The cell: the basic unit of life
FoundationAll living things are made of cells — one cell can be a whole organism, or many cells build a big one.
- All living things are made of cells
- Use a microscope to observe cells
- Cell → tissue → organ → organism
§Text Passage
1Place a thin sheet of onion skin under a microscope and you will see small room-like units arranged in neat rows. These are cells. Whether a tall tree, an insect or a human being, every living body is built from cells. The cell is the basic unit of structure and function in living things, and all life activities happen inside cells or through their combined work.
2Cells were first discovered in 1665, when the English scientist Robert Hooke examined a thin slice of cork with his own compound microscope. He saw many honeycomb-like little boxes and named them cells. Later, scientists found similar structures in plants and animals one after another, and the cell theory was proposed: all organisms are made of cells, and the cell is the foundation of life.
3Most cells measure only about 0.01 to 0.1 millimetre across, far too small for the naked eye, so a microscope is needed. The human body contains tens of trillions of cells: skin cells are constantly shed and replaced, red blood cells carry oxygen, and muscle cells can contract to produce movement. A hen's egg is actually one giant single cell — one of the few cells we can see unaided.
4Understanding cells explains how we grow, how wounds heal and where diseases come from. A wound heals because cell division replaces damaged cells, while cancer arises from cells that divide without control. Cell theory unified the study of animals, plants and microbes and stands as a cornerstone of biology, preparing us to learn about cell structure and function next.
1Summary 重點摘要
All living things are made of cells — one cell can be a whole organism, or many cells build a big one.
2Vocabulary cards Vocabulary
The basic unit of life, extremely small.
An instrument that magnifies tiny objects so they can be seen.
An organism made of one cell, like an amoeba.
Made of many cells, like humans and trees.
A complete living individual, made of one cell or many.
All living things are made of cells; the cell is life's basic unit; new cells come from existing cells.
A cell adapted in shape and structure for a particular job, such as a red blood cell carrying oxygen.
A group of similar cells working together, which then builds organs.
3Step by step Steps
- How small is a cell?A cell is typically only 0.01–0.1 mm across, far below what the naked eye can resolve, so it must be magnified tens to hundreds of times under a microscope — observing cells always depends on this instrument. For example, onion skin seen in the school laboratory shows cells neatly tiled like a brick wall. Understanding how tiny cells are also explains why this microscopic world was only discovered after the microscope was invented.
- One cell is a complete lifeAmoebae and some algae are single-celled organisms: one cell carries out every life function — nutrition, respiration, excretion and reproduction — living on its own in ponds or soil. A common mistake is to think of them as incomplete; in fact a single cell is already a complete organism, able to cope with changes in its surroundings entirely by itself, even though its structure is simple and its shape keeps shifting as it moves.
- How many cells in a human?The human body contains tens of trillions of cells, and they are not piled up at random but arranged in order: cells of similar shape form tissues, tissues build organs, organs build systems, and together these make up the whole organism. In humans, skin cells are constantly shed and replaced, and a wound heals as cells divide — everyday proof that activities at the cell level keep the whole body running.
- Specialisation and division of labourIn multicellular organisms, cells specialise, developing shapes and structures suited to their jobs: red blood cells are disc-shaped for carrying oxygen, nerve cells have long extensions that pass messages quickly, and muscle cells contract to move the body. Cooperating through the cell–tissue–organ hierarchy, these specialised cells make the whole organism far more efficient than many single cells each working alone.
- Cell theoryIn 1665 the English scientist Robert Hooke examined a thin slice of cork under his own microscope, saw a grid of tiny empty rooms and named them cells. Biologists later summarised the cell theory: all organisms are made of cells, the cell is the basic unit of life, and new cells arise only from the division of existing cells. This theory unites the huge variety of life on one common structural basis and remains a cornerstone of biology.
- Measuring and observingCells are measured in micrometres (µm), one thousandth of a millimetre: a human red blood cell is about 7–8 µm across, while most plant and animal cells fall between 10 and 100 µm — knowing the unit is essential for describing cell sizes accurately. When using a microscope, locate and focus the specimen on low power before switching to high power for a closer view. Afterwards, record an honest drawing with every part labelled, which is the basic standard of scientific recording.
4Diagrams Diagrams
5Worked example Example
Example: order from smallest to largest: heart, cardiac muscle cell, human, cardiac muscle tissue.
- Smallest: cardiac muscle cell
- Same cells together: cardiac muscle tissue
- Tissues build the heart (organ)
- Organs build the human (organism)
- Answer: From smallest to largest: cardiac muscle cell → cardiac muscle tissue → heart → human. The cell is the basic unit of structure and function; identical cardiac cells gather into tissue; several tissues build the beating organ we call the heart; and organs and systems together form the whole human. Remember the order: cell → tissue → organ → organism.
6Common mistakes Mistakes
7Quick check Quick check
Q1What tool is used to observe cells?
Q2An amoeba is made of one cell. What is it called?
Q3Order: organ, cell, organism, tissue.
Q4In which year, and on what specimen, did Hooke first discover cells?
Q5Why do red blood cells and nerve cells have different shapes?
8Recap Recap
- All living things are made of cells
- Use a microscope to observe cells
- Cell → tissue → organ → organism
- Multicellular organisms depend on specialised cells working together
- Robert Hooke first discovered cells in 1665 while examining cork