Photosynthesis: The Process of Converting Light Energy into Chemical Energy, Study notes of Biology

Download Photosynthesis: The Process of Converting Light Energy into Chemical Energy and more Study notes Biology in PDF only on Docsity! Photosynthesis Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed. Photosynthesis may be summarised by the word equation: carbon dioxide + water glucose + oxygen The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the violet-blue and reddish orange-red wavelengths, and little from the intermediate (green-yellow-orange) wavelengths. Chlorophyll All chlorophylls have: • a lipid-soluble hydrocarbon tail (C20H39 -) • a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different side-groups on the head The tail and head are linked by an ester bond. Leaves and leaf structure Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem vessels. Land plants must guard against drying out and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal of water is also lost. Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot desert days. The structure of the chloroplast and photosynthetic membranes The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast Phosphoric acid and ADP undergo a condensation reaction ° ° Water is eliminated and woot ATP is formed Synthesis of ATP from ADP Acondensation reaction has led to phosphorylation. Photolysis: a water molecule is split into oxygen and hydrogen ions, releasing two electrons H,0 %x0, + 2H* <e>7>o3m s PSI PSIl and PSI: the Z scheme PSIl electron transfer [ electron acceptor | Xe suds electron occurs and 2 transfer electrons are ae chain assed to photoionisation X Pi a another electons are scoepto a passed to an electron acceptor | (= light _.@ Gees light the electrons are transferred to ‘positive’ holes in chlorophyll molecules Sequence of reactions Non-cyclic phosphorylation (the Z scheme) Both adenosine triphosphate (ATP) and NADPH are produced. In the first photosystem (Photosystem II, PSII): • photoionisation of chlorophyll transfers excited electrons to an electron acceptor • photolysis of water (an electron donor) produces oxygen molecules, hydrogen ions and electrons, and the latter are transferred to the positively-charged chlorophyll • the electron acceptor passes the electrons to the electron transport chain; the final acceptor is photosystem PSI • further absorbed light energy increases the energy of the electrons, sufficient for the reduction of NADP+ to NADPH The oxidised form of nicotinamide adenine dinucleotide phosphate (NADP+) The reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) Chemiosmosis and ATP synthesis The components of non-cyclic phosphorylation are found in the thylakoid membranes of the chloroplast. Electrons passing through the transport chain provide energy to pump H+ ions from the stroma, across the thylakoid membrane into the thylakoid compartment. H+ ions are more concentrated in the thylakoid compartment than in the stroma. We say there is an electrochemical gradient. H+ ions diffuse from the high to the low regions of concentration. This drives the production of ATP. Chemiosmosis as it operates in photophosphorylation within a chloroplast