Nutrient digestion and absorption 2 Flashcards
What form is most of the fat in our diet?
triacylglycerol
What are the components of triacylglycerol?
3 stearic (fatty) acids and glycerol
What enzyme digests fat and where does this occur?
pancreatic lipase in the small intestine
Are triacylglycerols soluble or insoluble in water?
insoluble
Where on the triacylglycerol molecule can lipase act on and why?
on the outside only (surface digestion) as it is a water soluble enzyme
If lipase acting on TAG was the only digestion mechanism what would be the main problems?
Digestion would be very slow and there would be fat in the faeces (steathorrea)
What does TAG acted on by lipase form?
monoglyceride and 2 fatty acids
What is emulsification?
Dividing large lipid droplets into smaller droplets (1mm)
What does emulsification allow?
Increase in surface area and greater accessibility for lipase
What are the 2 requirements for emulsification
mechanical disruption and an emulsifying agent
What is the mechanical disruption in the form of fat digestion?
Smooth muscle contraction
What does an emulsifying agent prevent?
Remixing and reformation of the large lipid droplets
What are the 2 emulsifying agents in fat digestion?
bile salts and phospholipids
Explain in terms of charge why making the droplets amphipathic helps?
The like charges on each of the individual droplets repel each other to prevent reforming into a larger droplet
How is absorption further enhanced?
Forming micelles
What is the diameter roughly of a micelle?
4-7 micrometres
What are the constituents of a micelle?
monoglyceride, fatty acids, bile salts and phospholipids
Where are the polar portions and non polar portions located in a micelle
polar on the outside and non polar in the core
Where do the micelles breakdown and why?
Micelles are unstable and in the acid microclimate take on the H+ charge to release fatty acids and monoglycerides
Are micelles absorbed?
NO
What membrane do the micelles release the contents for absorption?
2nd part of duodenum
Where are TAG reformed? by what?
Smooth ER by enzymes
Why is the newly reformed TAG coated in phospholipids
As a form of emulsification
What happens after TAG are reformed in the Smooth ER?
vesicles are formed and go to Golgi apparatus to be exocytosed at serosal membrane
What are chylomicrons?
Extracellular fat droplets (1micrometre)
Where are the chylomicrons absorbed and where does this lead to?
lacteals in the small intestine villi
lymphatic system
What are the fat soluble vitamins?
A,D,E,K
What are the water soluble vitamins?
B,C, folic acid
How are water and fat soluble vitamins absorbed?
fat = same as fat water = passive diffusion or by carrier
Briefly describe a vitamin B12 molecule
large and charged
How is vitamin B12 absorbed?
Binds to intrinsic factor in stomach
this complex absorbed via specific transport mechanism in the distal ileum
What is vitamin B12 important for? What can happen if there is a deficiency?
maturation of red blood cells
pernicious anaemia
How much vitamin B12 store does the liver have?
3 years
How is iron transported across the brush border?
by DMT1
What happens once iron has been transported?
Now in duodenal enterocytes
incorporated into ferritin = now an iron store
What is ferritin levels regulated around?
Iron levels
What happens to unbound iron?
Crosses serosal membrane into the blood and binds to transferrin which goes to the liver
What is hyperaemia in terms of ferritin?
Increased ferritin levels - more bound iron
What is anaemia in terms of ferritin?
decreased ferritin levels - more iron released into the blood
Must iron be 3+ or 2+ to cross the membrane?
2+
