NUTRITION – 1
Nutrition is the process by which an organism provides itself or is provided with materials necessary for energy release, growth, repair of body tissues, and keeping the body in good condition.
FOOD
Food is any liquid or solid which provides the body with materials for growth and repair, energy release, or keeping the body in a healthy condition.
KINDS OR MODES OF NUTRITION
Basically, there are two kinds of nutrition:
- AUTOTROPHIC NUTRITION
- HETEROTROPHIC NUTRITION
AUTOTROPHIC NUTRITION
This is a mode of nutrition whereby organisms can make their own food from simple inorganic substances, such as carbon dioxide and water, using either light energy (photosynthesis) or chemical energy (chemosynthesis). Organisms which feed this way are known as AUTOTROPHS.
Example
- Green plants
- Iron bacteria and sulphur bacteria
HETEROTROPHIC NUTRITION
This is the mode of nutrition whereby organisms use organic materials as the only source of food. Organisms which feed this way are known as HETEROTROPHS. They feed on already made food.
There are three types of heterotrophic nutrition:
- Saprophytic nutrition
- Symbiotic nutrition
- Holozoic nutrition
1. Saprophytic nutrition
This is the mode of nutrition whereby organisms feed on dead, decaying body parts of animals or their excreta. Organisms feeding this way are known as SAPROPHYTES.
2. Holozoic nutrition
This is a mode of nutrition whereby organisms take food by mouth. It passes through a digestive system, is broken down, and finally absorbed into body tissues.
FORMS OF HOLOZOIC NUTRITION
- Herbivores – Animals which feed on plants only. Examples: cow, goat, zebra.
- Carnivores – Animals which feed on flesh only. Examples: lion, tiger.
- Omnivores – Animals which feed on a variety of foods (flesh, plants, insects, etc.). Examples: man, monkey, pig.
- Insectivores – Animals that feed on insects. Examples: shrews, ants.
3. Symbiotic nutrition
This mode of nutrition involves organisms of different species existing in a nutritional relationship with each other.
There are three kinds of symbiotic associations:
- Mutualism
- Commensalism
- Parasitism
Mutualism
This is a nutritional relationship in which both partners benefit by living together. Example: Nitrogen-fixing bacteria in the roots of legumes.
Commensalism
This is an association of two groups whereby the host does not get any benefit. Example: Epiphytes (plants that grow on other plants) such as mosses and algae which grow on upper parts of big trees to get sunlight easily.
Parasitism
This is a nutritional relationship between two organisms in which one organism lives inside or on another living organism and depends on it for food. The parasite causes harm to the host. The host provides food and shelter for the parasite.
Endoparasites live inside the body of the host. Examples: tapeworm, roundworm, plasmodia.
Ectoparasites live on the host’s surface (outside the body). Examples include some mites, fleas, and body lice.
The importance of nutrition
- Nutrition enables an organism to get nutrients and energy required for various life processes, including growth and development of cells.
- To protect the body against diseases.
- Enables replacement of worn-out tissues and dead cells.
NUTRITION IN MAMMALS
HUMAN NUTRITION
FOOD SUBSTANCES
There are several types of substances needed by the human body for proper functioning. The basic food substances include proteins, carbohydrates, lipids, vitamins, minerals, roughage, and water.
- CARBOHYDRATES
These are compounds containing the elements:
- Carbon (C)
- Hydrogen (H)
- Oxygen (O)
Carbohydrates are also known as SACCHARIDES.
There are three groups of saccharides:
- Monosaccharides
- Disaccharides
- Polysaccharides
MONOSACCHARIDES
These are the simplest form (basic unit) of carbohydrates which are absorbed directly into the blood. They are also known as REDUCING SUGARS.
Their general formula is C6H12O6.
Examples of monosaccharides:
- Glucose – Occurs freely in grapes, honey, tomato, and germinating maize.
- Fructose – Occurs freely in all ripe sweet fruits, e.g., banana, pineapple.
- Lactose – Found in mammalian milk.
DISACCHARIDES
These are also known as NON-REDUCING SUGARS. They are formed when two molecules of monosaccharides condense and release molecules of water. Their general formula is C11H22O11.
Examples of disaccharides:
- Maltose is formed when two molecules of glucose condense.
