Chapter 4
The Energy of Life
Photosynthesis
● Some organisms, known as producers, use energy
from the sun to make organic molecules
● Autotroph- organism that gets energy by making it
out of light or chemicals
● Heterotroph- organism that gets its energy by eating
other organisms
● Plants are producers and use energy from the sun to
make glucose out of atmospheric CO2
● Photosynthesis- process by which light energy is
converted to chemical energy
● Takes place in the chloroplast
● Chlorophyll- molecule that absorbs light energy in the
chloroplast
2 steps in photosynthesis
1. Light-dependent reactions- captures energy from
sunlight to make O2, energy carrying molecules, and
ATP
2. Light- independent reactions- use energy made in
light-dependent reactions to make sugars from CO2;
also called the Calvin cycle
I expect you to know where the processes happen,
what goes IN, and what goes OUT
Overall reaction:
6CO2 + 6H2O → C6H12O6 + 6O2
Notice how it is the cellular respiration equation
BACKWARDS!
● Plants are producers
● One of the main sources of energy for most organisms on Earth
● The energy for almost all organisms begins as sunlight
● Light energy comes from the sun
● Sunlight includes several kinds of energy, such as
ultraviolet radiation, microwaves, and the visible light
spectrum
● Visible light appears white but is made of several
colors/wavelengths of light
● The process that captures energy
from sunlight to make sugars that
store chemical energy is called
photosynthesis
● Photosynthesis- process by which
light energy is converted to
chemical energy; produces sugar
and oxygen from carbon dioxide
and water
● Plants absorb visible light for
photosynthesis
● Photosynthesis occurs in the chloroplast of plant cells
● The molecule in the chloroplast that absorbs the
energy in visible light is called chlorophyll
● 2 types of chlorophyll in plants: chlorophyll a and
chlorophyll b
● Chlorophyll absorbs mostly red and blue wavelengths
of light, NOT green light
● The green color of plants comes from the reflection
of the wavelength of light that shows green
● Plants absorb red/blue light, reflect green light
● Most chloroplasts are in leaf cells
● The 2 main parts of the chloroplast needed for photosynthesis are the
grana (singular: granum) and the stroma
● Grana are stacks of structures called thylakoids
● Thylakoids are coin shaped,
membrane-enclosed compartments
● Thylakoids contain chlorophyll,
light-absorbing molecules, and
proteins
● The inside of the thylakoid is
called the lumen
● Grana are surrounded by a fluid
called the stroma
Overall reaction for photosynthesis:
6CO2 + 6H2O + light → C6H12O6 + 6O2
1. Light-dependent reactions
● Depends on the presence of light to occur
● If there’s no light, there’s no light-dependent
reactions
● “Photo” part of photosynthesis
● Happens across the thylakoid membrane
● Chlorophyll captures energy from sunlight
● Water molecules are broken down to make hydrogen
ions, electrons, and oxygen gas
● Main function of the light-dependent reactions is to
capture and transfer energy
Summary of Light-dependent Reactions
● Energy is captured from sunlight by light-absorbing
molecules (chlorophyll). The energy is transferred to
electrons that enter the electron transport chain
● Water molecules are broken down into H+ ions, electrons,
and O2 molecules
● Energized electrons provide energy for H+ ion transport
and are added to NADP+ to form NADPH
● The flow of H+ ions through ATP synthase makes ATP
● Where is happens: across the thylakoid membrane
● What goes in: electrons and water
● What goes out: oxygen, NADPH, and ATP
2.Light-independent reactions
● Also called the Calvin Cycle
● Does not need sunlight to occur
● Can happen any time that energy from the light-
dependent reactions is available
● Energy sources are ATP and NADPH
● Uses carbon dioxide and energy to make simple
sugars
● “Synthesis” part of photosynthesis
Summary of Light-independent Reactions (Calvin Cycle)
● Carbon dioxide enters the Calvin cycle
● ATP and NADPH from the light-dependent reactions transfer
energy to the Calvin and keeps the cycle going
● One high-energy 3-carbon molecule is made for every three
molecules of carbon dioxide that enter the cycle
● Two high-energy 3-carbon molecules are bonded together to
make a sugar. 6 molecules of carbon dioxide must be added to
the Calvin cycle to make one 6-carbon sugar
● Where it happens: in the stroma
● What goes in: CO2, ATP, and NADPH
● What goes out: a sugar, NADP+, and ADP
