206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food? Green plants, in fact,
have to make or rather synthesise the food they need and all other organisms
depend on them for their needs. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
You have already learnt that the autotrophic nutrition is found only in plants
and all other organisms that depend on the green plants for food are
heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemical
process by which they use light energy to drive the synthesis of organic
compounds. Ultimately, all living forms on earth depend on sunlight for
energy. The use of energy from sunlight by plants doing photosynthesis is
the basis of life on earth. Photosynthesis is important due to two reasons: it
is the primary source of all food on earth. It is also responsible for the release
of oxygen into the atmosphere by green plants. Have you ever thought what
would happen if there were no oxygen to breath? This chapter focusses on
the structure of the photosynthetic machinery and the various reactions
that transform light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis. Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and exposed to light. On testing these leaves for the presence
of starch it was clear that photosynthesis occurred only in the green parts
of the leaves in the presence of light.
P
HOTOSYNTHESIS IN
H
IGHER
P
LANTS
C
HAPTER
13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2020-21