The structure and function of the various organelles within animal and plant cells
Many 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
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
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
The Golgi apparatus is a stack of membrane-bound sacs,
which looks very much like a pitta bread
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
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
A lysosome is a spherical sac surrounded by a single
membrane
Function
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
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
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
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
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
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
◄The nucleus and endoplasmic
reticulum
►Golgi apparatus
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◄Mitochondrion
► Chloroplast
There are some organelles which are non membrane-bound…
Structure
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
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
Function
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
These are used in cell division, they form fibres known as
spindle which move the chromosomes during nuclear
division
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