International Agriculture

Energy from Food: Photosynthesis and Cellular Respiration

Let's see how plants capture and use energy!

Part 1: Photosynthesis and Cellular Respiration
Cellular Respiration Activity

How does energy from the sun become the energy we use to ride a bike?
What is in our food that gives us energy?
How can people in other countries eat different food than us but still get the same nutrients?
Where does our food come from, and how do people feel about food around the world?


On our planet we find that most living things (plant or animal) are made up of tiny microscopic structures called cells. It is important to understand that all of the activities connected with living things occur within these microscopic structures....including life's biggest business, the transformation of energy. You might think that all the really important processes happen on the really big scale of you as a person, but in reality, the absolutely required processes are mainly happening on the scale of a single cell. Without the transformation of energy at a cellular level, life could not occur on our planet.

The reason for this is that there must be a method through which living things can obtain the energy provided by the non-living world. On Earth, they most reliable and intense source of energy is the SUN. So, the question is how does the radiant energy from the sun get transformed into a form of energy usable in the living world? This is what this reading assignment is all about.

If we examine most forms of life on our planet (plants and animals), we find that they all share something in common. Their cells use as their basic energy source a special type of carbohydrate, or sugar, called GLUCOSE. The energy found in the chemical molecules of glucose is the type of energy we earlier referred to as chemical energy. Think about this for a moment... chemical energy!

Question: If energy is stored in a chemical molecule, what must be true about the bonds which hold the molecule together? Are they strong bonds or weak bonds? How do you know?

The next obvious question is... where did the chemical energy that was used to form the sugar glucose come from in the first place? If sunlight is the most reliable source of energy, the sun should have something to do with this... in some way, the sun's energy is continuously being transformed into the chemical energy of glucose that can then be used as food by the living things.

Don’t assume too much!!

There are special types of living things on our planet that have the tools required to carry out this transformation, they are the GREEN PLANTS. Notice only green plants are mentioned. All other organisms [such as fungi (molds, mushrooms etc.) and animals, none of which are green] lack the tools to make this transformation of energy occur.

Green plants are important!

They perform the chemical activity (used to transform energy from sunlight into food for living things) called…



Photosynthesis is a chemical activity that can be understood at many levels… from simple explanations to very, very complex descriptions of the process. ISP takes the simplified approach.

It is easy to assume that plants absorb sunlight and use its energy to grow directly, after all sunlight is energy. However, if this were the case, why can’t all living cells do this trick? Green plants use specialized cells, and the process of photosynthesis, to capture the energy from sunlight and store it by creating sugar (glucose) molecules.

It is important for you to realize that this alone can not sustain life.

Plants, much less all the animals, can not stay alive if this was all that happened. Remember, glucose is like a storage house for energy, storing the chemical energy obtained from the sun.

A second important activity must occur to release this stored energy. This second reaction is called…


How Does Photosynthesis Work?
In a way, the word PHOTOSYNTHESIS defines itself:
Photo = light, or radiant ENERGY which provides the ENERGY to...
Synthesis = to build or construct
But what is constructed by the green plants? They use “simple” chemicals, meaning that they have a small number of atoms in each molecule, to construct more “complex” chemicals such as the sugar called glucose.
The simpler chemicals include carbon dioxide and water which must be available to the green plant. So, using the energy from sunlight, carbon dioxide and water molecules chemically react, meaning that their atoms are rearranged into the form called glucose. During this rearrangement, the sun's radiant energy is transformed into chemical energy which is stored in the glucose molecule.

Here is a simple word equation which summarizes the whole thing:

Carbon Dioxide + Water −−−> Glucose + Oxygen + Water

Question: How does sunlight fit into this equation? Where should it be written in?

To make all this happen a special substance is required. That substance is called...

CHLOROPHYLL — it is the green pigment found in green plants. Chlorophyll is what makes green plants green.
Chlorophyll has the ability to trap the sun's radiant energy, and use it to rearrange those carbon dioxide and water molecules into the glucose.

Only chlorophyll can do this.

No chlorophyll would mean no photosynthesis.

Let's summarize our story of photosynthesis:

Carbon Dioxide + Water + Sunlight   ==   Glucose + Oxygen + Water
captured by stored chemical given off as
chlorophyll energy waste products.

Use the photosynthesis lab to explore more.

Some form of respiration occurs in EVERY living thing, every cell of every plant AND animal.

Respiration releases the chemical energy stored in food.

Respiration releases the chemical energy stored in glucose, which is used by the cells to carry out their life activities (growing, moving, etc.). The following diagram attempts to summarize the important relationship between photosynthesis and respiration. (This reading is concerned with photosynthesis. Your next reading focuses on respiration.)

Sunlight (radiant energy)
is Transformed through the process of
Photosynthesis in Green Plants
Transforming the radiant energy into
GLUCOSE (chemical energy)
which can be
Respired by
Plants & Animals
to Transform chemical energy
Into other forms
Required to continue living.


