One of the major differences between plants and animals
on earth is the ability of plants to internally manufacture their
own food. To produce food for itself a plant requires energy from
sunlight, carbon dioxide from the air and water from the soil. If
any of these ingredients is lacking, photosynthesis, or food
production, will stop. If any factor is removed for a long period
of time, the plant will die. Photosynthesis literally means "to
put together with light."
Any green plant tissue is capable of photosynthesis.
Chloroplasts in these cells contain the green pigment called
chlorophyll which traps the light energy. However, leaves are
generally the site of most food production due to their special
structure. The internal tissue (mesophyll) contains cells with
abundant chloroplasts in an arrangement that allows easy movement
of water and air. The protective upper and lower epidermis (skin)
layers of the leaf include many stomata that are openings in the
leaf formed by two specialized guard cells on either side. Guard
cells regulate movement of the gases, (i.e. CO2 into
and O2 and H2O out of the leaf), involved
in photosynthesis. The lower epidermis of the leaf normally
contains the largest percentage of stomata.
Photosynthesis is the process of turning the energy of sunlight
into chemical energy from the raw products of CO2 and
H2O. This process is necessary to sustain nearly all
forms of life. Photosynthesis is divided in to two separate
reactions known as the light and dark reactions. They take place
when light is present but the dark reaction does not require
light. The whole process is begun by light reacting with pigments
in the leaf causing the splitting of water molecules. This is
called photolysis or the Hill Reaction which is not
completely understood. Three products are produced in this
reaction. Electrons from the hydrogen molecules and remaining H+
ions are used to form two separate energy storage molecules. The
air we breath is from the remaining oxygen portion of H2O.
The carbon dioxide molecules are transformed into sugars during
the dark reaction using the energy that was formed during the
This part of the photosynthetic process is also called the Calvin
Cycle. With one cycle of this reaction 3 carbon atoms are fixed or
placed in a sugar molecule. This pathway is called C-3
photosynthesis. This is the way that most dicots or broadleaf
plants make sugars during the dark reaction. C-3 photosynthesis
has a disadvantage though. Oxygen competes with CO 2 for a binding
site during the dark reaction. Sometimes sugars are not formed,
but energy is still expended to complete the cycle. This is called
Another dark reaction pathway is called C-4
photosynthesis because 4 carbons are fixed or placed in a sugar
molecule each time the cycle is completed. The dark reaction of
C-4 photosynthesis occurs inside of specialized parts of leaf
cells in the leaf called the bundle sheath, which exclude the
presence of O2. Because there is no oxygen present
photorespiration does not occur. The C-4 photosynthetic pathway is
what occurs in most monocots or grasses. This is a more efficient
pathway and allows grasses to grow faster than broadleaf plants.
Crassulacean acid metabolism or CAM photosynthesis is the dark
reaction type found in many cactus, succulents, bromeliads, and
orchids as well as a few other plants. CAM photosynthesis is
similar to C-4 photosynthesis. However, CAM plants open their
stomata only during the night to collect CO2, when air
temperatures are cooler, thus conserving water because of reduced
transpiration. The CO2 is converted into malic acid
and then converted back to CO2 during the day when
light is present, thus producing sugars, while the stomata are
closed and greatly reducing water loss.
Plants convert the energy from light into simple
sugars, such as glucose. This food may be converted back to water
and carbon dioxide, releasing the stored energy through a process
called respiration. This energy is required for growth in nearly
all organisms. Simple sugars are also converted to other sugars
and starches (carbohydrates) which may be transported to the stems
and roots for use or storage, or they may be used as building
blocks for more complex structures, e.g. oils, pigments, proteins,
cell walls, etc.
Photosynthesis is dependent on the availability of
light. Generally speaking, as sunlight increases in intensity
photosynthesis increases. This results in greater food production.
Many garden crops, such as tomatoes, respond best to maximum
sunlight. Tomato production is cut drastically as light
intensities drop. Only two or three varieties of "greenhouse"
tomatoes will produce any fruit when sunlight is minimal in fall
Water plays an important role in photosynthesis in
several ways. First, it maintains a plant's turgor or the firmness
or fullness of plant tissue. Turgor pressure in a cell can be
compared to air in an inflated balloon. Water pressure or turgor
is needed in plant cells to maintain shape and ensure cell growth.
