4. Communication within multicellular organisms

Question 1

How do multicellular organisms achieve coordination of communication?

Answer 1

• Multicellular organisms show division of labour, this means that different cells carry out different functions within defined areas of the body.
• Multicellular organisms achieve coordination of communication by means of extracellular signalling molecules.
• One tissue in the body will release a signalling molecule which will travel to another tissue with complementary receptors. This allows a signal to be passed from one cell to another.

Question 2

How does binding initiate a cellular response?

Answer 2

• Receptor molecules of target cells are proteins with a binding site for a signal molecule.
• Binding changes the conformation of the receptor, which initiates a response within the cell.

Question 3

Describe target cells

Answer 3

• Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor
• In a multicellular organism, different cell types may show a tissue-specific response to the same signal

Question 4

What are hydrophobic signalling molecules?

Answer 4

• Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors.
• They are able to pass through the phospholipid bilayer because the tails of the phospholipids in the plasma membrane are also hydrophobic and allow the molecules to pass across.
• Hydrophobic signals can directly influence transcription of genes.
• The steroid hormones oestrogen and testosterone are examples of hydrophobic signalling molecules.

Question 5

What are transcription factors?

Answer 5

• The receptors for hydrophobic signalling molecules are transcription factors which can be found in the cytosol or nucleus.
• Transcription factors are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription
• They do this by enhancing or blocking the binding of RNA polymerase to specific genes

Answer 6

• When a steroid hormone enters a cell it binds to and activates a specific receptor in the cytosol (or nucleus) forming a hormone-receptor complex.
• The hormone-receptor complex then moves to the nucleus where it binds to specific sites on DNA.
• Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.

Answer 7

Hydrophilic signals require transmembrane receptor molecules at the surface of the cell because they are not capable of passing across the hydrophobic plasma membrane and entering the cytosol.

Question 8

Describe the reception stage.

Answer 8

• Transmembrane receptors change conformation (shape) when the ligand (signalling molecule) binds to the extracellular face.
• The signal molecule does not enter the cell, but the signal is transduced across the membrane of the cell.

Question 9

Describe the transduction stage.

Answer 9

• Transmembrane receptor proteins act as signal transducers by convert an extracellular ligand-binding event to a specific intracellular response through a signal transduction pathway.
• Transduced hydrophilic signals often involve cascades of G-proteins or phosphorylation by kinase enzymes.

Question 10

Describe the cellular process of glucose uptake.

Answer 10

• Glucose passes into cells by travelling through transporter proteins in the plasma membrane (by facilitated diffusion).
• Binding of the peptide hormone 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 of fat and muscle cells.
• GLUT4 glucose transporter proteins allow glucose to pass across the plasma membrane and enter the cell.

Question 11

What causes type 1 diabetes?

Answer 11

A failure to produce insulin in the pancreas.

Question 12

What causes type 2 diabetes?

Answer 12

Loss of insulin receptor function. This type of diabetes is usually associated with obesity.

Question 13

How is type 2 diabetes treated?

Answer 13

• Type 2 diabetes may be treated with medications to lower blood glucose levels along with lifestyle changes.
• Exercise also triggers recruitment of GLUT4 so can improve uptake of glucose to fat and muscle cells in subjects with type 2 diabetes.

Question 14

What is resting membrane potential?

Answer 14

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

Question 15

What must occur to allow the transmission of a nerve impulse?

Answer 15

• The transmission of a nerve impulse requires changes in the membrane potential of the neuron’s plasma membrane.
• Nerve transmission is a wave of depolarisation of the resting potential of a neuron.

Question 16

What is depolarisaton?

Answer 16

• A change in the membrane potential to a less negative value inside.
• Depolarisation results from an action potential.

Question 17

What is an action potential?

Answer 17

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

Question 18

Action potential 1

Answer 18

Binding of a neurotransmitter triggers the opening of ligand-gated ion channels at a synapse.

Question 19

Action potential 2

Answer 19

Ion movement occurs and there is depolarisation of the plasma membrane.

Question 20

Action potential 3

Answer 20

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.

Question 21

Action potential 4

Answer 21

A rapid and large change in the membrane potential occurs.

Question 22

Action potential 5

Answer 22

A short time after opening, the sodium channels become inactivated.

Question 23

Action potential 6

Answer 23

Voltage-gated potassium channels open to allow potassium ions to move out of the cell to restore the resting membrane potential.

Question 24

Action potential 7

Answer 24

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

Question 25

How does an action potential affect surrounding cells?

Answer 25

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

Question 26

What is the retina?

Answer 26

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

Question 27

How are photoreceptors formed in animals?

Answer 27

• In animals the light-sensitive molecule retinal is combined with a membrane protein, opsin, to form the photoreceptors of the eye.
• In rod cells the retinal-opsin complex is called rhodopsin

Question 28

Generation of a nerve impulse 1

Answer 28

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

Question 29

Generation of a nerve impulse 2

Answer 29

Photoexcited rhodopsin activates a G-protein called transducin (a single photoexcited rhodopsin activates hundreds of molecules of transducin)

Question 30

Generation of a nerve impulse 3

Answer 30

Transducin activates the enzyme phosphodiesterase (PDE) (each activated molecule of transducin activates one molecule of PDE)

Question 31

Generation of a nerve impulse 4

Answer 31

PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP) (each active PDE molecule breaks down thousands of cGMP molecules per second)

Question 32

Generation of a nerve impulse 5

Answer 32

The reduction in cGMP concentration as a result of its hydrolysis causes the closure of ion channels in the membrane of the rod cells

Question 33

Generation of a nerve impulse 6

Answer 33

The inward leakage of positive ions (Na+ and Ca+) is halted so the membrane potential increases; hyperpolarisation (increasing charge) triggers nerve impulses in neurons in the retina.

Question 34

How does rhodopsin affect rod cells?

Answer 34

The rhodopsin absorbs a wide range of wavelengths, and a very high degree of amplification from a single photon of light results in sensitivity at low light intensities.

Question 35

How do opsins affect cone cells?

Answer 35

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