Photosynthesis
Chapter 8
Energy and life
• [1] __Energy___ is the ability to do work.
Autotrophs
• Plants and some
other types of
organisms are able to
use light energy from
the sun to produce
food. Organisms such
as plants, which
make their own food,
are called [2]
__autotrophs_.
Heterotrophs
• Other organisms,
such as animals,
cannot use the sun's
energy directly.
These organisms,
known as [3]
__heterotrophs__,
obtain energy from
the foods they
consume.
Living things
• To live, all organisms,
including plants,
must release the
energy in sugars and
other compounds.
• Autotrophs use [4]
_light__ energy from
the sun to produce
food.
Chemical energy and ATP
• Energy comes in
many forms,
including light, heat,
and electricity.
Energy can be stored
in [5] _chemical ____
compounds
ATP
• Living things use chemical
fuels as well. One of the
principal chemical
compounds that cells use
to [6] __store___ and [7]
_release__ energy is
adenosine triphosphate,
abbreviated ATP.
• ATP consists of adenine, a
5-carbon sugar called
ribose, and three
phosphate groups. Those
three phosphate groups
are the key to ATP's ability
to store and release energy
ADP
• Adenosine
diphosphate (ADP) is
a compound that
looks almost like ATP,
except that it has [9]
__two___ phosphate
groups instead of
three. This difference
is the key to the way
in which living things
store energy.
ADP to ATP
• When a cell has energy
available, it can store
small amounts of it by
adding a
[10] _phosphate__
group to ADP
molecules, producing
ATP, as shown in the
figure at right. In a
way, ATP is like a fully
charged battery, ready
to power the
machinery of the cell.
Release of energy
• How is the energy
that is stored in ATP
released? Simply by
[11] ___breaking__
the chemical bond
between the second
and third [12]
_phosphate group__,
energy is released
Using biochemical energy
• One way cells use the
energy provided by
ATP is to carry out [13]
__active transport__.
• Many cell membranes
contain a sodiumpotassium pump, a
membrane protein
that pumps [14]
_sodium ions__ (Na+)
out of the cell and [15]
__potassium__ (K+)
into it.
Protein synthesis
• Energy from ATP
powers other
important events in
the cell, including the
[17] __synthesis_ of
proteins and nucleic
acids and responses
to chemical signals at
the cell surface.
ATP storage
• Most cells have only a [18] _limited__ amount of ATP, enough
to last them for a few seconds of activity.
• Even though ATP is a great molecule for transferring energy, it
is not a good one for storing large amounts of energy over the
long term.
Glucose
• A single molecule of the sugar[19] _Glucose__ stores more
than 90 times the chemical energy of a molecule of ATP.
• Therefore, it is more efficient for cells to keep only a small
supply of ATP on hand.
• Cells can [20] __regenerate_ ATP from ADP as needed by using
the energy in foods like glucose.
ATP from glucose
8.2 - Photosynthesis
• The key cellular process identified with energy production is
[21] __photosynthesis___.
• In the process of photosynthesis, plants use the energy of
sunlight to convert [22] __water___ and carbon dioxide into
high-energy [23] ___carbohydrates____—sugars and
starches—and [24] _oxygen_, a waste product.
Photosynthesis
• [25] __Photosynthesis__ uses the energy of sunlight to
convert water and carbon dioxide into high-energy sugars and
oxygen.
• Plants then use the sugars to produce complex carbohydrates
such as starches.
• Plants obtain carbon dioxide from the air or water in which
they grow.
• Photosynthesis is a series of [26] __chemical reactions__ that
uses light energy from the sun to convert water and carbon
dioxide into sugars and oxygen.
Photosynthesis equation
• Although the equation tells you that [27] __water__ and
• [28] ___carbon dioxide__ are required for photosynthesis, it
does not tell you how plants use these low-energy raw
materials to produce high-energy sugars.
Light and pigment
• In addition to water
and carbon dioxide,
photosynthesis
requires light and [29]
__chlorophyll__, a
molecule in
chloroplasts.
