Photosynthesis

Photosynthesis

Plants are capable of converting inorganic substances (carbon dioxide and water) into organic sugar.

Biology

Keywords

photosynthesis, light phase, dark phase, chloroplast, catabolic process, autotróf, leaf, light, sunlight, oxygen, organic material, carbon dioxide, glucose, solar energy, water, dextrose, oxygen-production, carbon fixation, inner membrane, granum, thylakoid, matrix, Photosystem II, Photosystem I, photosynthetic pigments, ATP, ATPase, electron transport chain, glyceric acid-3-phosphate, glyceraldehyde 3-phosphate, pentose biphosphate, energy transformation, cycle, photon, atmospheric gases, carbohydrate, Sun, metabolism, plant, biochemistry, biology, _javasolt

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Scenes

The principle of photosynthesis

  • CO₂ - It is an inorganic molecule, from which the plant produces an organic molecule, that is, sugar. Plants are autotrophic organisms; they are capable of converting inorganic substances into organic substances. Heterotrophic organisms (animals and fungi) are not capable of this.
  • O₂ - It is released as a by-product during photosynthesis. On Earth, the oxygen needed by heterotrophic organisms is produced by photosynthesis.
  • light - Its particles are called photons. Plants use the energy of photons to produce organic sugar from inorganic CO₂.
  • C₆H₁₂O₆ - Glucose (also known as dextrose). Plants produce it from carbon dioxide using the energy of light.
  • H₂O - The plant absorbs it from the soil. During photosynthesis, its molecules split into oxygen, protons (H⁺) and electrons (e⁻).

Structure of a leaf

  • Vascular bundles: Xylem - It transports water and mineral salts. During photosynthesis, water molecules split into oxygen, protons (H⁺) and electrons (e⁻).
  • Vascular bundles: Phloem - It transports organic materials dissolved in water. During photosynthesis the sugars produced are transported to other parts of the plant by the phloem.
  • stoma - CO₂, necessary for photosynthesis in the assimilation tissue enters the leaf through the stoma. The oxygen produced is also released through it. Plants can close these openings to avoid drying out by evaporation
  • assimilation tissue (mesophyll) - Its cells contain a large number of chloroplasts, in which photosynthesis takes place. Its top layer consists of vertically elongated cells, while the bottom layer has a spongy texture.
  • epidermis - It consists of one layer of cells. Its cells (with the exception of the guard cells of the stomata) do not contain chloroplasts. It serves to protect the plant, and to maintain contact with the environment through the stomata.

Photosynthesis

  • Vascular bundles: Xylem - It transports water and mineral salts. During photosynthesis, water molecules split into oxygen, protons (H⁺) and electrons (e⁻).
  • Vascular bundles: Phloem - It transports organic materials dissolved in water. During photosynthesis the sugars produced are transported to other parts of the plant by the phloem.
  • stoma - CO₂, necessary for photosynthesis in the assimilation tissue enters the leaf through the stoma. The oxygen produced is also released through it. Plants can close these openings to avoid drying out by evaporation
  • mesophyll cell - It contains a large amount of chloroplasts, in which photosynthesis takes place.
  • CO₂ - It is an inorganic molecule, from which the plant produces an organic molecule, that is, sugar. Plants are autotrophic organisms; they are capable of converting inorganic substances into organic substances. Heterotrophic organisms (animals and fungi) are not capable of this.
  • O₂ - It is released as a by-product during photosynthesis. On Earth, the oxygen needed by heterotrophic organisms is produced by photosynthesis.
  • light - Its particles are called photons. Plants use the energy of photons to produce organic sugar from inorganic CO₂.
  • C₆H₁₂O₆ - Glucose (also known as dextrose). Plants produce it from carbon dioxide using the energy of light.
  • H₂O - The plant absorbs it from the soil. During photosynthesis, its molecules split into oxygen, protons (H⁺) and electrons (e⁻).

Cell

  • Golgi apparatus - It plays an important role in processing proteins.
  • endoplasmatic reticulum - A complex, interconnected network of membrane vesicles inside the cell. It plays an important role in protein synthesis, protein processing, lipid synthesis and the breaking down of certain substances.
  • vesicle - Substances within the cell are transported wrapped in membrane bubbles. One type of vesicles are the lysosomes, in which certain substances are digested and waste is broken down.
  • cytoplasm
  • vacuole - A bubble within the cell, filled with nuclear sap. It plays an important role in maintaining internal hydrostatic pressure or turgor within the cell, storing minerals and removing waste.
  • chloroplast - Photosynthesis takes place in it: the plant uses solar energy to produce sugar from carbon dioxide.
  • cell wall - It is composed of cellulose and serves to protect the cell, maintain its shape and make plant tissues firm.
  • nucleus - It is made up of chromatin, a combination of DNA and proteins. The cells of animals, plants and fungi are eukaryotes, that is, they contain nuclei. Prokaryote cells (bacteria) do not have nuclei, their DNA is found in the cytoplasm.
  • cell membrane - A lipid membrane that encloses the cell.
  • cytoskeleton - It plays important roles in the positioning and movement of vesicles and organelles, and provides animal cells - which do not have cell walls - with structure and shape.
  • mitochondrion - The cell´s power station: it produces ATP by breaking down organic molecules. ATP is the molecule of the energy transfer of cells.

