Photosynthesis and Respiration

Photosynthesis and Respiration  


SB3. Students will derive the relationship between single-celled and multi-celled organisms and the increasing complexity of systems. 

Learning Tasks


Key Terms

Click here to download a list of key terms for this module. 


Pre-Assessment Photosynthesis and Cellular Respiration

This activity is not for a grade and will not be considered as part of your course requirement. However, it will give you an idea of what you already know and topics where you need additional instruction. Please choose true or false for the following statements.

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Go to the navigation bar for the course and click on QUIZZES. Choose the quiz titled Pre-Assessment Photosynthesis and Cellular Respiration.


Cellular Respiration Notes

Cellular respiration is the process of using oxygen in the mitochondria to chemically break down organic molecules such as glucose to release the energy stored in its bonds. In the process molecules of water and carbon dioxide are released as waste products. Some of the energy is used to make Adenosine triphosphate, ATP, from Adenosine diphosphate and phosphate. ATP is the main energy currency of cells.


Below is the structural Formula for ATP (from Wikipedia). Notice the three phosphate molecules on the left. To release energy, one phosphate is removed leaving ADP. To store energy, the phosphate is replaced and ADP is restored to ATP.

Structural Formula for ATP; notice the three phosphate molecules on the left; to release energy, one phosphate is removed leaving ADP; to store energy, the phosphate is replaced and ADP is restored to ATP

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Heterotrophs as well as autotrophs utilize cellular respiration to supply the energy to power cellular activities. To understand cellular respiration, one must understand the principles of photosynthesis. Why do you think that is so?


If you need more information about photosynthesis, read Dr. Michael J. Farabee's useful online book about the topic. One simple explanation is this: through the process of photosynthesis the energy from sunlight is used to change water and carbon-dioxide into glucose with oxygen being tossed out as a waste byproduct.

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Image of photosynthesis equation  




Image of cell respiration equation


Cellular Respiration begins with a biochemical pathway called GLYCOLYSIS. This is a process in which one molecule of glucose is broken in half by enzymes in the cytoplasm, producing 2 molecules of pyruvic acid and only 2 molecules of ATP. Glycolysis releases a relatively small amount of the energy stored in glucose.

Diagram for glycolysis

If oxygen is present, glycolysis acts as the first step for 2 more processes, the Krebs Cycle and the electron transport chain. This series of reactions produces 36 molecules of ATP!


All processes that require oxygen are described as "aerobic". Cellular respiration is an aerobic process.

If sufficient oxygen is not present in the cell, glycolysis is followed by a different pathway called fermentation.  Fermentation does not produce much energy.    


Processes that do not require oxygen are "anaerobic" processes. Fermentation is an anaerobic process.

The following drawing depicts the events of  fermentation.  Notice that glycolysis is the first step in the process that results in 2 molecules of ethanol alcohol.

Diagram for process of fermentation


Cellular Respiration - Aerobic Processes


You can draw an analogy between the process of cellular respiration in your cells and a car. The mitochondria represent the engines of our cells where sugar is burned for fuel and the exhaust is CO2 and H2O. Note that in a car that burned fuel perfectly, the only exhaust should theoretically be CO2 and H2O also.


There are 3 steps in the process of cellular respiration:


1. Aerobic respiration – occurs when the two pyruvic acid molecules from glycolysis are modified and diffuse into the mitochondria where the next two processes occur.

a. The Krebs Cycle, also known as the Citric Acid Cycle, produces 2 ATP molecules, 10 energy carrier molecules, and CO2 from each glucose molecule. These energy carriers move on to the next step.

Diagram of cell respiration

b. The Electron Transport Chain is a series of chemical reactions that produces 34 ATP molecules and H2O from the carrier molecules that were produced in the Krebs Cycle.

Glucose is converted to pyruvate through a series of reactions that use 2 molecules of ATP and produce 4 molecules, for a net gain of 2 ATP.? Pyruvate enters the mitochondrion and is processed through the Krebs cycle; products from Krebs, diffuse through the inner mitochondrial membranes in the electron transport chain.? Oxygen atoms accept Hydrogen ions and form water to remove the hydrogen ions.

