Photosynthesis: Using Light to Make Food
Plants are autotrophs (“self-feeders”, producers) capable of making their own organic
material from inorganic nutrients
Since they do with through photosynthesis, they are photoautotrophs
Plants require inorganic nutrients, including:
1. CO2 from the air
2. Water and minerals from the soil
Plants can make all their own organic molecules
Carbohydrates, lipids, proteins, nucleic acids
Photosynthesis and cellular respiration are intimately tied to one another
There is an exchange of molecules and energy
Basics of photosynthesis
Photosynthesis takes place in chloroplasts that are found mainly in mesophyll cells in
the interior of leaves
Stomata are microscopic pores on the undersides of leaves that open and close to
allow CO2 in and H2O vapor and O2 out
Structure of chloroplasts
The chloroplast’s inner membrane encloses a compartment filled with stroma, a
thick fluid where sugars are made
Within the stroma are disk-like membranous sacs called thylakoids
Thylakoids are found in stacks called grana
These contain the chlorophyll
The two major sets of reactions of photosynthesis
Light reactions convert solar energy into chemical energy
ATP is produced as a source of chemical energy; NADPH as a source of high
energy electrons
The Calvin cycle makes sugars from carbon dioxide
The Nature of Sunlight
Sunlight is a type of energy called radiation, or electromagnetic energy
The distance between the crests of two adjacent waves is called a wavelength
The full range of radiation is called the electromagnetic spectrum
Electrons derived from water are energetically boosted by sunlight energy
The energized electrons are passed through a series of electron carriers, losing
energy along the way
Chloroplasts contain several kinds of pigments
Chlorophyll a, chlorophyll b, carotenoids
All are present in the thylakoid membranes (grana)
All are part of the light-harvesting complexes called photosystems
Light carries energy
Behaves as both waves and
Discreet packets of energy called photons, a fixed quantity of light energy
What happens in a photosystem?
A cluster of pigment molecules form a light-gathering antenna
When a photon strikes one pigment molecule, the energy is transferred from one
pigment to another until it reaches the reaction center chlorophyll a molecule
The reaction center of the photosystem contains a special chlorophyll a molecule
As the electron is excited in the chlorophyll a molecule, it is transferred to the
primary electron acceptor
When this occurs, the photosystem has just lost electrons
To continue, the electrons have to be replenished
That’s where the water comes in
The light reactions generate ATP and NADPH
Two types of photosystems work together
First Photosystem - water-splitting
Second Photosystem - NADPH-producing
Energy from the electrons moving through the electron transport chain is used to
pump H+ (protons) into the thylakoids
This generates a gradient
As the H+ move down the gradient, they drive the production of ATP
The photosystems and ATP synthases are localized in the thylakoid membrane
Calvin cycle
Produces sugars from carbon dioxide
The cycle uses
Carbon from CO2
Energy from ATP
High-energy electrons from NADPH
Produces
Glyceraldehyde 3-phosphate (G3P) - a 3C molecule
CO2 joins with the 5-C sugar RuBP (ribulose-1,5-bisphosphate) catalyzed by the
enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase)
How many ATP and NADPH are needed per G3P produced?
How many ATP and NADPH are needed per glucose produced?