3 Substance exchange- Digestion & Absorption Flashcards

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1
Q

What happens during digestion?

A

Large biological molecules are hydrolysed into smaller ones that can be absorbed across cell membranes, used for a number of functions

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2
Q

Why can’t biological molecules ingested be transported into the cells?

A

They’re too large, must first be broken down by hydrolysis

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3
Q

How are carbohydrates digested?

A

-Begins in the mouth
-Amylase (enzyme produced in salivary glands + pancreas) catalyses hydrolysis of starch into maltose. This breaks a glycosidic bond.
-Membrane bound disaccharidases (enzymes in cell membranes of epithelial cells in ileum; small intestine) catalyse hydrolysation of disaccharides into monosaccharides
-Monosaccharides are transported into epithelial cells in ileum using transporter proteins; glucose plus galactose by co-transporter proteins and fructose by facilitated diffusion
-Once inside, can be used for respiration

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4
Q

How are lipids digested?

A

Mostly takes place in small intestine
-Lipase (enzyme made by pancreas, released in small intestine) hydrolyses lipids into fatty acids and monoglycerides. This breaks an ester bond
-Bile salts made by the liver, help digest lipids by making small droplets; micelles (emulsification). These have a large surface area, allowing quicker hydrolysis
-Micelles allow monoglycerides and fatty acids to be absorbed by epithelial cells in ileum. They diffuse through the cell surface membrane into the cell cytoplasm

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5
Q

How are proteins digested?

A

Mostly in small intestine
-Endopeptidases (enzymes that catalyse hydrolysis of bonds) hydrolyse peptide bonds between amino acids in centre of protein, to make polypeptides
-Exopeptidases catalyse hydrolysis of peptide bonds at ends of polypeptide, making dipeptides and amino acids
-Dipeptidases hydrolyse dipeptides into amino acids
-Amino acids are then transported into epithelial cells in ileum by co-transporter proteins

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6
Q

How are amino acids/monosaccharides absorbed?

A

-Sodium-dependent co-transporter proteins are located in epithelial cell membranes and they actively transport sodium ions into the blood; causing the concentration of sodium ions in epithelial cells to decrease
-This causes sodium ions in the ileum to diffuse down their concentration gradient through a co-transporter protein
-When sodium binds to a co-transporter protein, amino acids/monosaccharides also bind; causing it to undergo a conformational change
-These are all transported into the epithelial cell cytoplasm
-Can now be used inside epithelial cells or absorbed into bloodstream

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7
Q

How are lipids absorbed?

A

-Micelles move to surface of epithelial cells, break down to release monoglycerides and fatty acids
-Diffuse into epithelial cell (as non-polar)
-Endoplasmic reticulum reforms them into triglycerides, travel to golgi apparatus; packaged into lipoproteins
-Move into lymphatic capillary by exocytosis, then to bloodstream where they’re hydrolysed into triglycerides ready for absorption

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