4 - communication and signaling

Question 1

How do multicellular organisms signal between cells? And give examples.

Answer 1

using extracellular signalling molecules for examples using steroid hormones, peptide hormones and neurotransmitters

Question 2

Describe the role of receptor molecules

Answer 2

proteins with a binding site for a specific signal molecule

Question 3

state

Answer 3

Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor.

Question 4

state

Answer 4

Signalling molecules may have different effects on different target cell types due to differences in the intracellular signalling molecules and pathways that are involved.

Question 5

state

Answer 5

In a multicellular organism, different cell types may show a tissue-specific response in the same signal.

Question 6

state

Answer 6

Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors.

Question 7

state

Answer 7

The receptors for hydrophobic signalling molecules are transcription factors.

Question 8

Describe the role of transcription factors.

Answer 8

Transcription factors are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.

Question 9

Give examples of steroid hormones which are hydrophobic signalling molecules.

Answer 9

oestrogen and testosterone

Question 10

state

Answer 10

Steroid hormones bind to specific receptors in the cytosol or the nucleus .

Question 11

state

Answer 11

The hormone-receptor complex moves to the nucleus where it binds to specific sites on DNA and affects gene expression.

Question 12

state

Answer 12

The hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs). Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.

Question 13

state

Answer 13

Hydrophobic signalling molecules bind to transmembrane receptors and so not enter the cytosol.

Question 14

Give examples of hydrophilic extracellular signalling molecules

Answer 14

peptide hormones and neurotransmitters

Question 15

Describe the role of transmembrane receptors in signal transduction.

Answer 15

Transmembrane receptors change conformation when the ligand binds to the extracellular face; the signal molecule does not enter the cell, but the signal is transduced across the plasma membrane.

Question 16

state

Answer 16

Transmembrane receptors act as signal transducers by converting the extracellular ligand-binding event into intracellular signals, which alters the behaviour of the cell.

Question 17

Describe the roles of G-proteins and cascades of phosphorylation by kinase enzymes in transducing hydrophilic signals.

Answer 17

Transduced hydrophilic signals often involve G-protein or cascades of phosphorylation by kinase enzymes.
G-proteins relay signals from activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels.

Question 18

Describe the effects of the binding of the peptide hormone insulin to its receptor.

Answer 18

Binding of the peptide hormone insulin to its receptor results in an intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.

Question 19

Describe the causes of Type 1 and Type 2 diabetes.

Answer 19

Diabetes mellitus can be cause by failure to produce insulin (type 1) or loss of receptor function (type 2)

Question 20

What type of diabetes is generally associated with obesity?

Answer 20

type 2

Question 21

Describe the role of exercise in the treatment of type 2 diabetes.

Answer 21

Exercise also triggers the recruitment of GLUT4, so can improve uptake of glucose to fat and muscle cells in subjects with type 2.

Question 22

Define the term resting membrane potential.

Answer 22

Resting membrane potential is a state where there is no net flow of ions across the membrane.

Question 23

state

Answer 23

Transmission of a nerve impulse requires changes in the membrane potential of the neuron’s plasma membrane

Question 24

define the term action potential

Answer 24

An action potential is a wave of electrical excitation along a neuron’s plasma membrane.

Question 25

state

Answer 25

Neurotransmitters initiate a response by binding to their receptors ( ligand-gated ion channels) as a synapse.

Question 26

state

Answer 26

Depolarisation of the plasma membrane as a result of the entry of positive ions triggers the opening of voltage-gated sodium channels, and further depolarisation occurs.

Question 27

state

Answer 27

Inactivation of the sodium channels and the opening of potassium channels restores the resting membrane potential.

Question 28

Describe how a wave of depolarisation passes along the length of a neuron.

Answer 28

Depolarisation of a patch of membrane causes neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened.

Question 29

Describe what happens when an action potential reaches the end of a neuron.

Answer 29

When the action potential reaches the end of the neuron it causes vesicles containing neurotransmitters to fuse with the membrane - this releases neurotransmitter, which stimulates a response in a connecting cell.

Question 30

state

Answer 30

Restoration of the resting membrane potential allows the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane.

Question 31

Describe how ion concentration gradients are re-established.

Answer 31

Ion concentration gradients are re-established by the sodium-potassium pump, which actively transports excess ions in and out of the cell.

Question 32

Describe the role of the retina.

Answer 32

The retina is the area within the eye that detects light and contains two types of photoreceptor cells: rods and cones

Question 33

state

Answer 33

In animals the light-sensitive molecule retinal is combined with a membrane protein, opsin to form the photoreceptors of the eye

Question 34

Name the retinal-opsin complex found in rod cells.

Answer 34

In rod cells the retinal-opsin complex is called rhodopsin.

Question 35

Describe the process which triggers nerve impulses in neurons in the retina beginning when retinal absorbs a photon of light.

Answer 35

Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin.

Question 36

state

Answer 36

A very high degree of amplification results in rod cells being able to respond to low intensities of light.

Question 37

Describe the importance of different forms of opsin in cone cells.

Answer 37

In cone cells, different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV.