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

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Model of a Plant Cell
(Magnification approx. 500,000-1,000,000)
English
This membrane is responsible for controlling the selective transport into and out of the cell. It has the
same function with regard to the organelles, too. The cytoskeleton in the cytoplasm, which is made up
of proteins, guarantees not just the stability of the cell but also the most diverse intracellular movements
(e.g. visible plasma streams).
Nucleus (3a) with nucleolus (3b)
The nucleus (approx. 5-25 µm) is the information centre of the cell. It is enclosed within a double mem-
brane lining with defined channels (= nuclear pores for controlling the metabolic flow between the nucle-
us and the cytoplasm) and contains the main part of the cell’s genetic information, present in the form of
chromatin. Only during cell division is the chromatin (normally not visible under a light microscope) trans-
formed into its more compact form, viz. chromosomes. In the process, the DNA, which is bound by prote-
ins, is strongly reduced by condensation and spiralisation. The nucleoli occur exclusively in the interior of
the nucleus and are the site for the synthesis of preliminary stages of cytoplasmic ribosomes (5).
Endoplasmic Reticulum (smooth ER (4a) and rough ER (4b)) Ribosomes (5)
All proteins of the cell are built at the “sewing machine” of proteins, the ribosomes. These extremely small
organelles (approx. 20-30 nm) can float freely in the cytoplasm or get attached to the sacciform or tubular
membrane system of the endoplasmic reticulum (rough ER). Inside the ER´s compartment, proteins are in
part transformed into helper proteins, commonly known as molecular chaperones, and are transported
to their biophase. The smooth ER, without ribosomes attached, is mainly responsible for the synthesis of
lipids. The structure of the ER is extremely dynamic and always subject to constant reorganisation. The ER
is also connected to the membrane coating of the nucleus. That is to say, the membrane and the lumens of
both of the organelles blend directly with one another.
Plasmodesmata (6)
Plasmodesmata constitute contact structures between neighbouring plant cells. In the process they form
connections, in the form of fine channels, between the living protoplasts through the cell wall and the
middle lamella. The coupling is built through tubular ER cisternae of both of the cells and its function is to
transfer low-molecular substances between cells.
Plastids
Plastids are compartments typical to plant cells. They are always surrounded by a double membrane. The
inner membrane is formed for the purpose of enlarging the reactive surface into the interior of the plas-
tids. Plastids emerge on their own from the division of young proplastids and spread themselves during
mitosis into daughter cells. Chloroplasts possess their own genetic information (ring-shaped, extrachromo-
somal genome; plastids DNA).
The green chloroplasts (7) are the site of photosynthesis and the synthesis of numerous plant constituents
(e.g. fatty acids). The colourless and fluid matrix is denoted as stroma; the enlarged lamellar/sacciform
inner membranes are called thylakoids. The stacked membrane areas are called grana thylakoids. The
protein-bound pigments responsible for photosynthesis are found in these membranes (chiefly chlorophyll
and carotenoids). These photosynthesis pigments are also responsible for the Hill reaction. The Calvin-
Benson cycle or the photosynthetic carbon reduction cycle (PCR) CO
2
fixation as well as the formation of
carbohydrates and starch take place in the stroma region.
Other plastids:
Chromoplasts: are inactive photosynthesis plastids responsible for the colouration of plant organs
Leucoplasts: are responsible for storing starch (amyloplasts), proteins (proteinoplasts), oils (elaioplasts)
Etioplasts: are the preliminary stage of chloroplasts and originate in the dark
Gerontoplasts: are all plastids at a very mature age
Mitochondria (8)
Mitochondria are the organelles responsible for cell respiration and energy conversion. They are there-
fore the “power plant” of every cell. Only mitochondria can give rise to mitochondria. Just like plastids,