Major food groups
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Major food groups. Carbohydrates = sugars = saccharides Lipids = fats Proteins nucleic acids  Carbohydrates, proteins and nucleic acids (but not fats) are long chains of smaller subunits, which are monomers. The combinations are polymers .

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Major food groups

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Major food groups

Major food groups

  • Carbohydrates = sugars = saccharides

  • Lipids = fats

  • Proteins

  • nucleic acids

    Carbohydrates, proteins and nucleic acids (but not fats) are long chains of smaller subunits, which are monomers. The combinations are polymers.

    Polymers in which the order of monomers provides information: proteins and nucleic acids.


Major food groups

Nucleic acid digestion- pancreatic ribonuclease & deoxyribonuclease; nucleosidases

& phosphatases; act in small intestines


Composition and function of pancreatic juice

Many enzymes are secreted in inactive form:

Examples include:

Trypsinogen is activated to trypsin

Pepsinogen activated to pepsin by HCl

Procarboxypeptidase is activated to carboxypeptidase

Composition and Function of Pancreatic Juice


Major food groups

  • Digestive enzymes that are produced by the small intestine are all brush border enzymes (membrane-bound)

  • Advantages and disadvantages of having membrane-bound enzymes


Figure 23 27 activation of pancreatic proteases in the small intestine

Figure 23.27 Activation of pancreatic proteases in the small intestine.

Stomach

Pancreas

Epithelial

cells

Membrane-bound

enteropeptidase

Trypsinogen

(inactive)

Chymotrypsinogen

(inactive)

Procarboxypeptidase

(inactive)

Trypsin

Chymotrypsin

Carboxypeptidase


Major food groups

Protein digestion

Enzymes

and source

Site of

action

Foodstuff

Path of absorption

Protein

Pepsin

(stomach glands)

in presence

of HCl

Stomach

Large polypeptides

Pancreatic

enzymes

(trypsin, chymotrypsin,

carboxypeptidase)

+

Small

intestine

Small polypeptides,

small peptides

• Amino acids enter

the capillary blood in the

villi, and are transported

to the liver via the hepatic

portal vein.

Brush border

enzymes

(small intestine)

Small

intestine

Amino acids


Figure 23 33 protein digestion and absorption in the small intestine

Figure 23.33 Protein digestion and absorption in the small intestine.

Amino acids of protein fragments

Lumen of

intestine

Brush border enzymes

Apical membrane (microvilli)

Pancreatic

proteases

1

Protein fragments (peptides)

are digested to amino acids by

brush border enzymes of

mucosal cells.

Absorptive

epithelial

cell

2

The amino acids are then

absorbed by active transport into

the absorptive cells

Amino

acid

carrier

3

The amino acids leave the

villus epithelial cell by facilitated

diffusion and enter the capillary

via intercellular clefts.

Active transport

Capillary

Passive transport


Major food groups

Carbohydrate digestion

Enzyme(s)

and source

Site of

action

Foodstuff

Path of absorption

Starch and disaccharides

Salivary

amylase

Mouth

Pancreatic

amylase

Small

intestine

Oligosaccharides

and disaccharides

Brush border

enzymes in

small intestine

• All monosaccharides

enter the capillary

blood in the villi, and

are transported to the

liver via the hepatic

portal vein.

Small

intestine

Lactose

Maltose

Sucrose

Galactose

Fructose

Glucose


Note fats are not made of a long chain of monomers unlike proteins and polysaccharides

Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides)

Fat digestion

Enzyme(s)

and source

Site of

action

Foodstuff

Path of absorption

Unemulsified

fats

• Fatty acids and glycerol

enter the intestinal cells via

diffusion.

Emulsification by

the detergent

action of bile

salts from the

liver

Small

intestine

• Fatty acids and monoglycerides

are recombined to form

triglycerides and then

combined with other lipids and

proteins within the cells to make

chylomicrons, which are

extruded by exocytosis.

Pancreatic

lipases

Small

intestine

fatty acids

glycerol

• The chylomicrons enter the

lacteals of the villi and are

transported to the systemic

circulation via the lymph in the

thoracic duct.


Figure 23 34 emulsification digestion and absorption of fats

Figure 23.34 Emulsification, digestion, and absorption of fats.

Fat globule

1

Large fat globules are emulsified

(physically broken up into smaller fat

droplets) by bile salts in the duodenum.

Bile salts

2

Digestion of fat by the pancreatic

enzyme lipase yields free fatty acids and

glycerol, still associated

with bile salts

Fat droplets

coated with

bile salts

Fatty acids

and bile salts

3

Fatty acids and glycerol diffuse into epithelial cells.

There they are recombined and packaged

with other lipids and proteins

to form chylomicrons.

4

Chylomicrons are extruded from the

epithelial cells by exocytosis. The

chylomicrons enter lacteals. They are

carried away from the intestine by lymph.

Epithelial

cells of

small

intestine

Lacteal


Major food groups

Nucleic acid digestion

Enzyme(s)

and source

Site of

action

Foodstuff

Path of absorption

Nucleic acids

Pancreatic ribo-

nuclease and

deoxyribonuclease

Small

intestine

• Subunits are absorbed into

capillary blood in the villi

and transported to the

liver via the hepatic portal

vein.

Brush border

enzymes

(nucleosidases

and phosphatases)

Small

intestine

Pentose sugars,

N-containing bases,

phosphate ions


In the large intestine

Other than digestion by enteric bacteria, no further digestion takes place

Bacteria synthesize B complex vitamins and vitamin K

water, and electrolytes are absorbed

In the large intestine:


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