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Glucose is converted to pyruvate through a series of reactions that use 2 molecules of ATP and produce 4 molecules, for a net gain of 2 ATP.  Pyruvate enters the mitochondrion and is processed through the Krebs Cycle; products from Krebs, diffuse through the inner mitochondrial membranes in the electron transport chain.  Oxygen atoms accept Hydrogen ions and form water to remove the hydrogen ions.

Remember exploring the organelles that are found in a cell? We learned about an organelle called the mitochondrion--the Powerhouse of the cell. Here's how that organelle got its nickname. The bulk of a cell's ATP is produced in the mitochondria.

Image of mitochondrion and link to Wikipedia entry on the topic

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To be energy efficient, particles must move through the membrane with little energy expense.

Electron transport produces an uneven charge on the mitochondria membrane. This allows protons to move through the membrane by electrical charge attraction rather than by using energy. Protons now move through the membrane and are used by ATP synthase enzyme to recycle ADP molecules and phosphate molecules to produce ATP.  


2. Anaerobic respiration - Fermentation is an Anaerobic Process. There are several forms of fermentation. Two of these forms are lactic acid fermentation and alcoholic fermentation.

Comparing energy yield:

Aerobic respiration (with oxygen) can produce 36 ATP molecules from each glucose molecule. Anaerobic respiration (without oxygen) only allows production of 2 ATP molecules from each glucose molecule. Therefore, aerobic respiration is significantly more efficient than anaerobic respiration.

3. Energy and Exercise

Cells normally contain small amounts of ATP produced during glycolysis and cellular respiration. At the start of a race, muscle cells only contain enough ATP for a few seconds of activity. When running a long race, muscle cells are producing most of their ATP by lactic acid fermentation. When the race is over, the only way to get rid of the extra lactic acid is to acquire more oxygen. This is known as an "oxygen debt" because oxygen must be "paid back" to the cells to remove the lactic acid. The debt is paid as you continue to breathe deeply for several moments after the activity stops.


For longer races, cellular respiration is the only way to generate a continuing supply of ATP. It releases energy more slowly than fermentation.

Glucose is converted first to pyruvate, then to lactic acid


In this image glucose is converted first to pyruvate, then to lactic acid. This image was based on a diagram published at Estrella MountainCommunity College.


Have you answered the earlier question about why you need to understand the principles of photosynthesis to fully understand cellular respiration? These two processes are the reverse of each other. Water, carbon dioxide, carbohydrates and oxygen cycle between these two processes. Energy moves through both processes in one direction

Click here for a pdf version of the chart below. 

Chart comparing photosynthesis to respiration--this image is also a link to a pdf containing the tabel and its information


Energy and ATP

Energy is the ability to do work or cause change.


In Science, when we use the word work, we're talking about moving something against a force. The key word here is moving--going from one place or position to another--and having to overcome a force to get there. The distance of the movement can be quite large (moving a rocket from the face of the earth to the moon or beyond...say, across the universe) or the distance can be very, very small (the space between a couple of molecules or from one cell to another). Whatever the distance, if we have to act against a force in order to move something from one place to another, work is being done.


A person dragging a heavy load from one spot to another location is work.

Image of man dragging a box--an example of work


The body absorbing tiny nutrients (moving extremely small particles from one place to another) is also an example of work.

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So, work isthe ability to change or move matter against other forces.


Work is done when a net force causes an object to change its state of motion or rest, causing it to speed up, slow down, or change direction. The direction of the applied force and the direction that the object moves must be the same for work to be done. The amount of work done is proportional to the size of the force and the distance that the object moves. The formula used to calculate work is


Work = Force x distance


Work for a Cell includes

All of which require ENERGY.


The ultimate source of energy for ALL organisms is the sun. Photosynthetic organisms convert the energy from sunlight into chemical energy that is stored in food. Heterotrophic organisms obtain energy from food but different types of organisms get their food in many different ways.


