Week 5 Bioscience

Question 1

1. Ingestion

Answer 1

• taking food and drink into the digestive tract
• usually occurs via the mouth
• food and drink that is swallowed is called a bolus

Question 2

2. Propulsion

Answer 2

• moving food or waste through the digestive tract
• swallowing (initiated voluntarily)
• peristalsis - alternate waves of muscle contraction and relaxation (involuntary)

Question 3

3. Mechanical Breakdown

Answer 3

• physically breaking the food up into smaller fragments so it can be chemically digested
• mouth: chewing and mixing food with saliva → bolus
• stomach: churning and mixing with gastric juice → chyme
• small intestine: segmentation → mixes food with digestive juices and aids in nutrient absorption

Question 4

4. Chemical Digestion

Answer 4

Food → specific enzymes →building block

Carbohydrate → amylases & brush border enzymes → glucose/ monosaccharides

protein → pepsin, proteases & brush border enzymes → amino acids

fats/ triglycerides → bile & lipases → glycerol & fatty acids

nucleic acid → nucleases & brush border enzymes → base, phosphate ion & sugar

Question 5

5. Nutrient Absorption

Answer 5

• passage of end products (monomers) of digestion from the lumen of the digestive tract into the blood or lymph
• majority of absorption occurs in the small intestine
• absorption occurs through absorptive epithelial cells of the small intestine
• small amount of absorption occurs in the
• stomach (lipid soluble substances, e.g. alcohol)
• large intestine (water, electrolytes and some vitamins)

Question 6

6. Defecation

Answer 6

• elimination of indigestible substances, in the form of faeces, from the body via the anus

Question 7

The Mouth

Answer 7

• oral cavity includes the lips and cheeks, the palate, the tongue and salivary glands
• modified mucosa: lined by multiple layers of epithelial cells
• continually replaced due to abrasion

Digestive functions of the mouth:
* ingestion
* mechanical breakdown by mastication (chewing)
* chemical digestion of carbohydrates initiated by enzymes (amylase) found in saliva
* propulsion (peristalsis) - swallowing food

Question 8

Salivary Glands

Answer 8

• salivary glands: the parotid, sublingual and submandibular
• produce saliva: contains mostly water, mucus, electrolytes, digestive enzymes and antimicrobial proteins (antibodies, lysozyme, defensins)

Functions of saliva:
* mucus moistens and lubricates food
* dissolves food chemicals and facilitates taste
* contains enzymes (amylase) that initiates chemical digestion of carbohydrates
* primarily controlled by parasympathetic nervous system

Question 9

The pharynx and oesophagus

Answer 9

Digestive functions of the pharynx and oesophagus:
* Propulsion(peristalsis) of food to the stomach

Question 10

The small intestine

Answer 10

• extends from stomach to large intestine
• three subdivisions:
• Duodenum - curls around the pancreas. Receives bile and pancreatic juice for chemical digestion
• jejunum - major site of mechanical and chemical digestion and absorption
• ileum - ends at the large intestine
• The small intestine is highly adapted for nutrient absorption
• three structural modifications of the mucosa/submucosa:
  
  • circular folds
  • villi
  • microvilli

→ all 3 modification increase the surface area more than 600 times

Circular folds
* deep, permanent folds of the mucosa and submucosa
* slow the movement of chyme, thus increasing time for nutrient absorption

Villi
* finger-like projections of the mucosa
* absorptive epithelial cells → nutrient absorption
* core contains a capillary bed and a wide lymph capillary (lacteal)
* contains Goblet cells to produce mucus
* enterocyte: produce intestinal juice

Microvilli
* very small, densely packed microvilli on the surface of individual absorptive epithelial cells → form the “brush border”
* plasma membrane bears enzymes that complete carbohydrate, protein and nucleic acid digestion (called brush border enzymes)

Digestive functions of the small intestine:
1. propulsion (peristalsis)
2. mechanical breakdown (segmentation)
3. chemical digestion of all 4 food classes: carbohydrates, proteins, fats and nucleic acids
4. nutrient absorption

