophic describes how they obtain organic carbon; it does not mean that their cells can use glucose without respiration.
Mistake 2: saying respiration only happens at night
Respiration occurs in light and darkness. During the day, it may be hidden by a greater net effect of photosynthesis on gas exchange.
Mistake 3: treating glucose as directly usable energy
Glucose contains stored chemical energy, but cells need controlled metabolic pathways to transfer that energy to ATP. Use the phrase energy is transferred to ATP, rather than saying respiration simply “creates energy.”
Mistake 4: claiming all plant cells photosynthesize
Only cells with suitable photosynthetic machinery can photosynthesize. Most living plant cells respire, including many cells in roots and other non-green tissues.
Mistake 5: calling the pathways exact opposites
Their simplified equations have opposite-looking inputs and outputs, but their reaction sequences and biological roles differ. Explain the complementary movement of matter while distinguishing the direction of energy transformation.
Mistake 6: confusing respiration with photorespiration
Cellular respiration transfers energy from organic substrates to ATP. Photorespiration is a different set of reactions associated with Rubisco binding oxygen instead of carbon dioxide, reducing the efficiency of carbon fixation. Do not use the terms interchangeably.
How to answer an IB exam question on this relationship
For a question asking why plants require both processes, build the answer as a causal sequence:
Photosynthesis absorbs light energy and converts it into chemical energy in carbon compounds.
It produces organic substrates that can be used in respiration and biosynthesis.
Respiration oxidizes organic substrates and transfers released energy to ATP.
ATP supplies energy for active transport, biosynthesis, growth and other cellular work.
Respiration can occur in non-photosynthetic cells and in darkness.
Therefore, photosynthesis supplies stored energy and organic matter, while respiration makes energy available to cells when and where it is required.
If the command term is compare, include both similarities and differences. If it is explain, connect each statement with a biological consequence instead of listing isolated facts. Avoid giving a detailed account of the Calvin cycle or Krebs cycle unless the question specifically requests mechanisms.
Plants need photosynthesis to capture light energy, fix carbon and produce organic compounds. They need respiration to transfer energy from those compounds to ATP, which supports cellular work throughout the plant. The two processes interact through their substrates and products, but they are distinct pathways rather than exact reversals.
For IB Biology, remember the central contrast: photosynthesis stores energy in organic molecules; respiration makes that energy available through ATP. RevisionDojo’s C1.2 and C1.3 Study Notes, Flashcards and Questionbanks can then help you practise expressing this relationship with the precision expected in exam answers.
Sarah holds a PhD in Cell Biology and taught IB Biology across Europe and Asia for 18 years, latterly as a science department lead. Outside of the papers, her focus lies with the Biology EE, especially with its new format, closing the gap between understanding and application.
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