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Class notes for chapter 2 of AS level Biology A OCR
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Basic Components of Living Systems Microscopy
Magnification and Calibration
Eukaryotic Cell Structure
The Ultrastructure of Plant Cells
Prokaryotic and Eukaryotic Cells
Microscopy
, Microscopes are used to magnify an object hundreds of thousands of times. They make
individual cells visible, allowing people to use them to understand the function of cells.
In the 16th and 17th centuries, the first light microscopes were developed. By the mid 19 th
century, they were powerful enough for individual cells to be seen, leading to the
development of cell theory. Cell theory states that
- both plant and animal tissue is composed of cells
- cells are the basic unit of all life
- cells only develop from existing cells
Year Development of cell theory
1665 Cells First Observed
Robert Hooke, an English scientist, observed the structure of thinly sliced
cork using an early light microscope. He described the compartments he
saw as cells. However, he only observed cell walls as this was dead plant
tissue.
1674-1683 First Living Cells Observed
Anton Van Leeuwenhoek, a Dutch biologist, developed a technique for
creating powerful glass lenses and using his handcrafted microscopes to
examine samples of pond water. He was the first person to observe
bacteria and protoctista and described them as ‘little animals’ or
‘animalcules’ - they are now called microorganisms. He went on to observe
red blood cells, sperm cells and muscle fibres for the first time.
1832 Evidence for the Origin of New Plant Cells
Bathélemy Dumortier, a Belgian botanist, observed cells division in plants,
providing evidence against the theories of the time, that new cells arise
from within old cells or that cells form spontaneously from non-cellular
material. However, it took several years for cell division to be accepted as
the theory of origin of all new cells.
1833 Nucleus First Observed
Robert Brown, an English Botanist, was the first to describe the nucleus of a
plant cell.
1837-1838 The Birth of Universal Cell Theory
Matthias Schleiden, a German botanist, proposed that all plant tissues are
composed of cells. Jan Purkyně, a Czech scientist, was the first to use a
microtome to make ultra-thin slices of tissue for microscopic examination.
Based on his observations, he proposed that not only are animals
composed of cells, but also that, “basic cellular tissue is clearly analogous to
that of plants”. Not long after this, and independently, Theodor Schwann, a
German physiologist, made a similar observation and declared that “all
living things are composed of cells and cell products”. He is credited with
the origin of cell theory.
1844 (1855) Evidence for the Origin of New Animal Cells
Robert Remak, a Polish/German biologist, was the first to observe cell
division in animal cells, disproving the theory that new cells originate from
within old cells. He wasn’t believed at the time however, and Rudolf
Virchow, a German biologist, published Remak’s findings as his own in
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