Chapter 5: Cellular Signals

Question 1

5 examples of chemical signals?

Answer 1

1. Hormones
2. Neural transmitters
3. Cytokines
4. Antibodies
5. Pheromones

Question 2

Stimulus response model

Generalised model steps?

Answer 2

1. Stimulus - a change in the environment.
2. Receptor - senses the stimulus.
3. Modulator - a signal is sent to the brain to interpret the stimulus.
4. Effector - produce a response to the stimulus.
5. Response - the change resulting from detecting the stimulus.

Question 3

3 stages of cellular communication

Answer 3

1. Signal reception - an extra cellular signal molecule binds to a specific receptor protein molecule on a target cell. Signalling molecule and binding site of receptor protein have complimentary shapes.
2. Signal transduction - the activates receptor triggers a cascade of events within the cell. Many different enzymes are involved.
3. Signal response - effector proteins are produced by gene activity that produces a specific cellular response. E.g. apoptosis, cell division.

Question 4

What are receptors?

Answer 4

Proteins that receive various signals.

Question 5

How can signals move from source to target?

4 ways?

Answer 5

1. Long-distance travel to target cells: hormones secreted by various endocrine glands - travel via the circulatory system.
2. Travel to nearby cells: move by diffusion through the interstitial fluid around cells.
3. One cell sends and receives signal: signal released by one cell may be received by the same cell. Immune cells.
4. Direct cell-to-cell contact: structural features enable signal to move directly from the cytosol of one cell to that of another cell.
   • gap junctions (in animal tissues): pores in the plasma membranes of adjacent cells. Transmission of various small molecules and electrical signals. E.g. heart muscle - spread of an electrical pulse.
   • plasmodesmata (in plant tissues): gaps through cell walls.

Question 6

What are target cells?

Answer 6

Cells that possess the receptors to bind a particular signal molecule.

Question 7

Types of cell signalling

4 types?

Answer 7

1. Endocrine signalling: carried in the bloodstream to target cells. Often long distance. May involve hormones or cytokines as signalling molecule. E.g. production of insulin —> stimulates cellular uptake of glucose.
2. Paracrine signalling: occurs between nearby cells. Neurotransmitters or cytokines are the signalling molecules. E.g. neurotransmitters travel across synapse (gap) to another cell —> response.
3. Autocrine signalling - cell produces and reacts to own signals. Growth, development, immune system. E.g. presence of foreign antibody —> T-cells produces a growth factor —> stimulate own production.
4. Pheromones: chemical signals released into external environment. Affect physiology or behaviour of members of same species. E.g. pheromone released by female silk moth —> detected by male’s antennae —> male’s pheromone binding protein binds it —> wing fluttering response.

Question 8

Types of signal transduction

Answer 8

1. Hydrophobic signalling molecule (lipophilic)
   • can cross phospholipid bilayer of plasma membrane.
   • binds to intracellular receptors - located in the cytosol or in the nucleus.
   • once bound to receptor, it becomes a hormone-receptor protein complex that can now act as a transcription factor and actives certain genes.
   • E.g. steroids.
2. Hydrophilic signalling molecules (lipophobic)
   • cannot cross phospholipid bilayer of plasma membrane.
   • binds to extracellular (cell-surface) receptors - located on plasma membrane of specific target cell.
   • signal molecule becomes the ‘first messenger’ - as it binds with the specific receptor in the cell surface.
   • this bonding activates an enzyme embedded in the inner surface of the cell membrane which in turn activates the production of multiple copies of a ‘secondary messenger’ which causes a cascade of events.
   • this relay of signals eventually reaches the nucleus where specific genes are activated, and a response is produced.
   • E.g. adrenaline.

Question 9

Two types of regulatory pathways

Answer 9

1. Hormonal (endocrine) system

2. Nervous system

Question 10

Human hormones

1. Three groups?
2. Travel through?
3. Released from?

Answer 10

1. • amino acid derivatives - readily dissolve in water, tend to end with -ine)
   • lipid-derived- not water-soluble, tend to end with -ol or -one
   • peptide and protein - hydrophilic (insulin, growth hormone)
2. Bloodstream. Specific receptor~specific hormone.
3. Endocrine glands, tissues and organs.

Question 11

Plant hormones

1. Use?
2. Two transport vessels?
3. Most hormones travel through?
4. Signals transmitted through~to?

Answer 11

1. Communicate between cells.
2. • phloem - transports molecules from top of plant (leaves) to roots.
   • xylem - transports water and ions from roots to top of plant.
3. Phloem.
4. Signal transduction pathways to the nucleus.

