Photosynthesis
BIO Chp 6
Overview of photosynthesis
• Flowering plants as photosynthesizers
– Photosynthesis occurs in the green parts of plants
– Leaves contain mesophyll tissue specialized for photosynthesis
– Water is taken up by roots and transported to leaves by veins
– Carbon dioxide enters through openings in the leaves called
stomata
– Light energy is absorbed by photopigments in thylakoids of
chloroplasts
Plants as photosynthesizers
Overview of Photosynthesis
• Photosynthetic reaction
Solar energy + 6CO2 + 6H2O  C6H12O6 + 6O2
– Carbon dioxide is reduced to form glucose
– Water is oxidized to form oxygen, therefore the oxygen given off
by photosynthesis comes from water
• Two set of reactions occurs in photosynthesis
– Light reactions occur first followed by the Calvin cycle
– NADP+ carries hydrogens from the light reactions to the Calvin cycle
– ATP formed in the light reactions is used in the Calvin cycle
Overview of photosynthesis
Captured Solar Energy
• Visible light
– Visible light is a part of the electromagnetic spectrum
– Visible light includes the colors blue, violet, green, yellow,
orange, and red
– Blue-violets have the shortest wavelengths and the highest
energy content
– Photopigments chlorophyll a and b and carotenoids absorb
specific portions of the light spectrum
– Blue, violet, and red are best absorbed
– Green is reflected
Solar energy capture
• Light reactions
– 2 types of photosystems composed of photopigments
– Reaction center of each photosystem is chlorophyll a
– Electrons of chlorophyll a absorb light energy and move to an
electron acceptor molecule
– Electrons then pass down one of 2 paths-noncyclic or cyclic
• The noncylic pathway
–
Produces ATP and NADPH
– Light energy is absorbed by chlorophyll a in PSII
– Excited electrons are removed & replaced by splitting a water
molecules
H2O  2H+ + 2e- + ½O2
– ATP is produced
• Organization of thylakoid
– Thylakoid space acts as reservoir of hydrogen ions
– Hydrogen ions flow down gradient through an ATP synthase
complex in thylakoid membrane to produce ATP
– Thylakoid membranes contain the following complexes:
• PS I and II
• ATP synthase complexes
Organization of the Thylakoid
Carbohydrate synthesis
Stages of the Calvin cycle
Carbon fixation
• carbon fixation occurs when a reaction occurs that attaches
CO2 to an organic compound
• Ribulose biphosphate (RuBP) combines with CO2 (carbon dioxide)
to form a 6C molecule
• Enzyme which catalyzes this reaction is RuBP carboxylase
The Calvin cycle reactions
Carbohydrate synthesis
• Occurs by reduction of carbon dioxide
• Importance of the Calvin cycle
– G3P is the Calvin cycle product that can be converted to glucose
phosphate
– Glucose phosphate can then be converted into many different
organic molecules
Carbohydrate synthesis
• Occurs by reduction of carbon dioxide
• Importance of the Calvin cycle
– G3P is the Calvin cycle product that can be converted to glucose
phosphate
– Glucose phosphate can then be converted into many different
organic molecules
Photosynthesis vs. Cellular Respiration
• Photosynthesis vs cell respiration
– Both plant and animal cells carry out cell respiration
– Only plant cells photosynthesize
– Both processes utilize an electron transport chain and
chemiosmosis for ATP production
– Photosynthesis reduces CO2 to carbohydrates and releases O2
– Respiration utilizes O2 and gives off CO2
Photosynthesis vs Cell Respiration