Plants build food from sunlight; all living cells then burn that food for energy. Photosynthesis and respiration are the two complementary engines of life, and the CDS examiner loves testing their equations, sites, by-products and the exact stage where energy is captured. This page makes both processes simple, accurate and fully exam-ready.
Why This Topic Matters for CDS
Biology questions in the CDS General Science paper are fact-dense and quick to score if your basics are clean. Photosynthesis and respiration appear almost every year — sometimes as a direct equation, sometimes hidden inside a question on chlorophyll, stomata, mitochondria or fermentation.
Both are metabolic processes: photosynthesis is anabolic (building up glucose, storing energy) while respiration is catabolic (breaking down glucose, releasing energy). Seeing them as mirror images is the fastest way to remember everything. Many students lose easy marks here simply because they mix up the raw materials, the products, or the cell organelle involved. Once you fix the four anchors — inputs, outputs, site and energy direction — every question on this topic becomes a quick recall exercise rather than a calculation.
These two processes also explain larger ideas the examiner may test indirectly: the carbon cycle, the oxygen cycle, why forests are described as carbon sinks, and how food chains transfer energy that originally came from the Sun. Treat this chapter as the foundation for a whole cluster of biology questions, not an isolated topic.
Photosynthesis stores energy in chemical bonds; respiration releases it as ATP. One feeds the other — together they keep the carbon and oxygen cycles running.
Photosynthesis: The Food Factory
Photosynthesis is the process by which green plants, algae and some bacteria make their own food (glucose) using carbon dioxide and water in the presence of sunlight and chlorophyll. Because they manufacture food themselves, such organisms are called autotrophs.
The overall balanced equation is:
6CO2 + 6H2O → C6H12O6 + 6O2
(carbon dioxide + water, in presence of sunlight and chlorophyll → glucose + oxygen)
The oxygen released comes from the splitting of water (photolysis), not from carbon dioxide — a favourite CDS trap.
Organisms that cannot make their own food and depend on others are called heterotrophs — this includes all animals, fungi and most bacteria. The entire living world ultimately runs on the glucose first manufactured by autotrophs, which is why photosynthesis is often called the most important chemical reaction on Earth. The glucose produced is either used immediately for energy, stored as starch, or converted into cellulose, proteins and fats for growth.
Besides the main green pigment chlorophyll, leaves also contain accessory pigments such as carotenoids (yellow and orange) that absorb extra wavelengths of light and pass the energy on to chlorophyll. These pigments become visible in autumn when chlorophyll breaks down, which is why leaves change colour.
Raw Materials and Conditions
Four things are essential for photosynthesis to occur:
- Carbon dioxide — enters leaves through tiny pores called stomata.
- Water — absorbed by roots and carried up by xylem.
- Sunlight — the energy source; most useful are the red and blue parts of the spectrum.
- Chlorophyll — the green pigment in chloroplasts that traps light energy.
The process happens mainly in the chloroplasts of leaf cells, especially in the mesophyll. The leaf is therefore often called the ‘kitchen of the plant’.
Glucose made during the day is stored in leaves as starch. The iodine test (starch turns blue-black) is the classic proof that photosynthesis has taken place.
Light and Dark Reactions
Photosynthesis occurs in two linked stages inside the chloroplast.
Light reaction (photochemical phase)
Takes place in the grana (thylakoid membranes). Chlorophyll absorbs light, water is split into hydrogen and oxygen (photolysis), oxygen is released, and energy carriers ATP and NADPH are produced. This stage needs light.
Dark reaction (Calvin cycle / biosynthetic phase)
Takes place in the stroma of the chloroplast. Here CO2 is fixed and reduced using the ATP and NADPH from the light reaction to form glucose. It is called ‘dark’ because it does not directly need light, though it usually runs in daytime.
The ‘dark reaction’ does not require darkness. It simply does not use light directly. Calling it a night-time process is wrong.
Cellular Respiration: Releasing Energy
Respiration is the process by which cells break down glucose to release energy stored in its bonds. This energy is captured as ATP (adenosine triphosphate), the universal energy currency of the cell.
The overall equation for aerobic respiration is essentially the reverse of photosynthesis:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP)
Do not confuse respiration with breathing. Breathing is the physical exchange of gases (inhaling O2, exhaling CO2); respiration is the chemical release of energy inside cells. Breathing is only the transport step that supplies oxygen for respiration and removes the carbon dioxide it produces.
Respiration happens in every living cell, all the time, day and night, because cells constantly need energy for growth, repair, movement and transport of substances. Unlike photosynthesis, it does not require sunlight, chlorophyll or green tissue. The energy released is not used directly; instead it is temporarily stored in ATP, which can then be spent wherever the cell needs it. When ATP releases its energy it becomes ADP, and respiration recharges ADP back to ATP — a continuous cycle.
Aerobic vs Anaerobic Respiration
Respiration can occur with or without oxygen.
Aerobic respiration
Occurs in the presence of oxygen, mainly in the mitochondria (the ‘powerhouse of the cell’). Glucose is completely oxidised to CO2 and water, releasing a large amount of energy — about 38 ATP per glucose molecule (commonly cited as 36–38).
Anaerobic respiration
Occurs without oxygen and releases far less energy. Examples:
- In yeast: glucose → ethanol + CO2 + energy (alcoholic fermentation, used in baking and brewing).
- In human muscles during heavy exercise: glucose → lactic acid + energy. The build-up of lactic acid causes muscle cramps and fatigue.
The reason anaerobic respiration yields so little energy is that glucose is only partially broken down. Without oxygen as the final electron acceptor, the cell cannot run the most productive stages, so most of the chemical energy stays locked inside ethanol or lactic acid. This is why a sprinter tires quickly, while a long-distance runner whose muscles stay aerobic can sustain effort for much longer. After heavy exercise, the body needs extra oxygen (the ‘oxygen debt’) to clear the accumulated lactic acid.
