Nutrition and Animal Digestive System

Nutrient

any substance taken in by an organism that is needed for survival, growth, development, tissue repair, or reproduction

Nutrition

process of consuming and using food and nutrients

4 steps of food processing in animals

1. ingestion

2. digestion

3. absorption

4. egestion

Dietary categories

• basic similarities in organ system function lead to similarities in nutritional requirements

• different animal physiologies can have different nutritional demands

1. carnivores

2. omnivores

3. herbivores

Organic nutrient purposes

1. to provide energy - for synthesis of ATP

2. to make new molecules - larger molecules are taken apart and rebuilt to provide ATP in a way that the body can digest

4 major organic nutrients in animals

1. carbohydrates - energy source

2. proteins - provides A.A. for protein production

3. lipids - major component of cell membranes

4. nucleic acids - provide sugars, based and phosphates used to make DNA/RNA/ATP

4 essential nutrient groups

1. essential amino acids

2. essential fatty acid

3. vitamins

4. minerals

Essential amino acids

• 9 A.A. used in human bodies

• 20 A.A. total

• aminos are useful for producing proteins

• not stores and cannot be metabolically synthesized

Essential fatty acids

• certain unsaturated fatty acids that cannot be synthesized by animal cells

• unsaturated found in plants

• obtain them from fish

• Unsaturated fatty acids – contain double bonds, lacking hydrogen --> kinked and bent --> bad packing --> typically remain as liquids (oils) 

• Saturated fatty acids – fully bonded hydrocarbon chain --> long, flat chains -> good for packing --> solid form (butters) 

 

Fatty acid – carboxylic acid and hydrocarbon chain 

Advantages of Omega 3

• source of energy

• reduce inflammation, cancer, clotting

• High and low density lipoproteins  - flow in body to help fuel physiological processes 

• Excess of bad will clog arteries, calcifying circulatory system, leading to heart attacks, low blood pressure, etc 

  • Omega 3's act as good cholesterols, containing proteins, that go and find excess deposists and break them down 

2 types of digestion

1. internal

2. external

Intracellular

1. Simple animals - vertebrates 

2. Single celled organisms 

   1. Sponges, amoeba 

3. Use phagocytosis to absorb nutrients (engulfs) 

4. Cannot store food 

5. Excretes by expulsion  

6. Inefficient mechanism for vertebrates 

Extracellular

1. Occurs in cavity  

2. Protects interior of cells from hydrolytic enzymes 

3. One opening in the extrance and exit  

   1. Food enters 

   2. Food is digested 

   3. Food is absorbed – slows down – maximizes the food essential nutrient extraction 

      1. More efficient and better support to a metabolically active organism 

   4. Food is excreted  

Vertebrate Digestive System 

• GI track – cannal 

  • Single elongated tube, food comes in, is processed, and excreted through secondary opening 

  • Starts in mouth , pharynx, eso, stom, intes, anus 

• Accessory structures that help in digestion 

  • Help in breakdown and absorption 

  • Tongue, teeth, salivary glands – external – help in the physical aspect of digestion 

  • Liver, gallbladder, pancrease – internal – release chemicals to help aid in the chemical process  

3 methods of absorption

1. simple diffusion —> natural movement of high concentration to low concentration

2. facilitated diffusion —> plasma membrane has structures and proteins that facilities movement from high to low concentration

3. active transport —> against concentration gradient, ATP useage, binding substrate and uses a conformational change to move the molecule across the plasma membrane

3 regions of the alimentary canal

1. anterior end

2. middle portion

3. posterior part

Anterior end

• functions primarily in ingestion

• mouth, pharynx, esophagus

Middle portion

• functions in storage and initial digestion

• 1+ more food storage or digestive organs

• upper part of small intestine and associated organs

posterior part

• lower part of intestine and large intestine

• undigested material defected through opening (anus or cloaca)

Structure of the GI Tract

• Epithelial cells line the GI tract

• Functions:

  • Secrete enzymes and mucus

  • Release hormones into blood

  • Absorb/transport nutrients

• Gland ducts:

  • Pass through epithelium

  • Release acid, enzymes, water, ions into lumen

• Tight junctions:

  • Prevent substances from passing between cells

  • Ensure controlled absorption

• Surrounding support:

  • Smooth muscle (movement)

  • Neurons (control)

  • Connective tissue + blood vessels

• Neural activation:

  • Triggered by sight/smell of food and food in tract

  • Controls secretion and motility

Structure of GI tract

Epithelial cells have tight junctions --> no space between neighboring cells --> inhibits movement of substances between these cells  

- body is regulating what enters blood stream 

Smooth cells --> flexible --> involuntary  --> provides contraction and release (peracellsis) --> forces food through elementary canal 

Mouth

• Presence of food stimulates salivary glands to produce saliva

• Saliva contains:

  • Water

  • Proteins

  • Mucus

  • Antibacterial agents

• Functions of saliva:

  • Moistens and lubricates food (helps swallowing)

  • Dissolves food for taste

  • Kills bacteria

  • Begins carbohydrate digestion (via salivary amylase)

Pharynx and esophagus

• Acts as a pathway to storage organs

• Does not participate in digestion or absorption

• Swallowing:

