Chapter 23 Photosynthesis and Chemosynthesis
Chlorophyll a is the most abundant and important photosynthetic pigment. It is the primary pigment for photoexcitation. • The other photosynthetic pigments are called accessory pigments. • They enables the plant to absorb sunlight of greater range of wavelength. • They transfer the energy of the light they have absorbed to chlorophyll a for photoexcitation. • They protect chlorophyll a from destruction by absorbing excessive sunlight.
Action Spectrum The action spectrum is highly correlated to the absorption spectrum.
Paper chromatography for separation of photosynthetic pigments
Different photosynthetic pigment has different Rf value. Rf value of a pigment = Distance travelled by the pigment Distance travelled by the solvent front The list below indicates the Rf value of the most common photosynthetic pigment using Petroleum ether: acetone(9:1) as a developing solvent: Carotene 0.95 Xanthophyll 0.71 Chlorophyll a 0.65 Chlorophyll b 0.45
Electromicrograph of Chloroplast Light dependent stage of photosynthesis takes place in grana and intergrana. Light independent stage of photosynthesis takes place in stroma.
Mechanism of Photosynthesis Light dependent stage 1: Chlorophyll molecule is excitated from its ground state by a process called photoexcitation. Ch Ch+ + e-
Mechanism of Photosynthesis Light dependent stage 2: The high energy electron is transferred via a series of electron transport chain.
Mechanism of Photosynthesis Light dependent stage 3: The energy is handed over to form ATP, a process called photophosphorylation.
Mechanism of Photosynthesis Light dependent stage 4: Water molecule is splitted up to give oxygen, hydrogen ion and electron, a process called photolysis. The H+ together with the e- via the electron transport chain reduce NADP to NADPH2. The electron returns the chlorophyll ion to its ground state.
Mechanism of Photosynthesis Light dependent stage Alternative explanation for step 4: Water is ionized to form hydrogen ion and hydroxide ion
Mechanism of Photosynthesis Light dependent stage 4 OH- 2H2O + O2 + 4e- The electron returns the chlorophyll ion to the ground state. The H+ together with the e- via the electron transport chain reduce NADP to NADPH2.
The main products formed by the light dependent stage are ATP and NADPH2 which will be used in the light independent stage of photosynthesis. The light dependent stage is a series of photochemical reactions which are NOT controlled by enzymes. The rate is therefore NOT temperature dependent. Light dependent stage takes place in grana and intergrana. Light independent stage takes place in stroma.
Products of Photosynthesis Some triose phosphate is to synthesize glucose and then sucrose and starch.
Products of Photosynthesis Fatty acids and glycerol are produced from the 3-C compound and triose phosphate respectively. Lipids are formed by combining glycerol and fatty acids,
Products of Photosynthesis Amino acids are formed from the 3-C compounds.
Factors affecting the rate of photosynthesis • Light intensity • Carbon dioxide concentration • Temperature
Concepts of Limiting factor The rate of photosynthesis is controlled by light intensity, CO2 concentration and temperature. The most unfavourable factor affects the rate of photosynthesis most. This factor is called Limiting factor. The change in other factors have no effect on the rate of photosynthesis. When the limiting factor becomes favourable, it will no longer be limiting and the rate of photosynthesis increases accordingly until the other factors become limiting.
Photo-autotroph: Sunlight energy Inorganic raw materials Organic food Chemical energy Chemo-autotroph: Inorganic raw materials Organic food Chemosynthesis Autotroph/ autotrophic organisms are able to synthesize their own food using the raw materials such as carbon dioxide and water. Inorganic raw materials Organic food
Iron bacteria: Fe2+ Fe3+ + Energy Inorganic raw materials Organic food Colourless sulphur bacteria: S SO4= + energy Inorganic raw materials Organic food Examples of Chemo-autotrophs/ Chemosynthetic organisms
Nitrite bacteria Nitrite bacteria oxidize the ammonium ion in the soil into nitrite ion with release of energy: 2 NH4+ + 3 O2 2 NO2- + 2 H2O + 4 H+ + Energy The energy is used to synthesize organic food for this bacteria.
Nitrate Bacteria Nitrate bacteria oxidizes nitrite ion into nitrate ion with release of energy. 2 NO2- + O2 2 NO3- + Energy The energy is used to synthesize organic food from inorganic raw materials.