More about digestion

How they are digested


– broken down into monosaccharides by pancreatic amylase and can then be absorbed into bloodstream

– under construction –


Malnutrition and resulting diseases


A condition when your body does not get enough nutrients.

Dietary-related diseases:

Vitamin A

  • Night Blindness –> cannot see well in dim light
  • Foods rich in Vitamin A: Dairy products (e.g. milk, oil, butter), Liver

Vitamin B1 (Thiamine)

  • Beriberi – Symptoms (Dry beriberi):
  1. anorexia (loss of appetite)
  2. heaviness, weakness, pain and numbness in the legs
  3. loss of muscle functions (e.g. paralysis, speech difficulties)
  4. subjects feel weak and get easily exhausted
  5. confusion
  6. nystagmus (involuntary eye movement)
  7. vomiting
  • Symptoms (Wet beriberi):
  1. swollen calf muscles
  2. fast and bouncing pulse
  3. weak heart leading to heart failure
  • Foods rich in Vitamin B: Brown rice, beans, corn, nuts, yellowfin tuna

Vitamin B3 (Niacin)

  • pellagra – symptoms:
  1. High sensitivity to sunlight
  2. Aggression
  3. Dermatitis, alopecia, edema
  4. Smooth, beefy red glossitis (tongue is swollen)
  5. Red skin lesions
  6. Insomnia
  7. Weakness
  • Foods rich in niacin: Whole grains, red meat, fish, peanuts

Vitamin C

  • Scurvy – Symptoms:

a) General weakness followed by shortness of breath, pain in bones, joints and muscles of the extremities.

b) Swollen and tender joints, haemorrhages in various tissues and pain in joints.

c) Bleeding gums and loose teeth.

  • Foods rich in Vitamin C: Citrus fruits (e.g. lemon)

Vitamin D

  • Rickets – Symptoms:


  1. The child is restless, fretful and pale with flabby and toneless muscles
  2. Potbelly
  3. Development is delayed
  • Vitamin D can be produced through exposure to sunlight.
  • Certain marine fish (Salmon, trout, mackerel etc.) are good sources. Dairy products are poor sources of Vitamin D.

Lack of protein

  • Kwashiorkor – symptoms:
  1. Shiny skin, changes in hair colour/texture
  2. Large belly that protrudes
  3. Decreased muscle mass, failure to gain weight and grow
  4. Swelling (edema)
  5. Increased and more severe infections due to damaged immune system (e.g. Diarrhea, rash)
  6. Shock (late stage)Fatigue, Irritability
    • Foods rich in protein: meat, dairy products

Lack of iodine

  • goiter (abnormal enlargement of thyroid gland)
  • In childhood, it causes reduced thyroid functioning which results in retarded physical and mental growth.
  • Foods rich in iodine: fish and shellfish

Lack of iron

  • Anemia – symptoms:
  1. Low red blood cell count
  2. Tiredness
  3. Pallor (pale appearance)
  4. Rapid heart beats (sensations of pounding heartbeats)
  5. Headaches
  6. Dizziness
  7. Shortness of breath
  8. Nervousness
  9. Depression
  10. Poor concentration
  11. Weak memory
  12. Easy bruising of body parts
  13. Slow healing of wounds
  • Foods rich in iron: Red meats, egg yolks, clams, spinach, nuts, liver


  • Caused by exceeding your calorie needs frequently over a period of time, and not burning off these calories through exercise.
  • This causes an increase in muscle-to-fat ratio as the body stores unused calories.


  • Occurs when the colon absorbs too much water or if the colon’s muscle contractions are slow or sluggish, causing the stool to move through the colon too slowly. The stool then becomes hard and dry, making it harder to pass out of the body.
  • Dietary fibre prevents this by increasing stool weight and decreasing gut transit time.
  • Read more about constipation here and here about dietary fibre

Anorexia nervosa

  • An eating disorder where people have an irrational fear of gaining weight and restrict the amount of food they consume.
  • Sufferers experience excessive weight loss as their calorie intake is drastically lower than their calorie needs, causing the body to convert the fat reserves in the body to energy.

– pictures will be placed at a later date –

More about the stomach

Before 1822, little was known about the stomach until an unusual event occurred. A patient of Dr. Beaumont was left with a permanent open gastric fistula (i.e. a hole on his body leading to the stomach). This allowed Dr. Beaumont to study the stomach by inserting and removing objects directly from the stomach cavity, leading to many breakthroughs.


A very detailed interactive account of the systems in the human body, as well as the organs’ functions.


Detailed description of the digestive system and the functions of different parts + diagrams.


Nutrients and a balanced diet

An ideal diet

Made up of foods which contain a balance of macro-nutrients and micro-nutrients: (of total calorie intake)

  2. FATS (30%)
  3. PROTEINS (10-15%)
  6. WATER


They are made up of sugars and are classified according to how many sugar units are combined in 1 molecule. Monosaccharides are the building blocks of carbohydrates.


Monosaccharides Disaccharides Polysaccharides
– Simple sugars –   When 2 simple sugars combine –   more than 10 sugar units, up to 1000s
–   Glucose, fructose, galactose (in fruits, veggies etc.) –  Sucrose (Table sugar)-  Lactose (in milk/dairy)-   Maltose (in malt) –  Starch (long chains of glucose, occurs as granules)-  Cellulose (major component of cell walls)-   Main components of dietary fibre

Main roles:

  • **Provide energy
  • The construction of the body organs and nerve cells
  • The definition of a person’s biological identity such as their blood group

How is it used?
Starches and sugars are the main energy-providing carbohydrate sources.
Simple sugars are absorbed directly by the small intestine into the bloodstream, where they are then transported to their place of use. Disaccharides are broken down by digestive enzymes into simple sugars. The body also needs the help of digestive enzymes to break down the long chains of starches into their constituent sugars which are then absorbed into the bloodstream.
The human body uses carbohydrates in the form of glucose.

Storage: Converted to glycogen, a polysaccharide similar to starch, which is stored in the liver and the muscles and is a readily available source of energy for the body.

Glucose may come directly from dietary carbohydrates or from glycogen stores.

As the brain needs to use glucose as an energy source, the level of glucose in the blood must be constantly maintained above at optimum level. Several hormones, including insulin, work rapidly to regulate the flow of glucose to and from the blood to keep it at a steady level.
Why important?

  • A diet containing an optimum level of carbohydrates may help prevent body fat accumulation;
  • Starch and sugars provide readily accessible fuel for physical performance;
  • Dietary fibre, which is a carbohydrate, helps keep the bowel functioning correctly.

As carbohydrates are found in a wide range of foods, they should be supplied from diverse food sources to ensure that the overall diet contains adequate nutrients. They also contribute to the taste, texture and appearance of foods and help to make the diet more varied and enjoyable.


90% of fats in the diet and body are in the form of triglycerides. Fatty acids and glycerol are the building blocks of fats.

Types of fatty acids

Saturated Monounsaturated Polyunsaturated
Definition no double bond in carbon atom chain One double bond Two or more double bonds
Characteristics (High proportion) –  relatively high melting temperature-  tend to be solid at room temperature

– usually liquid at room temperature

When oils are heated Most stable, most resistant to oxidisation More stable and can be re-used to a greater extent than polyunsaturated Less stable, cannot be re-used too often
Examples (Rich in) Dairy, meat, margarines, butter, lard, coconut/palm oil Nuts, olives, avocados, rapeseed and their oils Omega-3-  Fish (Salmon, mackerel, trout)-  Walnuts, soybean, flax seed and their oilsOmega-6

– Sunflower seeds, sesame, walnuts, soybean, corn and their oils

All fats are made up of a combination of all 3 fatty acids, but one type will generally predominate.

**Trans fatty acids –> Some frying and baking fats (e.g. hydrogenated vegetable oils) used in biscuits, cakes and pastries, dairy products, fatty meat from beef and sheep.

