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Cell Structure and Function

Introduction

Every living organism is made of one or more cells. A cell is the smallest unit that can carry out the basic processes of life. Your body contains many kinds of cells, and plants, fungi, protists, bacteria, and archaea are also made of cells. In this aiMOOC, you will explore how cell structures are connected to their functions and why different cells are built differently.

This course is designed for Grades 7–8. By the end, you should be able to identify major cell structures, explain what they do, compare plant, animal, and prokaryotic cells, use a light microscope safely, and apply the idea that structure supports function.

Cells as the Basic Units of Life

Cells are the basic structural and functional units of living things. Some organisms, such as many bacteria, consist of a single cell. Other organisms, including humans and trees, are multicellular and contain many specialized cells that work together.

Most cells share four important features:

Cell membrane: A thin boundary that controls movement of substances into and out of the cell.
Cytoplasm: The material inside the cell where many chemical reactions happen.
DNA: Genetic information that contains instructions for cell activities and inheritance.
Ribosome: A structure that builds proteins.

Cells are small because small cells can exchange materials with their surroundings efficiently. As a cell grows, its volume increases faster than its surface area. This makes transport across the cell membrane more difficult, so cells usually stay microscopic.

Cell Theory

Cell theory summarizes several major ideas in biology:

All living organisms are made of one or more cells.
The cell is the basic unit of structure and function in living things.
New cells arise from existing cells.

These ideas developed as microscopes improved and scientists were able to observe cells more clearly. Cell theory connects the study of tiny cells to the organization of whole organisms.

Prokaryotic and Eukaryotic Cells

Cells can be grouped into two broad types: prokaryotic and eukaryotic.

A prokaryotic cell does not have a membrane-bound nucleus. Its DNA is located in a region called the nucleoid. Prokaryotic cells also lack membrane-bound organelles such as mitochondria and chloroplasts. Bacteria and archaea are prokaryotes.

A eukaryotic cell has a nucleus that is surrounded by a membrane. Eukaryotic cells also contain membrane-bound organelles with specialized jobs. Animals, plants, fungi, and protists are eukaryotes.

A useful comparison is:

Feature	Prokaryotic cell	Eukaryotic cell
Nucleus	No membrane-bound nucleus	Membrane-bound nucleus present
Membrane-bound organelles	Generally absent	Present
DNA	In a nucleoid region	Mainly inside the nucleus
Typical examples	Bacteria and archaea	Animals, plants, fungi, and protists

Major Cell Structures and Their Functions

Cell structures are often called organelles when they carry out specialized functions inside eukaryotic cells. Think of a cell as a coordinated system: each part has a job, and the parts depend on one another.

Cell Membrane

The cell membrane forms the outer boundary of every cell. It is selectively permeable, which means some substances can cross more easily than others. This helps the cell maintain stable internal conditions.

Small molecules can sometimes move across the membrane by diffusion, from an area of higher concentration to an area of lower concentration. Osmosis is the diffusion of water across a selectively permeable membrane. Some substances require transport proteins, and active transport uses cellular energy to move substances against a concentration gradient.

Cytoplasm and Cytoskeleton

The cytoplasm fills much of the cell interior. It contains water, dissolved substances, and many cell structures. Numerous chemical reactions take place there.

The cytoskeleton is a network of protein fibers that helps a eukaryotic cell keep its shape, organize its internal parts, and move materials within the cell.

Nucleus

The nucleus contains most of a eukaryotic cell's DNA. DNA carries instructions for making proteins and regulating cell activities. The nuclear membrane separates the nucleus from the cytoplasm while allowing controlled exchange through nuclear pores.

Ribosomes

Ribosomes build proteins by linking amino acids in the order specified by genetic instructions. Ribosomes are found in both prokaryotic and eukaryotic cells. Some float freely in the cytoplasm, while others are attached to rough endoplasmic reticulum in eukaryotic cells.

Endoplasmic Reticulum and Golgi Apparatus

The endoplasmic reticulum, or ER, is a membrane network. Rough ER has ribosomes attached and helps make and process proteins. Smooth ER lacks attached ribosomes and has roles that include making lipids and processing certain chemicals.

The Golgi apparatus modifies, sorts, and packages proteins and lipids into small membrane-bound sacs called vesicles. These vesicles can transport materials to other parts of the cell or to the cell membrane.

Mitochondria

Mitochondria release usable energy from food molecules during cellular respiration. The energy is captured mainly in a molecule called ATP, which cells use to power many activities.

Cells that need large amounts of energy often contain many mitochondria. For example, muscle cells use large amounts of energy for contraction.

Chloroplasts

Chloroplasts are found in plants and many algae. They contain chlorophyll and carry out photosynthesis, using light energy to help make sugars from carbon dioxide and water.

