BIOL #09: Cell Communication (II) Flashcards

1
Q

How Do Adjacent Cells Connect and Communicate?

A
  • Unicellular organisms do not usually connect to one another
    + physical connections between cells are the basis of multicellularity.
  • Cells of multicellular organisms adhere to one another and have specific, distinct structures and functions.
    + Groups of similar cells performing similar functions comprise tissues.
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2
Q

Cell-Cell Attachments

A
  • The structures that hold cells together vary among multicellular organisms.
  • Cell-cell connections help adjacent cells adhere to each other.
  • Cell-cell gaps allow adjacent cells to communicate with each other.
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3
Q

Connections between Plant Cells

A
  • Plant cells are glued together by the middle lamella, which is:
    + Composed of gelatinous pectins.
  • Pectins are polysaccharides.
    + Continuous with the adjacent plant cells’ primary cell walls.
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4
Q

Connections between Animal Cells

A
  • A middle-lamella-like layer exists between cells in many animal tissues.
    + made of gelatinous polysaccharides
    + polysaccharide glue may be reinforced by cable-like proteins (e.g. collagen) that span the ECM to connect adjacent cells.
  • In animal cells there are two main types of cell junctions that hold cells together: Tight junctions & Desmosomes
    + These junctions are especially common in epithelial tissue, which lines the external and internal surfaces of the body.
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5
Q

Tight Junctions

A
  • Tight junctions
    + composed of specialized proteins in the plasma membranes of adjacent animal cells.
  • proteins line up and bind to each other and form a watertight seal between the two plasma membranes.
  • Tight junctions are usually found between cells in tissues that form a barrier, such as the tissue lining the stomach or bladder.
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6
Q

Desmosomes

A
  • Desmosomes are made of proteins that link the cytoskeletons of adjacent cells.
    + These proteins bind to each other and to the proteins that anchor cytoskeletal intermediate filaments.
  • Desmosomes are common in epithelial and muscle tissue.
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7
Q

The Molecular Basis of Selective Adhesion

A
  • Animal cells attach to each other selectively because there are several classes of cell adhesion proteins.
  • Each major cell type has its own cell adhesion proteins.
  • These cell-cell connections are also species and tissue specific.
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8
Q

Cell Communication via Cell-Cell Gaps

A
  • Direct connections between cells in the same tissue allow cells to communicate and work together in a coordinated fashion.
  • Plant cells are connected by plasmodesmata.
  • Cells in animal tissues are connected by gap junctions.
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9
Q

Plasmodesmata

A
  • Plant cell walls are perforated with plasmodesmata, membrane-lined channels filled with cytoplasm.
  • Cytosol passes through the plasmodesmata, joining the internal environments of adjacent cells.
    + These connection unify most of a plant into a single, continuous living system
  • Water and small solutes can pass freely between cells.
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10
Q

Gap Junctions

A
  • Gap junctions provide cytoplasmic channels between adjacent animal cells similar to plasmodesmata of plants.
  • Ions, sugars, amino acids and other small molecules can pass through gap junctions.
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11
Q

Overview of Cell Signaling

A
  • Signal transduction pathway

+ a series of steps by which a signal on a cell’s surface is converted into a specific cellular response

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12
Q

Where Do Cell Signals Come From?

A
  • The signals received by cells, whether originating from other cells or from changes in the physical environment, take various forms – including light and touch.
  • Most cells communicate with each other via chemical signals.
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13
Q

Evolution of Cell Signaling

A
  • Pathway similarities between prokaryotes and eukaryotes:
    + suggest that ancestral signaling molecules evolved in prokaryotes.
    + modified later in single-celled and multicellular eukaryotes.
  • Conclusion: a shared evolutionary history of cell signaling mechanisms in prokaryotes and eukaryotes.
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14
Q

Quorum Sensing in Bacteria

A
  • Unicellular organisms live together and communicate with one another.
    + Cell-cell communication in bacteria is called quorum sensing.
  • Bacteria release species-specific signaling molecules when their numbers reach a specific threshold.
    + Quorum sensing allows bacteria populations to carry out activities that are only productive when performed by a critical mass of cells in synchrony
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15
Q

Direct Contact

A
  • One type of local signaling
  • Cell junctions involve direct contact between the cytoplasm of cells
  • Cell-cell recognition involves direct contact between membrane molecules
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16
Q

Local Regulators

A
  • Another type of local signaling
  • Involves messenger molecules that are secreted by signaling cells and only travel short distances
    + Example: growth factors are compounds that stimulate nearby cells to grow and divide
  • In animal cells, local signaling involving regulators is called paracrine signaling
17
Q

How Do Distant Cells Communicate?

A
  • The activities of cells, tissues, and organs in different parts of a multicellular organism are coordinated by long-distance signals.
  • Hormones are long-distance messengers
  • Hormones are information-carrying molecules
    + Usually small
    + Typically present in minute concentrations
    + Have a large impact on the condition of the organism as a whole
18
Q

Endocrine Signaling

A
  • Hormonal signaling in animals is known as endocrine signaling
  • Pathway of endocrine signaling:
    + Secreted from a cell
    + Circulate in the body via the circulatory system
    + Act on target cells far from the signaling cell.
19
Q

Hormones Vary Widely in Effect and Structure

A
  • In addition to differences in their chemical structure and their effects on target proteins, hormones may be soluble or insoluble in lipids.
  • Lipid-soluble hormones
    + Diffuse across the plasma membrane into the cytoplasm of target cells.
  • Lipid-insoluble hormones
    + Are large or hydrophilic
    + Do not cross the plasma membrane but instead bind to a receptor on the plasma membrane of a target cell.
20
Q

