Lecture 2 - Digestion and Absorption of Nutrients Flashcards
Definition of digestion
Process by which foodstuffs are broken down in the GI tract into absorbable units
Are food and nutrients inside the body during digestion?
No, considered to be outside the body, GI tract is hollow tube outside body
What are the four types of digestive processes
Mechanical
Chemical
Enzymatic
Microbial
Mechanical digestion includes…
Mastication (chewing)
Grinding action of the gizzard (birds)
Movement of the GI tract (segmentation and peristalsis)
What is peristalsis
series of wave-like muscle contractions that move food through the digestive tract
Chemical digestion involves
Gastric acid (HCl; pH 1.5-3.5)
Enzymatic digestion occurs where? Involves what?
In the lumen and the mucosa
Digestive enzymes are secreted into the GI tract
Definition of absorption
Process of moving digested products through the gut mucosal wall
Two types of absorption
Transcellular (across cell)
Paracellular (across tight jxn)
Three types of transport
Passive
Active
Osmosis
Describe passive transport
Relies on concentration/electrochemical gradients
Transcellular or paracellular
No energy required
What is osmosis
Water moves from low solute concentration to high solute concentration (water follows solute)
The electrochemical gradient is the combination of what
Chemical driving force (e.g. Na, K from low to high conc)
Electrical driving force (charge force; neg charge inside)
Describe active transport
Moves against concentration/electrochemical gradient
Needs energy (ATP)
Primary or secondary
What is secondary active transport
Movement of a solute against its gradient by pairing it with facilitated diffusion of a different solute with its concentration gradient
What molecules use secondary active transport
Glucose, aa, B-vitamins, bile salts
How much water do humans absorb/reabsorb per day? Where does this happen?
2L/day from food and drink + 7L per day from gut secretion = 9L a day
95% of it is absorbed in the small intestine
What kind of transport does water absorption use
Passive transport (osmotic gradient; transcellular and paracellular)
Amount of water secreted depends on? Location of absorption depends on?
Whether the animal is a carnivore or herbivore
Depends on species
Distribution of absorption in small intestine vs colon or dog vs horse
Dog = majority absorbed by SI (88%), 12% by colon
Horse = more even (58% SI, 42% colon)
Types of carbohydrates
Monosaccharide
Disaccharide
Polysaccharide
Examples of mono, di, and polysaccharides
Mono = glucose, fructose, galactose
Di = sucrose, maltose, lactose
Poly = starch, cellulose
Sucrose, maltose and lactose composition
Sucrose = glucose + fructose
Maltose = glucose + glucose
Lactose = glucose + galactose
What kinds of bonds are in starch? Cellulose?
Starch = alpha-1,4 glycosidic bond
Cellulose = beta-1,4 glycosidic bond
How many carbons in glucose, fructose and galactose
Glucose/galactose = six
Fructose = five
Steps in amylose digestion
Amylose broken down into dextrins by salivary alpha-amylase
Dextrins broken down to maltose by pancreatic a-amylase
Maltose broken down into glycose by maltase
Slide 17**
Polysaccharide digestion
What transports glucose across the brush border membrane? Fructose?
Sodium glucose transporter 1
GLUT5
How does glucose transport vary in low-sugar vs high-sugar meal
Low-sugar = sodium glucose transporter 1 is used (secondary active transport)
High-sugar = GLUT2 allows glucose to be absorbed passively
What kinds of CHO are absorbed
Monosaccharides only
What happens to lactase activity over time? Enzyme activity varies with…
Constantly decreasing after birth
Age, region of SI
Parts of an amino acid
Amino group, carboxylic acid group, R group
Proteins are
Large molecules consisting of amino acids linked by peptide bonds
Primary, secondary, tertiary and quaternary protein structure
Primary = sequence of a chain of aa
Secondary = H bonds of the peptide backbone (B-sheet, a-helix)
Tertiary = 3D folding pattern of a protein due to side chain interactions
Quaternary = protein consisting of more than one amino acid chain
How are gastric enzymes activated
HCl in the stomach leads to denaturation of protein, this triggers the activation of pepsinogen into pepsin
Gastric protease enzyme is called (unactivated and activated)
Pepsinogen (proenzyme) -> pepsin
Pancreatic protease enzymes and how they’re activated
Trypsinogen is activated by enterokinase and becomes trypsin
Trypsin activates the rest:
Chymotrypsinogen -> chymotrypsin
Proelastase -> elastase
Procarboxypeptidase A -> Carboxypeptidase A
Procarboxypeptidase B -> carboxypeptidase B
What happens to large peptides at the brush border
Turned into di- and tri- peptides or free aa by brush border peptidases
What happens to di- and tri and free aa inside the brush border membrane
Small amounts of di- and tri- peptides will cross through, most turned into aa by cytoplasmic peptidases
Amino acids carried through membrane
How does brush border membrane transport differ for aa
Neutral, basic and acidic aa have different transport systems, use different ion gradients
Slide 29**
Protein and CHO recap
Most ingested fats are in the form of
triglycerides
What is a triglyceride composed of
Glycerol + 3 fatty acids
Where are bile salts produced and stored
Produced in liver
Stored in gallbladder
Two parts of bile salts…
One part is negatively charged (hydrophilic head), the other is hydrophobic tail (positive charge)
Slide 33***
Lipid digestion
Fat goes from…
Large fat drops -> + bile -> small, emulsified fat drops -> products of degradation absorbed by micelles -> free f.a. + monoglyceride diffuse into cell
What breaks down fats into monoglycerides and f.a.
Pancreatic lipase, colipase
What happens to the f.a. and monoglycerides that are produced by fat breakdown
Stored in micelles, move out of micelles and enter cell by diffusion
Absorbed fats + cholesterol + proteins inside the intestinal cell form what? Where does this go?
Form chylomicrons
They are released into the lymphatic system
Nucleotide vs nucleoside
Nucleotide = Sugar + base + phosphate group
Nucleoside = sugar + base
Purines? Pyrimidines?
Purines = adenine, guanine
Pyrimidines = thymine, cytosine, uracil
RNA has what instead of what?
Uracil instead of thymine
Nucleosomes are formed when what happens? Eventually leading to the formation of…
When DNA wraps around a protein (histone)
Eventually forming chromosomes
Steps in nucleic acid digestion/absorption
- Denatured by gastric acid
- Broken down to nucleotides by pancreatic nucleases
- Phosphatase on brush border cleave P ion
- Nucleosidase catalyzes breaking of covalent bond btw nitrogenous base and pentose sugar
Slide 38**
Major enzymes in nucleic acid digestion/absorption
Pancreatic nucleases
Phosphatase
Nucleosidase
Products of nucleotide breakdown are absorbed in the… Transported to…
Duodenum and jejunum
Liver and other tissues
what kind of transport are electrical and chemical driving forces
Passive transport
How does primary active transport work
Na+ binds transporter, stimulates phosphorylation by ATP = conformational change, K+ then binds causing release of P and back to original conformation
bile salts are required for __________ digestion
Lipid
