Fundamentals of Biology

Characteristics of Life

Have you ever wondered how we know if something is living or nonliving? A dog is living. A rock is nonliving. But what about a seed, a mushroom, or a tiny germ you cannot even see? Scientists look for special signs called the characteristics of life.

These characteristics help us decide if something is alive. All living things share these same big ideas. If something is living, it does these things in some way.

In this lesson, you will learn that living things:

• are made of cells,
• use energy,
• keep their bodies balanced, which is called homeostasis,
• grow and change,
• reproduce or make more of their kind,
• and respond to stimuli, which means they react to things around them.

1. Living things are made of cells

A cell is a tiny building block of life. All living things are made of one or more cells. Some living things, like bacteria, have only one cell. Other living things, like people, trees, and cats, have many cells.

You cannot usually see cells with just your eyes. They are very small. But even though they are tiny, cells do important jobs that help a living thing stay alive.

If something is not made of cells, it is not living. A soccer ball may roll and bounce, but it is not made of cells. It is nonliving.

2. Living things use energy

All living things need energy. Energy helps them do the work of living. Animals get energy by eating food. Plants make their own food using sunlight, air, and water.

Living things use energy to move, grow, stay warm, and do the jobs inside their cells. Even a plant sitting still is using energy. It is growing, making food, and staying alive.

A toy car may move fast, but it only moves if a battery or a person makes it go. It does not get and use energy the way living things do.

3. Living things keep balance in their bodies

Living things need to keep things inside their bodies just right. This is called homeostasis. That is a big word, but it means keeping a safe, steady balance.

For example, when you get hot, your body sweats to cool down. When you get cold, you may shiver to warm up. Your body is trying to keep your temperature balanced.

Plants also keep balance. They need the right amount of water and sunlight. If a plant does not get enough water, it may droop. If it gets what it needs, it can stay healthy.

4. Living things grow and change

All living things grow. A baby kitten grows into an adult cat. A tiny seed grows into a plant. As living things grow, they also change.

Some changes are easy to see. A caterpillar changes into a butterfly. A child grows taller. A tree grows new branches.

Nonliving things can change too, but not in the same way. A puddle may get smaller in the sun, but it is not growing. A wooden chair can get scratched, but it is not changing as a living thing does.

5. Living things reproduce

Living things make more of their own kind. This is called reproduce. Dogs have puppies. Birds lay eggs that hatch into chicks. Plants make seeds that can grow into new plants.

Reproduction is another sign of life. A living thing comes from another living thing. New living things do not appear from rocks, tables, or toys.

6. Living things respond to stimuli

Living things can react to changes around them. These changes are called stimuli. A stimulus can be light, sound, touch, heat, cold, or something else in the environment.

For example, if you touch something hot, you quickly pull your hand away. That is a response to a stimulus. If a dog hears its owner open a food bag, it may run over. That is also a response.

Plants respond too. Many plants grow toward sunlight. Some flowers open in the daytime and close at night.

How the characteristics work together

Living things do not show just one characteristic of life. They show all of them.

• A rabbit is made of cells.
• A rabbit eats food for energy.
• A rabbit keeps its body balanced.
• A rabbit grows from a baby to an adult.
• A rabbit can have babies.
• A rabbit responds to sounds, light, and danger.

Because it has all these characteristics, a rabbit is living.

Living, once living, and nonliving

Sometimes it helps to sort things into three groups:

• Living: alive right now, like a fish, tree, or ant.
• Once living: was alive before, like a fallen leaf, a wooden desk, or paper.
• Nonliving: was never alive, like a rock, glass, or metal spoon.

A wooden desk came from a tree, so it was once part of something living. But the desk itself does not grow, use energy on its own, or reproduce. So it is not living now.

Worked Example 1: Is a rock living?

Let us check the characteristics of life.

1. Is a rock made of cells? No.
2. Does it use energy like living things? No.
3. Does it grow as living things grow? No.
4. Does it reproduce? No.
5. Does it respond to stimuli on its own? No.

Answer: A rock is nonliving.

Worked Example 2: Is a sunflower living?

Let us look for the signs of life.

1. A sunflower is made of cells. Yes.
2. It uses energy by making food from sunlight. Yes.
3. It needs water and keeps itself healthy. Yes.
4. It grows from a seed into a tall plant. Yes.
5. It makes seeds for new sunflowers. Yes.
6. It responds to light. Yes.

Answer: A sunflower is living.

Worked Example 3: Is a toy robot living?

A toy robot may move, light up, or make sounds, so this one is trickier.

1. Is it made of cells? No.
2. Does it use energy? It may use batteries, but not like living things do.
3. Does it grow? No.
4. Does it reproduce? No.
5. Does it respond on its own like a living thing? No. It only follows its programming.

Answer: A toy robot is nonliving. Moving does not always mean living.

Worked Example 4: Is a seed living?

This can be confusing because a seed may look still and dry.

1. Is it made of cells? Yes.
2. Can it use energy when it begins to grow? Yes.
3. Can it grow into a plant? Yes.
4. Did it come from another living thing? Yes.
5. Can it respond to water, warmth, and light as it starts growing? Yes.

