Biology Unit 2

Average number of cells in the human body.

30-40 trillion.

Before microscope

Cells were so small that before the invention of the microscope, they were unknown.

Robert Hooke

In 1665, English scientist Robert Hooke made a simple microscope and observed a piece of cork. He published drawings of cells, fleas, and other minute bodies in his book Micrographia.

Robert Hooke’s first word for cells

Cellulae (Latin for small rooms). He called them this because the cork’s cell structure reminded him of the cells monks live in at a monastery.

Who the word cells came from

Robert Hooke

Cell definition

The basic structural and functional unit of all living organisms.

Anton van Leeuwenhoek

During the late 1600s, Dutch scientist Anton van Leeuwenhoek designed his own microscope after being inspired by Robert Hooke’s book.

Anton van Leeuwenhoek’s discovery

He found living organisms in pond water, milk, and various other substances.

Matthias Schleiden

In 1838, German scientist Matthias Schleiden studied plant tissues and concluded that all plants are composed of cells.

Theodor Schwann

In 1839, German scientist Theodor Schwann reported that animal tissues also consisted of individual cells.

Rudolph Virchow

In 1855, Prussian physician Rudolph Virchow proposed that all cells are produced from the division of existing cells.

Cell theory

One of the fundamental ideas of modern biology: All living organisms are composed of one or more cells. Cells are the basic unit of structure and organization of all living organisms. Cells arise only from previously existing cells, with cells passing copies of their genetic material on to their daughter cells.

Microscope technology achievement

Development in microscope technology have given scientists the ability to study cells in greater detail than earlier scientists.

Optical Microscopes

Uses visible light and lenses to magnify an object. An example is a compound light microscope. Each lens in the series magnifies the image of the previous lens. Example: When two lenses individually magnify 10 times, the total magnification is 100 times. Scientists often stain cells when using a light microscope because cells are so tiny, thin, and translucent.

Optical Microscopes Cons

Scientists have developed techniques for light microscopes, but the properties of visible light will always limit their resolution (the ability of the microscope to make individual components visible). Objects cause light to scatter, which blurs the image. The maximum magnification without blurring is around 1000X.

Electron Microscope

In the 1930s, Ernst Ruska developed the electron microscope. Instead of lenses, it uses magnets to aim a beam of electrons at thin slices of cells. This type of electron microscope is called a transmission electron microscope (TEM) because electrons are transmitted through a specimen of fluorescent screen. Thick parts of the specimen absorb more electrons than thin parts, forming a black-and-white shaded image of the specimen.

Transmission Electron Microscope pros/cons

They can magnify up to 500,000X, but the specimen must be dead, sliced very thin, and stained with heavy metals.

Scanning Electron Microscope (SEM)

A modified transmission electron microscope. It directs electrons over the surface of the specimen, producing a three-dimensional image. Disadvantage is only non-living cells and tissues can be observed.