Most AUTOTROPHS or PRODUCERS use PHOTOSYNTHESIS, to convert the energy in SUNLIGHT, CARBON DIOXIDE, and WATER into chemical energy or FOOD. (GLUCOSE)


We can write the overall reaction for photosynthesis as:

6H2O + 6CO2 C6H12O6+ 6O2


Most of us don't speak chemicalese, so the above chemical equation translates as:

Six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar (called glucose) plus six molecules of oxygen.

The foods made by AUTOTROPHS are stored in various organic compounds, primarily CARBOHYDRATES. One of the most important carbohydrates is a six carbon sugar called GLUCOSE.

 Because Heterotrophs must consume other organisms to get energy, they are also called CONSUMERS.

Image depicting various levels of consumers and one level of producer

Only part of the energy from the sun is used by Autotrophs to make food, and only part of that energy can be passed on to other Consumers. A great deal of the energy is LOST as heat.

Enough energy is passed from Autotroph to Heterotroph to give the Heterotroph the energy it needs.


Chemical Energy and ATP

The activities of the cell are powered by a chemical fuel called adenosine triphosphate (also known as ATP). 

An ATP molecule is made up of adenine, ribose (a sugar), and three phosphate groups.

Image of an ATP Molecule


Adenosine diphosphate (also known as ADP) is similar to ATP, except it only has 2 phosphate molecules. 

The difference in the 2 molecules is the way they store energy.  When a cell has energy available, it can store small amounts of energy by adding a phosphate group to and ADP molecule.  This will then produce an ATP molecule.

ATP will release or give off energy when it releases a phosphate molecule.  Since a cell can add or subtract a phosphate group, it has a way of storing and releasing energy.

Most cells have only a small amount of ATP.  It can't be used to store energy for long periods of time.    It is more efficient for a cell to keep a small supply of ATP.  Cells can regenerate ATP from ADP as needed by using the energy in carbohydrates like glucose.



Light and Photosynthesis Notes


Diagram of the visible spectrum

What is light?


Visible light is one component of the electromagnetic spectrum.  Although sunlight appears to be white light, it is actually made up of a variety of different colors. (You can separate White Light into its component colors by passing the light through a prism.)


The resulting array of colors, ranging from red at one end to violet at the other is called the visible spectrum. Each color of light has different wavelengths, and a different amount of energy.

When light strikes an object, its component colors can be Reflected, Transmitted, or Absorbed by an object.


An Object that absorbs all colors appears black.


A pigment is a molecule that absorbs certain wavelengths of light and reflects or transmits others. Objects or organisms vary in color because of their specific combination of pigments.


Wavelengths that are reflected by Pigments are seen as the object's color. For example, grass absorbs all of the colors of the spectrum except green.  Since grass reflects green, green light waves travel to our eyes.  Therefore, we see the grass as green.


Located in the membrane of the thylakoids are a variety of pigments. Chlorophyll is the most common and important pigments in plants and algae. The TWO most common types of Chlorophyll are designated

You can see in the diagram below that the chlorophylls are only slightly different in structure.

Image comparing the chemical structures of chlorophyll a and b

Chlorophyll ABSORBS VIOLET, BLUE AND RED LIGHT.  These are the wavelengths of light that photosynthesis uses. 

Chlorophyll a ABSORBS LESS BLUE light but MORE RED light than Chlorophyll b absorbs.

ONLY Chlorophyll a is directly involved in the light reactions of photosynthesis.  Chlorophyll b assists Chlorophyll a in capturing light energy and is called an accessory pigment.  By absorbing colors that Chlorophyll a CANNOT absorb, the accessory pigments enable plants to capture more of the energy in light. 

Another group of accessory pigments found in the Thylakoid Membranes, called the carotenoids,  includes yellow, red, and orange pigments that color carrots, bananas, squash, flowers, and autumn leaves. The carotenoids in green leaves are usually masked by chlorophyll until autumn when chlorophyll breaks down.