Answer: A seed is living.

Tips for telling if something is living

• Do not ask only, “Does it move?” Plants are living, but they do not move from place to place like animals.
• Check for all the characteristics of life.
• Remember that some things are once living, like wood or paper.
• If something needs food or makes its own food, grows, and comes from another living thing, it is likely living.

Quick review

• Living things are made of cells.
• Living things use energy.
• Living things keep a healthy balance called homeostasis.
• Living things grow and change.
• Living things reproduce.
• Living things respond to stimuli.

Summary

The characteristics of life are the special traits that all living things share. Living things are made of cells, use energy, keep balance in their bodies, grow and change, reproduce, and respond to the world around them.

When you want to decide if something is living, do not look at just one clue. Look for the full set of characteristics of life. That is how scientists tell living things from nonliving things.

Living, Nonliving, and Dead

In science, we sort things into groups so we can understand them better. One important way to sort things is by asking: Is it living, nonliving, or dead?

These three groups may sound similar, but they are different. Learning the difference helps us describe plants, animals, and objects correctly.

Living means something is alive right now.

Dead means something was once alive, but it is not alive anymore.

Nonliving means something was never alive at all.

Let’s learn how to tell the difference.

What makes something living?

Living things do certain things because they are alive. A living thing may be big or small, but it shows signs of life.

Living things usually:

• need food or energy
• need water
• grow and change
• respond to the world around them
• need air

Animals, plants, and people are living things. A puppy grows. A tree needs water and sunlight. A child grows taller over time.

Even if a living thing does not move from place to place, it can still be alive. For example, a flower stays rooted in the ground, but it grows, needs water, and responds to sunlight. That means it is living.

What is dead?

Something dead was once living, but now it has stopped doing the things living things do.

A dead thing does not grow anymore. It does not need food or water anymore. It is no longer alive.

Examples of dead things include:

• a fallen leaf that came from a tree
• a dead bug
• a piece of wood cut from a tree
• a feather that came from a bird

These things are not living now, but they came from something that was alive. That is why they are called dead, not nonliving.

What is nonliving?

Nonliving things were never alive. They do not grow, eat, drink, or need air.

Examples of nonliving things include:

• a rock
• water
• air
• a metal spoon
• a plastic toy

A rock may be big, small, smooth, or rough, but it was never alive. A toy car may move when someone pushes it, but it does not grow or need water. So both are nonliving.

A helpful way to think about it

You can ask two science questions:

1. Is it alive right now?
2. If not, was it ever alive before?

If the answer to the first question is yes, it is living.

If the answer to the first question is no, but the answer to the second question is yes, it is dead.

If the answer to both questions is no, it is nonliving.

We can show it like this:

Living = alive now
Dead = once alive, not alive now
Nonliving = never alive

Be careful with tricky examples

Some things can trick us. A car moves, but it is not living. It needs fuel, but fuel is not the same as food for a living thing. A car does not grow or heal, so it is nonliving.

A stick may look like just an object on the ground. But if it came from a tree, then it was once part of a living thing. That makes it dead.

A seed can be tricky too. A seed may look quiet, but it can grow into a plant when it gets water, air, and the right place to grow. That means the seed is living.

Worked Example 1: Dog, rock, and dead leaf

Let’s sort three things: a dog, a rock, and a dead leaf.

• Dog: A dog eats, drinks water, grows, and breathes. It is living.
• Rock: A rock was never alive. It is nonliving.
• Dead leaf: The leaf came from a tree, which was living. Now the leaf is no longer alive. It is dead.

Worked Example 2: Flower in the ground, wooden table, and plastic bottle

Now let’s try examples that are a little harder.

• Flower in the ground: It grows and needs water and sunlight. It is living.
• Wooden table: The wood came from a tree. The tree was once living, but the table is not alive now. It is dead.
• Plastic bottle: Plastic was never alive. The bottle is nonliving.

Worked Example 3: Bird feather, fish in a pond, and cloud

Let’s keep going.

• Bird feather: The feather came from a bird, which is a living thing. The feather itself is not alive now. It is dead.
• Fish in a pond: The fish swims, eats, grows, and needs oxygen in the water. It is living.
• Cloud: A cloud is made of tiny drops of water. It was never alive. It is nonliving.

Worked Example 4: Seed, paper, and bicycle

This set is even trickier.

• Seed: A seed can grow into a plant. It is living.
• Paper: Paper is made from trees. Trees are living things, but paper is not alive now. It is dead.
• Bicycle: A bicycle can move only if someone rides it. It does not grow or need water. It was never alive. It is nonliving.

How to classify something step by step

When you see an object, use these steps:

1. Ask: Is it alive right now?
2. If yes, classify it as living.
3. If no, ask: Was it ever alive?
4. If yes, classify it as dead.
5. If no, classify it as nonliving.

Quick practice ideas

Try sorting these on your own:

• cat
• sand
• apple on a tree
• fallen branch
• glass cup

Possible answers:

• Cat: living
• Sand: nonliving
• Apple on a tree: living
• Fallen branch: dead
• Glass cup: nonliving

Things to remember

• Living things are alive now.
• Dead things were alive before, but are not alive now.
• Nonliving things were never alive.
• Moving does not always mean living.
• Not moving does not always mean nonliving.

