Animal Nutrition and Digestion

Animal Nutrition

• Animals are heterotrophic, obtaining nutrients from other organisms.
• A diet refers to the food an animal eats every day.
• Most animals require seven types of nutrients: carbohydrates, proteins, fats, vitamins, minerals, water, and fiber.
• A balanced diet contains all nutrient types in correct portions to meet energy needs.

Energy Needs

• Daily energy requirement: The amount of energy one uses per day, derived from food.
• Factors influencing daily energy requirement:
  • Age: Teenagers need more energy for growth.
  • Gender
  • Activity level: Influenced by occupation or job.
• Diet changes throughout life to match daily energy requirements.
• Examples:
  • Pregnant women need extra calcium and iron for the baby's bone development.
  • People over 50-60 eat less due to slower metabolism.

Comparison of Daily Energy Requirements

• Figure 7.2 illustrates the daily energy requirements for different demographics.
• It shows variations in energy needs based on age (child vs. teenager vs. adult), gender (male vs. female), and condition (pregnant female).

Nutrients: Vitamins

• Vitamins are organic substances needed in small amounts.
• Insufficient vitamins can lead to vitamin deficiency diseases.
• Vitamin C:
  • Found in citrus fruits and raw vegetables.
  • Needed to make collagen, a protein found in skin, connective tissues, and bones.
  • Deficiency leads to scurvy, causing bleeding gums and joint pain. Historically common among sailors on long voyages without vegetables.

Nutrients: Vitamins (cont.)

• Vitamin D:
  • Found in butter and egg yolk; also produced by skin exposed to sunlight.
  • Needed to help the body absorb calcium for bones and teeth.
  • Deficiency leads to rickets, causing soft and deformed bones. Historically common in children in industrial areas with limited sunlight exposure.

Nutrients: Minerals

• Minerals are inorganic substances needed in small amounts.
• Mineral deficiency can lead to deficiency diseases.
• Calcium (Ca):
  • Found in dairy and bread.
  • Needed for bones, teeth, and blood clotting.
  • Deficiency leads to brittle bones and teeth and poor blood clotting.

Nutrients: Minerals (cont.)

• Iron (Fe):
  • Found in red meat, egg yolk, liver, and dark green vegetables (e.g., spinach, kale).
  • Needed for making hemoglobin, the red pigment in blood that carries oxygen.
  • Deficiency leads to anemia, a condition with insufficient red blood cells, resulting in inadequate oxygen delivery to tissues.

Nutrients: Fiber

• Fiber (roughage):
  • Plant-based material (cellulose) that is insoluble and indigestible by humans.
  • Helps move food through the alimentary canal by aiding peristalsis.
  • Stimulates muscles in the digestive tract, especially with hard, undigested foods.
  • Prevents constipation.

Foods High in Fiber

• Vegetables, fruits, husk of grains (oat & wheat bran), whole meal bread, brown rice.

Peristalsis

• Peristalsis is a series of involuntary wave-like muscle contractions that move food along the digestive tract.
• Involves the relaxation and contraction of circular and longitudinal muscles to squeeze the bolus of food forward.

Fat and Heart Disease

• Saturated fats are found in animal foods.
• These foods often contain cholesterol.
• Research suggests that high consumption of saturated fats and cholesterol increases the risk of heart disease.
• Coronary heart disease involves the formation of fat deposits inside arteries, limiting blood supply to the heart, leading to oxygen deficiency and impaired heart function.

Fat and Heart Disease (cont.)

• Fat deposits in arteries can cause blood clots, resulting in a heart attack.
• Foods high in saturated fat: dairy, red meat, and eggs.
• Foods low in unsaturated fat: vegetable oils, fish oil, and white meats (fish, chicken).

Obesity

• Obesity is defined as being very fat (BMI > 30).
• Caused by taking in more energy than is used up, often due to excessive carbohydrate and fat consumption.
• Health risks: heart disease, stroke, and joint problems (especially knees).
• Maintaining a balanced diet and regular exercise is essential for a healthy weight. Crash diets or starvation diets are not recommended due to the likelihood of regaining weight.

