Cell Communication

Question 1

Where does cell communication occur?

Answer 1

• Between cells: Electrical and chemical

- Within cells

Question 2

How are electrical transmissions sent between cells?

Answer 2

Direct contact via gap junctions:

• gap junctions are the type of cell junction in which adjacent cells are connected through protein channels
• These channels connect to the cytoplasm of each cell and allow molecules, ions and electrical signals to pass between them
• Gap junctions are found in between the vast majority of cells within the body because they are found between all cells that are directly touching other cells

Question 3

How do gap junctions work?

Answer 3

• They allow flow of current from cell-cell
• Rapid transmission in both directions
• Linked cells can act together as one unit e.g. cardiac and some smooth muscle

Question 4

Where can chemical transmission be sent from/to?

Answer 4

• Cell -> cell (synapse)
• Cell -> several cells (paracrine)
• Many cells -> many cells (endocrine)

Question 5

What happens at a chemical synapse?

Answer 5

• Cell -> cell using chemical neurotransmitter
• Effect depends on transmitter and the receptor it acts on
• One way transmission

Question 6

Where is the presence of chemical synapses common?

Answer 6

The nervous system

Question 7

What are the sequence of events at a chemical synapse?

Answer 7

• Impulse arrives at terminal of presynaptic cell
• Transmitter released from storage vesicles
• Transmitter diffuses across synaptic cleft
• Transmitter binds to receptor on post synaptic cell
• Alters postsynaptic cell

Question 8

What does an excitatory neurotransmitter do?

Answer 8

Generates an impulse: muscle contraction or gland secretion

Question 9

What does an inhibitory neurotransmitter do?

Answer 9

Switches off the cell

Question 10

What is paracrine communication?

Answer 10

One cell communicated with several cells locally

Question 11

What is autocrine communication?

Answer 11

Where the chemical acts on the cell that released it (feedback)

Question 12

What is endocrine transmission?

Answer 12

• Chemical sent to all parts of the body via the blood

- Hormone acts only on cells with the correct membrane receptor protein (target cells)

Question 13

Is neural communication specific or non-specific?

Answer 13

Very specific or localised

Quick impulse transmission so suitable for rapid responses

Question 14

Give examples of neural communication in the body?

Answer 14

• Control of voluntary muscle contractions
• Sensory systems
• Control of blood pressure

Question 15

What part of the body is affected by humoral communication?

Answer 15

• Can affect many cells in different parts of the body
• Coordinated, body-wide actions
• Slow to act but effect persists

Question 16

Give examples of humoral communication in the body?

Answer 16

• Gastro-intestinal gland responses
• Control of metabolism and growth
• Regulation of the menstrual cycle

Question 17

What is a ‘first messenger’ in a pathway?

Answer 17

• The chemical transmitter which acts on a receptor protein
• Some can pass through the target cell membrane, and act on receptors inside the target cell
• Others cannot enter the cell

Question 18

Explain the role of steroid transmitters?

Answer 18

• Steroids are lipids that CAN pass through the outer cell membrane
• They act on receptors inside the target cell
• The steroid-receptor complex acts o the DNA in the nucleus to initiate protein synthesis
• The protein then alters cell function
• These hormones have a delayed action because translation also needs to occur which takes time

Question 19

Can peptide transmitters pass through the cell membrane?

Answer 19

No, as they are too large and more hydrophilic than cell membranes
- Therefore they bind to plasma membrane receptors and set up a series of reactions controlled by ‘G-proteins’

Question 20

What does G-protein activation by peptide transmitters activate?

Answer 20

G-proteins activate 2nd messenger systems in the cell such as:

• Cyclic AMP
• Calcium ions

Question 21

Explain the process of G-protein and cyclic AMP activation?

Answer 21

• The transmitter binds to a membrane receptor
• G protein is activated by GTP (guanosine triphosphate)
• ATP is converted to cyclic AMP by the enzyme adenyl cyclase
• cAMP activates an enzyme, called protein kinase
• This, in turn, activates a specific enzyme which catalyses a chemical reaction resulting in the formation of a molecule e.g. protein

Question 22

Explain the role of calcium as a second messenger?

Answer 22

• Transmitter binds to a membrane receptor
• This activates a G-protein, which activates an enzyme: phospholipase C
• This in turn causes opening of calcium channels: calcium enters by diffusion
• Calcium binds to a protein in the cytosol; here, it is calmodulin (but there are other Ca-binding proteins)
• Calcium-calmodulin complex acts as a second messenger which then regulates an enzyme which regulates a chemical reaction
• In some cells, the calcium is released from stores within the cell, but the basic principle is much the same