Glucose + glucose = maltose + water - Sucrose is formed by condensation of glucose and fructose.
Glucose + fructose = sucrose + water - Lactose is formed by condensation of galactose and glucose.
Glucose + galactose = lactose + water
POLYSACCHARIDES
These are formed when several units of monosaccharides are linked together.
Examples of polysaccharides:
- Starch – Stored in plant cells; glycogen is stored in muscles and liver of vertebrates; chitin is found in exoskeleton of arthropods and fungal cells.
- Cellulose – Forms the building material of plant cell walls.
Source of Carbohydrates
- Cereals – e.g., maize, rice, wheat
- Sugar – e.g., honey, sugar cane, glucose, sweet fruits
Function of Carbohydrates in the body
- To provide the body with energy.
- Carbohydrates combine with proteins, glucose, and lipids which are important components of cell membranes.
2. PROTEINS
Proteins are compounds of carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain sulphur and phosphorus. Proteins are long chains of amino acids formed through condensation.
Approximately twenty amino acids occur naturally. The human body makes ten of these amino acids, called NON-ESSENTIAL AMINO ACIDS. The other ten amino acids must be obtained from the diet and are called ESSENTIAL AMINO ACIDS. We get essential amino acids by eating food rich in protein. Food that contains all the essential amino acids is called first-class proteins; most animal proteins fall in this group. Second-class proteins lack one or more essential amino acids; most plant proteins fall in this group.
Functions of protein
- The body uses proteins for tissue growth and repair, such as healing wounds and replacement of skin and mucous membranes.
- Antibodies are made of proteins and are important in offering immunity to the body.
- Enzymes that help digest food are proteins. Hormones, the chemical messengers in our bodies, are also proteins.
- Proteins serve as an alternative source of energy in the body.
Protein food
3. LIPIDS
Lipids are compounds of carbon, hydrogen, and oxygen. They are insoluble in water. The main forms of dietary lipids are fats and oils. Fats are solid at room temperature, while oils are liquid. Lipids are made up of fatty acids and glycerol. Fatty acids can be essential or non-essential. The body can make non-essential fatty acids but cannot produce essential fatty acids. Therefore, we need to eat food that contains essential fatty acids. Examples include oil fish, nuts, and oil seeds such as sunflower seeds, maize, avocados, and olives.
Functions of lipids
- Lipids are a source of energy; they produce more energy than all other food substances.
- They are important components of cell membranes.
- Fat deposits protect delicate organs such as the heart and kidneys.
- Fat stored under the skin helps insulate the body against heat loss.
- Essential fatty acids are important for the formation of substances that help control blood pressure.
4. ROUGHAGE
This dietary fiber is obtained from the indigestible parts of plants. Roughage has no nutritional value as it is not digested or absorbed by the body. However, it helps in the passage of food and feces through the gut by preventing contraction of the gut muscles.
Source of roughage: Whole grown cereals, fruits, beans, cabbage, spinach, cassava, and whole baked potatoes.
5. WATER
Water does not provide energy to the body but is important in several ways:
- It is used in the digestion and absorption of food.
- It is a medium of transport for food and hormones.
- It acts as a lubricant, e.g., in the eyeball and eyelids.
- It helps in the excretion of harmful by-products of metabolic processes, e.g., urine, sweat.
- It helps regulate heat loss (evaporation of sweat cools the body).
- It is used in the manufacture of different secretions, e.g., tears, saliva.
Water can be replaced in the body through:
- Direct drinking
- Eating foods and fruits
6. VITAMINS
Vitamins are complex organic micronutrients essential for growth and survival. Lack of vitamins leads to deficiency diseases. Vitamins are grouped into two categories: water-soluble and fat-soluble.
- Fat-soluble vitamins can be stored in the body and need not be consumed daily. Examples: Vitamins A, D, E, and K.
- Water-soluble vitamins are not stored in the body and should be consumed regularly. Examples: Vitamins B and C. Vitamin B has various forms, including B1, B2, B6, and B12.
Source, functions, and deficiency of vitamins
| Vitamin | Source | Function | Sign of deficiency |
|---|---|---|---|
| Vitamin A (retinol) | Liver, milk, carrots, orange, and yellow vegetables | Essential for formation of membranes of the eyes and respiratory tract | Night blindness, increased risk of infection |
| Vitamin B1 (thiamine) | Lean meat, liver, eggs, yeast extract, brown rice | Carbohydrate metabolism, coordination of muscles | Beriberi: loss of appetite, muscle cramps, heart failure |
| Vitamin B2 (riboflavin) | Liver, meat, whole grain cereals, yeast extract | Metabolism of all food and energy release | Cracked lips, sore mouth corners |
| Vitamin B3 (niacin) | Nuts, fish, meat, yeast extract, unpolished rice | Needed by enzymes to convert food into energy | Pellagra: skin lesions, loss of appetite, muscle weakness |
| Vitamin B6 (pyridoxine) | Meat, vegetables, yeast extracts, whole grown cereals | Essential in protein metabolism | Nerve irritability, sores in mouth and eyes, anemia |
| Vitamin B12 (cyanocobalamin) | Fish, meat, eggs, milk, liver | Builds genetic material, helps form blood cells | Anemia, nerve damage, weight loss |
| Vitamin C (ascorbic acid) | Citrus fruits, fresh green vegetables, tomatoes | Antioxidant, improves iron absorption, synthesis of collagen | Muscle weakness, bruising, joint pains, scurvy, poor wound healing |
| Vitamin D | Egg yolk, milk, oily fish, liver | Helps build and maintain teeth and bones | Rickets in children, osteoporosis in adults |
| Vitamin E | Corn, sunflower oil, butter, brown rice, peanuts | Antioxidant, prevents membrane damage | Nerve abnormalities, infertility in rats |
| Vitamin K | Green vegetables, liver | Needed for normal blood clotting | Defective blood coagulation, excessive bleeding |
7. MINERALS
Certain mineral elements are vital for the proper functioning of the body. Some are required in relatively large quantities and are called macro minerals. Others are required in very small quantities and are referred to as micro minerals or trace elements.
The following are examples of minerals, their sources, and their functions in the body.
a) Macro minerals
| Mineral | Source | Function | Deficiency symptoms |
|---|---|---|---|
| Calcium | Milk, yoghurt, cheese, sardines, egg, green vegetables | Builds strong bones and teeth, promotes muscle and nerve functions, important in blood clotting | Weak bones, bleeding easily |
| Phosphorus | Meat, milk, fish, eggs, nuts | Builds bones and teeth, helps muscle and nerve activity, aids formation of genetic materials | Impaired nerve activity, poor bone and teeth formation |
| Potassium | Peanuts, bananas, orange juice, green beans, meat | Maintains regular fluid balance, needed for nerve and muscle function | Poor muscle contraction |
| Iron | Liver, meat, beans, green vegetables | Essential for forming hemoglobin (red pigment in blood) | Anemia |
| Zinc | Oysters, shrimp, crab, meat, yeast extracts | Activates enzymes, helps heal wounds, necessary for a healthy immune system | Impaired teeth, poor immune response, skin problems |
| Sodium | Table salt | Necessary for nerve and muscle activity | Muscle cramps |
| Chlorine | Table salt | Maintains water and ionic balance, forms hydrochloric acid in the stomach | Poor digestion of proteins |
| Magnesium | Spinach, pumpkin seeds, sesame seeds, black beans | Relaxes nerves and muscles, strengthens bones | Muscle weakness, irregular heartbeat, weak bones |
b) Micro minerals
| Mineral | Source | Function | Deficiency symptoms |
|---|---|---|---|
| Iodine | Iodinated table salt, seafood | Production of thyroid hormone which regulates growth, development of bones and teeth, helps prevent tooth decay | Goiter (enlarged thyroid gland) |
| Fluoride | Fluoridated water and fluoride tablets | Development of bones and teeth, helps prevent tooth decay | Poor development of bones and teeth, tooth decay |
| Manganese | Kidneys, liver, tea, coffee, nuts, fruit | Formation of bones and activation of enzymes | Nausea, dizziness, loss of hearing, loss of bone mass |
| Copper | Meat, fish, liver | Synthesis of bones and hemoglobin, activation of enzymes | Bleeding under skin, easy rupturing of blood vessels, bone and joint problems, anemia |
A BALANCED DIET
A balanced diet refers to food containing all types of nutrients in the correct proportion. We should eat a diet low in fats, sugar, and salt but high in proteins, carbohydrates, vitamins, minerals, and roughage. More importantly, we should take in large amounts of water.
