Glossary: Photosynthesis

 

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Photosynthesis

During photosynthesis, light energy converts carbon dioxide and water (the reactants) into glucose and oxygen (the products).

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Photosynthesis

Photosynthesis is the process plants use to make their own food, which fuels all their activities. During photosynthesis, light energy from the sun converts groups of six carbon dioxide molecules and six water molecules (the reactants) into one glucose (sugar) molecule and six oxygen molecules (the products). 

Chemical Equation for Photosynthesis

Sunlight + 6 CO2 + 6 H2O → C6H12O6 + 6 O2

CO2 (carbon dioxide)

Carbon dioxide is a reactant of photosynthesis, and six carbon dioxide molecules are needed to produce one glucose molecule. Carbon dioxide molecules move from the surrounding air, entering a plant's leaf through the stomata in the epidermis. Once inside the leaf, these gas molecules move through the spongy mesophyll, entering the chloroplasts of palisade mesophyll cells. Within the stroma of the chloroplast, enzymes and the ATP, NADPH, and light energy harvested during the light-dependent reactions drive the light-independent reactions (also known as dark reactions or the Calvin cycle) that convert carbon dioxide into sugars.

H2O (water)

Water is a reactant of photosynthesis, and six water molecules are needed for each set of photosynthesis reactions. These water molecules move from the soil surrounding the plant, entering the roots through the root hairs and traveling through the xylem in the roots and stem to a leaf. Once inside the leaf, the water molecules move into the chloroplasts inside the palisade mesophyll cells. During the light-dependent reactions that occur in the thylakoids of the chloroplast, the water molecules are split to convert NADP+ to NADPH, to help drive the production of ATP, and to produce oxygen, a byproduct of photosynthesis.

C6H12O6 (glucose)

A glucose (sugar) molecule is the product of each light-independent reaction that occurs in the stroma of the chloroplast during photosynthesis. Glucose is made by joining two of the three-carbon sugar products of the Calvin cycle into a six-carbon sugar. Glucose can be made into larger sugars (sucrose) or carbohydrates such as starch and cellulose. Sugars exit the leaf through the phloem, traveling to the roots for storage or to other parts of the plant, where they're used as energy to fuel the plant's activities. 

O2 (oxygen)

Oxygen molecules are a byproduct of photosynthesis. They are produced during the light-dependent reactions that occur in the thylakoids of the chloroplast. This oxygen gas is released through the leaf's stomata into the surrounding air.

Photons

Photons are units of light energy from the sun that can power photosynthesis in plant cells. Within the chloroplast's thylakoid membranes, pigments—including chlorophyll—absorb photons, initiating the light-dependent reactions of photosynthesis.

 

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Dicot leaf

Leaves, and the structures they contain, play key roles in photosynthesis.

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Leaf

The epidermis is a single layer of epidermal cells that covers all the parts of plants, including roots, stems, leaves, and flowers. Unlike the other plant parts, leaves have two epidermal layers, the upper and lower epidermis, which surround the inner mesophyll. In the epidermis, small pores called stomata allow leaves to exchange gases with the surrounding air. During photosynthesis, carbon dioxide (a reactant) enters the leaf through the stomata and oxygen (a byproduct) exits through the stomata.

Phloem

Vascular bundles contain xylem and phloem—continuous tubes that transport water, nutrients, and other substances throughout the plant's roots, stem, and leaves. Positive hydrostatic pressure in the phloem moves dissolved sugars and organic compounds from the leaves downward to the stem and roots in a process called translocation.

Xylem

Vascular bundles contain xylem and phloem—continuous tubes that transport water, nutrients, and other substances throughout the plant's roots, stem, and leaves. The evaporation of water at the leaf's stomata (transpiration) creates negative hydrostatic pressure in the xylem that moves water and dissolved minerals from the roots upward to the stem and leaves.

Stomata

Stomata (singular: stoma) are small epidermal pores that allow gases and water vapor to move between interior leaf structures and the surrounding air. During photosynthesis, carbon dioxide (a reactant) enters the leaf through the stomata and oxygen (a byproduct) exits through the stomata.

