If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Intro to photosynthesis

ENE‑1 (EU)
ENE‑1.I (LO)
ENE‑1.I.1 (EK)
ENE‑1.I.1.i (EK)
ENE‑1.I.2 (EK)
ENE‑1.J (LO)
ENE‑1.J.1 (EK)
ENE‑1.J.5 (EK)
Conversion of light energy to chemical energy. Reactions of photosynthesis, where they take place, and their ecological importance.


Have you hugged a tree lately? If not, you might want to give it some thought. You, along with the rest of the human population, owe your existence to plants and other organisms that capture light. In fact, most life on Earth is possible because the sun provides a continuous supply of energy to ecosystems.
All organisms, including humans, need energy to fuel the metabolic reactions of growth, development, and reproduction. But organisms can't use light energy directly for their metabolic needs. Instead, it must first be converted into chemical energy through the process of photosynthesis.

What is photosynthesis?

Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct. The glucose molecules provide organisms with two crucial resources: energy and fixed—organic—carbon.
  • Energy. The glucose molecules serve as fuel for cells: their chemical energy can be harvested through processes like cellular respiration and fermentation, which generate adenosine triphosphate—start text, A, T, P, end text, a small, energy-carrying molecule—for the cell’s immediate energy needs.
  • Fixed carbon. Carbon from carbon dioxide—inorganic carbon—can be incorporated into organic molecules; this process is called carbon fixation, and the carbon in organic molecules is also known as fixed carbon. The carbon that's fixed and incorporated into sugars during photosynthesis can be used to build other types of organic molecules needed by cells.
In photosynthesis, solar energy is harvested and converted to chemical energy in the form of glucose using water and carbon dioxide. Oxygen is released as a byproduct.

The ecological importance of photosynthesis

Photosynthetic organisms, including plants, algae, and some bacteria, play a key ecological role. They introduce chemical energy and fixed carbon into ecosystems by using light to synthesize sugars. Since these organisms produce their own food—that is, fix their own carbon—using light energy, they are called photoautotrophs (literally, self-feeders that use light).
Humans, and other organisms that can’t convert carbon dioxide to organic compounds themselves, are called heterotrophs, meaning different-feeders. Heterotrophs must get fixed carbon by eating other organisms or their by-products. Animals, fungi, and many prokaryotes and protists are heterotrophs.
Besides introducing fixed carbon and energy into ecosystems, photosynthesis also affects the makeup of Earth’s atmosphere. Most photosynthetic organisms generate oxygen gas as a byproduct, and the advent of photosynthesis—over 3 billion years ago, in bacteria resembling modern cyanobacteria—forever changed life on Earthstart superscript, 1, end superscript. These bacteria gradually released oxygen into Earth’s oxygen-poor atmosphere, and the increase in oxygen concentration is thought to have influenced the evolution of aerobic life forms—organisms that use oxygen for cellular respiration. If it hadn’t been for those ancient photosynthesizers, we, like many other species, wouldn't be here today!
Photosynthetic organisms also remove large quantities of carbon dioxide from the atmosphere and use the carbon atoms to build organic molecules. Without Earth’s abundance of plants and algae to continually suck up carbon dioxide, the gas would build up in the atmosphere. Although photosynthetic organisms remove some of the carbon dioxide produced by human activities, rising atmospheric levels are trapping heat and causing the climate to change. Many scientists believe that preserving forests and other expanses of vegetation is increasingly important to combat this rise in carbon dioxide levels.

Leaves are sites of photosynthesis

Plants are the most common autotrophs in terrestrial—land—ecosystems. All green plant tissues can photosynthesize, but in most plants, but the majority of photosynthesis usually takes place in the leaves. The cells in a middle layer of leaf tissue called the mesophyll are the primary site of photosynthesis.
Small pores called stomata—singular, stoma—are found on the surface of leaves in most plants, and they let carbon dioxide diffuse into the mesophyll layer and oxygen diffuse out.
A diagram showing a leaf at increasing magnifications. Magnification 1: The entire leaf Magnification 2: Mesophyll tissue within the leaf Magnification 3: A single mesophyll cell Magnification 4: A chloroplast within the mesophyll cell Magnification 5: Stacks of thylakoids—grana—and the stroma within a chloroplast
Image credit: modified from "Overview of photosynthesis: Figure 6" by OpenStax College, Concepts of Biology, CC BY 3.0
Each mesophyll cell contains organelles called chloroplasts, which are specialized to carry out the reactions of photosynthesis. Within each chloroplast, disc-like structures called thylakoids are arranged in piles like stacks of pancakes that are known as grana—singular, granum. The membrane of each thylakoid contains green-colored pigments called chlorophylls that absorb light. The fluid-filled space around the grana is called the stroma, and the space inside the thylakoid discs is known as the thylakoid space. Different chemical reactions occur in the different parts of the chloroplast.

