Photosynthesis - CARNES AP BIO

Chapter 10
Chemical Energy & ATP
The activities of all cells
are powered by chemical
The principal compound
that living things use to
store energy is adenosine
triphosphate (ATP)
The characteristics of ATP
make it an exceptionally
useful molecule that is
used by all types of cells
as their basic source of
What is Photosynthesis?
Photosynthesis is the process whereby light energy is
converted to chemical energy and carbon is fixed into organic
In the presence of light, plants transform carbon dioxide and
water into carbohydrates and release oxygen
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
Figure 7.18 The chloroplast, site of photosynthesis
Inside a Chloroplast
Key structures INSIDE Chloroplasts
a stack of thylakoids
saclike structure in chloroplasts made of photosynthetic
membranes – these sacs are made up of lipid bilayers
region outside of the thylakoid membranes
molecules are embedded in the thylakoid membrane
Stages of Photosynthesis
The reaction that occurs during photosynthesis can be
broken into 2 stages:
1. Light Dependent Reactions
Take place within the thylakoid membranes inside a
“PHOTO” phase – make ATP & NADPH
Light Independent Reactions (Calvin Cycle)
Take place in the stroma of the chloroplast
“SYNTHESIS” phase – coverts CO2 to sugar
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):
Carotenoids (absorb light in the blue, green, and violet range):
Chlorophyll a - directly involved in transformation of photons to chemical
Chlorophyll b - helps trap other wavelengths and transfers it to chlorophyll a
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!
The Structure of Chlorophyll a
Chlorophyll a is a large molecule
with a single magnesium atom in
the head surrounded by alternating
double and single bonds.
The head is called the porphyrin
ring and is attached to a long
hydrocarbon tail
The double bonds are the source of the
electrons that flow through the electron
transport chains during photosynthesis.
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.
Photosystems are light-harvesting complexes
in the thylakoid membranes of chloroplasts.
Each photosystem consists of a reaction center
containing chlorophyll a and a region of many
atenna pigment molecules that funnel energy into
chlorophyll a.
Two types of photosystems cooperate during
Photosystem I
Photosystem II
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).
Stages of Photosynthesis
There are 2 stages in Photosynthesis:
1. Light dependent reactions
2. Light independent reactions (Calvin Cycle)
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
Visual Overview of Photosynthesis
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)
oxygen gas is produced as a by-product
Light Independent Reactions - Overview
do not require light directly – so also known
as the Dark Reactions or the Calvin Cycle
take place in the stroma of chloroplasts
ATP and NADPH produced during light
dependent reactions are used to make glucose
Stage 1 of Photosynthesis
The Light Reactions
Light is absorbed by PS I and PS II in the
thylakoid membranes and electrons flow
through electron transport chains.
There are 2 possible routes for electron flow:
Noncylic photophosphorylation
Cyclic photophosphorylation
Photophosphorylation is a method of
generating ATP by using light to add P to ADP
Occurs in Light Reactions
Figure 10.11 How a photosystem harvests light
Chlorophyll a
Noncyclic Photophosphorylaton
During noncyclic photophosphorylation,
electrons enter two electron transport chains,
and ATP and NADPH are formed.
The process begins in PS II and proceeds to
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)
A. Photosystem II -- Light is Figure
absorbed by
Energy is
transferred to e-, which goReactions
into ETC (electron transport
chain.) Hydrolysis breaks water up into e-, H+, and O2
Section 8-3
B. ETC moves H+ ions from stroma into inner thylakoid.
C. Photosystem I -- light is absorbed by pigments, energy
goes to e-, NADPH is formed
D. Hydrogen movement makes inside positively charged.
E. As H+ diffuses through ATP synthase, ADP is made into
Go to
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)
Cyclic Photophosphorylaton
Under certain conditions…photoexcited electrons take an shortcircuit path called cyclic electron flow.
During Cyclic Electron Flow, photo-excited electrons travel from
PS II electron transport chain to PS I, to a primary electron
acceptor, and then back to the cytochrome complex in PS II.
Involves Photosystem I only…no production of NADPH and no release of
oxygen…but ATP is produced.
Why Cyclic?
Because noncyclic electron flow produces ATP and NADPH in roughly
equal quantities…but Calvin cycle consumes more ATP than NADPH.
Cyclic flow makes up the difference for more ATP needed.
Figure 10.14 Cyclic electron flow
Cyclic vs. Noncyclic Electron Flow
Noncyclic – pg 186
uses Photosystem II, and ETC (with the electron
carrier plastoquinone, Pq) , Photosystem I, and
another ETC using an iron-containing protein called
produces ATP and NADPH
Cyclic – pg. 187
uses only Photosystem I and the second 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 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
(Calvin Cycle)
Stage 2 of Photosynthesis
The Dark Reactions (Calvin cycle)
Calvin cycle can be divided into 3 phases:
Phase 1: Carbon Fixation
Phase 2: Reduction
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.
Each molecule of 3-phosphoglycerate receives additional phosphate group
from ATP to become 1,3 biphosphoglycerate.
Pair of electrons donated from NADPH reduces 1,3 to G3P (a
sugar)….notice for every 3 molecules of CO2 there are six molecules of
Phase 3: Regeneration of CO2 acceptor (RuBP)
In a series of reactions, 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
Figure 10.17 The Calvin cycle (Layer 1)
Figure 10.17 The Calvin cycle (Layer 2)
Figure 10.17 The Calvin cycle (Layer 3)
A. 6 CO2’s combine with 6 5-C molecules – make
12 3-C molecules
C. 2 of the 12 3-C molecules are made into
D. Other 10 3-C molecules are broken down into
six 5-C molecules to start cycle over…
Figure 10.20 A review of photosynthesis
Factors Affecting Photosynthesis
Amount of water available
too little, stop photosynthesis
best between O°C and 35° C (too high, damage
enzymes; too low, stop photosynthesis)
Intensity of light
up to a point, increasing light intensity increases
rate of photosynthesis – after this point, the rate of
photosynthesis will NOT continue to increase
Metabolic Challenges – Water vs. CO2
How do we maintain photosynthesis while still
preventing water loss?
