Biology LESSON 9 - Photosynthesis

Lesson Objectives:

  • Student will learn why cells need energy.
  • Student will understand the relationship of light to energy.
  • Student will understand how photosynthesis converts light into usable energy.
  • Student will learn about the different types of chlorophyll.
  • Student will understand the byproducts of photosynthesis (oxygen released in the atmosphere).

1. Cells Need Energy

(1.1) Cells need energy. They need energy for tranport, for movement, for reproduction--any work the cell does needs to be powered by energy. How do cells get energy?

2. Light

(2.1) Photosynthesis is the process through which plants and some bacteria use sunlight (which is luckily abundant on Earth) to make sugar, which cell respiration transforms into ATP (adenosine triphosphate). Molecules of ATP are used by all living things to power the cells. In plants, photosynthesis is carried out in structures called chloroplasts, while bacteria use their cytoplasm to performs photosynthesis.

(2.2) To understand the process of photosynthesis we need to understand a little about light. The visible spectrum is only a small section of the electromagnetic spectrum, but it serves as a good way to examine wavelengths. See, different colors are also different wavelengths of light. (Wavelength is measured as the distance from peak to peak, or trough to trough, see the images below.) The shorter the wavelength, the higher the energy. The visible spectrum ranges from about 380 nanometers (violet) to about 750 nanometers (red). Wavelengths shorter than violet (which we can't see) are referred to as ultraviolet, wavelengths longer than red (which we also can't see) are referred to as infrared.

Image by Erin Donahoe, used with permission.

Light Spectrum

750 nm

(2.3) When an object reflects part of the spectrum of light, we see it as that color. So an object that looks red to us reflects the red part of the spectrum (so we receive that color) but absorbs the other parts of the spectrum. Color-mixing can also play a part--say an object reflects yellow and blue, we'd see the object as green. White is what our eye sees when all of the colors are reflected, black is what we see when all the wavelengths are absorbed. We can use a prism to bend the wavepaths in such a way that we can see the full spectrum of color.

Prism and Light
This image was created by NASA, and as a work of the U.S. federal government is in the public domain.

(2.4) Chorophyll, the main pigment contained in chloroplasts, absorbs every color except green, which is why most plants are green. Plants use the violet-blue part of the spectrum the most, but the other parts of the spectrum are also used.

3. Chloroplasts and Chlorophyll

(3.1) A chloroplast is a three membraned organelle, which only exists in eukaryotes (for bacteria, photosythesis takes place in the cytoplasm). The chloroplast contains thykaloids, disks in which the light reactions of photosynthesis take place. Stacks of thykaloids are contained in sacs called grana, and the areas between the grana are referred to as stroma.

Image from materials by TeachingPoint, used with permission.

(3.2) The thykaloids are filled with the photosynthetic pigment chlorophyll. (The word chlorophyll comes from ancient Greek: chloros = green and phyllon = leaf. Most photosynthesis occurs in the leaves of plants.) Chlorophyll has many different chemical make-ups, depending on the plant or bacterium that contains it. (There are also other photo-reactive pigments that can perform photosynthesis, but chlorophyll is the most common.) Chlorophyll is a complex molecule. Different naturally occurring types of chorophyll have been named by letter, chlorophyll a, chlorophyll b, etc.

Some examples:

Chlorophyll a Chlorophyll b Chlorophyll c
C55H72O5N4Mg C55H70O6N4Mg C35H30O5N4Mg
(universal) (plants) (algae)

Chloraphyll A Chloraphyll C2
Chorophyll a, b, and dChorophyll c1 and c2

click on the images for a larger view

These images were released into the public domain by the creator.

4. The Light Reactions

(4.1) In the chemical reaction for photosynthesis, six molecules of water and six molecules of carbon dioxide react in the presence of chlorophyll and sunlight to produce one molecule of sugar plus six molecules of oxygen.

