Content Objectives
Light Energy
The source of energy for most life on this planet comes from sunlight. When you think of light, one tends to consider only what you can see, but there is much more to ponder! When it's dark you need something to illuminate things so you can see, like a flashlight, a lamp, or a candle. You stand outside in the sun when it is daylight and you can see because it's not dark. When you feel the warmth of the sun on your skin, one quickly starts to question if you need sunscreen. You see a sunrise or sunset and wonder how something can be so beautiful. As a child, I can remember seeing a rainbow and wondering what kind of magic put that there. Was it a leprechaun leading you to their pot of gold? There is no magic or leprechauns; it's the natural world of Science and light traveling through a medium, being absorbed by matter, or reflecting off a surface. All these things that we can see are an effect of light, which is radiant energy from the sun. Yet the strange thing you need to comprehend is that this energy has characteristics of both a wave and a particle. The wave is based on the light energy's frequency and wavelength. The packet of light energy known as a photon is what we will first discuss.
Before opening a unit, I like to get an idea of the student's prior knowledge to gauge their understanding. This also helps address misconceptions that will arise during the lessons that can be used as teachable moments. I use interactive notebooks in class, so the first page is always entitled "What I Know". Our content standards are already written on the board at the start of each unit, so I ask the students to simply tell me anything they know about them and any connections they are aware of based on other content that relates to the discussed topic. After this informal assessment, we begin with the lesson.
Light is a form of energy coming from the sun, which is about 93 million miles away. It takes about 8 & ½ minutes to get here traveling at the speed of light, which is 186,282 miles per second (299,792 kilometers per second). It can exist as both a particle and a wave. “In physics, a photon is a bundle of electromagnetic energy making the basic unit that makes up all light and is sometimes referred to as a "quantum" of electromagnetic energy.”1 This principle eludes that some photons are absorbed giving off heat and some materials emit electrons after absorbing a photon. “This process is called the photoelectric effect. The photoelectric effect is a property of light that is not explained by the theory that light is a wave, leading scientists to treat light as both a wave and a stream of particles”2 With an idea as abstract as moving particles of energy, I need something to give students to link ideas to. A video segment from Discovery Education or BrainPOP comes in handy about photons and what they are!
Next, we consider the concept of light as a wave. To do this, we will discuss what a transverse wave is. “Transverse waves are waves in which the oscillations are at right angles to the direction of energy (wave) movement.”3 This back and forth movement is a way of measuring and comparing different wavelengths (distance between successive wave crests) based on comparisons to the X-axis and its frequency or number of wavelengths per second. Simply put, frequency is how many cycles have been completed in one second (usually crest to crest or trough to trough). This translates into comparing short, strong frequencies of energy to long, weak frequencies. An excellent way to model this is with a slinky on the floor by undulating it back and forth. Quick movements represent high energy, while slow movements represent low energy. I also include Flocabulary and Bill Nye the Science Guy video segments. My students respond very well to both video platforms because they are entertaining, presented on a child’s level, and understandable. Frequencies and wavelengths are the basis of understanding the next concept dealing with the Electromagnetic Spectrum.
Energy comes in many forms and light and its many parts are no exception. “Light is energy in motion and the amount of energy transmitted through light is based entirely on the light’s frequency or wavelength, so the higher the frequency, the more energy.”4 “Light consists of a combination of electric and magnetic fields called electromagnetic waves.”5 This spectrum consists of seven different parts based on their wavelengths which include: Radio waves, microwaves, infrared, visible light, ultraviolet, X-ray, and gamma rays. When you think of light, you think of being able to see it; well, the only part detectable to humans is visible light (hence the name). Radio waves, microwaves, infrared, and visible light are longer, weaker, lower energy wavelengths that are not harmful (non-ionizing radiation). On the other hand, ultraviolet, X-ray, and gamma rays are shorter, stronger, higher energy wavelengths that are quite harmful (ionizing radiation). “Ionizing radiation in high-frequency, tiny-wavelength light (from ultraviolet light to X-rays and gamma rays) pack such an energy wallop that it can knock electrons right out of their atoms, making for an unstable chemical situation. These reactive atoms are called free radicals and are infamous for their destructive effects.”6 What is important to the lesson is for students to distinguish the differences in wavelengths and types of energy, but more importantly they understand the visible light spectrum and what colors really are.
Figure 1. Electromagnetic Spectrum. https://commons.wikimedia.org/wiki/File:Spectre.svg (Accessed July 31, 2019)
Electromagnetic radiation is classified only by its differences in amounts of energy. The entire electromagnetic spectrum consists of sunlight. “Light consists of electromagnetic waves of particular frequencies and wavelengths but is commonly referred to and dramatically represented as rays.”7 A ray is the straight line, in this case representing the path of white light. With a prism white light can be separated into the colors of the rainbow. ROYGBIV is an acronym for the colors in wavelength order from longest to shortest. Red is the long, weak side of the color spectrum and violet is the short, strong side with the other colors following suit in between. Colors are measured in nanometers (nm) in wavelength intervals. The color red bottoms out at 740nm and violet extends to 380nm. “It’s the infrared that makes you sweat on a hot day, but it’s the ultraviolet that can give you a sunburn or worse.”8 We have come to the point where you may be asking yourself what different wavelengths have to do with plants. The answer is that plants like certain parts of visible light more than others in photosynthesis operations. Next, we will discuss how plants produce their own food and how this energy is part of the process.
