Photosynthesis and cellular respiration are important plant processes. Photosynthesis uses energy from sunlight to produce sugars from carbon dioxide and water. This fuels plant growth and also produces oxygen as a byproduct. Cellular respiration breaks down sugars to release the stored energy, producing carbon dioxide and water. Plant growth occurs through metabolic processes that use enzymes to break down and rebuild molecules. These plant processes are essential to human life as they produce oxygen, food, fuel, and play a role in rainfall and climate regulation.

1. Horticulture Science
Lesson 11
Understanding Plant Physiology

2. Interest Approach
Bring a drinking glass to class. Either teach briefly or
ask students to come up with a simple equation for
photosynthesis, such as “carbon dioxide and
water give you sugar and oxygen”. Partially fill the glass
with water and ask for an easy carbon dioxide source.
When your breath is suggested,
breathe out into the container, thus “combining” water
and carbon dioxide.

3. Interest Approach
Cover the container with your hand and shake it
vigorously. After anywhere from 5 to 60 seconds of
shaking, stop and take a drink. State that the equation
must be true because you just tasted the water and it
was sweet. This will prompt an active discussion on how
this is or is not possible. Stick to your story as long as
necessary.

4. Interest Approach
Another approach is to talk to a student before class
and have the student play along, first as an unbeliever
and then as a convert. You may lace another drinking
glass with sugar syrup so that when you fill it up with tap
water it tastes sweet. The object of the deception is to
get the students to think of exactly what it takes to
produce sugar. Of course, it only happens in plants.

5. Student Learning
Objectives
•Analyze the process of
photosynthesis.
•Examine the process of cellular
respiration.

6. Student Learning
Objectives
•Describe plant growth processes.
•Explain why photosynthesis and
respiration are important to
human beings.

7. Terms
• ATP
• cellular respiration
• cellulose
• chlorophyll
• chloroplast
• enzymes
• fructose

8. Terms
• glucose
• metabolism
• mitochondria
• NADPH
• photosynthesis
• starch
• transpiration

9. How does photosynthesis work?
• Photosynthesis is the process by which a plant turns
the light energy from the sun into chemical energy in
the form of sugar.
• Photosynthesis is a complex series of chemical
reactions that happens within the plant cells.
• The photosynthetic process can be broken down into
two stages: the light reaction and CO2 fixation.

10. How does photosynthesis work?
• The process of producing food begins with the light
reaction phase.
• Light energy is trapped and used to fuel the
photosynthetic process.
• The harnessing of light energy is made possible by
pigments found in chloroplasts, which are specialized
organelles within the individual plant cells.
• As light enters the chloroplast, chlorophyll, carotene,
and xanthophylls, pigments absorb the energy.

12. How does photosynthesis work?
• Chlorophyll, the primary
pigment involved in the
manufacture of food, uses
the light energy to make
the high-energy
compounds ATP and
NADPH.
– These compounds power
reactions in the cells.

13. How does photosynthesis work?
• Chlorophyll uses the energy to split water molecules,
H2O, during the first phase of the photosynthetic
reaction.
• Oxygen atoms from water molecules bond to form
O2. The O2 escapes through the stomata into the
atmosphere.
• Hydrogen atoms from the water molecule are
incorporated into sugar molecules as the process
continues in CO2 fixation.

14. How does photosynthesis work?
• During the CO2 fixation phase of the photosynthetic
reaction, energy in the form of ATP and NADPH is
used to form sugars.
• Carbon dioxide enters through the stomata and is
available to the chlorophyll in the cells.

15. How does photosynthesis work?
• The CO2 combines with hydrogen from the water
molecule.
• Two forms of sugar, glucose and fructose
(C6H12O6), are assembled.
• Although they both have the same formula, the
molecular structures differ.

16. How does photosynthesis work?
• Glucose and fructose fuel plant growth.
• They are the source of energy for the plant’s life
processes.
• All live cells in the plant benefit as the sugars are
transported through the phloem to the rest of the plant.
• The simple sugars can also be combined to form a
more complex sugar: sucrose (C12H22O11).

17. How does photosynthesis work?
• The sugar molecules may be processed further to
form starch and cellulose.
• These are huge molecules, resulting from the bonding
of thousands of glucose molecules.
• Starch serves as the principle way in which food is
stored for plants.
• When needed, it is easily broken down into glucose
or converted into other plant products by plant
enzymes.

18. How does photosynthesis work?
• The cellulose is applied to cell walls for strength and
rigidity.
• Once sugars are converted to cellulose, they are not
reclaimed for other purposes.

19. How does cellular respiration work?
• In many ways, cellular respiration is the opposite of
photosynthesis.
• Photosynthesis stores energy in chemical bonds in
the process of making sugars.
• Energy stored in the chemical bonds is released
during cellular respiration.

20. How does cellular respiration work?
• Cellular respiration takes place in structures
contained within cells called mitochondria.
• Cellular respiration is extremely important in the
growth and development of plants.
• All living plant cells respire or use energy to live and
function.

21. How does cellular respiration work?
• Cellular respiration involves sugars produced in the
photosynthetic process along with oxygen and water.
• In the reaction, chemical energy is released when the
molecular bonds of the sugar molecules are broken in
combination with oxygen gas.

22. How does cellular respiration work?
• The extracted energy, ATP, drives a variety of chemical
reactions in the cell.
• By-products of the reaction include carbon dioxide and
water.
• Water and carbon dioxide are released, as is all of the
energy that was contained in the bonds of the glucose
molecule.

23. How does cellular respiration work?
A COMPARISON OF PHOTOSYNTHESIS
AND RESPIRATION

24. How does plant growth occur?
• Actual plant growth is fueled by cellular respiration.
• It takes place primarily at night when photosynthesis
is shut down.
• With signals from hormones, enzymes or chemical
activators are produced.
• Each enzyme has a specific job.

25. How does plant growth occur?
• With split-second timing, enzymes break down sugars
and recombine them with nitrogen and other
minerals.
• Many complex molecules are produced, which
include starches, pectin (to bind cells), lignin (a
tough, durable substance), cellulose, lipids (fats),
proteins, pigments, hormones, vitamins, and
alkaloids, and tannins (materials that protect plants
from pests and diseases).

26. How does plant growth occur?
• All of the chemical reactions in a plant fall under a
term: metabolism.
• The speed at which chemical reactions occur is
influenced by temperature.
• Plant metabolism is slowed in cool or cold
temperatures and is more rapid in warmer
temperatures.
• For example, florists keep cut flowers in coolers to
slow metabolism that leads to aging.

27. How does plant growth occur?

28. How are photosynthesis and respiration
important to
human beings?
• Photosynthesis and cellular respiration are two of the
most important chemical reactions to human beings.
• All of our food and nutrition comes either directly or
indirectly from photosynthesis.
• We eat a combination of plants and things that ate
plants.
• Much of our heat and electricity comes either directly
or indirectly from byproducts of photosynthesis in
plants.

29. How are photosynthesis and respiration
important to
human beings?
• Fossil fuels that power our vehicles have their origin in
plants.
• Most of the oxygen that we breathe comes from
photosynthesis reactions.
• Transpiration is the release of water from leaves as
part of photosynthesis.
• It puts thousands of tons
of water into the air, cooling
our environment and
encouraging rainfall.

30. Review/Summary
•How does photosynthesis
work?
•How does cellular respiration
work?

31. Review/Summary
•How does plant growth occur?
•How are photosynthesis and
respiration important to human
beings?