The light-dependent reactions and the Calvin cycle

Photosynthesis in the leaves of plants involves many steps, but it can be divided into two stages: the light-dependent reactions and the Calvin cycle.
  • The light-dependent reactions take place in the thylakoid membrane and require a continuous supply of light energy. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, start text, A, T, P, end text—an energy storage molecule—and start text, N, A, D, P, H, end text—a reduced (electron-bearing) electron carrier. In this process, water molecules are also converted to oxygen gas—the oxygen we breathe!
  • The Calvin cycle, also called the light-independent reactions, takes place in the stroma and does not directly require light. Instead, the Calvin cycle uses start text, A, T, P, end text and start text, N, A, D, P, H, end text from the light-dependent reactions to fix carbon dioxide and produce three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules—which join up to form glucose.
Schematic of the light-dependent reactions and Calvin cycle and how they're connected.
The light-dependent reactions take place in the thylakoid membrane. They require light, and their net effect is to convert water molecules into oxygen, while producing ATP molecules—from ADP and Pi—and NADPH molecules—via reduction of NADP+.
ATP and NADPH are produced on the stroma side of the thylakoid membrane, where they can be used by the Calvin cycle.
The Calvin cycle takes place in the stroma and uses the ATP and NADPH from the light-dependent reactions to fix carbon dioxide, producing three-carbon sugars—glyceraldehyde-3-phosphate, or G3P, molecules.
The Calvin cycle converts ATP to ADP and Pi, and it converts NADPH to NADP+. The ADP, Pi, and NADP+ can be reused as substrates in the light reactions.
Image credit: modified from "Overview of photosynthesis: Figure 6" by OpenStax College, Biology, CC BY 3.0
Overall, the light-dependent reactions capture light energy and store it temporarily in the chemical forms of start text, A, T, P, end text and start text, N, A, D, P, H, end text. There, start text, A, T, P, end text is broken down to release energy, and start text, N, A, D, P, H, end text donates its electrons to convert carbon dioxide molecules into sugars. In the end, the energy that started out as light winds up trapped in the bonds of the sugars.

Photosynthesis vs. cellular respiration

At the level of the overall reactions, photosynthesis and cellular respiration are near-opposite processes. They differ only in the form of energy absorbed or released, as shown in the diagram below.
On a simplified level, photosynthesis and cellular respiration are opposite reactions of each other. In photosynthesis, solar energy is harvested as chemical energy in a process that converts water and carbon dioxide to glucose. Oxygen is released as a byproduct. In cellular respiration, oxygen is used to break down glucose, releasing chemical energy and heat in the process. Carbon dioxide and water are products of this reaction.
At the level of individual steps, photosynthesis isn't just cellular respiration run in reverse. Instead, as we'll see the rest of this section, photosynthesis takes place in its own unique series of steps. However, there are some notable similarities between photosynthesis and cellular respiration.
For instance, photosynthesis and cellular respiration both involve a series of redox reactions (reactions involving electron transfers). In cellular respiration, electrons flow from glucose to oxygen, forming water and releasing energy. In photosynthesis, they go in the opposite direction, starting in water and winding up in glucose—an energy-requiring process powered by light. Like cellular respiration, photosynthesis also uses an electron transport chain to make a start text, H, end text, start superscript, plus, end superscript concentration gradient, which drives start text, A, T, P, end text synthesis by chemiosmosis.
If those things don't sound familiar, though, don't worry! You don't need to know cellular respiration to understand photosynthesis. Just keep reading and watching, and you'll learn all the ins and outs of this life-sustaining process.

Want to join the conversation?

  • duskpin seedling style avatar for user Paarth Tara
    Okay, if the light dependent reactions can create the ATP itself, then why not just transport that ATP everywhere instead of forming Glucose then spending a lot of other time in transforming back that Glucose into ATP?
    (25 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user tyersome
      Excellent question.

