Difficulty: Introductory to Intermediate | Prerequisites: basic chemistry (atoms, electrons, redox reactions), cell organelle structure, understanding of ATP and energy currency in cells.
Photosynthesis is the process that underpins nearly all life on Earth, converting light energy into the chemical energy that organisms use to grow, reproduce and maintain themselves. This tutorial covers the first half of photosynthesis: the light-dependent reactions, which take place in the thylakoid membranes of chloroplasts. You should already be comfortable with basic redox chemistry (what it means for a molecule to be oxidised or reduced) and with the idea that ATP is the cell's energy currency. If you have covered cellular respiration and the electron transport chain in mitochondria, you will find useful parallels here.
Plants, algae and certain bacteria capture sunlight and use it to split water molecules, releasing oxygen and producing ATP and NADPH. These energy carriers then power the Calvin cycle (covered in Part II) to build sugars from CO₂. The light-dependent reactions happen in the thylakoid membranes via two photosystems working in series, linked by an electron transport chain.
Photosynthesis
The process by which light energy is converted into chemical energy (glucose), using carbon dioxide and water, with oxygen released as a byproduct. In simple terms, plants use sunlight to make food.
Photoautotroph
An organism that produces its own organic molecules using light energy (e.g. plants, algae, cyanobacteria). Think of it as a self-feeder that runs on sunlight.
Chemoheterotroph
An organism that obtains both energy and carbon from organic compounds made by other organisms. In simple terms, this covers most animals, fungi and many bacteria: they eat food rather than making it.
Chloroplast
The double-membraned organelle in plant and algal cells where photosynthesis occurs. Think of it as the solar panel of the cell.
Thylakoid
A flattened, membrane-bound sac inside the chloroplast. The light-dependent reactions take place in and across the thylakoid membrane. Think of thylakoids as the individual shelves inside the chloroplast where light is captured.
Granum (plural: grana)
A stack of thylakoids within the chloroplast. Stacking increases the surface area available for light absorption.
Stroma
The fluid-filled space surrounding the thylakoids inside the chloroplast. The Calvin cycle (light-independent reactions) takes place here.
Visible light
The portion of the electromagnetic spectrum that human eyes can detect, spanning wavelengths of roughly 380 nm (violet) to 700 nm (red). This is also the range of light energy that drives photosynthesis.
Photon
A discrete packet (quantum) of light energy. The energy of a photon is inversely proportional to its wavelength: shorter wavelength means higher energy.
Wavelength
The distance between successive peaks of a wave. In the context of light, wavelength determines colour and energy.
Electromagnetic spectrum
The full range of electromagnetic radiation, from gamma rays (short wavelength, high energy) to radio waves (long wavelength, low energy). Visible light is a small slice in the middle.
Pigment
A molecule that absorbs specific wavelengths of light and reflects others. The reflected wavelengths are what you see as the pigment's colour.
Chlorophyll a
The primary photosynthetic pigment found in all photosynthetic organisms that release oxygen. It absorbs mainly blue-violet and red light, and reflects green. It is the only pigment that can participate directly in the light reactions by passing excited electrons to the reaction centre.
Chlorophyll b
An accessory pigment that absorbs blue and orange-red light and passes that energy to chlorophyll a. It broadens the range of light wavelengths a plant can use.
Carotenoids
Accessory pigments (yellow, orange, red) that absorb blue-green light and transfer energy to chlorophyll a. They also protect chlorophyll from damage by excess light (photoprotection).
Absorption spectrum
A graph showing which wavelengths of light a particular pigment absorbs. Peaks on the graph indicate wavelengths that are strongly absorbed.
Action spectrum
A graph showing the rate of photosynthesis at each wavelength of light. It closely mirrors the combined absorption spectra of all photosynthetic pigments, confirming that absorbed light drives the process.
Most life on Earth depends on photosynthesis, either directly or indirectly.
Photoautotrophs (plants, algae, cyanobacteria) produce organic molecules and release O₂.
