Cells and Cell Theory

Grade 6 · Science · Study Guide

Explore the fundamental unit of life, the key scientists who developed cell theory, and the difference between prokaryotic and eukaryotic cells.

Every living thing on Earth — from bacteria to blue whales — is made of cells. The cell is the basic unit of structure and function in all living organisms. Some organisms, like bacteria, are made of just one cell. Others, like humans, are made of trillions of specialized cells working together.

Cell theory is one of the most important ideas in all of biology, and it took centuries to develop. In 1665, Robert Hooke used an early microscope to look at cork and saw tiny box-like compartments he called 'cells.' Anton van Leeuwenhoek later observed living cells in pond water. The modern cell theory, developed by Schleiden, Schwann, and Virchow in the 1800s, states three things: (1) all living things are made of one or more cells, (2) the cell is the basic unit of life, and (3) all cells come from pre-existing cells.

Cells come in two main types. Prokaryotic cells (like bacteria) have no nucleus — their DNA floats freely in the cell. Eukaryotic cells (like those in plants, animals, and fungi) have a true nucleus that holds the DNA, along with other membrane-bound organelles. Both types have a cell membrane, DNA, and ribosomes.

Key terms

Cell Theory
The scientific theory stating all living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells.
Prokaryotic Cell
A simple cell with no membrane-bound nucleus, like bacteria.
Eukaryotic Cell
A complex cell with a true nucleus and membrane-bound organelles, found in plants, animals, and fungi.
Organelle
A specialized structure inside a cell that performs a specific function (e.g., mitochondria, nucleus).
Cell Membrane
A flexible layer surrounding all cells that controls what enters and exits the cell.

Flashcards

Cell Theory
Three-part theory: cells are the basic unit of life, all organisms are made of cells, all cells come from existing cells.
Robert Hooke
Scientist who coined the word 'cell' after observing cork under a microscope in 1665.
Prokaryote
Organism with cells that have no membrane-bound nucleus (e.g., bacteria).
Eukaryote
Organism whose cells have a true nucleus (e.g., plants, animals, fungi).
Nucleus
The organelle that contains the cell's DNA and controls cell activities.
Cell Membrane
The thin, flexible barrier around every cell that controls what enters and exits.
Organelle
A small structure inside a cell that does a specific job, like mitochondria producing energy.
Mitochondria
The organelle that produces energy for the cell; called the 'powerhouse of the cell.'

Questions

  1. Who is credited with first observing and naming 'cells' using a microscope?

    1. Anton van Leeuwenhoek
    2. Robert Hooke
    3. Matthias Schleiden
    4. Rudolf Virchow
    Show answer

    Answer: Robert Hooke

    Robert Hooke first described cells in 1665 after observing thin slices of cork under a microscope.

  2. Prokaryotic cells have a membrane-bound nucleus.

    Show answer

    Answer: False

    Prokaryotic cells (like bacteria) do NOT have a membrane-bound nucleus. Their DNA floats freely in the cytoplasm.

  3. Which statement is part of modern cell theory?

    1. Cells only exist in animals
    2. All cells come from pre-existing cells
    3. Plants are not made of cells
    4. Cells do not contain DNA
    Show answer

    Answer: All cells come from pre-existing cells

    One of the three principles of cell theory is that all cells arise from pre-existing cells — cells cannot appear from nothing.

  4. The basic unit of structure and function in all living things is the ___.

    Show answer

    Answer: cell

    Cell theory establishes that the cell is the fundamental building block of all living organisms.

  5. Which organelle is the 'control center' of a eukaryotic cell, containing the DNA?

    1. Mitochondria
    2. Cell wall
    3. Nucleus
    4. Ribosome
    Show answer

    Answer: Nucleus

    The nucleus is the control center of eukaryotic cells, housing the DNA and directing cell activities.

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