Cellular Biology

Photosynthesis



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The term 'photosynthesis' is taught simply to young children as something that happens in a plant when there is sunshine that makes energy. As children progress through their school years, you can see their intellectual progress just by asking, "What is photosynthesis?" Their answers will undoubtedly grow more complex with each passing year.

DEFINITION


So what IS photosynthesis? Campbell Reece Biology 5th Edition defines it as the conversion of light energy to chemical energy that is stored in glucose or other organic compounds, and this process occurs in plants, algae, and certain prokaryotes. The generic formula for photo synthesis is:

Carbon dioxide + Water + Light -> Glucose + Oxygen + Water

STRUCTURE OF PLANT LEAVES

To understand the process of photosynthesis, you must first understand the structure where photosynthesis takes place: the leaf. A leaf is composed of several layers, the first being the cuticle, a wax layer on the surface that prevents UV damage and excess water uptake and loss. Next is the upper epidermis, a layer of cells that seal off the top layer and are thin enough to allow light through to the next layer, the palisade mesophyll. The palisade mesophyll is contains numerous chloroplasts, double membrane organelles that are the sites of photosynthesis. Chloroplasts are composed of an outer and inner membrane, and within the inner membrane are numerous membranous sacs called thylakoids, arranged in stacks collectively called grana. Thus, to be specific, photosynthesis occurs in the plant's leaf's palisade mesophyll's chloroplasts' thylakoid's membrane! Below the palisade mesophyll is another layer called the spongey mesophyll. The spaces between cells in this layer allows for gas exchanges to occur. Gases leave the leaf through the last layer, the lower epidermis. The lower epidermis is equipped with special structures called stomata, which open when water content in the cells are high, allowing gases to diffuse out.

Photosynthesis is divided into 2 parts: light dependent and light independent reactions.

LIGHT DEPENDENT REACTIONS

When a photon of light strikes the light gathering antenna complex of the light dependent reactions' photosystem II, an electron is excited to a higher energy level. This electron came from the chlorophyll A molecule and is then captured by the electron acceptor. Chlorophyll A, now lacking one electron, has become a strong oxidizing agent. An enzyme removes an electron from water within the cell and gives it to chlorophyll a. This causes water to split into 2 hydrogen atoms and oxygenm which is released as the air we breath. Meanwhile, the photoexcited electron passes down a chain of electron carriers plastoquinone, cytochrome complex. and plastocyanin. This process causes a release of energy that is harnessed by the thylakoid membrane to make ATP; the energy is used to pump H+ ions into the thylakoid space, and when these ions diffuse back out, they pass through ATP synthase.

The original photoexcited electron fills an electron "hole" in another pigment, chlorophyll b. It is then driven to another electron acceptor, passed on to another carrier, and is transferred to NADP+ to make NADPH and H+.

So at the end of the light dependent reactions, the products made are 1 molecule of ATP and 1 NADPH and H+

LIGHT INDEPENDENT REACTIONS aka THE CALVIN CYCLE

In this second part of photosynthesis, an enzyme called ribulose bisphosphate carboxylase (RuBisCo) catalyzes a reaction between atmospheric CO2 and ribulose bisphosphate, creating the molecule 3-phosphoglycerate. ATP is invested, making 1,3-bisphosphoglycerate. NADPH and H+ reduce this molecule to make G3P. G3P is analogous to a Lego building block; the cell can use it to assemble simple sugars, lipids, polysaccharides, or amino acids. G3P also continues in the Calvin Cycle; ATP is invested, changing it to ribulose bisphosphate and the entire cycle turns again.

OVERALL, to make one glucose molecule, it requires:

6 turns of the Calvin Cycle

18 ATP molecules

12 NADPH + H+

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