MET - Photosynthesis [LESSON]

Photosynthesis

All organisms, including humans, need energy to fuel the metabolic reactions of growth, development, and reproduction. But organisms can't use light energy directly for their metabolic needs. Instead, it must first be converted into chemical energy through the process of photosynthesis.

Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct.

 Image shows overall equation for photosynthesis: carbon dioxide & water as reactants & glucose and oxygen as products.

Note that there is a net input of solar energy in the overall reaction, so photosynthesis is an endergonic reaction.

Leaf Structure

The image shows the anatomy of the leaf.

Photosynthesis takes place in organelles called chloroplasts in the middle of the leaf. Small pores called stomata (singular form is stoma) are found on the surface of leaves in most plants, and they let carbon dioxide diffuse into the mesophyll layer and oxygen diffuse out. You may recall from previous modules that stomata also let water out during transpiration. Within each chloroplast, disc-like structures called thylakoids are arranged in piles like stacks of pancakes that are known as grana. The membrane of each thylakoid contains green pigments called chlorophylls that absorb light. The fluid-filled space around the grana is called the stroma, and the space inside the thylakoid discs is known as the thylakoid space.

Photosynthesis

Overall, photosynthesis has two major steps: the light-dependent reactions and the Calvin cycle. It is important to remember the overall connection between the two which is shown in the diagram below.

The overall cycle between the two major steps is shown in this image.

Notice that the light dependent reactions make ATP and NADPH (that’s the “loaded up”, or reduced, electron taxi in photosynthesis), and the Calvin cycle takes these products and uses them.  When ATP is used, it leaves the Calvin cycle as ADP + Pi and when NADPH is used, it leaves the Calvin cycle as NADP+ (the oxidized form of NADPH).

Why is there no ATP in the overall photosynthesis equation?

ATP is created in the light-dependent reactions (step 1) and consumed in the Calvin cycle (step 2), so there is NO net production of ATP in photosynthesis. The purpose is to make sugar, NOT energy.

Light and Excitation

Plants and other autotrophs have evolved to use the visible light spectrum (middle range of the electromagnetic spectrum). The pigments in the photosystems of the light-dependent reactions absorb light best in the 400-500nm (purple to blue) and 600-700nm (yellow to red) range. 

Why do we see green autotrophs as green?

Autotrophs do NOT absorb well in the green light range, thus the light is reflected back and we see the organism as green.

You may recall from chemistry that when atoms absorb energy, their electrons become “excited” and move to a higher energy state. This phenomenon is what drives photosynthesis. When a pigment molecule (say, chlorophyll) absorbs photons of light from the sun, the electrons become excited and move to a higher energy state, making them easier to be plucked from chlorophyll by another molecule.

Light-dependent Reactions

The light dependent reactions occur in the thylakoid membranes within the chloroplasts of cells. In order to capture light energy, the thylakoid membrane contains light harvesting centers called photosystems. Each photosystem consists of special chlorophyll molecules and accessory pigments bound to a group of proteins. When photons of light are absorbed by the pigment molecules, electrons of the molecules are excited and are transferred to a primary electron acceptor. Also, as energy is captured from sunlight, photolysis, the splitting of water, separates water into hydrogen ions, electrons, and oxygen. The hydrogen ions and electrons are used for energy storage, while the oxygen diffuses out of the chloroplast.

Image shows 2 photosystems embedded into thylakoid membrane & traces pathway of the light reactions of photosynthesis.

There are two photosystems important to the function of capturing light energy, photosystem II and photosystem I. Each photosystem has a unique reaction center based on the wavelength of light that is absorbed best within the molecules. These two photosystems work in tandem to capture the light energy necessary to synthesize ATP and NADPH.

Through the use of electron transport chains, both ATP and NADPH are generated as energy storage molecules. An electron transport chain between photosystem II and photosystem I creates the proton gradient necessary for chemiosmosis to phosphorylate ADP to ATP. Another electron transport chain after photosystem I uses the electron carrier NADP+ (nicotinamide adenine dinucleotide phosphate) to pick off the protons from photolysis to form NADPH. At the conclusion of the light dependent reactions, ATP and NADPH serve as chemical energy storage compounds needed to drive the Calvin cycle.

Please watch the Photosynthesis video below and take good notes to understand the light-dependent reactions of photosynthesis.

Now, practice what you have learned by placing the steps below in the correct order in the Light Reactions Ordering activity.

Calvin Cycle

Carbon fixation is the process of reducing inorganic carbon dioxide to form an organic compound. This is the second stage of photosynthesis. During the Calvin cycle, ATP and NADPH are used to convert carbon dioxide into stable energy storage molecules. This process requires carbon fixation and reduction. Within the stroma of the chloroplast, carbon dioxide is attached to ribulose bisphosphate (RuBP, a larger sugar) to enter the cycle. In a series of reactions, each molecule of RuBP is phosphorylated and reduced from NADPH and ATP to make G3P. Some of this G3P is later converted to glucose, while the remaining compounds are recycled to create more RuBP.

Watch the Photosynthesis Part 2 video below to learn more about the Calvin cycle. 

Photorespiration

This image depicts the two possible paths that rubisco can utilize

RuBP oxygenase-carboxylase (rubisco), a key enzyme in the Calvin cycle, is the molecular equivalent of a good friend with a bad habit. In the process of carbon fixation, rubisco incorporates carbon dioxide into an organic molecule during the first stage of the Calvin cycle. But rubisco also has a major flaw: instead of always using carbon dioxide (CO2) it sometimes picks up oxygen (O2). This side reaction initiates a pathway called photorespiration, which, rather than fixing carbon, actually leads to the creation of a waste product. Photorespiration therefore wastes a ton of cellular energy and the plant cannot survive.

Why do both oxygen and carbon dioxide fit into the active site of rubisco?

They are both very small, nonpolar molecules and this is a rare example of how the same enzyme can bind two substrates.

This image shows more detail about integration of oxygen into the Calvin cycle by rubisco.

Adaptations

Some plants have adaptations to minimize photorespiration. A "normal" plant—one that doesn't have photosynthetic adaptations to reduce photorespiration—is called a C3 plant. These plants will die in prolonged hot and dry weather. There are two major types of plants that have adaptations to get around this and live in extreme environments.

In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called bundle-sheath cells.

Instead of separating physically, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves.  It is stored as an organic acid until daylight can allow for photosynthesis to occur.

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