Summary

Scientists classify objects as living, dead, or nonliving. Living things are alive right now and show signs of life like growing and needing water. Dead things were once alive but are not alive anymore. Nonliving things were never alive at all.

If you forget, ask yourself: Alive now? Alive before? Those two questions can help you choose the correct group.

Essential Requirements for Life means the basic things living things need in order to stay alive. All living things, such as plants, animals, and people, have needs. If these needs are not met, the living thing may become weak, stop growing, or die.

There are three big things living things need:

• Water
• A source of nutrients or energy such as food or sunlight
• Gas exchange, which means taking in and giving off gases from the air

Let’s learn about each one.

1. Living things need water.

Water is very important for life. People and animals drink water. Plants take in water through their roots. Water helps living things grow, stay healthy, and do the jobs their bodies need to do.

If a plant does not get water, its leaves may droop and turn brown. If an animal does not get enough water, it can become very sick. That is why water is one of the most important needs for life.

2. Living things need nutrients or energy.

Animals and people get energy by eating food. Food gives their bodies the power to move, grow, and stay healthy.

Plants are living things too, but they do not eat food the way animals do. Plants use sunlight to make their own food. They also need water and materials from the soil. So, even though plants and animals get energy in different ways, they both need energy to live.

Here are some examples of energy sources:

• A rabbit eats grass.
• A bird eats seeds.
• A person eats fruits, vegetables, and other foods.
• A sunflower uses sunlight to make food.

3. Living things need gas exchange.

Gas exchange means taking in one gas and giving off another gas. Animals and people breathe in oxygen from the air and breathe out carbon dioxide.

Plants also exchange gases. Plants take in carbon dioxide from the air and give off oxygen. This is one more way plants are living things with needs.

Different living things get gases in different ways:

• People breathe with lungs.
• Fish get oxygen from water using gills.
• Plants exchange gases through tiny openings in their leaves.

Living things and nonliving things

Knowing the requirements for life helps us tell the difference between living and nonliving things. A living thing needs water, energy, and gas exchange. A nonliving thing does not.

For example:

• A puppy is living. It drinks water, eats food, and breathes air.
• A tree is living. It takes in water, uses sunlight for energy, and exchanges gases.
• A rock is nonliving. It does not need water, food, or air to stay alive.
• A toy car is nonliving. It does not grow or need life requirements.

All three needs work together.

A living thing cannot stay healthy with just one or two of its needs met. For example, a hamster may have water and air, but if it has no food, it cannot live. A plant may have sunlight and air, but without water, it will not survive.

We can think about it like this:

• Water helps life keep going.
• Energy helps life grow and move.
• Gas exchange helps living things use air in the ways they need.

Worked Example 1: Is it living?

Question: A cat drinks water, eats food, and breathes air. Is it living?

Step 1: Does it need water? Yes.

Step 2: Does it need energy from food? Yes.

Step 3: Does it exchange gases by breathing? Yes.

Answer: Yes, the cat is a living thing.

Worked Example 2: What does a plant need?

Question: A plant is placed by a sunny window, but no one waters it. What important need is missing?

Step 1: The plant has sunlight, so it has a way to make food.

Step 2: The plant is in air, so it can exchange gases.

Step 3: It is not watered, so it does not have water.

Answer: The missing need is water.

Worked Example 3: Living or nonliving?

Question: A bicycle is outside in the air and rain. Is it living?

Step 1: Does the bicycle need water to stay alive? No.

Step 2: Does it need food or sunlight for energy to grow? No.

Step 3: Does it exchange gases to live? No.

Answer: No, a bicycle is nonliving.

Worked Example 4: Compare two living things

Question: How do a fish and a flower both meet their needs for life?

Step 1: Does each need water? Yes. A fish lives in water, and a flower needs water from the soil.

Step 2: Does each need energy? Yes. A fish gets energy from food. A flower gets energy from sunlight.

Step 3: Does each exchange gases? Yes. A fish gets oxygen from water. A flower exchanges gases with the air.

Answer: Both are living because both need water, energy, and gas exchange.

Important idea: Different living things may meet their needs in different ways, but all living things still need the same basic things to survive.

Here is a simple way to remember the essential requirements for life:

• Water to help the body or plant work
• Energy from food or sunlight
• Gas exchange with the air or water

When you study a plant, animal, or other living thing, ask yourself these questions:

1. How does it get water?
2. How does it get energy?
3. How does it exchange gases?

If you can answer those questions, you are thinking like a scientist!

Summary

All living things have basic needs. They need water, a source of nutrients or energy, and gas exchange. Animals, people, and plants may meet these needs in different ways, but every living thing must have them to survive.

Autotrophs vs. Heterotrophs

All living things need energy to live, grow, and stay healthy. Energy helps living things move, grow new parts, heal, and do all the jobs their bodies need to do.

Living things get energy from food. But not all living things get food in the same way. Some living things make their own food, and some living things must eat other living things to get food.

That is where the big science idea comes in:

• Autotrophs make their own food.
• Heterotrophs cannot make their own food, so they get food by eating plants, animals, or both.