Chemoheterotrophs (animals, fungi, most bacteria) rely on those organic molecules for energy and carbon. Without photoautotrophs, they would have no food source and no atmospheric oxygen for aerobic respiration.
Plants are the most familiar photosynthesisers, but they are far from the only ones.
Algae and phytoplankton (aquatic photosynthetic organisms) produce as much, if not more, oxygen than land plants. They are responsible for roughly half of global O₂ production.
Certain bacteria, notably cyanobacteria, also carry out oxygenic photosynthesis. So the statement "only plants and some protists can photosynthesise" is false.
A common error is to think plants only photosynthesise and do not perform cellular respiration.
Plants carry out both processes. They photosynthesise when light is available and respire continuously (day and night) in their mitochondria.
The majority of a plant's ATP is produced by oxidative phosphorylation in mitochondria, not in chloroplasts.
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Water (H₂O) is oxidised: it loses electrons and hydrogen ions; oxygen is released as a byproduct.
Carbon dioxide (CO₂) is reduced: it gains electrons and hydrogen to form glucose.
This is the reverse direction of cellular respiration.
Chloroplasts have a double membrane (outer and inner) with an intermembrane space between them.
Inside the inner membrane is the stroma, a fluid-filled matrix.
Suspended in the stroma are thylakoids, flattened membrane sacs. Thylakoids stack into grana (singular: granum).
The thylakoid membrane is where the light-dependent reactions occur.
The thylakoid space (lumen) is the interior of the thylakoid sac; this is where H⁺ ions accumulate to drive ATP synthesis.
The stroma is where the Calvin cycle (light-independent reactions) takes place.
Sunlight travels as electromagnetic waves, but it also behaves as discrete packets of energy called photons.
When a photon strikes a pigment molecule in a leaf, the pigment absorbs the photon's energy if its wavelength matches the pigment's absorption profile. This energy excites an electron in the pigment to a higher energy level.
If the wavelength does not match, the photon passes through or is reflected. The colour you see in a leaf is the light it reflects, not the light it absorbs.
Visible light spans roughly 380 nm (violet) to 700 nm (red).
Chlorophyll a absorbs most strongly in the blue-violet (~430 nm) and red (~660 nm) regions.
Chlorophyll b absorbs strongly in the blue (~455 nm) and orange-red (~640 nm) regions.
Carotenoids absorb in the blue-green range (~400–500 nm).
The green-yellow wavelengths (~500–600 nm) are the least absorbed by all major photosynthetic pigments. This is why most plants appear green: they reflect green light rather than absorbing it.
The absorption spectrum shows which wavelengths a specific pigment absorbs. It is measured by shining light through a purified pigment solution.
The action spectrum shows the rate of photosynthesis at each wavelength. It is measured by exposing whole plants or chloroplasts to narrow bands of light and recording O₂ output.
The action spectrum closely matches the combined absorption spectra of all pigments, confirming that the wavelengths absorbed are the ones driving photosynthesis.
From the action spectrum, we can conclude that plants can use all colours of visible light for photosynthesis (answer B on the tutorial), though blue and red are the most efficient. Green-yellow light is least efficient, not most efficient.
Chlorophyll a alone cannot absorb all useful wavelengths.
Accessory pigments (chlorophyll b, carotenoids, and in some organisms phycobilins) broaden the range of wavelengths a plant can harvest.
Carotenoids also serve a photoprotective role: they dissipate excess light energy that could otherwise damage chlorophyll (photooxidation).
Understanding light absorption by pigments is the basis for designing artificial photosynthesis systems and solar cells that mimic how leaves capture energy. It also explains why greenhouse growers use red and blue LED grow lights rather than green ones: those wavelengths deliver the most photosynthetically useful energy per watt.
Students often think only plants can photosynthesise. In fact, algae, phytoplankton and cyanobacteria are major photosynthesisers, and phytoplankton alone account for roughly half of global oxygen production.
