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Photosynthesis - PowerPoint PPT Presentation

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Photosynthesis. How do plants grow?. Van Helmont - 1648. Joseph Priestley. Priestley – 1771 – plants restore “good quality” to air. Jan Ingenhousz – 1796 – plants only restore good quality to air in the presence of light. Water is source of oxygen released during photosynthesis.

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Presentation Transcript
how do plants grow
How do plants grow?

Van Helmont - 1648



water is source of oxygen released during photosynthesis
Water is source of oxygen released during photosynthesis

Van Niel was studying the activities of photosynthetic bacteria - he found that purple sulfur bacteria reduce carbon to carbohydrates but do not release oxygen; instead the purple sulfur bacteria use hydrogen sulfide in their photosynthesis - so for them the reaction is as follows:

CO2 + 2H2S + light energy =>(CH2O) + H2O + 2S

Van Niel then generalized this to the following reaction for all photosynthetic activity

CO2 + 2H2A + light energy=>(CH2O) + H2O +2A

C.B. van Niel


photosynthesis has two separate reactions
Photosynthesis has two separate reactions
  • Experiments by F.F. Blackman in 1905 demonstrated that photosynthesis has two stages or steps - one is a light-dependent stage and the other is a light-independent stage - due to changes in the effectiveness of the light-independent stage with increases in temperature, Blackman concluded that this stage was controlled by enzymes
the role of pigments
The role of pigments
  • A pigment is any substance that absorbs visible light - most absorb only certain wavelengths and reflect or transmit the wavelengths they don't absorb
  • Chlorophyll absorbs light primarily in the violet, blue and red wavelengths and reflects green wavelengths, and thus appears green
  • Absorption spectrum - the light absorption pattern of a pigment
  • Action spectrum - the relative effectiveness of different wavelengths for a specific light-requiring process
  • Chlorophyll is implicated as the principle pigment in photosynthesis because its absorption spectrum is the same as the action spectrum for photosynthesis
the photosynthetic pigments
The Photosynthetic Pigments
  • Chlorophyll a - found in all photosynthetic eukaryotes and cyanobacteria - essential for photosynthesis in these organisms
  • chlorophyll b - found in vascular plants, bryophytes, green algae and euglenoid algae - it is an accessory pigment - a pigment that serves to broaden the range of light that can be used in photosynthesis - the energy the accessory pigment absorbs is transmitted to chlorophyll a
  • carotenoids - red, orange or yellow fat-soluble accessory pigments found in all chloroplasts and cyanobacteria - caroteniods are embedded in thylakoids as are chlorophylls - two types - carotenes and xanthophylls (xanthophylls have oxygen in their structure, carotenes don't)
when pigments absorb light electrons are temporarily boosted to a higher energy level
When pigments absorb light, electrons are temporarily boosted to a higher energy level

One of three things may happen to that energy:

1. the energy may be dissipated as heat

2. the energy may be re-emitted almost instantly as light of a longer wavelength - this is called fluorescence

3. the energy may be captured by the formation of a chemical bond - as in photosynthesis

the photosystems
The Photosystems
  • The chlorophylls and other pigments are embedded in thylakoids in discrete units called photosystems
  • Each photosystem has 250 to 400 pigment molecules in two closely linked components - the reaction center-protein complex and the antenna protein complex
  • All pigments in the photosystem are capable of absorbing photons of light, but only one pair of those in the reaction center-protein complex can actually use the energy in a photochemical reaction
  • The other pigments in the antenna protein complex act like antenna to gather light and transfer that energy to the photochemically active pigments
the photosystems1
The Photosystems
  • There are two different kinds of photosystems –
  • Photosystem I - has chlorophyll a - has an optimum absorption peak of 700 nanometers of light - the chlorophyll a is called P700 because of this activity
  • Photosystem II - has special chlorophyll a active at 680 nanometers - the P680 chlorophyll a
  • In general the two photosystems work together simultaneously and continuously - however, photosystem I can work independently
calvin cycle details
Calvin Cycle - details
  • The Calvin cycle begins when CO2 enters the cycle and is joined to RuBP this forms a 6 carbon compound which immediately splits into two 3 carbon compounds (the 6 carbon intermediate has never been isolated) - the 3 carbon compound is 3-phosphoglycerate (PGA)
  • Because each PGA has three carbons, this is sometimes also called the C3 pathway
  • Each full turn of the Calvin cycle begins with entry of a CO2 molecule and ends when RuBP is regenerated - it takes 6 full turns of the Calvin cycle to generate a 6 carbon sugar such as glucose
  • Although we usually report glucose as the product of photosynthesis, the cell usually produces either sucrose or starch as its storage products
  • At night, sucrose is produced from the starch and it is transported from the chloroplast to the rest of the cell
the full calvin cycle equation
The full Calvin Cycle equation

6CO2 + 12NADPH + 12H+ + 18ATP =>

C6H12O6 (GLUCOSE) + 12NADP+ + 18ADP + 18 Pi + 6H2O

the c4 pathway
The C4 Pathway
  • In some plants the first carbon compound produced through the light-independent reactions is not the 3 carbon PGA, but rather is a 4 carbon molecule oxaloacetate - plants that use this pathway are called C4 plants
  • Leaves of C4 plants typically have very orderly arrangement of mesophyll around a layer of bundle sheath cells


with C4



why use c4 pathway
Why use C4 pathway?
  • A problem with C3 is that for all C3 plants, photosynthesis is always accompanied by photorespiration which consumes and releases CO2 in the presence of light - it wastes carbon fixed by photosynthesis - up to 50% of carbon fixed in photosynthesis may be used in photorespiration in C3 plants as fixed carbon is reoxidized to CO2
  • Photorespiration is nearly absent in C4 plants - so greatly increases their efficiency - this is because a high CO2: low O2 concentration limits photorespiration - C4 plants essentially pump CO2 into bundle sheath cells (or the products of its reduction) thus maintaining high CO2 concentration in cells where Calvin cycle will occur
  • Thus net photosynthetic rates are higher for C4 plants (corn, sorgham, sugarcane) than in C3 relatives (wheat, rice, rye, oats)
why use c4 pathway1
Why use C4 pathway?
  • C4 plants evolved in tropics and are well adapted to life at high temperature, high light intensity and dry conditions - optimal temperature for C4 photosynthesis is much higher than for C3 - efficient use of CO2 allows C4 plants to keep stomata closed longer and thus they lose less water during photosynthesis than do C3 plants
  • C4 monocots do especially well at high temperature
  • C4 dicots do especially well in dry conditions
crassulacean acid metabolism
Crassulacean Acid Metabolism
  • Crassulacean Acid Metabolism (CAM) has evolved independently in many plant families including the stoneworts (Crassulaceae) and cacti (Cactaceae)
  • Plants which carry out CAM have ability to fix CO2 in the dark (night) via the activity of PEP carboxylase - malic acid (malate) so formed is stored in the cell's vacuole - during the light (day) the malic acid is decarboxylated and CO2 is transferred to RuBP in Calvin cycle within the same cell
  • so CAM plants, like C4 plants, use both C4 and C3 pathways, but CAM plants separate the cycles temporally and C4 plants separate them spatially