The chemical activity occurring in all plant and animal cells releasing the energy from glucose.

Respiration is NOT the same as what we call breathing!

Respiration is a chemical reaction…

More on respiration in the next reading…




Background -
Leaves vary in size and shape from long, narrow blades of grass, to wide, lobed sugar maple leaves, to needle-like pine leaves. Although leaves do come in a variety of sizes and shapes, they share one common function -- they are the main organs of photosynthesis for a plant.
In Part A of this investigation, you will be examining the basic structure of leaves in order to see how they are adapted to the function of carrying out photosynthesis. Although there are many different types of leaves, you will be studying one particular type that best represents the "typical" leave of a green plant.
Below is a diagram that illustrates a variety of different types of leaves and possible leave arrangements.
Leaf Diagram
Use of the Compound Microscope -

In order to complete Part A of this lab, you will need to familiarize yourself with the use of the compound microscope. Since this is one of the only times during ISP that you will be using the microscope, your teacher will cover the basics quite quickly in demonstrating its proper use. During the brief class demonstration, be sure you are able to identify the following microscope parts and their use:
Microscope Stage Objective Lenses (zoom)
Ocular Lens (eyepiece) Coarse Adjustment Knob
Fine Adjustment Knob Diaphragm
It is very important that you handle your microscope with a great deal of care. Microscopes are very expensive pieces of laboratory equipment that are costly to maintain (and VERY costly to replace).
Materials -

Compound Microscope X-Sections of Leaves
Pencil and Eraser
Procedures -
  1. Turn on your microscope and adjust the LENs to set its magnification of an object to ~100 times. This will be a clearly marked position on the zoom lens or when the shorter objective lens is in position above the hole in the microscope stage.
  2. Obtain a PREPARED microscope slide of the cross-section of a typical leaf ----- place the slide on the stage and position the tiny leaf that you see embedded in the plastic underneath the lens of the microscope.
  3. Turn the COARSE ADJUSTMENT KNOB of the microscope clockwise so the objective lens is real close (but NOT touching) the top of the microscope slide.
  4. Looking through the microscope lens, slowly turn the COURSE ADJUSTMENT KNOB towards you (counter-clockwise) until you see the leaf x-section (you may have to re-position the microscope slide while doing this).

Try to get the image focused properly AND in the center of the microscope's field of view.

You can use the FINE ADJUSTMENT KNOB to fine focus the image for perfect sharpness! When properly done, the image that you see should look something like you see at the top of the next page.....

Inside of a Leaf
  1. Be sure the image is right-side-up! The vertical PALISADE LAYER cells should be at the top, with the SPONGY LAYER towards the bottom. This is what a perfect theoretical leaf might look like. Reality is never quite so neat and tidy.
  2. In the space below, carefully draw the cross-section of the real leaf AS YOU SEE IT THROUGH YOUR MICROSCOPE. Neatly label the parts that you are able to identify from the above drawing. YOU MUST USE PENCIL IN MAKING THIS DIAGRAM!!

X-Section Of A Leaf (high power on the microscope): Label the parts!!

  1. You may have some prepared microscope slides of the lower surface of a leaf showing the pores or STOMATA through which leaves are able to "breath". If so, be sure to view this slide through your microscope using 100 power. Compare the image you see with the diagrams below:
Drawing Conclusions -

Complete the following table concerning the functions of the various cell types in a leaf.
Tissue Table
Answer the following questions carefully...
  1. In which layers of the leave structure did you locate the structures known as chloroplasts?
  2. What is the FUNCTION of the chloroplasts of leaf cells?
  3. What might be a reason why the stomata are found ONLY on the lower surface of the leaf you studied -- explain carefully!


Background -

Plants are capable of growing in many different environments. Dandelions grow in cracks in a sidewalk, pine trees grow high on mountainsides, water lilies grow from shallow lake bottoms. Though plants flourish in very different environments, most plants have one thing in common -- they need light to survive! Unlike certain animal species, no species of green plant can survive for long in total darkness. To make sugar, plants capture light energy in the process of photosynthesis. In Part B of this investigation, you will observe how important light energy is to the process of photosynthesis.
Two Chemical Reactions You Need To Know -
Bromthymol Blue(Blue) + Carbon Dioxide + Water ��Bromthymol Blue(Yellow) + Carbonic Acid Carbon Dioxide + Water ��Sugar + Oxygen
Materials -

Beaker (250 ml) Four Test Tubes Distilled Water
Test Tube Caps (4) Bromthymol Blue Solution Straw
Elodea Plants (4) Test Tube Rack Masking Tape