It is recommended to eat more fish, poultry products, and legumes (such as peas, beans, and peanuts) instead of red meat as protein sources. The following should be done to maintain a healthy body:
- Physical activity can preserve and improve your health. It is important to balance food intake and exercise.
- Minimize intake of fats and oils by eating foods low in fats, sugars, and salts to reduce risks of heart attacks, tooth decay, and high blood pressure.
- Include plenty of grains, fruits, and vegetables in your diet.
- Eating a variety of foods provides the body with energy and nutrients required for proper health.
Nutritional requirements for different groups of people
Nutritional requirements differ for different groups of people. The ratio of nutrients varies depending on the state of the body. The following are some groups and their special nutritional needs:
1. Expectant and lactating mothers
Expectant and lactating (breastfeeding) mothers need to get enough nutrients. They should eat a balanced diet because they require nutrients for themselves and the growing fetus or baby.
Protein is needed for the build-up of the mother’s muscles, breasts, uterus, blood supply, and the fetus or baby’s tissues and organs.
Folic acid and vitamin B help lower the risk of birth defects such as spina bifida, a spinal disorder characterized by a hole in the spine.
Calcium helps in the development of the fetus or baby’s bones. If the mother’s diet lacks adequate calcium, it is derived from her bones, weakening them.
Zinc is important for proper labor progression and growth and development of the baby.
The mother requires iron for her blood supply; the fetus also needs to store iron for use during the first few months after birth.
Expectant mothers require adequate dietary fiber to reduce constipation, a common complaint during pregnancy.
2. Children
Children require enough proteins for growth and development of body tissues. Inadequate protein can lead to stunted growth.
Minerals like calcium are necessary for strong bones and teeth. Zinc is important for body growth and can be depleted by vigorous physical exercise.
Vitamin B12 is required for the formation of red blood cells. Vitamin C helps build immunity.
Children require more energy-giving foods because they are more active than adults.
3. Adolescents
- Need food rich in carbohydrates due to high body metabolism.
- Need food rich in protein and mineral salts such as calcium, iron, and phosphorus.
- Adolescent girls require additional iron to compensate for blood loss during menstruation.
4. The elderly
Elderly people require less energy-giving foods because they are generally less active. They should eat foods rich in fiber to reduce constipation and bowel problems.
They also need minerals such as iron, zinc, and calcium. Iron is necessary because anemia is common in older age; zinc is required for a healthy immune system and wound healing.
Old age leads to loss of calcium from bones, causing soft and weak bones. Adequate calcium and vitamin D are needed to counter this.
Elderly often have problems chewing tough foods due to weakened teeth and swallowing due to decreased saliva production. They should get nutritious, easy-to-chew and swallow foods, e.g., proteins from eggs or liver instead of meat.
5. Sedentary workers
These individuals stay in one place for long periods during their occupational activities, e.g., potters, weavers, clerks, receptionists, doctors.
They are encouraged to balance their diets with physical exercise. Obesity is common among them, so they should limit intake of foods rich in lipids.
6. The sick
Sick people require plenty of nutrients to help recover health. Those with incurable diseases such as HIV/AIDS should get food to manage their conditions.
Proper nutrition keeps the immune system strong and helps fight infections.
Rapid weight loss is a major problem in late-stage AIDS; adequate nutrients help compensate.
Proper nutrition helps withstand heavy medication and combat symptoms like fatigue, nausea, diarrhea, and high blood sugar.
Excess caffeine, sugar, dried foods, spicy foods, and alcohol should be avoided as they harm a weakened immune system.
They need plenty of fruits and water. Fruits provide vitamins to fight disease; water replaces losses from diarrhea or vomiting.
Using a food guide pyramid is advisable to know what to eat. It shows recommended amounts of different food types for healthy eating.
Nutritional deficiencies and disorders
Malnutrition
Malnutrition results from eating too little or the wrong food. Types include obesity, rickets, kwashiorkor, marasmus, anorexia nervosa, and bulimia nervosa.
Nutritional disorders
These are conditions of ill health arising from poor eating habits.
