Lipids + hormones

Question 1

Describe the solubility of lipids

Answer 1

insoluble in water

soluble in organic solvents (eg. alcohol)

Question 2

What are the classes of lipids?

Answer 2

fatty acids
triacylglycerols (fats + oils)
glycerophospholipids (membrane lipids)
sphingolipids (membrane lipids)
cholesterol

Question 3

Describe the structure of fatty acids

Answer 3

long chain carboxylic acids
saturated + unsaturated
most unsaturated = cis-configuration
majority = unbranched
as number of carbon atoms increases, melting point increases
as number of double bonds increases, melting point decreases

Question 4

What is an essential fatty acid?

Answer 4

fatty acids humans cannot synthesise and therefore must be ingested

Question 5

What are fatty acids precursors for?

Answer 5

prostaglandins, thromboxanes and leukotrienes

Question 6

Describe the structure of triacylglycerols

Answer 6

fatty acid triester of glycerols (AKA triglycerides)
transported by lipoproteins and stored in adipocytes
if body needs energy, hormone-sensitive lipase splits triacylglycerols into glycerol and fatty acids
fatty acids can then be released into the blood and transported in the form of fatty acid-albumin complex

Question 7

Describe VLDLs

Answer 7

very low density lipoproteins
lipids = triglycerides (endogenous)
source = liver
function = transport to fat tissue

Question 8

Describe LDLs

Answer 8

low density lipoproteins 
lipids = cholesterol
source = liver
function = transport to peripheral tissue

Question 9

Describe HDLs

Answer 9

high density lipoproteins 
lipids = cholesterol, phospholipids
source = liver
function = transport from peripheral tissue to liver

Question 10

Describe the structure of glycerophospholipids

Answer 10

main components of cell membranes
amphiphilic (have both hydrophilic and hydrophobic parts)
hydrophobic fatty acid chain + polar head group (phosphate + alcohol)
2 fatty acids bonded to a glycerol which is bonded to a phosphate which is bonded to an alcohol
cleaved by phospholipases in the pancreas after being emulsified with bile

Question 11

Describe the structure of sphingolipids

Answer 11

components of cell membranes
amphiphilic (have both hydrophilic and hydrophobic parts)
sphingomyelin = fatty acid - sphingosine - phosphate - choline
glycolipid = fatty acid - sphingosine - sugar

Question 12

Describe the structure of cholesterol

Answer 12

weakly amphiphilic
condensed 4 ring system = steroid ring system
almost all molecule = hydrophobic
polar -OH group = hydrophilic
sources = dietary intake, endogenous synthesis (sites = all cells, major sites = liver + intestine)
precursor for synthesis of steroid hormones + other molecules

Question 13

What is a hormone?

Answer 13

a substance that controls and regulates biological functions

chemical messenger

Question 14

How do hormones act?

Answer 14

binding to a specific receptor on cell surface or within a target cell
interaction with receptor triggers and coordinates biological effect

Question 15

Describe autocine signalling

Answer 15

local cell-cell diffusion
act on neighbouring cells of same type
eg. prostaglandins

Question 16

Describe paracrine signalling

Answer 16

local cell-tissue diffusion
act on different cells in same tissue
eg. histamine

Question 17

Describe endocrine signalling

Answer 17

distributed by blood
act on distant target cells
eg. insulin

Question 18

Describe the different hormone types

Answer 18

peptide hormones = encoded by genes
amino acid-derived hormones = not directly encoded by genes, indirectly by genes of enzymes that synthesise hormones
steroid + sterol hormones = synthesised from cholesterol
lipid hormones = synthesised from fatty acids by specific enzymes
gaseous hormone = nitric oxide

Question 19

What are the 4 membrane-bound receptors?

Answer 19

receptor that is also an ion channel
G protein-coupled receptor
receptor bound to guanylyl cyclase
receptor that is also a kinase

Question 20

Describe what happens at a receptor that is also an ion channel

Answer 20

hormone binds to receptor
receptor opens ion channel
ions move into cell and leads to membrane depolarisation
membrane depolarisation = biological effect triggered by hormone
eg. serotonin, GABA

Question 21

Describe what happens at a G protein-coupled receptor (adrenaline)

Answer 21

hormone binds to receptor
receptor changes conformation, switching GDP for GTP in alpha subunit of protein
exchange leads to dissociation of alpha subunit which binds to another effector protein (calcium channel)
calcium channel opens and calcium ions move into cell
increase in intracellular mediators
alpha subunit hydrolyses attached GTP —> GDP which leads to dissociation of alpha subunit from calcium channel
calcium channel closes, no more calcium ions enter
alpha subunit reassociates with beta and gamma subunits and a new activation cycle can begin

alpha subunit can also bind to PLC (phospholipase C)
PLC = enzyme that catalyses conversion of PIP2 into IP3 (inositol-triphosphate) + DAG (diacylglycerol)
IP3 + DAG regulate enzyme activity

Question 22

Describe what happens at a G protein-coupled receptor (glucagon)

Answer 22

hormone binds to receptor
receptor changes conformation, switching GDP for GTP in alpha subunit of protein
exchange leads to dissociation of alpha subunit which binds to another effector protein (adenylyl cyclase (AC))
AC = enzyme that catalyses cyclisation of ATP into CAMP (Cyclic AMP)
CAMP = 2nd messenger which activates protein kinases which will phosphorylate proteins and activate them

Question 23

Describe what happens at a receptor bound to guanylyl cyclase

Answer 23

hormone binds to receptor
GTP is converted to CGMP (cyclic GMP)
CGMP then causes activation of protein kinases
eg. atrial-natriuretic hormone

Question 24

Describe what happens at a receptor that is also a kinase

Answer 24

hormone binds to receptor
conformational change triggers intrinsic kinase activity of receptor
receptor phosphorylates specific proteins using ATP
phosphorylation leads to activation or inhibition of protein
eg. insulin

Question 25

Describe the 2 mechanisms that can occur at intracellular receptors

Answer 25

hormone diffuses into cytoplasm
binds to intracellular receptor
hormone-receptor complex relocates to nucleus
hormone-receptor complex binds to hormone response elements on DNA and acts as transcription factor, turning gene expression on or off
eg. glucocorticoids

hormone diffuses via cytoplasm into nucleus where it binds to nuclear-located receptor
hormone-receptor complex binds to hormone response elements on DNA and acts as transcription factor, turning gene expression on or off
eg. estrogens, androgens, thyroid hormones