Biology 8

What are the steps of the digestive process?

1. Ingestion

2. Digestion

3. Absorption

4. Elimination

The feeding methods used to take food into the digestive cavity

Ingestion

Breaking down of food into smaller particles; carbohydrates into monosaccharides, proteins into amino acids, and fats into glycerols + fatty acids

Digestion

Why is digestion necessary?

We need to break food down into small particles that can cross through 2 layers of cells

Digestion occurs through two processes

mechanical fragmentation

enzymatic hydrolysis

physical act of breaking down large chunks into small pieces; increases surface area for food to be exposed to enzymes

mechanical fragmentation

breakdown of food molecules into basic building blocks to be absorbed

enzymatic hydrolysis

Process of transporting molecules from digestive system to bloodstream (has to cross through epithelium)

Absorption

Epithelium separates…

lumen (hollow space of alimentary canal exposed to external environment) from bloodstream (internal environment)

Elimination occurs because…

Not everything you eat is digestible; we don’t have the enzymes to break down EVERYTHING

Process when undigested materials are expelled through the anus

Elimination

The inside of the digestive tract, external to all body tissues

lumen

Four tissue layers of alimentary canal

1. inner most layer = mucosa: simple, cuboidal epithelium tissue

2. submucosa: connective tissue containing blood vessels and nerves

3. muscularis — multiple layers of smooth muscle tissue

4. outer layer of connective tissue

esophagus —> anus

Slow rhythmic movement due to muscle that propels food along canal

Peristalsis

Muscular valves that control/prevent the passage of digestive contents from one region to the next

Sphincters

The first ~20cm of the small intestine

duodenum

Sphincter located at the junction between the stomach and the duodenum (of small intestine); Keeps food in the stomach while only allowing the passage of some food into small intestine; prevents acid chyme from entering small intestine

Pyloric Sphincter

Sphincter that is supposed to keep food in stomach but is sometimes weak, and some content from stomach will get up to esophagus causing heartburn

Esophageal Sphincter

Accessory glands include

salivary glands

pancreas

liver & gallbladder

• liver secretes bile salts

• gallbladder stores bile but doesn’t make anything

Produce, store, or secrete enzymes/fluids into the digestive tract to help break down food and absorb nutrients

Accessory glands

Digestive Process

Oral Cavity

Pharynx

Stomach

Small Intestine

Liver??

Large Intestine

Oral Cavity (notes)

aka buccal cavity

teeth where mechanical fragmentation increases the surface area of food

Saliva

• mixed with food (~1L of saliva produced a day)

• contains mucin, a glycoprotein, that coats and makes food slippery to there’s not as much friction when swallowing

  • Glycoprotein is sugar coated (has polysaccharides attached to it)

• Saliva contains salivary amylase that targets starches and glycogens, breaking them down into maltose and dextrin

  • not 100% broken down to maltose but the process is started by chewing

Tongue automatically rolls chewed up food into bolus which is swallowed and sent into pharynx

What’s the difference between glucose and cellulose?

• Glucose = alpha-1,4

• Cellulose = beta-1,4 — few animals have enzymes to break down cellulose

Glycoprotein that coats and makes food slippery to there’s not as much friction when swallowing

mucin

Sugar coated protein

Glycoprotein

Enzyme that hydrolyzes starches to disaccharide maltose in oral cavity

Salivary Amylase

What are starches and glycogens broken up into?

Dextrins (smaller polysaccharides) and Maltose (disaccharide)

Ball of chewed or liquid food in preparation for swallowing

Food Bolus

Pharynx (notes)

aka throat

passageway to windpipe and esophagus

PASSES trachea (if food goes through it you’ll choke ☹)

Glottis is covered by epiglottis so no choking happens and food goes into the esophagus where it travels towards the stomach

Windpipe

Trachea

The opening to the trachea

Glottis

flaplike valve (flap of tissue) that covers the opening to the trachea

Epiglottis

How does bolus travel through and gets pushed down the esophagus?

By peristalsis — involuntary movement of smooth muscle

Stomach (notes)

probably has capacity to store 2L of stuff

Mucosa tissue layer makes up walls with cells that secrete gastric juice

Gastric Juices:

• consists of HCl + Pepsinogen + mucus

• pH = 2

Protein digestion:

• Pepsinogen = precursor for pepsin

  • inactive form of enzyme

• Pepsinogen turned active by acid hydrolysis (cleaving by HCl) which turns it into pepsin

  • inactive form so that it does not digest proteins inside stomach cells

  • Pepsinogen doesn’t become active until it gets to the lumen of the stomach

• Pepsin:

  • breaks SOME peptide bonds (not all) — doesn’t completely break proteins down to AA but instead turns it into smaller peptides

Mucus protects stomach epithelium from acidic environment

Stomach lining replaced every three days

Muscles churn food — mechanical digestion

• ensures pepsin makes contact with everything

Food remains in stomach for several hours

Stomach Chyme = food mixed with gastric juices

• pyloric sphincter prevents it from getting into small intestine

In oral cavity what types of digestion occur?

mechanical fragmentation AND enzymatic hydrolysis

In esophagus what types of digestion occur?