Question 11

The Stomach

Answer 11

• continuous with oesophagus superiorly and small intestine inferiorly at the pyloric sphincter
• temporary storage of food
• muscularis externa: circular & longitudinal - mix, churn and propel food
• modified contains an additional oblique muscle layer in muscularis externa → pummels the food and rams chyme (liquified food) into the small intestine
• modified mucosa:
• the surface of mucosa is composed entirely of mucus producing cells (Goblet cells)
• mucosa indented into gastric pits → gastric glands
• gastric pits/gastric glands contain:
• Parietal cells: produce hydrochloric acid (HCl)- activates pepsin, denatures proteins and destroys most bacteria
• Chief cells: produce pepsinogen, the inactive form of pepsin
• Goblet cells: mucus – protect mucosa from HCl

Digestive functions of the stomach:
* mechanical breakdown – mixing food with gastric juice (food → chyme)
* chemical digestion of proteins initiated (pepsin)
* absorption (minor role) of fat-soluble chemicals, e.g. alcohol and aspirin
* propulsion (peristalsis) – to the small intestine

Question 12

The large intestine

Answer 12

• extends from ileum to anus
• subdivisions
• caecum – pouch lying below the ileocaecal valve
• appendix – lymphoid tissue
• colon – puckered by longitudinal muscle to form haustra
  
  • ascending, transverse, descending, sigmoid
• rectum
• expandable for temporary storage
• presence of faeces triggers the defaecation reflex
  
  • anus
  • final portion
  • internal and external anal sphincters
• modified mucosa = single layer of epithelial cells
• rich in goblet cells
• ease the passage of faeces
• protect the intestinal wall from bacterial acids and gases

Digestive functions of the large intestine:
* absorption
- water and electrolytes (Na+ and Cl- )
- vitamins produced by bacterial flora
* propulsion: haustral contractions and mass movements
* defaecation - parasympathetic reflex

Question 13

The liver

Answer 13

• Lobules- composed of hepatocytes that radiate from a central vein
• Portal triad at each corner
• arteriole: supplies O2 -rich blood from systemic circulation
• venule: supplies nutrient-rich blood from the small intestine
• bile duct: collects bile from hepatocytes
• Between the hepatocytes are leaky capillaries (liver sinusoids)
• allows hepatocytes to filter and process nutrient-rich blood
• Capillaries lined by macrophages
• remove debris (e.g. old RBC, bacteria)
• Bile produced by hepatocytes → collect and drain bile into the bile duct of the portal triad

1. digestive function → production of bile by hepatocytes
2. haematological functions → synthesis of plasma proteins, e.g. albumin, lipoproteins, clotting factors
3. detoxification: alcohol
4. drug and hormone metabolism: first pass metabolism
5. Metabolic functions
   * processing of nutrients
   - removing and storing excess nutrients, e.g. glucose → glycogen
   - use of amino acids to make protein or to yield energy
   - break down fatty acids to yield energy
   - vitamin and mineral storage
   * correcting nutrient deficiencies
   - regulating circulating levels of glucose, triglycerides, fatty acids and cholesterol

• produced by hepatocytes and stored in the gall bladder
• contains bile salts and phospholipids which
• emulsifies fats (break them into small droplets) → increases the surface area for digestive enzymes
• facilitates fat absorption
• released when chyme enters the duodenum
• reabsorbed in the ileum and recycled

Question 14

The gall bladder

Answer 14

• located on inferior surface of liver
• stores and concentrates unused bile
• bile is secreted into the duodenum directly from the liver or from that stored in the gall bladder

Question 15

The pancreas

Answer 15

• soft gland encircled by duodenum
• contains endocrine tissue → scattered “islets” of α and β cells which secrete hormones (glucagon and insulin) into the blood to regulate blood glucose levels contains cells that produces pancreatic juice →alkaline to neutralize chyme,
• contains enzymes for chemical digestion
• pancreatic juice released into duodenum through the hepatopancreatic ampulla and sphincter

Question 16

Enzymes in pancreatic juice

Answer 16

• pancreatic amylase → digest carbohydrates
• pancreatic proteases – secreted in inactive form, activated within the duodenum → digest proteins
• pancreatic lipases → digest lipids
• pancreatic nucleases → digest nucleic acids

Question 17

Metabolic states

Answer 17

• The availability of nutrients determines the overall metabolic state
• There are two states:
  1. The anabolic or absorptive state (“fed state”)– when nutrients are in abundance (during and for 4 hours after a meal) → anabolism exceeds catabolism and nutrients are used and excesses are stored
  2. The catabolic or post-absorptive state (“fasted state”)– when no food is being digested and body reserves must be broken down to provide nutrients → catabolism exceeds anabolism

