Class Notes (839,189)
Canada (511,223)
Biology (2,933)
BIO1140 (690)
Lecture

Extracellular Interactions

8 Pages
73 Views

Department
Biology
Course Code
BIO1140
Professor
Kathleen Gilmour

This preview shows pages 1,2 and half of page 3. Sign up to view the full 8 pages of the document.
Description
Topic 4: Extracellular Interactions -the ability for cells to recognize and bind to each other is important, especially in multicellular organisms (i.e.: animals, plants, fungi) -multicellular organisms have extracellular structures that are important for function -extra cellular structures are normally created by the cells they surround -the components of extra cellular structures are synthesized in the cell and exported to the extracellular environment Plant Cell Walls and Plasmodesmata -cell walls function to provide support, protection for the cell and regulates permeability, but limits the types of cell to cell connections that can occur -animal cells lack cell walls and so they can form a number of different types of cell to cell connections, depending on the function of the tissue -the cell wall is secreted by the cell -the cell wall is made up by 3 classes of molecules: 1) Structural Fibres -provide strength and rigidity to the cell wall -held in place by a hydrated, water-containing matrix -contain cellulose microfibrils -cellulose is a polysaccharide -consists of repeated units of B-D-glucose -linked by B-glycosidic linkages between C1 and C4 (stiff linkages) -cellulose is a relatively stiff, linear molecule -this makes it suitable for its function as reinforcing structural fibres in the cell wall -strings of cellulose molecules are lined up together to create a cellulose microfibril -cellulose is synthesized on the surface of the cell (the extracellular membrane) -enzymes in the cell membrane catalyze the synthesis of cellulose; these are integral transmembrane proteins known as rosettes -the rosette is an enzyme complex for cellulose synthesis -the extracellular surface of the protein (facing the external environment) has the catalytic activity for cellulose synthesis -the intracellular / cytosolic side of the rosettes link to microtubules by means of motor proteins -this creates a system that tow around the rosettes in the plasma membrane; they synthesize cellulose wherever they go -this system allows the cell to control the pattern of cellulose formation in the cell wall -in primary cell wall, you end up with a network / mesh of cellulose microfibrils in a variety of directions -in secondary cell walls, you end up with arrays of cellulose microfibrils -this is important in determining patterns of growth; where the cellulose microfibrils are rigid, the cell will not be able to grow in that direction -this is why the primary cell wall has a different organization than the secondary cell wall 2) Matrix -primarily made up of carbohydrates (90%) with some glycoprotein -main carbohydrates are hemicellulose and pectin -hemicellulose describes a collection of carbohydrates that all gave long side chains that link together to form a network -pectin is also a branched carbohydrate; the backbone of the molecule carries a lot of negative charges and tends to attract water molecules to keep the matrix hydrated, as well as to allow things to get embedded into it -pectin creates a gel-like mass (i.e.: jam) -serves as the glue / adhesive molecule that holds cell walls together and connects adjacent cell walls together -forms the bulk of the mass that surrounds the cells -cellulose microfibrils are inserted into the matrix of carbohydrates and provide the rigidity to the gel-like mass -the glycoprotein is extensin 3) Adhesive Molecules -involves pectin (see above) Primary and Secondary Cell Walls -the extracellular structures are synthesized by the cell on the external surface -in order from internal to external: 1) Middle Lamella -the first thing the cell does is lay down a layer of pectins; this is known as the middle lamella -middle lamella joins cell walls together 2) Primary Cell Walls -the cell secretes the primary cell wall after the middle lamella -the primary cell wall is the cell wall that has a network of cellulose microfibrils -this network is able to expand as the primary cell wall gets pushed further away from the cell 3) Secondary Cell Wall -layers of secondary cell wall are laid down after the primary cell wall -in each layer of secondary cell wall, the cellulose microfibrils have a different orientation -provides strength to the cell wall as a whole -the multiple layers are especially evident in woody plants and can be seen in their rigidity -secondary cell wall often contains lignin which contributes to the strength of the cell wall -the different patterns of cellulose microfibrils are achieved by the different movement of rosettes and their pattern of synthesis along microtubules and driven by motor proteins -the cell controls the pattern of microtubules and therefore the pattern of cellulose microfibrils Cell to Cell Connections in Plants 1) Plasmodesmata -cytoplasmic connections between cells -made up of a hole in the cell wall, lined with plasma membrane that allows the cytoplasm of the 2 cells to be in communication -as the cell wall is forming, there are bits of smooth endoplasmic reticulum that stretch across the location where the new cell wall is forming -the cell wall forms holes around those bits of tubule of the smooth ER (known as desmotubules which often remain in the plasmodesmata) -anything that can fit between the plasmodesmata can move from one cell to the next (i.e.: water, ions, larger molecules like plant hormones / RNA / signalling molecules, etc.) Cell to Cell Connections in Animals -animal cells have a variety of cell to cell connections because of the need to form cells into tissues -there are 3 broad types of connections: 1) Adhesive (Anchoring) Junctions: adherens, desmosomes -anchor cells together so that the tissue is integrated as a whole -hold animal cells within tissues together -adhesive junctions involve transmembrane proteins (i.e.: cadherins) -cadherins have large extracellular domains -extracellular domains contain repeated units of structure; these repeated units can be recognized by other cadherins of the same type -they are held together by non-covalent bonds -cadherins from 1 cell will bind with cadherins from the neighbouring cell and this holds the 2 cells together -the 2 sides have to match (recognize) and then link together -on the intracellular side, cadherins link to the cytoskeleton (i.e.: microfilaments or intermediate filaments) through clusters of linking proteins -when cadherins link to microfilaments, the junctions are called adherins junctions -they integrate the cytoskeleton into the junction and hold the cell together -bands of adherins junctions are usually found as a belt around the cell -adherins link to microfilaments intracellularly and cadherins extracellulary -punctate adherins junctions are spots of adherins junctions -the other type of anchoring junction is a desmosome -in this case, cadherins link to intermediate filaments -there are cadherins that are transmembrane proteins that link in the extracellular space and link to intermediate filaments on the intracellular side -they link through linking proteins; the linking proteins from dense plaques of proteins which make desmosomes strong connections -important in tissues that experience a lot of stress (i.e.: the skin, the heart, etc.) 2) Tight Junctions -seal up spaces between cells -are normally found in cell layers that separate 2 different compartments (i.e.: the gut, the kidney, etc.) -help to seal space between cells and prevent mixing of separate layers -the space between the cells is call
More Less
Unlock Document

Only pages 1,2 and half of page 3 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit