Agriculture Reference
In-Depth Information
deciduous plants or at the end of each leaf's useful life
for evergreen plants.
The energy released by aerobic respiration is stored
in a chemical form in a substance called ATP , which
can be transported to wherever energy is needed
in the cell, in effect acting like a 'battery'. Some
energy is also released as heat, similar to the cellular
respiration which produces heat in our bodies to
keep us warm.
The energy requirement of cells within the plant
varies: reproductive organs can respire at twice
the rate of the leaves for example. Also, in apical
meristems, the processes of cell division and cell
differentiation require high inputs of energy to create
new cells.
It would appear at first sight that respiration is
the reverse of photosynthesis. This is correct in
the sense that photosynthesis creates glucose
as an energy-harvesting strategy, and respiration
breaks down glucose as an energy-releasing
mechanism. It is also correct in the sense that the
simple equations representing the two processes
are mirror images of each other. It should, however,
be emphasized that the two processes have two
notable differences. The first is that respiration in
plants (as in animals) occurs in all living cells of all
tissues, in leaves, stems, flowers, roots and fruits.
Photosynthesis occurs predominantly in the palisade
mesophyll tissue of leaves but not in other plant
tissues such as the roots. Second, respiration takes
place continuously whereas photosynthesis only
operates when light is present, it cannot happen in
the dark.
Plant size and growth rate
It is important for anyone planning a garden
that the eventual size (in terms of both height
and width) of trees, shrubs and perennials
is considered. The impressive Ginkgo biloba
(maidenhair tree) can grow to 30 m in height
(at least twice the height of a normal house)
and is, therefore, not the plant to put in a small
bed. Similarly x Cuprocyparis leylandii (leyland
cypress), useful in rapidly creating a fine hedge,
can also grow to 30 m, and reach 5 m in width,
to the consternation of even the most friendly of
neighbours!
The eventual size of a plant is recorded in plant
encyclopaedias and specialist nurseries will
also give advice to potential buyers. It should
be remembered that the final size of a tree or
shrub may vary considerably in different parts
of the country, and may be affected within a
garden by factors such as aspect, soil, shade
and wind. Attention should also be given to
the rate at which a plant grows. Taxus (yew)
and Magnolia stellata (star magnolia) are two
notable examples of slow-growing species.
Respiration
In order that growth can occur, food manufactured by
photosynthesis must be broken down in a controlled
way to release the energy which was trapped from
sunlight for the production of useful substances such
as cellulose, the main constituent of plant cell walls,
and proteins for enzymes. This energy is also used
to fuel cell division and the many chemical reactions
that occur in the cell. Respiration is the process by
which these sugars and starches are broken down to
yield energy, releasing carbon dioxide and water as
waste products. It takes place in the mitochondria
of the cells and is often referred to as 'cellular
respiration'.
In order that the breakdown is complete, and the
maximum energy is released, oxygen is required in
the process of aerobic respiration (Figure 9.6).
Respiration is the process by which sugars are
broken down to yield energy, the end products
being carbon dioxide and water.
Factors affecting respiration
Two environmental factors which affect the rate of
respiration are:
X oxygen
X temperature.
Oxygen
Oxygen is essential for aerobic respiration as it is
needed to break down the carbohydrates to release
glucose + oxygen
carbon dioxide + water + energy
(ATP and heat)
Figure 9.6 Equation for aerobic respiration
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