Photosynthesis
Photosynthesis is the process by which plants, some bacteria and some protistans use the energy
from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into
pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.
Photosynthesis may be summarised by the word equation:
carbon dioxide + water glucose + oxygen
The conversion of usable sunlight energy into chemical energy is associated with the action of the
green pigment chlorophyll.
Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other
photosynthetic organisms. All photosynthetic organisms have chlorophyll a. 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). Chlorophyll
a absorbs its energy from the violet-blue and reddish orange-red wavelengths, and little from the
intermediate (green-yellow-orange) wavelengths.
Chlorophyll
All chlorophylls have:
• a lipid-soluble hydrocarbon tail (C20H39 -)
• a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different
side-groups on the head
The tail and head are linked by an ester bond.
, Leaves and leaf structure
Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf
may be viewed as a solar collector crammed full of photosynthetic cells.
The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the
products of photosynthesis, sugar and oxygen, leave the leaf.
Water enters the root and is transported up to the leaves through specialized plant cells known as
xylem vessels. Land plants must guard against drying out and so have evolved specialized structures
known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the
protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the
singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis
can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these
gases are moving between the inside and outside of the leaf, a great deal of water is also lost.
Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot
desert days.
The structure of the chloroplast and photosynthetic membranes
The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes
have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have
chloroplasts with a surrounding membrane.
Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between
grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast
has three, forming three compartments.
Structure of a chloroplast
Photosynthesis is the process by which plants, some bacteria and some protistans use the energy
from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into
pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.
Photosynthesis may be summarised by the word equation:
carbon dioxide + water glucose + oxygen
The conversion of usable sunlight energy into chemical energy is associated with the action of the
green pigment chlorophyll.
Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other
photosynthetic organisms. All photosynthetic organisms have chlorophyll a. 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). Chlorophyll
a absorbs its energy from the violet-blue and reddish orange-red wavelengths, and little from the
intermediate (green-yellow-orange) wavelengths.
Chlorophyll
All chlorophylls have:
• a lipid-soluble hydrocarbon tail (C20H39 -)
• a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different
side-groups on the head
The tail and head are linked by an ester bond.
, Leaves and leaf structure
Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf
may be viewed as a solar collector crammed full of photosynthetic cells.
The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the
products of photosynthesis, sugar and oxygen, leave the leaf.
Water enters the root and is transported up to the leaves through specialized plant cells known as
xylem vessels. Land plants must guard against drying out and so have evolved specialized structures
known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the
protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the
singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis
can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these
gases are moving between the inside and outside of the leaf, a great deal of water is also lost.
Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot
desert days.
The structure of the chloroplast and photosynthetic membranes
The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes
have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have
chloroplasts with a surrounding membrane.
Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between
grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast
has three, forming three compartments.
Structure of a chloroplast
