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Photosynthesis
- Chapter 8
8-1 Energy and Life
• Energy is the ability to do work
• Living things depend on energy to maintain
homeostasis
• Without the ability to obtain and use energy,
life would cease to exist
• Where does this energy come from?
Chemical Energy and ATP
• Energy comes in many forms
Exs. – light, heat and electricity
• ATP and ADP
– Cell activities are powered by chemical
fuels
– One of the principal chemical compounds
that living things use to store and release
energy is ATP (adenosine triphosphate)
• An ATP molecule consists of a
nitrogen-containing compound called
adenine, a 5-carbon sugar called
ribose, and three phosphate groups
Fig. 8-2 ATP is used by all types of cells as their basic energy
source. For example, the energy needed by the cells of a soccer
player comes from ATP.
pg. 202
• ADP (adenosine diphosphate) has a
structure that is similar to ATP
• There is one important difference: ADP
has two phosphate groups instead of
three
• This is the key to the way in which
cells store energy
• A cell can store small amounts of
energy by adding a phosphate group
to ADP molecules, producing ATP
molecules
• ATP is like a fully charged battery,
ready to power the machinery of the
cell.
pg. 203
Releasing Energy From ATP
• Energy stored in ATP is released when
ATP is converted into ADP and a
phosphate group
• Cells can add and subtract a third
phosphate group giving it a way of
storing and releasing energy as
needed.
iText Video – ATP Formation (pg. 203)
• Most cells have only a small amount of
ATP, enough to last for only a few
seconds of activity
• ATP is a great molecule for
transferring energy.
• It is not good for long term energy
storage
The ATP Cycle
(ATP Formation)
The ATP Cycle
Using Biochemical Energy
• ATP carries just enough energy to
power a variety of cellular activities
– Exs.:
• Active Transport (Protein Pumps)
• Movement (cilia, flagella, muscles)
• Light (Fireflies)
• The characteristics of ATP make it an
exceptionally useful molecule that is
used by all types of cells as their basic
energy source
Autotrophs and Heterotrophs
• Originally, nearly all energy in food
comes from the Sun.
• The energy that living things need
comes from food
• Autotrophs are organisms such as
plants, which make their own food
• Heterotrophs obtain energy from the
foods they consume
– Ex.: Impalas, leopards, & mushrooms
Fig. 8-1 Autotrophs vs. Heterotrophs Autotrophs use light energy from
the sun to produce food. These impalas get their energy by eating grass. A
leopard, in contrast, gets its energy by eating impalas and other animals.
Impalas and leopards are both heterotrophs.
pg. 201
8–2 Photosynthesis: An Overview
• Photosynthesis is a process in which
plants use the energy of sunlight to
convert water and carbon dioxide into
oxygen and high-energy
carbohydrates (sugars and starches)
that can be used as food.
An Overview of
Photosynthesis
An Overview of Photosynthesis
• The overall equation for photosynthesis
can be shown as follows:
6CO2 + 6H2O
sunlight
C6H12O6 + 6O2
chlorophyll
carbon dioxide +
water
sugar
+
oxygen
Investigating Photosynthesis
• The experiments of many scientists
have contributed to the modern
understanding of the process of
photosynthesis
Van Helmont’s Experiment (1600s)
• Experiment to find out if plants grew by taking
material out of the soil
• Van Helmont concluded that most of the
mass the plant gained had come from water,
because that was the only thing that he had
added to the pot.
• Priestley’s Experiment (100+ yrs
later)
– Took a candle, placed a glass jar over it,
and watched as the flame gradually died
out
– Placed a live sprig of mint under the jar
– Discovered that plants release oxygen
• Jan Ingenhousz
– Showed that the effect observed by Priestley
occurred only when the plant was exposed to
light
– Light is necessary for plants to produce oxygen
• The experiments performed by van
Helmont, Priestley, Ingenhousz, and
other scientists reveal that in the
presence of light, plants transform
carbon dioxide and water into
carbohydrates and release oxygen
Chlorophyll and Chloroplasts
• Energy from the Sun travels to Earth
as light.
• Our eyes perceive this as “white light”
which is actually mixture of different
wavelengths.
• We see these wavelengths as:
Red, Orange, Yellow, Green, Blue,
Indigo and Violet.
