Photosynthesis Student Learning Guide

Page outline

  1. Introduction
  2. Understanding Light
  3. Chlorophyll
  4. Absorption and Action Spectra
  5. Quiz: Light and Photosynthetic Pigments

1. Introduction

In the last module, we learned that photosynthesis occurs in two phases: the light reactions and the Calvin cycle. Here’s a summary of what happens in each.

  • The light reactions: Powered by energy from light, the chloroplast oxidizes water, reduces the electron carrier NADP+ to NADPH, and combines ADP and Pi to form ATP. Having lost electrons and protons, water becomes oxygen, which bubbles out of the chloroplast.
  • The Calvin cycle: The chloroplast uses energy from ATP and reducing power from NADPH to reduce carbon dioxide to the three carbon sugar G3P (glyceraldehyde phosphate), the key output of photosynthesis (and the molecule that could be considered to be the goal of the process). As carbon dioxide is reduced, NADPH is oxidized to become NADP+, and ATP loses energy to become ADP and Pi. 

Now, let’s delve into the details of the light reactions.

2. Understanding Light

Energy is the ability to do work or cause change. In photosynthesis, it takes energy to change carbon dioxide (a low energy exhaust product) into carbohydrate (a high energy fuel). Photosynthesis harvests this energy during the light reactions, which  can be thought of as a series of energy conversions.

  1. First, light energy is converted into a flow of electrons: essentially, an electrical current.
  2. Second, that electrical current is converted into the chemical energy of two short term energy-storage compounds: NADPH and ATP.

Light is a form of electromagnetic energy. We commonly experience other forms of electromagnetic energy in our daily lives, including radio waves, microwaves, infrared light, ultraviolet light, and X-rays.

As a biology student, you only need to know a few things about electromagnetic energy.

  1. It’s a type of energy that can move through the vacuum of space (unlike sound energy, for example, which needs to move through air, water, other fluids).
  2. It has a dual nature. On the one hand, electromagnetic energy consists of massless particles called photons. Thinking of this energy as particles is useful when you want to envision a photon hitting an electron and energizing it (which, as we’ll see, is what happens during photosynthesis). At the same time, electromagnetic energy is a wave, with a frequency and a wavelength.
  3. The wavelength and frequency of electromagnetic energy are inversely linked together: as frequency increases, the wavelength decreases (and vice versa). For example, X-rays have a very short wavelength (about 10-10 meters). Correspondingly, their frequency is very high (1018 Hertz, or waves/second).
  4. Higher frequency/shorter wavelength electromagnetic energy has more energy than lower frequency, longer wavelength energy. For example, X-rays have more energy than radio waves. As a result, we take protective measures against needless exposure to X-rays when X-rays are used in medical or dental settings. That’s because the energy from X-rays can disrupt DNA and proteins, causing tissue damage and mutations that have been associated with cancer. But we don’t worry about exposure to radio waves (which are constantly passing through our bodies).

The diagram above illustrates the connection between electromagnetic energy, wavelength, and frequency. In terms of photosynthesis, it also shows how visible light is just one portion of the electromagnetic spectrum. It’s the portion that our primate visual system evolved to see, and which photosynthetic organisms evolved to exploit for converting electromagnetic energy into chemical energy.

Like all electromagnetic energy, light consists of photons that have a wavelength and a frequency (though only the wavelength is indicated in this image). Note that red light has a longer wavelength (700 nm) than violet light (400 nm). That means that red light has less energy than violet light does.

Also, note that in our daily lives, we rarely experience light of only one wavelength. White light from a light bulb is a mixture of wavelengths, as is the light from the sun (which also includes many other wavelengths of electromagnetic energy, ranging from radio waves to X rays). When we shine a beam of white light through a prism, however, we can see how it breaks up into its constituent colors. In nature, that happens whenever we see a rainbow.

3. Chlorophyll: the key photosynthetic pigment

Green pigments like chlorophyll reflect green light, and absorb red and blue light

When light (and all electromagnetic energy) interacts with matter, the energy can be reflected or absorbed. Substances called pigments absorb certain light wavelengths and reflect others. So, what does it mean for something to be green, or blue, or yellow? Because we’re dealing with photosynthesis, which is carried out by green plants, let’s focus on green. A green object is perceived as green because it has pigments that reflect green light (which bounces off the pigmented object into our eyes) and absorbs other wavelengths of light. That means that the green leaves of plants are reflecting away green light, and absorbing other wavelengths, such as blue and red.

Chlorophyll a

In chloroplasts, the primary photosynthetic pigment is chlorophyll. Chlorophyll molecules have a central magnesium atom (at “3”), essential for the process of converting light energy into electrical energy. The magnesium atom is embedded within a nitrogenous porphyrin ring (“2”). A long hydrocarbon chain anchors chlorophyll in the thylakoid membrane.