Starvation & Malnutrition

• Starvation occurs in regions with food scarcity, often due to droughts or poor harvests.
• Malnutrition results from an unbalanced diet, even with sufficient food intake.
• Obesity can also be a form of malnutrition.

Malnutrition Examples

• Kwashiorkor:
  • Common in areas of extreme poverty.
  • Caused by a lack of protein in the diet.
  • Typically affects children aged 9 months to 2 years (after weaning from breast milk).
  • Symptoms: underweight with a swollen belly due to high carbohydrate intake.
  • Treatment: high-protein diet.
• Marasmus:
  • Diet lacks both protein and carbohydrates.
  • Results in a lack of energy.
  • The child is greatly underweight and appears emaciated (abnormally thin and weak).

Nutrients Providing Energy

• Macronutrients that provide energy:
  • Carbohydrates: $(17 \text{kJ/g})$
  • Fats: $(39 \text{kJ/g})$
  • Proteins: ~ $(17 \text{kJ/g})$
• The body uses these nutrients in the specified order to release energy.

Organic vs. Inorganic Nutrients

• Organic nutrients contain carbon, while inorganic nutrients do not.
• Organic nutrients: carbohydrates (including fibre), fats, proteins, and vitamins.
• Inorganic nutrients: water and minerals.

Textbook Questions Recap (pg. 78)

• 7.1 A balanced diet contains carbohydrates, fats, proteins, vitamins, minerals, and water.
  * (a) Which are organic & which are inorganic?
  * (b) Which can provide energy?
  * (c) What is the role of fibre in the diet?
• 7.2 List 3 health problems associated with obesity.
• 7.3 What is coronary heart disease?
• 7.4 What is the difference between starvation and malnutrition?
• 7.5 What is meant by ‘deficiency disease’?
• 7.6 Give 2 examples of deficiency disease.

Digestion

• Digestion is the breaking down of food molecules into smaller pieces so they can be absorbed from the alimentary canal into the bloodstream.
• Absorption is the process by which digested food is absorbed from the alimentary canal into our bloodstream.
• Food must be broken down into small molecules to pass through the wall of the alimentary canal.

Alimentary Canal

• The alimentary canal is a long tube running from one end of a mammal to the other, including the mouth, oesophagus, stomach, small intestine, large intestine, rectum, and anus.

Digestion (cont.)

• Carbohydrates, lipids, and proteins are large molecules that need to be broken down through digestion.
• Vitamins and minerals are already small and do not need digestion.
• Two types of digestion:
  • Mechanical digestion
  • Chemical digestion

Mechanical Digestion

• Mechanical digestion is a physical process where food is broken into smaller pieces.
• This occurs through chewing by teeth and the churning motion of the alimentary canal (in the stomach).

Chemical Digestion

• Chemical digestion is a chemical process where large, insoluble molecules are broken down into smaller, soluble molecules with the help of enzymes.
• The following table summarizes the process:
  • Carbohydrate (Starch) + amylase $\rightarrow$ Simple sugars (monosaccharides)
  • Protein + protease $\rightarrow$ Amino acids
  • Fat (lipids) + lipase $\rightarrow$ Fatty acids & Glycerol

Stages of Nutrition

• There are 5 main stages for human nutrition:
  1. Ingestion
  2. Digestion (mechanical & physical)
  3. Absorption
  4. Assimilation
  5. Egestion

Teeth

• Teeth aid with ingestion and mechanical digestion.
• Ingestion: Taking substances (food/drink) into the body through the mouth.
• Mechanical digestion: Breakdown of food into smaller pieces without chemical change to the food molecules.

Use of Teeth

• Teeth help grind and chew food particles, breaking them down into smaller pieces.
• This increases the surface area for enzymes to act on and helps soluble parts of the food dissolve.

Structure of Teeth

• Teeth are divided into three parts: crown, neck, and root.

Enamel

• Enamel:
  • Covers the crown and protects the tooth.
  • Outermost part of the tooth.
  • Made of calcium salts.
  • Hardest substance made by animals but can be dissolved by acid.

