Unit 3 AOS 2 - Cell Communication

Question 1

Stimulus response model

Answer 1

1. Reception
2. Transduction
3. Response

Reception: the detection of a signal due to a change in the internal or external environment
● A receptor’s reaction to a stimulus can generate a mechanical, electrical or chemical signal. The
generation of this signal falls under the second state of the stimulus-response model:
transduction

Transduction: the series
of events that occur after the reception of a signal which results in the generation of a response
➔Can involve sending a signal between organisms, across the body, to a neighbouring cell, or back to the original receptor cell

Response: the action of a cell, organ, or organism caused by a signal
➔For instance, increased or decreased protein production, the release of chemicals.
● The response will require the action of some structure, which is known as the effector
➔ For instance, response = opening up a protein channel, effector - the protein channel itself opening up

Question 2

Cell signalling molecules

Answer 2

• Cells use chemicals called signalling molecules to communicate with other cells.
• In eukaryotes, distant cells must communicate with one another, communication allows for a single cell that detects a change in the environment to initiate a myriad of responses in cells throughout the body.
• A cell can release chemical signalling molecules, which bind to specific receptors complementary in shape to the molecule, and initiate a response within a target cell
  ➔When binding to a cell receptor, chemical signalling molecules are the stimulus in the stimulus-response model of a cell. If a cell does not have a specialised receptor of the signalling molecule, it will not respond

Question 3

Cell signalling

Mode of transmission
APE

Answer 3

The transmission of signalling molecules from one cell to another can occur over short or long distances in the body. Chemical signalling can be broken up into three different modes of transmission:

● Autocrine signalling
When signalling molecules act on and initiate a response in the same cell that released them
● Paracrine signalling
When signalling molecules act of cells neighbouring the source cell
● Endocrine signalling
When signalling molecules are transported in the blood to act on cells far away from the source cell

Question 4

Animal hormones

Answer 4

Hormones are one of the main types of chemical signalling molecules. In animals, hormones are produced and released by glands, perform a variety of functions, and can act on nearby cells or be transported throughout the body in the bloodstream.

3 types:

• Lipid-based hormones (steroid)
• Peptide/protein hormones
• Amino-acid derived hormones

Sources:
Glands of the endocrine system including the adrenal glands, pituitary gland, and testes/ovaries

Mode of transmission:
Endocrine (in the blood), paracrine, or autocrine signalling

Question 5

3 types of animal hormones

Lipid-based (steroid)
Peptide and protein based
Amino-acid derived

Answer 5

Lipid-based (steroid):

• derived from fatty acids or cholesterol. When derived from cholesterol they are known as steroid hormones
• e.g. testosterone

Peptide and protein based:

• consist of short chains of amino acids, whereas protein hormones are longer polypeptides
• e.g. insulin, growth hormone

Amino-acid derived:

• derived from the amino acids tyrosine or tryptophan. Distinguished from peptide/protein hormones by differences in functioning
• e.g. adrenaline

Question 6

Plant hormones

Answer 6

• Plants also use hormones to communicate between their cells.
• Unlike animals, plants do not have an endocrine system or specialised glands. Instead, every plant cell is capable of producing a wide range of hormones which can be transmitted around the plant.

E.g. auxins, cytokinins

Sources: Majority of plant cells can make many different hormones

Mode of transmission: Variety, including cell to cell contact, and in xylem and phloem

Question 7

Pheromones

Answer 7

Pheromones are chemical signalling molecules produced in an organism that are transmitted through the air (or into an external environment) and detected by cells in the same organism, usually of the same species

➔Are typically detected by the olfactory (smell) receptors within the nose
➔Pheromones influence the behaviour and physiology of the receiving individual
➔Allows dogs to mark their territory, allows bees & ants to run their highly structured
communities.

Sources:
Seen in many insects and vertebrates such as cats and dogs.

Mode of transmission: Airborne, typically received by olfactory system

Question 8

Neurotransmitters

Answer 8

• Neurotransmitters are signalling molecules produced and secreted by neurons.
• Neurons are highly specialised cells that use electrical signals to trigger the release of neurotransmitters, allowing for rapid transmission of messages around an animal. Neurotransmitters are released in three steps:
  1) The neuron is excited by a stimulus
  2) A electric current passes along the neuron
  3) This leads to the production and release of neurotransmitters

Neurotransmitters act via paracrine signalling so, after release via exocytosis, they diffuse across a small gap before reaching complementary receptors on a target cell. This gap is known as the synapse. Neurotransmitter reception leads to a variety of responses:
● If the cell is a neuron, it can lead to a chain of neurons producing electrical impulses, releasing neurotransmitters, and stimulating the next neuron.
● If the cell is a muscle cell, it may cause muscle contraction.
● If the cell is part of a gland, it may cause hormone release.

Mode of transmission: Paracrine signalling across synapse

Question 9

Cytokines

Answer 9

Involved in communication within the immune system.