Let’s learn what each one means.

What is an autotroph?

An autotroph is a living thing that can make its own food. Many autotrophs use sunlight, air, and water to make food.

The most common autotrophs you know are plants. Grass, flowers, trees, and bushes are all examples of autotrophs.

Plants use sunlight to help them make sugar, which is a kind of food. This process is called photosynthesis. You do not need to memorize that big word right away, but it means using light to make food.

Think of it like this: a plant is like a tiny food factory. It does not need to eat a sandwich or chase food. It uses sunlight to make what it needs.

Examples of autotrophs:

• Grass
• Oak tree
• Rose bush
• Seaweed

What is a heterotroph?

A heterotroph is a living thing that cannot make its own food. It must get energy by eating other living things.

Animals are heterotrophs. People, dogs, cats, birds, fish, and insects all need to eat food to live.

Some heterotrophs eat plants. Some eat animals. Some eat both. But all heterotrophs must get food from somewhere else.

Think of it this way: a heterotroph cannot be its own food factory. It must find food outside its body.

Examples of heterotrophs:

• Rabbit
• Lion
• Human
• Frog

The biggest difference

The main difference is how they get food.

• Autotrophs make their own food.
• Heterotrophs eat other living things for food.

This means plants are often the start of many food chains. They make food first. Then animals get energy by eating plants or by eating other animals that ate plants.

Why plants are so important

Plants help many living things survive. Since plants make food, they give energy to many animals.

For example:

1. Grass makes its own food from sunlight.
2. A rabbit eats the grass.
3. A fox eats the rabbit.

The energy starts with the plant. Without autotrophs, many heterotrophs would not have food.

Easy way to remember

• Auto can help you remember own. An autotroph makes its own food.
• Hetero can help you remember other. A heterotroph gets food from other living things.

Are all plants autotrophs?

For 3rd Grade science, you can think of plants as autotrophs. They make their own food using sunlight.

Are all animals heterotrophs?

Yes. Animals are heterotrophs because they cannot make their own food. They need to eat plants, animals, or both.

Worked Example 1: Classify a sunflower

Question: Is a sunflower an autotroph or a heterotroph?

Step 1: Ask, “Does it make its own food or eat other living things?”

Step 2: A sunflower is a plant.

Step 3: Plants make their own food using sunlight.

Answer: A sunflower is an autotroph.

Worked Example 2: Classify a deer

Question: Is a deer an autotroph or a heterotroph?

Step 1: Ask, “Does it make its own food?”

Step 2: A deer is an animal, and animals do not make their own food.

Step 3: A deer eats plants like grass and leaves.

Answer: A deer is a heterotroph.

Worked Example 3: Follow the food chain

Question: In this food chain, who is the autotroph and who are the heterotrophs?

$$\text{grass} \rightarrow \text{mouse} \rightarrow \text{owl}$$

Step 1: Find the living thing that makes its own food. Grass is a plant, so it makes its own food.

Step 2: Find the living things that must eat to get energy. The mouse eats plants, and the owl eats animals.

Answer:

• Autotroph: grass
• Heterotrophs: mouse and owl

Worked Example 4: Think carefully

Question: A caterpillar eats leaves. Is it an autotroph or a heterotroph?

Step 1: Ask, “Does the caterpillar make its own food?” No.

Step 2: Ask, “Does it eat other living things?” Yes, it eats leaves from a plant.

Answer: A caterpillar is a heterotroph.

Common mistakes to avoid

• Mistake 1: “If an animal eats plants, it is an autotroph.”
  That is not correct. If it eats plants, it is still a heterotroph.
• Mistake 2: “Anything green is an autotroph.”
  Many green plants are autotrophs, but the most important rule is this: Can it make its own food?
• Mistake 3: “People are autotrophs because we eat healthy food.”
  People are heterotrophs because we cannot make our own food inside our bodies the way plants do.

Let’s compare them side by side

• Autotrophs:
  • Make their own food
  • Usually use sunlight
  • Example: plants
• Heterotrophs:
  • Cannot make their own food
  • Must eat plants or animals
  • Example: animals

Quick check

1. A tree is a(n) autotroph because it makes its own food.
2. A dog is a(n) heterotroph because it must eat food.
3. In a food chain, the plant is often the autotroph.
4. A cow is a heterotroph even though it eats plants.

Summary

All living things need energy. Autotrophs make their own food, and heterotrophs must eat other living things to get energy.

Plants are the most common autotrophs you will study. Animals, including people, are heterotrophs.

If you ever feel stuck, ask one simple question: Does it make its own food, or does it eat to get food? That question will help you decide if something is an autotroph or a heterotroph.

Domains and Kingdoms of Life

All living things can be grouped in ways that help us understand them. This is called classification. Classification means sorting living things by what they are like.

Scientists group living things from very big groups to smaller groups. Two very big groups are called domains and kingdoms. For 3rd grade, the most important kingdoms to know are animals, plants, fungi, protists, and bacteria.

A domain is a very large group of living things. A kingdom is a big group inside a domain. You can think of it like this: a domain is a giant box, and kingdoms are smaller boxes inside it.

