Cell-cell interactions involve the cell surface, extracellular matrix or cell wall, and connections between cells. Neighboring plant cells are joined by plasmodesmata, which allow sharing of cytoplasm, while animal cells use tight junctions and desmosomes. Gap junctions and plasmodesmata form direct connections, and hormones facilitate long-distance communication between distant cells.

1. Cell-Cell Interactions
Dr. Corl
BIOL 105
September 13, 2013

2. Cell-Cell Interactions
• The cell surface
• The extracellular layer
• Cell-cell connections
– Cell-cell attachments
– Cell-cell gaps
• Cell-cell communication (long distance)

3. The Cell Surface
• Recall the structure of the plasma (cell) membrane:
– Phospholipid bilayer w/ cholesterol molecules interspersed
– Both integral proteins and peripheral proteins
• Many of which have carbohydrate groups covalently attached!

4. The Extracellular Layer
• Most organisms have an extracellular layer just
exterior to the plasma membrane:
– Provides an extra layer of protection / defense.
– Helps define cell shape.
– Helps attach one cell to a neighboring cell.
• Broad types of extracellular layers:
– Cell wall:
• Surrounds plant, fungi, bacteria, and algal cells.
– Extracellular matrix:
• Surrounds animal cells.

5. The Extracellular Layer
• Usually “fiber composites”:
– Cross-linked network of long filaments (fibers)
surrounded by a stiff ground substance.
– Protects cell from stretching (tension) and compression.

6. The Plant Primary Cell Wall
• Fibrous components = Cellulose microfibrils.
• Ground substance = Pectins and other
gelatinous polysaccharides.

7. The Plant Cell Wall
• Primary cell wall:
– Defines shape of plant cell.
– Counteracts force of water entering the plant cell via osmosis: cell
wall exerts wall pressure.
• Secondary cell wall:
– Secreted by certain plant cells. (e.g. xylem cells, above)
– Secreted interior to the primary cell wall.
– Can provide tough structural support (contains lignin).

8. The Plant Cell Wall
• Secondary cell wall:
– Contains the durable polymer lignin.
– Found primarily in the xylem (water conducting)
tissue of plants with a true vascular system:
• e.g. Ferns, “evergreen plants,” and flowering plants.
– Adaptation that allows vascular plants to grow tall
and resist the force of gravity:
• Xylem system acts like an internal skeleton!

9. The
Extracellular
Matrix (ECM)
• Fiber composite secreted
by animal cells.
• Fibrous component:
– Cable-like collagen protein
• Ground substance:
– Rich in proteoglycan
complexes:
• Contain hundreds of
proteoglycan molecules:
– Core protein with many
hydrophilic carbohydrate
chains attached.

10. The Extracellular Matrix (ECM)
• Provides structural support.
• More pliable (flexible) than the plant cell wall.
• Helps cells adhere to each other.

11. The Extracellular Matrix (ECM)
• The cell’s internal cytoskeleton is physically connected to
the ECM via protein-protein interactions.

12. The Extracellular Matrix (ECM)
• Specifically, actin filaments are linked to transmembrane
proteins called integrins, which are linked to proteins (e.g.
fibronectins and laminins) which are linked to collagen proteins.

13. Cell-Cell Interactions
• The cell surface
• The extracellular layer
• Cell-cell connections
– Cell-cell attachments
– Cell-cell gaps
• Cell-cell communication (long distance)

14. Cell-Cell Connections
• Unicellular organisms may secrete
polysaccharide-rich biofilms, connecting
them to each other and to the substrate.
– e.g. Dental plaque in your mouth!

15. Multicellularity Through
Cell-Cell Connections
• In multicellular organisms (e.g. plants and
animals), various types cell-cell attachments
and cell-cell gaps help to connect neighboring
cells within a given tissue.

16. Cell-Cell Attachments
• Middle lamella (plants)
– Joins neighboring cell walls.
• Tight junctions (animals)
• Desmosomes (animals)

17. Middle Lamella (Plants)
• Gelatinous polysaccharides (pectins) glue
together neighboring plant cell walls.

18. Tight
Junctions
(Animals)
• Specialized proteins from adjacent cell
membranes line up and bind to each
other, “stitching” the cells together.

19. Tight Junctions
• Can form a watertight seal between cells.
• Common in cells lining your skin, stomach,
intestines, and bladder.

20. Desmosomes
(Animals)
• Anchoring and membrane proteins binding to
each other and to intermediate filaments link
the cytoskeletons of adjacent cells.