Steps of Cell-Cell Signaling

A
  • Cell-cell signaling occurs in four steps:
    1) Signal reception
    2) Signal transduction
    3) Signal response
    4) Signal deactivation (termination)
21
Q

Step 1: Signal Reception

A
  • Hormones and other cell-cell signals bind to signal receptors.
    + presence of appropriate receptor protein dictates which cells respond to a particular hormone, these are called target cells.
  • Identical receptors in diverse cells and tissues allow long-distance signals to coordinate the activities of cells throughout a multicellular organism.
22
Q

Signal Receptors

A
  • proteins that change their shape or activity after binding to a signaling molecule.
    + Receptors are dynamic and may change in their sensitivity to particular hormones.
    + Most signal receptors are located in the plasma membrane.
    + Receptors can be blocked.
    + Cells only respond to signals if they have the correct receptor molecules.
  • Signal receptors that bind to lipid-soluble hormones (e.g. testosterone) are located inside the cell (intracellular receptors)
    + Most signal receptors are located in the plasma membrane
  • Some signalers are small enough that they can pass through the membrane even if they are not lipid-soluble (e.g. nitric oxide, NO)
23
Q

Step 2: Signal Processing

A

Lipid-soluble hormones that cross the plasma membrane produce different cell responses than lipid-insoluble hormones that bind to membrane receptors.

24
Q

Signal Processing in Lipid-Soluble Hormones

A
  • Lipid-soluble steroid hormones bind to receptors inside the cell and trigger a change in the cell’s activity directly.
  • In this case the hormone-receptor complex is transported to the nucleus, where it alters gene expression.
25
Q

Signal Processing in Lipid-Insoluble Hormones

A
  • Hormones that cannot diffuse across the plasma membrane bind to membrane receptors.
  • When a signal binds to the cell surface it triggers a complex series of events, collectively called a signal transduction pathway and converts the extracellular hormone signal to an intracellular signal.
  • The message transmitted by a hormone can be amplified as it changes form.
    + Signal transduction occurs at the plasma membrane.
    + Amplification occurs inside.
26
Q

Signal Amplification in Lipid-Insoluble Hormones

A

Signal amplification: If some of the molecules in a multi-step pathway transmit the signal to numerous molecules at the next step, the result can be a large number of activated molecules by the end of the pathway.

27
Q

Fine-tuning with Lipid-Insoluble Hormone Signaling

A

The multi-step pathways that are involved with this type of signaling provides more opportunities for coordination and regulation, which allows fine-tuning of responses.

28
Q

Signal Transduction

A
  • Signal transduction involves G proteins or enzyme-linked receptors.
    + G proteins trigger the production of an intracellular messenger.
    + Enzyme-linked receptors trigger the activation of a series of proteins inside the cell.
29
Q

G Proteins

A
  • G proteins are intracellular peripheral membrane proteins that are closely associated with transmembrane signal receptors.
  • When G proteins are activated by a signal receptor, they trigger the production of messengers inside the cell.
    + G proteins link the receipt of an extracellular signal to the production of an intracellular signal.
  • G proteins are activated when they bind GTP (guanosine triphosphate) and are deactivated when they hydrolyze the bound GTP to GDP.
30
Q

G Proteins and Signal Transduction

A
  • Linking an external signal to the production of an intracellular signal involves three steps:
    1) Hormone binds to the membrane receptor, which changes shape and activates the G protein
    2) G protein exchanges GDP for GTP and splits into two parts.
    3) One part of the G protein activates a membrane enzyme, which catalyzes the production of a second messenger.
31
Q

Second Messengers

A
  • Second messengers
    + small molecules that diffuse rapidly throughout the cell, amplifying the hormone signal (e.g. cAMP).
  • Some work by activating protein kinases
    + which add a phosphate group to (phosphorylate) other proteins.
    + Can result in a phosphorylation cascade
32
Q

Phosphorylation Cascade

A
  • A phosphorylation cascade
    + The protein kinase is active when phosphorylated and inactive when the phosphate is removed – continuing along a chain to a receptor
33
Q

Results of Signal Processing

A
  • Many key signal transduction events in cells occur via G proteins or enzyme-linked receptors.
  • The signal transduction event has two results:
    1) Easily transmitted extracellular message is converted into an intracellular message.
    2) Original message is often amplified many times over.
34
Q

Step 3: Signal Response

A
  • The ultimate response to a cell-cell signal varies from signal to signal and from cell to cell, but fall into two general categories:

1) A change in which genes are being expressed in the target cell
2) Activate or deactivate a particular target protein that already exists in the cell

35
Q

Step 4: Signal Deactivation

A
  • Turning off cell signals is just as important as turning them on.
  • Cells have automatic and rapid mechanisms for signal deactivation.
    + These mechanisms allow the cell to remain sensitive to small changes in the concentration of hormones
    + Cells may vary in the number and activity of signal receptors.
36
Q

Other Types Of Signaling

A
  • ligand-gated ion channel receptor acts as a gate when the receptor changes shape
    + A ligand is a molecule that specifically binds to another molecule, often a larger one
  • When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor
37
Q

Interactions between Signaling Pathways

A
  • Each cell
    + has many intracellular and membrane signal receptors
    + receives an almost constant stream of different chemical signals due to changes in the environment.
  • When signal transduction pathways are triggered by many signals, their receptors can intersect and connect to form a complex network, which allows cells to have an integrated response to an array of extracellular signals.
38
Q

The Specificity of Cell Signaling and Coordination of the Response

A
  • Different cells have different collections of proteins
    + Different proteins allow cells to detect and respond to different signals
    + The same signal can have different effects in cells with different proteins and pathways
  • Pathway branching and “cross-talk”(interaction) between pathways further help the cell coordinate incoming signals
  • Cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal.