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Chloroplasts and photosynthesis
3.1 Why organisms undergo photosynthesis and how chloroplasts are adapted for their function
Photosynthesis: what is it?
Photosynthesis is a process whereby light energy is converted to chemical energy which can be used to synthesis large,
organic molecules from smaller, inorganic substrates. Plants and other organisms which are photosynthetic (for example,
bacteria) are known as autotrophs, that is, they make organic compounds from small inorganic precursors. More
specifically, those autotrophs which undergo photosynthesis are known as photoautotrophs. Animals, and many other
organisms, cannot synthesise their own food, but instead digest organic molecules. These are known as heterotrophs.
carbon dioxide + water glucose + oxygen
6CO2 + 6H2O C6H12O6 + 6O2
Photosynthesis is considered to be the most fundamental biochemical process, as all aerobes (organisms which respire
aerobically) require the products of photosynthesis to undergo aerobic respiration.
Where does photosynthesis take place?
Photosynthesis is a process which occurs in two stages, both entirely inside the chloroplast. A chloroplast is an organelle
within photosynthetic cells. The list below outlines some of the adaptations chloroplasts have which help them achieve
efficient rates of photosynthesis, and the explanations will become clear to you as you progress through the topic:
the granal membranes provide a large surface area for the attachment of the photosynthetic pigments (chlorophylls
and carotenoids), electron carriers and enzymes for the light-dependent reactions
a network of proteins in the grana hold the pigments in a very precise manner that forms the photosystems allowing
for maximum absorption of light
the granal membranes have many ATP synthase enzymes attached to them which, via chemiosmosis (see 4.4 Electron
transport chain) help to manufacture ATP molecules
the fluid of the stroma holds all of the enzymes needed to carry out the light-independent reactions
the stroma fluid surrounds the grana, and so the products of the light-dependent reactions can directly and readily
pass into the stroma for the light-independent reactions
chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture photosynthetic proteins
Structure of a chloroplast
Chloroplasts vary, but most are disc-shaped and approximately 2-10m long. Each has a double membrane (called an
envelope), consisting of the inner membrane and outer membrane.
intergranal lamella Chloroplasts contain stacks of
starch grain flattened membrane compart-
. . . . single
ments. Each stack is called a
. . . granum (plural: grana) and each
. . . thylakoid
compartment is a thylakoid.
Small, thin membranal
granum extensions connect different
. grana, called intergranal
. . lamellae. The fluid surrounding
. . . the grana is called stroma.
. . . . Starch grains can also be found
. . . . . in the stroma matrix, as well as
DNA DNA and ribosomes, which can
stroma be used to make proteins.
The two stages of photosynthesis are the light-dependent reactions (the first or `light' stage) and the light-independent
reactions (the second or `dark' stage). The first stage takes place in the grana, and the second stage in the stroma fluid of
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Above: transmission electron microscopic image (TEM) of a plant cell chloroplast (x50,000)
Photosynthetic pigments and photosystems
Embedded in the thylakoid membrane among grana are coloured compounds which absorb light of a short range of
wavelengths and reflect light of other wavelengths. These are called photosynthetic pigments. A photosynthetic pigment
can absorb some light energy of specific wavelengths. Various photosynthetic pigments are arranged into small structures
called photosystems in the granal membrane.…read more
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The graph to the left above shows the absorption spectra for photosynthetic pigments, and the diagram to the right
above shows the action spectrum for photosynthesis.
Chromatographic separation of pigments
A chromatogram can be used to separate the different photosynthetic pigments in a photosystem.…read more