Question 18

Anabolic state

Answer 18

• All cells need energy to do work ie: active transport, muscle contraction and chemical reactions
• Adenosine triphosphate (ATP) is the molecule that holds the energy within its bonds
• The breakdown of ATP releases energy that the cell uses for work
• Glucose sparing: ATP used by neurons. Skeletal muscles use triglycerides
• Cellular respiration – biochemical processes that lead to the production of ATP
• Cellular respiration involves three biochemical processes:
• glycolysis
• tricarboxylic acid (TCA) cycle (Kreb’s/citric acid cycle)
• electron transport chain (oxidative phosphorylation)

The monomers of carbohydrate digestion are monosaccharides or glucose so all carbohydrates are absorbed into the body as glucose and can be converted to ATP.

Foods other than glucose can be used to produce ATP:
* Triglycerides (fats) → fatty acids and glycerol
* Alcohol
* Proteins → amino acids

Question 19

The anabolic or absorptive state

Answer 19

• during and within 4 hours after eating
• nutrients are being absorbed from the small intestine → abundant supply in the blood
• Immediate use of nutrients in all body cells:
• ATP produced by glucose first, then other food sources
• amino acids used for protein synthesis
• Once ATP storage is full all excess nutrients are stored:
• glucose → converted to glycogen by glycogenesis or converted to triglycerides by lipogenesis
• amino acids → converted to triglycerides by lipogenesis
• glycerol and fatty acids → reconverted to triglycerides by lipogenesis

Question 20

The catabolic or post- absorptive state

Answer 20

• fasting state, more than 4 hours after eating
• nutrients are no longer being absorbed from the small intestine → blood nutrient levels decrease
• Stored nutrients broken down to maintain blood glucose levels (4-6 mmol/L)
• Sources of blood glucose:
• Glycogenolysis = breakdown of glycogen to glucose
• Gluconeogenesis = production of new glucose from non carbohydrate source
• triglycerides → glycerol (lipolysis) → glucose
• proteins are broken down → amino acids → glucose

Question 21

Summary: anabolic/absorptive state

Answer 21

• Controlled by insulin
• Occurs for up to 4 hours after eating. Fed state. Nutrients delivered from digestive system
• Body cells produce ATP → cellular respiration
• Amino acids used for protein synthesis
• Excesses of all nutrients stored
• Glucose → glycogen by glycogenesis
• Glucose → triglycerides by lipogenesis
• Fatty acids and glycerol → triglycerides by lipogenesis
• Amino acids → triglycerides by lipogenesis

Question 22

Summary: catabolic/post-absorptive state

Answer 22

• Controlled by glucagon
• Occurs 4 hours after eating. Fasted state. No nutrients delivered from digestive tract
• Blood glucose needs to be maintained between 4-6 mmol/L particularly for use by neurons
• Catabolic breakdown of molecules to produce glucose
• Glycogen → glucose by glycogenolysis
• Triglycerides → Fatty acids and glycerol by lipolysis → glucose by gluconeogenesis
• Proteins → Amino acids produce glucose during starvation → glucose by gluconeogenesis

Question 23

Summary: fate of carbohydrates

Answer 23

• Carbohydrates broken down into glucose and absorbed into bloodstream
• Absorptive state
  All cells → produce ATP
  Excess → glycogen and stored in liver and skeletal muscle
  Excess → triglycerides and stored in adipose tissue
• Post Absorptive state
  Blood glucose levels maintained by:
  → glycogenolysis
  → gluconeogenesis

Question 24

Summary: fate of proteins

Answer 24

• Proteins broken down into amino acids
• Absorptive state
  All cells → protein synthesis or produce ATP
  Excess → converted to triglycerides and stored
• Post Absorptive state
  breakdown of tissue proteins to release amino acids if prolonged starvation
  Amino acids used to produce:
  → glucose and produce ATP if glycogen and fat stores depleted

Question 25

Summary: fate of triglycerides

Answer 25

• Triglycerides broken down into glycerol and fatty acids absorbed into lymphatic system then into bloodstream
• Absorptive state produce ATP stored as fat
• Post Absorptive state glycerol and fatty acids → glucose and used by cells to produce ATP