Light phase

  • chloroplast - Photosynthesis (the production of glucose from carbon dioxide, using solar energy) takes place here. It has a double membrane; the internal membrane contains the enzymes necessary for the photosynthesis.
  • internal membrane - Thylakoid discs are formed by the invagination (or folding-in) of this membrane. They contain the key enzymes of the light phase of photosynthesis. Thylakoid discs form stacks, called grana.
  • granum
  • thylakoid
  • matrix
  • thylakoid membrane - It contains the key enzymes of the light phase of photosynthesis.
  • thylakoid space (lumen)
  • Photosystem II - It consists of proteins and light-absorbing pigments. Its absorption maximum is 680 nm. Its pigments are chlorophyll a, chlorophyll b and xanthophyll. The central pigment in its reaction centre is chlorophyll a. When chlorophyll a absorbs a photon, it enters an excited state and releases an electron, which enters the electron transport system.
  • Photosystem I - It consists of proteins and light-absorbing pigments. Its absorption maximum is 700 nm. Its pigments are chlorophyll a, chlorophyll b and carotene. The central pigment in its reaction centre is chlorophyll a. When chlorophyll a absorbs a photon, it enters an excited state and releases an electron. The Photosystem I replaces this electron with one accepted from the electron transport chain.
  • e⁻
  • H₂O - The plant absorbs it from the soil. During photosynthesis, its molecules split into oxygen, protons (H⁺) and electrons (e⁻).
  • O
  • H⁺
  • O₂ - It is released as a by-product during photosynthesis. On Earth, the oxygen needed by heterotrophic organisms is produced by photosynthesis.
  • PQ - Plastoquinone. It transports the electrons released by the Photosystem II to the cytochrome complex.
  • cyt - Cytochrome complex. There are iron-containing proteins in it. It accepts electrons from the PQ complex and transfers them to the plastocyanin. Meanwhile, it pumps hydride ions through the membrane into the thylakoid lumen.
  • PC - Plastocyanin. It accepts electrons from the cytochrome complex and tranfers them to Photosystem I.
  • Fd - Ferredoxin. It accepts electrons from the Photosystem I and transfers them to the FNR molecule.
  • FNR - Ferredoxin NADP reductase. It transfers electrons between the ferredoxin and the NADP, that is, it reduces the NADP.
  • phosphate
  • ADP
  • ATP - It forms by ADP and phosphate joining together. It is the main energy-supply molecule of cells. Organic sugar is produced from inorganic carbon dioxide, using ATP.
  • NADP - It is reduced to NADPH by accepting an electron (e⁻) from the FNR and a proton (H⁺) passing through the ATPase.
  • NADPH
  • ATPase - Enzyme protein that produces ATP. The protons (H⁺ ions) pass from the inner side of the thylakoid membrane to the external side through the ATPase. The protons pass from the inside to the outside due to the high proton concentration and the excess positive charge. While they pass through the ATPase, energy is released which is used in the production of ATP.
  • electron transport chain - The electrons (e⁻) excited by the Photosystem II migrate to the Photosystem I via the electron transport chain. Meanwhile protons pass through the membrane and accumulate inside the thylakoid.
  • force driving H⁺ ions

Dark phase

  • ATP
  • ADP
  • NADPH
  • NADP
  • 5C - A sugar molecule containing 5 carbon molecules (pentose bisphosphate).
  • CO₂ - Carbon dioxide is an inorganic molecule, from which the plant produces an organic molecule, that is, sugar. It increases the number of carbon atoms of pentose. The enzyme that catalyses carbon fixation (RuBisCo) is the key enzyme of dark reactions.
  • 3C
  • 3C - A molecule containing 3 carbon atoms (glyceraldehyde 3-phosphate).
  • 6C (Glucose) - The product of photosynthesis, it is formed from a five-carbon sugar molecule and an inorganic carbon dioxide molecule containing 1 carbon atom. The plant uses glucose in its further metabolic processes for starch synthesis or in their digestive processes for the production of ATP.
  • CO₂ fixation, formation of glyceric acid-3-phosphate - The key reaction of the dark phase. Here, the inorganic carbon dioxide is incorporated into the organic sugar molecule. The essence of autotrophic processes is that organic substances are formed from inorganic material. The number of carbon atoms per molecule increases from 5 to 6; the result of this reaction is two three-carbon glyceric acid-3-phosphate molecules. The catalyst of this reaction is the RuBisCO enzyme.
  • Formation of glyceric acid-1,3-diphosphate - The three-carbon glyceric acid-3-phosphate molecule is converted into a glyceric acid-1,3-diphosphate molecule, using ATP.
  • Formation of glyceraldehyde 3-phosphate - The three-carbon glyceric acid-1,3-diphosphate molecule is also converted into a three-carbon glyceraldehyde 3-phosphate molecule. The reaction uses NADPH; the molecule releases inorganic phosphate. (This is not shown in the animation, for simplicity.)
  • Release of glycerinaldehyde-3-phosphate from the cycle - One of the six glycerinaldehyde-3-phosphate molecules is released from the cycle and is used by the cell in the formation of glucose.
  • Formation of ribulose-1,5-diphosphate - Three-carbon glycerinaldehyde-3-phosphate molecules are converted into a five-carbon ribulose-1,5-diphosphate (a pentose-bisphosphate) in several steps, in reactions catalysed by enzymes, with the use of ATP. This phase is also called ribulose-1,5-bisphosphate regeneration. The cycle starts all over again.