6H2O + 6CO2sunlight
C6H12O6+ 6O2

This reaction occurs in two sections, which we'll call light reactions and dark reactions. There are a lot of reactions taking place, so you may want to read over this a few times to get a feel for what happens. (Believe it or not, this is the simple version--even more happens than we discuss here!) The light reactions take place in the grana, while the dark reactions take place in the stroma.

(4.2) In the light reactions, when chlorophyll is exposed to sunlight the light excites the electrons in the molecule, and each molecule of chlorophyll loses two electrons. These electrons are passed through proteins in the thykaloid membrane as part of what is called the electron transport process, and are eventually attached to NADP+ (nicotine adenine dinucleotide phosphate) and H+ to form NADPH, another energy carrier needed in the dark reactions phase. (NADP+ starts with a positive charge, adding two electrons makes a negative charge of 1, which attracts the positively charged hydrogen ion to form NADPH.)

NADP+ + H+ + 2e- --> NADPH

(4.3) This leaves the chlorophyll with an electron deficit. To replenish these electrons the plant breaks down two water molecules, stripping two electrons from each and leaving behind four H+ ions and one O2, which is then released by the plant into the air. Remember that H+ is really just a proton--the cell uses energy from the electron transport process mentioned above to transfer this proton through a protein channel (proton pump) into a confined space (usually a space between two membranes), where the only way the hydrogen ion can leave is through an ATP synthase (an enzyme). This second transfer is powered by diffusion. The cell uses energy to pump lots and lots of protons in (active transport), allowing them to come back out only by diffusion through the enzyme. The diffusion through this enzyme allows the cell to combine ADP (adenosine diphosphate) and inorganic phosphate to make ATP (adinosine triphosphate), the main energy carrying molecule of all cells. Since this process adds a phosphate group to the molecule ADP (to make ATP) it is called phosphorylation. The hydrogen ions haven't been used up, so they can next be used to make the NADPH and H+ needed in the dark reactions, or can be pumped back into the confined space by the proton pump to power the production of more ATP. The light reactions constantly (whenever there is light) "recharge" ADP into ATP, and constantly give off oxygen from the breakdown of water molecules. Each successful photosynthetic process produces 2 ATP, one NADPH, and one H+ (as well as half a molecule of diatomic oxygen).

ADP + PienergyATP

adenosine diphosphate + inorganic phosphate with energy yields adenosine triphosphate

(4.4) So the energy to power the proton pump is provided by the electron transport process, and the passage of the H+ ion through an enzyme back into the original compartment provides the energy to add a phosphate to ADP, forming ATP. This entire process is known as chemiosmosis.

5. The Dark Reactions

(5.1) Dark reactions are also referred as "light independent reactions" (since they can occur during day or night) or as "carbon-fixation" (since carbon dioxide is taken from the air and carbon is affixed to compounds in the plant) or as the Calvin Cycle (named for the person who discovered it). These reactions occur in the stroma of the chloroplast.

(5.2) The Calvin Cycle forms a circle. It starts with a compound called ribulose phosphate, a complex molecule with 5 carbons in it, so it's known as a 5-C chemical. Stimulated by enzymes, ribulose phosphate convinces ATP to give up one of its phosphate groups--making ribulose biphospate and leaving ADP to go back and be "recharged" into ATP by the light reactions.

(5.3) Ribulose biphosphate combines with water and carbon dioxide from the air. So now the molecule has 6 carbons. This configuration is unstable, so it splits into two identical molecules of phosphoglyceric acid, a 3-C chemical.

(5.4) The two molecules of phosphoglyceric acid each gain another phosphate (breaking down two more molecules of ATP into ADP) becoming biphosphoglyceric acid. NADPH and the hydrogen ions made in the light reactions are used to split a phosphate off of each (putting phosphates back to make more ATP) and to provide the energy and hydrogen to turn the remaining chemical into glyceraldehyde phospate, also called phosphoglyceraldehyde, or PGAL. (The NADPH and H+ combine with the phosphoglyceric acid to make PGAL, leaving behind NADP+ which will go back to the light reactions to be transformed again.) PGAL is the raw material for anything a plant needs. PGAL can be used to make sugar and to replenish the ribulose phosphate stores, so that this reaction can happen again.