Procedures -
  1. Carefully wash and dry four test tubes -- place them in your team's test tube rack.
  2. Using pieces of masking tape, label the test tubes 1, 2, 3, & 4. Make certain the NAME of one of your team members and the class period is on the label.
  3. Fill a clean beaker with 140 ml of distilled water. Note: Bromthymol Blue has been added to color the water a pale blue. Using a straw, blow gently into the solution until it turns a pale yellow color -- continue blowing for 20 seconds after the color change.
  4. Add equal quantities of the solution to each of the four test tubes (each should be ~2/3's full).
  5. Obtain TWO sprigs of Elodea from the front desk -- make sure they are of equal size. Place one into test tubes ONE and THREE, then carefully seal all four test tubes with Parafilm.
  6. Put the test tubes into the appropriate trays for 24 hours of exposure to a BRIGHTLY LIT environment or a nearly LIGHTLESS environment.

TEST TUBES 1 & 2 IN the LIGHTLESS environment

TEST TUBES 3 & 4 IN BRIGHTLY LIT environment

Test Tubes
    After 24 Hours -
  1. Obtain your team's four test tubes and place them in your test tube rack for observation -- place them in numerical order, one to four.
  2. Complete the following table:
  3. Test Tubes Table
  4. Carefully remove the Elodea from your two test tubes and place the sprigs back into the container provided by your teacher.
  5. Remove the masking tape from your four tubes and carefully wash the test tubes -- dry the outsides, and place them in the proper storage container as described by your teacher.

Analyzing Data -
  1. After exhaling through the solution, what color did the solution become?________________
  2. What caused the color change noted in question #1?
  3. What was the purpose of preparing Tubes 2 & 4 without the Elodea? EXPLAIN CAREFULLY!
  4. How do you account for the color changes that occurred in the test tubes that contained Elodea?
  5. What substance was the Elodea absorbing, how do you know?
  6. Why didn't you have a similar color change in the test tubes that did NOT contain Elodea? EXPLAIN CAREFULLY!
  7. Compare the color change in the light and dark tubes with Elodea. Explain their differences.
  8. Carefully explain what you found out in this lab investigation. Make certain you not only explain the findings, but also what is meant by a CONTROLLED experiment.


Click here to download the Photosynthesis Review Sheet in Microsoft Word format.

Photosynthesis Review

1-In the process of photosynthesis ______________ energy is transformed into ________ energy.

2-The reactants of photosynthesis are _____________, _____________ and _________.

3-The products of photosynthesis are __________, ___________, and _____________.

4-One of the waste products of photosynthesis is ______________. The plant does not need it so it _____________________________________________________________.

5-Photosynthesis happens in the _________________. The chemical in the chloroplast is called ________________________.

6- The prefix chloro- means ___________________.

7-The prefix photo- means ___________________.

8-The suffix –synthesis means ___________________.

9-After Oxygen is made it is stored in the ________________ layer and finally released through the __________________.

10-The __________________ opens at night to release gas and water.

11-The part of the leaf that is NOT a cell, that is made of wax is the ________________.

12-The vein transports materials in the leaf. The phloem transports _______________ and the xylem transports ____________________.

13-Three kinds of carbohydrates are ____________________, ______________ and ___________________.

14-The suffix –dermis means __________________________. (A Dermatologist studies ______________________)

**Remember to review WHERE the cell parts are and WHAT their functions are!

Cellular Respiration Activity

Click here to download theCellular Respiration Activity file in Microsoft Word format.

Cellular Respiration Activity

We have learned that living cells need to go through the process of Cellular Respiration to get energy from the food they eat in order to move, reproduce and function. In this activity we will use the Cellular Respiration equation to analyze what is happening when bread rises.

  1. Get in a group.
  2. Heat up the amount of water stated on the packet of pizza dough mix and heat up a separate beaker with 500 mL of water so that they are very hot.
  3. Put the pizza dough powder with the needed water and mix in a bowl.
  4. Place the 500 mL of hot water in a separate bowl. Place the bowl with the pizza mix on a plate and place the plate on top of the bowl of 5oo mL of hot water.
  5. Measure half of the circumference of the dough and put a paper towel on the top.
  6. Measue the circumference of the dough every minute for 5 minutes, covering with a paper towel after each measurement.

Time Circumference of Dough
Minute 0  
Minute 1  
Minute 2  
Minute 3  
Minute 4  
Minute 5  

  1. Is this Aerobic or Anaerobic? How do you know?
  2. What causes the dough to rise?
  3. How much did the dough rise?
  4. What percent increase of a rise did it show?
  5. Explain what happened using the equation for Cellular Respiration.
  6. What living cell was in the pizza mix that did Cellular Respiration?

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Keywords: Lynn, Annabel, student, photosynthesis, cellular respirationDoc ID: 590
Owner: Annabel G.Group: International Agriculture
Created: 2008-07-16 18:00 CSTUpdated: 2012-11-21 10:32 CST
Sites: International Agriculture