Two Types of Photosynthesis

Last week, we learned about the limiting factors of photosynthesis. Today I want to expand on this topic and discuss two common types of photosynthesis: light-dependent reactions and light-independent reactions. As you can already tell from the names, light-dependent photosynthesis happens in the presence of sunlight, whereas light-independent photosynthesis occurs without photons. We will look at these two concepts in greater detail.

Light-Dependent Photosynthesis

Overview of light-dependent photosynthesis

Light-dependent reactions begin when chlorophyll in photosystem II absorbs light (photoactivation). The electrons carrying energy is passed to chlorophyll molecules at the reaction center. Here, electrons are raised to a higher level. Consequently, excited electrons are produced, which are passed from carrier to carrier along the electron transport chain. Near the thylakoid membrane, different concentrations of protons form a gradient (chemiosmosis). ATP is produced when electrons pass through this gradient by ATP synthase. Some of this ATP is used during light-independent photosynthesis.

Light-Independent Photosynthesis

Overview of Light-Independent Photosynthesis (Calvin cycle)

Light-independent photosynthetic reactions take place in the stroma of the chloroplast. The main purpose is to produce carbohydrates. In this process, carbon dioxide is fixed, or added, to ribulose bisphosphate (RuBP) by RuBP carboxylase. This forms an unstable six-carbon compound because RuBP (a five-carbon compound) is put together with carbon dioxide (an organic material with one carbon molecule). This six-carbon compound splits into two molecules of glycerate-3-phosphate (GP). Next, ATP and NADPH produced in the light-dependent reaction are used. ATP provides the energy and NADPH provides hydrogen needed to reduce GP into triose phosphate (TP). Some of the three-carbon sugars form hexose sugars, while others are used to making more RuBP. This repeating cycle is called the Calvin cycle.

Works Cited

“Light-Dependent Reactions.” Wikipedia, Wikimedia Foundation, 18 Dec. 2018, en.wikipedia.org/wiki/Light-dependent_reactions.