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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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
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
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.