Second, water is split into hydrogen and oxygen by the energy of
the sun that has been absorbed by the chlorophyll in the plant
leaves. The oxygen is released into the atmosphere and the
hydrogen is used in manufacturing carbohydrates. Third, water
dissolves minerals from the soil and transports them up from the
roots and throughout the plant, where they serve as raw materials
in the growth of new plant tissues. The soil surrounding a plant
should be moist, not too wet or too dry. Water is pulled through
the plant by evaporation of water through the leaves
(transpiration). Photosynthesis also requires carbon dioxide (CO2)
which enters the plant through the stomata. Carbon and oxygen are
used in the manufacture of carbohydrates. Carbon dioxide in the
air is 350 parts per million (ppm) or 0.035% at sea level and is
plentiful enough so that it is not a limiting factor in plant
growth. However, since carbon dioxide is consumed in making sugars
and is not replenished by plants at a rapid rate, a tightly closed
greenhouse in midwinter may not let in enough outside air to
maintain an adequate carbon dioxide level. Under these conditions,
improved crops of roses, carnations, tomatoes and certain other
crops can be produced if the carbon dioxide level is raised with
CO2, generators or, in small greenhouses, with dry ice
or a natural gas flame.
Although not a direct component in photosynthesis,
temperature is an important factor. Photosynthesis occurs at its
highest rate in the temperature range of 65° to 85°F (18°
to 27°C) and decreases when temperatures are above or below
Carbohydrates made during photosynthesis are of value
to the plant when they are converted into energy. This energy is
used in the process of building new tissues. The chemical process
by which sugars and starches produced by photosynthesis are
converted into energy is called respiration. It is similar to the
burning of wood or coal to produce heat or energy. This process in
cells is shown most simply as:
This equation is precisely the opposite of that used to
illustrate photosynthesis, although more is involved than just
reversing the reaction. However, it is appropriate to relate
photosynthesis to a building process, while respiration is a
- Uses food for plant energy.
- Releases energy.
- Occurs in all cells.
- Uses oxygen.
- Produces water.
- Produces carbon dioxide.
- Occurs in darkness as well as light
If oxygen is limited or not present then anaerobic
respiration or metabolism occurs. The by products of this reaction
are ethyl alcohol or lactic acid and CO2. This process
is also know as fermentation or the Pasteur effect, (Louis Pasteur
was the first to describe the effect), which is used to
manufacture brewing and dairy products. It also occurs in muscle
tissue when they are over exerted. The muscle burning we feel
doing exercises is the accumulated lactic acid that forms in our
tissue because of limited oxygen. Plant tissues undergo the same
process, for example waterlogged soils limit the oxygen available
to roots and may cause them to rot because of fermentation.
- Produces food.
- Stores energy.
- Occurs in cells containing chloroplasts.
- Releases oxygen.
- Uses water.
- Uses carbon dioxide.
- Occurs in sunlight.
By now, it should be clear that respiration is the
reverse of photosynthesis. Unlike photosynthesis, respiration
occurs at night as well as during the day. Respiration occurs in
all life forms and in all cells. The release of accumulated carbon
dioxide and the uptake of oxygen occurs at the cell level. In
animals, blood carries both oxygen and carbon dioxide to and from
the atmosphere by means of the lungs, gills, spiracles etc. In
plants there is simple diffusion into the open spaces within the
leaf and exchange occurs through the stomata.
Transpiration is the process by which a plant loses
water, primarily through leaf stomata. Transpiration is a
necessary process that involves the use of about 90% of the water
that enters the plant through the roots. The other 10% of the
water is used in chemical reactions and in plant tissues.
Transpiration is necessary for mineral transport from the soil to
the plant for the cooling of the plant through evaporation, to
move sugars and plant chemicals, and for the maintenance of turgor
pressure. The amount of water lost from the plant depends on
several environmental factors such as temperature, humidity and
wind or air movement. An increase in temperature or air movement
decreases relative humidity and causes the guard cells in the leaf
to shrink, opening the stomata and increasing the rate of