Download C Photosynthesis

AN OVERVIEW OF PHOTOSYNTHESIS
Autotrophs are the
producers of the
biosphere
• Plants are
autotrophs,
producing their own
food and sustaining
themselves without
eating other
organisms
• Plants, algae, and some bacteria are
photoautotrophs
– Producers of food consumed by virtually all
organisms
Plants produce O2 gas by splitting water
• The O2 liberated by photosynthesis
– Is made from the oxygen in water
Photosynthesis uses solar energy
–
To produce glucose and O2 from CO2 and H2O
Sunlight energy
ECOSYSTEM
Photosynthesis in
chloroplasts
CO2
Glucose
+
+
H2O
O2
Cellular respiration in
mitochondria
ATP
(for cellular work)
Heat energy
Photosynthesis is the process by which certain
organisms use light energy
– To make sugar and oxygen gas from
carbon dioxide and water
Light
energy
6 CO2
+
Carbon dioxide
6
H2O
Water
C6H12O6
PHOTOSYNTHESIS
Glucose
+
6
O2
Oxygen gas
Electrons can capture light energy
Move into a higher-energy orbit in the atom
Fluorescence and phosphorescence occur when excited electrons
release their energy as light energy, to return to their original energy state
Can also release energy as heat
{Remember that energy transformations increase entropy}
Plants capture sunlight using
pigments = molecules
having electrons that absorb
light energy
• Chloroplast membranes
hold photosystems
containing pigments
Chlorophyll a molecules
fluoresce when their electrons
absorb UV light
Light energy is Electromagnetic Energy
Two models to describe behavior: Photon and
Wave
Wave Model of Electromagnetic Energy
Shorter wavelengths carry higher energy light
Our eyes sense part of this energy as visible light
Our brains interpret the different energies as colors
Pigments filter light waves
Wavelengths that the pigment
does not absorb are passed
through (reflected)
The color of the pigment (what we
see) are the colors of light the
pigment does not absorb
Chlorophyll is green because it absorbs
all visible wavelengths except green
Photosynthesis – Overall Reaction
– make sugar and oxygen gas
–
from carbon dioxide and water
Light
energy
6 CO2
+
Carbon dioxide
6
H2O
Water
C6H12O6
PHOTOSYNTHESIS
Glucose
+
6
O2
Oxygen gas
Photosynthesis occurs in chloroplasts
In plants, photosynthesis
– Occurs primarily in the leaves, in the
chloroplasts, which contain stroma, and
stacks of thylakoids called grana
Mesophyll Cell
Leaf Cross Section
LM 2,600 
Leaf
Mesophyll
Chloroplast
Vein
Stoma
CO2 O2
TEM 9,750 
Chloroplast
Grana
Stroma
Stroma
Granum
Thylakoid
Thylakoid
space
Outer
membrane
Inner
membrane
Intermembrane
space
Photosynthesis is a redox process, as is cellular
respiration
• In photosynthesis
– H2O is oxidized and CO2 is reduced
Reduction
6 CO2
+
6 H2O
C6H12O6
+
6 O2
+
6 H2O
Oxidation
Oxidation
C6H12O6 +
6 O2
6 CO2
Reduction
Overview: Photosynthesis occurs in two stages
linked by ATP and NADPH
• The complete process of photosynthesis consists
of two linked sets of reactions
– The light reactions and
– the Calvin cycle
• The light reactions
–
Convert light energy to chemical energy and
produce O2
• The Calvin cycle assembles sugar molecules from CO2
–
Using ATP and NADPH from the light reactions
H2O
CO2
Chloroplast
Light
NADP+
ADP
+P
CALVIN
CYCLE
(in stroma)
LIGHT
REACTIONS
(in thylakoids)
ATP
NADPH
O2
Sugar
THE LIGHT REACTIONS: CONVERTING
SOLAR ENERGY TO CHEMICAL ENERGY
Visible radiation drives the light reactions
• Certain wavelengths of visible light, absorbed by pigments
–
Drive the light reactions of photosynthesis
Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
1 nm
103 nm
UV
1m
106 nm
Microwaves
Infrared
103 m
Radio
waves
Light
Reflected
light
Visible light
380 400
500
600
700
750
Wavelength (nm)
650
nm
Chloroplast
Absorbed
light
Transmitted
light
• Each photosystem consists of
– Light-harvesting complexes of pigments
– A reaction center with a primary electron
acceptor that receives excited electrons from
a reaction-center chlorophyll
Photosystem
e–
Heat
Photon
Photon
(fluorescence)
Ground state
Chlorophyll
molecule
Photon
Excited state
Thylakoid membrane
Energy of electron
Light-harvesting Reaction
center
complexes
Primary electron