Inside a Chloroplast

A photosynthetic cell contains anywhere from one to several thousand chloroplasts. A chloroplast has two membranes. The inner membrane is folded into many layers.  A chloroplast's inner membrane layers fuse along the edges to form thylakoids. (Thylakoids are disk shaped structures that contain photosynthetic pigments.)

Image depicting the interior of a chloroplast  

Each thylakoid is a closed compartment surrounded by a central space. The thylakoids are surrounded by a gel like material called the stroma. The neatly folded layers of the thylakoids that resemble stacks of pancakes are called grana. The thylakoids are interconnected and are layered on top of one another to form the stacks of grana. (Each chloroplast may contain hundreds or more grana.)



Photosynthesis is the process that provides energy for almost all life. During photosynthesis, autotrophs use the sun's energy to make carbohydrate molecules from water and carbon dioxide, releasing oxygen as a byproduct.
The Process of PHOTOSYNTHESIS can be summarized by the following equation:

Equation showing water plus carbon dioxide plus energy in the form of light yeilds glucose and oxygen

 (Notice that the water is split to release oxygen; its hydrogen becomes part of the glucose; the carbon dioxide re-forms to include the hydrogen and makes glucose!)



Occurs in the thylakoids


Energy is captured from sunlight. Water is split into Hydrogen ions, electrons, and Oxygen (O2). Photolysis is the splitting of a water molecule.


The light reaction traps energy from sunlight and does not involve CO2 and does not make sugars. The O2 diffuses out of the chloroplast (Byproduct).



Occurs in the thylakoids

The light energy is converted to chemical energy, which is temporarily stored in ATP and NADPH.

The electrons that absorbed the energy are passed along the (electron transport chain) ETC and store energy in ATP. NADP picks of the H+ to form NADPH+ and electrons from photolysis and stores it for later use.



Occurs in the stroma


A series of reactions referred to as the CALVIN CYCLE

The chemical energy stored in ATP and NADPH powers the formation of Organic Compounds (Sugars), using carbon dioxide, CO2 .and the H+ from the NADPH+.

Photosynthesis occurs in the chloroplasts of plant cells and algae and in the cell membranes of certain bacteria.


Diagram of the Calvin Cycle

Image source


The rate at which a plant can carry out photosynthesis is affected by the plant's environment. Three things in the plant's environment affect the rate of photosynthesis:

  1. Light Intensity: this is one of the most important factors. As light intensity increases, the rate of photosynthesis initially increases and then levels off to a plateau.
  2. CO2 Levels: Increasing the level of CO2 stimulates photosynthesis until the rate reaches a plateau.
  3. Temperature: Raising the temperature accelerates the chemical reactions involved in photosynthesis. The rate of photosynthesis increases as temperature increases. The rate of photosynthesis generally peaks at a certain temperature, and photosynthesis begins to decrease when the temperature is further increased.


Introduction to Photosynthesis by Hippocampus

The following Hippocampus video overview of the process of photosynthesis explains how and why photosynthesis occurs.

Screenshot of and link to Hippocampus video Introduction to Photosynthesis

Energy and ATP

The video below,Energy and ATP, supplements the notes provided for this unit. In order to get a better idea of how ATP plays a part in the process of photosynthesis, take time to view the presentation.

Screenshot of and link to a short video about Energy and ATP


Illuminating Photosynthesis

Photosynthesis is an important process. You need as many opportunities as possible to understand what happens when plants harness light to make fuel. PBS's Illuminating Photosynthesis is an excellent resource for exploring why and how photosynthesis works.  Be sure to try the Puzzlers activities at the end!

Screenshot of and link to PBS interactive resource Illuminating Photosynthesis

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After exploring the resource, record the balanced equation for photosynthesis.  You will need to be able to write and explain this equation for future activities in this module

Cellular Respiration

The video below, Cellular Respiration, supplements the notes provided for this unit. In order to get a better idea of how energy comes into and moves out of cells, take time to view the presentation.