Chloroplasts connect cell structure to an organism's way of obtaining energy: plants make energy-rich food molecules using light, while animals obtain food by eating other organisms.

Vacuoles and Lysosomes

Vacuoles store substances such as water, ions, nutrients, and wastes. Plant cells often contain one large central vacuole. When filled with water, it helps support the cell by pressing outward against the cell wall.

Lysosomes contain digestive enzymes that break down worn-out cell parts and other materials. They are especially common in animal cells.

Plant and Animal Cells

Plant and animal cells are both eukaryotic, so both have a nucleus, cell membrane, cytoplasm, ribosomes, mitochondria, endoplasmic reticulum, and Golgi apparatus. They also have important differences.

Animal cells do not have a cell wall or chloroplasts. Their outer boundary is the cell membrane, and their shape may vary widely depending on function.

Plant cells have a cell wall outside the cell membrane. The cell wall provides support and helps maintain shape. Plant cells commonly contain chloroplasts for photosynthesis and a large central vacuole that stores water and helps maintain pressure.

Structure	Plant cell	Animal cell
Cell membrane	Present	Present
Nucleus	Present	Present
Mitochondria	Present	Present
Cell wall	Present	Absent
Chloroplasts	Present in photosynthetic cells	Absent
Large central vacuole	Usually present	Absent

Structure Supports Function

A major idea in biology is that the shape and internal organization of a cell help it perform its job.

A few examples:

Red blood cell: Its flexible shape helps it move through narrow blood vessels and carry oxygen.
Neuron: Long extensions help a nerve cell send signals over distances.
Muscle cell: Specialized protein structures and many mitochondria support contraction and high energy use.
Root hair cell: A long projection increases surface area for absorbing water and minerals from soil.
Palisade cell: Many chloroplasts help this plant cell absorb light for photosynthesis.

When you study a cell, ask two questions: What structure do I notice? and How might that structure help the cell do its job?

Observing Cells with a Microscope

Most cells are too small to see clearly with the unaided eye. A light microscope uses visible light and lenses to magnify specimens.

Important parts include the eyepiece, objective lenses, stage, light source, focus controls, and diaphragm. When using a microscope, begin with the lowest-power objective. Center the specimen before increasing magnification, and use fine focus at higher power.

Total magnification equals eyepiece magnification multiplied by objective magnification. For example, a 10× eyepiece used with a 40× objective gives 400× total magnification.

Microscope images can help you compare cell size, shape, arrangement, and visible structures. However, not every organelle can be seen with a classroom light microscope because some structures are too small or transparent.

From Cells to Organisms

In multicellular organisms, organization builds upward:

Cell: The smallest living unit.
Tissue: A group of similar cells working together.
Organ: A structure made of different tissues working together.
Organ system: A group of organs that cooperate in major body functions.
Organism: A complete living thing.

This hierarchy shows that cell function matters far beyond the individual cell. If cells cannot perform their roles, tissues and organs may not work properly.

Interactive Tasks

Quiz: Test Your Knowledge

Which structure controls what enters and leaves a cell? (Cell membrane) (!Cell wall) (!Nucleus) (!Vacuole)

Which structure contains most of the DNA in a eukaryotic cell? (Nucleus) (!Golgi apparatus) (!Mitochondrion) (!Cell wall)

Which structure builds proteins? (Ribosome) (!Chloroplast) (!Vacuole) (!Lysosome)

Which organelle releases usable energy during cellular respiration? (Mitochondrion) (!Nucleus) (!Cell wall) (!Golgi apparatus)

Which structure is found in plant cells but not animal cells? (Chloroplast) (!Cell membrane) (!Ribosome) (!Mitochondrion)

What is a main function of the Golgi apparatus? (Sorting and packaging cell products) (!Storing genetic information) (!Making light energy) (!Controlling microscope focus)

Which statement best describes a prokaryotic cell? (It lacks a membrane-bound nucleus) (!It always contains chloroplasts) (!It has many membrane-bound organelles) (!It is always part of a multicellular organism)

What is osmosis? (Movement of water across a selectively permeable membrane) (!Movement of proteins into the nucleus) (!Production of ATP in mitochondria) (!Breaking down food inside lysosomes)

Why does a root hair cell have a long projection? (To increase surface area for absorption) (!To store DNA outside the nucleus) (!To produce animal hormones) (!To replace the cell membrane)

What should you do first when viewing a slide with a light microscope? (Start with the lowest-power objective) (!Use the highest-power objective) (!Remove the stage) (!Close the light source completely)

Memory Game

Nucleus	Contains most DNA in a eukaryotic cell
Ribosome	Builds proteins
Mitochondrion	Releases usable energy from food molecules
Chloroplast	Carries out photosynthesis
Golgi apparatus	Sorts and packages proteins and lipids
Vacuole	Stores water and other substances

Drag and Drop

Match the correct terms.	Topic
Selective barrier	Cell membrane
Genetic control center	Nucleus
Protein builder	Ribosome
Energy-releasing organelle	Mitochondrion
Photosynthesis organelle	Chloroplast

...