21. Desmosomes
• Made of proteins that link the cytoskeletons of
adjacent cells.
• Common in epithelial and muscle cells.

22. Cadherins
• A major class of cell adhesion proteins.
• An important component of desmosomes.
• Different types of cells express different types
of cadherins on their plasma membranes.
– Selective adhesion: adjacent cells of the same
cell type often adhere to one another due to
interactions of their cell-type specific cadherins.

23. Cell-Cell Interactions
• The cell surface
• The extracellular layer
• Cell-cell connections
– Cell-cell attachments
– Cell-cell gaps
• Cell-cell communication (long distance)

24. Cell-Cell Gaps
• Create a direct connection between the
cytoplasm of adjacent cells.
• Allows neighboring cells to communicate directly
through membrane “holes” and channels.
• Two major types:
– Plasmodesmata (plants)
– Gap junctions (animals)

25. Plasmodesmata (Plants)
• Cell-cell gaps connecting adjacent plant cells.
• Lined with plasma membrane.
• Allows a plant cell to directly share cytoplasm with
neighboring plant cells.

26. Plasmodesmata (Plants)
• Function in movement of water:
– Speeds the movement of water from the root
exterior to the root interior (location of xylem).
• Function in movement of sugars:
– Speeds the movement of sugars between
adjacent phloem cells.

27. • Water, sugars, and other molecules
can travel through plant tissues via the:
– Symplastic route:
• Traveling via the symplast (continuous network of shared
cytoplasm between plant cells connected by plasmodesmata)
– Apoplastic route:
• Traveling around plant cells (e.g. through porous cell walls
and the middle lamella) without actually entering the
cytoplasm of individual cells.
• Apoplast: Extracellular space around cells.
Plasmodesmata
(Plants)

28. Gap Junctions (Animals)
• Each gap junction consists of many channels (made
of _______) that connect adjacent ______ cells.
• Allow water, ions, and small molecules to move
between adjacent cells.

29. Gap Junctions (Animals)
• Extensively found within _____ muscle tissue:
– Speeds conduction of electrical impulses throughout the heart,
coordinating heart muscle contraction (your heartbeat!).
• Also found (to a limited extent) within ________ tissue:
– Allow electrical impulses to directly flow from neuron to neuron.

30. Cell-Cell Connections:
Summary
______ junctions
__________
_____ junction

31. Cell-Cell Interactions
• The cell surface
• The extracellular layer
• Cell-cell connections
– Cell-cell attachments
– Cell-cell gaps
• Cell-cell communication (long distance)

32. Long Distance Communication
• Distant cells communicate with each other
via ________:
– Information carrying molecules that:
• Are secreted by a cell,
• ________ in the body, and
• Act on target cells far from the original cell.
– ____ concentrations of hormones can have a
large impact on target cells!
– Hormone function and structure vary widely.
• Lipid soluble (steroids) vs. non lipid soluble.

33. Hormone Signal Receptors
• Signal receptors are ________ that change
conformation (shape) upon hormone binding.
• Each hormone binds to a specific type of signal
receptor:
– Steroid receptors: Located in ______.
– Other hormone receptors: Located in cell ________.
• To _______ to a particular hormone, a cell must
express the appropriate signal receptor!

34. Steroid
Hormone
Receptors
• ______ diffuses across plasma membrane
and binds to receptor in cytosol.
• Hormone-receptor complex can enter ______
and change gene activity.

35. Steroid Hormone: Estradiol
• Estradiol, for example:
– Is released by follicle cells in the _______ of females.
– Binds to ________ within the ______ of various cell
types, ultimately causing target cells to:
• Differentiate (mammary gland cells during puberty).
• Proliferate (endometrial cells lining the uterine wall).
• Produce and secrete its own hormones (hypothalamic neurons).

36. Other Hormone Receptors
• Non-lipid soluble (non-steroid) hormones bind
to receptors on plasma ________.
• Signal ____________:
– Conversion of an extracellular signal (hormone) to
an intracellular signal.

37. Signal
Transduction
Pathways
• Involve several steps.
• Message is _________ as it changes form.

38. Non-Steroid
Hormone:
Epinephrine
• Epinephrine is a non-steroid hormone:
– Produced and released by the _______
glands in response to short-term stress.
– Binds to epinephrine ________ embedded
in the cell membranes of liver cells:
• Triggers a signal transduction cascade that
ultimately activates phosphorylase:
– Enzyme that helps convert glycogen to ________.

