Digestive Physiology, Biochemistry, & Metabolism

digestion

Food is mechanically and/or chemically broken down into absorbable units

Absorption

Uptake of nutrients into the body, typically by cells of the small intestine

small intestine vs large intestine absorption

Small intestine – nutrient absorption

Large intestine – water absorption

role of amylase

breaks down sugar

most fiber is _______ why

not digestive because amylase can’t attack it

Pathway of carbohydrates (starches), sugars, proteins, and fat (triglycerides) to blood

carbs > single sugars > portal vein to liver

sugars > sugars > portal vein to liver

proteins > polypep > amino acid > portal vein to liver

fat > fatty acids

1. fatty acid long chain > lymph vessel to blood

2. fatty acid short chain > portal vein to liver

Key characteristic about fat (triglycerides)

Insoluble

Must go through a process to make them polar and soluble using bile salts and pancreatic lipase > fatty acid

absorbable units of carbohydrates, sugars, proteins, fats

single sugars (fructose, glucose), sugar, amino acids, fatty acids and monoglycerides

Describe characteristics of monosaccharides

polar and soluble

dissolve and circulate in blood

can monosaccharides freely cross lipid cell membrane? why?

No

Because they are polar, they cannot freely cross the lipid cell membrane > need transport proteins

describe the cell membrane and what molecules cross easily

Cell membrane = lipid = nonpolar

Nonpolar molecules - cross easily (like dissolves like)

Polar molecules - cannot cross easily - need a transporter/channel

2 ways monosaccharides imported into cell

1. SGLTs = Secondary Active Transport

2. GLUTs = Facilitated Diffusion

Describe secondary active transport

Na⁺-coupled glucose transport

Uses the Na⁺ concentration gradient

Na⁺ gradient is created by the Na⁺/K⁺ ATPase

Therefore, SGLTs indirectly depend on ATP

Describe facilitated diffusion

Moves monosaccharides high > low []

Does NOT directly require ATP

Example: GLUT4 > found mainly in skeletal muscle + adipose (fat) tissue

Insulin stimulates GLUT4 movement to the cell membrane, increasing glucose uptake

How do amino acids dissolve in blood (2)

directly or binded to albumin

How do amino acids import into cells (2)

1. secondary active transport (coupled to Na+)

2. facilitated diffusion

Describe each of the following amino acid transporters: symporters, antiporters, uniporters

Symporter – similar to Na/glucose channel (same direction)

Antiporter – opposite switch amino acids which are similar in some way (both positive or soluble)

Uniporter – moves one amino acid in one direction

Difficult amino acid to move through channels

Proline - ring attached to side chain, bulky

Which amino acid is the smallest and is the only one with a Hydrogen side chain

glycine

Most lipids are hydro____ and nonpolar, so how do they dissolve?

phobic

lipoproteins (nonpolar) (chylomicron, VLDL, LDL, HDL)

Chylomicrons

biggest lipoprotein

from digestion first into lymphatics > blood

When chylomicrons in blood

attacked by lipoprotein lipases > break them down into fatty acids ang glycerol (can enter nearby tissues)

rest is broken down by liver

what happens to spent VLDL or VLDL remanent

returned to liver again or converted to LDL (CVD)

Majority of LDL is what

cholesterol

(high cholesterol is a problem)

LDL can bind to __________ and >

LDL receptors, be absorbed into tissues (ex: cholesterol can be used to make hormones)

If blood LDL exceeds LDL receptors

LDL can accumulate > increases risk of CVD

what happens to excess intracellular cholesterol

packaged into HDL by tissues > released into blood > HDL goes to liver > scavenges excess chol in blood

Summarize the key points of the different lipoproteins, why is all of this done

1. Chylomicrons and VLDL transport triglycerides and deliver them to body cells.

2. LDL transports and delivers cholesterol to tissues.

3. HDL returns cholesterol to the liver for reuse or elimination.

All because lipids are nonpolar/hydrophobic

Chylomicron characteristics

largest and least dense

almost entirely triglycerides

VLDL characteristics

about half triglycerides (very low density)

LDL characteristics

small

mostly cholesterol

HDL characteristics

mostly protein (very high density)

Absorbed nutrients participate in metabolism through ___________, where they are broken down for energy and/or ___________, where they are used to build new compounds

catabolic processes

anabolic processes

Catabolic processes generate, examples

ATP

Glycolysis, beta-oxidation, Krebs cycle, OXPHOS

Nutrients can be used as raw materials for anabolic processes such as ________ that require _______, and examples

growth, ATP

Glycogenesis, triglyceride synthesis, translation

Excess nutrients that are not immediately used can be stored as (2)

glycogen or triglycerides

metabolic pathway of glucose (carbohydrates)

Glycolysis > Krebs Cycle > ETS (energy)

OR Biosynthesis of many compounds such as glycoprotein, pentoses

Amino acids contain what

nitrogen containing compounds (NTs, nucleotides, NO)

metabolic pathway of amino acids

Urea cycle or Cahil cycle > Krebs Cycle > ETS

OR Can be converted into glucose via gluconeogenesis

metabolic pathway of fatty acids

Beta-oxidation > Krebs Cycle > ETS (energy)

OR Biosynthesis of many compounds, such as membrane lipids

Metabolism of carbohydrates, lipids, proteins to produce ATP occurs in what 2 phases

1. Fuel oxidation (preparatory) phase

2. Energy generation phase

Fuel oxidation (preparatory) phase describe, 3 cycles

Most processes anaerobic

where fuels are partially oxidized to generate NADH, FADH₂, and substrates for the ETS

Glycolysis, beta-oxidation, Krebs cycle

can yield small amount ATP

Energy generation phase describe, requires

Products from first phase used to create large amounts of ATP by oxidative phosphorylation in mitochondria

Metabolic pathways of ALL major macronutrients converge to allow entry into this phase

Requires O₂ (aerobic) as terminal electron acceptor

Glycolysis (carbs)

9 steps, one glucose (6-carbon molecule) broken into 2 pyruvate (3-carbon molecule), can be converted into acetyl-CoA and enter Krebs Cycle

Net yield of glycolysis

1. 2 ATP molecules (4 ATP generated less 2 ATP required to complete the pathway)

Produced by substrate-level phosphorylation: phosphate is transferred to ADP from a high-energy intermediate of glycolysis 2>3

2. 2 NADH; used by mitochondria during oxidative phosphorylation

3. 2 pyruvate; can be converted into acetyl-CoA and enter the Krebs/TCA cycle to generate more NADH

Does glycolysis require oxygen and is it always occuring?

Glycolysis always happens in cell with or without oxygen (it is NOT anaerobic)

During hypoxia the rate increases (goes up or down depending on how much oxygen)

this means it is present in all cell types always occurring at some level

describe beta-oxidation

Triglycerides are broken down into their components: glycerol molecule and three fatty acids (variable)

Through the iterative process of beta-oxidation, 2-carbon pieces of the fatty acids are individually broken off to form acetyl-CoA

• One fatty acid provides many acetyl-CoAs, exact yield depends on the fatty acid

• NADH and FADH₂

What happens to amino acids (2) > ______

Can be de- or trans-aminated, which removes their amino group

Depending on its side chain, the remaining carbon skeleton of the amino acid can enter the Kreb cycle as one of several metabolites, including acetyl-CoA, fumarate, and oxaloacetate

Why is it not good to break down amino acids, what does body prefer to use instead for energy

Turning them into carbon skeleton and removing nitrogen (waste)

Carbohydrates or lipids

Krebs Cycle (TCA) known as “______”, occurs where

last chance

mitochondria matrix

3 main steps of Krebs cycle

1. Acetyl-CoA starts the cycle

   • Pyruvate > Acetyl-CoA (2C)

   • Acetyl-CoA (2C) + oxaloacetate (4C) > citrate (6C)

2. 6C mol slowly broken down

   • Citrate (6C) > carbons removed as CO₂

   • energy/electrons captured in NADH + FADH₂

3. Oxaloacetate (4C) is regenerated > cycle repeats

   • NADH + FADH₂ carry electrons to electron transport system (ETS) to help make ATP

Electron Transport System (ETS) describe

• successive reactions transfers electrons across protein complexes embedded in inner mitochondrial membrane

• As electrons move towards increasingly electronegative electron accepters (terminal acceptor = oxygen), they release a small amount of energy

• Released energy is harvested by proton pumps (Complexes I, III, and IV) to create proton gradient by pumping H⁺ from matrix into inner membrane space

What does ATP synthase (complex V) do

Allows protons back into matrix (=down their concentration gradient)

Energy released by this exothermic reaction fuels ADP + Pi > ATP

Which cycle yields biggest amount of ATP and why? about how many molecules

ETS

amount of substrates, reducing equivalents, etc., produced from each molecule of fuel (e.g., glucose) in upstream pathways

30-36

Excretion

Disposal of unabsorbed or excess material, bacteria, and dead cells

Kidney vs liver excretion

Kidneys: excrete water-soluble waste, minerals, toxins

Liver: urea (amino acid and protein waste products)

How do you know if there’s a liver vs kidney issue based on blood work

High ammonia in blood (normal urea) – liver issue

High urea in blood (normal ammonia) – kidney issue