Lesson IV: Photosynthesis PDF

Lesson IV: PHOTOSYNTHESIS Photosynthesis - takes place in the cells of leaves within the organelle chloroplasts. The leaves, which are considered the plants' food factories, contain cells whose shapes and arrangements are well adapted to perform the photosynthetic function efficiently. ▪ The up...

Lesson IV: PHOTOSYNTHESIS Photosynthesis - takes place in the cells of leaves within the organelle chloroplasts. The leaves, which are considered the plants' food factories, contain cells whose shapes and arrangements are well adapted to perform the photosynthetic function efficiently. ▪ The upper and lower epidermis is composed of thin, transparent, and closely packed cells that allow light to pass through. ▪ The palisade and spongy tissue in dicot leaves, collectively called the mesophyll tissue, is the photosynthetic tissue since the cells composing this layer contain numerous chloroplasts. ▪ Gases (carbon dioxide and oxygen) enter and leave the spaces in between the leaf tissues via small openings called stomata (singular: stoma). ▪ The size of the stomata is regulated by the two kidney- shaped cells called guard cells. ▪ The veins that traverse the leaf blade containing the vascular tissues consist of the xylem and phloem. The xylem tissue transports water and minerals coming from the roots, and the phloem tissue transports food, the product of the photosynthetic process, to various plant parts where it is needed. Light energy 6𝐶𝑂2 + 6𝐻2 𝑂 𝐶6 𝐻12 𝑂6 + 6𝑂2 Chlorophyll The equation simply means that the carbon dioxide that enters the leaf stomata and the water that reaches the leaf via the xylem tissue chemically combine in the presence of light energy to form glucose (monosaccharide). Oxygen is released as a by-product of this reaction. The oxygen released by photosynthesis enhances the oxygen content of the atmosphere. Raw Materials of Photosynthesis Carbon dioxide - The reservoir of carbon dioxide is the atmosphere. The carbon part in carbon dioxide is important in the synthesis of sugar, the product of the photosynthetic reaction. When in the leaf, carbon dioxide dissolves in water and is stored in the leaf in the form of carbonic acid. Water - is primarily obtained from the soil. As water is transported to the leaf from the roots, it is photochemically split (photolysis) into hydrogen ions (H*) and hydroxyl ions (OH). Water is the source of electrons and ions (H* and OH-) needed in the synthesis of high-energy molecules. Other Factors Involved in the Photosynthetic Process Light energy powers the entire photosynthetic reaction. It comes from the sun in discrete packets of light called photons. Light consists of various or sun light wavelengths; the red, blue, and violet wavelengths are the most useful in photosynthesis. The green wavelength is reflected. This is the reason leaves appear green in color to our eyes. Visible light is only a small part of this color spectrum. As the photons are absorbed by the chloroplasts, the electrons in the chlorophyl molecule become "excited." This means that they move from their ground state to a higher energy level or excited state. Interestingly, only about 1% of the light energy that strikes the leaf is actually used in photosynthesis. This is because a large fraction of it is transmitted, reflected, and converted to heat energy. The chloroplasts are the actual sites of photosynthesis; thus, they are referred to as the "kitchen" in making sugar. Chloroplasts contain green, light-trapping chlorophyll pigments. Chlorophyll is a complex molecule. The type of chlorophyll present in all photosynthetic organisms is chlorophyll a. Chlorophyll a absorbs energy from the violet, blue, red, and orange wavelengths efficiently. Aside from chlorophyll a, other accessory pigments were discovered to be associated within the chloroplasts. These pigments absorbed energy, which chlorophyll a cannot absorb. These accessory pigments are chlorophyll b, c, d, and e, xanthophylls, and carotenoids. Chlorophylls c, a, and e are present in species of algae. Electron carriers and enzymes are chemical substances that are also contained in the chloroplast. ▪ Electron carriers are substances that can readily accept and release high-energy electrons. Examples are ferredoxin, plastocyanin, and cytochrome complexes. ▪ Enzymes are substances that catalyze specific chemical reactions to happen. An example is the enzyme rubisco found in the chloroplast. Photosystems I and II Photosystems - are aggregates of pigments and proteins organized within the chloroplasts. They are the structural and functional units that function for harnessing solar