The methyl group (-CH3) shown at “4” is what makes this chlorophyll a “chlorophyll a” molecule. Another chlorophyll, chlorophyll b, has the same structure, except for that at number “4” it would have a carbonyl group (a –C=O: carbon double bonded to an oxygen) instead of the methyl group. That chemical change has the effect of changing which wavelengths of light the chlorophyll absorbs, as we’ll see below.

4. Absorption and Action Spectra

Chlorophyll’s properties as a pigment can be quantified through its absorption spectrum. In a laboratory setting, you can take isolated chlorophyll, and, using a device like a spectrophotometer, you can measure which wavelengths of light chlorophyll best absorbs. Why is this important? Because the wavelengths of light that a pigment absorbs can be harvested to perform some work. In the case of photosynthesis, the work is creating an electrical current, which can be used to transform lower energy molecules into higher energy molecules.

Chlorophyll a absorption spectrum

Here’s the absorption spectrum of chlorophyll a. The X-axis shows the wavelength of light shining on the chlorophyll, and it ranges from short wavelength violet light at 400 nm (“nm” = nanometer, or 1 billionth of a meter) on the far left to 760 nm red light on the far right. The Y-axis is the percentage of absorption of light energy.

The part of the line bracketed by “b” shows how in the green and yellow part of the spectrum chlorophyll a absorbs hardly any light. By contrast, the highest point of the line (“a”) is where most light energy is absorbed (blue light). A second peak shows up in the red part of the spectrum at “c.”

The graph below adds two additional photosynthetic pigments: Chlorophyll b and carotenoids. Carotenoids are accessory pigments: they help absorb light energy during photosynthesis, and they protect the photosynthetic pigments (in much the same way that the pigment melanin protects DNA in our skin cells from damage from ultraviolet radiation).

Absorption spectra for Chlorophyll a, Chlorophyll b, and Carotenoids
Courtesy of the Exploratorium Teacher Institute

The absorption spectrum for a pigment shows how much light energy a pigment absorbs. By contrast, the action spectrum shows how much photosynthesis occurs at different wavelengths of light. In other words, an absorption spectrum is a property of a pigment. An action spectrum is a property of an photosynthesizing organism (or of isolated chloroplasts).

How could you measure how much photosynthesis occurs at different wavelengths of light? It’s pretty straightforward, and there are many AP, college, or high school level lab activities where you do exactly that. One involves using disks from spinach leaves (cut out with a paper hole punch), placed in water. As the disks photosynthesize, they produce bubbles of oxygen, which makes them float. You can measure how long it takes for the disks to float to the top of a cup of water, and that gives you an indication of how much photosynthesis is occurring.

The action spectrum for photosynthesis looks like what you see below.

The X-axis of this graph is the same as the absorption spectra graphs above. The Y-axis now shows oxygen production. You can see that the highest amount of photosynthesis occurs at about 700 nm, in the red part of the spectrum (at “C”). Yellow light (at “b”) generates the least amount of photosynthesis. Another peak is the blue part of the spectrum at “a.”

5. Quiz – Photosynthesis: Light and Pigments

So, at this point, we’ve identified a few key things about photosynthesis:

  • its energy conversions
  • its key pigment
  • the absorption spectra associated with some of its key pigments
  • its action spectrum.

And we’ve discussed a few key features about the light energy that drives photosynthesis

Let’s take a quiz, and then we’ll learn some more.

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-PSN: Light and Pigments”]

[h]Photosynthesis: Light and Pigments

[i]

[q multiple_choice=”true” dataset_id=”SMV_PSN_light_and_pigments|704d139a795e4″ question_number=”1″] Light energy directly drives the transformation of

[c] NADPH into NADP+

[f] No. NADPH into NADP+ is an oxidation, and it lowers free energy. Light energy is used to increase free energy. Next time, make another choice .

[c] NADP+ into NADPH

[f] You’re partially correct, but there’s a better answer (there’s something else that light energy does.

[c] ATP into ADP and phosphate

[f] No. That’s an exergonic reaction that doesn’t need any driver. Look for endergonic reactions

[c] ADP and P into ATP

[f] You’re on the right track, but there’s a better answer. Find it

[c*] ADP and P into ATP; and NADP+ into NADPH

[f] Excellent. Those are the two direct products of the light reactions.

[!!!!]Question 2 [/!!!]

 

[q multiple_choice=”true” dataset_id=”SMV_PSN_light_and_pigments|704925c6689e4″ question_number=”2″] Photosynthesis converts

[c] electrical energy to chemical energy to light energy

[f] No. Remember that photosynthesis means “making with light.” The process starts with light. Remember that the next time you see this question.

