photosynthesis only

• Photosynthesis is the process by which most autotrophs, including plants, algae, and some bacteria, manufacture their own food.
• Photosynthesis is made up of a series of chemical reactions that convert light energy into the chemical energy of organic food molecules.
• Photosynthetic autotrophs use light energy to produce glucose (C6H12O6, a simple carbohydrate that serves as a food molecule) from carbon dioxide and water, while releasing oxygen gas as a by-product.
• 6CO2 + 12H2O → C6H12O6 + 602 + 6H2O.
• Photosynthesis uses water as a reactant, but also yields water as a product.
• The water used in photosynthesis is not the same as the water used in the light-dependent reactions.
• Photosynthesis requires light energy, photosynthetic pigments, and water as basic requirements.
• Visible light is the only portion of the electromagnetic spectrum that can be detected by human eyes and is used by cells to power photosynthesis.
• Visible light is composed of different colors of light namely, red, orange, yellow, green, blue, and violet.
• Photosynthetic cells are able to absorb light by means of colored molecules called pigments.
• The most common and abundant of these pigments are the chlorophylls, especially the forms called chlorophyll a and chlorophyll b.
• Chlorophyll a is the universal chlorophyll because of its presence in all photosynthetic organisms and is the pigment that actually uses light energy during photosynthesis.
• Chlorophyll b serves as an accessory pigment, together with another family of pigments called carotenoids (which include the yellow xanthophylls and the yellow-orange carotenes).
• Accessory pigments are those that are basically involved in absorbing light, but not in using it.
• Plant cells may also contain nonphotosynthetic pigments called anthocyanins.
• Anthocyanins do not participate actively in photosynthesis, but protect plant parts from damage by ultraviolet light.
• The color of a pigment depends on the kind of visible light that it absorbs and reflects.
• Chlorophylls, for example, appear green because they reflect green light, while absorbing red and blue lights.
• Carotenoids owe their colors to the red, orange, and yellow lights that they reflect and the blue-green to green lights that they absorb.
• Only absorbed light can be used in photosynthesis, which means that if only chlorophylls are present in photosynthetic cells, the cells will only be able to use a very small portion of the visible light that reaches them.
• Accessory pigments allow the portions of visible light that chlorophylls cannot absorb also to become available for the cells to use.
• Two electron transport chains (ETCs) are closely associated with these two photosystems, the first ETC receives electrons from PS II and passes these to PS I, while the second one receives electrons from PS I and eventually gives these up to a molecule of NADP (nicotinamide adenine dinucleotide phosphate).
• Photosynthesis occurs in two sets of reactions, the light-dependent reactions (or simply- the light reactions), and the light-independent reactions (also known as the carbon reactions or the Calvin cycle and were once referred to as the dark reactions).
• In order for PS II to be functional again, the reaction center chlorophyll must be returned to its ground or unexcited state, this is done by getting electrons from water.</flash
• Each thylakoid is a flattened membranous sac with an inner compartment called the thylakoid space or lumen.
• The reaction center chlorophylls in turn use this light energy to jump-start the photosynthetic process.
• The light reactions, which occur in the thylakoid membranes, are the photo- part of photosynthesis, where light energy is converted into the chemical energy of ATP and NADPH.
• There are around 10 to 100 grana in a chloroplast and about 10 to 20 thylakoids comprise a granum.
• Embedded in the thylakoid membrane are numerous photosystems, a photosystem is a cluster of proteins and pigments including around 300 chlorophyll a molecules and 50 accessory pigments that play an essential role in photosynthesis.
• A photosystem has a reaction center, which contains a pair of chlorophyll a molecules, surrounded by a light-harvesting complex, in which the rest of the pigments are located.
• The Calvin cycle, on the other hand, happens in the stroma, it is the -synthesis part of photosynthesis because during this stage, simple carbohydrates are produced using CO2, ATP, and NADPH.
• The chloroplast is bound by an outer membrane and an inner membrane, between which is an intermembrane space.
• The excited electrons are then picked up by the first ETC, which connects PS II with PS I.
• Within the inner membrane is a fluid region called the stroma, and suspended in this are structures that look like stacks of coins, each stack is called a granum, and the coin-shaped forms that make it up are called thylakoids.
• Light energizes two electrons (2e-) in chlorophyll a, causing this molecule to release these excited electrons.
• The pigments in the light-harvesting complex, called antenna pigments, are in charge of absorbing light energy which they immediately pass to the chlorophyll a molecules in the reaction center.
• Photosynthesis in a eukaryotic cell takes place in the chloroplast, a type of plastid that owes its green color to the chlorophyll molecules it contains.
• Carotenoids increase the fraction of visible light that is useful in photosynthesis, hence they are always found together with chlorophylls in all photosynthetic cells.
• Photosynthesis starts when light strikes PS II, the antenna pigments of this photosystem capture light and transmit it to chlorophyll a in the reaction center.
• Photosynthetic autotrophs may be prokaryotic or eukaryotic, with the latter being the focus of this module.