Photosynthesis / Energy to Matter
Life on Earth exists because plants have the capability to make their own food. This process is called photosynthesis and is a vital link in the complex web of life. It is an essential process animals need plants to be able to do. “It is without a doubt, the most important biochemical reaction for life on Earth.”9 When I begin our discussions about plants, I tell the students that plants perform a miracle. They don’t have to hunt or scavenge for their food like animals do; this uses energy. They don’t have to go to the grocery store and get it from elsewhere like us humans do. They just stay right where they are and let all the materials they need come to them. Just the term photosynthesis itself means, to put together (synthesis) with light (photo). I tell my students that plants eat sunshine and poop out oxygen. In a generalized sense, that is only half and it is much more complicated, but it gets their attention and sticks in their head.
Photosynthesis is the autotrophic (self-feeders) process by which plants use sunlight, water, and carbon dioxide to live. These components are used along with chlorophyll to convert them into food, which is used for energy. Oxygen is a byproduct of this process which is released into the atmosphere because they have no use for it. All plants use photosynthesis, so they all need sunlight. The idea that every type of green plant makes their own food is essential to understand. Students will need the context of this material to further build on new and interrelated plant and life science materials.
Plants’ survival depends on their ability to make food. Food is made in the leaves of the plant. It is here where materials are collected and used so this complex process can take place. We will begin with the ingredients, so we know how the materials get to where they need to be (the leaf). Water, carbon dioxide, sunlight, and chloroplasts are needed for the process to begin. Water is sucked up from the soil through the root system. It is then transported through the stem to the leaf. Carbon dioxide enters the leaf through tiny microscopic holes or pores in the leaf called stomata. Sunlight’s energy is absorbed by the leaf and collects inside the plant cells. The main organelles (cell parts) in the cell are called chloroplasts. Chloroplasts contain a green pigment called chlorophyll. This is where the sun’s light energy is used to power photosynthesis. Amazingly these organelles can move around in the cell based on the intensity and direction of light being collected for the best absorption possible. In this process, a type of sugar (called glucose) and oxygen are made. Oxygen is not wanted, so it is released through the stomata. Water is released through the stomata as well. This water vapor is part of the water cycle we know as transpiration. The chemical formula for photosynthesis is:
6CO2 + 6H2O + Energy → C6H12O6 + 6O2.
If you remember the joys of balancing an equation you will first notice that it requires six molecules of both carbon dioxide and water to make one molecule of glucose (simple sugar). Leaves are the primary food-producing part of the plant and all photosynthetic activity takes place here. This paragraph is the simplified version of this chemical reaction. We will now discuss more detailed chemical aspects of photosynthesis for content knowledge so that an educator really knows what is going on.
As I have previously stated, “Photosynthesis a complex series of reactions, driven by light energy absorbed by chlorophyll and other pigments, that results in the synthesis of organic compounds from carbon dioxide and water.”10 We will now analyze these complex reactions in their simplest forms. There are two separate processes occurring, making the molecules and then storing them. Light absorbing chlorophyll captures energy from light and uses it to oxidize water. This causes water molecules to split apart (oxidation) causing it to energize and turns it from a low energy chemical to a high energy chemical. Oxygens from water molecules pair up and go one way and hydrogens then mix with parts of carbon dioxide to make carbohydrates (reduction) that the plant uses for food. The process gives us energized electrons to produce and store chemical energy in two high energy compounds. The first is called ATP (Adenosine triphosphate). The second is called NADPH (Nicotinamide adenine dinucleotide phosphate). There are two types of reactions in photosynthesis: light reactions (requiring light) that form energized electrons and dark reactions (not requiring light) that use energized electrons. The process where ATP & NADPH compounds are produced occurs in the light reactions. Next comes the complicated process of glucose synthesis called the Calvin Cycle. The light reactions process is known as the Z Scheme (a.k.a. energy transduction) where light energy is converted into electrical energy by exciting chlorophyll with photons. This is where those little packets of light energy (photons) allow electrons to transfer to NADPH. It takes two stages because the photon doesn’t carry enough power, so it must happen twice. There are two photosystems labeled PSI & PSII and there are two types of chlorophyll (types a & b) that allow it to occur. It occurs in two stages of a strong oxidant (PSII) and a weak oxidant (PSI). Chlorophyll a absorbs higher energy (blue, indigo, violet) wavelengths, while chlorophyll b absorbs lower energy (red, orange, yellow) wavelengths giving it the energy required to complete the reaction. It is here where solar energy is absorbed and electrons can transfer completing compound transformation from solar energy into chemical energy stored within carbohydrates. The final photosynthesis process is the dark reaction where carbon dioxide is reduced and electrons are accepted. This process requires a series of biochemical steps; therefore, it will not be covered in this material. This is a very complicated process for those of us who don’t have chemistry degrees, but incredibly important in the bigger picture of life on this planet. We have talked about sunlight, photosynthesis, and the energy involved in the process. We will now talk about how plants provide the energy for the rest of the organisms on the planet and how they use this energy and return it back into the system to be reused.
