2.5 Energy Flow in Ecosystems
The Sun plays an important part in sustaining life on Earth. It is
the source of almost all the energy living things need to survive.
The Source of Energy
Most of the Sun's energy is in the form of radiant energy, which
is energy that travels through empty space. Some of the radiant
energy from the Sun is invisible, such as ultraviolet energy.
Visible radiant energy is what we see in the form of light.
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DIG DEEP!
Did You Know?
Plants in the Dark
Scientists know of only a few plants that do not use
photosynthesis--the plants do not need sunlight to grow. Indian
pipe is a common plant in Ontario that gets its food from fungi in
the soil. Fungi get their food from the roots of nearby trees. The
leaves of Indian pipe do not have green chlorophyll, so they are
white.
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Where Does the Energy Go?
About 70 % of the Sun's radiant energy is absorbed by Earth's
land, atmosphere, and water, and is converted into thermal
energy (Figure 1). Thermal energy warms the atmosphere and
evaporates water. About 30 % of the radiant energy from the
Sun is reflected back into space. A very small amount of the
radiant energy that reaches Earth's surface is absorbed by living
organisms.
Figure 1 How the Sun's radiant energy is distributed
70 %
30 % reflected by clouds or
Earth's surface
19 % absorbed by the
atmosphere and clouds
51 % absorbed by land and
oceans
0.023 °% absorbed by plats in
photosynthesis
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If you do not eat breakfast one morning, you might feel tired or
weak because you do not have much energy. All organisms need
energy to live and grow, but not all energy works in the same
way. Thermal and radiant energy do not directly give organisms
all they need to live and grow. Energy from the Sun must first be
converted into a form that can be used by living things--chemical
energy (Figure 2). Chemical energy can be stored in living things
and used as needed. Some organisms absorb light energy to
make their own chemical energy. Other organisms absorb
chemical energy from the foods they eat.
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Figure 2 Humans use chemical energy to live, move, and grow.
We get the chemical energy we need by eating and digesting
food.
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Photosynthesis
When you eat an apple, your body gets energy from the apple.
But where did the apple get the energy to grow? Some organisms
use light to make their own food. An organism that makes its own
food is called a producer. Plants are producers. They convert
light energy to chemical energy in a process called
photosynthesis (Figure 3). The green colour of plants comes
from the chemical chlorophyll. Chlorophyll absorbs light energy.
The light energy is used to convert two chemicals (carbon dioxide
and water) to a different chemical (sugar), which is stored in the
plant (Figure 4). These sugars contain stored chemical energy.
During this process, the plant also releases oxygen into the air.
The word equation for photosynthesis describes what happens:
carbon dioxide + water light energy (ARROW POINTING RIGHT)
sugar + oxygen
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Figure 3 Chlorophyll in leaves traps the light energy used in
photosynthesis.
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Figure 4 Chemical energy is stored in plant parts.
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DIG DEEPER!
Learning Tip
How to Read a Word Equation
The + in a word equation can be read as "and." The arrow can be
read as "react to form." So, the word equation for photosynthesis
could read "carbon dioxide and water, with the addition of light
energy, react to form sugar and oxygen."
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Cellular Respiration
Unlike producers, which can make their own energy through
photosynthesis, humans are consumers and need to obtain
energy from other sources. Consumers obtain energy by eating
other organisms, producers and consumers, or their products.
The food you eat contains stored energy. When you digest, the
energy in the foods is released through a process called cellular
respiration. In this process, the sugar and oxygen are changed
into carbon dioxide and water. As this change occurs, energy is
released. The word equation for cellular respiration is
sugar + oxygen (ARROW POINTING RIGHT) carbon dioxide +
water + energy
Cellular respiration is different from breathing. Cellular respiration
is a chemical process that occurs in the cells of living things.
Breathing, or physical respiration, is the process of taking air into
lungs to get oxygen into cells. Both animals and plants need
oxygen to survive.
Complementary Processes
Photosynthesis and cellular respiration involve the same
chemicals--water, carbon dioxide, oxygen, and sugar. Both
processes also involve energy. But the reactions are the opposite
of each other and produce different products. While
photosynthesis produces sugars that contain stored energy,
cellular respiration uses the sugars and releases the stored
energy so that it can be used for life functions. Both processes
need to happen in order to sustain life. Figure 5 shows how
these processes work together.
BEGIN FIGURE CAPTION:
Figure 5 Photosynthesis and cellular respiration are processes
that depend on each other.
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2.5 Wrap Up
- Green plants are producers. They use photosynthesis to
produce food, which stores the Sun's energy as chemical energy.
- Consumers obtain energy when they eat other organisms or the
products of other organisms.
- Plants and animals perform cellular respiration to release
energy from food.
- Photosynthesis and cellular respiration are complementary
processes.
CHECK YOUR LEARNING
1. How much of the Sun's energy that comes to Earth is used in
photosynthesis? What happens to the rest of the energy? K/U
2. Why are photosynthesis and cellular respiration considered
complementary processes? K/U, C
3. What important substances do plants produce during
photosynthesis? How do humans use these substances? K/U, C
4. (a) How do producers get the chemical energy they need to
live and grow?
(b) How do consumers get the chemical energy they need to live
and grow? K/U
5. Plants can make their own food by the process of
photosynthesis. Animals need to find and eat food to obtain
energy. What are the advantages and disadvantages of each way
of obtaining energy? T/I
6. (a) Why are humans considered to be consumers?
(b) Name five other organisms that are consumers. K/U
7. Animals are unable to make their own energy, yet you obtain
energy when you animal food products. How does this illustrate
the flow of energy through an ecosystem? T/I