Cells and cell theory

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Lesson notes

Cell Theory

All living organisms are composed of one or more **cells**.
The **cell** is the basic structural and functional unit of life.
All cells arise from **pre-existing cells** through cell division.
Cell theory was developed in the mid-19th century by **Schleiden**, **Schwann**, and **Virchow**.
Some biologists debate whether viruses, which are not cellular, should be considered living.

Discovery of Cells

**Robert Hooke** first observed cells in 1665 using a compound microscope on cork slices.
He named them "cells" because they reminded him of small rooms (Latin×cella×).
Hooke actually saw empty **cell walls** of plant tissue, not living cells.
**Anton van Leeuwenhoek** improved microscopy with a single-lens microscope achieving 270x magnification.
Leeuwenhoek discovered living cells, such as bacteria and protozoa, which he called "animalcules".

Microscopes and Technology

**Compound microscopes** use two lenses (objective and eyepiece) for higher magnification.
Hooke's microscope had two convex lenses and used reflected light.
Leeuwenhoek's single-lens microscope used a small glass sphere for up to 270x magnification.
**Electron microscopes**, developed in the 1920s, use electron beams to view objects smaller than light wavelengths.
Improvements by Carl Zeiss, Otto Schott, and Ernst Abbe in the 1880s enhanced optical quality.

Types of Cells

**Prokaryotic cells** lack a nucleus and membrane-bound organelles (e.g., bacteria).
**Eukaryotic cells** have a nucleus and organelles (e.g., plant and animal cells).
Plant cells have a **cell wall**, **chloroplasts**, and a large central **vacuole**.
Animal cells have a **cell membrane**, **nucleus**, **mitochondria**, and other organelles.
Cells can be **unicellular** (one cell) or **multicellular** (many cells).

Importance of Cell Theory

Cell theory unifies all biology by stating that cells are the foundation of life.
It explains that all organisms are made of cells and that cells come from existing cells.
Understanding cells helps explain growth, reproduction, and disease.
Modern cell biology studies how cells function, communicate, and specialize.

Animal cell showing key organelles.

Plant cell with cell wall, chloroplasts, and large vacuole.

Prokaryotic bacterium with flagellum and plasmid.

Slides

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Practice questions

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1.What is the basic unit of structure and function in all living organisms?
Easy
- ACell
- BAtom
- CTissue
- DOrgan
2.Who is credited with first discovering cells by observing cork under a microscope?
Easy
- ARobert Hooke
- BAnton van Leeuwenhoek
- CMatthias Schleiden
- DTheodor Schwann
3.Which of the following is NOT a part of cell theory?
Easy
- AAll cells have a nucleus.
- BAll living things are composed of cells.
- CThe cell is the basic unit of life.
- DAll cells come from pre-existing cells.
4.What did Robert Hooke actually see when he observed cork under his microscope?
Medium
- ALiving cells with organelles
- BEmpty cell walls of plant tissue
- CNuclei of cork cells
- DLiving animal cells
5.Which two scientists are credited with formally developing cell theory in the mid-19th century?
Medium
- AHooke and Leeuwenhoek
- BSchleiden and Schwann
- CZeiss and Abbe
- DHooke and Schleiden
6.Why could Hooke not see organelles inside the cells he observed?
Medium
- AThe cells were too thick.
- BHis microscope had low magnification.
- CHe only looked at animal cells.
- DOrganelles did not exist in cork cells.
7.Anton van Leeuwenhoek achieved a magnification of 270× using a single-lens microscope. What was the typical maximum magnification of simple microscopes before his improvements?
Hard
- A10×
- B50×
- C100×
- D200×
8.What technological advancement in the 1920s allowed scientists to view objects smaller than the wavelength of visible light?
Hard
- ACompound microscope
- BElectron microscope
- CFluorescence microscope
- DConfocal microscope

Revision notes

## Cells and Cell Theory - **Cell theory** (mid-1800s): 1. All living organisms are made of cells. 2. Cells are the basic structural/organizational unit of all organisms. 3. All cells come from pre-existing cells. - Cells are the basic unit of structure and reproduction in all living things. - Some organisms are **single-celled** (e.g., bacteria), others are **multicellular** (e.g., humans). - **Robert Hooke** (1665) first observed cells in cork using a compound microscope; he named them "cells" (Latin *cella* = small room). - **Anton van Leeuwenhoek** improved microscopes (up to 270× magnification) and observed living cells. - **Matthias Schleiden** and **Theodor Schwann** later confirmed cells in both plants and animals. - Modern microscopes include **optical** (light) and **electron** microscopes, allowing us to see tiny cell structures. - Note: Some biologists debate whether viruses (non-cellular) are alive, but cell theory applies to all cellular life.

