Cooperative Extension
MG Manual Home
Basic Botany

  MG Manual Reference
Ch. 1, pp. 25 - 29
The three major plant functions that are the basics for plant growth and development are photosynthesis, respiration, and transpiration.


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."
Leaf Cross Section
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.
Light Reaction Top
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 light reaction.
Dark Reaction Top
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 photorespiration.
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 and spring.
How a Plant Grows
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 this range.


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 breaking-down process.


  1. Uses food for plant energy.
  2. Releases energy.
  3. Occurs in all cells.
  4. Uses oxygen.
  5. Produces water.
  6. Produces carbon dioxide.
  7. 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.


  1. Produces food.
  2. Stores energy.
  3. Occurs in cells containing chloroplasts.
  4. Releases oxygen.
  5. Uses water.
  6. Uses carbon dioxide.
  7. 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 transpiration.

Search Index Comment

This site was developed for the Arizona Cooperative Extension, College of Agriculture, The University of Arizona.
© 1998 The University of Arizona. All contents copyrighted. All rights reserved.