How Cells Capture Energy

Photosynthesis and Chloroplasts

Using chlorophyll and other pigments, photosynthetic organisms capture light energy and then extract carbon from CO2, which they use to make sugars, releasing oxygen in the process as a by-product.

All cells require energy to carry out the functions of life. A photoautotroph, also referred to simply as an autotroph, is an organism that makes its own food using light energy. It does this through a process called photosynthesis, the process by which autotrophs convert light energy into chemical energy that is stored in organic compounds, such as sugars. Photoautotroph comes from Greek roots photo, meaning "light," auto, meaning "self," and troph, meaning "food." In photosynthesis, light is captured in special cells, where it is converted to chemical energy. This chemical energy is then used to power the production of sugars. Photoautotrophs use energy provided by light, as well as carbon dioxide (CO2) and water from the atmosphere, to produce carbohydrates. Oxygen is given off during this process, and water is created as a by-product. The carbohydrates can later be broken down, releasing the energy stored within them. Plants, algae, and certain types of protists and bacteria are photoautotrophs. Since they make their own food, photosynthetic organisms make up the base of food chains by serving as producers.

In general, the process of photosynthesis can be explained using the following simplified equation:
6CO2+6H2O+Light energyC6H12O6+6O2{6{\rm{CO}}_2+6{\rm {H_2 O}}+{\text{Light energy}}\rightarrow{\rm {C_6H_{12}O}}_6+6{\rm {O}}_2}
The six-carbon sugar produced, C6H12O6, is glucose. The direct product of photosynthesis is actually a three-carbon molecule used to make sugar rather than glucose itself. Since the process that builds glucose from the three-carbon molecule immediately follows photosynthesis, the entire reaction is most easily summarized by the equation.


Through the process of photosynthesis, photoautotrophs are able to convert light energy into chemical energy. During this energy conversion, carbon dioxide (6CO2) and water (H2O) are used to make glucose (C6H12O6) and the by-product oxygen (O2).
In the cell, photosynthesis takes place in a structure called a chloroplast, which is a membrane–bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. Chloroplasts are green because they contain chlorophyll, a green pigment used in photosynthesis. Chloroplasts function in similar ways, regardless of the cell type in which they are found. All green parts of plants contain chloroplasts, but the main sites of photosynthesis are the leaves. This explains why leaves are green while other parts of the plant may be other colors. Leaves contain cells known as mesophyll cells, which are found in the interior of the leaf. Leaves also contain stomata found on the exterior portion of the leaf. A mesophyll cell is a cell in a leaf that contains chloroplasts, where photosynthesis takes place, or where carbon fixation occurs in C4 plants. A stoma (plural, stomata) is a small opening mostly found on the undersides of leaves that allows for gas exchange between the plant and the external environment. Chloroplasts consist of two membranes (an outer membrane and an inner membrane) surrounding stroma, a fluid inside chloroplasts that contains bundles of membrane-bound sacs called thylakoids. A thylakoid is a membrane-bound sac inside the stroma that may be stacked into columns. A stack of thylakoid sacs where the light reactions of photosynthesis take place is a granum (plural, grana). The space within each thylakoid sac is called lumen.

Structure of a Chloroplast

The chloroplast is enclosed by an inner and outer membrane. This membrane-bound organelle is also the site of photosynthesis. The thylakoid membranes in chloroplasts exist as sacs within the stroma. Stacks of thylakoids, known as the grana, contain chlorophyll, which gives chloroplasts their green color.