Carbon fixation is the process of building complex carbon compounds from simple carbon compounds.
Autotrophs organisms that fix carbon, using carbon dioxide as a carbon source, and combine it with water
Photoautotrophs provide nearly all the energy used by living systems on Earth
Overview of Photosynthesis
Photosynthesis is a redox reaction:
Carbon dioxide is reduced to sugar
Water is oxidized to molecular oxygen
Light energy converted into chemical energy
Producers contain chlorophyll
Chlorophyll can trap light energy (photons).
The chlorophyll convert this energy into chemical energy.
The chemical energy is transferred as bond energy (electrons)and is transferred in turn to other chemical energy stores called carbohydrates, lipids and protein.
These molecules are called organic molecules.
Photosynthic Active Wavelengths
Photosynthesis uses only small visible portion of the electromagnetic spectrum
Wavelengths of visible light most important for photosynthesis.
The symbol for wavelength is
The sun is a star
It has very high surface temperatures
It produces a vast amount of electromagnetic radiation of widely varying frequencies
Gamma-rays, which have very short wavelengths
To the far-infrared (much longer )
Leaves have a layered organization
The mesophyll tissue (middle layers of cells) is the main site of photosynthesis
Sap flows through the veins
Covered in a waxy cuticle which decreases water loss
filled with chloroplasts (usually several dozen of them)
Site of photosynthesis
(arrows point to stomata)
Spongy Mesophyll layer
irregular in shape and loosely packed.
their main function seems to be the temporary storage of sugars and amino acids synthesized in the palisade layer.
microscopic pores found on the under side of leaves.
bounded by two half moon shaped guard cells that function to vary the width of the pore.
The Chloroplast • Double outer membrane • Internal membranes called thylakoids which is the location of the light dependent reaction • Stroma surrounding the thylakoids and inside the double membrane. This is the location of the light independent reaction that includes the Calvin cycle. • The stroma often contains starch grains and oil droplets both products of photosynthesis
A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed).
Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms.
All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a.
Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.
Chlorophyll a is the main photosynthetic pigment in all organisms except bacteria
Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene).
Compare wavelength absorbed
1. Chlorophyll a: Light to medium green. Main photosynthetic pigment.
2. Chlorophyll b: Blue-green. Accessory Pigment.
3. Carotene: Orange. Accessory Pigment.
4. Xanthophyll: Yellow. Accessory Pigment.
Light absorption by chlorophyll
Absorption spectra are obtain
from samples of pigment.
Different wavelengths of
light are passed through
and the absorption is
measured using a colorimeter.
This absorption spectra for
• absorption of blue light
• absorption of red light
• green light is reflected.
Notice the Y-axis is rate of
• The rate of photosynthesis is
measured at different
• The maximum rate are at the
blue end and red end of the
• The lowest rates are in the
• Chlorophylls are absorbing blue
and red light well but not green.
Photolysis: Energy is absorbed to produce ATP , H + and O 2
Photolysis is the process in which:
• light is absorbed by the chlorophyll
molecules on the thylakoid
membranes of the chloroplast.
• Light energy is converted to
chemical energy in the form of
• The electron bring about chemical
changes that include the formation
of ATP , H + and O 2
Partition of Function in the Chloroplast
The light-dependent reactions (the harvesting of light) occur on the thylakoid membrane.
The carbon fixation reactions
(formation of carbohydrate) occur in the stroma.
Photosynthesis occurs in two main phases :
1. Light Dependent Reaction in which
A. Energy of sun is trapped by chlorophyll molecules (oxidation)
B. ADP is reduced to ATP with NADP + reduced to NADPH.
C. The reaction must have light to take place.
D. This reaction takes place on the thylakoid membranes.
2. The Light Independent Reaction which
A. Uses the chemical energy from the LDR to fix atmospheric carbon into organic molecules such as glucose.
B.The process does not require light and can occur in both the light and dark periods.
C.This reaction takes place in the stroma
Photosystem I and II
Light Dependent Reaction (LDR)
• Light passes through the leaf until until it reaches the chloroplast
• Light is absorbed by chlorophyll a in Photosystem II (a number of different chlorophyll's working together).
• The chlorophyll absorbs the light energy and converts this to chemical energy in the form of electrons.
• Photosystem II is oxidized, releasing electrons
Stages of Light Dependent reaction:
• The electrons from PS II pass along membrane proteins (in thylakoid)
• The membrane proteins like cytochrome are reduced
• The reduced membrane protein pumps H + from the stroma into the space inside the thylakoids.
Protons Build up Inside Thylakoids
• At the same time as a) Photosystem I (having a different chlorophyll combination) absorbs light with a peak absorption of 680nm.
• The chlorophyll molecule releases electrons in the oxidation of PS I
• PS I is now oxidized.
• The electrons pass from PSI to other membrane proteins named here as ferrodoxins
• These proteins bring about the reduction of NADP + to NADPH + H +
• NADPH is found in the stroma and is used in the light independent reaction
To continue absorbing light PSI must be reduced back to its 'ground state’
• The source of electrons for this reduction are those passed to cytochrome from PSII
• PS II must also be reduced and returned to its 'ground state' to maintain light absorption
• The source of electrons is water.
• Electrons are removed for PS II which leaves a source of H + and
• Oxygen is a waste product of photosynthesis
Summary diagram of Oxidative Photophosphorylation
Summary diagram of Oxidative Photophosphorylation
The arrangement of the membrane proteins and photosystem's
• The electron transfer along proteins
• sequence of coupled oxidations and reduction
• build up of protons (Hydrogen ions) in the thylakoid lumen
• Formation of NADPH, ATP and Oxygen
Cyclic Photophosphorylation: This is a biochemical adaptation to low light intensities
Light Independent Reaction (LIDR)
• The energy trapped from sunlight in the light dependent reaction (ATP and NADPH) is used to fix carbon from carbon dioxide into organic molecules.
• The reaction takes place in the stroma and is controlled by enzymes.
Ribulose bisphosphate carboxylase (Rubisco) allows carbon (carbon dioxide) to be fixed into an initial organic molecule
RBCase therefore can be seen as a link between inorganic (non-living) and the organic (living) e.g. Primary productivity
Light Independent Reaction
So-called because the reactions do not directly need light.
Occurs in the stroma of the chloroplast.
Fixes carbon to make carbohydrates.
The Calvin Cycle: Phases 1 & 2
1. Carbon uptake
Adds carbon dioxide to 5C ribulose bisphosphate (RuBP)
Catalyzed into RUBISCO ; ribulose bisphosphate carboxylase
2. Carbon reduction phase
Citrate is made and broken to form 2 phosphoglycerate (PGA)
PGA is rearranged and phosphorylated by ATP
NADPH reduces the backbone further to form glyceraldehyde-3-phosphate (G3P)
The Calvin Cycle: Phase 3
3. Reformation of RuBP:
G3P is rearranged,
With further investment of ATP…
To make RuBP, a bisphosphorylated compound
G3P is shuttled out of the cycle to produce glucose and other carbohydrates elsewhere