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BaConCell Lecture Notes: The Eucaryotic Cell.
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The eucaryotic cell
Reading: Alberts et al. Chapter 1
1.
From Procaryotes to Eucaryotes
It is thought that all organisms living now on earth derive from a single primordial cell
born more than 3 billion years ago. This cell, out-reproducing its competitors, took the
lead in the process of cell division and evolution that eventually covered the earth with
green, changed the composition of its atmosphere, and made it the home of intelligent
life. The family resemblances among all organisms seem too strong to be explained in
any other way. One important landmark along this evolutionary road occurred about 1.5
billion years ago, when there was a transition from small cells with relatively simple
internal structures - the so-called procaryotic cells, which include the various types of
bacteria - to a flourishing of larger and radically more complex eucaryotic cells such as
are found in higher animals and plants.
2.
Present-day living cells are classified as procaryotic (bacteria and their close
relatives) or eucaryotic. Although they have a relatively simple structure, procaryotic
cells are biochemically versatile and diverse - for example, all of the major metabolic
pathways can be found in bacteria, including the three principal energy-yielding
processes of glycolysis, respiration, and photosynthesis. Eucaryotic cells are larger and
more complex than procaryotic cells and contain more DNA, together with components
that DNA allow this DNA to be handled in elaborate ways. The DNA of the eucaryotic cell
is enclosed in a membrane-bound nucleus, while the cytoplasm contains many other
membrane-bounded organelles, including mitochondria, which carry out the oxidation of
food molecules, and, in plant cells, chloroplasts, which carry out photosynthesis.
Mitochondria and chloroplasts are almost certainly the descendants of earlier procaryotic
cells that established themselves as internal symbionts of a larger anaerobic cell.
Eucaryotic cells are also unique in containing a cytoskeleton of protein filaments that
helps organise the cytoplasm and provides the machinery for movement.
3.
The eucaryotic cell is bound by a membrane called the plasmalemma or plasma
membrane. All membranes in the cell are made of lipids, proteins, cholesterol and
glycoproteins in various quantities according to the function of the membrane. The
membrane is involved in transport of substances into and out of the cell and organelles,
as well as communication via signal transduction proteins.
4.
Organelles are small, membrane-bound components of the cell with specialist
functions. The fluid of the cell is called the CYTOSOL, and the cytosol plus organelles is
called the CYTOPLASM. The cytosol is full of proteins involved in cell function, and is
where all of the cell's protein synthesis occurs.
5.
The NUCLEUS is a double-membrane bound organelle that contains most of the
cell's DNA. There is also a NUCLEOLUS, which is involved in assembly of ribosomes, and
there are many kinds of enzymes and other DNA-associated proteins involved in DNA
replication and in transcription. During transcription, there is a large amount of mRNA in
the nucleus. The outer membrane of the nucleus has distinct pores involved in the
transport of molecules in and out of the nucleus.
6.
The ENDOPLASMIC RETICULUM (ER) is a labyrinthine membraneous organelle
involved in the transport and modification of proteins through the cell. It is closely
associated with the nucleus and the membrane of the ER is continuous with the outer
membrane of the nucleus. Proteins destined for the ER are made at the ER membrane
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BaConCell Lecture Notes: The Eucaryotic Cell.
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surface and inserted directly through the membrane. There is ROUGH ER (RER,
sometimes called RIBOSOMAL ER) and SMOOTH ER (SER), which have different functions
but are connected to each other.
7.
The GOLGI APPARATUS is made up of individual Golgi bodies (called dictyosomes
in plant cells) that modify proteins before transport to other parts of the cell or for
secretion. Golgi bodies consist of a layer (sometimes called stack) of sacs called
cisternae, and proteins are transported from layer to layer and gradually modified. The
most common modification is glycosylation.
8.
MITOCHONDRIA are double membrane-bound organelles that produce large
quantities of ATP for the cell's use. They are thus the power stations of the cell.
9.
CHLOROPLASTS are double membrane-bound organelles found only in plant cells.
They capture light and produce energy and carbon dioxide in a process called
photosynthesis.
10.
Both mitochondria and chloroplasts are endosymbionts, and they contain DNA that
codes for some of their functions, and ribosomes for translation of mRNA.
11.
LYSOSOMES are small vesicle-like organelles with a function in the degradation of
cell substances. A plant cell equivalent is the VACUOLE, which is also involved in
maintaining turgor pressure. Another kind of vesicle-like organelle is the PEROXISOME,
which is involved in the cell's hydrogen peroxide cycle.
12.
The CYTOSKELETON consists of three different kinds of filamentous proteins MICROTUBULES (MTs) MICROFILAMENTS (MFs) and INTERMEDIATE FILAMENTS (IFs).
Each kind of filament has a specialist function in the cell, but mostly involved in the
maintenance of cell shape and in intra-cellular movement. Cytoskeleton elements
generally cannot function without associated proteins. NOTE: cytoskeleton is covered in
the Autumn term course "Cell Dynamics".
13.
CENTRIOLES are found in animal cells only and are involved in the organisation of
the cytoskeleton during cell division, in association with microtubule organising centres
(MTOCs).
14.
RIBOSOMES are complexes of proteins involved in the translation of mRNA into
proteins. Ribosomes act in conjunction with tRNA and mRNA as well as other proteins to
assemble amino acids into protein sequences. Because the proteins produced are often
inserted directly into the ER, ribosomes are often seen on the surface of the ER (thus,
RER).
15.
Plant cells also have a CELL WALL, made of different kinds of cellulose compounds.
Its function is to maintain rigidity of the cell, but it is now defined as an extra-cellular
matrix. Cell wall fibres are laid down in alignment with microtubules, often in a crisscross fashion for strength.