Mod6 - Interactions Between Cells in Multicellular Systems Flashcards
What are the 4 major tissue types of the body?
Connective, Epithelial, Muscular, Nervous
Describe the amount of ECM in each of the 4 tissue types and how this relates to tissue strength
Epithelial, Muscular and Nervous: little ECM, intermediate filaments and cell-cell junctions provide strength
Connective: few cells, lots of ECM provides strength
Name the 5 types of cellular junctions in animals
Tight junctions, Adherens junctions, Desmosomes, Gap junctions, Hemidesmosomes
Describe the function of tight junctions
Seals adjacent cells together in an epithelial sheet to prevent leakage of extracellular molecules between them (also helps polarise cells)
Describe the function of adherens junctions
Joins an actin bundle in one cell to a similar bundle in a neighbouring cell
Describe the function of desmosomes
Joins the intermediate filaments in one cell to those in a neighbour
Describe the function of gap junctions
Form channels that allow small, intracellular, water-soluble molecules (e.g. ions and metabolites) to pass from cell to cell
Describe the function of hemidesmosomes
Anchor intermediate filaments in a cell to the basal lamina
Which type of cellular junction do plants have, and which animal junction is it most comparable to?
Plasmodesmata -> Gap Junctions
Describe the structure of vertebrate tight junctions
Formed of strands of occludin and claudin proteins, which allow lipids in the plasma membrane to diffuse freely, but not membrane proteins
How are tight junctions related to cell polarisation?
They allow the apical and basolateral membranes to be different in composition and function, as lipids in the membrane can diffuse but proteins cannot)
What type of protein is found in adherens junctions and desmosomes (and which ion is required for this to work)?
Cadherins (calcium, Ca2+)
What type of protein is found in hemidesmosomes?
Integrins
Describe the structure of adherens junctions, and the grander structure they can be involved in?
Cadherins link to actin filaments in two neighbouring cells via linker proteins; a continuous band of AJs forms an ADHESION BELT and an actin network across the epithelium (this CAN be contractile because of the presence of myosin II)
What is the function of contractile actin networks when myosin II is involved?
Allow epithelial sheets to move, invaginate, form tubes, etc., e.g., neurulation to generate neural tube
Describe the structure of desmosomes
Cadherins link to cytoplasmic plaque made of intracellular linker proteins; this cytoplasmic plaque has keratin filaments anchored to it
Where are desmosomes found in the body?
Abundant in tissues under high shear stress e.g., heart muscle (in addition to epithelia ofc)
What are the effects of desmin mutations?
Desmosomes in adjacent muscle cells are linked to DESMIN intermediate filaments; desmin is expressed in cardiac, smooth and skeletal muscle, and desmin mutations can cause muscular dystrophy and cardiac myopathy
Describe the structure of hemidesmosomes
Integrins in the basal plasma membrane bind to both laminin (NOT LAMIN) in the basal lamina, and cytokeratin intermediate filaments inside the cell (via linker proteins)
What is the function of cadherins and how do they achieve this?
Define which cells can interact with each other to form tissues; cadherins are transmembrane proteins in the plasma membrane which bind to an IDENTICAL CADHERIN in the next cell
What are the two types of cadherin mentioned, and where are they found?
Epithelial cells express E-CADHERIN
Muscle cells express N-CADHERIN
How are cadherins and cancer related?
Cancer cells often no longer express the specific cadherins that would normally keep a cell in the right place (e.g., expressing N-cadherin instead of E-cadherin makes cells highly motile)
What may cancerous cells secrete an increased amount of?
Matrix proteases which can digest the ECM, helping them to escape through the basal lamina
Where are connexon channels found and what do they do?
GAP JUNCTIONS - connexon channels allow direct transfer of ions and small water-soluble molecules (<1000 daltons) between cells
What is the main functional difference between plant plasmodesmata and animal gap junctions?
Plasmodesmata allow larger molecules through (e.g., SER, proteins, mRNAs)
Name the two proteins that link the cytoplasm with the nucleus
SUN and KASH
What characteristic is correlated with increased collagen content?
