Lecture 6 Flashcards
Cytoskeleton
Definition - A network of microtubules, microfilaments, and intermediate filaments that extend through the cytoplasm to serve a variety of functions (mechanical, transport, signalling).
Helps maintain cell shape and position organelles within cells
The cytoskeleton rapidly disassembles and reassembles, unlike the body’s skeletal system. This ability allows rapid changes in cell shape
The cytoskeleton is highly dynamic but still provides stability
What are the three main components that the cytoskeleton is made of?
Microtubules, microfilaments and intermediate filaments
Microtubules
A hollow rod composed of tubulin proteins that makes up part of the cytoskeleton in all eukaryotic cells and is found in cilia and flagella.
Microtubules are composed of tubular subunits (proteins)
They may radiate out from an organising centre (centrosome)
Microtubules resist compression thus help maintain cell shape
Microtubules are also involved in organelle motility within the cell (as well as entire cell movement). ATP-powered motor proteins (gives organelles and vesicles a route) can “walk” organelles, to be transported to specific targets within the cell (walk along until they get to where they are supposed to be)
Flagella and cilia
Microtubules can also provide cell motility (they can build special cellular features
Flagella is made up of microtubules which allows the snake like motility
Cilia- the microtubules gives cilia some strength and rigidity. If cells are fixed in place, beating of cilia moves fluid past them, if they are not fixed then movement of the actual cell would occur. These cilia produce a rowing like motion (power stroke)
Microfilaments
A cable composed of actin proteins in the cytoplasm of almost every eukaryotic cell.
Microfilaments are a double chain of actin subunits (protein subunits)
Microfilaments form - Linear strands (connect one part of a cell to another part of the cell and stop it being pulled apart (resisting tension) AND 3D networks (using branching proteins) (like cobwebs - microfilaments can have connections that allows it to build a 3D structure
Microfilaments resist tension
The cortical network under the plasma membrane helps make this region less fluid and thus maintains cell shape (lots of tension around the outer membrane, lots of pressure building up along that edge and the cell wants to resist the tension force that is trying to make it spread, so just under the cell membrane we find a cortical network of microfilaments, holding the cell together and allowing cell shape to be maintained.)
Interactions between actin and motor proteins such as myosin support cell movement
Actin-myosin interactions allow - muscle contraction, amoeboid movement (puts out projections and bunches its cytoplasm behind it and moving as a result) and cytoplasmic streaming in plants (material moves in the cell with the help of microfilaments)
Intermediate filaments
A component of the cytoskeleton that includes filaments intermediate in size between microtubules and microfilaments.
Intermediate filaments are made of various proteins including keratins in hair, lamins in the nucleus and neurofilaments in neurons
They are supercoiled into cables
Less dynamic than microtubules or microfilaments (these structures are made to last as they are often used in cell parts that need to be preserved/permanent)
Intermediate filaments form relatively permanent cellular structures
Has rigidity and can withstand ‘pulling’
Intermediate filaments help maintain cell shape and anchor organelles
They may also remain after the cell that made them has died, as in your hair and outer layer of skin
How are cells joined together?
Cell junctions
Three main types of cell junctions
Tight junctions
Desmosomes
Gap junctions
Each differs in structure and function
All of the junctions are membrane proteins
Tight junctions
A type of intercellular junction between animal cells that prevents the leakage of material through the space between cells.
Holding neighbouring cells tightly pressed together
May form a continuous seal around each cell
Prevents movement of fluid across cell layers (controls fluids moving across the cell layers)
Desmosomes
A type of intercellular junction in animal cells, functioning as a rivet, fastening cells together.
Anchoring junction
Provides attachments between sheets of cells e.g. muscle (pull one cell, pulls the one next to it etc.)
Acts like rivets (a ‘torn muscle’ is a torn desmosome) (riveted one cell membrane to another cell membrane, the phospholipids that make up the cell membrane are not very strong therefore desmosomes connect to the intermediate filaments within the cell (the desmosome is locking into the strongest part of the cell’s cytoskeleton))
Connected into the cell by intermediate filaments
Gap junctions
A type of intercellular junction in animal cells, consisting of proteins surrounding a pore that allows the passage of materials between cells.
A point of cytoplasmic contact between two cells (cytoplasm of one cell is connected to the cytoplasm of another cell)
Has little ‘tunnelways’ which allows communication
Ions and small molecules can pass from cell to cell (can fit though tunnel ways)
Allows rapid cell to cell (intercellular) communication
Extracellular matrix
ECM: The meshwork surrounding animal cells, consisting of glycoproteins, polysaccharides, and proteoglycans synthesised and secreted by cells.
In many tissues cells do not make direct contact with other cells (but they still form a solid substance). Cells lie within an ECM, the composition of which varies between tissues.
The ECM is composed of materials secreted by cells. This secretion occurs by constitutive exocytosis.
Most ECM proteins are glycoproteins (proteins with added carbohydrates). The most abundant ECM glycoprotein is collagen. Collagen fibres have great tensile strength (made of supercoil as this structure can resist large forces).
Collagen fibres are embedded in a proteoglycan complex matrix. Proteoglycans are proteins with extensive sugar additions. Proteoglycans trap water within the ECM. (sugars are good at getting water to bring into tissues) Water resists compression and thus helps to retain tissue shape
Other glycoproteins (fibronectins) attach cells to ECM
Membrane proteins (integrins) connect the ECM to cytoskeleton
Providing a communication link from ECM to the cell interior ( the integrin will connect the fibronectin to the integrin and the integrins connect to the cytoskeleton and there is now a direct communication link) (although the outside is not an alive structure it can still communicate to the alive interior of the cell
ECM in bone, cartilage and connective tissue
Connective tissue - very loose ECM as it is a softer tissue
Cartilage - more rigid ECM than CT
Bone - ECM becomes so hard that it becomes mineralised and therefore hasn’t got much flexibility to it
