Homeostasis, signalling and biological fuel

Question 1

Describe the stages of cellular respiration

Answer 1

Glycolysis:

• Means sugar splitting
• Take the 6 carbons, and split it into two 3 carbons = pyruvate
• This is done with 2 ATP, and produces 4 ATP, having a net production of 2 ATP
• It is a redox reaction because the NAD becomes NADH, done without oxygen

Pyruvate oxidation:

• When oxygen is present, the NADH and pyruvate move into the mitochondria
• Move into the inner membrane
• Completely oxidises pyruvate (removes all remaining hydrogen) into Carbon Dioxide
• First, it oxidises into acetoyl CoA, which is turning the 3 carbons into 2
• The lost carbon becomes CO2
• As this oxidation occurs, NAD carriers become reduced NADH (they gain the removed hydrogen)

Citric Acid Cycle/ Krebs cycle:

• Here, acetyl CoA binds to a 4 carbon molecule. It becomes a six carbon molecule
• It goes through a series of reactions, releasing ATP, NADH and FADH2 and a Carbon dioxide molecule
• The cycle restarts with the 4 carbon molecule attaching to another CoA

Oxidative Phosphorylation & (NADH, H+ gradient, ATP):

• TheNADH andFADH2 made in other steps deposit their electrons (hydrogen ions) in the electron transport chain, turning back into their “empty” forms NAD+ and FAD+
• This forms a H+ gradient
• The H+ flows back into the matrix through an enzyme called ATP synthase
• ATP synthase uses the flow of H+ across the gradient to power ATP production
• 26-38 ATP are produced

Question 2

List the functions of biological membranes

Answer 2

• A barrier between the inner and outer surface
• Boundaries among organelles / inner compartments
• Protect the cells
• Control the movement of substances into and out of cells
• Regulate the composition within individual cells
• Control the flow of information between cells (recognizing or sending signals) – Capture and release energy (e.g. mitochondria)
• Cell adhesion
* Synthesize steroids

Question 3

Describe the general steps of intercellular signalling

Answer 3

• Cells communicate via chemical signals
• These chemical signals are in the form of proteins or other molecules. They are known as ligands, which is a term for molecules that bind to other molecules
• These ligands are produced and secreted by the sending cell into the extracellular space
• In order to be a target cell/ responding/ receiving cell, the cell must have the right receptor for that signal
* When a ligand binds to a receptor, the receptors shape and activity changes, triggering a change within the cell
* The message carried by a ligand is often relayed through a chain of chemical messengers inside the cell
* Ultimately, it leads to a change in the cell, such as alteration in the activity of a gene or even the induction of a whole process, such as cell division

Question 4

Distinguish between direct, endocrine, paracrine and autocrine signaling.

Answer 4

Endocrine:
Cells transmit signals over long distances using circulatory system. Signals that are produced in one part of the body and travel through the circulation to reach far-away targets are known ashormones.

Paracrine:
Cells next to each other can communicate through the chemical release of ligands which can diffuse through the short space between the cells

Autocrine:
A cell signals itself, releasing a ligand that binds to receptors on its own surface

Question 5

How do G-proteins coupled receptors work in intracellular signalling?

Answer 5

• GPCR’s have seven transmembrane alpha helices (seven green thingos)
• They interact with G proteins; specialized proteins that bind guanosine triphosphate and guanosine diphosphate
• All G proteins that bind to GPCR’s are heterotrimeric: they are made of three different subunits; alpha, beta and gamma
• The alpha and gamma subunits are bound to the membrane by lipid anchors
• In its inactive form, the alpha subunit is bound to a GDP. In its active form, it is bound to GTP
• When a ligand bind to the GPCR, the GPCR undergoes a conformational change where the alpha helix changes its GDP to GTP
• When the alpha unit binds to the GTP, it disassociates from its beta and gamma sub units
• The alpha subunit is now able to regulate target proteins
• The target proteins relay a second messenger
* The GTP is hydrolyzed (loses one phosphate) into GDP, stopping the cycle

Question 6

Describe enzyme linked receptors

Answer 6

• They are cell surface receptors that have an extracellular domain (where ligand binds to receptors) and an intracellular domain (within the cell, it is the part of the receptor that can act as an enzyme)
• When a ligand binds to the extracellular domain, the catalytic activity of the intracellular domain is activated
• Intracellular proteins go and dock onto the enzymes of the receptor
* The signal will be relayed by these intracellular proteins into the cells interior

Question 7

Briefly describe the 3 types of secondary messengers

Answer 7

Calcium ions Ca+:
In most cells, Ca+ levels are low. For signalling purposes, Ca+ may be stored in compartments such as the E.R. Ligands will bind to ligand-gated Ca+ channels, allowing Ca+ into the cell. Once in the cell, calcium 2+ ions binds to proteins with complementing binding sites. When the ion binds, it changes the proteins shape and thus function.

cAMP:
Cyclic adenosine monophosphate is a small molecule made from ATP. Enzymes called adenylyl cyclase converts ATP into cAMP. The cAMP activates a protein called protein kinase (PKA) which adds phosphates to target proteins, changing its structure and function

Inositol triphosphate IP3 diacylglycerol DAG:
When a ligand binds to the GPCR, it activates it, which in turn activates phospholipase. The phospholipase is an enzyme which then separates pip 2 into IP3 and DAG. IP3 is freed, and goes and binds to the IP3 Ca+ channel. The Ca+ channel is opened, allowing Ca+ to travel into the cytosol. The increased level of Ca+ along with DAG, activate Protein Kinase C, it will begin to phosphorylate target proteins

Question 8

Outline the physiological process of fever in relation to homeostasis

Answer 8

• Chemical substances called pyrogens (pryo= heat, gen= generate) released by white blood cells (endogenous) and/or invaded microorganisms (exogenous) raise the set point of the thermoregulatory centre causing the whole body temperature to increase
  • Tm > 38 D
• Protection mechanism – inactive enzymes, kill bacteria, inhibit viruses