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Module: BIOM - 1006

Lecturer: Dr Martin

Date: 26/10/16

The Cytoplasm, Cytosol and Cytoskeleton
o

The cytoplasm is everything outside of the nucleus but still within the plasma membrane
 This includes all organelles bar the nucleus

o

The cytosol is the part of the cytoplasm outside of all membrane-bound organelles
 It is a water-based gel consisting of ~20% protein
 It is the site of many biochemical reactions, such as protein synthesis
 It contains the cytoskeleton

o

The cytoskeleton is the mechanical strength of the cell which
controls cell shape and drives the movement of organelles
through the cell
...
The cytoskeleton
fulfils a number of functions:
 Large scale:
 Whole cell movements
 Contraction of muscle cells
 Changes in cell shape
 Small scale/within the cell
 Chromosome alignment during cell division
 Organelle movement

o

There are 3 protein filaments that form the cytoskeleton (all 3 shown below):
 Intermediate filaments
 These provide mechanical strength to the cell
 Microtubules
 These organise the cytoplasm (organelles and chromosomes)
 Actin filaments
 These support the cell and allows whole cell movement

o

Intermediate filaments (see right)
 These provide the cell with mechanical strength and allow it to
withstand stretching
 However, they do tend to deform under stress, but do
not rupture
 They are rope-like fibres ~10nm in diameter
 They surround the nucleus and extend to the periphery
 They are anchored at the plasma membrane at cell-cell
junctions called desmosomes
 They are also found within the nucleus
 This provides strength to the nuclear envelope and is
known as the nuclear lamina
 Intermediate filaments are found where mechanical stresses
are high, such as:
 Nerve axons
 Muscle cells
 The structure of an intermediate filament consists of intermediate filament protein twisted
together into an extended α-helix
 One end consists of a NH2 group and the other end has a carboxylic acid group
 The α-helices are then arranged in a staggered formation to provide rigidity and to
form a bigger structure called a tetramer
 There are 4 types of intermediate filament:
 Keratin filaments
 Found in epithelial cells
 Vimentin and vimentin related filament
 Found in connective tissue, muscle tissue and glial cells
 Neurofilaments
 Found in nerve cells
 Nuclear lamins
 Found in the nuclear envelope to strengthen it

o

Microtubules (see right)
 They play an organisation role within cells
 They grow out from the cell centre
 Structurally, they are a system of tracks
 They transport organelles and other components via motor
proteins
 They form the mitotic spindle used during mitosis which allows the
movement of chromosomes
 The spindle must be assembled and disassembled during cell
division
 They are composed of subunits made up of:
 α-tubulin and β-tubulin
 Both are globular proteins and are very similar in
structure
 They bind together to form a protofilament, this has a polarity
o The +ve end terminates with β-tubulin
o The -ve end terminates with α-tubulin
o The microfilament is a hollow, tubular structure and is made up from 13
protofilaments
o They grow faster at the +ve end compared to the -ve end
 Motor proteins drive organelles along the microtubules
 Kinesins drive towards the plus end
 Dyneins drive towards the negative end

o

Actin filaments (see right)
 They are responsible for cell movement
and rigidity
 They have a thread-like appearance under
an electron microscope
 They are ~7nm in diameter
 They are structured as a twisted chain
 All the monomers of actin point the same
way
 However, the actin filaments are also
polarised with a +ve and -ve end
 Like microfilaments, actin filaments
grow much faster at the positive
end than the negative end and can
grow very fast when actin monomer concentration is high
 However, when the concentration of actin monomers is moderate, a
phenomenon occurs by which the filament will have monomers added to the
positive end faster than the negative end
o This causes the positive end to grow faster than the negative end



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