      The major reasons that I know of:
      1) The high energy bonds in ATP are (by definition) unstable, so for long term storage of energy ATP is not a good choice.
      2) In many situations phosphate is a limiting nutrient, so needing to make more ATP could severely limit the plants ability to store energy.
      3) Fixed carbon (e.g. glucose) can be converted into other molecules the plant needs including:
      • cellulose for structure
      • lipids for long term energy storage, cell membranes, etc.
      • proteins for structure, catalysis, etc.
      (41 votes)
  • piceratops seed style avatar for user Zepeda, Denver
    what is hydrolysis
    (16 votes)
    Default Khan Academy avatar avatar for user
  • leafers ultimate style avatar for user Lydia M
    Why is the first photosystem depicted in photosynthesis diagrams called "photosystem II" and the second photosystem called "photosystem I"? Are the names arbitrary or do they tell us something about the nature of how the photosystems work?
    (8 votes)
    Default Khan Academy avatar avatar for user
    • primosaur tree style avatar for user Rick
      The reason for this is simply because Photosystem I was discovered first, and Photosystem II was discovered second. You're right, it is confusing because the Photosystem II process occurs first, followed by Photosystem I.
      (24 votes)
  • blobby green style avatar for user 18plaitay
    Why would you consider photosynthesis important ?
    (0 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Laurent
      Photosynthesis is extremely important! It is the process in plants that allows it to harness energy from sunlight and convert it into chemical energy that can be used by plants and other organisms. In fact all the energy we get from food is derived from the energy we get directly from plants or indirectly from animals that ate plants. Hence without the sun or plant's ability to carry out photosynthesis, there would be no energy to sustain most of the life on earth.
      (28 votes)
  • blobby green style avatar for user Sanglap Nahá
    The reactions occur without any dependence on light...so can it run during night time?...if so,in night time, the guard cells of the stomata close, so how can it take in carbon-dioxide to continue the cycle?...
    (4 votes)
    Default Khan Academy avatar avatar for user
    • leafers sapling style avatar for user Marianne
      Both reactions, the light-depended reaction and the Calvin's cycle OCCURS ONLY in the light (and out of color spectrum, mainly blue and red colors are used thus green reflected into your eye).
      1. Light-depended reaction gives you the NADPH
      2. You need NADPH in Calvin's cycle
      And you don't get the NADPH without light.
      EDIT after a comment brought up by Safwan: to be exact, The Calvin cycle needs light to start, but can continue for a while even without the light.
      (5 votes)
  • duskpin seedling style avatar for user Noelia Cano
    Wait, so:ATP=Three Phosphates. ADP=Two Phosphates. What if there is only one Phosphate?
    What would it be called? And what would happen if there was only one phosphate?
    (0 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user N Peterson
      ATP is Adenosine TriPhosphate, with three phosphates, and lots of energy stored in bonds.
      ADP is Adenosine DiPhosphate, with two phosphates, and some energy stored in bonds.
      AMP is Adenosine MonoPhosphate, with a single phosphate group. These do not have energy stored in the bonds between phosphates, as there is only one.
      Biological processes add/subtract phosphates, changing these into each other.
      A related molecule, cAMP (cyclic AMP), has a cyclic structure, and rather than an energy storage role, it functions as a messenger in cell signaling pathways.
      (10 votes)
  • leafers seedling style avatar for user Aatmann Patel
    What happens after the plants form glucose and oxygen? What happens to the oxygen when it is released?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • stelly blue style avatar for user Morgan
    What does the Pi stand for in the pictures describing light reactions and the Calvin cycle?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • winston baby style avatar for user sheikhyahya4299
    do all other biological molecules are derived from carbs.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Arjo Dasgupta
    In our school, we are doing an experiment where the rate of photosynthesis is being measured using different coloured waters. We mixed blue, red and green food colouring with water and then light was shone on them including clear water. Elodea plant was used. The data measured using an oxygen probe shows that the plant in clear water produces oxygen faster and green comes in second but blue and red produces oxygen slower. Why is that? Shouldn't red produce oxygen faster as red has the highest wavelength among other colour? Why does clear water produce oxygen fastest and why does green produce oxygen faster even though the colour of the plant is green?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • boggle blue style avatar for user Davin V Jones
      It is likely that your colored water is not purely filtering those individual wavelengths. As such, the green water is still allowing some blue and red wavelengths to pass, while the blue and red water is isolating more to only their ends of the spectrum.

      Since green still allows some of both blue and red to pass, chlorophyll from both ends of the spectrum still reacts with light and thus you have production on both ends. Whereas, with the blue or red water, primarily only the chlorophyll associated with those individual spectra can react.

      For a rough illustration, if you assume the light curves below and chlorophyll A absorbs blue light and chlorophyll B absorbs red. For the green curve, A and B get about 50% light. For the blue curve, the A is getting about 75% light and B is getting 0%, and vice versa for Red curve. So green is getting a weighted average of 50% reaction rate while Blue or Red get about 37.5% reaction rate.
      | A - B    |- A  B  |A    B-
      | / \ | \ | /
      | / \ | \ | /
      |/ \ | \ | /
      Green Blue Red
      (5 votes)