• Plants gather the sun's
energy with lightabsorbing molecules
called [30] _pigments_.
Chlorophyll
• The plants' principal
pigment is chlorophyll.
• There are two main
types of chlorophyll:
chlorophyll a and
chlorophyll b.
• Chlorophyll [31]
___absorbs__ light
very well in the blueviolet and red regions
of the visible
spectrum.
Chlorophyll – light absorption
• However, chlorophyll
does not absorb light
well in the green
region of the
spectrum.
• Green light is
reflected by leaves,
which is why plants
look green.
Chlorophyll – light absorption
• Plants also contain red and orange pigments such as carotene
that absorb light in other regions of the spectrum.
• Photosynthesis requires light and chlorophyll.
• Because [32] ___light___ is a form of energy, any compound
that absorbs light also absorbs the [33] __energy___ from that
light.
Chlorophyll – light absorption
• When chlorophyll
absorbs light, much of
the energy is
transferred directly to
[34] _electrons_ in the
chlorophyll molecule,
raising the energy
levels of these
electrons.
• These [35] _highenergy__ electrons
make photosynthesis
work.
Van Helmont's Experiment
• In the 1600s, the Belgian physician Jan van Helmont devised
an experiment to find out if plants grew by taking material out
of the soil.
• Determined the mass of a pot of dry soil and a small seedling.
• Planted the seedling in the pot of soil and watered it regularly.
• At the end of five years, the seedling, which by then had
grown into a small tree, had gained about 75 kg
• The mass of the soil, however, was almost unchanged. He
concluded that most of the gain in mass had come from water,
because that was the only thing that he had added.
Priestley's Experiment
• English minister Joseph Priestley performed an experiment
that would give another insight into the process of
photosynthesis.
• Priestley took a candle, placed a glass jar over it, and watched
as the flame gradually died out. Something in the air, Priestley
reasoned, was necessary to keep a candle flame burning. That
substance was oxygen.
• Priestley then found that if he placed a live sprig of mint under
the jar and allowed a few days to pass, the candle could be
relighted and would remain lighted for a while.
• The mint plant had produced the substance required for
burning. It released oxygen.
Jan Ingenhousz
• Dutch scientist Jan Ingenhousz showed that the effect
observed by Priestley occurred only when the plant was
exposed to light.
• The results of both Priestley's and Ingenhousz's experiments
showed that light is necessary for plants to produce oxygen.
• The experiments performed by van Helmont, Priestley, and
Ingenhousz led to work by other scientists who finally
discovered that in the presence of light, plants transform
carbon dioxide and water into carbohydrates, and they also
release oxygen.
8-3 The Reactions of Photosynthesis
• Inside a Chloroplast
• photosynthesis takes place inside chloroplasts.
• Chloroplast, contain saclike photosynthetic membranes called
thylakoids (THY-luh-koydz).
• Thylakoids are arranged in stacks known as grana (singular:
granum).
• Proteins in the thylakoid membrane organize chlorophyll and
other pigments into clusters known as photosystems.
• These photosystems are the light-collecting units of the
chloroplast.
Photosystem
• The reactions of
photosystems in two
parts:
• the light-dependent
reactions [takes place
in the thylakoid
membranes]
• the light-independent
reactions, or Calvin
cycle [takes place in
the stroma]
Electron Carriers
• Sunlight excites electrons in chlorophyll, the electrons gain a
great deal of energy.
• Electron carriers are used to transport high-energy electrons
from chlorophyll to other molecules.
• A carrier molecule is a compound that can accept a pair of
high-energy electrons and transfer them along with most of
their energy to another molecule.
• This process is called electron transport, and the electron
carriers themselves are known as the electron transport chain
Electron carriers
Carrier molecule
• One of these carrier
molecules is a compound
known as NADP+
(nicotinamide adenine
dinucleotide phosphate).
• NADP+ accepts and holds 2
high-energy electrons along
with a hydrogen ion (H+).