Dentine

• Dentine:
  • Middle layer under the enamel.
  • Bone-like.
  • Hard, but not as hard as enamel.
  • Has strands of living cytoplasm.

Pulp Cavity

• Pulp cavity contains:
  • Living cells
  • Blood vessels: Supply cytoplasm in dentine with oxygen and nutrients and remove waste.
  • Nerves: Detect temperature and pressure.

Root

• The root is covered with cement, which has fibres growing out of it.
• These fibres attach the tooth to the jawbone, allowing slight bending when biting or chewing.

Types of Teeth

• Most mammals have 4 types of teeth, each shaped to suit their function:
  • Incisors
  • Canines
  • Premolars
  • Molars

Incisors

• Incisors:
  • Located at the front of the mouth.
  • Used for biting off pieces of food.
  • Sharp-edged and chisel-shaped.
  • One root.

Canines

• Canines:
  • Located at either side of incisors.
  • Used for tearing flesh.
  • Pointed and curved.
  • One root.

Premolars

• Premolars:
  • Located towards the back of the mouth.
  • Used for grinding food.
  • Broad surface with cusps.
  • One or two roots.

Molars

• Molars:
  • Located at the back of the mouth.
  • Used for grinding food.
  • Large surface with cusps.
  • Two or three roots.
  • Wisdom teeth (3rd molar) are molars right at the back that generally appear during late teens to early twenties, if at all.

Deciduous Teeth

• Humans are unique from other mammals in having two sets of teeth (deciduous & permanent teeth).
• Deciduous teeth (milk teeth/primary teeth):
  • First set of teeth.
  • Grow through the gums when a child is around 5 months old.
  • Full set of 20 deciduous teeth by age 24-30 months.
  • Begin to fall out at around age 7.

Permanent Teeth

• Also called adult teeth.
• Second set of teeth.
• 20 teeth will grow to replace the lost milk teeth + an additional 12 teeth (32 in total).
• By age 17, most people should have a full set of permanent teeth.
• Many adults do not have all 32 teeth as it is not uncommon for some or all 4 wisdom teeth not to emerge. Some people also have hypodontia, where excluding the wisdom teeth, other permanent teeth are missing.

Milk vs. Permanent Teeth

• Primary teeth (milk teeth) include central incisors, lateral incisors, cuspids (canines), first molars, and second molars.
• Adult teeth include central incisors, lateral incisors, cuspids (canines), bicuspids (premolars), first molars, second molars, and third molars (wisdom teeth).

Plaque

• We have many bacteria in our mouths, many of which are harmless.
• Some of these bacteria combined with substances from our saliva form a sticky film over our teeth.
• Plaque usually forms near the gums or between teeth.
• Plaque is soft and easily removed by brushing, but if left unremoved, it will harden into tartar, which is difficult to remove.

Dental Decay: Tooth Decay

• Bacteria in plaque feed on sugar left on teeth.
• The sugar is converted to acid (lactic acid) during respiration carried out by the bacteria.
• This acid dissolves the enamel and eventually the dentine (dissolves faster than the enamel).
• If left untreated, the tooth will need to be removed.

Dental Decay: Gum Disease

• Bacteria in the plaque, if not removed, can infect the gums.
• Gums become inflamed and swollen.
• Gums may bleed when brushing teeth.
• Usually painless; however, if no action is taken, bacteria can reach the roots, hence loosening the tooth.

How to Maintain Healthy Teeth and Gums

• Avoid eating too much sugar.
  • Eat sugary foods together with meals.
  • Do not leave sugary food in the mouth for long periods of time (e.g., sucking on candies).
• Brush teeth often with fluoride toothpaste (fluoride makes teeth more resistant to tooth decay).
• Visit a dentist regularly for check-ups.

Alimentary Canal

• A long tube which runs from mouth to the anus.
• Is part of the digestive system.
• The wall of the alimentary canal is made of muscles which perform peristalsis to move food along.
• When food is required to be kept in one place, sphincter muscles can close the alimentary canal in certain places.

Peristalsis (revisited)

• Peristalsis is a series of involuntary wave-like muscle contractions that move food along the digestive tract.
• Involves the relaxation and contraction of circular and longitudinal muscles to squeeze the bolus of food forward.