There are 3 domains:

• Bacteria
• Archaea
• Eukarya

Most of the kingdoms you will learn about belong in the domain called Eukarya. The kingdom Bacteria belongs in the domain Bacteria.

Let’s learn the 5 main kingdoms.

1. Animal Kingdom

Animals are living things that need food from other living things. They do not make their own food. Most animals can move from place to place at some time in their lives.

• Examples: dog, fish, bird, ant, human
• Animals need food, water, air, and space to live
• Animals are part of the domain Eukarya

2. Plant Kingdom

Plants are living things that usually stay in one place. Most plants make their own food using sunlight, air, and water.

• Examples: tree, grass, flower, cactus
• Plants need sunlight, water, air, and space to grow
• Plants are part of the domain Eukarya

3. Fungi Kingdom

Fungi are living things that do not make their own food like plants do. They get food from things around them. Many fungi grow in dark, damp places.

• Examples: mushroom, mold, yeast
• Fungi are not plants and not animals
• Fungi are part of the domain Eukarya

4. Protist Kingdom

Protists are living things that do not fit well into the plant, animal, or fungi kingdoms. Many protists are tiny and live in water or wet places.

• Examples: algae, amoeba
• Some protists make their own food, and some do not
• Protists are part of the domain Eukarya

5. Bacteria Kingdom

Bacteria are very tiny living things. They are so small that we usually need a microscope to see them. Some bacteria are helpful, and some can make us sick.

• Examples: bacteria in yogurt, bacteria in soil
• Bacteria are part of the domain Bacteria
• They are living things even though they are tiny

What about Archaea?

Archaea are tiny living things, like bacteria, but they are in a different domain. Many archaea live in places that are very hot, very salty, or otherwise hard for most living things to live. For this lesson, it is enough to know that Archaea is a domain.

How are the kingdoms different?

• Animals eat food and usually move
• Plants make their own food
• Fungi do not make their own food and often grow on dead matter or damp places
• Protists are a mixed group, often tiny and found in water
• Bacteria are very tiny living things

A helpful way to think about it

If something is a dog, it is in the animal kingdom. If something is a rose bush, it is in the plant kingdom. If something is a mushroom, it is in the fungi kingdom. If something is algae in pond water, it is in the protist kingdom. If something is tiny bacteria in soil, it is in the bacteria kingdom.

Worked Example 1: Sorting easy examples

Question: Is a cat an animal, plant, fungi, protist, or bacteria?

Think: A cat moves, eats food, and does not make its own food.

Answer: A cat is in the animal kingdom.

Worked Example 2: A plant or not?

Question: A sunflower grows in soil and uses sunlight to make food. Which kingdom is it in?

Think: It makes its own food using sunlight.

Answer: A sunflower is in the plant kingdom.

Worked Example 3: A tricky one

Question: A mushroom grows in a damp forest. Is it a plant?

Think: Mushrooms do not make their own food like plants do.

Answer: No. A mushroom is in the fungi kingdom.

Worked Example 4: Tiny living things

Question: You look through a microscope and see tiny living things in yogurt. Which kingdom do they belong to?

Think: Helpful tiny living things in yogurt are bacteria.

Answer: They belong to the bacteria kingdom.

Tips for remembering the kingdoms

• Animals = eat and move
• Plants = make food with sunlight
• Fungi = mushrooms and mold
• Protists = tiny living things, often in water
• Bacteria = very tiny living things seen with a microscope

Why classification matters

Classification helps us study life. When we know what group a living thing belongs to, we can learn what it needs to live, how it grows, and how it is like other living things.

Brief Summary

Living things are sorted into groups called domains and kingdoms. The 3 domains are Bacteria, Archaea, and Eukarya. The 5 main kingdoms you should know are animals, plants, fungi, protists, and bacteria. Animals eat food, plants make food, fungi are things like mushrooms, protists are often tiny and live in water, and bacteria are very tiny living things.

Microorganisms: Bacteria, Viruses, and Fungi

Have you ever wondered if tiny living things are all around you? They are! Some are so small that we cannot see them with just our eyes. These tiny things are called microorganisms, or microbes.

Microorganisms are found in soil, water, air, on plants, and even inside your body. Some microorganisms are helpful, some can make us sick, and some do both in different ways.

In this lesson, you will learn about three kinds of microorganisms: bacteria, viruses, and fungi. You will also learn how they help nature and how some can cause disease.

What Are Microorganisms?

Microorganisms are very tiny. Most are too small to see without a microscope. Even though they are tiny, they do important jobs in the world.

Some microorganisms help break down dead plants and animals. Some help our bodies digest food. Others can spread sickness. That is why it is important to learn about them.

1. Bacteria

Bacteria are tiny living things with just one cell. They are almost everywhere on Earth.

Many bacteria are helpful. For example, some bacteria live in your intestines and help your body break down food. Some bacteria are used to make foods like yogurt and cheese.

Other bacteria help in nature. They break down dead plants and animals. This returns nutrients to the soil so new plants can grow.

But some bacteria can be harmful. Harmful bacteria can cause infections and make people sick. This is one reason we wash our hands and clean cuts.

