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Chapter 8
8.1 Energy and Life
Energy
• Plants and some other types of organisms
are able to use light energy from the sun to
produce food.
• Organisms that make their own food are
called autotrophs or producer.
• Other organisms cannot use the sun’s
energy directly. These organisms, called
heterotrophs or consumer, obtain energy
from the foods they consume.
List some Autotrophs
List some Heterotrophs
ATP
• One of the principal chemical compounds
that cells use to store and release energy is
adenosine triphosphate, or ATP.
• ATP consists of adenine, a 5-carbon sugar
called ribose, and three phosphate
groups.
ADP
• Adenosine diphosphate (ADP) is a similar
compound that has only two phosphate groups
instead of three.
• When a cell has energy available, it can store small
amounts of energy by adding a third phosphate
group to ADP, producing ATP.
• The energy stored in ATP can be released by
breaking the bond between the second and
third phosphate groups.
• Because a cell can subtract this third phosphate
group, it can release energy as needed.
• The characteristics of ATP make it exceptionally
useful as the basic energy source of all cells.
• Cells use energy from ATP to carry out many
important activities, including active transport,
synthesis of proteins and nucleic acids, and
responses to chemical signals at the cell surface.
8.2
• Photosynthesis: An Overview
PHOTOSYNTHESIS
What is Photosynthesis?
Plants using high-energy carbohydrates
– sugars and starches – and oxygen, a waste product.
What do you need for Photosynthesis?
Van Helmont
Priestley’s
Experiment
• 100 years
after Van
Helmont
• Candle, Jar,
plant
Ingenhousz
• showed that the effect observed by Priestley
occurred only when the plant was exposed to light.
• 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.
PHOTOSYNTHESIS
The sun
Carbon dioxide
from the air
A plant
Water from the
soil
PHOTOSYNTHESIS
What does photosynthesis make?
PHOTOSYNTHESIS
Oxygen
(waste)
Food for
plant (sugar)
PHOTOSYNTHESIS
water + Carbon dioxide
sugar + Oxygen
• Photosynthesis uses the energy of sunlight
to convert water and carbon dioxide into
high energy sugars and oxygen.
• Plants use the sugars to produce complex
carbohydrates such as starches.
• Plants obtain the carbon dioxide they need
for photosynthesis from the air or from the
water in which they grow.
The sun
PHOTOSYNTHESIS
Oxygen
(waste)
Carbon dioxide
from the air
Food for plant
(sugar)
Water from the
soil
PHOTOSYNTHESIS
Where does photosynthesis
take place?
PHOTOSYNTHESIS
vacuole
Cell wall
nucleus
Cell
membrane
Chloroplasts!!!
Plant Cell
• In addition to water and carbon dioxide,
photosynthesis requires light and
chlorophyll.
• 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 and chlorophyll b.
• Chlorophyll absorbs light in the blue-violet and
red regions very well.
• But it does not absorb light in the green region
well.
• Green light is reflected by leaves, which is why
plants look green.
• Any compound that absorbs light absorbs the
energy in light.
• When chlorophyll absorbs sunlight, much of
the energy of the light is transferred directly to
the electrons in the chlorophyll molecule,
raising the energy level of the electrons
8.3
• The reactions of Photosynthesis
PHOTOSYNTHESIS
Chloroplasts are the GREEN parts of a plant cell.
They contain a chemical called CHLOROPHYLL.
Chlorophyll is a catalyst.
A catalyst speeds up a chemical reaction,
but doesn’t get used itself.
PHOTOSYNTHESIS
So PHOTOSYNTHESIS takes place in the
Chloroplasts
• Inside each chloroplast is a saclike structure
known as a thylakoid. A stack of thylakoids
forms a granum
• Proteins in the thylakoid membrane
organize chlorophyll and other pigments
into clusters known as photosystems.
• These photosystems are the light
collecting units of chlorophyll.
• The reactions of photosynthesis occur in
two parts:
– (1) the light-dependent reactions and
– (2) the light-independent reactions, which are
also called the Calvin cycle.
• (Calvin cycle takes place in the stroma outside the
thylakoid membranes)
Light and Dark
The Reactions of Photosynthesis
• When sunlight excites electrons in chlorophyll, the
electrons gain a great deal of energy.
• electron carriers transfer 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.
• Formation of
NADPH When
NADP+ accepts
a pair of highenergy
electrons, it
becomes
NADPH.
Light-Dependent Reactions
• light-dependent reactions require light
• The light-dependent reactions produce
oxygen gas and convert ADP and NADP+
into the energy carriers ATP and NADPH.
• The light-dependent reactions take place
within thylakoid membranes of
chloroplasts.
• H+ ions cannot cross the membrane
directly. However, the membrane
contains a protein called ATP synthase
(sin-thays) that allows H+ ions to pass
through it. As H+ ions pass through this
protein, the protein rotates like a turbine
being spun by water in a hydroelectric
power plant. As it rotates, ATP synthase
binds ADP and a phosphate group
together to produce ATP.
The Calvin Cycle
• The Calvin cycle uses ATP and NADPH
from the light-dependent reactions to
produce high-energy sugars.
• the Calvin cycle does not require light,
these reactions are also called the lightindependent reactions.
Light-Dependent
Reactions
• Remove carbon dioxide
• Begin with six carbon
dioxide molecules
• Produce high-energy
sugars
• Produce H+ ions within
the thylakoid membrane
Calvin cycle
Reactions
• Convert ADP and
NADP+ into ATP and
NADPH
• Produce Oxygen gas
• Occur in stroma of
chloroplast
• Occur in thylakoid
membranes of
chloroplast
Factors Affecting Photosynthesis
• Because water is one of the raw materials of
photosynthesis, a shortage of water can
slow or even stop photosynthesis.
• Temperature is also a factor. Photosynthesis
depends on enzymes that function best
between 0°C and 35°C.
• The intensity of light also affects the rate at
which photosynthesis occurs.
• After the light intensity reaches a certain
level, however, the plant reaches its
maximum rate of photosynthesis. The level
at which light intensity no longer affects
photosynthesis varies from plant type to
plant type.
The Steps in Photosynthesis
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1) The light energy strikes the leaf, passes into the leaf and hits a chloroplast inside an
individual cell.
2) The light energy, upon entering the chloroplasts, is captured by the chlorophyll inside a
grana.
3) Inside the grana some of the energy is used to split water into hydrogen and oxygen.
4) The oxygen is released into the air.
5) The hydrogen is taken to the stroma along with the grana's remaining light energy.
6) Carbon dioxide enters the leaf and passes into the chloroplast.
7) In the stroma the remaining light energy is used to combine hydrogen and carbon dioxide
to make carbohydrates.
8) The energyrich carbohydrates are carried to the plant's cells.
9) The energyrich carbohydrates are used by the cells to drive the plant's
life processes.
Review
What is ATP?
• adenosine triphosphate
• 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.
Can you explain this?