Extracellular components
and connections between
cells
Plasma membrane
• Regulates what goes in and out of the cell…
• Outside of cell, there is the ECM – Extracellular matrix.
• Glycoproteins
• Collagen – forms strong fibers outside cells, 40% of protein in
body
• Integrins – cell surface receptor proteins, bind to associated
proteins attached to microfilaments of the cytoskeleton.
• Transmit signals between the ECM and the cytoskeleton,
integrate changes inside and outside of cell.
Figure 4.26a
Cell junctions in animal cells
• Especially common in epithelial cells
• Tight junctions: plasma membranes tightly pressed against
each other, prevent leakage of extracellular fluid across a layer
of epithelial cells. Ex. Between skin cells – watertight.
• Desmososmes – (anchoring junctions) – like rivets, fastening
to cells into strong sheets. Attach muscle cells to each other
• Gap junctions (like plasmodesmata in plant cells) –
Communicating junctions, cytoplasmic channels from one cell
to an adjacent cell. Membrane protein surrounding a pore
that ions, sugars and AA pass. Ec. Heart muscle, animal
embryos.
Tight junctions prevent
fluid from moving
across a layer of cells
Tight
junction
TEM
0.5 m
Tight
junction
Intermediate
filaments
Desmosome
TEM
1 m
Gap
junction
Ions or small
molecules
Space
between cells
Plasma membranes
of adjacent cells
Extracellular
matrix
TEM
Figure 4.27
0.1 m
Cell wall of plants
• Protection, maintains shape, prevents excessive uptake of
water
• Thicker than cell membrane
• Contains cellulose
Primary cell wall – in young plants
Middle lamella – thin layer between primary walls of cells,
contain pectins that are sticky and glues adjacent cells together.
When mature, the cell may reinforce the primary cell wall or add
a secondary cell wall between the plasma membrane and
primary wall. Ex. Wood consists mainly of secondary walls
Figure 4.25
Secondary
cell wall
Primary
cell wall
Middle
lamella
1 m
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
Cell Junctions in plant cells
• Plasmodesmata – in plant cells
• Non-living cell walls are perforated with membrane-lined
channels filled with cytosol.
• Make cells continuous with each other.
• Water and small solutes pass freely from cell to cell, proteins and
RNA may as well.