All organisms can be classified as either being:
Autotrophic- Meaning they can make their own food. Most autotrophic organism make their own food by photosynthesis. They use carbon dioxide, water, and the sun’s energy to accomplish this.
Heterotrophic- Meaning the organism obtains food from the environment. Heterotrophs can be further catagorised as being parasitic if they obtain nutrition from a live host or saprophytic if they obtain their nutrition from dead organisms.
Saprophytes are herbivores if they live off of plant matter, carnivores if they live off of animal matter, or omnivores if they obtain their nutrition from both.
Some lower animals have no digestive system. Each cell of their body contains digestive enzymes that break down certain types of food. Examples of these enzymes in humans are:
Amylase- Digests starch
Protease- Digests protein
Lipase- Digests lipids or fats
The Digestive System are the parts of the body that takes in the nutrition and then breaks it down so that our body cells can use it. Each body part has a specific role to play in digestion and use of food. In this way the nutrition is usable when transported to the cells. It consists of the alimentary canal. This is the long tube within out bodies that starts with the mouth and ending with the anus. There are also glands such as the salivary glands, the liver, and the pancreas that are attached to the alimentary canal.
The major functions of the digestive system are:
breakdown of complex food into their simple soluble absorbable sub-units.
Absorption: the passage of the products of digestion into the blood or lymph.
Movement of Food: controlled by sphincter muscles, longitudinal and circular muscles in the gut wall.
In order to accomplish its functions the digestive system begins with the mouth where food enters. This process is called ingestion. In this process food is placed into the alimentary canal.
After the food enters the alimentary canal it must be digested. This is the breakdown of complex food into their simple soluble absorbable subunits.
They must be broken down so that they can be absorbed into the bloodstream and then taken to the body cells. This process is called absorption. Through absorption the products of digestion enter into the blood or lymph.
After the food is absorbed the nutrients are brought to the body cells. Here the process of assimilation converts the absorbed nutrients into complex molecules for growth, repair and defence.
Finally the waste products which remain behind must be excluded from the body. This is done by the process of egestion.
To review: The five stages of human nutrition are:
Parts of the Digestive System
The mouth is where food is mechanically digested. Here, the teeth physically break down the food. The food is broken down so that it can be easier to swallow and has a greater surface are for enzymes to act on.
Starting from the front of the mouth the four types are:
incisors — These are chisel shaped teeth and are used for cutting, slicing and biting.
canines — These are long, pointed teeth that resemble fangs. They grip, stab and tear food.
premolars —These are teeth with projections on the surface called cusps. They crush and chew food.
molars — These are large teeth located in the back of the mouth. They also crush and chew food.
When we are young we have a first set of teeth called milk teeth. These eventually fall out and our permanent teeth grow in. There are 32 permanent teeth and 20 milk teeth. There are no molars in the set of milk teeth.
The first teeth to appear at around six months of age are usually the two lower central incisors - or ‘cutting teeth’. These are followed by the two upper central incisors. Once the teeth start to come through, they appear at the rate of around one a month. So by his first birthday, your baby may have six teeth.
Four central incisors (cutters) start appearing at around 6-7 months
Four more side incisors appear at around 8 months
Four back molars (chewing teeth) appear at between 10-14 months
Four canines (pointed teeth) appear at around 16-20 months.
Four more back molars appear at between 24-30 months.
The mouth contain the enzyme amylase (also called ptyalin). This enzyme is contained in the saliva. The saliva is secreted by 3 salivary pairs of salivary glands located under the tongue, at the back of the jaws, and in the cheeks. (See diagram below) Saliva also contains lysozyme which helps destroy micro-organisms. The saliva helps to soften the food so that it is easy to swallow. Amylase digests starch into maltose.
After the food is chewed it is formed into a ball or bolus and pushed back to the pharynx. This is where the food will leave the mouth and enter the oesophagus.. To stop food from entering the trachea (or windpipe) there is a small flap called the epiglottis. This flap closes over the trachea to stop food from entering.
The initial stages of eating and swallowing are under voluntary control. This means that it is governed by the brain.