Artificial leaf

  • nitride semiconductor - It is a cheap and widely used semiconductor. It splits water molecules using the energy of light, which is the equivalent of the light phase of photosynthesis.
  • metal catalyst - It catalyses the reduction of carbon dioxide, which corresponds to the dark phase of photosynthesis. It produces organic material (formic acid) from carbon dioxide.
  • H₂O
  • O₂
  • H⁺
  • e⁻
  • CO₂
  • HCOOH (formic acid)

Animation

Narration

During photosynthesis, plants produce organic material, glucose, from inorganic material, carbon dioxide, using the energy of light. Oxygen is also formed in this process.

Photosynthesis takes place in the green parts of plants, that is, in the leaves, and often in the soft stem. The green colour of plants results from the large amount of chloroplasts in the cells of the assimilation tissue. These chloroplasts are where photosynthesis takes place.

Chloroplasts have a double membrane. The internal membrane forms the disc-like thylakoids, which form stacked membranous structures called grana. The thylakoid membrane contains the key enzymes for the light phase of photosynthesis.

The most important of these are the two photosystems and the electron transport chain between them.
The photosystems contain protein-bound light-absorbing pigments, the most important being chlorophyll.
The central chlorophyll-a molecules of Photosystem II are excited by photons and release electrons, which enter the electron transport system.
The oxidised, electron-deficient chlorophyll replaces its missing electrons from water molecules, that is, it splits water. The oxygen atoms in water molecules combine to form molecular oxygen, while protons accumulate inside the membrane.
The first member of the electron transport chain is plastoquinone, which transfers the electrons to the cytochrome complex. Cytochrome is an iron-containing protein, which transfers electrons to the Plastocyanin while pumping more protons into the thylakoid lumen.
The electrons are transferred to Photosystem I from the electron transport chain. The central chlorophyll molecule of Photosystem I is in an electron-deficient state, since it has previously released electrons, being excited by photons. The electrons are then transferred to the ferredoxin NADP reductase by ferredoxin molecules.
In the light phase, protons accumulate on the inside, that is, the proton concentration of the thylakoid lumen increases and thus becomes positively charged. This creates an outward driving force. Protons pass to the outside through the ATPase, while energy is released, since the system enters a lower-energy state from a higher-energy state due to the equalisation of charge and concentration. The energy released is used in the production of ATP. The protons and electrons released are accepted by the NADP, which converts into NADPH.
To sum up, the energy of the photons causes an unequal distribution of protons. This creates a driving force, which is used for the production of ATP.

The reactions of the dark phase are light-independent. In this phase carbon dioxide is incorporated into an organic compound using the energy of the ATP and the hydrogen ions of the NADPH produced in the light phase.
Let's start with 3 five-carbon sugar molecules. They have 15 carbon atoms altogether. An enzyme protein incorporates 1 carbon dioxide molecule into each sugar molecule, while the products split and 6 three-carbon molecules are formed, with a total of 18 carbon atoms. Then, by using 1 NADPH and 1 ATP for each molecule, 6 glyceraldehyde 3-phosphate molecules are formed. One of these quits the cycle, while the others convert back to 3 five-carbon sugar molecules using 3 ATPs, and the cycle starts all over again. That is, by using ATP and NADPH produced in the light phase, 1 three-carbon molecule is produced in this cycle. Two cycles produce 2 three-carbon molecules, which attach and form a six-carbon glucose molecule. The plant uses glucose in its further metabolic processes for starch synthesis or in its digestive processes for the production of ATP.

Experiments have been carried out in order to create artificial systems that mimic photosynthesis. In an artificial leaf the light reactions and the dark reactions take place in two separate vessels. The light reactions take place in a nitride semiconductor, which decomposes water when exposed to light. Oxygen is released as bubbles, while protons and electrons are transferred into the other vessel, with the latter being transferred through a conductive wire. This vessel is the site of the dark reactions. Here a metal catalyst is used to produce formic acid from carbon dioxide and water. This system makes it possible to use the energy of sunlight, and it may also be helpful in reducing the carbon dioxide content of the atmosphere, which would help to reduce the greenhouse effect and thus global warming.

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