3B Scientific Plant cell model, magnified 500,000-1,000,000 times - Manual (Page 4)

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Model of a Plant Cell
(Magnification approx. 500,000-1,000,000)
mitochondria, too, are enclosed in a double membrane coating and possess their own genetic information.
The components/proteins responsible for the respiratory chain (ATP synthesis) are located on the inner side
of the membrane. The citrate cycle and the fatty acid oxidation cycle take place inside the mitochondrial
matrix.
Endosymbiont theory
The endosymbiont theory attempts to explain the origin of mitochondria and plastids. According to this
theory, mitochondria and plastids go back to intercellular protozoan (bacterial) symbiosis. In other words:
plastids have developed from cyanobacteria, and mitochondria have developed from purple bacteria. At
some point in the course of evolution, a “prototype“ cell with a nucleus (urocyte) incorporated prokaryotes
and integrated them into its cellular functions. A strong indication of this is the fact that mitochondria and
plastids have the following in common:
• A double membrane coating (inner and outer membranes are quite different in their chemical composi-
tion; the inner membrane resembles bacterial membranes)
• Inherent ring-shaped genome
• Inherent ribosomes (correspond to bacterial ribosomes, differ from cytoplasmic ribosomes)
Dictyosomes/Golgi apparatus (9)
Dictyosomes are disc-shaped, membranous hollow cavities (cisternae). The sum of all dictyosomes in a cell
are termed the Golgi apparatus. They are closely connected to the ER and are responsible for the conver-
sion, storage and transfer of the products of the ER. Consequently, a distinction can be drawn between a
generation side (facing the ER, regeneration from the ER) and a secretion side (facing away from the ER)
which forms a significant cellular transport system responsible for exocytosis (elimination of substances
from the cell), the construction of biomembranes and is also involved in cell-wall formation.
Vacuole (10)
The vacuole is an organelle only to be found in plant cells. It is a space filled with fluid and is surrounded
by a simple membrane (= tonoplast). In mature plant cells, the volume of the central vacuole can consti-
tute up to 80% of the total volume of the cell. In the cell, vacuoles serve as reaction, storage (e.g. of ions,
organic acids, saccharides, proteins, pigments), transport and deposit compartments (for substances that
can be harmful to the cell, e.g. toxins, tanning agents). The breakdown of macromolecules (lytic compart-
ment) is also carried out in the vacuoles.
Microsomes/Microbodies (11)
Microsomes are organelles with a homogenous structure (simple membrane, spherical, size: 1 µm, granular
matrix) on the one hand, and strong biochemical and functional differences on the other.
Different functions:
− Lysosomes: are responsible for the break-up of proteins, polysaccharides and nucleic acids
− Glyoxysomes: play an important part in converting depot fats to carbohydrates
− Oleosomes (oil globules): are responsible for the break-up of fats and oils
− Peroxisomes: play an important part in photorespiration. Peroxisomes also break up the glycollate
which is inevitably created during CO
2
fixation. Carbon is fed back into the photosynthesis cycle, and two
amino acids are produced for protein synthesis.
Cell wall (12)
Possessing a rigid cell wall is an additional feature which distinguishes plant cells from animal cells. The
cell wall gives the plant cell rigidity and form (exoskeleton) by resisting the interior osmotic pressure
(= turgor pressure) of the cell. It is a product secreted by the protoplasts (apoplast). From a chemical point
of view, the cell wall is made up of polysaccharides and proteins.
The cell wall is made up of up to three layers.
Middle lamella: a gelatinous layer, only a few nm in thickness, made up of pectin compounds with a low
quantity of proteins. It has no fibril structure and is therefore elastic.
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