Biology rulebook stating that: (1) all living things are made of one or more cells, (2) the cell is the basic unit of structure and function, and (3) all cells come from pre-existing cells.

A small, simple cell without a nucleus or membrane-bound organelles; examples include bacteria and archaea.

A larger, more complex cell with a nucleus and membrane-bound organelles; found in plants, animals, fungi, and protists.

Thin, flexible boundary made mainly of phospholipids and proteins that controls what enters and leaves the cell.

Membrane-bound organelle that stores DNA and controls the cell’s activities.

Adenosine triphosphate; the main energy-carrying molecule used by cells.

Membrane-bound organelle that contains DNA and controls the cell’s activities.

The diffusion of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.

Cells: The Basic Units of Life — SHS Science Foundations: Integrated Chemistry, Biology, and Physics

Q: You see a cell under a microscope. It has a nucleus, many mitochondria, no cell wall, and no chloroplasts. What type of cell is it most likely?

Animal cell. The cell has a nucleus and mitochondria, so it is eukaryotic. It has no cell wall and no chloroplasts, which fits an **animal cell**. Plant cells usually have a cell wall and, in green tissues, chloroplasts.

From Atoms to Cells: Zooming In on Life

From Atoms to Cells

Atoms bond to form molecules. Molecules build up cells, which are the basic units of life. We are zooming out one level: from chemistry to living structures.

The Microscopic City Idea

Picture a cell as a tiny city. Organelles are power plants, factories, and storage centers. The cell membrane is a city wall with gates controlling what goes in and out.

Everything Alive Has Cells

A bacterium is 1 cell. You have about 30 trillion cells. Trees, animals, and fungi are all made of cells organized into tissues and organs.

What You Will Learn

We will explore cell theory, prokaryotic vs eukaryotic cells, key organelles, plant vs animal cells, and how diffusion and osmosis move substances across cell membranes.

Cell Theory: The Three Big Ideas

Cell Theory Overview

Cell theory is a rulebook for life: it explains what living things are made of and how new cells appear. It has three main statements that are still accepted today.

Idea 1: Made of Cells

All living things are made of one or more cells. Bacteria and many protists have just one cell; plants, animals, and fungi have many cells.

Idea 2: Basic Unit of Life

The cell is the smallest unit that can do all life processes: use energy, respond, reproduce, and keep internal conditions stable.

Idea 3: Cells from Cells

All cells come from pre-existing cells by cell division. This rejected the old idea that life could suddenly appear from nonliving matter.

Prokaryotic vs Eukaryotic Cells: Two Basic Designs

Two Cell Designs

Cells come in two main types: prokaryotic and eukaryotic. The big differences involve whether they have a nucleus and membrane-bound organelles.

Prokaryotic Cells

Prokaryotic cells (bacteria and archaea) are usually small and simple. They have no nucleus and no membrane-bound organelles. DNA sits in a nucleoid region.

Eukaryotic Cells

Eukaryotic cells (plants, animals, fungi, protists) are larger and more complex. They have a nucleus and organelles like mitochondria and, in plants, chloroplasts.

Memory Trick

Remember: prokaryotes = "pro–no" nucleus. Eukaryotes = "eu–true" nucleus. Both designs are successful and exist all around you.

Classify That Cell!

Use this quick mental activity to practice spotting prokaryotic vs eukaryotic cells.

For each description, decide: Prokaryotic (P) or Eukaryotic (E). Then check yourself with the hints.

A cell with DNA in a nucleus, many mitochondria, and no cell wall.

Your answer: P or E?
Hint: Nucleus present, many organelles.

A single-celled organism with no nucleus, a cell wall, and a circular DNA molecule.

Your answer: P or E?
Hint: DNA not in a nucleus.

A cell with chloroplasts, a large central vacuole, and a rigid outer wall made of cellulose.

Your answer: P or E?
Hint: Chloroplasts and cellulose wall.

A tiny cell living in your gut, helping digest food, with no membrane-bound organelles.

Your answer: P or E?
Hint: Lives as bacteria in your body.

Self-check:

E (likely an animal cell)
P (typical bacterium)
E (plant cell)
P (gut bacterium)

If you missed any, look back at the “Two Basic Designs” step and focus on the presence or absence of a nucleus and organelles.

Key Cell Structures: The Main Organelles

Organelles as Departments

Eukaryotic cells contain organelles, each with a specific job, like departments in a company: boundary, control center, power plants, food factories, and storage.

Membrane and Cytoplasm

The cell membrane is a flexible boundary made of phospholipids and proteins, controlling what enters and leaves. The cytoplasm is a jelly-like fluid where organelles sit and reactions occur.

Nucleus and DNA

The nucleus is the control center. It is surrounded by a nuclear envelope and contains DNA, which stores the instructions for making proteins and running the cell.

Mitochondria and Chloroplasts

Mitochondria release energy from food as ATP. Chloroplasts (in plants and some protists) capture light energy to make glucose by photosynthesis.

Vacuoles

Vacuoles are storage sacs. Plant cells often have a large central vacuole for water and support. Animal cells usually have smaller vacuoles for storage and transport.

