Respiration in plants

Topic Objectives: Upon completion of this lesson, the student
will be able to:

Learning Outcome #:

(B-1) - Compare and contrast the processes of photosynthesis
and respiration.

(B-2) - Write the balanced formulas for both photosynthesis
and respiration and discuss how energy is acquired
and released through these reactions.

Special Materials and Equipment:

References: Hartmann, H. T., Flocker, W. J., & Kofranek,
A. M. (1981). PLANT SCIENCE: GROWTH,
DEVELOPMENT, AND UTILIZATION OF CULTIVATED
PLANTS. Englewood Cliffs, NJ: Prentice-Hall.

Evaluation: Unit Exam

TOPIC PRESENTATION: RESPIRATION IN PLANTS

A. The Importance of Respiration in Plants: Much is often made of the
unique ability of plants to photosynthesize, with the importance of
plant respiration proportionately overlooked. Those involved in
ornamental horticulture cannot afford to lack understanding of the
respiration process in plants. Some points to keep in mind:

1. While only green parts of a plant exposed to light photosynthesize,
RESPIRATION IS ESSENTIAL FOR SURVIVAL OF ALL LIVING PARTS OF THE
PLANT, INCLUDING THE ROOTS, THE STEMS, THE LEAVES, AND THE SEEDS!

2. Photosynthesis occurs during the time of day that light intensities
are high enough to drive the light reaction. RESPIRATION IS ONGOING
24 HOURS A DAY, AS LONG AS THE PLANT IS ALIVE!

3. Restricting a plant's photosynthesis will delay its growth.
RESTRICTING RESPIRATION WILL KILL PLANT TISSUE.

In essence, we can equate "respiration" with "life." The importance of
plant respiration in horticulture cannot be overstated.

B. Plant Requirements for Energy: All plants require energy
for growth, movement, and conducting essential metabolic processes.
In short, all plant life processes are energy consumptive.

1. Growth refers to an increase in size resulting from division and
expansion of cells. Energy is consumed during growth by the
complex reproductive processes of mitosis, by the enzymatic
activity associated with protein synthesis, and by the active
transport processes associated with cell expansion.

2. Cells also expend energy to accumulate and concentrate required
nutrients, and to move certain materials from one part of the plant
to another.

3. Plant movements, such as phototropic and geotropic responses,
consume energy, again because of the requirements of active
transport.

C. Respiration and Energy Release: To release and use the energy stored
as a result of photosynthesis, plant cells depend on the process of
respiration.

1. Respiration occurs in all cells where energy is required.

2. In respiration, foods are broken down in a long series of chemical
reactions that release energy a little at a time.

3. The overall process is called oxidation because carbon atoms from
the food molecules become bound to atoms of oxygen from the air.

4. Carbohydrates are the main foods used in respiration. Carbohydrates
can take many forms:

a. Carbohydrates include sugars and larger molecules made by
linking sugars into chains called polysaccharides.

b. There are many kinds of sugars. Examples are glucose, fructose,
and galactose.

c. Sucrose (table sugar) consists of glucose linked to fructose.
Sucrose circulates throughout most plants, serving
as a mobile energy source.

d. Starch consists of many glucose molecules linked together.

e. Other organic materials which plants consume in respiration
include malic acid, glycolic acid, proteins, fats, and lipids.
Building blocks of these materials include carbohydrates.

f. Before these substrates can be used in respiration, cells must
break these complex materials down to release the individual
sugar molecules.

5. The Respiration Process:

a. In respiration, cells convert sugars and oxygen (O2) into
carbon dioxide (CO2), water (H2O), and energy.

b. The actual process involves complex enzymatic activity,
requiring many steps to complete. While these are beyond the
scope of this lesson, the net result can by expressed in the
formula below:

C6 H12 O6 + (6) H2O + (6) O2 ----> (6) CO2 + (12) H2O + energy

One molecule of sugar combines with six molecules of water
and six molecules of oxygen to produce six molecules of carbon
dioxide, twelve molecules of water, and a release of energy.

Note that water appears on both sides of the equation. This
is to remind us that the oxygen does not interact directly
with the sugar, as it might in a high-temperature reaction.
There is, of course, a net production of six water molecules
produced for every molecule of glucose consumed.

c. This is the same overall process that occurs in combustion
(fire), except that cells conduct the process in small steps
that utilize energy in chemical reactions and the gradual
release of heat instead of releasing all the energy quickly
in the form of high temperatures and light.

d. Most energy released in respiration is used by the plant for
driving metabolic processes. Some energy is lost in the form
of heat.

D. Comparing Respiration in Plants and Animals: It is worth mentioning
here that the meaning of "respiration" as we apply it to plants, and
the same term as we usually apply it to animals and ourselves, have some
very important similarities AND differences. These similarities and
differences can be confusing for those learning about respiration
in plants, and deserve some attention.

1. Similarities:

a. The essential inputs (carbohydrate and oxygen) and products
(carbon dioxide, water, and energy) are the same in both cases.

b. All animal cells, like plant cells, need to respire at all times
to survive.

c. Both plants and animals require gaseous exchange for
respiration to occur.

2. Differences:

a. Plant respiration takes place more slowly. Demand for
substrates is not as high, and by-products, including heat,
do not accumulate as quickly.

b. Higher animals require much more extensive circulatory systems
to deliver the required food and oxygen to respiring cells in
the amounts needed. Because of this, we often equate the term
"respiration" with our process of breathing. This can lead to
confusion. While plants require gaseous exchange through
stomata and diffusion, they have no process equivalent to
animal "breathing." To say they respire is NOT to say they
breathe. However, this can help us remember the importance of
oxygen to all plant cells.

E. Environmental Factors Affecting Respiration: The rate of respiration
is influenced by environmental conditions such as moisture, temperature,
and oxygen concentration.

1. Too much moisture around the plant roots can inhibit oxygen
exchange and cut respiration.

2. Temperature increases result in dramatic respiration increases.
Respiration can as much as quadruple for each 10 degree Centigrade
increase in temperature.

3. Respiration decreases in direct proportion to decreases in atmospheric
oxygen concentration.

4. Respiration is also affected by the general health of the plant
with regard to such factors as macro- and micronutrient
availability. Another consideration is the stage of development of
the plant (growing plants use more energy than mature ones).

F. Photosynthesis and Respiration Compared: Photosynthesis and respiration
are often said to be the opposite of each other. Their main features are
contrasted in the following chart:


Photosynthesis Respiration
===========================================================================

Fixes chemical energy Releases chemical energy
Absorbs C02 Forms C02
Forms 02 Uses 02
Makes sugar Uses up sugar (and other substances)
Increases weight Decreases weight
Only in green cells In all living cells
Only in light At all times (in both light and
darkness)

===========================================================================