3B Scientific Animal cell model - Manual (Page 3)

®
The Animal Cell
English
Introduction
Cells in animal multicellular organisms principally only occur in groups of similar cells or together with
other differentiated cells, or embedded in the intercellular substrate (intercellular substance, extracellular
matrix). The surrounding environment of the unicellular and primitive multicellular organisms (the “pri-
mordial soup“, so to speak) also surrounds the cells of more complex highly organized animal (human)
organisms, and ensures its nutrition via the blood vessels that penetrate throughout the tissues (down to
the capillaries).
The following basic characteristics distinguish the cells of living organisms: they possess higher complexity
of organization than their surroundings, they can react to stimuli from within and from their environment,
and they have the ability to reproduce (reduplication).
Overview of the construction and function of cells
The cell membrane (plasma membrane) encloses the cell and also provides a barrier to the external envi-
ronment allowing the maintenance of its own internal environment. Within the cell, certain structures and
small organs (organelles, see list below) are also enclosed by a plasma membrane. The plasma membrane
itself consists of polar lipids that form a semi-permeable membrane. Thus the individual compartments
and organelles are separated from one another and from the specific molecules and ions they contain.
The plasma membrane is also connected to a fine framework of structural proteins, the filaments of the
cell skeleton (cytoskeleton). This cytoskeleton consists of fine actin filaments (7 nm diameter), hollow
microtubules (25 nm diameter) and, lying in between in diameter, the intermediary filaments. The micro-
tubules develop from an organization centre, usually the centriole. They are also responsible for transport
processes along their length, to and from the organization centre (directional active transport, which also
occurs in the axons of nerve cells). The centriole itself is an organelle consisting of two groups of tubes
perpendicular to one another, from which the microtubules extend – this also occurs in newly formed cells.
During cell division the separation of the chromosomes is carried out by the “marionette threads”, the
microtubules emanating from the centriole.
As the name implies, the cytoskeleton ensures overall stability for the cell along with a corresponding
degree of flexibility. Furthermore the cytoskeleton enables extreme versatility in the active movements of
the cell: from stretching out foot-like appendages (e.g. filopodia) to make major changes in shape of the
entire cell (also the basis of active muscle contraction for example) to active movement of the cell (cell
migration). Moreover, the elements of the cytoskeleton propagate the tension lines within a cell via the
so-called cell-cell connections (e.g. desmosomes, see below) to the neighbouring cells and so mechanically
connect different areas of cells e.g. in the epidermis of the skin – particularly clear in the prickle cells.
Within the cell-cell connections (intercellular contact) structures with predominantly mechanical function
(contact adhesion: zonula; punctum; fascia adhaerens; macula adhaerens = desmosome) can be distinguis-
hed from those with an active metabolic and electro-coupling function (nexus, macula communicans = gap
junction; synapse). Finally, there are cell connections that seal off the intercellular area (contact barrier:
zonula occludens). Connections to the extracellular membrane form focal contacts and to the basal memb-
rane the hemidesmosome.
All proteins, which make up the components of the cytoskeleton, are made by the “sewing machine“ of
the proteins, the ribosomes. These can be suspended in the cytoplasm or may be bound onto the vacuole
system of the rough endoplasmic reticulum (rough ER). Information is carried to the ribosomes from the
cell nucleus, where genetic information is stored on the chromosomes by means of the mRNA. The ribo-
some couples amino acid to amino acid to order and “sews” them onto a peptide or protein. Peptides and
proteins are further modified by auxiliary proteins within the ER, e.g. sugar groups may be added to the
protein (glycosylation). The smooth ER can synthesize lipids (cholesterol, triglycerides, steroid hormones),
lipoproteins and phospholipids. Furthermore the smooth ER makes fat-soluble compounds water-soluble
3