Obesity
Obesity is excess body fat. Factors include sex, age, activity level, and frame size. Women need more body fat for fertility and store more fat in breasts, hips, and thighs.
Obesity mostly results from overeating and lack of exercise. Body mass index (BMI) relates body fat to disease risk.
BMI = Body mass (kg) / (height in meters)2
Example: Height = 1.65 m, mass = 60 kg
BMI = 60 / (1.65)2 = 60 / 2.7225 = 22.04
BMI 20–25 is healthy; over 30 indicates risk of obesity-related diseases.
Table BMI guide
| BMI | Body condition |
|---|---|
| Below 20 | Underweight |
| 20 – 25 | Advisable range |
| 25 – 30 | Overweight |
| 30 – 35 | Obese |
| Above 35 | Very obese |
Obesity increases the likelihood of high blood pressure, diabetes, certain cancers, stroke, and respiratory problems.
Prevention includes proper eating, regular exercise, and avoiding high-fat foods and unhealthy dieting.
Anorexia nervosa and Bulimia nervosa
Anorexia nervosa (slimmer’s disease or self-starvation syndrome) occurs when a person intentionally refuses to eat enough, leading to severe body mass loss.
Signs and symptoms of anorexia
- Muscle wasting (including weakening of heart muscle)
- Excessive loss of body mass
- Extreme fear of being fat
- Disturbed body image or feeling fat even when very thin
Bulimia nervosa involves excessive eating followed by efforts to remove food from the body, such as self-induced vomiting, fasting, excessive exercise, or use of laxatives or diuretics. Bulimics usually have normal body mass and keep their behavior secret.
Causes of Anorexia and Bulimia
Both have underlying psychological causes such as depression, low self-esteem, bottled-up emotions, and societal beauty standards. Mostly affect women.
Effects of Anorexia and Bulimia
- Heart problems due to weak cardiac muscles or mineral salt imbalance.
- Impaired mental function due to lack of glucose.
- Dehydration.
- Damage to teeth and gums from stomach acid during vomiting.
- Anemia, stomach ulcers, abdominal cramping, gut inflammation, irregular or absent menstrual periods, dry skin.
Treatment for Anorexia and Bulimia
Treatment involves resolving psychological problems, medical consultation, and lifestyle and dietary changes.
Nutritional deficiencies
These arise when the body lacks sufficient supply of a particular nutrient.
Marasmus
Marasmus is malnutrition in children caused by lack of adequate food.
Signs and symptoms of Marasmus
Weight loss, slowed growth, decreased activity, lack of energy, wrinkled skin, irritability, extreme hunger.
Treatment of Marasmus
Provide adequate food containing all nutrients in the right proportions.
Kwashiorkor
Kwashiorkor is caused by protein deficiency, mostly affecting children after stopping breastfeeding.
Signs and symptoms of Kwashiorkor
Thin arms and legs, poor growth, swollen abdomen due to enlarged liver, reddish or yellowish thin and weak hair, weakened immunity, diarrhea, anemia, dry cracked skin.
Treatment for Kwashiorkor
Provide a diet with adequate protein.
Rickets
Rickets is a condition where bones soften, leading to fractures and deformities, caused by lack of vitamin D, phosphorus, and calcium.
Signs and symptoms of Rickets
Bow legs, knock knees, odd-shaped skull, deformed spine, bone pain, dental problems, weak muscles, easy fractures, slow growth, muscle spasms and cramps.
Prevention of Rickets
Increase vitamin D, phosphates, and calcium in diet and ensure sufficient sunlight exposure.
FOOD TEST
Food tests determine which nutrients are present in a food specimen. Tests for carbohydrates, proteins, and lipids are described below.
