Living things: cells, systems, and heredity (grade 6) · lesson

All living things are made of cells

💡 Cells are the basic units of life: every living thing is made of cells, and new cells come from old cells.

Every living thing is made of one or more cells. Nonliving things, like sand or plastic, are not made of cells.

Most cells are too small to see with your eyes alone. A microscope makes them look much bigger.

All cells have a cell membrane. And new cells come only from cells that already exist. These ideas are called the cell theory.

🧠 Let's work it out

To decide if a speck is living, look for the one sign that only living things have.
Moving, being green, or being small can all be true of nonliving things.
Being made of cells is not.

What do you think comes next?

cell: the smallest unit of a living thing
microscope: a tool that makes tiny things look bigger

🔗 Khan Academy: Cells and organisms · Khan Academy ↗

Cell parts and their jobs

💡 Each part of a cell has a job, like a gate, a control center, a power plant, or a storage tank.

The cell membrane is the gate. It lets some things in and keeps others out. The nucleus holds the DNA, which directs the cell.

Mitochondria are the power plants. They break down food to release energy the cell can use.

A vacuole stores water and other materials. Plant cells have a large vacuole. When it loses water, the plant goes limp and droops.

🧠 Let's work it out

Match the clue to the job.
Something got sorted at the edge of the cell? That is the membrane.
Something about storing water? That is the vacuole.
Something about energy from food? That is the mitochondria.

What do you think comes next?

cell membrane: the thin outer layer that controls what goes in and out
nucleus: the part that holds the cell's DNA
mitochondria: parts that release energy from food
vacuole: a storage space for water and other materials

Plant cells and animal cells

💡 Plant cells have a cell wall and often chloroplasts, which animal cells do not have.

Plant and animal cells both have a membrane, a nucleus, and mitochondria.

Plant cells also have a stiff cell wall outside the membrane. It gives them a boxy shape.

Many plant cells have chloroplasts. These green parts use light to make food. Plant parts that grow in the dark, like an onion bulb, have no chloroplasts and are not green.

🧠 Let's work it out

Use clues like a detective.
Boxy walls mean a plant.
Green chloroplasts mean a plant part that gets light, like a leaf.
No wall means an animal cell.

What do you think comes next?

cell wall: a stiff layer outside the membrane of plant cells
chloroplast: a green part of a plant cell that makes food from light

From cells to organisms

💡 Cells form tissues, tissues form organs, organs work together in systems, and systems make an organism.

In a body, cells that do the same job group into a tissue. Muscle tissue is one example.

Different tissues form an organ, like the heart. Organs that work together form an organ system. All the systems together make an organism.

Cells in one body come in many shapes. A cell's shape fits its job. Nerve cells are long so they can carry signals far.

🧠 Let's work it out

When you see an odd cell shape, ask what the cell does.
Red blood cells are small, flat, and round.
That shape helps them slip through tiny blood vessels.

What do you think comes next?

tissue: a group of similar cells doing one job
organ: a body part made of different tissues, like the heart
organism: a whole living thing

Body systems work together

💡 Body systems work together to bring every cell what it needs.

The respiratory system brings oxygen into your lungs. The circulatory system, your heart and blood, carries it to every cell.

The digestive system breaks food into small nutrients. The nutrients pass into the blood, and the blood carries them to the cells.

Your cells then use the oxygen and nutrients to get energy. When you run, your muscles need more of both.

🧠 Let's work it out

Trace the path.
Food: mouth and stomach break it down, then the small intestine passes nutrients to the blood, then blood carries them to muscles, then cells use them.
Each step needs the one before it.

What do you think comes next?

respiratory system: the lungs and airways that bring in oxygen
circulatory system: the heart and blood vessels that carry blood
digestive system: the organs that break down food