• Helpful bacteria: help digestion, help make some foods, help decompose dead things
• Harmful bacteria: can cause sickness or infections

2. Viruses

Viruses are even smaller than bacteria. They are so tiny that they need very strong microscopes to be seen.

Viruses are different from bacteria. A virus cannot grow or make more viruses by itself. It needs to get inside a living thing, like a plant, animal, or person, to make more copies.

Many viruses cause disease. Some viruses can give people colds, the flu, or other illnesses. This is why covering coughs, washing hands, and staying home when sick can help stop germs from spreading.

When 1 virus gets inside a body, it can make many more. We can show that it grows in number like this:

$$1 \rightarrow 2 \rightarrow 4 \rightarrow 8$$

This does not happen exactly the same way every time, but it helps us understand that viruses can spread quickly.

3. Fungi

Fungi are living things too. Some fungi are large, like mushrooms, but some are tiny microorganisms.

Mold and yeast are kinds of fungi. You may have seen fuzzy mold growing on old bread or fruit. That mold is a fungus.

Many fungi are helpful decomposers. A decomposer is something that breaks down dead plants and animals. Fungi help clean up nature by breaking down things that have died.

Some fungi are also useful to people. Yeast helps bread rise. Other fungi can sometimes cause problems, like athlete's foot or moldy food.

• Helpful fungi: decompose dead things, help make bread rise
• Harmful fungi: can spoil food or cause some skin problems

How Are They Alike?

Bacteria, viruses, and fungi are all very small. They can all be found in many places around us.

They can also all affect living things. Some help people and nature, while others can cause harm.

How Are They Different?

1. Bacteria are tiny living things made of one cell.
2. Viruses are smaller than bacteria and need a living thing to make more viruses.
3. Fungi can be tiny or larger, and many help decompose dead matter.

Why Are Microorganisms Important?

Microorganisms are important because they help keep nature balanced. Without decomposers like some bacteria and fungi, dead plants and animals would pile up.

Some bacteria help our bodies. They help us digest food so we can get energy and nutrients.

We also need to know about harmful microorganisms so we can stay healthy. Good habits like washing hands, covering sneezes, and eating safe food can help protect us.

Worked Example 1: Helpful or Harmful?

Question: A kind of bacteria lives in your body and helps break down food. Is it helpful or harmful?

Step 1: Think about what it is doing.

It is helping your body break down food.

Step 2: Decide if that job is good or bad for you.

Helping digest food is good for you.

Answer: It is helpful.

Worked Example 2: Which One Needs a Living Thing?

Question: Which microorganism needs to get inside a living thing to make more of itself: bacteria, virus, or fungus?

Step 1: Remember the special fact about each one.

• Bacteria are living one-celled organisms.
• Viruses need a living thing to make more viruses.
• Fungi break down dead matter and can grow on food or in soil.

Step 2: Match the fact to the question.

The one that needs a living thing is the virus.

Answer: Virus.

Worked Example 3: Decomposer Detective

Question: Old leaves fall to the ground. Later, they start to rot and break apart. Which microorganisms might be helping?

Step 1: Think about what happens to dead leaves.

They are being broken down.

Step 2: Ask which microorganisms are decomposers.

Some bacteria and some fungi are decomposers.

Answer: Bacteria and fungi may be helping break down the leaves.

Worked Example 4: Sorting Microorganisms

Question: Put each example in the correct group: mold on bread, a germ that causes a cold, bacteria in your intestines.

Step 1: Identify each example.

• Mold on bread is a fungus.
• A germ that causes a cold is a virus.
• Bacteria in your intestines are bacteria.

Step 2: Sort them.

• Fungi: mold on bread
• Viruses: a germ that causes a cold
• Bacteria: bacteria in your intestines

Answer: Mold on bread = fungus, cold germ = virus, intestines helper = bacteria.

Ways to Stay Healthy Around Microorganisms

• Wash your hands with soap and water.
• Cover your mouth when you cough or sneeze.
• Do not eat moldy food.
• Keep cuts clean.
• Rest and stay home when you are sick, if an adult says you should.

Remember: Not all microorganisms are bad. Many are helpful and necessary. The world needs decomposers, and our bodies need helpful bacteria.

Quick Review

• Bacteria are tiny one-celled living things. Some are helpful, and some are harmful.
• Viruses are smaller than bacteria and need a living thing to make more viruses. Many cause disease.
• Fungi include molds and yeast. Many fungi are decomposers.
• Microorganisms can help digest food, break down dead things, and sometimes cause sickness.

Summary

Microorganisms are tiny things all around us. Bacteria, viruses, and fungi are three kinds of microorganisms. Some help people and nature, like bacteria in our bodies and fungi that decompose dead matter. Some can also cause disease, so healthy habits help keep us safe.

Form Follows Function in Biology

Have you ever noticed that a duck has webbed feet, but a cat does not? Have you seen that a cactus looks very different from a rose bush? In biology, these body shapes and parts are not random.

A big idea in science is form follows function. This means the shape, size, or structure of a living thing's body part helps it do a certain job.

In simple words, form means what something looks like, and function means what it does. A body part's form helps it do its function.