Students often think plants do not perform cellular respiration because they photosynthesise. Plants respire continuously in their mitochondria, day and night. Photosynthesis and respiration are complementary, not mutually exclusive.
Students often assume green light is the most useful for photosynthesis because leaves are green. The opposite is true: leaves are green precisely because green wavelengths are reflected (not absorbed), making green light the least efficient driver of photosynthesis.
Students sometimes confuse absorption spectrum with action spectrum. The absorption spectrum is for a single purified pigment. The action spectrum measures the actual rate of photosynthesis across wavelengths for the whole organism.
⚠️ Be able to write and explain the overall photosynthesis equation, identifying what is oxidised (H₂O) and what is reduced (CO₂).
⚠️ Know that the oxygen released in photosynthesis comes from water, not from CO₂. This is a classic exam question.
⚠️ Understand the difference between absorption spectrum and action spectrum and be able to interpret both graphs.
⚠️ Be prepared to explain why plants appear green in terms of pigment absorption.
⚠️ Know the role of accessory pigments and why having multiple pigment types is advantageous.
⚠️ Be able to label chloroplast anatomy: outer membrane, inner membrane, thylakoid, granum, stroma, thylakoid lumen.
True or False: Algae and phytoplankton produce as much, if not more, oxygen than land plants. (True)
True or False: Only plants and some protists can undergo photosynthesis. (False, cyanobacteria also photosynthesise)
Fill in the blank: The oxygen released during photosynthesis comes from _______ (water / H₂O).
True or False: Green light drives photosynthesis more efficiently than red light. (False)
Fill in the blank: The light-dependent reactions take place in the _______ membranes of the chloroplast. (thylakoid)
Q: Justify the statement that most life on Earth could not exist without photosynthesis. How are chemoheterotrophs dependent on photoautotrophs?
A: Photoautotrophs convert light energy into organic molecules (glucose) and release O₂. Chemoheterotrophs depend on those organic molecules as their source of both energy and carbon. They also depend on the O₂ produced by photoautotrophs for aerobic respiration. Without photosynthesis, there would be no food chain base and no atmospheric oxygen to sustain aerobic life.
Q: What is the result of photosynthesis? Write the equation and identify what is oxidised and what is reduced.
A: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Water is oxidised (it loses electrons, releasing O₂). Carbon dioxide is reduced (it gains electrons and hydrogen atoms, forming glucose).
Q: Which wavelengths have the lowest relative light absorbance for all photosynthetic pigments, and why is this significant for a plant's appearance?
A: Green-yellow wavelengths (~500–560 nm) have the lowest absorbance across chlorophyll a, chlorophyll b and carotenoids. Because these wavelengths are reflected rather than absorbed, most plants appear green to the human eye.
Q: True or False: Plants undergo photosynthesis but not cellular respiration, meaning the majority of ATP is produced within chloroplasts.
A: False. Plants perform both photosynthesis and cellular respiration. Cellular respiration occurs continuously in mitochondria. The majority of a plant cell's ATP comes from oxidative phosphorylation in mitochondria, not from the light-dependent reactions in chloroplasts.
Q: From the photosynthetic action spectrum, we can correctly conclude that: (A) Chlorophyll absorbs more green than red, (B) Plants can use all colours of visible light, (C) Every colour yields equal rates, (D) Green-yellow is most efficient. Which is correct?
A: B. Plants can use all colours of visible light for photosynthesis, though blue and red wavelengths drive it most efficiently. The action spectrum shows some photosynthesis at every visible wavelength, but the rate is lowest in the green-yellow range.
This material connects directly to cellular respiration (Tutorial on the ETC and oxidative phosphorylation), since both processes use electron transport chains and chemiosmosis to produce ATP. Understanding redox reactions here reinforces the same principles you learned in metabolism. The chloroplast structure discussion also ties back to the endosymbiont theory, as chloroplasts (like mitochondria) are thought to have originated from free-living prokaryotes.
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