energy. It consists of a series of electron carriers that forms the electron transport chain and a reaction center. ▪ A reaction center is a specific type of chlorophyll molecule surrounded by antenna complexes. These antenna complexes are composed of carotenoid pigments and other accessory pigments that can gather and absorb light energy and convey it to the reaction center. Two types of photosystems involved in harnessing energy: ▪ In photosystem I, the reaction center is a type of chlorophyll a known as phytochrome 700 (P700) that can absorb photons with a wavelength of 700 nm. ▪ In photosystem II, the reaction center is another type of chlorophyll known as phytochrome 680 (P680) that can absorb photons with a wavelength of 680 nm. The Photosynthetic Reactions Photosynthesis occurs in two stages: the light or photoreaction and the dark or synthesis reaction. The light reaction takes place in the grana (singular: granum), while the dark reaction takes place in the stroma of the chloroplast. Two sub-stages of light reaction: 1a. Cyclic Light Reaction - involves photosystem I where the reaction center is P700. When a photon of light reaches P700, it becomes excited, releasing a high- energy electron. The high-energy electron is received by the primary electron acceptor and then passes along a series of electron carriers. The energy that is released from the high-energy electron as it passes along the electron transport chain leads to the formation of ATP, a high-energy molecule. As the electron moves along the chain, it loses energy, meaning it goes back to its ground state and eventually returns to the chlorophyll molecule where it came from. Thus, in the cyclic light reaction, light energy is transformed into molecules of ATP, a high-energy compound. 1b. Noncyclic Light Reaction - involves both photosystems I and II. This time, the reaction center is P680. Likewise, P680 is excited as it absorbs light energy, releasing a high-energy electron. The high-energy electron also cascades along a series of electron carriers, leading to the formation of ATP. The electron goes back to its ground state as it loses its energy, but it will not go back to the original chlorophyll molecule where it came from. Instead, it is donated to the reaction center o photosystem I (P700) which has lost an electron. - Since P680 still misses an electron, it remains positively charged. This positive charge causes the water molecules to split, forming hydrogen ion (H-), hydroxyl ion (OH), two electrons, and oxygen. The hydrogen ion and an electron bind with nicotinamide adenine dinucleotide phosphate ion (NADP*), an unstable chemical substance found in the chloroplast forming the stable compound NADPH, another high-energy substance like ATP. The hydroxyl ions recombine to form oxygen 0, and water vapor that exit the stomata and are released into the atmosphere as by-products. P680 goes back to its ground state from the other electron that is released from the splitting of water. Thus, the overall products of the noncyclic light reaction are ATP, NADPH, oxygen, and water. 2. The Dark or Synthesis Reaction - is the stage where carbon dioxide that enters the leaf is converted to sugar. This reaction is also known as carbon fixation or the Calvin cycle. - In the chloroplast is a space called a stroma where a chemical substance called ribulose bisphosphate (RuBP5) is found. RuBP5 is a five-carbon sugar referred to as a carbon dioxide acceptor. Carbon dioxide (CO,) that enters the leaf via the stomata chemically combines with RuBP5, becoming RUBP6, an unstable six-carbon sugar. Such a reaction is catalyzed by the enzyme rubisco, which is also  present in the stroma. RuBP6 splits enzymatically forming two molecules of phosphoglycerate (PGA). PGA plus hydrogen ion (H+) from NADPH and a phosphate group (PO4) from ATP (formed during the light reaction) becomes phosphoglyceraldehyde (PGAL), which is the key material in the synthesis of glucose. This part of the dark reaction can be summarized as: 𝑃𝐺𝐴 + 𝑁𝐴𝐷𝑃𝐻 + 𝐴𝑇𝑃 → 𝑃𝐺𝐴𝐿 + 𝑁𝐴𝐷𝑃 + 𝐴𝐷𝑃 + 𝐻2 𝑂 - Assuming that six CO2 molecules enter the dark reaction, 12 PGAL molecules will be formed. Of these,10 PGAL will be enzymatically rearranged to replace the lost carbon dioxide acceptor (RUDP5) to begin the cycle again, and only the remaining two PGAL will chemically combine to form, becoming one glucose molecule, which is the final product of the dark reaction. - Therefore, photosynthesis is an anabolic pathway that traps light energy and transforms this energy into molecules of ATP and NADPH that are used in the production of glucose, a high- energy carbon-based compound.