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CYTOSKELETON OF CELL

Cytoskeleton - network of tiny protein fibres in cytoplasm

- Provides shape and support to cell

- Involved in movement of organelles and sometimes the cell itself

-certain cells only

- Can be dismantled and reassembled as required.

Three types of cytoskeleton fibres

Microtubules Intermediate Microfilaments

Filaments

largest smallest

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Microtubules

-Tiny hollow rods of variable length made of proteins called tubulins

[ 25nm in diameter]

Can be assembled and disassembled in cell as needed

Usually grow from region of cell called centrosome

Centrosome also contains a pair ofcentrioles (animals only) = barrel

shaped rings of microtubules

Two centrioles always at right angles to each other

Microtubule functions

- Cell support - Act as girders supporting cell and giving it shape

- Act as tracts along which small organelles move.

- During cell division centrosome produces spindle apparatus made ofmicrotubules along which chromosomes move

- Form cilia and flagella may cause movement of cell

CILLIA: Short hair like projections from surface of cell. A cell has many

cilia which beat in unison.

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FLAGELLA: long whip like extension of cell 1-2 per cell - cause

movement of cell.

CILIA

Unicellular organisms Man

Movement of cell movement of fluid

over surface of cell

eg paramecium eg Trachea [windpipe]

FLAGELLA

Unicellular organisms Sperm of many Animals + man

Cilia and Flagella - Structure identical

Each made of a ring of 9 pairs of microtubules ( fused) with 2 in centre .

Covered with plasma membrane.

= 9+2 pattern

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(a) cilium showing 9+2 pattern and basal body

Cilia and flagella are anchored to cell by basal body - structure identical tocentriole - 9 triplets of microtubules

Movement caused by tiny arms attached to microtubule pairs dynein arms

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MICROFILAMENTS

Smallest of cytoskeleton fibres

Made from actin proteins which form 2 strands which twist around each

other

can be assembled and disassembled as required

Functions

- Maintenance of cell shape and structures such as microvilli

- Changes of cell shape including formation of pseudopodia (cellextensions).

- Division of cell in two in animals.

- Essential for muscle contraction - with thicker myosin filaments

INTERMEDIATE FILAMENTS

Form more permanent framework providing shape

to cell- are not assembled or disassembled

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Made of different keratin proteins coiled together

Function

- Provide shape to cell

- Anchor certain organelles in place

eg Nucleus sits in cage of intermediate filaments

- Axons of neurons strengthened by intermediate filaments

- Forms nuclear lamina

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CELL JUNCTIONS

Human cells can have 3 types of intercellular junctions

tight junctions desmosomes gap junctions

Not all cells have junctions between them.- common in tissues such as

epithelium and muscle tissue

Tight junctions [Form seal]

Membranes of neighbouring cells are fused at the junction forming a seal

that prevents leakage between cells

Form continuous belts around cell eg intestine

Desmosomes [ anchoring ] junctions

Function like rivets holding cells together

Get circular plaques or discs on inside of each membrane. Also appear to

be protein filaments between cells which extend into cell on either side

plaques are reinforced by keratin fibres in cell

Gap Junctions

Provide communication between cells

Eg muscle cells, gland cells, embryos

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-Tiny cell to cell channels made of special membrane proteins

-Channels permit ions, nutrients, some hormones and electric signals to

pass through.