8 Chapter 8

Learning Objectives

  1. Describe reactions vital to photosynthesis
  2. Identify cellular location, reactants and products for the two stages of photosynthesis
  3. Describe the energy cycle

Photosynthesis

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living things had sufficient energy to build new structures and achieve biodiversity. All organisms capable of photosynthesis are considered producers, as they are capable of generating carbohydrates from carbon dioxide. Producers support life on earth for consumers, either directly or indirectly.

Only certain organisms can perform photosynthesis and require chlorophyll, a specialized pigment to absorb certain wavelengths of the visible spectrum. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and releases oxygen as a byproduct. Eukaryotes, like plants and algae, have chlorophyll-containing organelles called chloroplasts to provide a location for photosynthetic reactions. The chlorophyll is embedded in the thylakoid seen in the image below. In prokaryotes like cyanobacteria, there are no chloroplasts, so the process is less localized.

This illustration shows a chloroplast, which has an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Inside the inner membrane are flat, pancake-like structures called thylakoids. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoid is called the thylakoid lumen.

Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of chlorophyll-embedded thylakoids called grana form a third membrane layer.

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the Calvin cycle. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the Calvin cycle reactions, that stored chemical energy drives the assembly of sugar molecules from carbon dioxide.

This illustration shows a chloroplast with an outer membrane, an inner membrane, and stacks of membranes inside the inner membrane called thylakoids. The entire stack is called a granum. In the light reactions, energy from sunlight is converted into chemical energy in the form of A T P and N A D P H. In the process, water is used and oxygen is produced. Energy from A T P and N A D P H are used to power the Calvin cycle, which produces G A 3 P from carbon dioxide. A T P is broken down to A D P and Pi, and N A D P H is oxidized to N A D P superscript plus sign baseline. The cycle is completed when the light reactions convert these molecules back into A T P and N A D P H.

Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light reactions take place in the thylakoid membrane, using light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make G3P from CO2. Credit: Rao, A., Ryan, K., Fletcher, S., Hawkins, A. and Tag, A. Texas A&M University.

The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the pigments to initiate photosynthesis. The energy travels to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of hydrogen ions. The hydrogen ions flow through ATP synthase to form molecules of ATP, which are used for the formation of sugar molecules in Calvin cycle.

Light reactions convert light energy to chemical energy. This happens in the chloroplast across the thylakoid membrane. Water is split to provide hydrogen needed for carbohydrate construction. Hydrogens trickle through ATP synthase to join ADP + Pi generating ATP.  Oxygen atoms from water splitting form pairs (O2) which diffuse as a byproduct into the air. ATP generated will be contributed to the Calvin cycle. NADP+ delivers hydrogen to the Calvin cycle.

This illustration shows that A T P and N A D P H produced in the light reactions are used in the Calvin cycle to make sugar.

Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place. Credit: Rao, A., Ryan, K., Tag, A., Fletcher, S. and Hawkins, A. Department of Biology, Texas A&M University.

Using energy carriers formed in the first steps of photosynthesis, Calvin cycle converts CO2 to carbohydrates in a series of enzyme-driven reactions. An enzyme, RuBisCO, catalyzes a reaction with CO2 and another organic compound, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which then reacts with more CO2. Photosynthesis forms an energy cycle with the process of cellular respiration. Because plants contain both chloroplasts and mitochondria, they rely upon both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites.

Stomata are tiny openings, or leaf pores, that open and close to allow for gas exchange. They are found on the surface of leaves. Carbon dioxide will diffuse into leaf tissue and oxygen out. Some plants have evolved alternate methods of photosynthesis called C3 and C4 photosynthesis and carassulacean acid metabolism (CAM). These alternate methods allow the plants to survive in dry, hot climates. CAM plants keep stomata open at night. Chloroplasts can have symbiotic relationships with other organisms.

The energy cycle supports life on the surface of the earth, by the relationship of photosynthesis and cellular respiration. Photosynthesis absorbs light energy to build carbohydrates, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria. Both processes use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in biological harmony to allow organisms to access the sun’s energy.

Exercises

Reading Quiz

 

Key Takeaways

  1. Photosynthesis includes light-dependent reactions and the Calvin cycle.
  2. The reactants of photosynthesis are carbon dioxide and water. The products are glucose and oxygen. Light is required for the reactions to begin.
  3. Light-dependent reactions occur across the thylakoid membrane of the chloroplast, producing ATP. Calvin cycle uses CO2 to produce sugars in the stroma and requires ATP.
  4. Photosynthesis consumes carbon dioxide and produces oxygen, while cellular respiration in the presence of oxygen does the opposite.
Biology-2e. (2018). Houston, RX: website: OpenStax Book title: Biology 2e .

License

Icon for the Creative Commons Attribution 4.0 International License

Introductory Biology Copyright © 2023 by Mona Easterling is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book