Overview of Photosynthesis
• Light energy is converted to chemical energy and carbon is
fixed into organic compounds.
– Photosynthesis uses the energy of sunlight to convert
water and CO2 into O2 and high energy sugars
– 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
– carbon dioxide + water + light → sugar + 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.
Inside a Chloroplast
Photosynthetic Pigments
• Photosynthetic pigments absorb light energy and use it to provide energy to
carry out photosynthesis.
– Chlorophylls (absorb light in the red, blue, and violet range):
• Chlorophyll a - directly involved in transformation of photons to
chemical energy
• Chlorophyll b - helps trap other wavelengths and transfers it to
chlorophyll a
– Carotenoids (absorb light in the blue, green, and violet range):
• xanthophyll - Yellow
• beta carotene - Orange
• Phycobilins – Red
– Chlorophyll b, the carotenoids, and the phycobilins are known as
ANTENNA PIGMENTS – they capture light in other wavelengths and pass
the energy along to chlorphyll a.
– Chlorophyll a is the pigment that participates directly in the light
reactions of photosynthesis!
During photosynthesis, chlorophylls absorb free energy from light,
boosting electrons to a higher energy level in photosystems I and II.
Figure 10.9 Location and structure of chlorophyll molecules in plants
The pigment molecules have a
large head section that is
exposed to light in the surface of
the membrane; the hydrocarbon
tail anchors the pigment
molecules into the lipid bilayer.
Stages of Photosynthesis
This reaction can be broken into 2 stages:
1. Light Dependent Reactions
• Within the thylakoid membranes inside a chloroplast
• “PHOTO” phase – make ATP & NADPH…USE LIGHT
ENERGY TO PRODUCE ATP & NADPH
2. Light Independent Reactions (Calvin Cycle)
• Take place in the stroma of the chloroplast
• “SYNTHESIS” phase – converts CO2 to SUGAR
•
BOTH REQUIRE LIGHT (SOMEWHAT):
– Even the dark reactions in most plants occurs during daylight
because that is the only time the light reactions can operate
AND the dark reactions depend on the light reactions!!!
Light Reactions:
-carried out by molecules in thylakoid
membranes
-convert light E to chemical E of ATP and
NADPH
-split H2O and release O2 to the atmosphere
Calvin Cycle Reactions:
-take place in stroma
-use ATP and NADPH to convert CO2
into the sugar G3P
-return ADP, inorganic phosphate, and
NADP+ to the light reactions
Light Dependent Reactions - Overview
• require presence of light
• occur in thylakoids of chloroplasts
• use energy from light to produce ATP and
NADPH (a temporary, mobile energy source
that helps store even more energy)
• water is split during the process to replace
electrons lost from excited chlorophyll
• oxygen gas is produced as a by-product
The Light Reactions
•
Light is absorbed by PS II and PS I in the
thylakoid membranes and electrons flow
through TWO electron transport chains.
– There are 2 possible routes for electron flow:
1.Noncylic photophosphorylation
2.Cyclic photophosphorylation
•
Photophosphorylation is a method of
generating ATP by using light to add P to ADP
Photosystems
Photosystems I and II are embedded in the internal membranes of
chloroplasts (thylakoids) and are connected by the transfer of higher free
energy electrons through and electron transport chain (ETC).
PS I and PS II
• Named in the order they were discovered –
however, PS II occurs first, followed by PS I.
– PS I absorbs light best in the 700nm range (so
called P700).
– PS II absorbs light best in the 680nm range (so
called P680).
Cyclic vs. Noncyclic Electron Flow
http://highered.mcgraw-hill.com/olc/dl/120072/bio12.swf
• Noncyclic Electron Flow
– uses Photosystem II, and ETC (with the electron carriers B6f
comlex and Pq) , Photosystem I, and another ETC using an
iron-containing protein called ferredoxin & NADP reductase
– produces ATP and NADPH
• Cyclic Electron Flow
– uses only Photosystem II and the first ETC – no production of
NADPH and no release of Oxygen
– DOES produce ATP to be used to make up the difference
needed due to Calvin cycle demands.
Figure 10.11 How a photosystem harvests light
Chlorop
hyll a
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)
Figure 8-10 Light-Dependent
Reactions
Section 8-3
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Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)
Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
http://bcs.whfreeman.com/thelifewire/content/chp08/0802002.html
Light Independent Reactions - Overview
• Do not require light directly (Dark Reactions or the
Calvin Cycle)
• Stroma of chloroplasts
• ANABOLIC process – and therefore requires ENERGY
ATP and NADPH (light dependent reactions)
• Divided into 3 phases:
– Phase 1: Carbon Fixation
– Phase 2: Reduction
– Phase 3: Regeneration of CO2 Acceptor (RuBP)
Figure 10.17 The Calvin cycle (Layer 1)
Phase 1: Carbon Fixation
CO2 is incorporated and attached to RuBP
(catalyzed by enzyme rubisco).
Product of reaction is 6-carbon intermediate so
unstable that it splits in half to form two
molecules of 3-phosphoglycerate.
Phase 2: Reduction
Each molecule of 3phosphoglycerate
receives additional
phosphate group from
ATP to become 1,3
bisphosphoglycerate.
A pair of electrons donated
from NADPH reduces 1,3
bisphosphoglycerate into
Glyceraldehide-3phosphate (a sugar).
One of the G3P molecules is
exported and used to build
glucose.
Figure 10.17 The Calvin cycle (Layer 3)
Phase 3
The carbon skeletons of 5 molecules of G3P are rearranged by the
last steps of the Calvin cycle into three molecules of RuBP.
The RuBP is now prepared again to receive CO2…and the cycle
continues.
The regeneration phase requires ATP.
Conserved Core Processes
• Photosynthesis first evolved in prokaryotic
organisms;
• Scientific evidence supports that prokaryotic
(bacterial) photosynthesis was responsible for
the production of an oxygenated atmosphere;
• Prokaryotic photosynthetic pathways were the
foundation of eukaryotic photosynthesis.