Figure 2. Photosynthesis. https://commons.wikimedia.org/wiki/File:Photosynthesis_en.svg
(Accessed July 30, 2019)
Ecosystem / Energy Transfer
All living things must have energy to survive. We have spoken in depth of the sun emitting electromagnetic radiation and plants using certain parts of that energy to make food for their own survival in the process of photosynthesis. We will now begin to look at how this energy is transferred to other organisms. Consumers eat food to get the energy they need to survive. When food is consumed by organisms it is digested and the bonds within the food are broken, releasing the energy they need. This process is known as cellular respiration and amazingly it is the exact opposite of photosynthesis. The chemical formulas are:
Photosynthesis
6CO2 + 6H2O + Energy → C6H12O6 + 6O2
Cellular Respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Instead of making bonds and storing energy in photosynthesis, cellular respiration breaks the bonds and releases the energy. Also, instead of organisms releasing oxygen like plants do, they release carbon dioxide, which is exactly what plants need to continue their process of photosynthesis. This balancing act is a symbiotic relationship also known as interdependence. We are now ready to consider a bigger picture in this energy transfer across organisms known as ecosystems.
An ecosystem is the relationships within a community between living and nonliving things. Plants produce the food and animals consume plants and other animals. Decomposers recycle all living organisms in this revolving process. “Two basic laws underlie ecosystem function: Nutrients constantly cycle and recycle within and among ecosystems, while energy moves through ecological communities (the various populations of interacting organisms that inhabit ecosystems) in a continuous one-way flow.”11 Detritivores (earthworms) and decomposers are the main organisms that contribute to this nutrient cycle. Returning nutrients back to the soil allows for this process to ever continue by giving plants back the nutrients they need. As previously stated in the first law of thermodynamics, energy can neither be created nor destroyed only transformed from one form to another. Decomposers are a vital link in an ecosystem’s ability to recycle needed matter, not only by putting it into the soil, but by providing matter for the carbon and nitrogen cycles as well.
Ecosystems consist of food chains and food webs. Food chains consist of organisms consuming (eating) one another for sustenance in the ecosystem. This is based on predator, prey relationship where one organism is the hunter and the other is the hunted. Last there are three types of organisms starting with a producer, a consumer, and then ending with a decomposer. Hence, what an organism eats determines where they are in the order of the food chain. We also have three types of consumers ranging from herbivores (plants only), carnivores (meat eaters), and omnivores (both meat and plants). Each level of a food chain represents a different trophic level, or number of steps the organism is from the start of the food chain. An example of a food chain would be as follows:
-Grass (producer) is eaten by a grasshopper (herbivores/omnivores which are primary consumers).
-Grasshopper (herbivore) is eaten by a frog (carnivores/omnivores which are secondary consumers).
-Frog (carnivore) is eaten by a snake (carnivores/omnivores (sometimes) which are tertiary consumers).
-Snake (carnivore) is eaten by a hawk (carnivores/omnivores (sometimes) apex predator).
-Everything listed above is decomposed by fungi. Nutrients are broken down and returned to the soil.
The energy is transferred from the producer to the different types of consumers, on to the top predators, and then to the decomposers to be broken down and put back into the system so the plants can start over. Complex overlapping food chains then create food webs, leading to the broader idea of a biosphere or the sum of all ecosystems. Finally, this energy transfer process can also be viewed in a different way. The small amount of energy that is passed on can be observed as an energy pyramid.
Figure 3. Energy Pyramid. https://commons.wikimedia.org/wiki/File:Ecological_Pyramid.svg (Accessed July 29, 2019)
In an energy pyramid, we model the energy that is transferred through the system. Producers go on the bottom and predators on the top (trophic levels). Only about 10% of the actual energy stored in an organism is passed on when it is consumed by another. This pyramid shape represents the flow of energy because of the way that energy is used up and lost throughout the system. We will model the food chain moving a trophic level up the pyramid each time. The grass is the primary producer at 100%. The grasshopper being the primary consumer is at 10%. The frog is the secondary consumer at 1%. The snake is the tertiary consumer at 0.1% and the falcon is the apex predator at 0.01%. Please note that 90% of the energy is lost each time you go up a level in the pyramid which is all from heat loss! This means that the falcon will need to eat several snakes to get enough energy to survive and so on, and so on, back down the pyramid for each level down.
One final note, all my examples have been that of terrestrial creatures. Please don’t leave out our aquatic examples! The main terrestrial producer for land is green plants. The main aquatic producer for the ocean is phytoplankton. Both producers are at the beginning of the food chain, but both are enormous manufacturers of oxygen as well! So please keep in mind, all organisms are dependent upon photosynthesis, their organic structure, their oxygen production, and their energy whether they exist on the land or in the sea!
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