• This converts the NADP+ into
NADPH.
• The NADPH can then carry
high-energy electrons
produced by light absorption
in chlorophyll to chemical
reactions elsewhere in the
cell.
Light-Dependent Reactions
• The light-dependent
reactions use energy
from light to produce
ATP and NADPH.
• The light-dependent
reactions produce
oxygen gas and
convert ADP and
NADP+ into the
energy carriers ATP
and NADPH.
Light-dependent reactions
• Photosynthesis begins
when pigments in
photosystem II absorb
light.
• That first photosystem is
called photosystem II
because it was discovered
after photosystem I.
• The light energy is
absorbed by electrons,
increasing their energy
level.
• These high-energy
electrons are passed on to
the electron transport
chain.
Light-dependent reactions
• The thylakoid membrane
contains a system that provides
new electrons to chlorophyll to
replace the ones it has lost.
• These new electrons come from
water molecules (H2O).
• Enzymes on the inner surface of
the thylakoid membrane break
up each water molecule into 2
electrons, 2 H+ ions, and 1 oxygen
atom.
• The 2 electrons replace the highenergy electrons that chlorophyll
has lost to the electron transport
chain.
• As plants remove electrons from
water, oxygen is left behind and
is released into the air.
Light-dependent reactions
• High-energy electrons
move through the
electron transport chain
from photosystem II to
photosystem I.
• Energy from the
electrons is used by the
molecules in the
electron transport chain
to transport H+ ions from
the stroma into the inner
thylakoid space.
Light-dependent reactions
• Pigments in
photosystem I use
energy from light to
reenergize the
electrons.
• NADP+ then picks up
these high-energy
electrons, along with
H+ ions, at the outer
surface of the
thylakoid membrane,
plus an H+ ion, and
becomes NADPH.
Light-dependent reactions
• As electrons are passed from
chlorophyll to NADP+, more
hydrogen ions are pumped across
the membrane.
• After a while, the inside of the
membrane fills up with positively
charged hydrogen ions.
• This makes the outside of the
thylakoid membrane negatively
charged and the inside positively
charged.
• The difference in charges across
the membrane provides the
energy to make ATP.
• This is why the H+ ions are so
important.
Light-dependent reactions
• H+ ions cannot cross the
membrane directly.
• However, the cell membrane
contains a protein called ATP
synthase (SIN-thays) that
spans the membrane and
allows H+ ions to pass through
it.
• As it rotates, ATP synthase
binds ADP and a phosphate
group together to produce
ATP.
• Because of this system, lightdependent electron transport
produces not only highenergy electrons but ATP as
well.
Light-dependent reactions
• The light-dependent
reactions use water,
ADP, and NADP+, and
they produce oxygen
and two high-energy
compounds: ATP and
NADPH.
Calvin cycle
• Six carbon dioxide
molecules enter the
cycle from the
atmosphere.
• The carbon dioxide
molecules combine
with six 5-carbon
molecules.
• The result is twelve
3-carbon molecules.
Calvin cycle
• The twelve 3-carbon
molecules are then
converted into
higher-energy forms.
• The energy for this
conversion comes
from ATP and highenergy electrons
from NADPH.
Calvin cycle
• Two of the twelve 3carbon molecules are
removed from the
cycle.
• The plant cell uses
these molecules to
produce sugars, lipids,
amino acids, and other
compounds needed for
plant metabolism and
growth.
Calvin cycle
• The remaining ten 3carbon molecules are
converted back into
six 5-carbon
molecules.
• These molecules
combine with six new
carbon dioxide
molecules to begin
the next cycle.
Photosynthesis
Photosynthesis - summary
• Light-dependent
reactions
• Purpose – produce
energy for the Calvin
cycle
• Reactant
• H2O
• Products
• ATP
• NADPH
• O2
• Calvin cycle
• Purpose – produce
food for the plant
• Reactant
• CO2
• Products
• High-energy sugars
• Sucrose
• Glucose
• Fructose