EFB325 Cell Physiology

The extracellular matrix

Tissues in multicellular organisms are assembled from individual cells. The structure and integrity of those tissues relies on structures and supports within (the cytoskeleton) and outside (the extracellular matrix) those individual cells, as well as junctions that hold cells together.

Tissues and cells in animals are structurally diverse

• although all cells have a extracellular matrix (ECM) outside of the plasma membrane, some cells deposit a thick and mechanically important ECM; others cells have a very thin ECM around the cells and rely instead on the cytoskeleton for mechanical support
• animal tissues are primarily of four types: epithelial, connective, nervous, and muscle
• connective tissues have extensive ECM, which carries mechanical load (other tissues have little ECM)

The extracellular matrix of animal cells is a mesh strengthened by collagen protein

Collagen protein

• collagen is a fibrous protein, which provides the tensile strength of the ECM
• it is the most abundant protein in a mammal (~25% of total protein mass)
• prominent in connective tissue-cartilage, bone, tendons, ligaments
• collagen proteins bundle together as three strands in a helix in the ER - these are actually procollagen strands, these helices pack together as fibrils once they are secreted out of the cell and the procollagen is cleaved to its mature form, these further aggregate into thick collagen fibers (each fiber may have more than 700 individual collagen protein strands)
• different tissues produce different types of collagen

Elastic tissues have fibers of elastin

• found in skin, lungs, intestine
• forms loose framework of cross-linked chains (see Figure 5-21 on page 153)
• during aging, collagen is more cross-linked, rigid; while less elastin accumulates-bones brittle, skin responds to deformation more slowly (pinching)

Collagen fibers are embedded in matrix of glycoproteins-up to 95% carbohydrate

• the carbohydrates are mainly glycosaminoglycans (GAGs)=polymers of disaccharides
• one of the sugars in the disaccharide is always an amino sugar (like N-acetylglucosamine, abbreviated GlcNAc or N-acetylgalactosamine, abbreviated GalNAc)
• GAG chains are linked to a core polypeptide

IN CARTILAGE core polypeptide/GAGs associate with long carbohydrate polymers to form a huge complex which associates with collagen

• the glycoproteins absorb and hold alot of water making the cartilage resistant to compression and able to regain shape after distortion (makes cartilage flexible)

Q. What holds the ECM in place?

A. A protein link between the collagen/glycoprotein matrix and actin microfilaments in the cytoskeleton (this protein link passes through the plasma membrane)

• fibronectin proteins bind to collagen fibers in the ECM (they have a collagen-binding domain)
• the fibronectin proteins also have a domain that is recognized by a cell surface receptor protein
• integrins are the receptor proteins that bind to fibronectin-they have a fibronectin binding domain
• integrin proteins pass through the plasma membrane and bind to actin microfilaments (with the help of adaptor proteins)
• the connection between integrin and the actin microfilaments can be regulated through development to allow changes in cell shape

There is a special layer of ECM that supports a sheet of epithelial cells=basal lamina

• the basal lamina is made up of a special type of collagen
• laminin binds to the collagen in the basal lamina and also binds to integrins (serves a very similar function as the fibronectins)

The extracellular matrix of plants = cell walls

Plant cell walls play a major role in defining the shape of the plant and in protecting it from injury and pathogens.

• cell walls are rigid and very sturdy
• can withstand the turgor pressure within the cell (force exerted by the cytoplasm against the cell wall)
• the cell wall is generally permeable to the flow of water, small ions, and metabolites-but blocks movement of macromolecules
• cells in different types of tissues and at different stages of growth and development can have cell walls with different properties (sometimes the walls are flexible, some are rigid, some are thin, some are thick, some are waxy, . . .)

Plant cell walls are made of cellulose microfibrils in a matrix of polysaccharides and proteins

Cellulose

• unbranched polymer of glucose (500-6000 glucose per molecule) linked by beta(1,4) bonds
• long, linear molecules, stabilized by hydrogen bonding
• many (60-70) cellulose molecules are hydrogen bonded together to form a microfibril
  
• the orientation of the cellulose microfibrils in the cell wall is determined by microtubules in the cytoskeleton
• this orientation is important, because cell expansion and division will occur perpendicular to the orientation of the cellulose microfibrils (they are separated, not broken)

Cellulose microfibrils are embedded in a hemicellulose, pectin, and protein matrix

• hemicelluloses are beta(1,4) linked glucose chains with side chains of sugars linked on (xylose, galactose, arabinose, fucose)
• pectins are branched chains of mainly galacturonic acid, also rhamnose sugar; pectins trap alot of water, making a gel; pectins are negatively charged & bind Ca²⁺, which cross-links pectins to other cell wall components
• cell wall proteins (main ones are called extensins) are often rich in lysine (lysines are positively charged & bind to pectins); cell wall proteins are often glycosylated (have short carbohydrate chains attached)-act to further cross-link wall components
• mature cell walls (in woody tissues) also contain lignin - a large polymer of aromatic alcohols which is highly insoluble and makes the cell walls rigid and strong

Primary cell wall

• deposited during, just after cell division
• flexible, extensible

Secondary cell wall

• deposited during final differentiation, cell maturity
• deposited inside the primary cell wall
• includes the addition of more cellulose, also lignin
• very rigid, final shape of cell

Plant cells are connected by a gel of polysaccharides- mainly pectin=middle lamella

• may also be open spaces between cells

Adjoining plant cells are also connected by plasmodesmata (see discussion below-together with gap junctions)

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