Role of fats

  • Main energy store in the body
  • The most concentrated source of energy in the diet
  • The body’s fat deposits are used to meet energy demands when dietary energy is limited, for example where people have a poor appetite or during starvation. They may also be needed when energy requirements are high such as during high levels of physical activity and for growing babies and children.
  • Fat deposits cushion, protect vital organs and help insulate the body
  • Carrier for vitamins A, D, E and K, and enables their absorption. They provide the essential fatty acids, linoleic acid (omega-6) and alpha-linolenic acid (omega-3).

Fats are also a major factor causing obesity as it has low satiety potential. But, obesity is also caused by lack of exercise. Thus only reducing fats in the diet is not sufficient for weight loss.
How it is used

Fat taken in the diet is digested in the small intestine with the aid of bile acids and salts which act as emulsifiers and break down the larger fat droplets into smaller ones.

  • Bile does not contain digestive enzymes!

These are then acted upon by “lipases” which are secreted in the intestinal digestive. juices. Ultimately the fats are converted into Fatty acids and Glycerol. These are then absorbed in the form of “chylomicrons (small packages of fat and protein)” into the lymphatic channels via the lacteals and poured into the blood. From here chylomicrons are taken to liver and converted into cholesterol and other forms of fats which are transported to other parts of the body in the form of VLDL, LDL, IDL and HDL.



Made up of amino acids linked together. A typical protein may contain 300 or more amino acids.

  • Each protein has its own specific number and sequence of amino acids.
  • The shape of the molecule is important as it often determines the function of the protein.

Amino acids

Essential Non-essential
indispensable amino acids that cannot be produced during metabolism by the body–> must be provided by our diet dispensable amino acids that can be produced endogenously in the body from other proteins

Importance of proteins

  1. Essential elements for growth (in children) and repair, ensuring all living cells perform their functions
  2. Control blood sugar levels — hormones (e.g. insulin)
  3. Digestion of foods — enzymes (e.g. amylases, lipases, proteases)
  4. Fight infections (antibodies), allow contraction (muscle proteins) etc.

How are they used? – protein cycle

Proteins in our body are constantly being built and disposed of. After we eat, proteins are broken down by digestion into amino acids. Amino acids are then absorbed and used to make other proteins in the body. Adequate protein and energy intake, on a daily basis ensures the cycle continues.
Where are they found?

  1. High biological value proteins –> Animal sources (e.g. meat, poultry, fish, eggs, milk, cheese and yogurt)
  2. Low biological value proteins –>  Plants (e.g. legumes, grains, nuts, seeds and vegetables)

Vitamins, minerals


  • generally help regulate body processes.


  • provide important growth and repair functions in bones and teeth (e.g. calcium, magnesium and phosphorus)
  • maintain the composition of body fluids (e.g. sodium, chloride and potassium)


Water is a solvent for numerous biochemical molecules giving solutions and enabling:

  • Transport of nutrients, e.g. glucose and amino acids in blood, and sucrose in phloem
  • Removal of excretory products, e.g. ammonia, urea
  • Secretion of substances, e.g. hormones, digestive juices.

– under construction –


Calorie needs counter:

Factors affecting calorie needs:.

  1. Age
  2. Gender
  3. Body composition
  4. Activity level
  5. Metabolism
  6. Hereditary factors

Age: Babies need around 800 calories. Calorie needs peak at 25 and decreases as you get older –> The aging body replaces muscle with fat, which burns fewer calories than muscle does.

Gender: Men generally need more calories than women. Women require more calories during pregnancy.

Body composition: People with more muscles will burn more calories.

Activity level: If you do more exercise, you will burn more calories.




Reflection questions:

Were your predictions correct? What might have accounted for any differences between your predictions and the actual values?
No, my predictions were not correct. I had the misconception that chicken contained more calories compared to grains.

EvaluateHow might multiple trials improve the accuracy of your results?
They will remove human error and make the results more reliable.

ApplyHow can this information be used to help plan a healthful diet?
We can choose foods with more calories if we are doing vigorous activities and vice versa, so that we will not excessively store fats in our bodies.

