Behold the power of light

This exercise is a part of Educator Guide: SN 10: Scientists to Watch / View Guide

Directions: After students have had a chance to review the articles “Photosynthesis reinvented” and “How plants hunt water,” lead a classroom discussion based on the questions that follow.


Discussion questions:

1. What are chloroplasts?

Chloroplasts are organelles within the cells of plants and green algae that convert light energy into chemical energy via photosynthesis. Inside a chloroplast are disk-shaped structures called thylakoids. Thylakoids contain light-absorbing chlorophyll molecules that allow the energy from sunlight to be converted and stored as chemical energy. Thylakoids are arranged into stacks (each stack is called a granum) and connected by bridges (called lamellae). Like the bacteria from which they descended, chloroplasts have a DNA nucleoid, ribosomes and storage granules for nutrients (starch).

Extension prompts:

2. What are the light reactions in chloroplasts and what do they make?

Light reactions are the parts of photosynthesis that can only happen while there is a light energy source. When exposed to light, thylakoid disks embedded in chloroplasts absorb photons of light. This light energy initiates the reaction that splits water (H2O) into hydrogen (H2) and oxygen (O2), and ultimately generates adenosine triphosphate (ATP). ATP powers the cell’s metabolic activity.

3. What are the dark reactions in chloroplasts and what do they make?

Dark reactions, also called light-independent reactions, are steps of photosynthesis that do not require photons to proceed. In a process called the Calvin cycle, dark reactions in thylakoids combine hydrogen (from the light reactions) with CO2 to make the sugar building block glyceraldehyde 3-phosphate.

4. What are the dark reactions outside of chloroplasts in plant cells and what do they make?

Dark reactions that occur outside of chloroplasts in plant cells’ cytosol convert glyceraldehyde 3-phosphate, a product of the Calvin cycle, into glucose and other saccharides.


You may want to explore the “Chemistry and Other Physical Sciences Discussion Questions” in the Built for Speed Educator Guide if you are interested in additional questions about catalysts and activation energy.

Discussion questions:

1. How does an atom absorb light energy? What about molecules?

Based on an atom’s structure, it can absorb photons of light at specific wavelengths. An electron within an atom absorbs the incident photon of light — as the electron absorbs the light energy it is promoted from its ground state energy level to a higher energy level called the excited state. The electron is not stable in the excited state, so it generally returns back to the ground state releasing the energy initially absorbed in the form of heat. Generally, the same process occurs when molecules absorb light. Once an electron in a molecule absorbs energy, the high-energy electron generally falls back down to the ground state, but if an electron acceptor is nearby, the excited electron can move to the acceptor. This electron transfer can initiate a series of other chemical reactions.

2. Write the chemical formula and draw the skeletal or shorthand chemical structure of chlorophyll a. Circle all the double bonds in your structure.

Chlorophyll a has the chemical formula of C55H72MgN4O5. See the National Institutes of Health PubChem Open Chemistry Database for the shorthand molecular structure of chlorophyll a.

Extension prompts:

3. What is chlorophyll a and how does its structure relate to its function?

Chlorophyll a is the molecule that initially absorbs light energy in photosynthesis. Chlorophyll a belongs to a family of biological molecules called porphyrins, which also includes heme (from hemoglobin). Like other porphyrins, chlorophyll a is a flower-shaped molecule with a metal ion at the center of the flower (magnesium for chlorophyll, iron for heme), and it has alternating single and double bonds around the rings that make up the “petals” of the flower. Alternating double and single bonds, or conjugated multiple bonds, allow electrons to move freely. Double bonds become single bonds and single bonds become double bonds, then back again. When these “free electrons” absorb red and blue light, they get promoted to higher energy levels that coincide with the amount of energy gained.

4. What is electrolysis and how can electrolysis of water be demonstrated?

Electrolysis, or electrical splitting of water into hydrogen and oxygen, can be accomplished by connecting two wires to electrodes of a six-volt or nine-volt battery and immersing the wires (not touching each other) in a glass of water. Adding some salt to the water increases the electrical conductivity and speeds up the process. Bubbles of oxygen gas form at the positive electrode and bubbles of hydrogen gas form at the negative electrode. If desired, the gases can be collected by placing inverted test tubes over the ends of the wires.


Discussion questions:

1. How much energy is in light? Explain and give specific examples for red and violet light.

Light is electromagnetic waves with velocity c = 3×108 m/sec (approximately), frequency f and wavelength l. Equivalently, light can be thought of as quantum particles called photons. The energy of each photon, and thus, the maximum energy imparted to a molecule or device that absorbs that photon, is E = hf = hc/l, in which h » 4.14×10⁻15 eV sec is Planck’s constant. (eV is an electron volt or the energy of one electron coming from a one-volt battery.) For red light with a wavelength l = 7×10-7 m, each photon has an energy E = 1.77 eV. For violet light with a wavelength l = 4×10-7 m, each photon has an energy E = 3.11 eV.

2. Why does chlorophyll a appear green? What wavelength(s) of light does it absorb?

Chlorophyll a appears green because it reflects (does not absorb) wavelengths of green light. It absorbs wavelengths of light that correspond to the red (approximately 650 nm to 700 nm) and blue/violet regions (approximately 400 nm to 450 nm). See the Chlorophyll Absorption Spectrum on University of Illinois chemist Patricia Shapley’s website.

Extension prompts:

3. How do photovoltaic panels (solar cells) work?

Photovoltaic panels use semiconductors (usually silicon) that convert light energy to electrical energy. When an atom in the semiconductor absorbs a photon, the energy from that photon knocks an electron out of a tight (valence) orbit around the atom. This frees it to roam around as a conduction electron with an overall negative charge. The atom that is now missing one electron has a net positive charge. That absence of an electron can be thought of as a positively charged holethat can wander from atom to atom, as one atom steals a replacement electron from a neighboring atom. In an electric field, negatively charged electrons go one way and positively charged holes go the other way. If the photovoltaic panel is connected via wires to a circuit, charges can flow out of the panel into the circuit, do useful work and then the electrons and holes can be reunited in the panel.


Discussion questions:

1. What are some possible extensions and applications that you can think of for Chong Liu’s technology?

Chong Liu’s technology could allow scientists to convert sunlight to sugar or other food ingredients more efficiently, convert sunlight to fuel more efficiently, convert sunlight to electricity more efficiently, remove CO2 from earth’s atmosphere, recycle air in a spacecraft or spacesuit, recycle air in a submarine or diving suit and produce ammonia and isopropanol in more energy-efficient ways.

2. What are some possible extensions and applications that you can think of for José Dinneny’s technology?

José Dinneny’s technology could allow scientists to genetically engineer plants that glow various colors in response to various conditions. This ability could indicate the health of agricultural crops or houseplants. Scientists could also use these genetically engineered glowing plants as sensors in the environment to warn of air pollution, toxins in the soil or water, landmines or other explosives. Using the GLO-Roots system could also allow scientists to engineer plants to respond to environmental conditions so that they can better survive heat, cold, droughts or pests.

Extension prompts:

3. What would inspire more students to become scientists? What has inspired you to pursue an academic endeavor in your life?

Student answers will vary.