acceptor
To electron
transport chain
e–
Pigment molecules
Transfer of energy
Chlorophyll a molecule
In the light reactions, electron transport chains
generate ATP and NADPH
• Two connected photosystems absorb photons
of light in pigments that transfer the energy to
chlorophyll molecules in each of the
photosystems {P680 and P700}
• Photosystem regains electrons by removing
electrons from water molecules, releasing O2
gas
• The excited electrons
– Are passed from the primary electron
acceptor to electron transport chains
Photon
Photon
Photosystem II
1
6
Thylakoid membrane
Stroma
Photosystem I NADP+ + H+
e–
2 e–
4
5
P700
P680
Thylakoid
space
3
H2O
1 O
+
2 + 2 H
2
Electron transport chain
Provides energy for synthesis of
by chemiosmosis
ATP
NADPH
Chemiosmosis powers ATP synthesis in the light
reactions
• The electron transport chain
– Pumps H+ into the thylakoid space
• The diffusion of H+ back across the membrane
through ATP synthase
– Powers the phosphorylation of ADP to
produce ATP (photophosphorylation)
Chloroplast
Stroma (low H+ concentration)
Light
H+
Light
ADP + P
H+
NADP+ + H+
H+
NADPH
H+
Thylakoid
membrane
H2 O
1
O +2 H+
2 2
Photosystem II
Thylakoid space
(high H+ concentration)
H+
H+
Electron
transport chain
H+
H+
H+
H+
Photosystem I
H+
H+
H+
H+
ATP synthase
ATP
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
ATP and NADPH power sugar synthesis in the
Calvin cycle
• The Calvin cycle
– Occurs in the chloroplast’s stroma
– Consists of carbon fixation, reduction, release of
G3P, and regeneration of RuBP
Input
CO2
ATP
NADPH
CALVIN
CYCLE
Output:
G3P
• Using carbon from CO2, electrons from NADPH, and
energy from ATP
– The cycle constructs G3P, which is used to build
glucose and other organic molecules
Step 1 Carbon fixation. An enzyme
called rubisco combines CO2 with a
five-carbon
sugar
called
ribulose
bisphosphate (abbreviated RuBP). The
unstable product splits into two
molecules of the three-carbon organic
acid, 3-phosphoglyceric acid (3-PGA).
For three CO2 entering, six 3-PGA
result.
Input: 3
CO2
1
3
Step 2 Reduction. Two chemical reactions (indicated by
the two blue arrows) consume
energy from six molecules of
ATP and oxidize six molecules
of NADPH. Six molecules of 3PGA are reduced, producing
six molecules of the energyrich three-carbon sugar, G3P
Step 3 Release of one molecule
of G3P. Five of the G3Ps from step
2 remain in the cycle. The single
molecule of G3P you see leaving
the cycle is the net product of
photosynthesis. A plant cell uses
two G3P molecules to make one
molecule of glucose.
In a reaction
catalyzed by rubisco,
CO2 is added to RuBP.
P
P
P
6
RuBP
3-PGA
6
ATP
3 ADP
3
6
CALVIN
CYCLE
ATP
4
5
ADP +
2
P
6
G3P
6
NADPH
6
NADP+
P
G3P
3
Step 4 Regeneration of RuBP. A
series of chemical reactions uses
energy from ATP to rearrange the
atoms in the five G3P molecules (15
carbons total), forming three RuBP
molecules (15 carbons).These can
start another turn of the cycle.
Output:
P
1
G3P
Glucose
and other
compounds
P
PHOTOSYNTHESIS REVIEWED AND EXTENDED
Review: Photosynthesis uses light energy to
make food molecules
H2O
CO2
Chloroplast
Light
NADP+
ADP
+P
RUBP
Photosystem II
Thylakoid
membranes
Electron
transport
chains
Photosystem I
CALVIN
CYCLE 3-PGA
(in stroma)
ATP
NADPH
Stroma
G3P
Cellular
respiration
Cellulose
O2
LIGHT REACTIONS
Sugars
CALVIN CYCLE
Starch
Other organic
compounds
PHOTOSYNTHESIS, SOLAR RADIATION, AND
EARTH’S ATMOSPHERE CONNECTION
Photosynthesis moderates global warming
• Greenhouses used to grow plants
– Trap solar radiation, raising the temperature inside
Average composition of the atmosphere up to an altitude of 25 km.
Gas Name
Chemical Formula
Percent Volume
Nitrogen
N2
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.93%
*Carbon Dioxide
CO2
0.0360%
Neon
Ne
0.0018%
Helium
He
0.0005%
*Methane
CH4
0.00017%
Hydrogen
H2
0.00005%
N2O
0.00003%
O3
0.000004%
*Nitrous Oxide
*Ozone
* variable gases
http://www.physicalgeography.net/fundamentals/7a.html
• Excess CO2 in the atmosphere
– Is contributing to global warming
Some heat
energy escapes
into space
Sunlight
ATMOSPHERE
Radiant heat
trapped by CO2
and other gases