Animals and plants need to stay alive. They need to get food, water, air, and shelter. Their body parts help them do these things. That is why many living things have special shapes that match their needs.

Let’s break down the idea:

• Form = shape or body part
• Function = job or use
• Form follows function = the body part is shaped in a way that helps it do its job

Think about your own body. Your teeth are not all the same shape. Front teeth are flat and sharp for biting. Back teeth are wider for chewing. Their form matches their function.

This idea helps scientists understand why living things look the way they do. If we know what a body part does, we can often guess why it has that shape. If we see a shape, we can also guess its job.

Main Teaching Point 1: Animal body parts have shapes that help them survive.

Animals use their body parts to move, eat, hide, and stay safe. Different animals live in different places, so their body parts may look very different.

For example, birds have different kinds of beaks. A hummingbird has a long, thin beak. This helps it reach into flowers for nectar. A hawk has a sharp, curved beak. This helps it tear food.

Both are birds, but their beaks have different forms because they have different functions.

Feet are another good example. Ducks have webbed feet that help them paddle through water. Eagles have strong claws that help them catch prey. A duck's feet and an eagle's feet do different jobs, so they have different shapes.

Body covering can also match function. Polar bears have thick fur to keep warm in cold places. Fish have scales that help protect their bodies in water. The form of the body covering helps with survival.

Main Teaching Point 2: Plant parts also have shapes that help them do their jobs.

Plants are living things too, and their parts have important jobs. Roots take in water. Stems hold plants up. Leaves help plants make food from sunlight.

Different plants may have different kinds of leaves. A cactus has thick, small parts instead of big, soft leaves. This helps it live in a dry place and keep water inside.

A water lily has broad, flat leaves that float on water. This shape helps the plant spread out on the surface and get sunlight.

Roots also show form follows function. Some plants have deep roots to reach water far underground. Some plants have many spreading roots near the top of the soil to quickly soak up rain.

Flowers can have bright colors and special shapes. These forms help attract bees, butterflies, and other animals that help with pollination.

Main Teaching Point 3: We can ask two helpful questions.

When you study a plant or animal part, ask:

1. What does it look like? This tells you about its form.
2. What job does it do? This tells you about its function.

Then connect the two ideas. Ask, How does this shape help it do its job?

For example, if a bird has a long beak, you can ask how that long shape helps it get food. If a plant has wide leaves, you can ask how that shape helps it get sunlight.

Worked Example 1: Duck Feet

Question: A duck has webbed feet. How does the form of its feet help its function?

Step 1: Identify the form. The duck's feet have skin between the toes. This makes the feet wide and flat.

Step 2: Identify the function. Ducks swim in water.

Step 3: Connect them. The wide, webbed feet push against the water better than narrow feet. This helps the duck paddle and swim.

Answer: The duck's webbed feet are shaped to help it swim.

Worked Example 2: Cactus

Question: A cactus has thick stems and very small spines instead of big leaves. How does its form help its function?

Step 1: Identify the form. The cactus has a thick body and very small spines.

Step 2: Identify the function. The cactus needs to live in a dry place and save water.

Step 3: Connect them. The thick stem helps store water. The small spines help the cactus lose less water than big leaves would.

Answer: The cactus has parts shaped to help it keep water in a dry habitat.

Worked Example 3: Eagle Claws

Question: An eagle has sharp, curved claws. Why are these claws useful?

Step 1: Identify the form. The claws are sharp and bent.

Step 2: Identify the function. The eagle uses its feet to catch and hold food.

Step 3: Connect them. Sharp, curved claws can grab tightly and help keep prey from slipping away.

Answer: The eagle's claws are shaped to help it catch and hold food.

Worked Example 4: Water Lily Leaves

Question: A water lily has broad, flat leaves that float. How does this form help the plant?

Step 1: Identify the form. The leaves are wide, flat, and float on the water.

Step 2: Identify the function. The plant needs sunlight to make food.

Step 3: Connect them. Floating, flat leaves spread out on top of the water where they can get lots of sunlight.

Answer: The water lily's leaves are shaped to help the plant get sunlight.

More Examples of Form and Function

• Giraffe neck: A long neck helps reach leaves high in trees.
• Rabbit ears: Large ears help the rabbit hear danger.
• Fish fins: Fins help fish swim and steer.
• Tree bark: Bark helps protect the tree.
• Roots: Roots hold the plant in the ground and take in water.

Be Careful!

Not all body parts do just one job. Some parts can help in more than one way. For example, bird feathers can help with flying, warmth, and protection.

Also, living things that do similar jobs may have parts that look alike. This is because the shape helps with the same function. For example, many swimming animals have body parts that help them move through water.

How to Solve Questions About Form Follows Function

1. Look at the body part carefully.
2. Name its shape, size, or special feature.
3. Think about what job the part does.
4. Explain how the shape helps with that job.

You can use this sentence frame:

The _____ has a _____ shape, and this helps it _____.

Example: The duck has webbed feet, and this helps it swim.

Quick Check

• If a flower is bright and colorful, what job might that help with? It may help attract pollinators.
• If a mole has strong front paws, what job might that help with? It may help dig underground.
• If a plant has deep roots, what job might that help with? It may help reach water deep in the soil.

