Download PHOTOSYNTHESIS INTRODUCTION

PHOTOSYNTHESIS INTRODUCTION
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All photosynthetic organisms use a green coloured pigment called chlorophyll
which absorbs light energy
The two most common photosynthetic pigments are chlorophyll a and
chlorophyll b
Both pigments have a porphyrin ring attached to a long hydrocarbon tail
The porphyrin ring contains a magnesium atom at the centre surrounded by a
hydrocarbon ring with alternating double bonds.
Delocalized electrons in the alternating double-single bonds absorbs light energy
and start the photosynthetic process
Chlorophyll molecule
Figure 2
Chlorophyll molecules contain a
porphyrin ring and long hydrocarbon
tail. The hydrophobic tail
anchors the molecule into a membrane.
The porphyrin ring contains
electrons that absorb light energy
and begin the process of photosynthesis.
Chlorophyll a has a methyl
group (_CH3) at position _R in the
porphyrin ring, whereas chlorophyll
b contains an aldehyde group(_CHO) in that position
Types of Photosynthetic Organisms
1) Prokaryotic Autotrophs: Cyanobacteria (Bluegreen algae)
 Evolved 2.5 to 3.4 BYA, very important because excess
O2 put itno atmosphere and formed protective ozone
layer
 Uses chlorophyll a and d found in folds of the cell
membrane
 No cell walls, nucleus, or organelles
2) Eukaryotic Autotrophs: True Algae, Photosynthetic Protists, and Plants
 Have chlorophyll a, b, and / or c along with accessory pigments in membrane
bound chloroplasts
SUMMARY of PHOTOSYNTHETIC ORGANISMS
Photosynthesis is carried out by plants, algae, photosynthetic protists, and
cyanobacteria.
• Electrons in the porphyrin ring of chlorophyll absorb light energy and begin the
process of photosynthesis.
• The endosymbiotic theory proposes that an ancestor of cyanobacteria was
engulfed by an ancestor of today’s eukaryotic cell and gave rise to plant cells.
• Algal and plant cells contain chloroplasts. They are most abundant in the mesophyll
and guard cells of plant leaves.
• The following is an overall equation for photosynthesis:
chlorophyll
6CO2(g) _ 6H2O(l) _ light energy [CH2O](aq) _ 6O2(g)
• Stomata are open in the daytime and closed at night. Light-activated proton pumps
in guard cell membranes cause potassium ions to move from neighbouring epidermal
cells into guard cells. As a result, water moves by osmosis into guard cells,
causing them to swell. Increasing turgor pressure within guard cells causes their
membranes to buckle and stomata to open. As the concentration of sucrose in
guard cells decreases in the evening, water moves out of the cells and stomata close.
• Chloroplasts have an outer membrane and an inner membrane. The interior
space contains a semiliquid material called stroma with a system of membranebound
sacs called thylakoids, some of which are stacked on top of one another to
form grana. Thylakoid membranes contain chlorophyll molecules and electron transport chain.
transport chains.