Feeding and Digestion

We acquire energy through feeding and digestion. We often pay a loss of energy in tears, urine, hair, and this must be understood in the mass balance equation. The leftover is used in production, metabolism, digestion, synthesis, and activity costs.

Ways of gettings food

Direct absorption (simplest way to energy): sugars, amino acids coming through transporters, such as gills.

  • Endocytosis: larger food particles are invaginated and then get digested intracellularly.

Animals (not sponges) undergo gastrulation (gastrula formed from blastula)

  • Blastula: sperm and egg fused

  • Gastrula: embryo, cup shape, three layers.

  • This invagination forms the anus first.

  • Diploblastic: Have single entry/exit (mouth is the anus), two cell layers form. Only the orifice remains.

    • Batch reactor: take in little food stuff, can’t have door going both ways, so it is a slower process.

  • Triploblastic: Different exit and entry, 3 cell layers form (they have an extra mucus layer called mesoderm). Mouth forms between archenteron and ectoderm, allowing for unidirectional flow.

    • Plug flow reactor: Small intestine, continuous input and continuous output, allows for storing elements.

    • Continuous flow stirred tank receptor: Stuff comes in and some stuff is being already mixed, what leaves is a mix of older and newer stuff that’s not completely broken down (stomach)

  • More complex = less primitive. This separates and distinguishes what are more important organs are, and what are more important. We see specialized organs showing up, with increased efficiency and surface area.

The Digestive System or “Gut”

Most digestive system (more primitive, and more complex), have a digestive passage that is separated secretory organs. With more complex ones, that have increased efficiency and increased surface area.

The gut is made up of the headgut (mouth), foregut (esophagus and stomach), midgut (small intestine), hindgut (large intestine, colon, rectum, cloaca).

Headgut/foregut

These two parts (mouth, esophagus/stomach) are specialized for diet, like darwin’s beak shape, or like depending on what they eat, the nutrients they’re provided is most beneficial for their energy use.

Hummingbird don’t have a muscly stomach, birds eat rock to help with destroying certain animals in their stomach (added muscle).

The esophageal crop: can serve as a storage organ, like how mama bird chews up food and stores it here in order to feed baby birds. Can also be used to slow down and make digestion more gradual.

The Mammalian Stomach

The stomach is held closed by the esophageal sphincter (blocking esophagus) and the pyloric sphincter (blocking duodenum).

They are monogastric stomachs (carnivores and omnivores), and have rugae made up of gastric pits. Gastric release secretions to mix with food in order to form chyme. The top of the gastric pit is called the lumen, and the inside is made of specialized cells which all release their own thing. Goblet cells release mucus, parietal cells release HCl, and chief cells release pepsinogen. The pepsinogen can only be active in low acidity, so the HCl activates it to form pepsin, which will cleave itself (autocatalysis) back to pepsinogen.

There are also digastric stomachs (ruminants: cows, sheep) for animals who graze of veggies and hard to digest things. Fermentation occurs upwards (rumen) and microflora (organism that can break down grass specifically and other hard to break down things) are very involved. There is a lot of digestion in the Omasum, and stomach must be separated into these segments a little bit. They are doing secondary chewing first.

Intestine (midgut and hindgut)

Intestines are directly close to veins and arteries in order to help in circulation and transporting of energy absorbed (whisking away the energy). When muscles constrain, they are scrunching the intestine to move stuff.

The intestines are lined with cilia, which are what help move food. This is okay in small animals, but bigger ones need additional aid in terms of “peristalsis” (rhythmic movement) that pushes food through. Villi are tiny finger like things that line intestine, which are in close association with capillaries, and are what is doing the absorbing.

Hindgut

Midgut was where a lot of stuff was going on, and now the hindgut is like finishing touches.

Ileocecal sphincter: connects to small intestine.

There is reuptake of the fluids made in small intestine in the large intestine. When not enough is absorbed we get diarrhea, when too much is absorbed we get constipation.

Fibre can stop constipation with cellulose, inulin, and poly-fructose glucose molecules. They are hydrophilic and attract water to keep stool more hydrated and soft.

Blood glucose homeostasis: blood glucose levels rise after a meal, as glucose is broken down from complex and/or simple sugars and absorbed. Chronic elevated levels can lead to “glycation stress”, considered in diabetic people.

  • It’s struggling to go back down to a normal issue. This is managed by insulin production, however the issue comes from insulin being created in the pancreas and quickly turned over. Constant need to take and retake insulin if you are diabetic. Glucagon is reciprocal function to insulin, triggering production of it.

Gilia: Poisonous saliva, can go months without food. Saliva has exendin-4, similar to GLP-1, which enables digestion to occur more slowly, leading to its weight loss effect. It suppresses hunger.

Diabetic medication

Ozempic/Wegovy are taken less often and have a longer lasting effect.

  • Glucagon like peptide-1 receptor agonist (chemical that activates a receptor). GLP-1 released by specialized intestinal epithelial cells, and GLP-1 has upstream effects on insulin secretion. Is quickly degraded and removed from circulation (like insulin)

  • Inspired by gilia

Digesting meals

Some things are harder to digest than others. Simple things (simple sugars, amino acids) are the easiest to digest (assuming we have the proper enzymes like lactase). Complex things cost more to break down, may give more energy. In general proteins and lipids are harder to breakdown than simple sugars.

