Written by tutor Jonathan D.
When we think of all the different sources of energy available to us, the Sun stands out to be the primary source of energy to our planet. Sun light, which is a form of electromagnetic energy, is essential for a series of reactions that allow plants (and other photoautotrophic organisms) to create and store sugar molecules for energy. This process is called photosynthesis.
Photosynthesis is the process where light energy is converted into a form of chemical energy that can be stored and used later on. In plants, the photosynthesis-driving organelles, known as chloroplasts, are able to absorb sunlight to make sugar (C6H12O6) and oxygen (O2) from carbon dioxide (CO2) and water (H2O). Photosynthesis can be broken down into two different reactions: light-dependent and light-independent reactions. The light-dependent reactions are the initial stages of photosynthesis that allow chloroplasts to store energy from sunlight. The light-independent reactions, which takes place after the light-dependent reactions, essentially converts the carbon dioxide molecules into sugar.
If you look at the molecular formula for this reaction, it is essentially the reverse reaction for cellular respiration! The simplified equations for photosynthesis and cellular respiration respectively are represented by the following formulas:
Photosynthesis: 6CO2 + 6H2O + energy (i.e. light) --> C6H12O6 + 6O2
Cellular respiration: C6H12O6 + 6O6 --> 6CO2 + 6H2O + energy
As you can see, these two processes are associated with one another. For instance, a photoautotroph (e.g. plants) undergoes photosynthesis during the day to store up an adequate amount of sugar. Whenever sunlight is not present, this stored sugar can then be used as a source of energy through cellular respiration. Now that you have a better understanding of this process, let’s look at the machinery that makes photosynthesis work.
Chloroplasts are able to absorb light energy by a green pigment called chlorophyll. Plant chlorophyll looks green due to its ability to absorb red and blue light, but NOT green light. Therefore, chloroplasts utilize the red and blue light for making chemical energy while green light is visible to our eyes since it is not utilized by the plant chloroplasts.
This organelle is enclosed by an envelope with two membranes—an outer and inner—that serves as a protective barrier. Within this envelope is a dense, aqueous fluid known as the stroma. This fluid, similarly to cytoplasm within the plasma membrane of cells, contains important chloroplast structures such as the thylakoids. The thylakoids are flattened membrane structures where photosynthesis actually takes place within the chloroplast. These sac-like structures usually form stacks that are referred to as grana (singular = granum).
Summary of Photosynthesis
Photosynthesis is the process where light energy is converted into chemical energy so that photosynthetic organisms can make food for themselves. This process takes place within chloroplasts, which contain a green pigment called chlorophyll. Photosynthesis consists of two parts: a light-dependent reaction that converts light energy into chemical energy and the light-independent reaction that converts CO2 to sugar.