• In addition to water and carbon dioxide,
photosynthesis requires light and
chlorophyll, a molecule in chloroplasts
• Plants gather the sun’s energy with lightabsorbing molecules called pigments
• The plants’ principal pigment is chlorophyll
• There are two main types of chlorophyll:
– chlorophyll a
– chlorophyll b
• Chlorophyll absorbs the blue and red
regions of the visible spectrum
• Chlorophyll does not absorb light well in the
green region of the spectrum
• This is why plants are green
• Plants also contain red and orange
pigments that absorb light in other regions
of the spectrum
– Ex.: carotene
pg. 207
Fig. 8-5 Photosynthesis requires light and chlorophyll, which absorbs
light energy. In the graph, notice how chlorophyll a absorbs light in the violet and
red regions of the visible spectrum, while chlorophyll b absorbs light in the blue
and red regions of the visible spectrum.
Chloroplasts
• iText Video, Pg. 208
• Inside a Chloroplast
– Photosynthesis takes place inside
chloroplasts
– Chloroplasts contain saclike
photosynthetic membranes called
thylakoids
• Thylakoids:
– Are arranged in stacks known as grana
(singular: granum)
– The fluid portion of the chloroplast outside
the thylakoids is known as the stroma.
– Contain clusters of chlorophyll and other
pigments and photosystems (proteins that
are able to capture the energy of sunlight.
High-Energy Electrons
• Sunlight excites electrons in
chlorophyll and the electrons gain a
great deal of energy
• These high-energy electrons require a
special carrier
• One of these carrier molecules is a
compound known as NADP+
• The conversion of NADP+ into NADPH
is one way in which some of the
energy of sunlight can be trapped in
chemical form.
Fig. 8-4 Photosynthesis is a series of reactions that uses
energy from the sun to convert water and carbon dioxide
into sugars and oxygen. Photosynthesis takes place in a plant
organelle called the chloroplast.
pg. 206
• The photosynthesis reaction has two
stages:
– The Light-Dependent reactions
• take place within the thylakoid membranes
– The Light-Independent reactions (Calvin
Cycle)
• The Calvin Cycle takes place in the stroma,
the region outside the thylakoid membranes.
The Process of Photosynthesis
Light-Dependent
Reactions
Light-Independent
Reactions
pg. 209
iText Video - Pg. 209
8–3 The Process of
Photosynthesis
Light-Dependent Reactions
(ATP and NADPH)
• Require light
• Why plants need light to grow
• Use energy from light to produce
oxygen gas, ATP and NADPH
iText Video – Light-Dependent Reactions (Part 2)
Pg. 211
Light-Independent Reactions
(Sugars)
• The light-independent (dark) reactions use
ATP and NADPH from the light-dependent
reactions to produce high-energy sugars
• These reactions are also called the
Calvin Cycle
• The Calvin Cycle does not require light
iText Video – Calvin Cycle
Pg. 212
pg. 212
Factors Affecting Photosynthesis
• Many factors affect the rate at which
photosynthesis occurs
• Temperature:
– Temperatures above or below 0°C and
35°C may slow down the rate of
photosynthesis
– At very low temperatures, photosynthesis
may stop entirely
– Plants at these temperatures can carry
out photosynthesis only on sunny days
• Intensity of Light
– Affects the rate at which photosynthesis
occurs.
– Increasing light intensity increases the
rate of photosynthesis
– At a certain level, the plant reaches its
maximum rate of photosynthesis
– This level varies from plant to plant.
• Water:
– A shortage of water can slow or even
stop photosynthesis
– Plants that live in dry conditions (desert
plants and conifers) have a waxy coating
on their leaves that reduces water loss
Photosynthesis Under
Extreme Conditions
• Plants have small openings (stomata) in
their leaves that admit CO2 for
photosynthesis
• To prevent the plant from drying out, these
openings must close to conserve water.
• This may slow down or stop the process of
photosynthesis
C4 and CAM Photosynthesis
• Some plants have adapted to extremely
bright, hot conditions
• There are two major groups of these
specialized plants:
– C4 plants
– CAM plants
• These processes minimize water loss while
still allowing photosynthesis in intense
sunlight
• C4 plants have an added step during the
carbon-fixing stage to preserve moisture in
hotter climates.
C4 plants capture CO2 so that plants can
keep working under intense light and high
temperatures.
• C4 Plants:
–
–
–
–
Corn
Soy Beans
Sugar Cane
Crabgrass
• CAM plants take in CO2 only at night,
trapping carbon in the leaves.
• During the day, when the openings in
the leaves are tightly closed, the CO2
is released allowing photosynthesis to
take place.
• CAM Plants:
– Pineapple
– Cactus