Question 12

5 examples of what plant hormones are responsible for.

Answer 12

1. Phototropism - growth in response to light.
2. Geotropism - growth in response to gravity.
3. Apical dominance - inhibition of lateral branches.
4. Ripening of fruit - conversion of starches to sugars.
5. Abscission - shedding of leaves and fruit.

Question 13

5 examples of plant hormones and their major action.

• 1st one: source and action

Answer 13

1. Auxin
• source: tips of plant shoots
• major action: bending of the plant towards the light
2. Cytokinins: stimulates growth
3. Gibberellins: stimulates growth
4. Abscisic acid: inhibits growth
5. Ethylene (gas): ripening of fruit

Question 14

Neurotransmitters

1. Outline when sensory receptor is stimulated.
2. Outline process of neurotransmission

• able to label a diagram of a neuron
• diagrams of page 19

Answer 14

1. • When sensory receptor is stimulates (cell becomes less negative) and if this depolarisation is strong enough, sodium ion channels (+) open in the membrane and sodium ions flood the cell causing the cell to become more positive.
   • postassium ion channels (+) then open causing potassium ions to diffuse out of the cell, causing the cell to become more negative again.
   • negative to positive to negative = action potential.

2.
• Ca2+ enters the cell.
• Ca2+ signalsnto vesicles.
• vesicle fuses with plasma membrane.
• vesicle releases neurotransmitter molecules by exocytosis.
• Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic cell membrane. This interaction opens or closes ion channels.
• this might be excitatory (excites the cell) or inhibitory (inhibits the cell).

Question 15

Cytokines

1. Are what?
2. Different effects (3)?

Answer 15

1. Cell signalling molecules that aid cell-to-cell communication in immune responses and stimulate the movement of cells towards sites of inflammation, infection, trauma. Some are peptides, proteins or glycoproteins.
2. Stimulatory, inhibitory, regulatory.

Question 16

Apoptosis

1. Role in an adult
2. Role in embryos

• genetically controlled and highly regulated.

Answer 16

1. In adults: it ensures that the balance exists between the rates of cell renewal and cell death (damaged, diseased, end of life cells).
2. In embryos: it helps finalise the shape of the organs by using apoptosis to get rid of excess cells.

Question 17

Steps of apoptosis

Answer 17

1. Cell shrinks
2. Plasma membrane forms blebs
3. Nucleus and organelles condense and fragment
4. Cell breaks up and it’s contents form into parcels known as apoptotic bodies
5. These are engulfed by phagocytic cells

Question 18

Steps of necrosis

Answer 18

1. Cell swells
2. Cell becomes leaky and plasma membrane forms blebs
3. Cell membrane ruptures and releases the cell’s content. Cellular and nuclear lysis causes inflammation.

Question 19

Two pathways of apoptosis and steps involved in each when activated.

Answer 19

1. Intrinsic pathway (mitochondrial pathway)
   • initiation from within a cell.
   • factors released from the mitochondria begins apoptosis.
   • damaged, viral infected and stressed cells.
   Steps involved when activated:
   • pores form in the outer membrane of mitochondria.
   • cytochrome c, an enzyme, is released.
   • it combines with other compounds which activates a cascade of caspase molecules (protein digesting enzymes).
   • these enzymes cleave specific proteins in the cytoplasm or nucleus that initiates apoptosis.
   W
2. Extrinsic pathway (death receptor pathway)
   • initiated from outside the cell.
   • molecule binds to death receptors on plasma membrane.
   • cell’s are not stressed.
   • excess cells, no longer needed cells, old cells.
   Steps involved when activated:
   • external signalling molecules (ligands) bind to death receptors.
   • this activates a cascade of capsases that begin to cleave the cell and initiate apoptosis.
   • this pathway can trigger the intrinsic pathway by activating a BID protein that causes changes to the mitochondria.

Question 20

Malfunctions in apoptosis

Answer 20

1. Apoptosis is reduced to inadequate levels = too little cell loss will occur = cells will accumulate abnormally.
   • E.g. cancer, autoimmune diseases (arthritis - cells build up in joints)
2. Apoptosis increases above normal cell levels = excessive loss of cells.
   • E.g. neurodegenerative disorders (Parkinson’s disease and Alzheimer’s disease - progressive loss of nerve cells), AIDS - excessive loss of immune system cells.