UNIT 2: Metabolic Processes
Chapter 5: Photosynthesis: The Energy of Life pg. 210 - 240
All living organisms require energy to survive. All living organisms perform
cellular respiration to meet their energy needs. Some organisms (animals)
are heterotrophs (consumers) therefore they must consume food to obtain
their energy or molecules used in cellular respiration.
Plants are living organisms therefore they must cellular respire to stay alive.
Unlike animals that are heterotrophs, plants are autotrophs (producers) they
have the ability to make their own food through a process called
photosynthesis. Photosynthesis is the process where sunlight energy is used
to create glucose.
Green plants and their cells have specialized organelles that are responsible
for the process of photosynthesis. Chloroplasts are organelles that contain a
pigment called chlorophyll (green) which absorbs sunlight energy used to
produce starch (polysaccharide) made up of glucose molecules.
Photosynthesis is responsible for the fixation of carbon into energy rich
organic compounds.
sunlight energy + water + carbon dioxide → glucose + water + oxygen
The atmosphere on Earth as life developed may have been 3% oxygen and
10% carbon dioxide. The atmosphere today is less then 0.04% carbon
dioxide and 21% oxygen. Photosynthetic organisms used light energy to
produce stored energy in organic compounds, using hydrogen atoms from
water. The by product was oxygen released into the atmosphere.
UNIT 2: Metabolic Processes
Chapter 5: Photosynthesis: The Energy of Life pg. 210 - 240
5.1: Photosynthesis: An Introduction
pg. 212 - 219
Photoautotroph – is an organism that makes its own food using energy from
the Sun.
producer
→
consumer →
decomposer
(herbivore, carnivore, omnivore)
Producers use inorganic material to produce organic molecules, in which
they use as an energy source and building material for their own activities.
Producers serve, directly or indirectly, as food sources to consumers
Consumers used the organic molecules from the producers for energy and
building materials for their cells.
The waste products from both producers and consumers and the dead
decaying material is used by decomposers to obtain energy, and building
materials. Decomposers eventually return simple inorganic material back
into the environment.
The Two Stages of Photosynthesis
Photosynthesis is made up of two distinct stages; the light dependent and the
light independent phases.
Light-dependent reaction – is the first stage of photosynthesis, during which
water molecules are split as light energy is absorbed and transformed into
chemical energy in ATP and NADPH.
In the light dependent phase of photosynthesis sunlight is captured to
produce ATP and reduce NADP+ into NADPH (nicotinamide adenine dinucleotide phosphate). The ATP from the light dependent phase will enter
the light independent phase as an energy source.
During this phase water is broken down, releasing hydrogen electrons and
proton to create a proton gradient and release oxygen as a waste product. An
electron transport chain, found in the membrane to the Thylakoid discs will
deliver hydrogen protons and electrons to reduce NADP+ to NADPH. It is
here that ATP is also synthesized by oxidative phosphorylation.
Light Independent reaction (Calvin Cycle) – is the second stage of the
photosynthesis process that uses ATP and NADPH to convert CO2 to sugar
(C6H12O6).
The electrons and protons carried by NADPH and the high energy molecules
of ATP, reacts with CO2 to produce a high energy organic compound
(glucose). This process is called carbon fixation.
Figure 2: Both stages of photosynthesis occur in the chloroplast of photosynthetic
eukaryotes as well as in photosynthetic prokaryotes.
Chloroplasts: Photosynthesis Machines
In eukaryotic cells photosynthesis occurs in the chloroplast. The Light
dependant and independent reactions occurs here. The chloroplast is made
up of three membranes, creating three distinct compartments. The outer
membrane, inner membrane, and the thylakoid membrane create these three
compartments. The Thylakoid membrane is the location where the light
dependent phase occurs. The stroma is the compartment created between the
inner membrane and the thylakoid lumen. It is in this aqueous environment
that the Calvin cycle occurs (carbon fixation).
The Thylakoid discs create stacks called grana which are lined with many
light absorbing chlorophyll molecules. As sunlight passes through the
Thylakoid membrane, light energy is used to create NADPH and ATP.
Figure 3: The Chloroplast, the membranes and compartments of chloroplasts. pg. 214
Capturing Light Energy
The light dependent phase traps light energy to generate the production of
NADPH and ATP. This occurs in the membrane of the thylakoid disc where
many chlorophyll pigment molecules.