Increased tissue stiffness?
What are the two main components of the basal lamina?
Laminin and Collagen IV
What types of cells produce collagen?
Connective tissue cells (osteoblasts in bone and fibroblasts in skin/tendons)
State the 4 stages of collagen synthesis and where they occur
- Procollagen polypeptide chain
- Triple-stranded helical procollagen molecule (trimerisation occurs in the ER)
- Collagen fibril (OUTSIDE THE CELL)
- Collagen fibres (OUTSIDE THE CELL)
What vitamin is required for procollagen trimerisation?
Vitamin C (ascorbic acid) -> hence scurvy when deficient
What happens to convert procollagen to collagen?
Procollagen cannot assemble into fibrils until it is cleaved by a PROTEASE (which mainly happens outside the cell)
Describe how collagen is organised as it is synthesised (idk how to phrase this better)
Collagen fibres must be properly aligned:
- cells deposit collagen in an oriented way
- cells rearrange the fibres after secretion by pulling on them
Name 3 types of diseases due to ECM protein mutations
Abnormally stretchy skin, Brittle bone disease, Skeletal abnormalities (e.g. achondroplasia)
Describe the function of integrins (apart from those found in hemidesmosomes)
Focal Adhesions: they link the Extracellular Matrix to the Cell’s Cytoskeleton
What is the linker protein by which cells attach to collagen?
Fibronectin
Describe the role of fibronectin
It is the linker protein which links collagen to integrins - the integrins then bind to actin filaments inside the cell via adaptor proteins
In what two ways is integrin function regulated
- Binding to an intracellular protein - or an extracellular matrix molecule - causes integrins to switch to an activated state
- Phosphorylation can INACTIVATE integrins (e.g. in mitosis)
What is in the “spaces” between collagen in the ECM?
Gels of polysaccharide (Glycosaminoglycans) and glycoprotein (Proteoglycans)
Describe the structure of Glycosaminoglycans (GAGs)
- Large, negatively charged polysaccharides
- Strongly hydrophilic
- Occupy a large volume for their mass
Describe the structure of proteoglycans
They are extracellular (secreted) proteins with covalently-linked GAGs, forming a “toilet brush” structure
How do GAGs and collagen make cartilage tough and resistant?
GAGs generate swelling pressure because they bind water molecules; this pressure is resisted by collagen fibres
How and where are proteoglycans synthesised?
Protein component is made in the ER, where glycosylation starts; Glycosylation is completed in the Golgi apparatus; proteoglycan is delivered to plasma membrane by constitutive secretion
Which proteoglycan is made by a different synthesis process, and why?
Hyaluronan - it is made of ONLY carbohydrate, containing no protein at all
Why must plants have cell walls to provide strength, while animal cells do not?
PLANT CELLS LACK INTERMEDIATE FILAMENTS
Describe the structure and function of plant cell walls
They resist turgor pressure (both compression and tension); long fibres are oriented along the lines of stress
Distinguish between primary and secondary cell wall structure in plants
Primary: relatively thin, laid down first and allows the cell to grow (cell expansion is driven by turgor pressure)
Secondary: laid down after, composition determines cell properties (e.g., hard and thick in wood due to lignin; thin, flexible and waxy in leaves)
What molecules/structure actually gives the plant cell wall its tensile strength?
Cellulose fibres (these are interwoven with other polysaccharides, e.g., pectin for compression resistance and lignin in wood)
What is the role of pectin in plant cells?
It fills spaces, resists compression and sticks neighbouring cells together
Describe the process of cellulose synthesis in plants
Cellulose fibres are synthesised at the cell membrane (similar to hyaluronan synthesis in animals):
Cellulose Synthase is made in the ER and transported via the Golgi to the plasma membrane, where it makes many cellulose molecules and assembles them into a microfibril
State how cellulose is ORGANISED in the plant cell wall
Microtubules inside the cell determine the orientation of fibre deposition outside the cell