None! it’s just a passageway for food to get passed along to stomach

What is the purpose of the pH of stomach being 2 (which is a low value, very acidic)?

To disrupt any extracellular matrixes of the food you eat

• (such as steak which has connective tissue, blood vessels, etc. that have extracellular matrixes)

To kill bacteria

Enzymes have specific ranges of operation when it comes to pH/acidity. When an enzyme is in an environment where the pH is not favorable, it gets denatured and won’t work. This is known as…

pH optimum

Pepsinogen is an example of…

zymogen

Any protein produced in an inactive form that must be breaking of peptide bonds that changes conformation to turn it into the active form

Zymogen

How do you turn off zymogens?

By destroying it

Small Intestine (notes)

where majority of enzymatic digestion takes place

Lining = simple columnar epithelium

~6 meters long

Duodenum — first 25 cm that receives secretions from liver/gallbladder/pancreas

• where hydrolysis of all macromolecules takes place

Pancreas (notes)

Produces hydrolytic enzymes for ALL four groups of macromolecules

Secretes sodium bicarbonate (buffer) that changes pH of acid chyme to about 7-8

Structures:

• Sits under stomach, alongside duodenum

• liver’s bile duct connects to pancreatic duct that leads into the duodenum

Hydrolysis of carbohydrates (notes)

broken down by pancreatic amylase which breaks dextrin down into maltose

maltase breaks down maltose (disaccharide — consists of two units of glucose) to glucose

• maltose is too big to get across epithelium so it has to be broken down more

sucrase hydrolyzes sucrose to get fructose and glucose

lactase hydrolyzes lactose (only in newborn mammals) to get galactose and glucose

disaccharidases secreted by mucosa of small intestine (just on the outside of the apical membrane of epithelial cells)

Hydrolysis of carbohydrates requires which hydrolytic enzymes?

pancreatic amylase

maltase

sucrase

lactase

Where are enzymes secreted from in hydrolysis of carbohydrates

pancreas: pancreatic amylase

mucosa of small intestine: disaccharidases

Where are disaccharidases secreted and why?

Just outside apical membrane of epithelial tissue because it’s closer to surface where they can rapidly be taken up to bloodstream

Hydrolysis of Proteins (notes)

Pancreas secretes many proteinase zymogens (inactive form of enzymes)

1. Trypsinogen (ia) —> Trypsin (a)

2. Chymotrypsinogen (ia) —> Chymotrypsin —cuts polypeptides into smaller chains

3. Procarboxypeptidase (ia) —> Carboxypeptidase

Lining of small intestine secretes…

1. aminopeptidase (removes amino acids from amino end)

2. dipeptidase (attached to intestinal lining)

Zymogen activation

• intestine secretes enterokinase which phosphorylates trypsinogen —> trypsin

• trypsin activates all the other zymogens and also other trypsinogens

• kind of like a domino effect?

Deactivation of enzymes by digesting themselves/other enzymes

What are the zymogens of hydrolysis of proteins?

Trypsinogen

Chymotrypsinogen

Procarboxypeptidase

What are the active enzymes of hydrolysis of proteins?

Trypsin

Chymotrypsin

Carboxypeptidase

Aminopeptidase

Dipeptidase

Enterokinase

Which enzyme activates trypsinogen?

Enterokinase

What does the activation of trypsinogen to trypsin do?

It creates a domino effect where all the other zymogens and trypsinogens are activated

How are zymogens that are activated destroyed?

The enzymes will digest themselves and other enzymes

Hydrolysis of Nucleic Acids

nucleases from the pancreas hydrolyze DNA and RNA into nucleotides

sometimes nucleotides are hydrolyzed further into bases

Hydrolysis of Lipids (notes)

Pancreas secretes lipase

Lipase:

• hydrolyzes lipids into glycerol and fatty acids

Problem: lipids are hydrophobic

• all digested fluid is aqueous so lipase will only be able to attack surface —> takes more time

Solution: bile salts from liver (stored in gallbladder) emulsify large droplets into small ones

• breaks down large fat droplets into smaller one —> increases surface area to be attacked by lipase

What are the enzymes that are used in hydrolysis of lipids (and where do they come from)?

Lipases from prancreas

What breaks down large fat droplets into smaller ones and why does it do that?