[c] chemical energy to light energy to electrical energy

[f] No. To begin with, photosynthesis starts with light energy. Remember that and make another choice next time.

[c*] light energy into electrical energy into chemical energy

[f] Excellent. Light to electricity to chemical energy: that’s photosynthesis.

[!!!!]Question 3 [/!!!]

 

[q dataset_id=”SMV_PSN_light_and_pigments|70458273d45e4″ question_number=”3″] Light is a type of [hangman] energy

[c] electromagnetic

[f] Excellent!

[q dataset_id=”SMV_PSN_light_and_pigments|7041df21401e4″ question_number=”4″] Particles of light energy are called [hangman].

[c] photons

[f] Good!

[q dataset_id=”SMV_PSN_light_and_pigments|703e610f6a1e4″ question_number=”5″] Unlike sound, electromagnetic energy can move through the [hangman]of space.

[c] vacuum

[f] Correct!

[q dataset_id=”SMV_PSN_light_and_pigments|703ae2fd941e4″ question_number=”6″] As the frequency of light increases, its [hangman]shortens

[c] wavelength

[f] Correct!

[q dataset_id=”SMV_PSN_light_and_pigments|70371a6a419e4″ question_number=”7″] The wavelength of  blue light is [hangman]than the wavelength of red light.

[c] shorter

[f] Correct!

[q dataset_id=”SMV_PSN_light_and_pigments|70337717ad5e4″ question_number=”8″] The type of electromagnetic radiation that we can see is[hangman]

[c] light

[f] Great!

[q dataset_id=”SMV_PSN_light_and_pigments|702fd3c5191e4″ question_number=”9″] Substances that absorb or reflect electromagnetic radiation (including light) are called [hangman]

[c] pigments

[f] Good!

[q dataset_id=”SMV_PSN_light_and_pigments|702c307284de4″ question_number=”10″] The key pigment in photosynthesis is[hangman].

[c] chlorophyll

[f] Excellent!

[q dataset_id=”SMV_PSN_light_and_pigments|7028429e741e4″ question_number=”11″] The part that anchors chlorophyll in the thylakoid membrane is

[textentry single_char=”true”]

[c*] 1

[f] Excellent. “1” is a hydrocarbon tail, which anchors chlorophyll in the thylakoid membrane.

[c] *

[f] No. Think about how membranes are made of phospholipids, which include long hydrocarbon chains. What part of a chlorophyll molecule is like a long hydrocarbon chain?

[!!!!]Question 12 [/!!!]

 

[q dataset_id=”SMV_PSN_light_and_pigments|70242f89a51e4″ question_number=”12″] The porphyrin ring is

[textentry single_char=”true”]

[c*] 2

[f] Nice. “2” is a porphyrin ring. These structures are key to life, as they play a role in biological pigments like chlorophyll, and also in molecules like hemoglobin, myoglobin, and cytochromes.

[c] *

[f] No. This is just pure memorization really, and porphyrins are worth knowing about, because they appear not only in chlorophyll, but also in molecules like hemoglobin, myoglobin, and cytochromes. Next time, note that it’s called a porphyrin ring. Where do you see anything that looks like a ring of a particular element?

[!!!!]Question 13 [/!!!]

 

[q dataset_id=”SMV_PSN_light_and_pigments|70201c74d61e4″ question_number=”13″] Shifting one functional group in chlorophyll changes its absorption spectrum. Which number below indicates the changeable functional group?

[textentry single_char=”true”]

[c*] 4

[f] Excellent. “4” is a methyl group, the presence of which makes this chlorophyll a.

[c] *

[f] No. Here’s a hint. A functional group is a small group of atoms that changes the properties of the larger molecule that it’s attached to. There’s only one functional group shown in the diagram above.

[!!!!]Question 14 [/!!!]

 

[q dataset_id=”SMV_PSN_light_and_pigments|701b999dcc5e4″ question_number=”14″] In the diagram below, the letter indicating the lowest absorption is

[textentry single_char=”true”]

[c*] b

[f] Excellent. Letter “b” shows the lowest level of absorbance, which for this molecule (chlorophyll a) is in the yellow green part of the spectrum.

[c] *

[f] No. You can handle this question by remembering that yellow/green light is the light wavelength least absorbed by chlorophyll. Or, just take a good look at the graph, and note the lowest level of absorbance.

[q multiple_choice=”true” dataset_id=”SMV_PSN_light_and_pigments|701373742e5e4″ question_number=”15″] Students set up an experiment in which they shine blue, green, and red light on disks of spinach leaves, and determine the amount of photosynthetic activity by measuring oxygen the disks produce. Which of the colors below will generate the least photosynthetic activity

[c] blue

[f] No. Next time, think about the action spectrum for photosynthesis, and choose a different color.