Once food enters the mouth the teeth break it down into smaller and smaller pieces. This has the dual function of making the food easier to swallow and increasing the surface area of food on which the saliva can act. The tongue, lips and cheeks assist the teeth in the process by allowing the food to be "rolled" around the oral cavity. The mechanical action described above produces a softened bolus of food which is now ready to be swallowed.
Here the bolus is pushed into the upper part of the pharynx by the action of the tongue.
As the bolus pushes it's way into the oesophagus it automatically pushes the epiglottis downwards further closing off the airway.
The bolus then enters the oesophagus.
Click here to see a full animation of swallowing with explanations.
The oesophagus connects the mouth with the stomach. Food moves through the oesophagus by involuntary wavelike muscular contraction called peristalsis.
Peristalsis does not only occur in the oesophagus. It continues through the stomach, small intestine, and large intestine. Finally, peristalsis forces food into the rectum. Here it is ready to be discharged out of the anus.
Click here to view an animation of peristalsis
The stomach is a muscular sac that stores and digests food. There is a sphincter muscle at the end of the oesophagus that opens and closes to allow food into the stomach and to stop food from going back up into the oesophagus.
The stomach lining is called the mucosa. This mucosa has many folds in it forming millions of gastric glands. These glands secrete gastric juice. This liquid consists of:
1. Mucous- this is produced by goblet or mucous cells of the mucosa. This substance prevents the stomach from digesting itself.
2. Pepsinogen- This is produced by zymogen or chief cells. Pepseinogen is converted into pepsin by acid in the stomach. Pepsin converts protein into smaller peptides. Pepsin is said to be a protease- i.e. an enzyme that digests protein.
3. Hydrochloric Acid (HCl)- This is made by oxyntic or parietal cells. This gives the stomach a pH of 1 to 2. This acid kills many bacteria, loosens food, converts pepsinogen into pepsin, and denatures salivary amylase. When this acid is overproduced it sometimes rises up into the oesophagus. This causes heartburn. Neutralising the acid with a base such as Rennies can control this condition.
Below is a microscopic view of the above diagram:
As the stomach wall contracts the food is churned and thus digested mechanically. The food becomes a thick milky mixture called chyme. This chyme will leave the stomach and go into the small intestine. To do so, it goes through the pyloric sphincter.
#1 is the oesophagus, #2 is the stomach, #4 is the pyloric sphincter, #3 is the duodenum (the start of the small intestine).
The cells of the stomach are replaced very rapidly. ½ million are produced per minute. In this way, damaged stomach cells are constantly being replaced.
If the defence mechanisms to stop self-digestion of the stomach fail, then peptic ulcers of the stomach form. These cause bleeding and may dissolve the stomach lining all the way through the stomach wall. It is now called a perforated ulcer. This is very dangerous as bacteria may travel through the ulcer and into the body cavity. In some cases it can cause death.
The small intestine is where the food is absorbed into the blood to be taken to the body cells. The small intestine is about 6 metres long. It is made up of 3 parts. The duodenum, jejunum, and ileum.
1. The duodenum- The main function of the duodenum is to finish the digestive process. The cells lining the duodenum produce digestive enzymes. Also, products from the pancreas and the liver enter the duodenum to do their work.
The lining of the small intestine consists of many folds called villi. Each villus has about 600 microvilli. These foldings and microvilli increase the surface area of the small intestine for increased digestion or absorption.
Intestinal glands are located between the villi. These glands produce numerous enzymes, which are collectively called intestinal juices.
The intestinal gland is at the arrow, which is at the base of a villus.
The Pancreas secretes pancreatic juice for the digestive system. It consists of:
1. The salt sodium hydrogen carbonate which neutralises chyme in the stomach.
2. The digestive enzymes:
A. Amylase which changes starch to maltose
B. Lipase which changes lipids to fatty acids and glycerol.
These enzymes enter the and do their digestive work at the duodenum. They enter the duodenum through the pancreatic duct.
