Photosynthesis
Chapter 7
Photosynthesis
• In photosynthesis, organisms trap radiant
energy from sunlight and convert it into the
energy of chemical bonds in large molecules.
• Chlorophyll is a green pigment that absorbs
light energy.
• Photosynthesis transforms light energy into
chemical bond energy in the form of ATP.
• ATP is then used to form complex organic
molecules such as glucose.
Chloropasts
• Chloroplasts are the organelles in which
photosynthesis takes place.
• Two distinct regions in chloroplasts:
– Grana – stacks of membranous stacks containing
chlorophyll.
– Stroma – the spaces between the membranes.
Summary of Photosynthetic Events
• Light energy + carbon dioxide + water 
glucose + oxygen
• Light energy + 6CO2 + 6H2O  C6H12O6 + 6 O2
3 Distinct Events In The
Photosynthetic Pathway
• Light-Capturing events – chlorophyll and other
pigments absorb specific wavelengths of light.
– Light energy is captured by photosynthetic and
other pigments resulting in excited electrons.
– When specific amounts of light are absorbed by
the photosynthetic pigments, the electrons
become “excited” and can enter into chemical
reactions responsible for the production of ATP.
– Takes place in the grana of the chloroplast.
3 Distinct Events In The
Photosynthetic Pathway
• Light-dependent reactions (light reactions)
– Energy from the excited electrons is used to
dissociate water molecules into hydrogen and
oxygen.
– use the excited electrons to produce ATP.
– Takes place in the grana of the chloroplast.
• Light-independent reactions (dark reactions)
– Organic molecules such as glucose is produced.
– Takes place in the stroma.
Light-Capturing Events
• Chlorophyll and other pigments such as
carotenoids use light energy to drive
photosynthesis.
• Chloroplasts are membranous, saclike
organelles containing many thin, flattened
sacs called thylakoids.
Light-Capturing Events
• The thyylakoids contain chlorophyll and are
stacked in groups called grana.
• The pigments embedded in the thylakoids
capture light energy and some of the
electrons in the pigments become excited.
Light-Capturing Events
• The chlorophylls are arranged in clusters
called photosystems.
• Photons of light energy  excited electrons
from chlorophyll.
Light-Dependent Reactions
• The excited electrons from the light-capturing
stage are passed through a series of electrontransport steps.
Light-Dependent Reactions
• The chloroplast in the electron transport
system regains electrons from water; thereby
producing hydrogen ions, electrons, and
oxygen gas.
• Excited electrons + H20 + ADP + NADP+  ATP
+ NADPH + O2
Light-Independent Reactions
• The light-independent reactions are a series of
oxidation-reduction reactions, which combine
hydrogen from water with carbon dioxide
from the atmosphere to form simple organic
molecules such as sugar.
• ATP + NADPH + ribulose + CO2  ADP + NADP+
+ complex organic molecule + ribulose
Plant Metabolism
• Photosynthetic organisms are able to
manufacture organic molecules (including fats,
proteins, and complex carbohydrates) from
inorganic molecules.
• Vitamins are another important organic
molecule derived from plants. Vitamins are
organic molecules that we cannot
manufacture, but must have in small amounts
to maintain health.
Interrelationships Between
Autotrophs and Heterotrophs
• Autotrophs can capture energy and create
organic molecules from inorganic molecules.
• Heterotrophs must have organic molecules as
starting points.
• All organisms must do some form of
respiration.
Interrelationships Between
Autotrophs and Heterotrophs
• Plants, like animals, require oxygen for the ETS
portion of aerobic cellular respiration.
• Plants give off more oxygen to the
atmosphere than they take in during
respiration.
Interrelationships Between
Autotrophs and Heterotrophs
• The surplus oxygen given off is the source of
oxygen for cellular respiration in both plants
and animals.
• Animals supply the raw materials of CO2, H20,
and nitrogen needed by plants, and plants
supply the raw materials of sugar, oxygen,
amino acids, fats, and vitamins needed by
animals.