Alimentary Canal (Detailed)

• The alimentary canal includes the mouth, oesophagus, stomach, small intestine (duodenum, ileum), large intestine (colon, caecum, appendix, rectum), and anus.
• Other associated organs include salivary glands, liver, gall bladder, bile duct, pancreas, and diaphragm.

Mouth

• Food is ingested in the mouth cavity by teeth, tongue, and lips.
• Food is then masticated (bitten and ground down) by teeth, increasing its surface area.
• Mechanical digestion begins in the mouth.

Mouth (cont.)

• The tongue mixes the chewed food with saliva (containing enzyme amylase, water & mucus) to form a bolus.
• The bolus is then swallowed and travels down the oesophagus (gullet).

Mouth: Saliva

• Saliva is produced by salivary glands and it contains:
  • Water: Helps dissolve substances in food, allowing us to taste.
  • Mucus: Helps the food particles bind together to form the bolus.
  • Amylase: An enzyme which breaks down starch into maltose.
  • When you leave a biscuit in your mouth, it becomes mushy as the starch is broken down by salivary amylase. It will also taste sweet from the maltose produced if you leave it longer.

Oesophagus

• Tube leading from the mouth to the stomach.
• Located behind the trachea (windpipe).
• When we swallow, the epiglottis moves to block the entrance to the trachea so that food does not travel into our lungs.
• The bolus travels down the oesophagus and through a sphincter (ring of muscle) which guards the stomach. This muscle relaxes to allow food to enter the stomach.

Bolus Movement

• During swallowing, the epiglottis blocks the entrance of the trachea.

Lower Oesophageal Sphincter

• Relaxes to allow the bolus to pass through into the stomach. Located between the oesophagus and stomach.

Stomach: Churning

• Has muscular walls which contract and relax to churn food à mechanical digestion.
• Churning helps mix food with mucus and enzymes, forming a mixture called chyme.

Stomach: Gastric Juice

• The stomach wall contains:
  • Goblet cells: Produce mucus to protect the stomach from acid and enzymes.
  • Chief cells: Produce protease enzymes.
  • Parietal cells: Produce hydrochloric acid to:
    • Kill harmful microorganisms in food by denaturing their enzymes.
    • Help break down food.
    • Provide an acidic environment (optimum pH) for pepsin to work.
• The combination of these secretions forms Gastric juice.

Stomach: Pepsin

• Protease enzyme in the stomach is PEPSIN.
• Pepsin digests proteins into smaller polypeptides.
• Works best in acidic condition (optimum pH ~1.5-2).
• Peptide bonds are hydrolysed.

Rennin

• Also known as Chymosin, is an enzyme produced in the stomach of young, mostly herbivore mammals.
• Rennin causes the milk they drink to clot, and proteins in the milk are then digested by pepsin.
• Humans do not produce rennin.

Stomach Structure

• Gastric pit - where gastric juice is made. Contains cells that produce mucus, acid, and protease enzymes.

Stomach (cont.)

• The stomach can store food for a long time.
• After one to two hours, the sphincter at the bottom of the stomach (pyloric sphincter) will open and allow the chyme to pass into the duodenum (first section of small intestine).

Pyloric Sphincter

• Relaxes to allow chyme to enter the small intestine. Located between the stomach and small intestine.

Small Intestine

• Part of the alimentary canal between the stomach and the colon.
• Around 5m long
• Relatively thin hence the name ‘small’
• Section closest to the stomach is called the duodenum.
• Section closest to the colon is called the ileum.
• The middle section between the duodenum and the ileum is called the jejunum*.

Small Intestine Sections

• The small intestine consists of the duodenum, jejunum, and ileum.
• It connects the stomach to the colon.

Small Intestine (cont.)

• Several enzymes are secreted in the duodenum of the small intestine.
• These enzymes are made in the gland called the pancreas.
• A tube called the pancreatic duct transports these enzymes to the duodenum.
• A fluid called pancreatic juice is secreted by the pancreas, containing the enzymes.