[c*] green

[f] Yes. If you think of the action spectrum for photosynthesis, green is the color of light that drives the least amount of photosynthetic activity.

[c] red

[f] No. Next time, think about the action spectrum for photosynthesis, and choose a different color.

[/qwiz]

6. Photosynthesis: The Light Reactions Part 1, Interactive Lyrics

You might want to watch my music video, Light Reactions Part 1, before completing the interactive lyrics below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-PSN, Light Rxns 1, Interactive Lyrics”]

[h]Photosynthesis: The Light Reactions Part 1, Interactive Lyrics

[i]These are the lyrics to Photosynthesis: The Light Reactions, Part 1, organized into a fill-in-the-blanks quiz. Reading like this will deepen your understanding of how photosynthesis works. Give it a try! 

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d1c5f92487bb4″ question_number=”1″]

Organisms like plants that ________________

Chemically combine water with carbon dioxide

Making ______________ and oxygen too,
That’s why photosynthesis should matter to you!

 

The light reactions are photosynthesis’s first phase
The output’s ATP and ________
O2‘s the by-product, and as you can see
The inputs are water, and ____________.

 

Phase 2 is the _______ Cycle makes carbohydrate,
Using up the ATP and the NADPH,
While absorbing CO2 (it’s carbon ________)
Photosynthesis, what a sensation

[l]Calvin

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]carbohydrates

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]fixation

[fx] No. Please try again.

[f*] Great!

[l]light energy

[fx] No. Please try again.

[f*] Correct!

[l]NADPH

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]photosynthesize

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d1bf9203d4bb4″ question_number=”2″]

Light reactions
happening in leaves
Making _______
NADPH and ATP.
Through a _____-charged
current of electricity
Converted into _________ energy

[l]chemical

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]oxygen

[fx] No. Please try again.

[f*] Great!

[l]solar

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d1b8261dedfb4″ question_number=”3″]

Chloroplasts are the photosynthesizing organelle.
You can find them in leaves, in a ___________ cell,
Inside chloroplasts is a fluid called  ______,
it’s true everywhere, from Tokyo to Roma.

 

The stroma bathes tiny sacs called __________,
Membrane bound sacs, with a tiny space inside,
The thylakoid’s membrane is loaded with protein
It’s the  _____________’ main scene

 

It also has pigments like green ____________
Orange carotenoids and yellow xanthophylls
These pigments _______ light that’s mostly red and blue
The green is not absorbed it reflects back to you.

[l]absorb

[fx] No. Please try again.

[f*] Correct!

[l]chlorophylls

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]light reactions

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]mesophyll

[fx] No. Please try again.

[f*] Good!

[l]stroma

[fx] No. Please try again.

[f*] Good!

[l]thylakoids

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d1b129fa41fb4″ question_number=”4″]

Every chlorophyll has a __________ ring,
With magnesium atoms in the that center of that thing.
In the light reactions that magnesium gets pinged,
By photons, making its _________ zing,

 

Notice the chlorophyll’s tail — a ____________ chain
Which anchors chlorophyll into the thylakoid membrane.
A molecule with style, so pretty,
In the thylakoid, it makes __________.

[l]electricity

[fx] No. Please try again.

[f*] Excellent!

[l]electrons

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]hydrocarbon

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]porphyrin

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d1aa2dd695fb4″ question_number=”5″]

Engelmann showed us the _______ spectrum
He used a prism to break the sun’s
Light into its many different _______,
Shone it on an algae called Cladophora

 

The O2 algae makes will stimulate bacteria
Which will grow in any _______ rich area
Bacteria loved it over blue and red
But hardly grew over ______, they might as well be dead!

[l]action

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]colors

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]green

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]oxygen

[fx] No. Please try again.

[f*] Good!

[q labels = “top” dataset_id=”SMV_Light_Rxns_Part_1_Interactive_Lyrics|d19eaedbe03b4″ question_number=”6″]

This line of growth, it’s a reflection,
Of _______________ action spectrum
Note this doesn’t match exactly
The absorption spectra of ___________ a or b.

 

‘Cause the carotenoids and ____________ we´ve met
Absorb light frequencies the chlorophylls can´t get
And in the _________, they all cooperate!
In making ATP and NADPH

[l]chlorophyll

[fx] No. Please try again.

[f*] Excellent!

[l]photosynthesis’s

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]thylakoid

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]xanthophylls

[fx] No. Please try again.

[f*] Correct!

[/qwiz]

Links

  1. Photosynthesis 4: The Light Reactions (the next tutorial in this series)
  2. The Light Reactions Part 1 (The Big Picture): Music Video and Interactive Lyrics (covers what you’ve learned above)
  3. Photosynthesis Main Menu