Photosynthesis
Honors Biology
What you will learn…
 1. How do plants get food?
 2. Photosynthesis overview
 3. Leaf structure
 4. Chloroplast structure
 5. Pigments
 6. Overview of Two Stages
 6a. Overview of Light Reactions
 6b. Overview of Dark Reactions
 7. Light Reactions
 8. Dark Reactions (Calvin Cycle)
 9. Relationship Between the 2 Stages
 10. Factors Affecting the Photosynthetic Rate
 11. Alternate Pathways
1.How do plants get food?
 Plants are autotrophs (meaning “self-feeders” in Greek)
 Often referred to as the producers of the biosphere
because they produce its food supply
 All organisms that produce organic molecules from
inorganic molecules using the energy of light are called
photoautotrophs.
2.Photosynthesis Overview
 Photo, from the Greek word for light, refers to the first
stage.
 Synthesis, meaning “putting together” refers to the sugar
construction in the second stage
2.Photosynthesis: Overview
 The main purpose of photosynthesis is to make organic
molecules (carbohydrates).
 Overall equation:
6 CO2 + 6 H20  C6H12O6 + 6 O2
 Occurs in the leaves of plants in the chloroplasts.
 Oxygen is also produced in this process.
3.Leaf Structure
 Most photosynthesis occurs in the mesophyll layer of the leaf.
 Gas exchange of CO2 and O2 occurs at openings called stomata
surrounded by guard cells on the lower leaf surface.
 Stomata are able to open and close because water is also
evaporated through them into the atmosphere from the plant.
3.Leaf Structure
4.Chloroplast Structure
•
Similar to mitochondria, chloroplast has an outer
membrane and an inner membrane, with an
intermembrane space between them.
•
Inner membrane is filled with a thick fluid called
stroma
*Stroma is where sugars are made from carbon
dioxide and water
4.Chloroplast Structure
 Within stroma is a system of interconnected membranous sacs
called thylakoids
 contains thylakoid space
 Built into thylakoid membranes are the chlorophyll molecules
that capture light energy
 concentrated in stacks called grana.
5. Pigments
 Pigment is any molecule that is able to absorb light .
 Only light that is absorbed by pigments is useful for photosynthesis.
 Chlorophyll a is the most important photosynthetic pigment.
 Other pigments called antenna or accessory pigments are
also present in the leaf.
 Chlorophyll b
 Carotenoids (orange / red)
 Xanthophylls (yellow / brown)
 These pigments are embedded in the membranes of the
thylakoid in groups called photosystems.
6.Two Stages of Photosynthesis
 1. Light Reactions
 2. Dark Reactins (Calvin Cycle)
6a.Light Reactions overview
1.
Light reactions

Include the steps that convert light energy to chemical
energy stored in ATP and NADPH and produce O2 gas as a
waste product.

Occur in thylakoid membranes

Light energy absorbed by chlorophyll is used to make ATP
from ADP and phophate.


Also used to drive a transfer of electrons from water to NADP+, an
electron carrier similar to NAD+ that carries electrons in cellular
respiration.
NADP+ gets reduced to NADPH via enzymes by adding a pair of
light-excited electrons along with an H+
 Reaction temporarily stores energized electrons which
originally came form water that is split and O2 is released.
6b.Dark Reactions overview
Dark reactions, or Calvin Cycle

Occurs in the stroma

Does not require light directly

Cyclic series of reactions that assembles sugar molecules
using CO2 and the energy-containing products (NADPH
and ATP) of the light reactions.

Incorporation of carbon from CO2 into organic compounds
is called carbon fixation.

After carbon fixation, enzymes of the cycle make sugars by
further reducing the carbon compounds.
7. Light Reactions
7. Light Reactions
 Light energy is transformed into the chemical energy of ATP
and NADPH
 In this process, electrons removed from water molecules pass
from photosystem II to photosystem I to NADP+
 Between the two photosystems, the electrons move down an
electron transport chain and provide energy for ATP
production.
7. Light Reactions
 Consists of two Photosystems:
 Contain clusters of chlorophyll molecules along with other
pigments and proteins in the thylakoid membrane
 Consists of a number of light-harvesting complexes surrounding
a reaction center.
 Have chlorophyll a, chlorophyll b, and carotenoid pigments that
function collectively as a light-gathering antenna.
 Pigments absorb photons and pass the energy from molecule to
molecule until it reaches the reaction center.
 A protein complex that contains a chlorophyll a molecule and a molecule
called the primary electron acceptor:
 Captures a light-excited electron from the reaction-center chlorophyll
molecule and passes it to an electron transport chain
7. Light Reactions
 Two types: Photosystem I and Photosytem II:
 Photosystem I:
 Occurs second in light reactions
 Reaction center is called P700 because the wavelength of light it
absorbs best is 700 nm
 Photosystem II:
 Occurs first in light reactions
 Chlorophyll a molecule in reaction center is called P680 because the
light it absorbs best is red light with a wavelength of 680nm
7. Light Reactions
 Flow of electrons in light reactions (Figure 7.8A):
1. A pigment molecule in a light-harvesting complex absorbs a
photon of light. The energy is passed to other pigment
molecules and finally to the reaction center of Photosystem
II, where it excites an electron of chlorophyll P680 to a
higher energy level.
2. The electron is captured by the primary electron acceptor.
3. Water is split, and its electrons are supplied one by one to
P680, replacign those lost to the primary electron acceptor.
The oxygen atom compbines with an oxygen from another
split water molecule to form a molecule of O2.
7. Light Reactions
 4. each photoexcited electron passes from photosystem II to photosystem I
via an electron transport chain. The exergonic “fall” of electrons provides
energy for the synthesis of ATP.
 5. Meanwhile, light energy excites an electron of chlorophyll P700 in the
reaction center of photosystem I. The primary electron acceptor captures the
excited electron and an electron from the bottom of the electron transport
chain replaces the lost electron in P700.
 6. The excited electrons of photosystem I is passed through a short electron
transport chain to NADP+, reducing it to NADPH
7. Light Reactions
 Chemiosmosis
 Drives ATP synthesis using the potential energy of a concentration