In most vertebrates, metabolic rate goes up by around 2x when digesting, compared to resting.

Gastric Secretions

We see broad patterns of gastric secretion throughout digestive tract: saliva is everywhere, bicarbonate buffers pH of saliva to a 6.5 pH, there are dramatic shifts in pH (pepsin, also trypsinogen and chymotrypsinogen undergo certain process to act right).

Gastric secretion may help digestive process or help with excretion (brown poop cuz of old red blood cells).

Succus entericus: final breakdown enzymes to cleave disaccharide bonds (maltase, lactase, lipase).

You have more gastric juice if you eat more.

Carbohydrate digestion

We don’t have stuff to breakdown cellulose, but gastric microbes (symbiotic gut bacteria) can. There is no direct enzymatic degradation of carbs in the stomach. They mostly get broken down in small intestine to a monosaccharide. The exception is cellulose which is eaten as cellulose and exits as cellulose.

Glycogen and starch may potentially be broken down into oligosaccharides in the mouth, then into disaccharides, then into monosaccharides.

It can be hard to break up string linkages because a-1,6- linkages are not breakable, while a-1,4 linkages can be cleaved. This is by salivary amylase, or pancreatic amylase (it originates in mouth and is brought down).

Inulin - poly-fructose molecule, found in some plants instead of starch. Found in diet bars, hard to breakdown. Rely on symbiotic gut microflora, we get gassy.

Where do we get symbiotic gut microflora - vaginal birth, rabbits eat mother’s feces, termites need for help eating cellulose and lignin.

Absorption of carbohydrates at the Brush Border

Sodium is being transported into interstitial fluid. Occurs in small intestine, with brush border referring to the pits in the lumen lining.

Potassium is being pumped into cell, while Sodium is being pumped out. This is an ATP dependent process. There is the gradient now, and the Na+ near the brush border is co-transported into the cell with SLGT1 cotransporter. Then GLUT2 just allows the glucose down concentration gradient

Basolater Sodium/Potassium ion pumps set up sodium gradient. SGLT1 co-transports sodium and glucose into cell.(GLUT5 for fructose). Glucose, galactose, fructose are transported to interstitial fluid by GLUT2 (down conc. gradient)

We ensure high rates of uptake by attaching it to glucose in tight junction that connect epithelial cells.

You are sending more blood to your digestive tract.

We are not specialized for high sugar diets.

Flying animals need a lot of energy to fly because of nutrient absorption rates being high with high metabolic rates. Their digestive tracts are in turn, more complicated. They are good at weight saving, with less surface area for absorption, this makes FMR higher. There is high rates of cellular mediated transport per surface area (in brush border) for nectarivores. Even higher rates for paracellular glucose absorption.

One way we measure and track carbohydrates being processed is through radio-isotope labels. Take blood samples to measure bicarbonate, lactate, etc.

Carbon-13 is a stable isotope that did not decay, and if it is converted to CO2 completely we can absorb infrared radiation. Measures stable isotope breath. This can be done by collecting breath in a chamber. This is used to track fuel oxidation (exercise), and where the radioactivity carbon ended up (it should decay, but we can see the other isotope giving a hint to where it ended up).

Nectarivores levels are changed between very quickly, and while hovering they are oxidizing sugar (equivalent to us), a bottle of coke every minute.

In humans, an exercising person has exogenous and endogenous glucose and fructose stores (still using lipids), but if you ingest them together, mechanisms of absorption, there are different transporters with different affinities. More fructose goes to liver, blood, kidneys. Most oxidation of fructose is actually in the stomach.

Nectarivores burn sucrose, fructose and glucose. usually a 1:1 ratio of glucose and fructose, they have 100% of tracer carbon 13. With all glucose or all sucrose, hummingbirds never reached exercise budget. They can absorb fructose very easily, bypassing liver, they express a transporter on heat. Can move stuff very fast.

Some animals may go months between meals.

Not necessarily hibernation, but some animals can wait for a meal if they have very slow metabolism. Snakes eat enormous meals and the go awhile. They have to digest it, if they don’t it’s game over. They eat fast to forego microflora eating it and creating gas.

We’re swollen because we ate, but the extra body requires extra energy to move, and so keeping a strong heart helps. Our hearts expand after a big meal. Hypertrophy (more blood flow, more pumping required), causes growth of new heart tissue.

Cost of digesting meals: spike and fall of metabolic rate, going up by 50 x what it was at rest.

Post-Absorption Processing

Toxins and pharmalogical agents are subjects to biotransformation. We detoxify chemicals to make them easier to excrete. This causes toxin/medicine activity to decline.

Makes chemicals more hydrophilic so we can pee them out. May activate or inactivate a drug.

Sudafed and meth anecdote. Phenylephrine rapidly biotransformed via conjugation of a sulfate group, while pseudoephedrine did not.