Biology
Lesson 9.2:
Photosynthesis: An
Overview
OBJECTIVES:
1. Describe the role pigments play in the
process of photosynthesis.
2. Describe what electron carrier molecules
do.
3. Identify the reactants and products of
photosynthesis.
CHLOROPHYLL & CHLOROPLASTS
➢ Light: sunlight is a mixture of different wavelengths
of light
○ Wavelengths that are visible to our eyes are known
as the visible spectrum.
➢ Pigments: light-absorbing compounds
○ Photosynthetic organisms use pigments to capture
the energy in sunlight
○ Chlorophyll: principal pigment of green plants
■ Chlorophyll doesn’t absorb light well in the green
region of the spectrum, so it reflects it instead.
CHLOROPHYLL & CHLOROPLASTS
➢ Chloroplasts
○ Photosynthesis takes place inside organelles called
chloroplasts
○ Chloroplasts are surrounded by two envelope
membranes and they are filled with saclike
chlorophyll-containing membranes called thylakoids
■ Thylakoids are interconnected and arranged in
stacks known as grana
■ Stroma: fluid portion of the chloroplast, outside
the thylakoids
CHLOROPHYLL & CHLOROPLASTS
➢ Energy collection
○ Chlorophyll
transfers light
energy to its own
electrons
■ Then these highenergy electrons
are available to
do chemical
work
HIGH ENERGY ELECTRONS
➢ High-energy electrons produced by chlorophyll are
highly reactive and require a special “carrier”
○ An electron carrier is a compound that can accept a
pair of high-energy electrons and transfer them,
along with most of their energy, to another molecule
HIGH ENERGY ELECTRONS
➢ Example: NADP+ accepts and holds two highenergy electrons, along with a hydrogen ion (H+).
■ This converts NADP+ into NADPH and NADPH
can carry the high-energy electrons that were
produced by light absorption in chlorophyll to
chemical reactions elsewhere in the chloroplast.
■ The high-energy electrons can help build sugars
like glucose from nothing more than carbon
dioxide and water.
AN OVERVIEW OF PHOTOSYNTHESIS
➢ Photosynthesis uses the energy of sunlight to convert
water and carbon dioxide (low-energy reactants) into
high-energy sugars and oxygen (products)
AN OVERVIEW OF PHOTOSYNTHESIS
➢ Light-dependent reactions: first set of reactions that happen
during photosynthesis
○ Requires direct involvement of light and light-absorbing
pigments
○ Takes place in thylakoid membranes and uses energy from
sunlight to add a third phosphate to ADP to make ATP
○ Light-dependent reactions also take low-energy electrons
from water molecules and uses solar energy to raise them to
a much higher energy level
■ High-energy electrons are then transferred to the electron
carrier NADP+, which is converted to NADPH
AN OVERVIEW OF PHOTOSYNTHESIS
➢ Light-independent reactions: second set of reactions that
happen during photosynthesis
○ Plants absorb carbon dioxide from the atmosphere and
complete the process of photosynthesis by producing
carbon-containing sugars and other carbohydrates
○ No light is required to power the light-independent reactions,
which take place outside the thylakoids, in the stroma of the
chloroplast
➢ Light-dependent and light-independent reactions work
together to capture the energy of sunlight and transform
into energy-rich compounds such as carbohydrates