All living organisms are made up of cells and new cells
are produced when live cells divide. The cell is the smallest unit of life in
an organism. The cell lives and, as a result, the organism lives. Whatever an
organism does for survival it does for the survival of its cells.
The first microscopes were composed of a
single lens just like a magnifying glass. They were called simple
microscopes. Later microscopes were designed using 2 lenses. They are
called compound microscopes. The
top lens through which you look is called the eyepiece while the lower
lens that is close to the slide is called the objective lens. They are
called compound microscopes. The magnification power of a compound
microscope is calculated by multiplying the magnification of the 2 lenses.
Since these microscope use light to see the objects they are called light
microscopes.
Below is a diagram of a compound light
microscope. Learn the various parts. You will be using the microscope in your
biology study.
Robert Hooke In 1665
Robert Hooke used a light microscope to study cork. He noticed that the cork
was composed of many small boxes. He named the compartment cells because
they reminded him of prison cells. It is now
known that all living things are composed of cells. They are measured using the
unit micrometer.
The symbol for micrometer is µm. A µm is one thousandth of a millimetre. Animal Cells The diagram below is an animal as may be seen using a light microscope.
All the living matter of a cell is called protoplasm. The cell is
surrounded by a cell or plasma membrane. The nucleus is the
control centre of the cell. The cytoplasm surrounds the nucleus. The cytoplasm
is everything within the cell except for the nucleus. There are many small organelles
within the cytoplasm. This is where most of the cell’s activities take place.
The cytoplasm is composed of 90% water. They cannot be seen using a light
microscope. They will be discussed later.
Plant Cells The diagram below is a plant cell as may be seen using a
light microscope. All the living matter of a plant cell is also called protoplasm.
The cell is surrounded by a cell or plasma membrane. Unlike the animal
cell the plant cell also has a cell wall surrounding it. This is made of
cellulose and is very rigid. It supports the plant cell. The nucleus
is the control centre of the cell. The cytoplasm surrounds the nucleus.
The vacuole is a storage area for the plant cell. The vacuole contains cell
sap. This is made of sugars, salts, and pigments. The chloroplasts
contain chlorophyll. This is where photosynthesis occurs within the cell. The Ultrastructure
of Cells With the invention of the electron microscope a whole new world
was open up to scientists. Most light microscopes will enlarge a specimen up to
1000 times (1000X) but the electron microscope enlarge the specimen 250,000X
and higher! Using these microscopes scientists were able to discover parts of
the cell never seen or known of before. The fine detail of a cell when seen by
an electron microscope is called ultrstructure. Cell Membrane (Plasma Membrane) Below is a diagram of a part of the plasma membrane. Note that they are
composed of phospholipid molecules and protein. The phosphate heads
of each molecule is on the outside of the structure while the lipid tail of
each are on the inside part of the membrane. The protein location varies along
the membrane. Note that there are pores along the membrane. This is
where materials enter and leave the cell. The parts of the membrane move around
constantly. That is why the membrane is said to be fluid. This is called
the fluid mosaic model of the plasma membrane. Click here to
view an animation of the structure of the plasma membrane Plasma Membrane
Functions The nucleus is
the control centre of the cell. It is surrounded by a nuclear membrane that
allows molecules to enter and leave the nucleus similar to the plasma membrane.
The nucleus contains DNA (deoxyribonucleic acid).
The DNA is arranged in groups called chromosomes. This is the genetic material
of the cell. Every organism has a specific number of chromosomes in each
nucleus of each of its cells. Humans have 46 chromosomes in every cell while
roundworms have 2. When not dividing the chromosomes are called chromatin.
They become elongated and interwoven at this stage. A molecule of DNA when the cell is
not dividing (in chromatin form) A chromosome as seen at the start
of cell division Genes are located on the chromosomes. These are the
structures that control the production of protein. In this way the genes
determine the characteristics of the living thing. Nuclear
pores are openings through which materials enter and leave the nucleus. Large molecules can pass between the cytoplasm and the
nucleus through these pores. 1 example is RNA from nucleus to cytoplasm and
nucleotides from cytoplasm to nucleus. This will be discussed later in another
chapter of your text. The arrows show a nuclear pore. The “N” labels the
nucleus. The
nucleolus is the area where ribosomes are made. Ribosomes are made of
RNA. The nucleolus contains a lot of rRNA (ribosomal RNA) Mitochondria The mitochondria are called the “powerhouses” of the cell.