There are many functions of the liver. Some of the most important are:
1. The production of bile
2. Detoxifying the body, i.e. breaking down alcohol and drugs
3. Breaking down excess amino acids to form urea
4. Converting glucose to glycogen for storage
5. Converting excess carbohydrates to fats
6. Storing vitamins
7. Storing minerals such as Iron, Copper, and Zinc.
8. Making plasma proteins such as fibrinogen which is used in blood clotting
9. Making cholesterol which is used to form many hormones
10. Producing heat for the blood and body
Bile is a yellow-green liquid that is produced in the liver and stored in the gall bladder. Bile enters the duodenum through the bile duct. Bile has the following functions:
1. Emulsifies lipids- It breaks down large fat and oil molecules into tiny droplets. In this way, the surface area of the lipids is increased and enzyme action is more efficient.
2. Helps neutralise chyme in the stomach with its sodium hydrogen carbonate.
3. It excretes the pigments biliverdin and bilirubi which are made from dead red blood cells.
By the time the food enters this part of the small intestine it is fully digested. The job of the jejunum and ileum is to absorb the food. The lining of these sections of the small intestine contain many villi.The walls of the villi are only 1 cell thick. There is a rich supply of blood located inside each villus. The capillaries of each villus absorb nutrients from the food and, in this way, the food gets into the blood supply. Foods such as glucose, amino acids, vitamins, and minerals are absorbed into the blood here. The food is then taken to the hepatic portal vein which takes it to the liver. Here the food is stored and released when needed by the body. Amino acids are not stored but, those not used by the body, are broken down by the liver and form urea. This process is called deamination. Wastes created at the liver go from the liver to the hepatic vein and then to the kidneys. Here they are excreted.
Fats (fatty acids and glycerols) are absorbed into the villis’ lacteals. The lacteals have lymph fluid in them. (see diagram below) From the lacteal the fats are transported by the lymph and carried to the bloodstream at the subclavian veins near the base of the neck.
The large intestine is the last part of the alimentary canal. Here is where the final phase of the nutrition process occurs.
The large intestine is larger in diameter than the small intestine. That is why it is called “large”. It has a diameter of about 6 cm. As compared to the small intestine’s diameter of about 3 cm. It’s length is about 1.5 metres long while the small intestine is about 6 metres long. Food stays in the large intestine for varying lengths of time from 10 hours to as long as a few days.
The caecum is right below the junction of the large and small intestine. The appendix is at the end of the caecum. The function of them in humans is not known. In herbivores they serve as an area where bacteria exist and aid in the digestion of cellulose. The organs are said to be vestigal organs. They have lost their prior use. If bacteria do grow in the appendix or there is a faecal blockage pain is produced in the appendix. If not treated the appendix may burst and cause infection the the abdominal lining. This condition is called peritonitis. Usually the appendix is removed to avoid this serious condition.
The colon absorbs water from the remaining food that has past through the alimentary canal. The waste then becomes a semi-solid called faeces. Faeces are stored in the rectum and then egested through the anus.
Bacteria that live in the colon feed on the waste and produce some B vitamins as well as Vitamin K. These vitamins are absorbed into the blood stream at the colon. There are other bacteria in the digestive system that help break down food, especially cellulose. This is a symbiotic relationship. The bacteria obtain nourishment and we obtain vitamins and digested food.
A balanced diet must have seven components:
From the food we get energy for our bodies to function and protein for growth. The energy content is measured in joules (J) or kilojoules (kJ). The old units of energy were calories or kilocalories. 1 calories = 4.2 joules. Fats contain about twice as many joules as carbohydrates.
There are four food groups. The foods in each food group contain similar nutrients. See the table below for the food groups and the suggested servings per day for each food group.
Two common eating disorders are anorexia nervosa and bulimia nervosa. In each of these the person has an excessive desire to be thin and to control their weight and shape.
Anorexia nervosa has symptoms that include severe weight loss, a fear of being fat, and an over interest in food and weight. Bulimia nervosa involves binge eating, self-induced vomiting, and periods of starvation. Treatment for both of these disorders involves psychological counselling and education for both the victims and their families.
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