PHOTOSYNTHESIS
Photosynthesis is the metabolic pathway that takes raw materials and converts it to sugar. Sugar is a form of
chemical potential energy. It will be used by all consumers and by plants, during cell respiration, to make ATP (cell
energy)
The process is completed in two phases.
The Light reaction: It uses the electrons from chlorophyll in a series of redox steps to make ATP and NADPH.
These products are mandatory for the formation of sugar.
The Dark Reaction: sugar is made from carbon dioxide using the energy of ATP and NADPH (made in the light
reaction)
Photosynthetic organisms are known Autotrophs. This means “Self Feeders”. Autotrophs are also known
as Producers. They occupy the bottom Trophic Level in an Ecosystem and represent the large source of energy in
the biotic component of an ecosystem.
Photosynthesis takes place in the leaves in the chloroplast of the cells of plants. Photosynthesis occurs in
Plants, and green protista and prokaryotes.
Eq
CO2
+
Carbon dioxide
H2O
water
+
SunLight
sunlight
makes
C6H12O6
glucose
+
O2
oxygen
Chloroplast is an organelle with a double membrane system. Outer membrane and inner membrane system called
(thyllakoid membrane and grana stacks)
There are two phases of photosynthesis, the light dependent and light independent reactions Calvin Cycle). They
occur in different parts of the chloroplast:
Light Dependent Reactions: This is where sunlight energy is harvested and converted into cellular energy.
It occurs in the thyllakoid membranes (grana stack) and requires pigments to harvest light. The light
reaction.accomplishes the following.
There are two energy generating units of the light reaction called Photosystem II and Photosystem I
The two systems harvest light energy to produce a high energy electron.
Splits water into 2H (e-) + 1/2O2 (this is the source of molecular oxygen)
High energy electrons are released from chlorophyll molecules
These electrons travel down the electron transport chain (a series of redox reactions) to make ATP &
NADPH
NADPH is the final electron acceptor
Both NADPH and ATP are needed to drive the DARK reactions
This stage is light dependent and only works in the presence of light
This figure shows the two photosystems, the chlorophyll pigments, the redox proteins in the electron transport chain.
Light Independent Reactions = Dark Reactions: This is the sugar generating step. It occurs in the
aqueous phase of the chloroplast called the “stroma”, is can occur in the presence or absence of light. The light
independent reaction is also known as the Calvin Cycle.
Calvin Cycle is a sequence of chemical reactions that produces Glucose from a 3-C compound, known as
G3P. Carbon dioxide is fixed to a 5 C compound (Ribulose bisphosphate,RuBP) and forms an unstable 6C
intermediate that immediately forms 2 molecules of G3P. G3P is the direct precursor to Glucose. During the
Calvin Cycle carbon dioxide is reduced to Glucose. This is an uphill reaction and requires and input of energy (ATP
and NADPH).
LIGHT is packets (photons) of electromagnetic radiation. Visible light is white light and is the light that plant
pigments harvest for energy. The wavelengths of white light range from 400 -750 nm, the shorter wavelengths are
the most energetic while longer wavelength are the least energetic. Plants absorb mainly blue and red light and
reflect green light.
PIGMENTS:
Pigments are used to harvest sunlight. They are lipid based, hydrophobic and are found in the thylakoid membranes
of the chloroplast clustered in a complex called the reaction center. There are several types of photosynthetic
pigments.
a. Primary Pigments: These harvest the majority of the light. The green pigments called “Chlorophyll”.
Prefer red (600-700nm) and blue (400-500nm). There are two chlorophyll molecules (A &B). It is chlorophyll a that
releases the electron in response to specific wavelengths of light. Each absorbs maximally one specific wavelength
of light (either 680 or 700 nm).
b. Accessory Pigments. Non-chlorophylls. Harvest other wavelengths (colors) of light. Protect plant and
chlorophylls from harmful light.
Stomata: These are small pores on the undersurface of leaves in plants. They are the site of Gas exchange. When the
stomata are open mostly O2 and water vapor goes out and CO2 comes into the plant. On hot sunny days the plant
may close its stomato to prevent water loss, when it does CO2 levels drop inside the plant.
ADAPTATIONS IN PHOTOSYNTHESIS
C3 photosynthesis is what you just learned about. It is called C3 because when these plants fix CO2 they
form a 3C compound. PGA using and enzyme called ribulose bisphosphate carboxylase. On hot sunny days this
pathway is inefficient because the stomata close. This closure unintentionally reduces the CO2 levels inside the plant
and increases the O2 level inside the plant. When the level of O2 is > the level of CO2 “Photorespiration”.occurs.
They loose the CO2 they just fixed.
PHOTORESPIRATION REDUCES THE EFFICIENCY OF PHOTOSYNTHESIS!
C4 photosynthesis is common in monocots like Corn and SugarCane. This adaptation is a spacial separation and
avoids Photorespiration by fixing CO2 first in the mesophyll cells using a different enzyme which converts CO 2 into
a 4C organic compound (malic acid). When the time is correct, it later removes CO 2 and uses it again in the Calvin
Cycle this time inside the Bundle Sheath Cells. C4 plants have highly developed Bundle sheath cells.
CAM photosynthesis is common in xerophytic plants, succulent desert adapted plants. These plants form a
4 C organic acid compound. Their strategy is to keep the stomata closed during the heat of the day and avoid water
loss then open the stomata at night when it is cool. They fix CO2 at night into a 4-C organic acid then make glucose
using the Calvin Cycle during the day by removing the CO2 from the organic acid.
All three method still rely of the ATP and NADPH produced in the Light Dependent Phase to complete the
Calvin Cycle. C3 plants are most likely to suffer on hot days.