Crossword Puzzle

Nucleus	Which organelle contains most DNA in a eukaryotic cell?
Ribosome	Which structure builds proteins?
Vacuole	Which structure stores water and other substances?
Cytoplasm	What material fills much of the cell interior?
Chloroplast	Which organelle carries out photosynthesis?
Membrane	What cell boundary controls movement of substances?

LearningApps

Cloze Text

A cell is the smallest unit that can carry out the basic processes of

. Every cell has a boundary called the

. The material inside the cell includes the

. Genetic information is stored in

. In eukaryotic cells, most DNA is enclosed inside the

. Proteins are built by

. Mitochondria help release usable energy in the form of

. Plant cells can use light energy because they contain

. A plant cell wall helps provide shape and

. Water moving across a selectively permeable membrane is called

. A prokaryotic cell lacks a membrane-bound

. Scientists use a light microscope to observe cells that are too small to see with the

.

Open-Ended Tasks

Easy

Cell sketch: Draw a simple animal cell and label at least six structures. Add one short function beside each label.
Plant cell comparison: Create a two-column chart showing four structures shared by plant and animal cells and three features that differ.
Organelle analogy: Choose five organelles and compare each one to a part of a school, city, or factory. Explain why each comparison fits.
Microscope vocabulary: Make an illustrated vocabulary card for five microscope parts and explain what each part does.

Standard

Onion cell investigation: Observe prepared onion cells or a teacher-provided microscope image. Record shape, arrangement, visible structures, and two questions raised by your observation.
Cell model: Build a three-dimensional model of a plant or animal cell from safe household or craft materials and provide a legend explaining each structure.
Specialized cell profile: Research one specialized cell such as a neuron, red blood cell, muscle cell, or root hair cell. Create a one-page profile explaining how its structure supports its function.
Diffusion demonstration: Carry out a safe teacher-approved diffusion demonstration using water and food coloring. Record observations and explain the movement using concentration differences.

Advanced

Microscopy comparison: Compare two microscope images of different cell types. Write a short scientific report explaining which structures are visible, which are not visible, and why.
Cell interview: Interview a biology teacher, laboratory worker, medical professional, or researcher about how understanding cells matters in their work. Summarize three insights and connect them to this course.
Cell transport investigation: Design a teacher-approved investigation using dialysis tubing or another safe model membrane to explore movement of water or dissolved substances. Identify variables and explain your results.
Cell documentary: Produce a two- to four-minute video that follows a protein from its production at a ribosome through processing and transport in a eukaryotic cell. Use correct scientific vocabulary and a clear visual sequence.

Learning Assessment

Structure and function explanation: Choose three cell structures and explain how the physical features of each structure help it perform its function.
Unknown cell analysis: You observe a cell with a cell wall, chloroplasts, and a large central vacuole. Identify the cell type and justify your answer with evidence.
Prokaryote comparison: Explain two features shared by prokaryotic and eukaryotic cells and two important structural differences between them.
Transport reasoning: Predict what may happen to a cell when water moves rapidly across its membrane, and explain your prediction using osmosis.
Microscope problem: A student begins with the highest-power objective and cannot find the specimen. Explain what the student should change and why.
Specialized cell transfer: Invent a cell with a specific job and describe at least three structural adaptations that would help it perform that job effectively.

Evidence of Learning

Strong evidence of learning can include:

Knowledge: Accurate use of terms such as cell membrane, cytoplasm, DNA, nucleus, ribosome, mitochondrion, chloroplast, vacuole, cell wall, diffusion, and osmosis.
Understanding: Clear explanations of how organelles work together and why plant, animal, and prokaryotic cells differ.
Scientific skills: Safe microscope use, careful observation, labeled biological drawings, comparison of evidence, and accurate recording of results.
Products: A cell model, scientific diagram, investigation report, comparison chart, interview summary, or short educational video.
Reasoning: Ability to connect a structure to its function instead of only naming cell parts.
Transfer: Ability to apply cell concepts to unfamiliar examples, specialized cells, laboratory observations, health, plants, or environmental systems.

OERs on the Topic

You can also deepen your learning with Cell biology, Organelle, Microscope, Diffusion, Osmosis, Photosynthesis, and Cellular respiration.

Linked Learning Areas

Cell Structure and Function

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