Membrane Transport And Chemical Signalling (Physiology)

Question 1

Explain what a first messenger is

Answer 1

Signalling molecules released into the extracellular fluid with the aim of affecting the functioning of other cells.

Question 2

Identify the four main types of first messenger and give examples of each

Answer 2

Neurotransmitters
- Molecules released from neurones to allow communication between excitable cells e.g. acetylcholine.

Endocrine hormones
- Molecules released from glands and enter the blood stream to reach their target cells e.g. insulin, adrenaline.

Paracrine molecules
- Molecules that affect cells in close proximity e.g. NO from vascular endothelium.

Autocrine molecules
- Molecules that act on the cell that has secreted it e.g. interleukin released from monocytes.

Question 3

Explain how cells use receptors to recognise first messengers

Answer 3

• Receptors are protein molecules capable of recognising specific first messengers.
• The receptor can be located on the cell surface, or it can be intracellular.
• Receptor activation occurs when the first messenger combines with the receptor.
• The first messenger which causes receptor activation is known as an agonist.

Question 4

Explain why the physiological effect of a first messenger can be different in different tissues

Answer 4

• Even though there are the same first messenger receptors in both tissues, they are of different types.
• Activation of each receptor type results in different cellular events occurring since they are linked to different subcellular effectors.

Question 5

Explain the term signal transduction

Answer 5

When receptors are activated and the activity of intracellular molecules is modified to create a response.

Question 6

Describe how activation of ion channel linked receptors affect cellular function

Answer 6

• These are membrane bound proteins which allow the movement of ions across the cell membrane.
• When a receptor is linked to an ion channel (ligand-gated channel), it’s activation causes the channel to open and ions to enter or leave the cell.
• This can lead to hyperpolarisation or depolarisation of the membrane.

Question 7

Describe how activation of G protein linked receptors affect cellular function

Answer 7

• G proteins are membrane bound proteins which recognise when their associated receptors are activated and pass that message on to the effector system that generates the physiological response.
• G proteins associate with the molecule guanosine triphosphate when active (GTP), and the molecule guanosine diphosphate (GDP) when inactive.
• When activated, the G protein dissociates into the alpha subunit and the beta-gamma subunit.
• These subunits diffuse through the cell to affect ion channels or membrane bound enzymes.
• When G proteins affect enzyme activity they change the intracellular concentration of chemicals called second messengers.
• The second messengers trigger the preprogrammed series of biochemical events within the cell which lead to the correct response.

Question 8

Explain the difference between active and passive transport

Answer 8

Active transport requires the use of adenosine triphosphate (ATP) by the cell whereas passive transport does not.

Question 9

Describe the following types pf passive transport: diffusion, osmosis, and facilitated diffusion

Answer 9

Diffusion

• Refers to the movement of molecules from an area where they are in high concentration to an area where they are in low concentration.
• Rate of diffusion is affected by temperature, molecular weight, size of the concentration gradient, membrane surface area, and membrane permeability.

Osmosis

• When water moves down its concentration gradient across a semi-permeable membrane.
• A solution with a water concentration different from that of normal body fluids can influence cell size via osmosis. The ability to do this is referred to as tonicity.

Facilitated diffusion

• Involves the movement of molecules down their concentration gradients but a membrane bound protein is needed to facilitate their passage.
• The molecule attaches to a binding site on the carrier, then the carrier changes shape and releases the molecule on the other side of the membrane.
• Transports solutes such as glucose and amino acids which are too large to cross the cell membrane by simple diffusion.

Question 10

Describe the following types of active transport: primary and secondary active transport

Answer 10

Primary active transport

• A process in which a carrier moves molecules from an area where they are in low concentration to an area where they are in high concentration.
• ATP supplies the energy needed by this process by transferring a phosphate group to the transport protein.

Secondary active transport

• Requires an indirect energy input from ATP.
• Depends upon the ATP used by a primary active transport process.
• An example are the sodium-glucose transporters found in certain kidney tubules.

Question 11

Describe the following types of vesicular transport: endocytosis and exocytosis

Answer 11

Endocytosis

• This process brings matter into a cell.
• Phagocytosis is the process of engulfing and particles such as bacteria, dust, and cellular debris. Phagocytes surround a bacterium with its pseudopods and traps it in a phagosome. A lysosome fuses with the phagosome to form a phagolysosome. Hydrolytic enzymes are released to destroy the bacterium.
• Pinocytosis is the process of taking in some droplets of extracellular fluid containing molecules of use to the cell. The plasma membrane caves in at points to form pits. The pits separate from the surface membrane and form membrane bounded vesicles called pinocytotic vesicles in the cytoplasm which contain ECF fluid and the molecules it contains.
• Receptor-mediated endocytosis enables a cell to take in specific molecules from the ECF with a minimum of unnecessary matter. Extracellular molecules bind to receptors on the cell membrane and the receptors cluster together. The plasma membrane sinks inward and forms a clathrin-coated pit. The pit separates from the membrane to form a clathrin-coated vesicle containing the useful molecules from the ECF.

Exocytosis

• Process of discharging material from a cell.
• A secretory vesicle in the cell migrates to the surface and docks on peripheral proteins of the plasma membrane.
• These proteins pull the membrane inward and create a dimple that eventually fuses with the vesicle and allows it to release its contents.