Cell theory: A scientific theory stating that all living organisms are made of cells, cells are the basic unit of structure and organization, and all cells come from pre-existing cells.
Cell: The basic structural and functional unit of all living organisms; can be single-celled or part of a multicellular organism.
Robert Hooke: English scientist who first observed and named 'cells' in 1665 using a compound microscope.
Anton van Leeuwenhoek: Dutch scientist who improved microscopes and was the first to observe living cells.
Matthias Schleiden: German botanist who concluded that all plants are made of cells.
Theodor Schwann: German zoologist who concluded that all animals are made of cells.
Microscope: An instrument that uses lenses to magnify small objects, enabling the study of cells.

Flashcards

What is the cell theory?

Cell theory states that all living organisms are made of cells, cells are the basic unit of structure and function in living things, and all cells come from pre-existing cells.

Who is credited with discovering cells?

Robert Hooke is credited with discovering cells in 1665 when he observed thin slices of cork under a microscope and saw tiny pores he called 'cells'.

What did Robert Hooke actually see when he observed cork cells?

Hooke saw the empty cell walls of plant tissues, not living cells with internal components.

Why did Hooke not think cells were alive?

Because his microscope had low magnification, he could not see internal components like the nucleus or organelles, so he thought the 'cellulae' were empty rooms.

What is the meaning of the Latin word 'cella'?

'Cella' means 'a small room', like monks lived in.

Who improved microscope lenses and achieved 270x magnification?

Anton van Leeuwenhoek made a single-lens microscope with a small glass sphere that achieved 270x magnification.

What was the maximum magnification before Leeuwenhoek's improvements?

Before Leeuwenhoek, the maximum magnification was about 50x.

What invention made it possible to view objects smaller than the wavelength of visible light?

The electron microscope, developed in the 1920s, made it possible to view objects smaller than optical wavelengths.

What did Matthias Schleiden and Theodor Schwann contribute to cell theory?

They studied both plant and animal cells and concluded that cells are fundamental to both plants and animals, supporting the idea that cells are the basic unit of life.

What is the basic structural and organizational unit of all organisms?

The cell is the basic structural and organizational unit of all organisms.

Are viruses considered living organisms under cell theory?

Some biologists consider viruses living, but cell theory traditionally applies only to cellular life; viruses are not made of cells.

What is the name of the book in which Hooke described his cell observations?

Micrographia.

What did Hooke mistakenly suggest about the origin of mold?

Hooke suggested spontaneous generation, the idea that life arises from non-living matter, because he could not see seeds or spores.

What is a compound microscope?

A compound microscope uses an objective lens and an eyepiece to achieve higher magnification than a simple microscope.

When did compound microscopes first appear in Europe?

Around 1620.

Who manufactured microscopes and hired Otto Schott and Ernst Abbe to improve lenses?

Carl Zeiss, a German engineer.

What limitation do optical microscopes have?

Optical microscopes can only focus on objects the size of a wavelength of light or larger.

What is the difference between a single-celled and a multicellular organism?

A single-celled organism consists of one cell that carries out all life functions, while a multicellular organism is made of many cells that work together.

What does MS-LS1-1 standard cover?

MS-LS1-1 covers evidence that living things are made of cells, and that cells are the basic unit of life.

What did Hooke use to illuminate specimens in his microscope?

He used reflected light for some observations and directly transmitted light for others.

What is the significance of the cell theory in biology?

Cell theory is the fundamental principle that all life is composed of cells, which are the basic units of structure, function, and reproduction.

Why did Hooke name the pores 'cells'?

He named them 'cells' because they reminded him of the small rooms (cellulae) in a monastery or the six-sided cells of a honeycomb.

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