Photosynthesis
• Why don’t bushes or other trees usually
grow underneath large trees?
Lesson Objectives
• Identify the kind of energy that powers life.
• State why living things need energy.
• Evaluate the importance of autotrophs for providing energy
to all life.
• Describe how autotrophs and heterotrophs obtain energy.
• Define chemosynthesis.
• Compare and contrast glucose and ATP.
• Outline how living things make and use food.
• Outline the stages of photosynthesis.
• Describe the chloroplast and its role in photosynthesis.
• Identify the steps of the light reactions and the Calvin
cycle.
Terminology Review
• What is energy?
The ability to do work.
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• What is the ultimate source of all energy?
Sunlight
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Kinds of energy which power life
• What forms can energy come in?
Light, heat, chemical, nuclear, magnetic, and electrical
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• Where is energy stored?
In chemical bonds
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• How is this energy released?
By breaking the chemical bonds
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How Do Organisms Get Energy?
Autotrophs vs. Heterotrophs
Living organisms obtain chemical energy in one
of two ways.
They make it themselves OR
They consume those who can make it themselves
CAN MAKE IT THEMSELVES
Autotrophs—Photosynthesize
• Plants, algae, and some bacteria
• Producers, begin food chains which feed
all life
– Store chemical energy in carbohydrate food
molecules
• Organic molecules made through photosynthesis
store chemical energy (food)
Photosynthesis
• Provides over 99 percent of the energy
supply for life on earth
– Uses solar energy to convert water and
carbon dioxide into oxygen and glucose
CONSUMERSHeterotrophs
• Animals, fungi, and many protists and
bacteria
• Consumers, cannot make their own food
– Obtain energy through food consumption
• Autrotrophs or other Heterotrophs
– Highly diverse organisms
Chemosynthesis
• Other autotrophs: mostly bacteria in dark
or low-oxygen environments
– produce food using the chemical energy
stored in inorganic molecules such as
hydrogen sulfide, ammonia, or methane.
Tubeworms deep in the Gulf of Mexico get
their energy from chemosynthetic bacteria
living within their tissues. No digestive
systems needed!
Food to Energy Molecules:
Glucose and ATP
• Two of the most important energy-carrying
molecules
• Glucose: simple carbohydrate; energy-rich product
of photosynthesis; chemical formula C6H12O6
– “deliverable” form of energy; carried in blood through
capillaries and taken up by trillions of cells
– nearly universal food for life.
• ATP: store smaller quantities of energy; product of
first stage of photosynthesis and used during
second stage to make glucose
– provides cells with energy for cellular processes
– “useable” form of energy for your cells
Why Organisms Need Both
Glucose and ATP
• Glucose more chemical energy in a smaller
‘‘package” than a molecule of ATP
– more stable than ATP; better for storing and
transporting energy
– BUT too powerful for cells to use.
• ATP  right amount of energy to power life
processes within cells
– like a rechargeable battery
• energy released when broken down into ADP and
phosphate
• “worn-out battery” ADP recharged using new energy to
attach a new phosphate; rebuilds ATP.
ATP and ADP
• ATP: adenosine triphosphate; principle
chemical compound in which living things
store energy.
Adenine: nitrogen-containing
compound
Ribose: a 5-carbon sugar
3 phosphate groups
• ADP: adenine diphosphate; structural
similar to ATP but with one important
difference: ADP has only two phosphate
groups. ADP is converted to ATP when
available energy is used to add a
phosphate group to it.
ADP
two phosphates
Adenine: nitrogen-containing
compound
Ribose: a 5-carbon sugar
2 phosphate groups
Releasing Energy from ATP
• Energy stored in ATP is released when it
is converted to ADP and a phosphate
group.
