Chapter 7
Biological PathwaysPhotosynthesis
Biology 100
Energy Pathways
• Ultimately, the source of all
energy in almost all
ecosystems is the energy of
fusion in the sun.
• Primary Producers
(autotrophs) capture this
energy in photosynthesis
and use it to create organic
molecules.
• Consumers and
decomposers ingest these
organic molecules, rework
them to build their own
tissues, break some of them
down to support their own
metabolism.
Primary Production in Other Ecosystems
• While the open ocean is the largest ecosystem (65% of the
Earth’s surface), it contributes only 24% of the net primary
production.
• In contrast, tropical rain forests are a small ecosystem (3.3%),
but contribute over 21% of the net primary production.
Fig. 54.3
Photosynthesis
• Photosynthetic autotrophs use light as an energy
source
– Include some cyanobacteria prokaryotes, some
protozoa, all algae and green plants
– 99.9% of life on Earth relies on photosynthesis for
energy needs
• Light is a good source of energy, since everyday
we have the equivalent of 1 million Hiroshimasized atomic bombs of radiant energy that
reaches the Earth
Photosynthesis
• Photosynthesis is essentially the reverse of
aerobic respiration
– Energy + 6CO2 + 6 H2O → C6H12O6 + 6O2
– This is an endergonic reaction
– In plants, photosynthesis takes place in the leaves.
Photosynthesis
• Photosynthesis will
occur in the chloroplast.
•The light-capturing
event will occur in the
thylakoids
(membranous sacs,
stacked as the grana)
•The light-dependent
reactions will occur in
the thylakoids
•The light-independent
reaction will occur in the
stroma
Photosynthesis
• Photosynthesis takes place in three stages:
– Light-capturing events
• Capture energy from the sunlight
– Light-dependent reactions
• Using energy to make ATP
– Light-independent reactions
• Using ATP to power the synthesis of plant molecules
from CO2 in the air.
Light-Capturing Event
• In the light-capturing event, photosynthetic
pigments, especially chlorophyll, absorb photons of
light
• This excites some electrons in chlorophyll to the point where
they are passed to molecules in the light-dependent reactions
Photosynthesis
• In the light-dependent reactions, the energy of some of these
excited electrons is used to generate ATP.
• Some of the energy is used to split H2O into O2 and H+.
• Oxygen is released as a waste product
• The H+ and excited electrons are loaded onto a carrier, NADP+.
Photosynthesis
• The light-independent reaction, also known as the Calvin
Cycle is where CO2 is used to synthesize plant molecules
• The end product is sugar, a food source for the plant. This
makes the plant an autotroph.
What is Light?
• Light is actually tiny packet of energy, known
as photons.
• Some of these photons in light carry more
energy than others. Light, is a form of
electromagnetic energy, and is thought of as a
wave. The range of the electromagnetic
energy is represented by the electromagnetic
spectrum.
Electromagnetic
Spectrum
Notice the highestenergy photons(gamma
rays) have the shortest
wavelengths.
Electromagnetic Spectrum
Only a narrow range
of the electromagnetic
radiation (EMR)
reaches the Earth’s
surface. They are in
the form of visible
light from 380 nm to
750 nm range.
Our eyes/brain
interpret these
wavelengths as colors.
•400 nm = blue
•550 nm = green
•670 nm = red
What is Color?
• Sunlight (and “white” light) is actually composed of photons
travelling at many different wavelengths.
• When photons reach an object, some are absorbed, while
other are reflected.
– This is due to the characteristics of molecules on the
surface.
• If red photons are reflected and all others are absorbed, the
object will look red to us.
Chlorophyll
• Chlorophyll is the main pigment in plants that
absorb light.
– Chlorophyll exists in two forms
• Chlorophyll a
• Chlorophyll b
• Carotenoids are another group of pigments
that capture light of wavelengths not
efficiently absorbed by chlorophylls.
Pigments
• Chlorophyll will absorb
red and blue portions of
the electromagnetic
spectrum, while reflecting
green light.
• Carotenoids will absorb
blue and blue-green light,
while reflecting oranges
and yellows.
– When chlorophyll
disentegrates in the fall,
this pigment causes the
red, yellow and orange
colors to show through.
Anatomy of a Leaf
• The leaf of a plant is
covered by a waxy cuticle
over a layer of epidermal
cells
• Gases, such as CO2 in and
O2 out, pass through special
pores, called stoma on the
underside of the leaf.
• The mesophyll cells conduct
most of the photosynthesis
for the plant.
Photosynthesis
• Photosynthesis begins when
photosynthetic pigments
intercept photons with the
appropriate
• The energy of the absorbed
photons excites some of the
electrons of the chlorophyll
wavelengths.
• The excited electrons are
passed to an electron
carrier.
Photosynthesis: Light-dependent Rxn
• Ultimately, the energy from excited electrons is used to
generate ATP or carried off with NADP+ to form NADPH.
• Water is split into O2, H+, and electrons
• The electrons replace the ones carried off by NADPH.
• The O2 is released to the atmosphere
Light-independent Reaction
• The light independent
reactions (Calvin Cycle) use
ATP and high energy
electrons from the light
reactions to add one
carbon, from CO2, to a 5C
organic molecule.
• Each time around the cycle
adds one more carbon
• To build a glucose molecule
would require 6 cycles
Fig. 7.8,
pg. 138.
Photosynthesis: Light Dependent Rxns
• The key enzyme is the light-independent phase of
photosynthesis is RuBisCO
• Full name: ribulose bis phosphate carboxylase
oxygenase.
• Molecules of this enzyme account for 16% of the
protein content of chloroplasts
• It is probably the most abundant enzyme on the
planet.
• It begins the process of fixing carbon by adding
carbon dioxide to a 5C sugar.
• By enzyme standards, RuBisCO is slow, adding only a
few carbon dioxide molecules per second