Two points about light energy and pigments molecules:
1. The absorption of a photon by a pigment molecule excites a single
electron, going from ground state (low energy) to excited state (high
energy).
2. The difference between the energy level of ground state and excited
state is equivalent to the energy of one photon absorbed.
There are three possible outcomes for excited electrons in a pigment;
1. The excited electron returns back to ground state releasing energy as
thermal energy or fluorescence, which is the release of light.
2. The energy of the excited electron is transferred to an electron of a
neighbouring pigment molecule, and the electron returns back at
ground state. The second pigment molecule now has an excited
electron.
3. The excited state electron is transferred to a nearby electron accepting
molecule. This step is important to photosynthesis.
Primary electron acceptor – is a molecule capable of accepting electrons
and becoming reduced during photosynthesis.
How Chlorophylls and Carotenoids Cooperate in Light
Absorption
Chlorophyll is the primary light absorption molecule in plants, green algae,
and cyanobacteria. Chlorophyll a and b are the most dominant types of
chlorophyll. The secondary photosynthetic pigments are Carotenoids.
Antenna complex – is a cluster of light-absorbing pigments embedded in the
thylakoid membrane able to capture and transfer energy to special
chlorophyll a molecules in the reaction centre.
Reaction centre – is a complex of proteins and pigments that contain the
primary electron acceptor.
Chlorophyll a is excited by light rays and passes an electron to the primary
electron acceptor, where carotenoids and chlorophyll b donate their excited
electrons to chlorophyll a at the reaction centre. These pigments are known
as accessory pigments and they make up the antenna complex.
The pigments are able to absorb all wavelengths of light. Wavelengths not
absorbed are reflected away or pas right through. Leaves look green because
green and yellow light are reflected because they are not absorbed by the
pigments found in the chloroplasts.
Figure 6: The Antenna complex captures light energy and transfers it to the reaction
centre.
Absorption Spectrum – is a plot of the amount of light energy of various
wavelengths that a substance absorbs.
A spectrophotometer is a machine that analyzes a sample of pigment and its
absorption of light wavelengths. Each pigment has the ability to only absorb
a certain wavelength of light energy.
Chlorophyll a – magenta and red light is absorbed
Chlorophyll b – blue-green and orange light is absorbed
Carotenoids – only magenta – green light is absorbed
Figure 8: Absorption spectrum, these graphs show a) the absorption spectra of the
photosynthetic pigments, chlorophylls a and b and carotenoids, and b) the action
spectrum in higher plants, representing the combined effects of chlorophylls and
carotenoids. pg. 217
Action spectrum – is a plot of the effectiveness of light energy of different
wavelengths in driving a chemical process.
The absorption of light by chlorophyll and Carotenoids are used to drive
photosynthesis, are plotted on a graph and is known as the action spectrum.
Figure 9: Englemann’s 1882 experiment revealed the action spectrum of light used in
photosynthesis by Spirogyra, a green alga. The bacteria clustered in areas where the most
oxygen was produced. Oxygen is a product of photosynthesis.
Pigments and Photosystems
Photosystem I – is a collection of pigment proteins that includes chlorophyll
a and absorbs light at the 700 nm wavelength.
Photosystem II – is a collection of pigment proteins that includes
chlorophyll a and absorbs light at the 680 nm wavelengths.
Pigment molecules do not float around freely in the thylakoid membrane,
they are attached to specific proteins, organized into Photosystems.
Each Photosystem is made up of a large antenna complex, which has a
reaction centre with 250 to 400 proteins surrounding the reaction centre.
Each reaction centre has a small number of proteins with a pair of
chlorophyll a molecules, and a primary electron acceptor.
The light energy (photons) absorbed by the antenna complex is transferred
to the chlorophyll molecules of the reaction centre. The excited electron is
passed to the chlorophyll molecule which donates an electron to the electron
acceptor, and then to the electron transport chain.
There are two types of Photosystems; photosystem I (PSI) and photosystem
II (PSII).
Photosystem I – contains chlorophyll a molecules, also known as P700
molecules. This pigment absorbs light with a 700 wavelength.
Photosystem II – contains chlorophyll a molecules, also known as P680
molecules. This pigment absorbs light with a 680 wavelength.
** in photosynthesis it is important to remember that photosystem II occurs
before photosystem I.