Screenshot of and link to a short video about Cellular Respiration  


Harvesting Energy Interactive

Visit the interactive resource called Harvesting Energy. Make sure to click on the arrows. Doing so will take you through information related to the process of cellular respiration. You need to know this information if you are really going to understand the process of photosynthesis.

Screenshot of and link to an interactive resource called Harvesting Energy

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The Powerhouse of the Cell

The mitochondria plays a big part producing energy. That's why you need to learn more about it. Watch the video Powerhouse of the Cell at Teacher's Domain.


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After viewing Powerhouse of the Cell, please answer the questions below.

  1. What is an important difference between muscle cells of athletes and older persons?

  2. What are the two kinds of muscle fibers?

  3. What is it that tells the muscle cells to make more mitochondria?

  4. What is ATP

  5. Why would ballerinas be so exhausted after a performance?

  6. Why does mitochondrial DNA mutate more frequently than nuclear DNA?

  7. As a person ages, what changes occur that contributes to aging and lack of stamina?

A student handout is available in the sidebar. Have your handout ready so you can stop the video as needed to answer questions.


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When you have completed your work, submit it to the dropbox entitled Powerhouse of the Cell.


Cellular Respiration Study Guide

Online Activities for Practice

Cellular Respiration


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Screenshot of and link to interactive drag and drop fill in the blannk activity

Click here to download a copy of the text (and the accompanying word list) in the activity above.

Click here for a printable study guide.


Cellular Respiration Assignment Check

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You should have your Cellular Respiration Study Guide finished and with you as you complete this assignment check. You will have 10 minutes to complete this assignment check. When your are ready, go to the navigation bar, proceed to QUIZZES and choose the quiz titled Cellular Respiration AC.


Discussion: Energy

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Think about the importance of energy. 

Without a source of energy organisms cannot respond, build, reproduce, or carry out most of the functions of living things.  From where does our energy come?

Explain how Photosynthesis and Cellular Respiration are important in your everyday life. You must also respond to 2 other classmates. Be Scientific!

Post your responses to the Discussion titled Discussion: Energy.


Photosynthesis Explained

View the following short video about photosynthesis. As you watch the brief movie, there are a few key bits of information you should be looking for before the end of the presentation. In particular, focus on how a determined scientist used the power of the scientific method and a clever experiment to help him uncover a fact about plants that many people had never noticed. As you do you'll also be learning about:


Now that you have a chance to watch the video above, use what you've learned to check your understanding as you answer the questions below.

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Virtual Photosynthesis Lab

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Click here to download a worksheet for completing the lab.


Next, go to the following interactive photosynthesis lab. Once there, answer the questions IN A DIFFERENT COLOR as you work through the lab.  Submit the completed assignment to the dropbox entitled Photosynthesis Virtual Lab.


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Read the experimental introduction on the left sidebar, then answer the following questions after manipulating the experimental growth chamber with different types of light and plants.  You will need to try a number of combinations to answer the questions below.


Lab Questions:

1.      Which color lamp/s allow for the least amount of growth?  (2 points) Which color lamp/s allows the most growth?    (2 points)


2.      Explain your findings in #1.  Why do plants grow better in some colors of light than others? (2 points)


3.      Given that white light contains all the colors of the spectrum, what growth results would you expect under white light?  (2 points)


4.      How is growth of the plants related to photosynthesis? (Hint: think energy)  (2 points)


When your work is completed submit the finished assignment to the dropbox titled  Photosynthesis Virtual Lab.



Energy and the Chemistry of Life Video

Watch the Discovery Education video clips below. The video clips use physical and chemical concepts to analyze two metabolic processes: aerobic cellular respiration and photosynthesis.




Now, check your understanding of the videos you've just watched. Answer the questions in the crossword puzzle below.


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Plant Pigment Experiment

Watch the Pigment in Leaves video presentation below.  You will be asked to select answers to questions as you watch the presentation.  After viewing, answer the questions below.


Screenshot of and link to Hippocampus movie about plant pigments

Read the questions below. Mouse over the questions to found out more information.

What type of experiment is performed to separate the pigments?

What pigment traveled the greatest distance?

