CYTOSKELETON
Cells contain a network of fibres made of protein. These fibres keep the cell’s shape stable by providing an internal framework called the cytoskeleton:
-->Some of the fibres, called microfilaments (made of actin filaments) are able to move against each other – these cause the movement seen in some white blood cells, and they move some organelles around inside cells. Movement is side-to-side like a wind-shield wiper.
-->There are other fibres, called microtubules. These are cylinders about 25nm in diameter made of a protein called tubulin, and may be used to move a microorganism through a liquid or to waft a liquid past a cell. Movement is circular like a helicopter propeller.
Comparing Micortubule & Microfilaments:
Arrangement of microtubules- arranged in a 9+2 arrangement seen below:
UNDULIPODIA & CILIA
Structurally, flagella of eukaryotes (correctly named undulipodia) and cilia are the same. Each one is made up of a cylinder than contains nine microtubules arranged in a circle and another two microtubules in a central bundle. Undulipodia are longer than cilia.
The undulipodium that forms the tail of a sperm cell can move the entire cell. Undulipodia and cilia can move because the microtubules can use energy produced by ATP (adenosine triphosphate).
Some bacteria have flagella. These look like the same as eukaryotic undulipodia, but their internal structure is different. These are true motors; they are made of a spiral of protein, called flagellin, attached by a hook to a protein disc at the base. Using energy from ATP, the disc rotates, spinning the flagellum
Cell ultrastructure and the importance of the cytoskeleton of cells.
any of the organelles found within cells are membrane-bound, this means that they have their own surrounding membranes to separate them from the rest of the contents of the cell. They have the same structure as the main cell membrane. The organelles form separate compartments within the cell, a process called compartmentalisation.
Structure
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Function
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The nucleus is the largest
organelle in the cell. When stained, it shows darkened patches known as chromatin. It is surrounded by a nuclear envelope. This is a structure made of
two membranes with fluid between them. A lot of holes, called nuclear pores, go right through the envelope.
These holes are large enough for relatively large molecules to pass through.
There is a dense, spherical structure, called the nucleolus,
inside the nucleus
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The
nucleus stores the majority of the cell’s genetic material. The chromatin
consists of DNA and proteins. It contains the instructions for making
proteins. Some of these proteins regulate the cell’s activities. When a cell
divides, chromatin condenses into visible chromosomes. The nucleolus makes RNA
and ribosomes.
These pass into the cytoplasm and proteins are assembled at them
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Endoplasmic reticulum (ER)
consists of a series of flattened membrane-bound sacs called cisternae. They are continuous with the outer
nuclear membrane. Rough ER is studded
with ribosomes, smooth ER does not have
ribosomes
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Rough ER transports
proteins that were made on the attached ribosomes. Some of these proteins may
be secreted from the cell. Some will be placed on the cell surface membrane.
Smooth ER is involved in making the lipids that the cell needs
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The Golgi apparatus is a stack of
membrane-bound sacs, which looks very much like a pitta bread
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The Golgi
apparatus is responsible for receiving proteins and modifying them. It
receives proteins from the ER and may add sugar molecules to them. It then
packages the modified proteins into vesicles that can be transported. Some
modified proteins go to the cell surface so they can be secreted
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A single mitochondrion is
spherical or sausage-shaped. It has two membranes separated by a fluid-filled
space. The inner membrane is highly-folded to form cristae. The central part of the mitochondrion is the matrix
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Mitochondria are the site
where ATP
is produced during respiration.
ATP is sometimes called the universal carrier energy as it drives most of
the cellular processes
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Chloroplasts are only found in plant cells, and have two
membranes separated by a fluid-filled space. The inner membrane is continuous,
with an elaborate network of flattened membrane sacs called thylakoids. A stack of thylakoids is a granum (plural: grana).
Chlorophyll molecules are present on the thylakoids membranes and in the
intergranal membranes
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These are
the site of photosynthesis
in plant cells. Light energy is used to drive the reactions, in which
carbohydrate molecules are made from carbon dioxide and water
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A lysosome is a spherical sac
surrounded by a single membrane
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These contain powerful digestive enzymes
which are there to break down materials. For example, white blood cell
lysosomes help to break down invading microorganisms; and the specialised
lysosome in the head of a sperm cell helps penetrate the female egg cell
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►Golgi apparatus
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◄The nucleus and endoplasmic reticulum  |
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◄Mitochondrion
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Chloroplast
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There are some organelles which are non membrane-bound…
Structure
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Function
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A ribosome is a tiny organelle
that consists of two subunits. They can be found in the cytoplasm or attached
to the ER making rough ER
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Ribosomes
are the site of protein synthesis in the cell (where new
proteins are made). They act as an assembly line where coded information (mRNA)
from the nucleus is used to assemble proteins from amino acids
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Centrioles are small tubes of protein fibres (microtubules)
which are present only in animal cells and cells of some protoctists. They
are found in a pair next to the nucleus
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These are used in cell
division, they form fibres known as spindle which move the chromosomes during
nuclear division
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