CHAPTER 5-2
PHOTOSYNTHESIS
Biochemical Pathways
‘ Photosynthesis and
Cellular Respiration are
examples of
Biochemical Pathways
– a series of connected
reactions where the
product(s) of one
reaction becomes the
reactant(s) of the next
HOW ENERGY CYCLES
‘Photosynthesis
and respiration
form a continuous
cycle because the
products of one
process are the
reactants for the
other.
HOW ENERGY CYCLES
‘ Photosynthesis uses carbon dioxide, water and
light energy and produces carbohydrates and
oxygen.
CO2 + H2O + light energyÆ Glucose + O2
‘ Cellular respiration uses carbohydrates and
oxygen and produces carbon dioxide, water and
energy for cell activities.
Glucose + O2 Æ CO2 + H2O + energy (ATP)
LIGHT
‘ Light is a form of radiant energy. It is packaged as photons
‘ It is transmitted in waves and can travel through a vacuum.
‘ The complete range of radiant energy is called the
electromagnetic spectrum.
Different forms of radiant
energy differ in both
wavelength and amount of
energy they transmit.
A portion of the
electromagnetic spectrum
makes up the visible
spectrum - ROYGBIV.
5-2 HOW PHOTOSYNTHESIS WORKS
‘ Overall reaction of photosynthesis:
CO2 + H2O + light ---------> CH2O (C6H12O6) + O2
chlorophyll and enzymes
‘ Photosynthesis takes place in two stages:
1. Light energy is captured and converted to chemical
energy.
2. Organic molecules (glucose) are produced.
‘ Stages 1 is often referred to as the light (dependent)
reaction.
‘ Stage 2 is called the light-independent (dark) reaction.
DIAGRAM OF 3 STAGES
Chloroplasts
‘ In plants and most other
photosynthetic organisms,
photosynthesis occurs
inside chloroplasts.
‘ Chloroplasts have very
complex internal structures.
– flattened membrane-bound
sacs= thylakoids
– stacks of thylakoids = grana
– fluid matrix surrounding
stacks = stroma
Photosynthetic Pigments
‘ A pigment is a compound that absorbs light.
‘ Plants contain a variety of pigments including
– Chlorophylls - a and b (green)
• Chl. a is the primary photosynthetic pigment
– Carotenoids (red, orange and brown)
– Other pigments (blue, yellow, etc.)
‘ Chlorophyll b and carotenoids are considered
accessory pigments because they assist
chlorophyll a in photosynthesis
Why are plants green?
‘ Absorption vs reflection –
– Green light is reflected (into our eyes)
– The other colors are absorbed (used in photosyn.)
‘Plants appear green because
most of the pigments in plants
are chlorophylls; the other
pigments are hidden (except in
the Fall).
STAGE 1: LIGHT IS CAPTURED
‘ Occurs in grana of chloroplast
‘ Plant pigments are arranged in molecule clusters
called photosystems.
‘ Most photosynthetic organisms have two kinds of
photosystems: photosystem I and photosystem II
which absorb light at slightly different
wavelengths.
‘ Energy from photons is funneled to special
“reactive” chlorophyll a molecules
STAGE 1: LIGHT IS CAPTURED
‘electrons in chlorophyll a are boosted to a higher energy
level.
‘Water molecules in the thylakoid space split releasing
oxygen (O2), hydrogen ions (H+, protons) and electrons
(passed to pigments in Photo II). The electrons replace
those boosted by photons.
STAGE 1: LIGHT ENERGY IS CONVERTED TO
CHEMICAL ENERGY
The boosted electrons are accepted by the primary electron
acceptor.
STAGE 2: LIGHT ENERGY IS CONVERTED
TO CHEMICAL ENERGY
The electrons are
passed between
proteins in the
electron
transport chain
producing ATP
through
chemiosmosis.
‘ The electrons are passed to photosystem I where they
are boosted by photons again.
‘ Electrons are passed through another ETC, producing
NADPH from NADH+ (coenzyme)
Electrons lost
by photosystem I are
replaced by efrom photo II.
Chemiosmosis
‘ Formation of ATP through
the use of a H+
concentration gradient
across the thylakoid
membrane
– H+ ions are moved into
thylakoid space with energy
from electrons
– As concentration builds up,
they are allowed out of the
space; the energy released as
they are allowed out is used
to make ATP from ADP
using ATP synthase.
Review
– excited electrons in Photosystem II are used to
make ATP
– excited electrons in Photosystem I are used to
make NADPH
– these two products fuel the light-independent
reactions
STAGE 2: CHEMICAL ENERGY IS STORED IN
ORGANIC MOLECULES (“Light-Independent”
Reactions)
‘ ATP and NADPH are used to fix carbon in the
Calvin cycle (a.k.a. “carbon fixation”).
‘ Reactions do not need light - only the products of
the “light” reactions and CO2
‘ ATP powers the “dark” reactions
‘ NADPH provides H needed to make CHO
‘ CO2 provides the raw materials C and O2
– CO2 usually enters through stomata by day
Calvin Cycle
‘ Calvin Cycle occurs in stroma
‘ For every 6 carbon dioxide molecules that enter the
carbon cycle, 1 six-carbon sugar molecule can be
produced.
OVERVIEW
Other Photosynthesis Pathways
‘ Most plants are C3 plants because the first stable
molecule they produce (PGA) has 3 carbons.
‘ Plants that have evolved in hot, dry climates need
a mechanism to prevent water loss and still
complete photosynthesis
– Problem: open stomata let CO2 in but H2O out.
‘ Solution: Other types of plants “fix” carbon into
other compounds and then in a different location
or at a different time release the CO2 into the
Calvin cycle.
Other Photosynthesis Pathways
‘ C4 plants fix carbon first into 4 carbon molecule.
This reaction can be done in much lower CO2
levels than C3 plants. Then these compounds are
moved, and CO2 is released into the Calvin cycle
– examples: crops; corn, sugar cane
‘ CAM plants open stomata at night, and close
them during the day. The CO2 collected at night
is fixed into other compounds, and enters the
Calvin Cycle during the day when stomata are
closed.
– examples: cacti (succulents) and some aquatic plants
Factors Affecting Photosynthesis Rate
‘ Light - rate of photosynthesis ↑ as light intensity
↑ up to a certain point
‘ CO2 = similar pattern
‘ Temperature - rate of reaction doubles for every
10o C ↑ in temperature until enzymes denature.