If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

ChemWiki: The Dynamic Chemistry E-textbook > Biological Chemistry > Photosynthesis > Photosynthesis overview

Copyright (c) 2006-2014 MindTouch Inc.

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa

Photosynthesis overview

Photosynthesis is a process that occurs in plants, algae, and some bacteria. These photosynthetic organisms (called autotrophs)use the sun's energy to convert carbon dioxide (CO2) into organic compounds, such as carbohydrates. An example of carbohydrates would be simple sugars such as glucose, mannose, or galactose.


Photosynthesis by these organisms is essential for life on Earth. They take in CO2 ,which are waste products from animals and humans, and create oxygen so that we can breathe. However, there are certain bacteria that perform anoxygenic photosynthesis, meaning they consume CO2 but do not release O2.


Essentially, photosynthesis is the opposite of cellular respiration, which is carried out through glycolysis, the Krebs cycle, and the electron transport chain (ETC). All processes of photosynthesis are carried out in the chloroplasts of plants.


Figure 1. The photosynthesis equation. Can be found at http://commons.wikimedia.org/wiki/Fi...C3%ADntese.jpg

The process

The process of photosynthesis occurs through:

  1. Light reactions, which contain Photosystem I and Photosystem II. During the light reactions, energy from light is used to make ATP and NADPH, which are used to provide energy for the making of glucose, for exmple. During the dark reactions, carbon is synthesized into glucose, sucrose, and starch.
  2. Dark reactions occurs through the Calvin-Benson-Bassham cycle, which results in CO2 fixation.

Where Photosynthesis occurs: The Chloroplast


Figure 2. The parts of a chloroplast. Used with permission from Wikipedia Commons.

The chloroplasts are organelles in plants that harvest light energy to produce ATP and fix carbon in eukaryotic photosynthetic cells. Chloroplasts that exist in green plants are usually globular or discoid. They have a dual membrane system. Important features of the chloroplast include:

  1. Chlorophyll: Chlorophyll is a pigment that absorbs light in the blue and red wavelengths, which appears green to our eyes.The primary types are chlorophyll a and chlorophyll b.  
  2. Thylakoid membranes: This is where the chlorophyll, which are carriers for electron transport and are the required components for synthesis of ATP and NADPH, are stored.
  3. Thylakoids: Essentially, they are flattened sacs made up of an extensive network of membranes.
  4. Stroma: This is the fluid matrix of the chloroplast. It is analogous to the cytoplasm of a cell. It contains most of the necessary enzymes to carry out the dark reactions.
  5. Grana: These are stacks of thylakoids. They look like poker chips stacked up. (Sing. Granum).
  6. Lumen: This is the inside of a thylakoid.
  7. Lamella: This is the linkage between one thylakoid in a granum to another. (Pl. Lamellae.)
  8. Photosystems: These are light-capturing units of the thylakoid membrane.

Photosynthetic prokaryotes such as cyanobacteria do not have chloroplasts but they do have photosynthetic apparati bound to their plasma membrane. In the chloroplasts, electrons flow from H2O, through several electron acceptors, to NADP+, which serves as the final electron acceptor. This entire process requires energy, which is provided by the sun.

Photons and Light

Quantities of light are defined in terms of photons, which are particles of light. They are represented by the symbol hv. When photons strike the molecules and are absorbed, the electrons of those molecules get excited to a higher energy level. When the light source is removed, the electrons may return to their original states, give off energy as heat or light, or be transferred to other molecules. Visible light is the region of importance in photosynthesis. The range is from 350 to 800 nm (nanometers). The energy of light increases with shorter wavelengths and decreases with longer wavelengths.


Figure 3. The electromagnetic spectrum. Used with permission. Can be found at http://commons.wikimedia.org/wiki/File:Electromagnetic-Spectrum.png

Primary Pigments

In the thylakoid membranes of photosynthetic cells, there are light-absorbing molecules called pigments. Green pigments are in a class called chlorophyll. Chlorophylls a and b are found in most green plants. Bacteriochlorophyll is found in photosynthetic bacteria.  


Figure 4. Structure of chlorophyll a, chlorophyll b, and bacteriochlorophyll, respectively. Used with permission from Wikipedia Commons.

Secondary Pigments

Photosynthetic cells also contain secondary, or accessory, pigments, which absorb light where chlorophyll is not as useful. There are two types of secondary pigments: carotenoids and phycobilins. All carotenoids have 40 carbons and absorb in the range of 400 to 500 nm. That is why their colors are red, orange, and yellow. Two categories that represent carotenoids are xanthophylls and beta-carotenes. Phycobilins absorb in the range of 550 to 630 nm.


Figure 5. Example: Zeaxanthin, a xanthophyll which is one of the carotenoids.


  1. Zubay, Geoffrey. Biochemistry. New York: Macmillan Publishing Company, 1988.        


1.  True or False: Sucrose is a direct product of photosynthesis.

2. Chlorophyll can be found in the:

a) Grana

b) Lumen

c) Stroma

d) Thylakoid membrane

e) Outer membrane

3. Photosynthesis can be observed in the region of:

a) Ultraviolet light

b) Infrared

c) Visible light

d) Gamma rays

e) Radio waves

4. True or False: Chlorophyll a, chlorophyll b and bacteriochlorophyll are found in green plants.

5. Fill in the blank: The color of carotenoids is usually _____, _____, or _____.


1. False

2. d

3. c

4. False

5. red, orange, yellow


  • Tiffany Lui, University of California, Davis

Last modified
08:15, 2 Oct 2013



(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use