Pancreas and Liver

• The pancreas is a cream-coloured gland located behind and slightly under the stomach.
• The liver produces bile, which is stored in the gall bladder.
• The pancreatic duct transports enzymes to the duodenum.

Pancreatic Juice

• The pancreatic juice contains the following enzymes:
  • Amylase: (breaks down starch $\rightarrow$ maltose)
  • Trypsin (type of protease): (further breaks down proteins into peptides)
  • Lipase: (breaks down lipids $\rightarrow$ fatty acids + glycerol)
• These enzymes do not work well in acid environments
• The chyme, which contains hydrochloric acid from the stomach, is partially neutralised by sodium chloride contained in pancreatic juice.

Bile

• Besides pancreatic juice, another fluid, bile, flows into the duodenum.
• Bile is made in the Liver and stored in the Gall bladder and is:
  • Yellowish-green in color
  • Watery
  • Alkaline
• Bile flows from the gall bladder to the duodenum via the Bile duct

Bile Composition

• Bile contains:
  • Sodium hydrogencarbonate: helps neutralise the hydrochloric acid found in chyme.
  • Bile salts: emulsifies lipids (breaks them into smaller droplets) to increase their surface area for lipase to act on. This is a type of mechanical digestion.
  • Bile pigments: yellowish pigment made from haemoglobin when the liver breaks down old red blood cells, which are not required by the body and are excreted in faeces.

Emulsification

• Bile salts emulsify lipids (fats) into smaller droplets, increasing the surface area for lipase to act on, which is crucial for digestion.

Gall Bladder Location

• Gall bladder is connected to the liver, stomach, pancreas, and duodenum via the bile duct.

Digestion: Villi

• The small intestine also makes its own enzymes.
• The inner wall of the small intestine is covered with millions of finger-like projections (1mm) called villi.
• Specialised cells on the villi produce enzymes.
• These enzymes stay on the cells that produce them and they are not secreted as they do not enter the lumen (hollow space inside the intestine).

Small Intestine Structure - Villi

• The small intestine lining includes the mucosa, submucosa, circular muscle, longitudinal muscle, and serosa.
• Villi are folds of the intestinal lining, covered with epithelial cells having microvilli.
• Each villus has a lacteal, a capillary network, and an arteriole.

Villi Structure (Detailed)

• Villi consist of an epithelial cell layer with microvilli, a capillary network, a lacteal (lymph vessel), and goblet cells (which produce mucus).

Intestinal Enzymes Location

• The enzymes remain on the villi of the small intestine.

Enzymes Produced in the Small Intestine

• Enzymes produced by the villi of the small intestine are:

  • Carbohydrase enzymes:
    • Maltase: breaks down maltose into glucose
    • Sucrase: breaks down sucrose into glucose & fructose
    • Lactase: breaks down lactose into glucose & galactose
    • Lactase production stops in adulthood of lactose intolerant people, hence they cannot digest milk properly
  • Protease enzymes:
    • Peptidase: finish breakdown of polypeptides into amino acids
  • Lipase:
    • Breaks down lipids into fatty acids and glycerol

Disaccharides Breakdown

• Maltase breaks down maltose into glucose.
• Sucrase breaks down sucrose into glucose and fructose.
• Lactase breaks down lactose into glucose and galactose.
• These sugars are broken down on the epithelial lining of the small intestine since the enzymes remain on the villi, which are part of the stomach lining.

Digestion Summary

• Breakdown of Carbohydrates:
  • Teeth break down large pieces of food into smaller ones.
  • Amylase breaks starch molecules down to maltose molecules.
  • Maltase breaks maltose down to glucose molecules.
• Breakdown of Proteins:
  • Teeth break down large pieces of food into smaller ones.
  • Proteases break down protein molecules to polypeptide molecules.
  • Peptidases break down polypeptides to amino acid molecules.
• Breakdown of Fats:
  • Teeth break down large pieces of food into smaller ones.
  • Bile salts break down large drops of fat into smaller ones.
  • Lipase breaks down fat molecules to fatty acid and glycerol molecules.