Evaluate–What are some other pieces of information about food that this experiment did not provide? What else do you need to know to make healthful decisions about your diet?
The vitamins, minerals and fibre content in the foods. Besides these, we also need to know our activity level.

Compare and Contrast–What is the relationship between the release of energy in the form of heat and Calories?
The greater the release of energy in the form of heat, the greater the number of calories.

Digestive System


  • Feeding (or ingestion)
  • Digestion: breakdown of large food particles into soluble, diffusible molecules (so that the cells can absorb them)
  • Absorption: cells taking in digested food particles
  • Assimilation: food materials converted into new parts for the cell or used to provide energy

Five Stages of Human Nutrition

  • Ingestion: placing food into the alimentary canal at the mouth.
  • Digestion: breakdown of complex food into their simple soluble absorbable subunits.
  • Absorption: the passage of the products of digestion into the blood or lymph.
  • Assimilation: conversion of the absorbed nutrients into complex molecules for growth, repair and defence.
  • Egestion: expulsion of the undigested and unabsorbed material from the alimentary canal.

From Ingestion to Egestion – The Simplified Diagram

Now, about Digestion…

Major Functions of Digestive System

Digestion: breakdown of complex food into their simple, soluble and absorbable sub-units.

  • Physical processing: by breaking it up (chewing), mixing, adding fluid etc.
  • Chemical processing: by adding digestive enzymes to split large food molecules into smaller ones.

Absorption: the passage of the products of digestion into the blood or lymph.
Movement of food (Transportation): controlled by sphincter muscles, longitudinal and circular muscles in the gut wall.

Digestion is an enzyme-catalysed process

– Proteins called enzymes cause the rate of production of “products” (e.g. glucose, nutrients needed by body) to be much greater

– In other words, it catalyses chemical reactions. Read more here

Useful links for revision:

National Geographic


Animated digestive system

Group work:



The digestive process


Explain the length of the various parts of the digestive system
Esophagus → It has to squeeze food past our lungs and into the stomach, hence its length
Small intestine → It is the major site of chemical digestion which takes a long time to carry out, so it has the greatest length to allow complete digestion of the bolus before it leaves.
Large intestine → Its length maximises the amount of water and minerals absorbed into the bloodstream. As most of the nutrients have already been absorbed in the small intestine, the large intestine has an easier job and is comparatively shorter.

How is the digestive system adapted to suit its function of absorption?


  • Highly folded mucous membrane (mucosa). Allows for expansion during digestion.
  • Gastric pits secrete digestive juices to break down the bolus into chyme.
  • Muscular walls contract to mix bolus with the digestive juices.
  • Mucosa prevents the acidic digestive juices from destroying the stomach walls.
  • These maximise the amount of food digested and make digestion more efficient. This allows for better absorption of nutrients from the food.

Small intestine

Increased surface area -> rate of diffusion

  • Highly folded mucosa arranged in villi. Epithelial cells lining the small intestine have a folded cell membrane, microvilli. This increases surface area to volume ratio to allow for effective absorption of nutrients.
  • Long –> Maximises absorption of nutrients.

Increased rate of diffusion

  • Thin epithelial wall – The permeable villi lining its walls are only one cell thick,  allowing for quick diffusion of nutrients into the capillaries in the walls.
  • Dense network of thin-walled blood capillaries, creating a constant steep diffusion gradient. Allows for quick transportation throughout the body as well.
  • Circular and longitudinal muscles which allow bi-directional peristalsis. This improves the amount of food digested.

Why do we need food?

We need the substances in food to live and grow. (stated below)

Why do we need to digest food?

We need to break down the food into useable substances, such as:

  1. proteins needed for growth, bodily functions (e.g. cell repair, RBC production)
  2. nutrients, vitamins, minerals to keep our organs healthy, for growth and bodily functions (e.g. vitamins)
  3. glucose for respiration to provide energy for bodily functions (e.g. muscle movement)