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Carbon Fixation in Photosynthesis: Definition & Reactions

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  • 0:00 What is Carbon Fixation?
  • 0:50 The Calvin Cycle
  • 2:40 Photosynthesis
  • 4:00 C3 vs. C4 Cycle
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson, you will discover what carbon fixation means, when it occurs and what enzyme is involved. The lesson will discuss where the process of carbon fixation falls in photosynthesis as well as give a brief idea of a process by which carbon can be freed from fixation.

What is Carbon Fixation?

What does it mean to fix something? It can mean to repair something that's broken, like fixing a car or a bike. But that's not the only meaning. You can fix something in place. If you take a thumb tack or push pin and put a piece of paper on a bulletin board it 'fixes' the piece of paper so that it won't fall off.

Carbon Fixation is a process that involves fixing a carbon into place. Just like you take that piece of paper and tack it to the bulletin board so that it won't float around or end up on the floor, plants want to take carbon dioxide (CO2) out of the air and tack it to carbohydrates so it's not floating around all over the place. The process of taking inorganic carbon (usually CO2) and tacking it to an organic molecule (usually a carbohydrate) is known as carbon fixation.

Carbon fixation
Carbon Fixation fixes inorganic carbon to organic molecules

The Calvin Cycle

So, how exactly do plants go about taking the inorganic CO2 and tacking it onto a carbohydrate? This process of carbon fixation is actually the first step in the Calvin Cycle, also known as the Carbon Fixation Cycle or C3 cycle.

The first thing the cell needs is a molecule called ribulose-1,5-bisphosphate or RuBP for short.

RuBP structure.
RuBP structure

The next thing the cell needs is a special enzyme. Enzymes are proteins that speed up chemical reactions. The enzyme the cell needs for carbon fixation is called ribulose-1,5-bisphosphate carboxylase. Ribulose-1,5-bisphosphate carboxylase is quite a mouthful so it's often shortened to RuBisCO.

RuBisCO is an enzyme.
Ribulose-1,5-bisphosphate carboxylase (RuBisCO) is an enzyme

RuBisCO is responsible for fixing the carbon, rather like you are responsible for tacking the paper onto the bulletin board. However, this enzyme works so slowly (in cellular terms) that plants need a whole lot of it in order to fix enough carbon for its needs. Because plants contain so much of this enzyme, it could be in the Guinness Book of World Records as Earth's Most Abundant Enzyme!

With RuBP, CO2 and RuBisCO the plant cell is ready to fix carbon. RuBisCO takes the CO2 and adds it to the RuBP, creating a temporary intermediate molecule.

RuBisCO takes RuBP and CO2 and forms an intermediate.
Carbon fixation: RuBP and CO2 are temporarily joined as one.

After the intermediate is formed, the whole molecule is separated into two 3-carbon molecules (hence the name C3 cycle) called 3-phosphoglycerate (3PG). An equation for the reaction might look something like this: RuBP + CO2 = 2(3PG).

Carbon fixation forms 2 (3PG) molecules.
Carbon fixation: Structures of RuBP, CO2, intermediate and 3-PG

The 3PG is made into glyceraldehyde 3-phosphate (Ga3-P), which is used by the plant to produce sugar or starch, or to be cycled back to make RuBP, which again allows for carbon fixation.

3PG can form Ga3-P
3-PG can be used to form Ga3-P.

Photosynthesis

So where does photosynthesis fit into all of this? Photosynthesis takes place inside organelles called chloroplasts and is the process whereby plants convert sunlight into energy and oxygen. The inside of the chloroplast is called the stroma.

Basic Chloroplast Structure

Photosynthesis takes place in the chloroplast and has two phases: a light reaction and a light-independent reaction (LIR). During the light reaction, water is used in conjunction with a very excited electron to make ATP (the molecule animals and plants use for energy) with the waste product oxygen. Yes, oxygen is a waste product to plants, like CO2 is our waste product when breathing. Another molecule: NADP+ is converted to NADPH because it ultimately accepts excited electrons.

In the LIR, the cell uses the produced ATP and NADPH to make organic molecules like sugar and starch which can be used by the cell for energy and growth. The Calvin Cycle, of which carbon fixation is the first step, also occurs as part of the LIR and takes place in the stroma. ATP, CO2, NADPH, and H2O are all utilized in the LIR. Three molecules of carbon dioxide are required for a full Calvin Cycle.

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