Absorption

• At this point:
  • Most of the large molecules have been broken down into small molecules i.e. amino acids, monosaccharides, fatty acids + glycerol
• Molecules are small enough to pass through wall of small intestine and into the blood stream- Absorption
• Absorption mostly occurs in the ileum of the small intestine

Adaptations of Small Intestine for Absorption

• The small intestine is well suited for absorption and has several adaptations to achieve this:

Adaptations of Small Intestine: Length

• The small intestine is long, around 5m in adult humans.
• This allows for sufficient time for digestion to be completed and for food to be absorbed.

Adaptations of Small Intestine: Folds & Villi

• The inner wall of the small intestine has many folds with millions of villi.
• This greatly increases the surface area.
• Larger surface area means that absorption can occur at a faster rate.

Adaptations of Villi

• The cell membrane of villi epithelial cells has microvilli.
• This further increases the surface area for absorption.
• Carbohydrase and protease enzymes are present on the microvilli.
• Folds and presence of villi + microvilli increases surface area by 500x compared with a smooth surface.

Adaptations of Villi: Epithelium

• The epithelium of villi is only ONE cell thick.
• This provides a short distance for food molecules to diffuse into the blood.
• Hence, there is a rapid rate of absorption.

Villi: Blood Capillaries

• Villi contain a network of blood capillaries.
• Monosaccharides, amino acids, water, vitamins, and some fats can pass into the blood quickly.
• A steep concentration gradient is kept across the wall of the small intestine, increasing the rate of diffusion of food molecules.

Villi: Lacteal

• At the center of the villi is a Lymph vessel called a LACTEAL.
• The lacteal is surrounded by blood capillaries.

Lacteal Function

• The lacteal and surrounding network of capillaries ensure that absorbed food molecules are carried away quickly.

• A steep concentration gradient is maintained across the wall of the small intestine, increasing the rate of diffusion of food molecules.

How is Digested Food Absorbed?

• Digested food is absorbed either into the BLOOD in the BLOOD CAPILLARY or into the LYMPH in the LACTEAL.

Absorption into the Blood via Capillaries

• This type of absorption is for WATER-SOLUBLE molecules, which include:
  • Monosaccharides
  • Amino acids
  • Minerals
  • Water-soluble vitamins (Vitamin C)

Absorption into Blood:

• Water soluble molecules are absorbed into the blood in the capillaries by DIFFUSION and ACTIVE TRANSPORT.

Absorption into Blood

• Monosaccharides, amino acids, minerals, and water-soluble vitamins are absorbed into blood capillaries via diffusion and active transport.

Absorption in the Lacteal

• This type of absorption is for FAT-SOLUBLE molecules (i.e. fatty acids and glycerol, and fat-soluble vitamins which are Vitamins A, D, E and K):

Absorption in the Lacteal

• Fat soluble molecules are Absorbed by DIFFUSION

• Lymph $\rightarrow$ lymphatic vessels $\rightarrow$ heart

• Blood from heart (aorta) Blood to liver via hepatic portal vein

Absorption of Water

• Most water (90%) is absorbed in the small intestine.
• The rest is absorbed in the Colon (large intestine).
• Some minerals and vitamins are also absorbed in the colon, but very slowly.

Absorption in the Large Intestine

• Not all food can be digested.
• This undigested food moves through the caecum, past the appendix, and into the colon.
• The caecum and appendix have no function in humans.
• Remaining water is absorbed in the colon.

Large Intestine Sections

• The large intestine consists of the caecum, ascending colon, transverse colon, descending colon, appendix, and rectum.

Egestion

• Remaining indigestible food such as fibre (roughage) makes its way to the rectum.
• Bacteria and dead cells from the alimentary canal are also present.
• This mixture forms faeces, which is passed out through the anus. This is called EGESTION.

Faeces Storage

• Faeces is stored in the rectum, the final part of the large intestine, until it leaves the body via the anus.

Diarrhoea

• Diarrhoea is the loss of watery faeces.
• Occurs when not enough water is absorbed from faeces.
• Can be deadly if it occurs frequently as too much water can be lost from the body, leading to organ failure.
• 2nd leading cause of death in young children

Diarrhoea Treatment

• Can be treated by Oral rehydration therapy:
  • Drinking water with a small amount of salts and sugar dissolved in it.
• E.g., Coconut water works effectively.
• There are many causes of diarrhoea, a common one is cholera caused by infection of cholera bacteria called Vibrio cholerae.

