Topic 6

Question 1

Ways of signalling molecules travelling

Answer 1

• long distance travel to target cells- signalling molecules travel through circulatory system (blood) over long distances to reach target cell
• travel to nearby cells- signalling molecules diffuse through extracellular space around cells and travel short distances to target cells
• once cell sends and receives signal- chemical signals released by one cell may be received by the same cell
• direct cell to cell contact- signal can move directly from cytosol of one cell, to that of another-involves direct cell to cell contact
• signalling cells carry signals- signalling cells are highly mobile- deliver signal directly to target cells- signalling cell carries, on its surface a chemical signal and target cell has specific receptor for this signal

Question 2

gap junctions

Answer 2

• protein lined pores in plasma membranes of adjacent cells

- allow various small molecules to pass between cells and enable electrical signals

Question 3

plasmodesmata

Answer 3

• occur between adjacent cells in plant tissues
• plasmodesmata goes through cell wall
• plasma membranes of cells are continuous and form plasmodesmata lining

Question 4

three stages of cell communication

Answer 4

reception- signalling molecules binds with specific protein receptor and causes changes in receptor
transduction- signalling pathway in which series of steps are triggered that is brought about by change in receptor molecule
response- transduction process brings about cell response, can include activation/inhibition of genes in nucleus, activation of enzymes etc.

Question 5

SRM in terms of cellular communication

Answer 5

stimulus- signalling molecule
receptor- protein on cell surface or inside cell
transmission and amplification
effector- intracellular chemicals
response- change in cell functioning.

Question 6

signal reception

Answer 6

• when signals reach target cells, they bind to specific receptor
• receptors are highly specific protein- binds to 1 signalling molecules with specific shape- due to complimentary shape
• receptors are located on plasma membrane- cell surface receptors, or located in cytosol or in nucleus- intracellular receptors

Question 7

Signal transduction

Answer 7

• second stage, converts signal from outside a cell to cellular response
• signal is relayed in one form, changed to another molecule, and relayed to appropriate target cell within cell.
• links signal reception with cellular response

Question 8

Transduction of hydrophobic signal

Answer 8

-can readily diffuse through plasma membrane- thus protein receptor is located inside cytosol or in nucleus

Question 9

transduction of hydrophilic signal

Answer 9

• cannot readily diffuse across phospholipid bilayer- due to it being hydrophilic
• signal must be transferred from cell surface receptor tp second messenger in cytoplasm
• transfer of signal from receptor to nucleus occurs through signal transduction pathway in cytoplasm- involves production of large numbers of second messenger molecules
• second messengers aren’t enzymes, but activate key enzyme at start of enzyme relay.

Question 10

difference in chemical nature of signalling molecules

Answer 10

hydrophilic- soluble in water so require no carrier in travel through blood, unable to pass through phospholipid bilayer, bind to cell surface receptors, indirectly act on genes by producing second messengers, shorter response
hydrophobic- insoluble in water so require carrier proteins for transport on blood, are able to pass through phospholipid bilayer, bind to intracellular receptors, directly regulates gene expression with no second messengers, longer lasting response

Question 11

signal cellular response

Answer 11

final stage in cellular signalling

-effector proteins produced by gene activity- these produce cellular response to original external signal molecule

Question 12

what happens to hormones after they have delivered their signal

Answer 12

• they are deactivated by cell enzymes and excreted via kidneys or fesces.

Question 13

neurotransmitters

Answer 13

• chemical signalling molecules or nerve cells (neurons)
• communication within nerve cells depends on action potentials-transmit nerve impulse along axon
• produced by local movement of charged particles
• the communication between nerve cells are called neurotransmitters, which diffuse across small gaps called synaptic clefts.