39. Signal Transduction Pathways
• G-protein cascades:
– Binding of hormone to receptor activates a _____
inside the cell, which then in turn activates other
proteins inside the cell.
– e.g. Epinephrine binding to epinephrine receptor on
liver cell membranes.
• Enzyme-linked receptor cascades:
– Binding of hormone to receptor triggers a cascade of
phosphorylation events inside cell.
• Usually, the hormone-bound receptor is the first target to be
phosphorylated. (Autophosphorylation)
– e.g. _______ binding to insulin receptor on liver cell
membranes.

40. G-Protein Cascades
• G-protein initially in “___” conformation.
• Signal (hormone) binds to _______.

41. G-Protein Cascades
• Receptor changes _____ and activates.
• G-protein activates (turns on) and ____.

42. G-Protein Cascades
• Activated G-protein binds to and activates an _____.
• Enzyme catalyzes formation of a ______ messenger.
• Second messenger triggers a ______.

43. Second
Messengers
• ________ intracellular signaling molecules.
• May open ion ______ or activate protein kinases.
• Protein _______:
– Enzymes that activate/inactivate other proteins by
adding phosphate groups to them (phosphorylation).

44. Epinephrine Action
• 1.) Epinephrine binds to
and activates the
epinephrine _______ on
liver cell membranes.
• 2.) Receptor activates an
intracellular _______:
– G-protein activates an
enzyme, adenylyl cyclase.
• 3.) Adenylyl cyclase
catalyzes the formation of
a second messenger,
cyclic AMP (_______).

45. Epinephrine Action
• 4.) cAMP activates the
enzyme protein _____ A.
• 5.) Protein kinase A
activates phosphorylase
kinase.
• 6.) Phosphorylase kinase
activates phosphorylase.
• 7.) Activated
phosphorylase catalyzes
the cleavage of _______
into _______ monomers!

46. Enzyme-linked Receptors
• Hormone binding to receptor results in
autophosphorylation and __________ of
receptor.
• Activated receptors then induce
phosphorylation of many other _______ in
the cell: a phosphorylation cascade.
• Cascade causes _________ of signal.
• Best understood subgroup:
– Receptor tyrosine kinases (RTKs)

47. Enzyme-linked Receptors

48. Enzyme-linked Receptors

49. Enzyme-linked Receptors
Signal amplification!

50. Insulin Action
• Insulin is a non-steroid hormone:
– Released by the _______ in response to
elevated blood glucose levels.
– Binds to insulin _______ on the cell
membrane of ______ cells:
• Enzyme-linked receptors that initiate a
“phosphorylation” cascade within the liver cell.

51. Insulin Action
• 1.) Insulin binds to insulin
receptor on liver cells.
• 2.) Insulin ______
becomes phosphorylated.
• 3.) _____ protein
becomes activated.
• 4.) Ras activates an
______ called MAPKKK.
• 5.) MAPKKK activates
another enzyme: MAPKK.

52. Insulin Action
• 6.) MAPKK activates
another enzyme: MAPK.
• 7.) MAPK activates a
transcription factor, which
enters the ______.
• 8.) Transcription factor
increases the the
expression of _______
involved in glycogen
synthesis.
• 9.) Liver synthesizes more
_________ from glucose
monomers.

53. Signal
Transduction
Pathways
• Convert an
extracellular signal to
an intracellular signal.
• Original message is
__________ as it
changes form.
• May ultimately lead to
the activation of:
– Intracellular _______
– _________ factors
– Membrane channels

54. Signal Deactivation
• How are cell signals turned off?
– Hormone ______ away from receptor.
– G-proteins turn back “____” - deactivate.
– Second messengers are degraded.
– Phosphatases remove _______ groups
from proteins.

55. Signal Transduction Pathways
• As a biologist, you
will encounter
signal transduction
pathways often,
especially when
studying:
– The _______
system
– The _______
system
– The nervous
system

56. Cell-Cell Interactions
• The cell surface
• The extracellular layer
• Cell-cell connections
– Cell-cell attachments
– Cell-cell gaps
• Cell-cell communication (long distance)

57. Review Questions
• Contrast the extracellular matrix in animals
versus the plant cell wall.
• What are some different ways that
neighboring cells can be joined to one
another?
• How do plasmodesmata differ from gap
junctions?

58. Review Questions
• How do steroid hormones differ from
non-steroid hormones?
• Draw out a G-protein signaling cascade.
• Draw out an enzyme-linked receptor
signaling cascade.