Download 4.2 Overview of Photosynthesis TEKS 4B, 9B

What is photosynthesis?
• The process in
which plants use
the energy of the
sun to convert water
and carbon dioxide
into high-energy
carbohydrates
(sugars and
starches) and
oxygen (a waste
product).
4.2
Overview of Photosynthesis
• Chlorophyll is a molecule that
absorbs light energy.
chloroplast
• In plants, chlorophyll is
found in organelles called
chloroplasts.
leaf cell
leaf
TEKS 4B, 9B
4.2
Overview of Photosynthesis
TEKS 4B, 9B
Photosynthesis in plants occurs in chloroplasts.
• Photosynthesis takes place in two parts of chloroplasts.
– grana (thylakoids)
– stroma
grana (thylakoids)
chloroplast
stroma
4.2
Overview of Photosynthesis
TEKS 4B, 9B
• The light-dependent reactions capture energy from sunlight.
–
–
–
–
take place in thylakoids
water and sunlight are needed
chlorophyll absorbs energy
energy is transferred along thylakoid membrane then to
light-independent reactions
– oxygen is released
4.2
Overview of Photosynthesis
TEKS 4B, 9B
• The light-independent reactions make sugars.
– take place in stroma
– needs carbon dioxide from atmosphere
– use energy to build a sugar in a cycle of chemical
reactions
4.2
Overview of Photosynthesis
• The equation for the overall process is:
6CO2 + 6H2O  C6H12O6 + 6O2
granum (stack of thylakoids)
1
chloroplast
6H2O
thylakoid
6CO2
3
6O2
2
energy
stroma (fluid outside the thylakoids)
1 six-carbon sugar
4
C6H12O6
TEKS 4B, 9B
Light Energy
Chloroplast
CO2 + H2O
Sugars + O2
Light and Pigments
• Photosynthesis requires:
– water
– carbon dioxide
– light
– chlorophyll (a pigment molecule in
chloroplasts; two types)
• chlorophyll a
• chlorophyll b
ROYGBIV
• Sunlight is perceived as white light, but is
really a mixture of different wavelengths of
light (like a rainbow). The visible light we can
see is a very small portion of the
electromagnetic spectrum.
• Red Orange Yellow Green Blue Indigo Violet
Red: long
wavelength,
less energy
Violet: Short
wavelength, high
energy
• Pigments are molecules that absorb light at
different wavelengths.
– Chlorophyll absorbs light in the visible
spectrum, except the green wavelengths.
– Green light is reflected by the leaves making
plants look green.
• The high energy that is absorbed makes
photosynthesis work.
Factors affecting photosynthesis
– Shortage of water
– Temperature (0-35 degrees Celsius)
– Intensity of light
Inside a Chloroplast
• Photosynthesis takes place inside chloroplasts.
• Chloroplasts contain:
– thylakoids: saclike photosynthetic membranes
containing pigments
– grana (singular: granum): stacks of thylakoids
– stroma: region of chloroplasts outside of the
thylakoid membranes
– inner membrane
– outer membrane
4.3
Photosynthesis in Detail
• Photosystem II captures and transfers energy.
– chlorophyll absorbs
energy from sunlight
– energized electrons
enter electron
transport chain
– water molecules are
split
– oxygen is released as
waste
– hydrogen ions are
transported across
thylakoid membrane
TEKS 4B, 9B
Electron Carriers
•
•
Sunlight excites electrons in chlorophyll,
causing them to gain energy.
An excited electron is like a hot coal, and
cannot be easily carried from one place to
another- a protein called an electron carrier
is required to transport excited electrons.
Electron transport: An electron carrier molecule
can accept a pair of high-energy electrons
and transfer them to another molecule.
– Electron transport chain: Series of electron
carriers
– Example: NADP+ (nicotinamide adenine
dinucleotide phosphate)
NADP+ + 2 electrons + H +  NADPH
• NADPH can carry high-energy electrons to other
chemical reactions in the cell that need energy
4.3
Photosynthesis in Detail
TEKS 4B, 9B
• Photosystem I captures energy and produces energycarrying molecules.
– chlorophyll absorbs
energy from sunlight
– energized electrons
are used to make
NADPH
– NADPH is transferred
to light-independent
reactions
Light Dependent Reactions
• The light-dependent reaction splits water,
produce oxygen gas as waste, and
converts ADP and NADP+ into ATP and
NADPH.
4.3
Photosynthesis in Detail
TEKS 4B, 9B
The second stage of photosynthesis uses energy from
the first stage to make sugars.
• Light-independent
reactions occur in the
stroma and use CO2
molecules.
4.3
Photosynthesis in Detail
TEKS 4B, 9B
• A molecule of glucose is formed as it stores some of the
energy captured from sunlight.