  • Starts voluntarily in the pharynx

  • Continues involuntarily in the esophagus

• Peristalsis:

  • Rhythmic smooth muscle contractions

  • Moves food through the digestive tract

The Crop

• in some cases, food moves down the animal esophagus to the crop

• it is a storage organ that is a dilation of the lower esophagus

• purpose of storing and softening food with little or no digestion

• sizes varies off diet

• regurgitated material for offspring comes from the crop

Stomach

• C-shaped organ located on the left side of the abdominal cavity

• food enters the cardioesophageal sphincter from esophagus

• empties into small intestine at pyloric valve

functions of the stomach

1. temporary storage tank for food

2. digestion site

3. chemical breakdown of proteins using enzymes and acid

4. chyme delivered to duodenum of S.I.

composition of the stomach

• gastric glands with gastric pits of the sub-mucosa of the stomach

• cells produce bicarbonate-rich alkaline mucus, balancing pH

• chief and parietal cells

Chief cells

produces protein-digesting enzymes

• pepsinogen = activates pepsin to start chemical digestion of proteins

parietal cells

produce hydrochloric acid (HCl)

stomach specialization in birds

1. proventriculus - The part of the stomach that releases acid and enzymes to start digestion.

2. gizzard - A strong, muscular part that grinds food into small pieces, often using swallowed sand or small stones.

Stomach specialization in ruminants

• Cellulose: A main part of plant cell walls that humans can’t digest, so it passes through the body.

• Ruminants (herbivores like cows): Can digest cellulose because microorganisms in their gut break it down into simple sugars they can absorb.

Glycosidic bonds

• monosaccharides are joined to others or to alcohols and amines by glycosidic bonds

• produced by dehydration reactions

Ruminants

Have a stomach with multiple chambers to help digest plant material.

Rumen & reticulum

Contain microbes that break down cellulose.

Omasum

Absorbs water and nutrients from partially digested food (cud).

Cud

Food that is brought back up, chewed again, and swallowed.

Abomasum

The “true stomach” that uses acid and enzymes to continue digestion.

Small Intestine

• Where most digestion and absorption of nutrients and water happens.

• Enzymes - Break food into small molecules (like sugars and amino acids).

• Absorption - Nutrients pass through intestinal cells into the blood.

• Also absorbed - Vitamins, minerals, and water.

Surface Area Specializations

• Small intestine lining: Has folds, villi, and microvilli to increase surface area.

• Villi: Finger-like projections that stick into the intestine.

• Microvilli: Tiny projections on cells that form a “brush border.”

• Purpose: Greatly increases surface area (about 600×) to absorb more nutrients.

Absorption by villi

• centre of each villus has capillaries and a lymphatic vessel called a lacteal

• most fat particles are too bulky for capillaries, and enter larger, wider lacteal instead

  • empty into circulatory system

Accessory organs

1. pancreas

2. liver

3. gallbladder

Pancreas

secretes digestive enzymes and fluid rich in bicarbonate ions, which neutralizes acidity of chyme as it enters the small intestine

Liver

produces bile

• contains bicarbonate, cholesterol, phospholipids, organic waste, amphipathic bile salts

• bile is crucial for fat digestion and emulsification

Gallbladder

stores bile and secretes it during a meal

Larger intestine

• Large intestine: Includes cecum, colon, rectum, and anus.

• Function: Stores waste, absorbs water and salts.

• Bacteria: Live here and produce vitamins and gas.

• Appendix: Attached to the cecum.

Site of digestion; carbohydrates

• Carbs eaten: Mostly complex carbs (like starch); some simple sugars too.

• Digestion: Starts in mouth, mostly happens in small intestine (amylase breaks them down).

• Breakdown: Disaccharides → monosaccharides.

• Absorption: Simple sugars enter the blood through the intestine.

Digestion of lactose

• Lactose: Sugar in milk, broken down by lactase.

• Infants: Produce lactase; most adults don’t.

• Most people: Can’t fully digest lactose after childhood.

• Some populations: Evolved to keep producing lactase → can digest milk (lactose tolerant).

Proteins

• Proteins: Broken down in stomach (pepsin) and small intestine (trypsin).

• Pancreas: Releases inactive enzymes that are activated in the small intestine.

• Final breakdown: Into amino acids by enzymes on the intestine surface.

• Absorption: Amino acids enter the blood.

Lipids

• Fats (lipids): Mostly eaten as triglycerides.

• Digestion: Happens in small intestine by lipase (from pancreas).

• Breakdown: Into fatty acids and monoglycerides.

• Emulsification: Breaks fat into tiny droplets to help digestion.

• Absorption: Enter lymph system (too large for direct blood entry).

Formation of micelles and chylomicrons

• Micelles: Bile salts group fats into tiny, soluble clusters to help them be absorbed.

  • Absorption: Lipids slowly enter intestinal cells.

  • Inside cells: Re-formed into triglycerides.

• Chylomicrons: Fat droplets packaged with proteins.

  • Transport: Released into lymph, then eventually enter the blood.

Vitamins, minerals and water

• do not require digestion - absorbed completely

• water-soluble vitamins absorbed by diffusion or active transport in SI

• fat-solube vitamins follow pathway for lipid absorption

• small water amounts absorbed in stomach but mostly in SI