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biology - 1.4 Flashcards

1
Q

How do multicellular organisms signal between molecules?

A

Using extracellular signalling molecules.

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2
Q

What are examples of extracellular signalling molecules?

A

Steroid hormones, peptide hormones and neurotransmitters.

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3
Q

What are receptor molecules of target cells?

A

Proteins with a binding site for a specific signal molecule.

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4
Q

What does binding change?

A

The conformation of the receptor, which initiates a response within the cell.

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5
Q

What do different cell types produce?

A

Specific signals that can only be detected and responded to by cells with the specific receptor.

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6
Q

Why may signalling molecules have different effects on different target cell types?

A

Due to differences in the intracellular signalling molecules and pathways that are involved.

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7
Q

In a multicellular organism, what may different cell types show?

A

A tissue-specific response to the same signal.

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8
Q

What can hydrophobic signalling molecules do?

A

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

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9
Q

What are the receptors for hydrophobic signalling molecules?

A

Transcription factors.

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10
Q

What are transcription factors?

A

Proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.

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11
Q

What are examples of hydrophobic signalling molecules?

A

The steroid hormones oestrogen and testosterone.

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12
Q

What do steroid hormones do?

A

They bind to specific receptors in the cytosol or the nucleus.

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13
Q

What does the hormone-receptor complex do?

A

It moves to the nucleus where it binds to specific sites on DNA and affects gene expression.

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14
Q

What does the hormone-receptor complex bind to?

A

Specific DNA sequences called hormone response elements (HREs).

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15
Q

What does binding at hormone response elements do?

A

It influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.

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16
Q

What do hydrophilic signalling molecules do?

A

They bind to transmembrane receptors and do not enter the cytosol.

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17
Q

What are examples of hydrophilic extracellular signalling molecules?

A

Peptide hormones and neurotransmitters.

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18
Q

When the ligand binds to the extracellular face, what happens to transmembrane receptors?

A

They change conformation. The signal molecule does not enter the cell, but the signal is transduced across the plasma membrane.

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19
Q

What do transmembrane receptors act as?

A

Signal transducers by converting the extracellular ligand-binding event into intracellular signals, which alters the behaviour of the cell.

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20
Q

What do transduced hydrophilic signals often involve?

A

G-proteins or cascades of phosphorylation by kinase enzymes.

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21
Q

What do G-proteins do?

A

They relay signals from activated receptors to target proteins such as enzymes and ion channels.

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22
Q

What do phosphorylation cascades allow?

A

More than one intracellular signalling pathway to be activated.

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23
Q

What do phosphorylation cascades involve?

A

A series of events with one kinase activating the next in the sequence and so on.

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24
Q

What can phosphorylation cascades result in?

A

The phosphorylation of many proteins as a result of the original signalling event.

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25
Q

What does binding of the peptide hormone insulin to its receptor result in?

A

An intracellular signalling cascade that triggers recruitment of GLUT4 glucose transporter proteins to the cell membrane of fat and muscle cells.

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26
Q

What does binding of insulin to its receptor cause?

A

A conformational change that triggers phosphorylation of the receptor.

27
Q

What does phosphorylation of the insulin receptor start?

A

A phosphorylation cascade inside the cell, which eventually leads to GLUT4-containing vesicles being transported to the cell membrane.

28
Q

What can diabetes mellitus be caused by?

A

Failure to produce insulin (type 1) or loss of receptor function (type 2).

29
Q

What is type 2 generally associated with?

A

Obesity.

30
Q

What does exercise do?

A

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

31
Q

What is resting membrane potential?

A

A state where there is no net flow of ions across the membrane.

32
Q

What does the transmission of a nerve impulse require?

A

Changes in the membrane potential of the neuron’s plasma membrane.

33
Q

What is an action potential?

A

A wave of electrical excitation along a neuron’s plasma membrane.

34
Q

What do neurotransmitters do?

A

They initiate a response by binding to their receptors at a synapse.

35
Q

What are neurotransmitter receptors?

A

Ligand-gated ion channels.

36
Q

What does depolarisation of the plasma membrane as a result of the entry of positive ions trigger?

A

The opening of voltage-gated sodium channels, and further depolarisation occurs.

37
Q

What is depolarisation?

A

A change in the membrane potential to a less negative value inside.

38
Q

What does inactivation of the sodium channels and the opening potassium channels do?

A

Restores the resting membrane potential.

39
Q

What does binding of a neurotransmitter trigger?

A

The opening of ligand-gated ion channels at a synapse. Ion movement occurs and there is depolarisation of the plasma membrane.

40
Q

If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, what happens?

A

The opening of voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient. This leads to a rapid and large change in the membrane potential.

41
Q

A short time after opening, what happens?

A

The sodium channels become inactivated. Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.

42
Q

What does depolarisation of a patch of membrane cause?

A

Neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened.

43
Q

When the action potential reaches the end of the neuron, what does it cause?

A

Vesicles containing neurotransmitter to fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell.

44
Q

What does restoration of the resting membrane potential allow?

A

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

45
Q

How are ion concentration gradients re-established?

A

By the sodium-potassium pump, which actively transports excess ions in and out of the cell.

46
Q

Following depolarisation, what happens?

A

The sodium and potassium ion concentration gradients are reduced. The sodium-potassium pump restores the sodium and potassium ions back to resting potential levels.

47
Q

What is the retina?

A

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

48
Q

In animals, what form the photoreceptors of the eye?

A

The light-sensitive molecule retinal is combined with a membrane protein, opsin.

49
Q

In rod cells, what is the retinal-opsin complex called?

A

Rhodopsin.

50
Q

What does retinal do?

A

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

51
Q

What amplifies the signal?

A

A cascade of proteins.

52
Q

What does photoexcited rhodopsin activate?

A

A G-protein, called transducin.

53
Q

What does the G-protein, transducin, activate?

A

The enzyme phosphodiesterase (PDE).

54
Q

What does a single photoexcited rhodopsin activate?

A

Hundreds of molecules of G-protein.

55
Q

What does each activated G-protein activate?

A

One molecule of PDE.

56
Q

What does PDE catalyse?

A

The hydrolysis of a molecule called cyclic GMP (cGMP).

57
Q

What does each active PDE molecule break down?

A

Thousands of cGMP molecules per second.

58
Q

What does the reduction in cGMP concentration as a result of its hydrolysis affect?

A

The function of ion channels in the membrane of rod cells.

59
Q

What does the hydrolysis of cGMP result in?

A

The closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina.

60
Q

What does a very high degree of amplification result in?

A

Rod cells being able to respond to low intensities of light.

61
Q

In cone cells, what give different photoreceptor proteins?

A

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.

62
Q

Where do rods function?

A

In dim light but do not allow colour perception.

63
Q

Where do cones function?

A

Only in bright light and are responsible for colour vision.

advanced higher biology (33 decks)

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