Adding or subtracting a
3rd phosphate group allows
the cell to store and release
energy as it is needed
O
Using Biochemical Energy
• How the cells use ATP:
– To conduct active transport; like the sodiumpotassium pump
• It moves sodium ions (Na+) out of the cell and potassium ions
(K+) into the cell
• A single ATP molecule provides the energy to move three
sodium ions and two potassium ions in different directions
– Powers movement within the cell
• Moves cell organelles along microtubules by motor proteins
that use energy from ATP to generate force
http://student.ccbcmd.edu/~gkaiser/biotutuorials/eustruct/sppump.html
Photosynthesis: The Most Important
Chemical Reaction for Life on Earth
• Necessary conditions include:
– enzymes - proteins to speed up chemical
reactions
– chlorophyll - a pigment which absorbs light
– chloroplasts – which contain chlorophyll,
accessory pigments, and enzymes in patterns
which maximize photosynthesis
Stages of Photosynthesis
• Two stages:
– light reactions uses water; changes light
energy into chemical energy
• releases oxygen as a waste product.
– Calvin cycle uses chemical energy in ATP
and NADPH to make glucose
Chloroplasts: Theaters for
Photosynthesis
• Chloroplast contain:
– neat stacks called grana (singular, granum).
• consist of sac-like membranes, known as thylakoid
membranes
– Thylakoid membranes
• contain photosystems
– groups of molecules that include chlorophyll
• light reactions occur in thylakoid membranes.
– stroma
• space outside the thylakoid membranes
• reactions of the Calvin cycle occur here
Chloroplasts Function
• Work with enzymes and two basic
molecules: pigments and electron carriers
• Electron carrier molecules are usually
arranged in electron transport chains
(ETCs).
Photosynthesis Stage I:
The Light Reactions
Chloroplasts Capture Sunlight’s Energy
Light-Dependent Reactions
• Require LIGHT
• Use light energy to produce
– Oxygen gas
– Convert ADP to
energy carrying
ATP
– Convert NADP+ to
energy carrying
NADPH
Photosynthesis Stage II: The Calvin Cycle
Making Food “From Thin Air”
• Three major steps:
– Carbon fixation
– Reduction
– Regeneration
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Why is Carbon Dioxide “Fixed”
• Life on Earth is carbon-based
– needed in building blocks of biological molecules
– ultimate source of carbon is carbon dioxide
• Animals and most other heterotrophs cannot take in
CO2 directly
• Only autotrophs can build low energy inorganic CO2
into high-energy organic molecules like glucose
Three Pathways for Carbon
Fixation—Calvin Cycle
1. C-3 pathway
a. Most common
b. 6-C molecule splits into two 3-C molecules
Dry air, hot temperatures, bright sunlight lead to below two
pathways:
2. C-4 pathway
a. creates a 4-C molecule
3. CAM (Crassulacean Acid Metabolism)
a. cacti and succulents
b. Fix carbon dioxide at night
Factors Affecting Photosynthesis
• Shortages of water slow process down; can
stop it
– Plants that live in dry areas have a waxy coating on
their leaves that reduces water loss
• Temperature also can slow or stop it
– Enzymes used by plants for photosynthesis function
best between 0°C and 35°C (32°F to 95°F)
• Intensity of light
– More light = greater rate of photosynthesis until
maximum reached
• Maximum rate varies from plant to plant
Let’s Review
• Where does photosynthesis occur?
In the chloroplasts
_________________
• What are the saclike photosynthetic membranes
thylakoids
in the chloroplast called? _________________
• The thylakoids are arranged in stacks called
grana
__________.
• What is found inside the grana?
Clusters of chlorophyll and other pigments
______________________________________
• What are photosystems?
Proteins found in the grana
______________________________________
• What do photosystems do?
They capture the energy of sunlight
______________________________________
• How many stages does photosynthesis
2
have? ______
• What are they?
Light reactions
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Light-independent reactions (Calvin cycle)
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• Where do the light-dependent reactions
take place?
Thylakoid membrane
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• Where do the light-independent reactions
take place?
Stroma; region outside of the thylakoid membrane
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