CO2 comes in via stomata, but water goes out at
same time (transpiration):
So, plants close stomata during hot, dry days.
BUT, closing stomata decreases photosynthetic yield
because if stomata are closed then CO2 can’t enter the
plant leaf and O2 can’t exit!
Normal Pathway - C3 Plants
In most plants, initial fixation of carbon occurs using rubisco – the
enzyme in Calvin cycle that adds CO2 to ribulose biphosphate
These plants are called C3 plants because the first organic product of carbon
fixation is a three-carbon compound: 3-phosphoglycerate:
Ex. rice, wheat, soybeans
The declining level of CO2 in the leaf starves the Calvin cycle.
Making matters worse, rubisco can accept O2 in place of CO2 – and as O2
concentrations overtake CO2 concentrations, rubisco adds O2 to the Calvin
cycle instead of CO2.
The product formed splits, leaves the chloroplast, and is broken down by
mitochondria and peroxisomes – KNOWN AS PHOTORESPIRATION
(because it occurs in the light – photo AND because it consumes oxygen –
The environmental conditions that foster
photorespiration are hot, dry, bright days (the
conditions that cause stomata to close).
Photorespiration occurs in the light and consumes O2!
When plants close stomata they get declining
levels of CO2 – which starves the Calvin Cycle
If CO2 levels are low, rubisco can also accept O2
in place of CO2.
When CO2 levels are high CO2 fixation dominates.
When CO2 levels are low and O2 levels are high, respiration dominates.
Photorespiration DOES NOT PRODUCE ANY ATP – or SUGARS!!!
Process actually takes organic materials AWAY from the Calvin Cycle – NOT
Alternative Methods of Carbon Fixation –
Controlling Photorespiration
In certain plants, alternative methods of carbon
fixation that minimize photorespiration have
All plants do not use RuBP directly to fix their
 C4
C4 Plants
Modification for DRY ENVIRONMENTS
(combats photorespiration)
A unique leaf anatomy is required here - spatial
separation of processes (mesophyll cells v/s bundlesheath cells)
C4 Plants
In C4 plants, there are TWO distinct types of photosynthetic
cells: bundle-sheath cells and mesophyll cells.
The Calvin cycle is confined to the chloroplasts of bundle sheath
cells, HOWEVER, the cycle is preceded by incorporation of CO2 into
organic compounds in the mesophyll cells.
C4 Plants
Preface the Calvin Cycle with carbon fixation that forms a 4-C
compound as its first product – CO2 is added to PEP to form
Requires unique leaf anatomy – see page 192
PEP has a very high affinity for CO2, so when it is hot and dry
(and stomata close), PEP can fix CO2 when rubisco cannot.
Mesophyll cells of a C4 plant pump CO2 into the bundle sheath,
keeping [CO2] high enough for rubisco to accept carbon dioxide
C4 photosynthesis minimizes photorespiration and enhances
sugar production
Ex. Sugarcane, corn, members of the grass family
Figure 10.18 C4 leaf anatomy and the C4 pathway
In mesophyll cells, the enzyme
PEP carboxylase fixes carbon
dioxide (instead of RuBP).
A 4-carbon compound conveys the
atoms of the CO2 into a bundlesheath cell via plasmodesmata.
In bundlesheath cells,
CO2 is released
and enters the
Calvin cycle.
C4 Plants
In effect, the mesophyll cells of a C4 plant pump
CO2 into the bundle-sheath, keeping the CO2
concentration in the bundle-sheath cells high
enough for rubisco to accept carbon dioxide
rather than oxygen.
In this way, C4 photosynthesis minimizes
photorespiration and enhances sugar production.
This adaptation is especially advantageous in hot regions
with intense sunlight – and these environments are where
C4 plants thrive today!
CAM Plants
Adaptation for dry environments (combats photorespiration).
Open stomata during night and close them during day:
Temporal separation – night v/s day – closing stomata during the day
helps desert plants conserve water, but it also prevents CO2 from entering
the leaves.
During the night when their stomata are open, CAM plants take up CO2
and incorporate it into a variety of organic acids.
This is a mode of carbon fixation called CRASSULACEAN
Succulents (water-storing plants), cacti, pineapples are CAM plants.
The mesophyll cells of CAM plants store the organic acids they make
during the night in their vacuoles until day when light reactions can supply
ATP and NADPH needed for the Calvin Cycle.
During day, the CO2 is released from these organic acids and
becomes incorporated into sugar via the Calvin cycle.
Figure 10.19 C4 and CAM Photosynthesis Compared
BOTH pathways are
evolutionary solutions to
the problem of maintaining
photosynthesis with stomata
partially or completely
closed on hot, dry days.
Spatial Separation of Steps: In
C4 plants, carbon fixation
and the Calvin cycle occur in
different types of cells
(mesophyll and bundlesheath).
Temporal Separation of Steps:
in CAM plants, carbon
fixation and the Calvin cycle
occur in the same cells at
different times.
C3 (Normal) v/s C4 (Adaptation)
Light Reactions:
-carried out by molecules in thylakoid
-convert light E to chemical E of ATP and
-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
Photosynthesis Concept Map
Instrument used to measure what wavelengths
of light are being absorbed by pigments
Directs beams of light through a solution of
pigment and measures the fraction of light
transmitted at each wavelength
Figure 10.7 Determining an absorption spectrum
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