Guard cells

Each stoma is surrounded by two guard cells, specialized epidermal parenchyma cells that open and close the pore. These guard cells regulate transpiration, photosynthesis, and respiration, facilitating gas exchange by opening the stomata and preventing excess water loss by closing them.

Palisade mesophyll

In the palisade mesophyll, parenchyma cells are tightly packed, and their shape is usually polyhedral, elongated, or lobed. This structure and the presence of chloroplasts facilitate photosynthesis.

Spongy mesophyll

In the spongy mesophyll, parenchyma cells are loosely arranged, with spaces between them, and their shape tends to be spherical or stellate. This structure allows the leaf to receive carbon dioxide from the air and to release oxygen and water vapor into the air.

Different reactions during photosynthesis take place in different parts of the chloroplast.

Chloroplast

Chloroplasts are specialized, membrane-bound organelles, located in the cytoplasm of plant cells, that carry out photosynthesis. Plant cells can have anywhere from one to a hundred chloroplasts, which move around the cell and can divide to replicate themselves. Surrounded by two membranes, chloroplasts contain DNA, ribosomes, enzymes, grana, and a thick fluid called stroma. Chloroplasts are also involved in the plant's synthesis of fatty acids and amino acids and in its immune response.

Outer membrane

Chloroplasts are surrounded by outer and inner phospholipid membranes, which are separated by a narrow intermembrane space. The outer membrane is the external layer, which maintains the chloroplast's shape and protects its inner structures.

Inner membrane

Chloroplasts are surrounded by outer and inner phospholipid membranes, which are separated by a narrow intermembrane space. The inner membrane surrounds the stroma, forming a selectively permeable barrier that allows molecules to move into and out of the chloroplast.

Thylakoids

Chloroplasts contain stacks of disc-shaped thylakoids, where the light-dependent reactions of photosynthesis occur. Each thylakoid consists of a thylakoid membrane and an inner lumen. The thylakoid membrane contains the pigment chlorophyll, which is stored in protein complexes called photosystems I and II. These chlorophyll molecules absorb light energy from the sun, which the cell converts into chemical energy through a series of chemical reactions.

Stroma

The stroma is the thick fluid that fills the space between the inner membrane and the grana of a chloroplast. The light-independent reactions (also known as dark reactions or the Calvin cycle) of photosynthesis occur in the stroma. These reactions require carbon dioxide molecules (reactants) from the air surrounding the plant, as well as ATP and NADPH, energy-storing molecules that are produced by the light-dependent reactions that occur in the thylakoids.

Granum

Each chloroplast has from ten to a hundred grana (sing. granum), connected to each other by lamellae. Each granum is a stack of disc-shaped thylakoids, where the light-dependent reactions of photosynthesis occur.

Light-dependent reactions

During the light-dependent reactions that occur in the thylakoids of the chloroplast, the water molecules are split to convert NADP+ to NADPH and to produce oxygen, a byproduct that is released through the leaf’s stomata into the surrounding air. Additionally, ATP is formed using ATP synthase, ADP, and a free phosphate.

Chlorophyll

The thylakoid membrane contains the pigment chlorophyll, which is stored in protein complexes called photosystems I and II. These chlorophyll molecules absorb light energy from the sun, which the cell converts into chemical energy through a series of chemical reactions.

Light-independent reactions (Calvin cycle)

In the light-independent reactions (also known as dark reactions or the Calvin cycle) that occur within the stroma of the chloroplast, enzymes, ATP, and NADPH drive the conversion of carbon dioxide into sugars. Three carbon dioxide molecules enter each round of the Calvin cycle, and it takes two rounds to produce a glucose molecule. Two rounds of the Calvin cycle will produce two G3Ps that the plant cell converts into one glucose molecule, or it can use them to make sucrose and other organic molecules. ADP and NADP+ exit the Calvin cycle and return to the thylakoids for the next set of light-dependent reactions.

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