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Lesson Objectives:
By the end of the lesson (s) I can,
1. Distinguish between the processes of photosynthesis and cellular respiration.
2. Explain the process of photosynthesis.
Vocabulary:
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)
Autotroph
Calvin cycle
Carbohydrate
Cellular respiration
Chloroplast
Light reactions
Pigment
Photosystem
Electron transport chain
Carbon Fixation
C4 Pathway
Lesson Questions:
Electron
Photosynthesis
Solar energy
Stroma
Thylakoids
Chlorophyll
Heterotroph
Granum
Caratenoid
Primary electron acceptor
Chemiosmosis
Stomata
CAM pathway
1. Explain the relationship between photosynthesis and cellular respiration.
2. Summarize the process of how photosynthesis works?
3. What is the relationship between autotrophs and heteratrophs and their
dependence on photosynthesis to obtain the energy they need for life
processes?
4. What is the impact of chlorophylls and other pigments in the biochemical
pathways of photosynthesis?
5. Summarize the main events of the light reactions.
6. Construct a model of how ATP is made during the light reactions
7. Construct a model of the light reactions that take place in the thylakoid
membrane.
8. Construct a model of the Calvin cycle
9. Explain why light reactions and the Calvin cycle are dependent on each other?
10. Explain how the world would be different if C4 plants and Cam plants had not
evolved?
Focus Question:
1. How do the structures of organisms enable life’s functions?
Overarching Question (s):
1. How and why do organisms interact with their environment and what are the
effects of these interactions?
2. How do organisms live, grow, respond to their environment, and reproduce?
3. How is energy transferred and conserved?
The Light Reactions
 Obtaining Energy
 Organisms can be classified according o how they obtain energy.
 Autotrophs are organisms that use energy from sunlight or from chemical
bonds in inorganic substances to make organic compounds.
 Most use photosynthesis to convert light energy from the sun into
chemical energy in the form of organic compounds, primarily
carbohydrates.
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 Heterotrophs are organisms that must get energy from food instead of
making it directly from sunlight or inorganic compounds.
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Photosynthesis involves a complex series of chemical reactions in which the
product of one reaction is consumed in the next reaction.
 A series of chemical reactions linked in this way is referred to as a
biochemical pathway.
Photosynthesis
 The image below shows how autotrpohs use photosynthesis o produce organic
compounds from CO2 and water. The oxygen and some organic compounds
produced are then used by cells in cellular respiration. During cellular respiration
CO2 and water are produced.
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Therefore the products of photosynthesis are reactants in cellular respiration. And
conversely the products of cellular respiration are the reactants in photosynthesis.
Photosynthesis can be divided into 2 stages:
 Light Reactions – light energy is converted into chemical energy, which
is temporarily stored in ATP and the energy carrier molecule NADPH.
 Calvin Cycle – Organic compounds are formed using CO2 and the
chemical energy stored in ATP and NADPH.
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The above equation does not explain how photosynthesis occurs.
Capturing Light Energy
 The first stage of photosynthesis is the light reactions
 Light reactions require light to happen.
 Light reactions begin with the absorption of light on chloroplasts
 Chloroplasts are organelles found in the cells of plants and algae.
 They are surrounded by 2 membranes
 Inside the inner membrane is another system of membranes called
thylakoids that are arranged as flattened sacs or pancakes.
 They are connected to and layered to form stacks called grana or
granum (singular).
 Surrounding the grana is a solution called the stroma.
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Light and Pigments
 Light from the sun appears to be white. However, it is actually made up
of many colors.
 As shown below light can be separated into its component colors
by passing it through a prism. The result is an array of colors
ranging from red at one end to violet at another.
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This is known as the visible spectrum.
Many plants contain pigments that absorb light.
These pigments absorb certain colors more strongly than others.
By absorbing certain colors pigments subtract those colors from
the visible spectrum. This causes the light being reflected or
transmitted by the pigment no longer appear white.
 Example: Lenses tinted green in sunglasses contain a
pigment that reflects and transmits green light and absorbs
all other colors. Thereby causing them to look green.
Chloroplast Pigments
 Within the thylakiods are several pigments. The most important
one is chlorophyll.
 2 types of chlorophyll
 Chlorophyll a which absorbs more red light than
blue light.
 Chlorophyll b assists chlorophyll a in absorbing
light. It absorbs more blue light but more red light.
 Known as an accessory pigment.
 Other pigments that serve as accessory pigments are known as
carotenoids.
 Yellow, orange, and brown.
 These carotenoids capture sunlight that chlorophyll a and b
cannot absorb. This allows the plant to capture more
energy in light.
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Chlorophyll is generally more abundant and hides other pigments but in
the fall many plant lose their chlorophyll and other pigments can be seen.
Converting Light Energy To Chemical Energy
 Once pigments in chloroplasts capture light energy it must be converted to
chemical energy.
