Download Lesson 8 A Photosynthesis

PHOTOSYNTHESIS AND RESPIRATION
Lesson No. 8 (2B)
The yield of vegetables ultimately depends on the size and efficiency of their
photosynthetic system. Crop management practices in order to increase yield
are based on this assumption. Since photosynthesis is the basis of vegetable
production, it is important to know its associated physiological processes and
how the plant parts act together to capture and store energy from the sun
 The solar energy that a plant stores in carbohydrates during
photosynthesis is used to run and maintain processes in the plants, such as
absorption of water & nutrients, transporting them to leaves, and
converting other products of photosynthesis to cell walls and other cellular
parts; so that the plant can grow and develop.
 For the carbohydrates to be utilized, their energy must be released in the
process of respiration.
 The products of photosynthesis are the raw materials for respiration and
the fuels for work in the plant are the products of respiration.
 Since the general overall reaction of respiration is the breakdown of
carbohydrates into CO2 and water, it would seem that it is photosynthesis
in reverse, which is not actually so.
 While photosynthesis is a building up reaction, respiration is a breaking
down reaction.
CO2
CO2
CO2
Energy
Solar
Water
Chlorophyll
Respiration happens
inside the plant. After
carbohydrates are
created by
photosynthesis, this food
is then converted to grow
the different parts of the
plant. It converts its food
to water and carbon
dioxide plus energy.
Developing Fruit
Water
Photosynthesis involves
components that are
external to the plant –
sun, carbon dioxide,
water, etc. They come into
the plant to form
carbohydrates as the
basis for plant food.
 The product of photosynthesis may be used immediately or stored as
starch. They may also be converted into more complex compounds, such
Notes
as cellulose, proteins, fats, pigments, flavor components, and many other
kinds of compounds.
 This is accomplished by joining several units of glucose, converting them
into sugars, then joining the converted sugars or adding other nutrient
elements to the above substances to form new compounds.
 The energy will come from the breakdown of the simple substances into
carbon dioxide, water and heat. Collectively all these transformations are
called metabolism.
Photosynthesis is the process by which carbohydrates and energy-rich
chemicals are formed from carbon dioxide and water, using light energy with
the release of oxygen. In simple form, the process could be represented as:
light
CO2 + H2O
CH2O + O2
chlorophyll
 A very important feature of leaves is the presence on their surfaces of a
large number of tiny openings called stomata. Carbon dioxide (CO2) used
in photosynthesis enters and O2 a by-product of photosynthesis, exits
through the stomata.
 Photosynthesis is composed of many reactions, which are all triggered and
speeded up by protein bodies called enzymes.
 The energy from the sun increases the energy level of the chlorophyll. As
sufficient light strikes a sufficient number of chlorophyll, the energy is
transferred to water, causing it to split into its hydrogen and oxygen
components.
See Requirements Of Photosynthesis
Respiration involves the release of energy from carbohydrates:
CH2O + O2
CO2 + H20 + energy
 Respiration is composed of many reactions, which are all activated by
enzymes.
 When the plant needs to do work like build cells, produce flowers, etc. all
it needs to do is to break down the glucose, so the ‘batteries’ (ATP) will be
released to provide energy for the soil processes.
 The rate of respiration varies in different parts of the plant. It is high in
growing or immature organs and low in seeds and in specialized storage
organs.
 Oxygen is hardly a problem for respiration in the leaves; but it is a problem
in the roots when the plant is grown in waterlogged soils.
 The rate of respiration increases rapidly with the increase in temperature.
Notes

Respiration has to be reduced when storing seeds or vegetables for long periods of time. Reducing
temperature and excluding oxygen will slow down respiration.