Real-World Connections: Organelles in Action

Mitochondria and Exercise

During running or sports, muscle cells need lots of ATP. Their many mitochondria release energy from glucose and oxygen to power your movement.

Chloroplasts and Light

Houseplants in low light become weak because their chloroplasts cannot capture enough light to make glucose, reducing the plant’s energy supply.

Vacuoles and Wilting

When plants lack water, central vacuoles shrink, cells lose pressure, and the plant wilts. After watering, vacuoles refill and the plant stands upright again.

Membranes and Medicine

Many medicines must cross the cell membrane to work. Scientists design drugs to diffuse through or be transported across membranes effectively.

Plant vs Animal Cells: Spot the Differences

Shared Features

Both plant and animal cells are eukaryotic. They share a nucleus, mitochondria, cell membrane, cytoplasm, and usually some vacuoles.

Cell Walls

Plant cells have a rigid cell wall made mostly of cellulose, outside the membrane. Animal cells lack a cell wall, so they are more flexible in shape.

Chloroplasts

Chloroplasts are found in plant cells (and some protists) for photosynthesis. Animal cells do not have chloroplasts and must get food by eating.

Vacuole Size

Plant cells usually contain a large central vacuole for water storage and support. Animal cells have smaller vacuoles or sometimes none.

Visual Clues

Boxy cell, wall, chloroplasts, and big vacuole = plant cell. Rounded or irregular shape, no wall, no chloroplasts = animal cell.

Quick Check: Plant or Animal?

Use what you know about plant and animal cells to answer this question.

You see a cell under a microscope. It has a nucleus, many mitochondria, no cell wall, and no chloroplasts. What type of cell is it most likely?

Plant cell
Animal cell
Bacterial cell

Show Answer

Answer: B) Animal cell

The cell has a nucleus and mitochondria, so it is eukaryotic. It has no cell wall and no chloroplasts, which fits an **animal cell**. Plant cells usually have a cell wall and, in green tissues, chloroplasts.

Diffusion and Osmosis: How Stuff Moves In and Out

What Is Diffusion?

Diffusion is the spreading out of particles from higher concentration to lower concentration. It happens with gases, liquids, and dissolved particles.

Diffusion Example

Perfume sprayed in one corner of a room spreads until you can smell it everywhere. In cells, oxygen diffuses from high concentration areas into cells that use it.

What Is Osmosis?

Osmosis is the diffusion of water across a selectively permeable membrane, from more water (more dilute) to less water (more concentrated).

Effects on Cells

In pure water, cells gain water; in very salty or sugary solutions, cells lose water. Too much water in or out can damage cells.

Plant Cell Protection

Plant cells have a cell wall and large central vacuole that help manage water flow and keep the plant firm without bursting.

Thought Experiment: Raisins, Potatoes, and Water

Use this thought experiment to apply diffusion and osmosis.

Raisin in pure water

Imagine a dry raisin (a shriveled grape) dropped into a cup of pure water and left overnight.
Predict what happens to the raisin’s size.
Explanation: Inside the raisin there is a concentrated sugar solution. Water moves into the raisin by osmosis, so it swells.

Potato in salty water

Two potato slices: one in pure water, one in very salty water.
Predict how each slice feels after 30 minutes.
In pure water: water moves into the cells; the slice feels firm.
In salty water: water moves out of the cells; the slice feels soft and flexible.

Link to your cells

Think about why drinking extremely salty seawater is dangerous.
Hint: It changes the concentration around your cells, pulling water out of them by osmosis.

If you can explain each scenario in terms of “water moves from where it is more concentrated to where it is less concentrated,” you are getting the core idea of osmosis.

Key Term Review: Cells and Transport

Use these flashcards to review the main vocabulary from this module.

Cell theory: Biology rulebook stating that: (1) all living things are made of one or more cells, (2) the cell is the basic unit of structure and function, and (3) all cells come from pre-existing cells.
Prokaryotic cell: A small, simple cell without a nucleus or membrane-bound organelles; examples include bacteria and archaea.
Eukaryotic cell: A larger, more complex cell with a nucleus and membrane-bound organelles; found in plants, animals, fungi, and protists.
Cell membrane: Thin, flexible boundary made mainly of phospholipids and proteins that controls what enters and leaves the cell.
Nucleus: Membrane-bound organelle that stores DNA and controls the cell’s activities.
Mitochondrion (plural: mitochondria): Organelle that releases energy from food molecules to produce ATP; often called the powerhouse of the cell.
Chloroplast: Green organelle in plant cells and some protists where photosynthesis uses light energy to make food (glucose).
Vacuole: Membrane-bound storage sac; plant cells usually have a large central vacuole for water and support.
Diffusion: Movement of particles from an area of higher concentration to an area of lower concentration.
Osmosis: Diffusion of water across a selectively permeable membrane from an area of higher water concentration to lower water concentration.

Final Check: Putting It All Together

Test your understanding of cells, organelles, and transport.

Which statement best combines cell structure and transport concepts from this module?

Only plant cells use diffusion because they have cell walls.
All cells use diffusion and osmosis across the cell membrane to move substances, regardless of whether they are prokaryotic or eukaryotic.
Eukaryotic cells do not need diffusion because they have organelles.