The first stage, glycolysis (glucose → pyruvate), happens in the cytoplasm and is common to both aerobic and anaerobic pathways. Only the later stages differ.
Stages of Aerobic Respiration
Aerobic respiration is completed in three broad stages:
- Glycolysis — in the cytoplasm; one glucose (6 carbon) splits into two molecules of pyruvate (3 carbon), giving a small ATP yield.
- Krebs cycle (citric acid cycle) — in the mitochondrial matrix; pyruvate is fully oxidised, releasing CO2 and electron carriers.
- Electron transport chain (oxidative phosphorylation) — on the inner mitochondrial membrane; oxygen is the final electron acceptor and most ATP is produced here, with water formed as a by-product.
If asked where most ATP is generated, the answer is the electron transport chain in the mitochondria, not glycolysis.
Photosynthesis vs Respiration at a Glance
Set the two processes side by side to lock in the differences:
- Raw materials: photosynthesis uses CO2 + water; respiration uses glucose + O2.
- Products: photosynthesis gives glucose + O2; respiration gives CO2 + water + energy.
- Energy: photosynthesis stores energy; respiration releases it.
- Site: photosynthesis in chloroplasts; respiration in mitochondria (and cytoplasm).
- Occurs in: photosynthesis only in green cells with light; respiration in all living cells, all the time.
A useful exam habit is to read photosynthesis and respiration as a balanced loop. The glucose and oxygen made by plants are exactly what plants and animals consume during respiration, and the carbon dioxide and water released by respiration are exactly what plants take in for photosynthesis. This circular relationship is what keeps the atmospheric proportions of oxygen and carbon dioxide roughly stable over time, and it is the biological basis of the carbon cycle that often appears in environment questions.
Plants respire 24 hours a day. They photosynthesise only in light. At night, plants only respire, so they release CO2 — which is why they are not kept in hospital rooms at night.
Worked Example: Oxygen and Carbon Balance
If a plant cell carries out photosynthesis using 12 molecules of carbon dioxide, how many molecules of oxygen and glucose are produced, and how much water is required?
So 12 CO2 and 12 H2O give 2 glucose and 12 O2. Notice CO2 in and O2 out are equal in number here — a neat check.
High-Frequency CDS Facts
These one-liners appear repeatedly in objective papers:
- Chlorophyll contains magnesium (Mg) at its centre.
- Mitochondrion is the powerhouse of the cell; chloroplast is the site of photosynthesis.
- Stomata are guarded by guard cells that control opening and closing.
- Photolysis (splitting of water) releases the oxygen of photosynthesis.
- ATP is the energy currency of the cell.
- Anaerobic respiration in muscles produces lactic acid; in yeast it produces ethanol.
- Photosynthesis is anabolic (builds up); respiration is catabolic (breaks down).
- Plants get raw materials from roots (water) and stomata (carbon dioxide).
- Glycolysis is common to both aerobic and anaerobic respiration and occurs in the cytoplasm.
When revising, do not just memorise these as loose facts. Tie each one to its ‘why’: magnesium sits at the heart of chlorophyll because the molecule needs a central metal ion to trap light, and ATP is the energy currency because it can be made and broken quickly in small, usable packets. Understanding the reason makes the fact much harder to forget under exam pressure, and lets you answer twisted versions of the same question.
Net daytime effect: photosynthesis exceeds respiration, so plants are net producers of oxygen. This is why forests are called the lungs of the Earth.
Previous-Year Style Question
Q. During photosynthesis, the oxygen released by green plants comes from which of the following?
Answer: From water. In the light reaction, water is split by photolysis (2H2O → 4H+ + 4e− + O2), releasing oxygen. The oxygen does not come from carbon dioxide — a classic distractor in CDS papers.
Q. The site of the Krebs cycle in a eukaryotic cell is the:
Answer: The mitochondrial matrix. Glycolysis occurs in the cytoplasm, but the Krebs cycle and the electron transport chain are located in the mitochondria.
Quick Revision
- Photosynthesis: 6CO2 + 6H2O → glucose + 6O2, in chloroplasts, needs sunlight and chlorophyll.
- Light reaction in grana makes ATP, NADPH and O2; dark reaction (Calvin cycle) in stroma fixes CO2 into glucose.
- Respiration: glucose + O2 → CO2 + H2O + ATP, mainly in mitochondria.
- Aerobic gives ~38 ATP; anaerobic gives lactic acid (muscle) or ethanol (yeast) with little energy.
- Glycolysis is in the cytoplasm; oxygen released in photosynthesis comes from water.
Drill these equations and sites until they are automatic — they convert directly into marks. The Cavalier wishes you a confident attempt.
Frequently asked questions
Is photosynthesis the exact reverse of respiration?
In terms of overall raw materials and products they are opposites, but the actual biochemical pathways, enzymes and sites are completely different. So they are complementary, not literally reverse reactions.
Why is the dark reaction called 'dark' if it happens in daytime?
Because it does not directly use light energy. It runs on the ATP and NADPH produced by the light reaction, so it can proceed even without light, though it normally occurs during the day.
How much ATP does aerobic respiration produce per glucose?
About 36 to 38 ATP molecules per glucose, with most generated in the electron transport chain. Anaerobic respiration yields only about 2 ATP, which is why it is far less efficient.
Where does the oxygen released in photosynthesis come from?
From water, which is split during photolysis in the light reaction. It does not come from carbon dioxide, which is a common exam misconception.
Do plants respire only at night?
No. Plants respire continuously, day and night. During the day photosynthesis usually masks respiration, but at night only respiration occurs, so plants release carbon dioxide.
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