Sources: cells of the immune system

Mode of transmission: Typically autocrine or paracrine signalling

• Cytokines are chemical signals typically released by cells of the immune system that communicate with a variety of cell types throughout the body.
• Specialised cells of the body’s immune system release certain cytokines that bind to and guide other immune cells to a site of infection or injury
  ➔Cytokines cause a number of different responses, and are typically transmitted via autocrine or paracrine signalling
• Cytokines differ from hormones as all cytokines are protein-based, they are made by a variety of cells rather than specialised glands, and they circulate in lower concentrations than hormones

Question 10

Hydrophobic vs Hydrophillic signalling molecules

Answer 10

• Hydrophobic signalling molecules: are able to passively cross the plasma membrane and bind to intracellular receptors, initiating a response in the target cell.
• Hydrophilic (typically polar) signalling molecules: cannot cross the plasma membrane and instead bind to transmembrane receptors exposed to the extracellular space

Question 11

Signal transduction

Answer 11

The series of events that occur after the reception of a signal which results in the generation of a response

Question 12

Signal transduction of hydrophobic molecules

Answer 12

The majority of the plasma membrane is composed of non-polar fatty acids
- Because of this, hydrophobic signalling molecules can freely diffuse across the plasma membrane of a target cell.

Hydrophobic signalling molecules bind to intracellular receptors in the cytosol or in the nucleus.
To initiate a response in a cell, there are two common modes of action for hydrophobic signalling molecules after binding to receptors:
1. The molecule-receptor complex travels to the DNA and binds to regulatory regions to amplify or repress gene expression. In this case, the signalling molecule acts as a transcription factor.
2. The molecule-receptor complex may repress or activate enzymes in the cell.

• Can passively cross the plasma membrane
• Bind to intracellular receptors in the cytosol or nucleus
• Causes a cellular response, often related to gene expression or enzyme inhibition/activation

Question 13

Signal transduction in hydrophilic molecules

Answer 13

• Hydrophilic signalling molecules cannot readily cross the plasma membrane, so they bind to transmembrane receptors and rely on secondary messengers to pass the signal through a transduction cascade to bring about a cellular response.
• Hydrophilic signalling molecules include protein-based hormones, most amino-acid derived hormones, neurotransmitters, and cytokines.
• Cannot cross the plasma membrane
• Bind to transmembrane receptors, triggering changes in the receptor and within the cell
• Second messengers and proteins relay the signal (often with amplification) in a cascade to the cytosol or nucleus
• Causes a cellular response, often related to gene expression or enzyme inhibition/ activation, or protein channel function

E.g. Protein-based hormones (insulin), amino acid derived hormones, neurotransmitters, cytokines

Question 14

Example of hydrophobic signal transduction

Steroid hormones

Answer 14

Steroid hormones like testosterone and oestrogen are types of hydrophobic signalling molecules because they are lipid-based (and lipophilic).

• One target of oestrogen is endometrial cells, which are located in the uterus and help produce and maintain the uterine lining.
• Being hydrophobic, oestrogen passively crosses the membrane of endometrial cells and leads to the activation of genes that are necessary for normal uterine lining development.

Question 15

Apoptosis

Answer 15

The natural and controlled death of cells within our body which plays an important role in our development and day-to-day lives.

• It can be initiated by one of two pathways: the mitochondrial pathway or the death receptor pathway
• Commonly known as programmed cell death
  ➔When a cell begins to malfunction or is damaged, it will receive signals that initiate apoptosis, causing the eventual deal of the cell
  ★ Cannot be reversed

Question 16

The mitochondrial pathway (intrinsic)

Answer 16

When internal components of the cell (such as DNA) are damaged, specific proteins cause the initiation of the mitochondrial pathway of apoptosis by acting
directly on the mitochondria of a cell

★The mitochondria release cytochrome c into the
cytosol, which binds with cytosolic proteins to form the apoptosome
- This begins a cascade of reactions which result in the activation of caspase enzymes

Question 17

The death receptor pathway (extrinsic)

Answer 17

• Certain extracellular signals and molecules are
  recognised by ‘death receptor proteins’ on the membrane
  surface.
• These molecules are often cytokines released by
  T cytotoxic (Tc) cells, or cytokines released by various other immune cells.
• Following activation, the death receptor proteins begin a cascade of reactions within the cell which result in the activation of caspase enzymes.

Question 18

Process of apoptosis

Answer 18

1. Caspase activation
2. Digestion of cell contents
3. Cell shrinks
4. Blebbing and breakage
5. Signalling of macrophages
6. Both the mitochondrial and death receptor pathways initiate the activation of
   intracellular caspase enzymes by cleaving certain amino acid sequences from their structure. Once activated, caspase enzymes travel around the cell, digesting specific proteins.
7. The caspase–mediated digestion of proteins initiates a cascade of reactions that cause the digestion of all organelles.
8. The cell and nucleus shrink as intracellular material is digested
9. As the cytoskeleton is digested, the structural integrity of the cell is weakened.
   The membrane warps and detaches from the cell in membrane - enclosed vesicles known as apoptotic bodies which contain the digested intracellular contents.
10. As the cell breaks apart, chemicals are
    released. These chemicals attract phagocytes such as macrophages to the broken down cell.
    ➔It is only after apoptosis is complete that phagocytes consume apoptotic bodies by phagocytosis

Question 19

Malfunctions in apoptosis

Info (increase, decrease)
Examples

Answer 19

Too much apoptosis and we become sick and die. Too little apoptosis and we also become sick and die. It’s all about the perfect balance.

• Increasing apoptosis causes the death of too many cells and may lead to neurological disorders
• Insufficient apoptosis can allow cells to replicate exponentially, allowing the formation of
  tumours and cancers

● Cancer – result of too little apoptosis. Mutations present in the cancer cells enable these cells to grow in an uncontrolled fashion (neoplasia), forming tumours.
● Autoimmune diseases appear to involve disruption of apoptosis. Cells do not respond to the extrinsic pathway so little apoptosis is occurring
● Parkinson’s disease – loss of nerve cells due to unregulated activation of caspases that leads to excessive cell death