Cholera

• Enters the body via ingesting infected water/food (e.g., dirty drinking water contaminated by sewage).
• V. cholerae bacteria will:
  • Attach to the wall of the small intestine and release a toxin.
  • Toxin stimulates cells lining the small intestine to release chloride ions into the lumen $\rightarrow$ lowers the water potential in lumen.
  • Water leaves blood (higher water potent.) from capillaries in the wall of the small intestine and into the intestine (lower water potent.) via osmosis.
  • Water is lost from the body via watery faeces.
  • Blood contains not enough water and ions.
  • Results in dehydration.

Assimilation

• Assimilation is the movement of digested food molecules into cells of the body where they are used, becoming part of the cell.

• Water soluble* nutrients absorbed into the blood are taken to the liver via the hepatic portal vein.

• Some nutrients are processed at the liver.

• Some of the nutrients can be:

  • Broken down (e.g., excess amino acids)
  • Converted into other substances (e.g., excess glucose $\rightarrow$ glycogen)
  • Stored (lipid-soluble vitamins)

• Others remain unchanged

Assimilation (cont.)

• The nutrients, then dissolved in the blood plasma are transported to the rest of the body to become assimilated as part of a cell e.g.
  • Glucose for energy or stored as glycogen
  • Lipids to make cell membrane, hormones, or energy storage
  • Amino acids to make protein for cell development or make enzymes & antibodies

Nutrient Transport During Assimilation

• Nutrients which are Absorbed into blood vessels of villi of small intestine are transported via the Hepatic portal vein to Liver. From the liver these nutrients are Transported to rest of body
• Nutrients that are not stored in the liver are transported to the heart via the hepatic vein and vena cava to be transported all over the body

Assimilation

• Process of digested food is moved into blood vessels of villi in small intestine
• Hepatic portal vein transports to the liver and the nutrients are then transported to other parts of the body

Egestion (revisited)

• Faeces is stored in the rectum and expelled through the anus.

Recap of Digestion

Different parts of the alimentary canal and their functions summary.

• Mouth
  • Juice secreted - saliva
  • Gland where the juices are made - salivary glands
  • Enzymes in juice - amylase
  • Substrate - starch
  • Product - maltose
  • Functions of other substances - water, mucus, sodium chloride
• Oesophagus
  • Juice secreted - none
• Stomach
  • Juice secreted - gastric juice
  • Gland where the juices are made - pits in the stomach
  • Enzymes in juice - pepsin, rennin (only in young mammals)
  • Substrate - proteins, milk protein
  • Product - polypeptides, curdled milk protein
  • Functions of other substances - hydrochloric acid
• Duodenum
  • Juice secreted - pancreatic juice and bile
  • Gland where the juices are made - pancreas, liver (stored in the gall bladder)
  • Enzymes in juice - amylase, trypsin, lipase
  • Substrate - starch, proteins, fats
  • Product - maltose, polypeptides, fatty acids and glycerol
  • Functions of other substances - sodium hydrogencarbonate, bile salts, bile pigments
• Ileum
  • Juice secreted - no juice secreted but enzymes are made by cells covering the villi
  • Enzymes in juice - maltase, sucrase, lactase, peptidase, lipase
  • Substrate - maltose, sucrose, lactose, polypeptides, fats
  • Product - glucose, glucose and fructose, glucose and galactose, amino acids, fatty acids and glycerol

All of the digestive juices contain water and mucus.

##### Water is used for the digestion of large molecules to small ones.

It is also a solvent for the nutrients and enzymes. Mucus acts as a lubricant and prevents autodigestion.

Recap Diagram of the Digestive System

• Mouth: chewing + amylase
• Pharynx: epiglottis blocks windpipe route to nasal cavity close
• Esophagus: peristalsis squeezes food down
• Stomach: churning + protease (pepsin)
• Small Intestines: duodenum,