Question 14

synapse

Answer 14

point where one nerve cell connects with another neuron, muscle cell or gland

• neurotransmitters are packaged in vesicles when action potentials reach nerve terminal
• released through exocytosis at presynaptic membrane and diffuse across synaptic cleft, and bind to receptors on post synaptic membrane

Question 15

action of neurotransmitters

Answer 15

• arrival of nerve impulse at end of axon causes influx of calcium ions- causes vesicles to move towards presynaptic membrane and release neurotransmitters into synaptic cleft via exocytosis
• neurotransmitter diffuses across synaptic cleft to specific receptors on post synaptic membrane
• neurotransmitters binds to specific receptors on postsynaptic membrane
• ion channels in postsynaptic membrane open, causing influx of Na+
• cause receiving cell to respond to arrival of neurotransmitter- type of response depends on nature of cell
• neurotransmitters are broken down by enzymes once message is delivered
• component of neurotransmitters are actively reabsorbed back into synaptic knob.

Question 16

Pheromones

Answer 16

• chemical signalling molecule released by one animal that can carry signal to second member of same species
• if pheromone signal is received, second animal produces physiological or behavioural change.

Question 17

plant chemical signals

Answer 17

-plant growth influenced by internal and eternal factors and interaction of these factors.

Question 18

similarities and differences between plant and animal hormones

Answer 18

similarities
-hormones produced in response to stimulus
-hormones bind with specific receptor proteins and initiate sequence of events in target cells
produced in particular parts of body- travel to target cells in different parts of animal or plant in which a response is initiated
- hormones produced in relatively small amount
differences
-plant hormone may cause one response at low concentration, and opposite effect at high concentrations
-plant do not possess specialised secretory organs or cells.- messages are secreted in right conditions to growing regions of shoots and roots.
-distribution of hormones doesn’t occur through specialised pathway such as nervous system or blood stream- instead these signals usually diffuse from one cell to another through extracellular fluid or transported through xylem and phloem.
-plant’s do not maintain a stable internal environment.

Question 19

auxin and its actions

Answer 19

• found in apical bud and developing seeds
• promotes stem and coleoptile elongation
• produces trophic response to light and gravity

Question 20

cytokinins and its actions

Answer 20

• found actively dividing meristematic regions of stems, roots and leaves
• it promotes cell division

Question 21

gibberellins and its action

Answer 21

roots, young leaves and seeds

-break dormancy of some seeds, promotes germination and mobilises food store in seeds

Question 22

Ethylene and its action

Answer 22

most parts of a plant, with highest concentrations during ageing, leaf abscission and fruit ripening
-promotes flower opening and fruit ripening

Question 23

Abscisic acid and its action

Answer 23

• roots and terminal buds

- promotes stomatal closure in times of water stress

Question 24

apoptosis

Answer 24

form of programmed cell death in multicellular organisms

-controlled by diverse range of extracellular and intracellular signals- results in caspase activation

Question 25

intracellular signal- mitochondrial pathway

Answer 25

activated when:

• DNA is severely damaged, malfunction of oxidative enzymes or when a virus enters the cell
• proteins on surface of mitochondria are activated, and mitochondrial membrane breaks
• caspases are enzymes which once activated- enter nucleus and breaks DNA into smaller pieces

Question 26

extracellular signal- death receptor pathway

Answer 26

• message from outside of cell tells it to self destruct
• may be given message if:
• hasn’t fully developed
• cells are in excess of whats needed
• cells are no longer required
• specific cell surface receptor receives messages for self destruction- activates a cascade of events

Question 27

apoptosis cascade of events

Answer 27

• many different enzymes are activated within cell and at same time, message goes out to cells in the area
• all cells that have received death signal- begin to shrink and develop small blebs on their surface
• caspases enter through nuclear membrane pores, DNA and proteins in nucleus are degraded and mitochondria break down.
• organelles other than nucleus and mitochondria are generally preserved, as cell breaks into small membrane-enclosed fragments- some organelles are recycled
• small fragments bind to receptors on phagocytic cells that have responded to dying cell’s messages.
• phagocytes engulf the bodies, also secret cytokines, compounds that inhibit inflammation so that surrounding cells are not damaged-

Question 28

disease and apoptosis

Answer 28

• too much apoptosis- neurodegenerative disease e.g Parkinsons or Alzheimers
• too little apoptosis- cancer and rheumatoid arthritis