Overview of the Calvin Cycle pathway. Original work by Mike Jones. Licensed under The Creative Commons Attribution-ShareAlike License 2.5

(5.5) Here's an outline of how this process might work:

Through a series of reactions, these 30 carbons can be recombined to make the 6 molecules of ribulose phosphate we started with--and the plant gained two molecules of PGAL to be used as energy, stored as starch, or to be used in numerous other ways.

6. Bacteria

(6.1) Bacteria, since they are prokaryotes, don't have the internal membranes to power chemiosmosis (the process plants use to make ADP into ATP). Bacteria do this through a process known as substrate-level phosphorylation. An enzyme in the cytoplasm attaches a phosphate to the ADP to form ATP, and a separate enzyme works to form NADH and a hydrogen ion. The bacterium will still take in H2O and CO2 to perform essentially the same reactions as we saw in a plant's dark reactions.

Grading for this lesson:

  • 10: Your answers need to be correct and you can have no grammatical errors, within the second revision of this lesson. Answers are correct, complete, and clear; all lesson requirements have been met.
  • 9: You can have 1 or 2 errors (factual, spelling, punctuation, capitalization, wrong word, etc) on the second revision of the report or you can have no errors on the third revision. Answers are correct, complete, and clear; all lesson requirements have been met.
  • 8: You can have 1 or 2 errors on the third revision of this lesson or you can have no errors on the fourth revision. Answers are correct, complete and clear; all lesson requirements have been met.
  • 5: Plagiarism Ė purposeful or mistaken which will lower your final grade for the course (so be very careful when posting your work!); lack of effort, disrespect, or attitude (we are here to communicate with you if you donít understand something); or 9 or more errors; or lesson requirements have not been met.

Also be aware that you will have a chance to revise your work. More than 2 revisions will result in a lower grade, so read the directions carefully and make sure you meet the requirements.

No lesson is complete without the approval of the instructor, and all revisions must be completed before a grade is assigned. No grade will be given for incomplete work.

Assignment for Lesson 9

You are to answer the following questions in your own words. Please post the questions with your answers in the text box below to submit your work. Remember to use complete sentences, use proper grammar, and donít forget to proofread and spell check your work before submitting it. This may require additional internet research, so be sure to cite your sources.

Do not submit text that you have copied from sources, including websites. All of your work should be in your own words. Using copied text would be considered plagiarism. For more information, review our page on Plagiarism and Citation.

1. There are two methods we use to see in the dark.  They are image enhancement and thermal imaging.  Find out how each of these devices works and report your findings here.

2. In the lesson you learned about the visible spectrum, which is only one part of the electromagnetic spectrum. Find out eight sections of the electromagnetic spectrum, and list them in order according to wavelength. (For example, three of the sections will be ultraviolet, visible spectrum, and infrared, find out five more.) There is an easy to read chart here:

3. Can the dark reactions happen if the light reactions don't? Explain.

4. Two kinds of cell-transport are used in the process that forms ATP. Name the two types of transport and describe how they take place. Electron transport is not a form of cell transport.

5. Discuss some of the advantages to having a cycle that partially replenishes itself. In the Calvin cycle, what would happen to the cycle if there was no carbon dioxide available?

Use complete sentences to define the following terms or phrases in your own words. (Not all the terms are in the lesson. Using a dictionary or outside site for help is fine, and sometimes necessary, but be sure to use your own words and cite your source!)

Do not submit text that you have copied from sources, including websites. All of your work should be in your own words. Using copied text would be considered plagiarism. For more information, review our page on Plagiarism and Citation.

6. Chloroplast

7. Synthesize

8. Stroma

9. ADP

10. ATP

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