– carbon dioxide molecules enter the Calvin cycle
– energy is added and carbon molecules are rearranged
– a high-energy three-carbon molecule leaves the cycle
4.3
Photosynthesis in Detail
TEKS 4B, 9B
• A molecule of glucose is formed as it stores some of the
energy captured from sunlight.
– two three-carbon molecules bond to form a sugar
– remaining molecules stay in the cycle
Light Dependent Reactions
1.
Light hits Photosystem II in the thylakoid
membranes. Two electrons are excited and
these excited electrons are passed onto the
electron transport chain
a.
To replace the lost electrons, the thylakoid
membrane obtains low-energy electrons by splitting
water
2H2O  4H+ + O2 + 2 e•
•
The O2 is released as waste
The hydrogen ions (4H+) are released inside the thylakoid
membrane
Light Dependent Reactions
2. Electron transport chain (ETC)
a. Electrons are passed from Photosystem II to
Photosystem I from one electron carrier to
the next until they reach Photosystem I
b. Energy from the electrons is used by the
electron carriers in the ETC to force H+ ions
from the stroma into the inner thylakoid
space- build up of H+ will be used to drive
ATP synthase
Light Dependent Reactions
3. Light hits Photosystem I
a. Pigments in Photosystem I use energy from
light to energize two electrons, making them
high-energy
b. They are passed to NADP+ Reductase
which catalyzes the reaction of NADP+ take
combining with the high-energy electrons
and hydrogen ions (H+) to become NADPH
Light Dependent Reactions
4. Hydrogen Ion Movement
–
The inside of the thylakoid membrane fills
up with positively charged hydrogen ions
(H+) as electrons are passed from
Photosystem II to I
ATP synthesis
–
•
•
The thylakoid membrane contains a protein
called ATP synthase that spans the membrane
and allows H+ ions to pass through it
As H+ ions pass through ATP synthase, the
protein rotates and binds ADP and a phosphate
group to produce ATP.
Light-independent reactions (The
Calvin Cycle)
•
•
During the Calvin cycle, plants use ATP
and NADPH from the light-dependent
reactions to produce high-energy sugars
for long-term storage.
The Calvin cycle does NOT require light.
light
Chloroplast
Chloroplast
Thylakoids
CO2
H2O
NADP+
ADP + P
LightDependent
Reactions
Calvin
Cycle
ATP
NADPH
Sugars
O2
The Calvin Cycle: (4 Steps)
1. CO2 enters the cycle
–
–
Six carbon dioxide molecules enter and
combine with six 5-Carbon molecules.
Result: 12 3-carbon molecules
The Calvin Cycle
• Energy input
– The 12 3-carbon molecules are converted into
high energy forms using ATP and NADPH
• During this process, 12 ATP  12 ADP
• During this process, 12 NADPH  12 NADP+
The Calvin Cycle
•
6-carbon sugar produced from two 3carbon molecules removed to produce
sugar
5-carbon molecules regenerated
•
–
–
–
10 remaining 3-carbon molecules converted
into six 5-carbon molecules
This requires 6 ATP  6 ADP
These 5-carbon molecules can be reused in
step A.
The Calvin Cycle
CO2 Enters the Cycle
Energy Input
5-Carbon Molecules
Regenerated
6-Carbon Sugar
Produced
Photosynthesis
includes
Lightdependent
reactions
use
H2O and Energy
from
sunlight
Calvin cycle
Thylakoid
membranes
to produce
ATP
NADPH
takes place in
take place in
O2
Stroma
uses
NADPH
and ATP
CO2
of
to produce
Chloroplasts
High-energy
sugars
Photosynthesis
includes
Lightdependent
reactions
Calvin cycle
use
take place in
Energy from
sunlight
Thylakoid
membranes
to produce
ATP
NADPH
O2
takes place in
Stroma
uses
ATP
NADPH
of
to produce
Chloroplasts
High-energy
sugars
Photosynthesis is important for almost
all life on Earth because it —
A produces oxygen
B uses simple elements
C is responsible for most decay
D releases usable forms of nitrogen
A
B
1. What is this picture showing? chloroplast
stroma
2. What is letter A? __________
thylakoid
3. What is letter B? __________
Light
Chloroplast
Chloroplast
Thylakoids
CO2
H2O
NADP+
ADP + P
LightDependent
Reactions
Calvin
Cycle
ATP
NADPH
Sugars
O2
Light Dependent or Calvin
Cycle?
•
•
•
•
•
•
•
•
Occurs in the stroma Calvin Cycle
Calvin Cycle
Needs CO2
Produces ATP and NADPH Light Dependent
Occurs in the thylakoid membranesLight Dependent
Light Dependent
Needs sunlight
Calvin Cycle
Can occur in the dark
Uses ATP and NADPH Calvin Cycle
Produces a 6-Carbon Sugar Calvin Cycle