Saturday, December 7, 2013

Two Major Cell Types

According to their structure, cells can be of two types:
• Prokaryotes eg. Bacteria
• Eukaryotes eg. Fungi, Plants, Animals
Prokaryotic cells are so called because they have no nucleus (‘prokaryote’ comes from the Greek,
meaning ‘before the nucleus’). They also have no organelles (internal structures), so there is little
compartmentalization of function within them. From the mid-20th century, when the electron microscope
was developed, it became possible to study the internal detail of cells.
• The cell wall surrounds the cell. It protects the cell from bursting and is composed of peptidoglycan,
which is a mixture of carbohydrate and amino acids.
• The plasma membrane controls the movement of materials into and out of the cell. Some substances
are pumped in and out using active transport.
• Cytoplasm inside the membrane
contains all the enzymes for the
chemical reactions of the cell. It also
contains the genetic material.
• The chromosome is found in a
region of the cytoplasm called the
nucleoid. The DNA is not contained
in a nuclear envelope and also it is
‘naked’ – that is, not associated with
any proteins. Bacteria also contain
additional small circles of DNA called plasmids. Plasmids replicate
independently and may be passed
from one cell to another.
• Ribosomes are found in all prokaryotic cells, where they synthesize proteins. They can be seen in very
large numbers in cells that are actively producing protein.
• A fagellum is present in some prokaryotic cells. A flagellum, which projects from the cell wall, enables a cell to move.
• Some bacteria have pili (singular pilus). These structures, found on the cell wall, can connect to other bacterial cells, drawing them together so that genetic material can be exchanged between them.
Prokaryotic cells are usually much smaller in volume than more complex cells because they have no
nucleus. Their means of division is also simple. As they grow, their DNA replicates and separates into two different areas of the cytoplasm, which then divides into two. This is called binary fission. It differs slightly from mitosis (a type of cell division) in eukaryotic cells.

The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the metabolic function it performs. In general, they are large in comparison to prokaryotic cells.

Animal cells
• Plasma membrane
• Nucleus
• Ribosomes
• Endoplasmic reticulum
• Golgi body
• Vesicles
• Mitochondria
• Cytoskeleton

Plant cells
• Plasma membrane
• Nucleus
• Ribosomes
• Endoplasmic reticulum
• Golgi body
• Vesicles