Test for carbohydrates
| Procedure | Observation | Conclusion |
|---|---|---|
| Dissolve specimen in water. Add equal amount of Benedict’s solution. Boil the mixture. | Colour changes from blue to green to yellow then orange. | Reducing sugar is present. |
| Dissolve specimen in water. Add dilute hydrochloric acid and boil to hydrolyze disaccharides to monosaccharides. Neutralize with sodium hydroxide. Add Benedict’s solution and boil. | Colour changes from blue to green to yellow to orange. | Non-reducing sugar is present. |
| Add a few drops of iodine solution to the specimen. | Colour changes to blue-black. | Starch is present. |
Test for Protein
| Procedure | Observation | Conclusion |
|---|---|---|
| Specimen should be in solution form. Add sodium hydroxide solution and a drop of copper sulfate solution, shaking after each addition. | Colour changes to purple. | Proteins are present. |
Test for lipids
| Procedure | Observation | Conclusion |
|---|---|---|
| Rub specimen on dry filter paper and hold against light. | A translucent mark is formed. | Specimen contains lipids. |
| Ensure specimen is in solution form. Add equal amount of acetone, benzene, or ethyl alcohol. Shake mixture. Add equal volume of water. | Clear mixture turns cloudy, forming a milky suspension. | Specimen contains lipids. |
| Add some drops of Sudan III test to specimen solution. | Droplets of oil turn red. | Specimen contains lipids. |
DIGESTIVE SYSTEM IN HUMAN
Some common terms related to the movement of food along the alimentary canal:
- INGESTION – Taking food into the mouth.
- DIGESTION – The process by which food is broken down into small particles which are absorbed and assimilated.
- ABSORPTION – The process by which soluble end products of digestion diffuse into the bloodstream.
- ASSIMILATION – Incorporation of digestion products into cell metabolism.
- EGESTION – Removal of undigested food particles from the body through the anus.
The digestive system consists of the alimentary canal (gut) and associated organs such as the pancreas and liver.
PARTS OF ALIMENTARY CANAL
The alimentary canal is a long hollow tube running from the mouth to the anus, also known as the digestive tract.
- It consists of the mouth, pharynx, gullet, stomach, duodenum, ileum, caecum, colon, and rectum.
DIGESTION
Digestion is the process by which food is broken down into forms that can be absorbed and used by the body. It involves both mechanical and chemical breakdown.
Mechanical digestion occurs in the mouth where teeth chew food into small pieces to increase surface area for enzymes.
Chemical digestion is achieved by digestive enzymes. Digested food is absorbed and assimilated; undigested materials are egested as feces.
THE DIGESTIVE SYSTEM AND DIGESTION PROCESS
Major regions where digestion occurs are the mouth, stomach, duodenum, and ileum.
DIGESTION IN THE MOUTH
Food is chewed by teeth and mixed with saliva to form a bolus, increasing surface area for enzymatic activity.
- Saliva is alkaline, making food alkaline in the mouth.
- Secretion of saliva is controlled by the nervous system; smell, taste, sight, or thought of food stimulates saliva flow.
- Saliva is a mixture of mucus, water, salts, and the enzyme salivary amylase.
FUNCTION OF SALIVA
- Water acts as a solvent for dissolving food substances.
- Mucus lubricates food for easy swallowing.
- Salivary amylase converts starch to maltose.
The tongue rolls food into a round mass called a bolus (plural: boli), which is pushed down the esophagus.
THE PHARYNX
- The region crossing the air passage is called the glottis.
- The epiglottis prevents food from entering the windpipe (trachea).
- No digestive enzymes are present here.
THE OESOPHAGUS (GULLET)
- Tube connecting the pharynx and stomach.
- Food passes rapidly by contraction and relaxation of muscles, a process called peristalsis.
- Peristalsis moves food in the form of bolus through muscle valves called sphincters.
- No digestive enzymes are present.
DIGESTION IN THE STOMACH
- Food mixes with hydrochloric acid and gastric juice to form chyme (semi-solid mass).
- Gastric glands secrete gastric juices containing water, hydrochloric acid, mucus, and enzymes (pepsin and rennin).
FUNCTION OF HYDROCHLORIC ACID
- Provides an acidic medium for enzymes to work best.
- Hydrolyzes food into simple particles.
- Kills bacteria present in food.
Pepsin breaks down proteins into peptides.
Rennin coagulates soluble milk protein (casein) into an insoluble form for pepsin action; found mostly in young mammals.
Mucus protects the stomach lining from corrosion by hydrochloric acid.
GENERAL FUNCTIONS OF THE STOMACH
- Temporary storage of food.
- Protein digestion starts here.
- Mixes food during churning; absorbs water, alcohol, and some vitamins.
A muscle valve called the pyloric sphincter controls passage of chyme from stomach to duodenum.
