Chapter 4. Chapter 5: Energy Flow and Photosynthesis

4.1 Introduction

Welcome to the Interactive Study Guide for Chapter 5: Energy Flow and Photosynthesis! This Study Guide will help you master your understanding of the chapter's Driving Questions, using interactive Infographics and activities, as well as targeted assessment questions. Click "Next" to get started, or select a Driving Question from the drop-down menu to the right.

DRIVING QUESTIONS

• What are the photosynthetic organisms on the planet, and why are they so important?
• What are the different types of energy and what transformations of energy do organisms carry out?
• How do plants and algae convert the energy in sunlight into energy-rich organic molecules? (And why can’t humans do this?)
• How do algal biofuels compare to other fuels in terms of costs, benefits, and sustainability?

4.2 Driving Question 1:

Why should you care?

Do you notice plants? Many people don’t register more than a green background when they are outside. The next time you are outdoors, look for plants and notice their variety; even if you live in a city, there are likely several kinds just growing through the pavement cracks. In every terrestrial ecosystem a great variety of plants make up the largest group of organisms by weight—usually at least 90%. This preponderance of plants exists because they are producers, or autotrophs—organisms that can grow by making organic molecules from inorganic molecules.

Land plants are the youngest group of photosynthetic autotrophs; they were preceded by algae, which is a general term for photosynthetic organisms that are mostly single-celled and mostly aquatic. The first photosynthetic organisms were the prokaryotic bacteria, primarily the cyanobacteria, formerly called the blue-green algae. It is thought that the first eukaryotic algae arose from symbiotic relationships between a heterotrophic eukaryotic organism and a cyanobacterium that became a semiautonomous organelle living inside the eukaryote’s cells. Eventually, this cyanobacterium became what we now call chloroplasts.

The three groups of photosynthetic organisms—modern land plants, eukaryotic algae, and photosynthetic bacteria—are responsible for both the oxygen in our atmosphere and the organic molecules that are the foundation of nearly every food chain. Whether or not you notice them, you could not live without them.

What should you know?

To fully answer this Driving Question, you should be able to:

1. Compare and contrast the main types of photosynthetic autotrophs.
2. Explain the general importance of photosynthetic organisms to the biosphere

Infographic Focus

The Infographics most pertinent to the Driving Question are 5.3 and 5.6.

4.3 Driving Question 2:

Why should you care?

It is sometimes hard to conceptualize, but energy never goes away. When a car drives up a hill, it converts chemical energy in gasoline to kinetic energy. If it parks at the top of the hill, it has converted kinetic energy to potential energy, since it now has the potential to roll down the other side. Through photosynthesis, algae can take light energy, water, and carbon dioxide and convert them to chemical energy. This energy can be stored in the form of carbohydrates, oils, or proteins. Heterotrophs, such as humans, must obtain chemical energy from the food we eat. All organisms convert carbohydrates, proteins, and fats into high-energy molecules that their cells can use to power essential functions. To humans, an important use of those high-energy molecules is to convert chemical energy to kinetic energy, or heat. The heat can be used to maintain a constant body temperature, but much of it is lost to the surrounding air. Thus, we cannot use the chemical energy in our food without some of it escaping to our surroundings, but the original amount of energy never changes. Understanding that energy is both conserved and transformed is essential to understanding how you can conserve energy resources.

What should you know?

To fully answer this Driving Question, you should be able to:

1. Differentiate among the types of energy.
2. Explain in general terms the ways that organisms convert energy from one form to another and the efficiency with which they do it.

Infographic Focus

The Infographics most pertinent to the Driving Question are 5.4 and 5.5.

4.4 Driving Question 3:

Why should you care?

When organisms use light to convert carbon dioxide and water to carbohydrates, they are transferring the sun’s energy into the food chain. You are part of a food chain too, and even if you hate veggies, you rely on photosynthesis to feed you and provide you with oxygen to breathe and the food you eat. This is because we heterotrophic consumers cannot convert inorganic molecules to organic molecules that we can derive energy from. Photosynthesis requires a specific set of cellular machinery that can capture carbon dioxide, harness light energy, and process them with water into carbohydrates. Once you know more about it, you will see that photosynthesis is a minor miracle that happens all around us every day.

What should you know?

To fully answer this Driving Question, you should be able to:

1. Describe the relationship between energy level and color of light.
2. Write the general equation of photosynthesis.
3. Explain how the light (photo) reactions and carbon (synthesis) reactions work together to make photosynthesis happen.

Infographic Focus

The Infographics most pertinent to the Driving Question are 5.7 to 5.9.

4.5 Driving Question 4:

Why should you care?

Recently, solar panels have become a popular way to produce electricity from the sun, but photosynthetic organisms have been using sunlight to produce energy for billions of years, so we weren't the first to have the idea. Indeed we live on a solar-powered planet. Ancient photosynthesis is the basis for our fuel economy. The energy we get from fossil fuels is derived from the bodies of long-dead photosynthetic organisms: every time you drive a car or take a plane or a bus, you are releasing energy captured on a sunny day millions of years ago. Unfortunately, we are rapidly using up the world’s stores of fossil fuels.

People are concerned about the world's dependence on fossil fuels for energy. It is evident in the popularity of hybrid cars, the number of new businesses providing solar or wind power to homes, and the rest of the recent trends to go green. Why do people care about using oil, natural gas, and coal? Four primary reasons are climate, pollution, scarcity, and security. These fuels are called fossil fuels because they were made millions of years ago from the bodies of plants and other organisms. When they are burned, they release into the atmosphere carbon dioxide that has been trapped underground for just as many years. The planet naturally cycles carbon between living things and carbon dioxide in the atmosphere, but burning fossil fuels adds more carbon dioxide to the atmosphere than the global carbon cycle can handle. Using photosynthetic organisms to produce fuel oil is another way to harness the sun for energy that is carbon neutral; no new carbon is added to the atmosphere when it is burned.

By now, you've looked at photosynthesis and energy from several perspectives. One of the best ways to learn an important lesson is to tie several parts together in your mind. Photosynthesis and the potential for solar energy and biofuels to help solve our energy crisis are important things to care about.

What should you know?

To fully answer this Driving Question, you should be able to:

1. Differentiate between fossil fuels and biofuels and between renewable and nonrenewable energy sources.
2. Describe the pros and cons of the sources of biofuels.

Infographic Focus

The Infographics most pertinent to the Driving Question are 5.2 and Table 5.1.