What is a Cell?
The cell is the smallest structural and functional unit of life — every living organism is made of one or more cells. Cells carry out all the processes needed for life, from generating energy to reproducing and responding to their environment.
A cell is the basic building block of all living organisms, a membrane-bound unit containing genetic material and structures that carry out life's essential functions.
- •No true nucleus (nucleoid region)
- •No membrane-bound organelles
- •Smaller (1–10 µm)
- •Found in bacteria and archaea
- •Circular DNA
- •True membrane-bound nucleus
- •Has membrane-bound organelles (mitochondria, ER, Golgi)
- •Larger (10–100 µm)
- •Found in animals, plants, fungi, protists
- •Linear DNA in chromosomes
Step-by-step worked examples
A human cheek cell is about 60 micrometers (µm) in diameter. How many of these cells would fit end-to-end across a 6 cm ruler?
6 cm = 60,000 µm 60,000 µm ÷ 60 µm per cell = 1,000 cells So about 1,000 cheek cells would fit end-to-end
Classify the following as prokaryotic or eukaryotic: a bacterium, a human skin cell, a yeast cell, an archaeon.
Bacterium: prokaryotic (no nucleus) Human skin cell: eukaryotic (has nucleus) Yeast cell: eukaryotic (fungus, has nucleus) Archaeon: prokaryotic (no nucleus)
According to cell theory, if a tissue sample shows structures without a nucleus or clear cell membrane, could it be classified as living cells?
Cell theory states all living things are made of cells Cells must have a membrane and (usually) genetic material organized within Structures lacking these features (e.g., a virus) are not classified as true cells
Flashcards
Quick quiz
Q1.Which statement is part of cell theory?
Q2.What is the main difference between prokaryotic and eukaryotic cells?
Q3.Which of these organisms has prokaryotic cells?
Q4.What structure controls what enters and exits a cell?
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Common mistakes
All cells have a nucleus. — Correct: Only eukaryotic cells have a true, membrane-bound nucleus; prokaryotic cells do not.
Bacteria are eukaryotes. — Correct: Bacteria are prokaryotes — they lack a nucleus and membrane-bound organelles.
Cells can arise spontaneously from non-living matter. — Correct: Cell theory states all cells come from pre-existing cells (no spontaneous generation).
Viruses are cells. — Correct: Viruses are not cells — they lack a cell membrane and independent metabolism.
FAQ
What is a cell?
A cell is the smallest structural and functional unit of life, capable of carrying out all processes needed to sustain an organism.
What is cell theory?
Cell theory states that all living things are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
What are examples of eukaryotic cells?
Animal cells, plant cells, fungal cells, and protist cells are all eukaryotic, meaning they have a true nucleus.
How do you tell prokaryotic and eukaryotic cells apart?
Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have both, and are generally larger.