Bile salts from liver and gallbladder break down large fat droplets to increase surface area to be attacked by lipase

Absorption

Efficient absorption requires large surface area

Lining small intestine — 300m²

• has large folds in lumen covered with villi

• villi covered in turn by microvilli

• each villus have a capillary bed

In order for food particles to be absorbed into bloodstream, they have to pass mucosal layer (2 plasma membranes: apical and basal) into the interstitial fluid, then through capillary cell (2 plasma membranes: apical and basal), into blood

• in total passes through 2 epithelia, 4 plasma membranes

Capillary bed has a lacteal — blunt end vessel capillary of lymph system

Nutrients cross epithelium through

• facilitated diffusion: uses transport protein/channel to carry ions/molecules

• simple diffusion: doesn’t require any sort of transport, follows gradient

• active transport: cell expends energy

• co-transport

Cotransport Glucose from small Intestine (after a meal is digested)

1. Na+/K+ ATPase — pumps 3 Na+ out, 2 K+ in

2. Na+/Glucose transporter — as sodium enters cell (following gradient high outside Na+ conc —> low Na+ conc in cell), it allows Glucose in with it

3. GLUT-2 — in basal membrane facilitated diffusion with favorable gradient (high Glucose conc. in cell, low Glucose conc. outside cell)

AFTER DIGESTION OF STARCH = Glucose concentration is high outside of cell

4. GLUT-2 — inserted into apical membrane where facilitated diffusion takes place with favorable gradient

Lipid Absorption (notes)

Lipids are hydrophobic

Glycerol and long chain fatty acids reconstitute in epithelial cells

• attach to carrier protein to form a micelle (any cluster of hydrophobic materials carried by proteins)

• they are secreted to the lacteal (kinda middle section where capillaries surround)

• micelles will enter lymph system and enter vena cava

Short chain fatty acids cross membrane (same path as amino acids and monosaccharides)

• Goes to interstitial fluid then into capillaries

• Capillaries join into hepatic portal vessel where they transport to the liver

Large Intestine (notes)

thicker than small intestine

reabsorbs water — 7L of water used throughout the day during digestion but 90% is reabsorbed

water removed causes the stuff there to become solid —> feces

infection/irritation of large intestine affects water absorption —> diarrhea

too much water being taken out —> constipation

Caecum — blind pouch at the junction of small and large intestine

• filled with symbiotic bacteria that breaks down things we can’t digest

Feces = 40% bacteria + unabsorbed material

• some bacteria…

  • produce Vitamin K

  • produce gas (fermenters) H2S/CO2

• stored in rectum until elimination

Herbivores

Adapted to high cellulose diet — animals can’t digest usually

Foregut = mouth, teeth, stomach

1. Bacteria/protozoa in fermentation chambers break down their food — foregut fermenters

Hindgut = colon, caecum, rectum

2. Enlarged caecum or colon — hindgut fermenters

   1. no absorption so they tend to eat their own feces

Ruminant digestion

remarkable adaptation—special fermentation pouches (multichambered stomachs) where symbiotic microorganisms will hydrolyze cellulose for fermentation reactions —> alcohols, amino acids, fatty acid nutrients but also methane

Why is gas exchange required?

Cells require O2 for cellular respiration and gets rid of CO2 as a waste product

What form of gas exchange is effective for very short distances?

Simple diffusion

Limit for diffusion

1mm

Respiratory Medium

Sources of O2

ex. air or water

Respiratory surface

epithelium where gas exchange with a respiratory medium takes place

Why must a respiratory surface remain moist?

O2 gas has to dissolve in a layer of water before crossing epithelial layer

Name the advantages of air as a respiratory medium compared to water

1. More abundant amounts of O2 — 200ml/L compared to 10ml/L

2. faster diffusion rates for gases compared to liquids

3. less energy required for ventilation — movement of respiratory medium across respiratory surface (movement of air across epithelium)

4. O2 concentration more consistent with varying air temperatures — increased water temperatures, O2 significantly decreases

What is the disadvantage of air as a respiratory medium?

Water is always lost when breathing air

Which organisms/groups have no respiratory system and only rely on simple diffusion?

Porifera, Cnidarians, and Platyhelminthes

What is commonly seen in large animals?

They have special respiratory surfaces to deliver O2 from respiratory medium to blood OR hemolymph

Typically how many layers of epithelia must be crossed for O2 to enter blood system or CO2 to leave?