Summary

In biology, form follows function means a living thing's body parts are shaped in ways that help them do important jobs. Animals and plants have many special parts that help them get food, move, stay safe, and live in their habitats.

When you look at a beak, leaf, claw, root, or any other part, ask what it looks like and what it does. Then connect the shape to the job. That is how scientists use the idea of form follows function.

Using Dichotomous Keys

Have you ever tried to figure out what something is by answering questions about it? In science, we can do that with a tool called a dichotomous key.

A dichotomous key helps us identify, or name, a living thing by making choices. Each step gives us two answers to choose from. Then we follow the choice that matches what we see.

The word dichotomous means divided into two parts. That is why each step in the key has only 2 choices.

Scientists use dichotomous keys to sort plants, animals, and other living things. They look at observable traits. Observable traits are features we can notice with our eyes, like color, number of legs, wings, or body covering.

When we use a dichotomous key, we should use objective traits. Objective means something that is true and can be checked. For example, has 6 legs is objective. But looks cute is not objective, because different people may think different things.

Using a dichotomous key is like playing a guessing game, but in science we use careful observation and clear facts.

Main Ideas to Remember

• A dichotomous key helps identify a living thing.
• Each step has 2 choices.
• We choose the answer that matches what we observe.
• We keep following the steps until we reach the name of the organism.
• We use traits we can see clearly and describe fairly.

How to Use a Dichotomous Key

1. Look carefully at the organism.
2. Read both choices at step 1.
3. Pick the choice that matches what you see.
4. Go to the next step the key tells you to follow.
5. Keep choosing between 2 answers.
6. Stop when you reach a name.

Helpful Tips

• Read both choices before deciding.
• Do not guess too fast.
• Look for traits that are easy to check, like wings, fur, or number of legs.
• If one choice does not fit, try the other one.
• Use what you see, not what you think might be true.

Example 1: A Simple Key

Let’s identify an animal using this dichotomous key.

1. Has feathers → bird
   Does not have feathers → go to 2
2. Has fur → cat
   Does not have fur → fish

Question: What is an animal with feathers?

Worked Example: We start at step 1. The animal has feathers, so we choose that answer. The key tells us it is a bird.

Answer: bird

Example 2: Follow More Than One Step

Use the same key again.

1. Has feathers → bird
   Does not have feathers → go to 2
2. Has fur → cat
   Does not have fur → fish

Question: What is an animal that does not have feathers and does not have fur?

Worked Example: At step 1, we choose does not have feathers, so we go to step 2. At step 2, we choose does not have fur. The key tells us it is a fish.

Answer: fish

Example 3: A Plant Key

Now let’s try a dichotomous key with plants.

1. Has flowers → go to 2
   Does not have flowers → fern
2. Flower is yellow → sunflower
   Flower is not yellow → rose

Question: A plant has flowers, and the flowers are yellow. What is it?

Worked Example: At step 1, we see the plant has flowers, so we go to step 2. At step 2, the flower is yellow. The key says it is a sunflower.

Answer: sunflower

Example 4: A Bigger Animal Key

Let’s use a key with more steps.

1. Has wings → go to 2
   Does not have wings → go to 3
2. Has feathers → eagle
   Does not have feathers → butterfly
3. Lives only in water → fish
   Does not live only in water → go to 4
4. Has fur → dog
   Does not have fur → frog

Question: An animal does not have wings, does not live only in water, and has fur. What is it?

Worked Example: At step 1, we choose does not have wings, so we go to step 3. At step 3, the animal does not live only in water, so we go to step 4. At step 4, it has fur. The key tells us it is a dog.

Answer: dog

Why Dichotomous Keys Are Useful

• They help us organize living things.
• They help us compare traits.
• They help us identify unknown organisms.
• They help scientists describe life in a careful way.

Good Traits to Use in a Key

• has wings / does not have wings
• has flowers / does not have flowers
• has fur / does not have fur
• has 6 legs / does not have 6 legs
• lives in water / does not live in water

Traits to Avoid

• pretty / not pretty
• funny / not funny
• nice / mean

These are not good science traits because different people may disagree. A dichotomous key works best when the choices are clear and easy to observe.

Make Your Own Small Key

Here is one way to build a simple dichotomous key.

1. Pick a small group of living things, like a bird, a fish, a cat, and a butterfly.
2. Choose a trait that splits them into 2 groups.
3. For each new group, choose another trait that splits it again.
4. Keep going until each living thing has its own name.

For example, if you start with 4 organisms, you can split them into 2 groups of 2, then split each group again. That means:

$$4 = 2 + 2$$

This shows how a dichotomous key breaks a group into smaller groups by using two choices each time.

Watch Out for These Mistakes

• Choosing too quickly without looking carefully
• Using a trait that is hard to see
• Forgetting to read both choices
• Using opinions instead of facts

Let’s Review

A dichotomous key is a science tool for identifying living things. It gives two choices at each step. You look at the organism, choose the matching trait, and follow the path until you find its name.

When using a dichotomous key, remember to observe carefully, use clear traits, and take one step at a time. If you do that, you can identify many plants and animals like a scientist!