Unit 1: Key area 4 Communication and signalling

Question 1

How do multicellular organisms signal between cells

Answer 1

Multicellular organisms signal between cells using extracellular signalling molecules

Question 2

Name examples of signalling molecules

Answer 2

Steroid hormones, peptide hormones, and neurotransmitters are examples of extracellular signalling molecules.

Question 3

What are receptor molecules

Answer 3

Receptor molecules of target cells are proteins with a binding site for a specific signal molecule

Question 4

What does the binding cause and what does this initiate

Answer 4

Binding changes the conformation of the receptor, which initiates a response within the cell

Question 5

Describe how signalling molecules work

Answer 5

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

Question 6

Why could signalling molecules have different effects on different target cells

Answer 6

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 7

Remember this statement
In a multicellular organism, different cell
types may show a _________ to
the same signal

Answer 7

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

Question 8

Describe Hydrophobic signalling molecules

Answer 8

Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors
The receptors for hydrophobic signalling molecules are transcription factors
Steroid hormones bind to specific receptors in the cytosol or the nucleus
The hormone-receptor complex moves to the nucleus where it binds to specific sites on DNA and affects gene expression
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 9

What are transcription factors

Answer 9

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

Question 10

What are examples of hydrophobic signalling molecules

Answer 10

The steroid hormones oestrogen and testosterone are examples of hydrophobic signalling molecules

Question 11

Describe hydrophilic signalling molecules

Answer 11

Hydrophilic signalling molecules bind to transmembrane receptors and do not enter the cytosol
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 12

Name examples of hydrophilic extracellular

signalling molecules.

Answer 12

Peptide hormones and neurotransmitters are examples of hydrophilic extracellular signalling molecules.

Question 13

Describe transduction

Answer 13

Transmembrane receptors act as signal transducers by converting the extracellular ligand-binding event into intracellular signals, which alters the behaviour of the cell
Transduced hydrophilic signals often involve G-proteins 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 14

Describe the phosphorylation cascade

Answer 14

Phosphorylation cascades allow more than one intracellular signalling pathway to be activated
Phosphorylation cascades involve a series of events with one kinase activating the next in the sequence and so on. Phosphorylation cascades can result in the phosphorylation of many proteins as a result of the original signalling event.

Question 15

Describe insulin and GLUT4

Answer 15

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
Binding of insulin to its receptor causes a conformational change that triggers phosphorylation of the receptor. This starts a phosphorylation cascade inside the cell,which eventually leads to GLUT4 containing vesicles being transported to the cell membrane.

Question 16

What causes Diabetes mellitus

Answer 16

Diabetes mellitus can be caused by failure to produce insulin (type 1) or loss of receptor function (type 2)
Research health effects associated with type 2 diabetes and the success rate of treatment programmes.

Question 17

What is type 2 diabetes generally associated with

Answer 17

Type 2 is generally associated with obesity

Question 18

What triggers the recruitment of GLUT4

Answer 18

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

Question 19

Describe the resting membrane potential

Answer 19

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

Question 20

What does the transmission of a nerve impulse require

Answer 20

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

Question 21

What is an action potential

Answer 21

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

Question 22

How do neurotransmitters initiate a response

Answer 22

Neurotransmitters initiate a response by binding to their receptors at a synapse

Question 23

Describe the structure of neurotransmitters

Answer 23

Neurotransmitter receptors are ligand-gated ion channels.

Question 24

What does depolarisation of the plasma membrane trigger

Answer 24

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 25

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

Answer 25

Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse. Ion movement occurs and there is depolarisation of the plasma membrane. If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, 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. A short time after opening, 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.

Question 26

What is depolarisation

Answer 26

Depolarisation is a change in the membrane potential to a less negative value inside

Question 27

What does depolarisation of a patch of membrane cause

Answer 27

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 28

What happens when the action potential reaches the end of the neuron

Answer 28

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

Question 29

What does the restoration of the resting membrane potential allow

Answer 29

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

Question 30

What happens after repolarisation

Answer 30

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

Question 31

What is the function of the retina within the cell

Answer 31

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

Question 32

Describe the features/functions of rods and cones

Answer 32

Rods function in dim light but do not allow colour perception. Cones are responsible for colour vision and only function in bright light.

Question 33

In animals what combined with the light-sensitive retinal and what is formed as a result

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

In rods what is the retinal-opsin complex called

Answer 34

In rod cells the retinal-opsin complex is called rhodopsin

Question 35

What happens to the rhodopsin when Retinal absorbs a photon of light

Answer 35

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

Question 36

What amplifies the signal

Answer 36

A cascade of proteins amplifies the signal

Question 37

What happens when photoexcited rhodopsin activate. Describe the process

Answer 37

Photoexcited rhodopsin activates a Gprotein, called transducin, which activates the enzyme (PDE) single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE.

Question 38

What does PDE catalyse. Describe the process

Answer 38

PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)
Each active PDE molecule breaks down thousands of cGMP molecules per second.
The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells.

Question 39

After the reduction in cGMP concentration as a
result of its hydrolysis affects the function of
ion channels in the membrane of rod cells, what happens

Answer 39

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

Question 40

What does a very high degree of amplification result in

Answer 40

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

Question 41

In cone cells, different forms of opsin

combine with retinal to give what?

Answer 41

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