2 Epithelium

Types of Respiratory organs

1. Skin

2. Gills

3. Trachea

4. Lungs

Respiratory organ that is moist with a dense network of capillaries, seen in annelids and amphibians

Skin

Respiratory organ that are evaginations (outside folding) of the body wall to attain O2 from water

Gills

Respiratory organ found in terrestrial insects and relatives, independent of circulatory system

Trachea

Respiratory organ that are invaginations (internal folding) of body surface restricted to one location in body

Lungs

Types of lungs

book lungs (spiders — looks like book pages)

mantle lung (snails — in mantle cavity)

vertebrate lung

ALL LUNG TYPES ARE NOT HOMOLOGOUS

Gills in Fish (notes)

Have four gill arches

each gill arches have 2 columns of gill filaments

the gill filaments are covered with flaps of lamellae (one cell layer epithelia)

Within lamellae are capillary beds (one cell layer epithelia)

No pumps for gas exchange —> instead it moves by pressure concentration (high to low pressure)

When blood supply flowing through lamellae goes in the opposite direction of water

Counter current exchange

Counter Current exchange

as blood flows through capillary, O2 from water diffuses into the blood stream to the point that once blood reaches the end and leaves the capillary, nearly 100% of O2 is extracted from water coming into gills

When blood and water enter a system from the same end and travel in the same direction; tends to be less efficient from countercurrent exchange

Cocurrent exchange

Describe countercurrent exchange in birds

When a bird stands on cold surface, artery and veins of feet are close to each

because of this, the blood coming into artery is close enough to warm the blood going out of vein so even if some heat is lost, blood in vein retains most of the heat of the blood coming in through artery

Ventilation in gills

water must be kept moving for ventilation to occur

Ram ventilation = swimming with mouth open

Stationary ventilation = mouth is the positive pressure pump while operculum is the negative pressure

• Mouth opens, water comes in, Volume up Pressure down

• Mouth closes, Volume down, Pressure up

  • Built up pressure forces water to move across gills and out to operculum

Trachea (notes)

circulatory system plays no role in gas exchange

air enters through spiracles and into trachea

trachea branches into tracheoles that lead directly to cells

Trachea ventilation caused by…

body movement

Mammalian lungs (notes)

for any exchange system to be effective, it requires large surface area

air movement from what we breathe in:

1. Pharynx

2. Glottis

3. Larynx (voice box that is apart of trachea; has cords that reduce vibrations as you breathe in air)

4. Trachea

5. Bronchus

6. Bronchioles (23 levels of branching)

7. alveoli (air sacs where gas exchange takes place)

   1. capillary bed close to surface of alveoli

Where does gas exchange occur (aka what is the respiratory surface)?

Alveoli

Alveoli

one cell layer thick

very small

water coats their surface so that more water can be attracted to them

• water is cohesive however, so it will cause them to collapse

alveoli secretes surfactant that prevents surface tension

Cells lining tubes are ciliated and secrete mucus

• pollen, fungal spores, etc. from air gets caught by mucus

• mucus moves toward pharynx where it can be swallowed or spit out

Gas exchange takes place in alveoli and into alveolar capillary

Water coats the alveoli, causing…

collapse due to high surface tension

To prevent alveoli from collapsing, they have…

surfactant, a lipoprotein, that reduces surface tension

Ventilation in frogs

positive pressure breathing

• muscles of bottom of throat lowers, increases volume, lowers pressure

• air moves in (due to lower pressure)

• nostrils shut, neck muscles contract up, decreasing volume, increasing pressure

• air is swallowed

• elastic lungs will spread

• when exhaling, elastic lungs push air out

When frogs breathe, they hold it in for some time. Why do humans breathe continuously when frogs don’t need to?

They are poikilothermic and source their body heat from environment

We need O2 always to regulate our own body temp

Ventilation in mammals (notes)

negative pressure breathing (suction)

thoracic cavity separated from abdominal cavity by diaphragm

thoracic cavity needs to be air tight

Between body wall and lungs is:

• Pleura — 2 epithelia, slippery fluid between

  • change of volume causes lungs to slide past other structures (creating friction)

  • prevents tearing of lungs

Main tool of ventilation — contracting diaphragm and rib muscles

• breathing in increases volume of thoracic cavity —> creates low pressure

• when you relax rib muscles/diaphragm, they move up, decreasing volume of thoracic cavity —> creates high pressure

• lungs expel O2

Amount of air inhaled and exhaled during normal breathing

Tidal Volume ~500ml

The maximum amount of air that can be exhaled after maximum inhalation

Vital Capacity

~3400ml for females

~4800ml for males

Amount of air still left in lungs AFTER forced exhalation

Residual volumes

Bird respiration system (notes)

homeothermic

lungs don’t have alveoli, instead they have channels of parabronchi

parabronchi are tubes in lungs where gas exchange takes place

one way air flow — air goes through system without stopping

• possible by air sacs (though no gas exchange takes place in there)

Air breathed in goes through two cycles

1. Inhale 1 — trachea —> posterior air sacs

2. Exhale 1 — posterior air sacs —> parabronchi

3. Inhale 2 — in lungs —> anterior air sacs

4. Exhale 2 — anterior air sacs —> trachea —> out