This is where cellular respiration occurs. The end product of cellular
respiration is energy. Muscle and liver cells have many mitochondria and
produce a lot of energy. The mitochondria have 2 membranes. The inner membrane
is folded. Here, at the folds, is where the energy is releases. The more folds
it has the more energy is released. Foldings increase during exercise and
activity while they decrease during rest. In general: 1. The aerobic steps of
respiration occur here. 2. 36 of the 38 ATPs (energy molecules) from
one molecule of glucose are produced in the mitochondrion. 3. Liver, muscle and nerve cells are rich in
mitochondria. 4. Bone and fat cells have low numbers of
mitochondria. 5. Root hair cells and meristematic cells of
plants have large numbers of mitochondria. 6. Stem and root ground tissue cells of
plants are low in mitochondria. This
is where photosynthesis takes place in green plants. The green pigment is
called chlorophyll and is stored in the chloroplasts. Below is the ultrastructure of a chloroplast. Ribosomes Ribosomes are composed of
RNA and protein. They function in protein synthesis. They make protein using
amino acids. This will be discussed in a later chapter of your text. Below is a generalised ultrastructure of an animal and a plant cell. We
have not discussed all the parts labelled since they are not part of your
syllabus or will be discussed later in the syllabus. Below the diagrams are
definitions to the terms not discussed. Golgi Apparatus - a flattened, layered, sac-like
organelle that looks like a stack of pancakes and is located near the nucleus.
It produces the membranes that surround the lysosomes. The Golgi body packages
proteins and carbohydrates into membrane-bound vesicles for "export"
from the cell. Endoplasmic Reticulum- The endoplasmic reticulum contains a network of branching and joining
tubules 400 to 700 angstroms in diameter (1 angstrom equals 10-9 m). It has
been calculated that 1 ml of liver tissue contains about 11 square meters of
endoplasmic reticulum. The encircling membranes are about 50 to 60 angstroms
thick and have the same substructure as the plasma membrane. Two patterns are
found in the cell, the smooth endoplasmic reticulum and the rough endoplasmic
reticulum. The rough endoplasmic reticulum is covered by an evenly spaced
arrangement of ribosomal granules. The smooth endoplasmic reticulum lacks
ribosomes, which synthesize proteins. The smooth endoplasmic reticulum, rich in
a wide variety of enzymes, is most common in cells that are involved in the
synthesis of lipids, triglycerides, lipoprotein complexes, and steroids. Centriole- Centrioles generally
appear in animal cells as two cylinders at right angles to one another, close
to the nucleus. When viewed with an electron microscope, the cylinders show up
as nine bundles of tiny microtubules arranged in a circle. The centrioles help
to form the spindle fibres. Spindle fibres are microtubules that move
chromosomes around when the cell is dividing. Differences Between
a Plant and an Animal Cell Below is a chart
showing the general differences between a plant and an animal cell. Plant cells Animal cells Possess rigid cell walls No cell walls Have green chloroplasts No chloroplasts Contain chlorophyll Do not contain chlorophyll Thin lining of cytoplasm Most of cell is cytoplasm Vacuole filled with cell sap Small (if any) vaculoes Prokaryotic and Eukaryotic Cells All living things
can be categorized as being prokaryotic or eukaryotic. Prokaryotic
cells have no nucleus. They do
not have membrane-bound organelles such as nuclei, mitochondria or
chloroplasts. All prokaryotes are placed in the Kingdom Monera i.e. the
bacteria. Eukaryotic
cells have a membrane-bound
nucleus. Membrane-bound organelles such as nuclei, mitochondria and
chloroplasts are only present in eukaryotic cells. The Protista, Fungi, Plants
and Animals are eukaryotic organisms. Very Useful Links Cells Alive- An
excellent interactive cell model of both plant and animal cells. Cell Jigsaws- Try to put
and animal or plant cell puzzle together. Animal
Cell Flythrough- Take a short flight through an animal cell! Take an online
quiz on the parts of the cell. Take another
online quiz. Cell
Structure interactive lesson and quiz Another
animal cell interactive lesson and quiz Target Practice quiz on cell parts
TOPTOP
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TOPTOP
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TOPNucleus
TOPNuclear Pores
TOPNucleolus
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TOPChloroplasts
TOPOther Cellular
Structures
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