Digestive System

Humans as HeterotrophsHetero=another Trophe= nutrition

• As heterotrophs we cannot create carbon, therefore we need to ingest carbon in our food.

• All of our cells require nutrition for;

1)Growth

2)Repair

3)Maintenance

Functions of the Digestive System

1.Break down of food into nutrients.

2.Nutrients absorbed in the body.

3.Transported throughout the circulatory system.

Nutrients in the cell can be used as a

source of energy

OR

Used for the synthesis of chemical compounds

Stages of the Digestive System

1.Ingestion: taking in nutrients, eating/drinking.

2.Digestion: breaking down complex molecules into smaller ones.

3.Absorption: taking up of digested molecules

4.Egestion: removal of waste.

1. Ingestion • Ingestion occurs when food enters the mouth.

• It is mechanically broken down by teeth.

Question: Is this a form of digestion?What’s wrong with this?

1. Ingestion

Chewing stimulates the release of saliva from the salivary organs.

Saliva contains a mixtures of water, mucus and enzymes.

1. Ingestion: Amylase

• Enzyme that breaks starch down into simple sugars.

1. Ingestion:Bolus

• Food that has been chewed and mixed with saliva becomes a bolus.

The epiglottis, a flap of skin directs food down the esophagus.

1. Ingestion: Peristalsis • When the bolus has entered the

esophagus muscle contractions called peristalsis move it towards the stomach.

2. Digestion:Stomach

• Mechanical and chemical digestion occur in the stomach.

• The stomach is J-shaped, it stores food, and has a capacity of 1.5 L

2. Digestion: Sphincters

• Food enters and exits the stomach through sphincters.

• Sphincters are muscles surrounding a tube-like structure

• The bolus enters the stomach when the esophageal sphincter relaxes.

2. Digestion: Gastric Juice• The stomach contains gastric juice

secreted by the stomach lining.

• Gastric juice contains a mixture of:

HCl (hydrochloric acid)

Enzymes

Mucus

2. Digestion: Gastric Juice

• HCL

Breaks down fibres & Bacteria

• Mucus

Protect stomach lining from HCL

Why?

• If stomach lining is destroyed ulcers form.

2. Digestion • The stomach absorbs:

Water

Alcohol

Some medications (Aspirin)

Chief Cells

• Gastric chief cells found in the stomach produce pepsin, which breaks down large proteins into peptides.

• Replaced every 2-4 days.

Parietal Cells

• Stomach cells found in the glands.

• Secrete gastric acid.

• HCl acid secreted by active transport in the stomach.

2. Digestion: Chyme

• Chyme travels out of the stomach by the pyloric sphincter into the small intestine.

• Chyme is a mixture of:

Water

Digested food

Gastric Juices

2. Digestion: Small Intestine

• After the pyloric sphincter chyme enters the duodenum

• 7m in length, 2.5cm in diameter.

• Duodenum is first 25cm of the small intestine.

2. Digestion: Duodenum

• Majority of digestion occurs here.

• Secretions from pancreas and gallbladder enter the duodenum via a duct.

2.Digestion: Liver

• Has four functions in digestion:

1.Synthesis

2.Breakdown

3.Detoxification

4.Storage

Liver: Synthesis

• Produces bile which breaks down fats.

• Bile is stored in the gallbladder

Gallbladder

• Organ

• Stores bile before it is released in the small intestine.

• Bile aids in the digestion of lipids.

Synthesis: Bile

• Bile is an emulsifier (a compound used to mix immiscible substances)

• This allows for the breakdown of fats and for travel through the digestive system.

Detoxification

• Removes toxins from the body including

Alcohol

Chemicals

Storage

• Vitamins and carbohydrates (as glycogen) are stored in the liver.

Pancreas

• Increases the pH of chyme from 2.5 to 9 due to the production of sodium bicarbonate, which is an alkaline (base)

• Secretes enzymes for digestion

Lipase (fats)

Trypsin (proteins)

Small Intestine

• Secretes

Maltase (maltose - > glucose)

Peptidase (protein breakdown)

• Duodenum (beginning), Jejunum (middle), Ileum (end).

Overall: Food & Enzymes

• Carbohydrates -> Glucose

(amylase & maltase & lactase)

• Proteins -> Amino Acids

(pepsin & peptidase)

• Fats (triglycerides) -> Fatty Acids & Glycerol

(bile & lipase)

3. Absorption

• The jejunum and ileum contain villi (finger like projections) which increase the surface area.

• The villi help with absorption of

Glucose

Amino Acids

Fatty Acids

Glycerol

3. Absorption

• Once nutrients are absorbed through the villi they enter the bloodstream and are dispersed throughout the body.

Capillaries• Small blood vessels.

• Transports monosaccharides, amino acids, and water throughout the bloodstream.

• Are a component of the stomach, the large, and small intestine.

Lymphatic Vessels

• Absorb fatty acids and monoglycerides and transports them to the bloodstream.

• Why necessary?

3. Absorption: Large Intestine

• Water absorption

• Bacteria living in colon releasing Vitamin K & B, also absorbed.

4. EgestionRectum and Anus:

•The rectum stores and compacts waste.

•Cellulose aids in the ejection of material from the bowels. It is the main source of our dietary fibre.

•Lack of cellulose in the diet can lead to colon cancer.

Digestive System

• The entire process takes 24-36 hours.