 Chemical energy is stored in ATP and NADPH.
 Chloroplasts and carotenoids are grouped in clusters of a few hundred pigment
molecules in the thylakoid membrane.
 Each cluster of pigment molecules and the proteins that the molecules are
embedded in are referred to collectively as a photosystem.
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Light reactions begin when accessory pigment molecules in both
photosystems absorb light.
By absorbing light, those molecules acquire some of the energy carried by
light.
In each photosystem the energy acquired is passed quickly to the other
pigment molecules until it reaches a specific pair of chlorophyll a
molecules.
 STEP 1
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Light energy forces electrons to enter a higher energy level
in the two chlorophyll a molecules of photosytem II.
 These energized electrons are said to be excited.
 The excited electrons have enough energy to leave
chlorophyll a molecules.
 Because they have lost electrons, the chlorophyll a
molecule has undergone an oxidation reaction.
 Remember that each oxidation reaction must be
accompanied by a reduction reaction.
 Therefore, some substances must accept the
electrons that chlorophyll a has lost.
 STEP 2
 The acceptor of the electrons lost from chlorophyll a is a
molecule in the thylakoid membrane called the primary
electron acceptor.
 STEP 3
 The primary electron acceptor donates the electrons to the
first of a series of molecules located in the thylakoid
membrane.
 These molecules are called the electron transport chain
because they transfer electrons from one molecule to the
next.
 As electrons move through the electron transport chain,
they lose most of the energy they acquired when they were
excited.
 The energy they lose is used to move protons (H+)
into the thylakoid.
 STEP 4
 Light is absorbed by photosystem I.
 This happens at the same time that light is absorbed by
photosystem II.
 Electrons move from a pair of chlorophyll a molecules in
photosystem I to another primary electron acceptor.
 The electrons lost by these chlorophyll a molecules are
replaced by the electrons that have passed through the
electron transport chain in photosystem II.
 STEP 5
 Primary electron acceptor of photosystem I donates
electrons to a different electron transport chain.
 This chain brings the electrons to the side of the thylakoid
that faces the stroma.
 Once facing the stroma electrons combine with the protons
and NADP+, an organic molecule that accepts electrons
during oxidation/reduction reactions.
Making ATP in Light Reactions
 Important part of light reactions is the synthesis of ATP through chemiosmosis.
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Chemiosmosis relies on the concentration gradient of protons across the thylakoid
membrane. Therefore the amount of protons is greater inside the thylakoid than
inside the stroma
Concentration gradient of protons represents potential energy.
Energy is harnessed by enzyme called ATP synthase.
ATP is made by ATP synthase by adding a phosphate group to adenosine
diphosphate, ADP
Energy driving this reaction comes from the movement of protons inside the
thylakoid to the stroma.
See below.
The Calvin Cycle
 Calvin Cycle is a series of enzyme assisted chemical reactions that produce a 3 carbon
ring
 Carbon fixation is when atoms from CO2 are bonded or fixed into an organic compound.
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STEP 1
 CO2 diffuses into the stomata from surroundings.
 Enzyme combines each CO2 molecule with a five carbon ring
called ribulose biphosphate (RuBP).
 Resulting 6 carbon molecule is very unstable and immediately
splits into 2 three carbon molecules called 3-phosphoglycerate (3PGA)
STEP 2
 Each molecule of 3-PGA is converted into another smaller 3
carbon molecule called glyceraldehyde 3-phosphate (G3P) in a 2
part process.
 First each 3-PGA molecule receives a phosphate group
from ATP. Then it receives a proton (H+) from HADPH
and releases a phosphate group.
 The resulting ADP, NADP+ and phosphate that are
produces can be used again in the light reaction to make
more ATP and NADPH.
STEP 3
 One of the G3P molecules leaves the Calvin cycle and is used to
make organic compounds (carbohydrates) to be stored as energy
for later use.
STEP 4
 The remaining G3P molecules are converted back to RuBP
through the addition of phosphate groups from ATP.
 They will then enter the Calvin Cycle again.
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Alternative pathways
 Many plants that live in hot dry climates fix carbon in alternative ways
 Under these conditions they lose water rapidly through small pores called
stomata.
 Located on the undersurface of leaves.
 Plants can reduce the loss of water by partially closing the stomata when it
is hot and dry.
 Stomata are the pathways in which CO2 and O2 gas enter and leave plants.
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C4 Pathway
 Fixing of CO2 into 4 carbon compounds.
 The CAM Pathway
 Crassulacean Acid Methabolism, CAM Pathway, occurs at night because
of climate and temperature.
 At night CAM plants thank in CO2 and fix it to a variey of organic
compounds.
 During the day, CO2 is released from these compounds and enters
the Calvin Cycle.
 They lose less water during the day because their stomata are
closed and are opened at night.
 They tend to grow slower than other plants.
 Include Cactuses, pineapples, and jade plants.
Factors that affect photosynthesis