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Biology IB Notes standard level
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Biology1 Cell Biology
1.1 Introduction to cells
1.2 Ultrastructure of cells
1.3 Membrane Structure
1.4 Membrane Transport
1.5 The Origin of Cells
1.6 Cell Division
,1.1 Introduction to Cells
According to the cell theory, all living organisms, from bacteria to
human, are composed of cells, the smallest unit of living matter
Cell size
Cells are often too small to be seen without a microscope, and
they come in all sizes and shapes. Their small size allows for a
large surface-area-to-volume ratio that enables a more efficient
exchange of nutrients and waste with the environment
Types of cell
There are three basic types of cells: archaea, bacteria and
eukaryotes. Both archaea and bacteria are classified as
prokaryotes, whereas cells of animal, plants, fungi, and protists are
eukaryotes. Archaea are a unique group of organisms and likely
evolved in the harsh conditions of early Earth and are still
prevalent today in extreme environments, such has hot springs
and polar regions.
Common ancestry
All three types of cells share features that reflect their evolution
from a common ancestor. These features are:
1. A plasma membrane that separates the cell from its
environment
2. Cytoplasm, comprising the jelly-like cytosol inside the cell
3. Ribosomes that are important for the synthesis of proteins
4. DNA to store and transmit hereditary information
Prokaryotes vs Eukaryotes
,The term “prokaryote” means “before nucleus” and prokaryotes do
not have nuclei. Rather, their DNA exists as a single circular
chromosome in the central part of the cell called the nucleoid.
Eukaryotic cell possess many features that prokaryotes cells lack,
including a nucleus with a double membrane that encloses DNA. In
addition, eukaryotic cells tend to be larger and have a variety of
membrane-bound organelles that perform specific
compartmentalized functions.
1.1.2 Cell size
At 0.1 to 0.5 μm in diameter, prokaryotic cells are significantly
smaller than eukaryotic cells, which have diameters ranging from
10 to 100 μm. The small size of prokaryotes allows ions and organic
molecules that enter them to quickly diffuse to other parts of the
cell. Similarly, any waste produced within a prokaryotic cell can
quickly diffuse out. This is not the case in eukaryotic cells, which
have developed different structural adaptations to enhance
intracellular transport
Importance of size
Small size, in general, is necessary for all cells, whether prokaryotic
or eukaryotic. Not all cells are spherical in shape, but tend to
approximate a sphere.
, You may remember from geometry that the formula for the surface
area of a sphere is 4πr,2 while the formula for its volume is 4πr3/3.
Thus, as the radius of a cell increases, its surface area increases as the
square of its radius, but its volume increases as the cube of its radius (ie
much more rapidly).
Importance of size- 2
So, as a cell increases in size, its surface are-to-volume ratio
decreases. This same principle would apply if the cell had the
shape of a cube. If the cell grows too large, the plasma membrane
will not have sufficient surface area to support the rate of
diffusion required for the increased volume. In other words, as a
cell grows, it becomes less efficient. One way to become more
efficient is to divide; another way is to develop organelles that
perform a specific task. These adaptations lead to the
development of more sophisticated cells called eukaryotic cells.
Importance of size - example
Notice that as a cell increases in size, its surface area-to-volume
ratio decreases. When there is insufficient surface area to support
a cell’s increasing volume, a cell will either divide or die. The cell on
the left has a volume of 1 mm3 and a surface area of 6 mm2 with a
surface area-to-volume ratio of 6-1, whereas the cell on the right
has a volume of 8 mm3 and a surface area of 24mm3 with a surface
area-to-volume ratio of 3 to 1.
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