Transcript PHOTOSYNTHESIS

PHOTOSYNTHESIS
BIO11
Photosynthesis
• Photosynthesis is the process by which
carbohydrates (an organic nutrient) are
synthesized from inorganic sources using
the energy of light.
PHOTOSYNTHESIS OVERVIEW
6CO2 + 6H2O + energy (sun)  C6H12O6 + 6O2
• In order for photosynthesis to take place the
following conditions must be met:
1. Light – plants use the visible light spectrum
2. Photosynthetic pigments – there are various
photosynthetic pigments found in chloroplasts
including: chlorophyll A, chlorophyll B, chlorophyll
C, carotene, and xanthophyll (the most important
being chlorophyll A)
3. Thylakoid membranes – specialized membranes of
the chloroplasts that are embedded with chlorophyll.
These are in stacks called grana, surrounded by a
gel-like substance called the stroma.
Chloroplast
Photosynthesis overview
•
Photosynthesis occurs in two distinct phases:
1. The light reactions – known as the photophase
because it requires light energy.
2. The dark reactions (Calvin cycle) – during this
phase, light is not required; however, the ATP and
NADPH produced by the light reactions are.
STRUCTURE OF A LEAF
• Leaves are a plant's main
photosynthetic organs.
• Leaves must permit carbon dioxide
access to the photosynthetic cells but
impede water from diffusing out.
• The oxygen that is a waste product of
photosynthesis must be allowed to
escape from the leaf.
STRUCTURE OF A LEAF
• In
which cells does photosynthesis take place?
STRUCTURE OF A LEAF
• Mesophyll cells are specialized for
photosynthesis.
• These cells in the middle of the leaf
contain many chloroplasts, the
organelles that perform
photosynthesis.
STOMA
• The stoma allows for the plant to
exchange gasses with its environment.
Stoma
Guard cells
PIGMENTS & CHLOROPHYLL
• Pigment: any substance that absorbs
light of a specific wavelength.
• Photosynthetic Pigments absorb light
(solar) energy and convert it to
chemical energy.
• Chlorophyll by itself can’t carry out
photosynthesis - it will absorb light
but radiate it back at a different
wavelength.
PIGMENTS & CHLOROPHYLL
• Electron Acceptors must also be
present for photosynthesis.
• Chlorophyll is found embedded in
flattened membranes called
Thylakoids in the Chloroplast.
• Thylakoids are arranged I
interconnected stacks called Granna.
Photosynthesis: An Overview of the
Light and ‘Dark’ Reactions
• Occurs in Photoautotrophs (organisms
that can make their own using energy
from the sun).
• Photosynthesis takes place in the
chloroplasts.
• Photosynthesis includes two processes…
http://simple animation
• LIGHT REACTIONS
• Requires sunlight
• Occurs in the granna of
chloroplasts
• Produces ATP and NADPH (used
to power the Calvin cycle)
• ‘DARK’ REACTIONS
(a misnomer…aka Calvin cycle)
• Doesn’t require sunlight (happens
24/7).
• Occurs in the stroma of
chloroplasts
• Produces PGAL (which can later
be used to make glucose)
Light reactions
• Chlorophyll absorbs light energy into the thylakoid
membrane and is said to be ‘energized’. The energized
chlorophyll causes the following reactions to occur:
– A water molecule is split into hydrogen ions and oxygen
atoms. This split occurs due to chlorophyll’s strong
attraction for electrons. The hydrogen ions are then held
by the coenzyme NADP+. The oxygen atoms are released
to the atmosphere as a by-product.
– ADP adds a phosphate group to become ATP
(phosphorylation). The energy stored in ATP is used
during the Calvin cycle.
Overview of light dependent
reactions
The Calvin Cycle
• In Photosynthesis, ATP and NADPH are produced in
photophosphorylation, aka the Light Reactions. This happens in the
thylakoid but notice that the products are actually produced in the
stroma. This sets up the next series of reactions, the Calvin cycle
which happens completely in the stroma. This is where sugars are
manufactured. Melvin Calvin discovered this cycle in 1940.
THE CALVIN CYCLE
• The reactions that convert carbon dioxide
(CO2) into carbohydrate (glucose)
molecules and H2O is given off to the
atmosphere.
• Occurs in the stroma of chloroplasts.
• The process occurs by a cyclic series of
reactions.
… The Calvin Cycle
• The end product of photosynthesis isn’t really glucose; it’s
PGAL (phosphoglyceraldehyde). PGAL can be used to
manufacture glucose, or other sugars, fatty acids or amino
acids and as an energy source for cellular respiration.
• During a series of reactions, ATP and NADPH
produced by the light reactions help convert CO2 into
PGAL.
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•
The Calvin Cycle has three phases:
1st phase: Carbon Fixation
2nd phase: Reduction
3rd phase: Regeneration of the Carbon acceptor molecule
Phase 1: CARBON FIXATION
3 x CO2
1
3x
(5-C)
2
Rubisco
6x
(3-C)
1st Phase: Carbon
Fixation
1. Three five-carbon sugar
molecules are the
acceptors that bind 3 CO2
molecules (dissolved in
the stroma). This reaction
is catalyzed by the
enzyme rubisco.
2. Three unstable 6-C
molecules are produced
(not shown) which
quickly break down to
give six 3-carbon
molecules of the
Phosphate
carbon
Animation: Calvin cycle
Phase 2: REDUCTION REACTIONS
2nd Phase: Reduction
3. The six 3-C molecules are
3 x CO2
phosphorylated and
reduced to PGAL
1
2
(phosphoglyceraldehyde),
a three-carbon sugar. This
6 x 3-PGA
3 x RuBP Rubisco
6 x ATP
(3-C)
phosphate bond is then
(5-C)
6 x ADP
broken and hydrogen is
3
added from NADPH.
6 x 1,3 BPG
4. Six molecules of PGAL are
produced. However, only
6 x NADPH
one of the six molecules
6 x NADP
exits the cycle as an output
6 x Pi
6 x PGAL
(to make sugar, etc.)
(3-C)
while...
4
Animation: Calvin cycle
1 x PGAL (3-C)
NOTE: PGAL is also
referred to as G3P
3rd Phase: Regeneration
of the Carbon
acceptor molecule
5. ...the remaining five
Phase 3: REGENERATION
3 x CO2
1
3 x RuBP
(5-C)
2
6 x 3-PGA
Rubisco
(3-C)
6 x ATP
6 x ADP
3 x ADP
3 x ATP
6 x 1,3 BPG
6
6 x NADPH
5
enter a complex process
that regenerates more
five-carbon sugar
molecules to continue
the cycle....
5 x PGAL
(3 C)
6 x PGAL
(3-C)
6 x NADP
6 x Pi
4
1 x PGAL
(3-C)
Animation: Calvin cycle
3
6. In this process, ATP is
used to convert the
five PGAL’s to three
5-C molecules.
7. Summary...
9 ATP used
6 NADPH used
1 PGAL produced
5-C regenerated
Glucose formation
• The sixth PGAL will be used to form a
glucose molecule.
• Since each glucose molecule (C6H12O6) is
formed from two PGAL’s, two complete
turns of the cycle are required.
• NADP+, ADP and Pi are then released by
the Calvin cycle and are recycled back to
the thylakoid membrane to be transformed
yet again by the light reactions.