DIGESTION IN THE DUODENUM
- First part of the small intestine.
- Associated with the liver and pancreas.
THE LIVER
- Secretes bile stored in the gall bladder and released through the bile duct.
- Bile is greenish-yellow and contains bile salts.
FUNCTION OF BILE
- Emulsifies fats into tiny droplets to increase surface area for enzymes.
- Provides an alkaline medium for enzymes.
- Neutralizes acidic food from the stomach.
THE PANCREAS
Secretes pancreatic juice containing:
- Pancreatic amylase – breaks down starch into maltose.
- Pancreatic lipase – digests fat droplets into fatty acids and glycerol.
- Sodium hydrogen carbonate (NaHCO3) – provides a basic medium for enzymes, neutralizing acidic chyme.
- Trypsin – breaks down proteins into peptides.
DIGESTION IN THE ILEUM (SMALL INTESTINE)
- Largest section of the alimentary canal.
- Contains secretory cells that produce mucus and intestinal juice (succus entericus).
- Intestinal juice contains enzymes:
- Erepsin or peptidase – digests peptides to amino acids.
- Maltase – breaks down maltose to glucose.
- Lipase – breaks down fat droplets to fatty acids and glycerol.
- Sucrose – breaks down sucrose to glucose.
The ileum has two main functions:
- Digests all types of food.
- Absorbs end products of digestion into the bloodstream.
Note: The ileum wall has numerous finger-like structures called villi (singular: villus) which increase surface area for digestion and absorption.
Adaption of ileum to its functions
- It is long to provide a large surface area for digestion and absorption.
- It is highly coiled to increase surface area.
- It has villi and microvilli to increase surface area for absorption.
- It has a dense network of blood capillaries to facilitate diffusion of digested materials.
ASSIMILATION
Assimilation is the process by which digestion end products are incorporated into cell metabolism.
Glucose: some is oxidized during respiration to produce energy (ATP), some is stored as glycogen in muscles, and some is converted to fats stored beneath the skin.
AMINO ACIDS
Used in protein synthesis, growth, and repair of cells. In absence of glucose and fats, they may be oxidized for energy.
FATTY ACIDS AND GLYCEROL
- Oxidized to release energy during respiration.
- Stored as adipose tissue beneath the skin, helping insulate the body.
THE CAECUM AND THE APPENDIX
- No function in humans.
- In herbivores, they contain bacteria that secrete cellulose enzyme.
THE LARGE INTESTINE (COLON)
The large intestine has four functions:
- Absorbs water from undigested materials.
- Absorbs small amounts of digested food.
- Glandular lining produces mucus to lubricate feces passage.
- Temporary storage of feces until defecation.
EGESTION
Undigested and indigestible materials, known as feces, are removed from the body through the anus by egestion.
SUMMARY OF DIGESTION
| Part of alimentary canal | Enzymes secreted | Substance digested | Product of digestion |
|---|---|---|---|
| Mouth | Salivary amylase | Carbohydrates | Maltose |
| Stomach | Pepsin | Protein | Peptides |
| Stomach | Rennin | Soluble milk protein (caseinogen) | Insoluble milk protein |
| Stomach | Trypsin | Protein | Peptides |
| Duodenum | Pancreatic amylase | Starch | Maltose |
| Ileum | Pancreatic juice | Lipids | Fatty acids and glycerol |
| Ileum | Maltase | Maltose | Glucose |
| Ileum | Sucrase | Sucrose | Glucose and fructose |
| Ileum | Lactase | Lactose | Glucose and galactose |
ABSORPTION
Absorption is the process by which soluble end products, e.g., glucose, diffuse into the bloodstream.
- Absorption mainly takes place in the small intestine; some water, soluble vitamins B and C, and soluble salts are absorbed in the stomach.
PROCESS OF ABSORPTION
- Amino acids and glucose are absorbed into the bloodstream by active transport. They diffuse into the blood and are carried by the hepatic portal vein to the liver, then to general circulation.
- Fatty acids and glycerol are absorbed into the lacteals of the villi, drain into lymphatic vessels, and finally join the bloodstream at the vena cava.
Note: The small intestine wall has numerous finger-like villi that increase surface area for digestion and absorption.
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