Photosynthesis, Respiration, Transpiration
Plants are self-sufficient.
They make their own food thru the
process of photosynthesis using
light energy to make sugars from
carbon dioxide (C02)
and water (H20).
The three major functions that
are basic to plant growth and
• Photosynthesis – the process
of capturing light energy and
converting it to sugar energy,
in the presence of chlorophyll
• Respiration – the process of
metabolizing (burning) sugars to
yield energy for growth,
reproduction and other life
• Transpiration – the loss of
water vapor through the stomata
A primary difference between
plants and animals is the plant’s
ability to manufacture its own food.
carbon dioxide from the air and
water from the soil react with the
sun’s energy to form carbohydrates
(sugars and starches).
Photosynthesis literally means
to put together with light.
The photosynthetic process occurs
only in the chloroplasts,
tiny subcellular structures
contained in the cells of leaves and
green stems. In photosynthesis, the
sun’s energy combines hydrogen from
with carbon dioxide (CO2)
turning them into carbohydrates.
is given off as a by-product of
photosynthesis. The chemical
equation for the process of
photosynthesis is: 6CO2
+ light C6H12O6
This process is directly
dependent on the supply of water,
light and carbon dioxide. Any
one of the factors
on the left side of the equation
(carbon dioxide, water, or light)
can limit photosynthesis regardless
of the availability of the other
factors. If any one of these factors
is limiting, then the whole process
slows down or stops. An implication
of drought or severe restrictions on
landscape irrigation is a reduction
in photosynthesis and thus a
decrease in plant vigor.
In a tightly closed greenhouse
there can be very little fresh air
infiltration and CO2
levels can become limiting. This in
turn limits plant growth because the
production of sugars needed to do
the work involved with growing is
limited. Many greenhouses provide
to stimulate plant growth.
The rate of photosynthesis is
somewhat temperature dependent. For
example, with tomatoes, when
temperatures rise above 96 degrees
Fahrenheit the rate of food used by
respiration rises above the rate
that food is manufactured by
photosynthesis. Plant growth comes
to a stop and produce loses its
In respiration, plants
(and animals) convert the sugars
back into energy for growth and to
energize life processes (metabolic
processes). The chemical equation
for respiration shows that the
sugars from photosynthesis are
combined with oxygen. Notice that
the equation for respiration is the
opposite of photosynthesis.
Chemically speaking, the process
is similar to the oxidation
that occurs as wood is burned,
producing heat. When compounds
combine with oxygen, the process is
often referred to as burning.
For example, athlete’s burn
energy (sugars) as they exercise.
The harder they exercise, the more
sugars they burn so the more oxygen
they need. That’s why at full speed,
they are breathing very fast.
Athletes take up oxygen through
their lungs. Plants take up oxygen
through the stomata in their leaves
and through their roots.
Again, respiration is the burning
of sugars for energy to grow and do
the internal work of living. It is
very important to understand that
both plants and animals (including
microorganisms) need oxygen for
respiration. This is why overly wet
or saturated soils are detrimental
to both root growth and function,
and the decomposition processes
carried out by microorganisms in the
The same principles regarding
limiting factors are valid for both
photosynthesis and respiration.
Comparison of Photosynthesis
sugars from energy
in cells with chloroplasts
dioxide is used
dark and light
Water in the roots is pulled
through the plant by
transpiration (loss of water
vapor through the stomata of the
leaves). Transpiration uses about 90
percent of the water that enters the
plant. The other 10 percent is an
ingredient of photosynthesis and
Transpiration serves three
• Movement of minerals up from
the root (in the xylem) and
sugars (products of
photosynthesis) throughout the
plant (in the phloem). Water
serves as both the solvent and
the avenue of transport.
• Cooling – 80 percent of the
cooling effect of a shade tree
is from the evaporative cooling
effects of transpiration. This
benefits both plants and humans.
• Turgor pressure – Water
maintains the turgor pressure in
cells much like air inflates a
balloon, giving the non-woody
plant parts form. Turgidity is
important so the plant can
remain stiff and upright and
gain a competitive advantage
when it comes to light.
Turgidity is also important for
the functioning of the guard
cells, which surround the
stomata and regulate water loss
and carbon dioxide uptake.
Turgidity also is the force that
pushes roots through the soil.
Water movement in plants is also
a factor of osmotic pressure and
capillary action. Osmotic
pressure is defined as water
flowing through a permeable membrane
in the direction of higher salt
concentrations. Water will continue
to flow in the direction of the
highest salt concentration until the
salts have been diluted to the point
that the concentrations on both
sides of the membrane are equal.
A classic example is pouring salt
on a slug. Because the salt
concentration outside the slug is
highest, the water from inside the
slug’s body crosses the membrane
that is his skin. The slug becomes
dehydrated and dies. Envision this
same scenario the next time you
gargle with salt water to kill the
bacteria that are causing your sore
Fertilizer burn and dog urine
spots in a lawn are examples of salt
problems related to gardening. The
salt level in the soil’s water is
higher than in the roots, and water
flows from the roots into the soil’s
water in an effort to dilute the
concentration. So what should you do
if you accidentally over apply
fertilizer to your lawn?
refers to the chemical forces that
move water as a continuous film
rather than as individual molecules.
Water molecules in the soil and in
the plant cling to one another and
are reluctant to let go. You have
observed this as water forms a
meniscus on a coin or the lip of a
glass. Thus when one molecule is
drawn up the plant stem, it pulls
another one along with it. These
forces that link water molecules
together can be overcome by gravity
Whiting, Colorado State University
Cooperative Extension consumer
horticulture specialist and Colorado
Master Gardener Coordinator; M.
Roll, Extension horticulture agent,
Arapahoe County; and L. Vickerman,
Extension horticulture agent, El
Paso County. 12/03.
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