Is there more chlorophyll in the green leaf than in the red or yellow leaf?

When are red pigments produced in the leaves?

Carotene is a pigment that is found in green, yellow, and red leaves. In what color leaves is there a greater amount of chlorophyll? In what color leaves is there a greater amount of anthocyanin?



Poetic Justice

Here's an opportunity to get your creative talents soaring, or at least skipping. Make a poem about photosynthesis and/or cellular respiration. Here's an example:


I think that I shall never see

A thing so lovely as a tree.

But what you need to know is this,

It's busy doing photo-syn-the-sis!

Shout the news across the nation,

It also does cell respiration!


Now it's your turn.

Assignment icon

Think about the process of photosynthesis and/or cellular respiration. What are the most important thoughts everyone should know about these processes?


Write a poem of at least twelve couplets (two rhyming lines) that describes the steps and substances used and produced by photosynthesis and/or cellular respiration (the steps/details are listed in the rubric.)



The three couplets above are an example. They should get you thinking. Make sure that your work is entirely original. Copying someone else's work is not only forbidden, it is rude and makes those who stoop to such a low act look foolish.


You may refer to either photosynthesis and/or cellular respiration or both, combined. For an extra touch, to show that you have got style, make your poem into a song and record it.  Your instructor may choose the read or play the most creative works at the next Elluminate session! Use this rubric to assess your work before submitting it.

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When your work is completed submit the completed assignment to the DROPBOX entitled Poetic Justice.

Image of a lovely tree

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Photosynthesis Study Guide

Click here for a printable version of the study guide.


Autotrophs and Heterotrophs

1. Where does the energy of food originally come from?

2. Complete the table of types of organisms.

types of organisms





Organisms that make their own food



Organisms that obtain energy from the food they eat



Chemical Energy and ATP

. What is one of the principal chemical compounds that living things use to store energy?

4. How is ATP different from ADP? 

5. Be able to label each part of the ATP molecule illustrated below.

Unlabeled ATP molecule                                 

6. When a cell has energy available, how can it store small amounts of that energy?

7. When is the energy stored in ATP released? 

ATP and Glucose

8. Identify the molecules used to regenerate the energy in ATP.

a. ADP

b. phosphates

c. carbohydrates

d. organelles


The Photosynthesis Equation

9. Write the overall equation for photosynthesis using words. 

10. Write the overall equation for photosynthesis using chemical formulas.

11. Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy___________.


Light and Pigments

12 What does photosynthesis require in addition to water and carbon dioxide? 
What is the principal pigment of plants? 

Inside a Chloroplast

14. Chloroplasts contain saclike photosynthetic membranes called __________________.
What is a granum?  
The region outside the thylakoid membranes in the chloroplasts is called the_____________.
What are the two stages of photosynthesis called?


Factors Affecting Photosynthesis

18. What are three factors that affect the rate at which photosynthesis occurs? 

19. Is the following sentence true or false? Increasing the intensity of light decreases the rate of photosynthesis. 


Photosynthesis Study Guide Assignment Check

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You should have your Photosynthesis Study Guide finished and with you as you complete this assignment check. You will have 10 minutes to complete the assignment check. Go to the navigation bar, proceed to QUIZZES and choose the quiz titled Photosynthesis Study Guide AC.


Photosynthesis and Cellular Respiration Review Quiz

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Go to the navigation bar and click on QUIZZES. Choose the selection titled Photosynthesis and Cellular Respiration RQ.



Before continuing on to the Unit Test, be sure the following graded items have been completed. Click on a link below to return to the page in the module containing the assignment.

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Powerhouse of the Cell

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Cellular Respiration AC

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Discussion: Energy

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Photosynthesis Virtual Lab (counts as lab)

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Poetic Justice

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Photosynthesis Study Guide AC

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Photosynthesis and Cellular Respiration RQ



Photosynthesis and Cellular Respiration Unit Test

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Go to the navigation bar and click on QUIZZES. Choose the selection titled Photosynthesis and Cellular Respiration Unit Test.