gradient of hydrogen ions across a membrane
Gradient is created when an electron transport chain pumps hydrogen
ions across a membrane as it passes electrons down the chain.
Relationship between chloroplast structure and function in light
reactions:
The two photosystems and e.t.c. are all located in the thylakoid
membrane of a chloroplast.
As photoexcited electrons are passed down the e.t.c. connecting the two
photosystems, H+ are pumped across the membrane from the stroma
into the thylakoid space. This generates a concentration gradient across
the membrane.
7. Light Reactions
 Chemiosmosis (continued):
 Similar ATP synthase complex in mitochondria
 Energy of concentration gradient drives H+ back across the membrane
through ATP synthase
 ATP synthase couples the flow of H+ to the phosphorylation of ADP:
called photophosphorylation
7. Light Reactions
 Photosynthesis vs. Cell Respiration:
 In photosynthesis, light energy is used to drive electrons to the top
of the transport chain (whereas, cell respiration, high-energy
electrons pass down the e.t.c. coming from oxidation of food
molecules)
 Chloroplasts transform light energy into the chemical energy of
ATP (whereas, mitochondria transfer chemical energy from food to
ATP)
 In photosynthesis, the final electron acceptor is NADP+ (whereas,
in cell respiration, O2 is)
 In photosynthesis, electrons are stored in at a high state of potential
energy in NADPH (whereas, in cell respiration, they are at a low
energy level in H20)
8. Dark Reactions
 During this process, carbohydrates are formed.
 This is the only process on the earth that can form organic molecules
from inorganic ones. All other organic molecules (big 4) form from
carbohydrates!
 This cycle requires ATP, NADPH and CO2 to take place in the
stroma of the chloroplast.
 ATP, NADPH are from the light reaction, while CO2 has to
be taken in from the atmosphere through the stomata of the
leaves.
8. Dark Reactions
 Figure 7.10A: Overview of Calvin Cycle
 CO2 (from air), energy from ATP and high energy electrons
from NADPH (both generated by light reactions) , the Calvin
Cycle constructs an energy-rich, three-carbon sugar,
glyceraldehyde-3-phosphate (G3P).
 A plant cell uses G3P to make glucose and other organic
molecules as needed.
8. Dark Reactions
 Figure 7.10B: Details of the Calvin Cycle
1.
Carbon fixation: the enzyme rubisco attaches CO2 to
RuBP (5-C). The unstable 6-C product splits into two
molecules called 3-PGA.
1.
2.
For three CO2, six 3-PGA result
Reduction: NADPH reduces the organic acid six 3-PGA
to six molecules G3P with the assistance of ATP
8. Dark Reactions
3. Release of one molecule of G3P:
1.
Five G3Ps remain in the cycle, and one G3P will leave. Plant cells
use two G3P molecules to make one molecule of glucose.
4. Regeneration of RuBP
energy from ATP drives a series of chemical
reactions
to rearrange the atoms in the five G3P
molecules to form
three RuBP molecules. These can start another turn of the
cycle.
8. Dark Reactions
http://www.science.smith.edu/departm
ents/Biology/Bio231/calvin.html
9. Relationship between the 2
Stages
10. Abiotic Factors Affecting
Photosynthetic Rate
 Photosynthetic rate is depended on environmental factors:
 Amount of light available
 Level of carbon dioxide
 temperature
10. Abiotic Factors Affecting
Photosynthetic Rate
 Light intensity
 Up to a certain intensity, photosynthesis increases as more light
is available to the chlorophyll.
 When all the chlorophyll molecules are activated (saturated) by
the light, more light has no further effect.
10. Abiotic Factors Affecting
Photosynthetic Rate
 Temperature:
 Increased temperature increases photosynthetic rate until an
optimal temperature is reached.
 Above the optimal temperature, enzymes cannot function
properly and photosynthesis will decrease.
10. Abiotic Factors Affecting
Photosynthetic Rate
 Carbon Dioxide Levels:
 Increased carbon dioxide levels increases photosynthesis, unless
limited by another factor, then levels off.
11. Alternate Pathways
 These pathways adapt to perform photosynthesis in dry and
hot environment
 They are more efficient than the traditional C3 pathway
which use CO2 directly from the air
 Plants with alternative pathways have a slightly different
Calvin cycle.
 In C4 plants the location of the Calvin cycle is different
 In CAM plants the timing is different
11. Alternate Pathways
 C4 Pathway:
 CO2 fixation and the Calvin cycle take place in two separate
location.
 CO2 fixation is in the mesophyll cells of the leaf, even when
CO2 levels are low, producing a C4 product used for the Calvin
Cycle
 C4 product acts as a carbon shuttle to…
 The Calvin cycle takes place in the bundle sheath cells (around
the veins of the leaf) where sugars are made
 Examples of C4 plants: corn, sugar cane
11. Alternate Pathway
 CAM
 Occurs in succulent plants (cacti)
 Carbon fixation (trapping CO2) takes place at night when the
stomata are open
 Calvin cycle takes place during the day, when stomata are closed
 This way plants do not lose much water during hot and dry
days.
11. Alternate Pathway