digestion in humans
TRANSCRIPT
Digestion in Human Beings
26.1 Why We Need Food?• We need food to:
o provide us with energy for our daily activities like walking, and to maintain our body temperature;
o maintain a healthy body;o grow new cells and tissues; and o repair worn out tissues.
26.2 What is in the Food We Eat?
• You need the essential nutrients in the food you eat.
• The types of nutrients can be found in the food label on a food package.
• The food label lists the nutrient content of the food.
Main types of nutrients
• There are three main types of nutrients: carbohydrates, proteins and fats.
• Carbohydrateso The main carbohydrates in food are starch and
sugars. Starch can be found in rice, bread, noodles and potatoes.
o Sucrose, glucose and maltose are also forms of sugars. They can be found in fruits like bananas and apples
o Cellulose is another type of carbohydrate, which can be found in plants. Cellulose make up part of the fibre (roughage) that cannot be digested by the body but instead passed out from the body.
Main types of nutrients
• Proteinso Proteins are very large molecules that are
made up of several small molecules called amino acids.
o Proteins are needed to:• build new cells for body growth and for the repair of
worn-out tissues.• make more complex proteins such as enzymes that
carry essential functions in the body. o Proteins can be found in meat, fish and eggs.
Main types of nutrients
• Fatso Fats are large insoluble molecules that are
made up of glycerol and fatty acids.o Fats provide us with twice as much energy as
carbohydrates.o Fats can be found in food like butter and
cheese.o Fats are stored under our skin to insulate our
body against too much heat loss.
26.3 Why Must Food be Digested?
• We can only use the nutrients in the food we eat when they pass through the gut walls into the blood vessels.
• Here, the nutrients can be carried through the bloodstream to all parts of the body.
• The gut walls and blood vessels are made up of cells. • The cell membrane has small openings (or pores) that
allow small molecules to enter, but not large molecules. • Nutrients like glucose and amino acids are small
molecules. They can pass through the cell membranes easily and enter the bloodstream.
26.3 Why Must Food be Digested?
• But most major nutrients in food are large molecules which cannot pass through the cell membranes.
• They must be broken down into molecules that are small enough to pass through the cell membranes.
• This process is called digestion.• Digestion is the breaking down of large, complex
food molecules into small, simpler molecules. • Our body is able to carry out digestion by producing
complex proteins called enzymes. • The enzymes involved in digestion are called
digestive enzymes.
Nutrients like starch, proteins and fats, are large, complex molecules. They cannot pass through the cell membrane.
Nutrients like glucose and amino acids, are small, soluble molecules. They can pass through the cell membrane.
26.4 What are Enzymes?• Enzymes are complex proteins that speed up
the rate of chemical reactions. Enzymes remain unchanged at the end of chemical reactions.
• Enzymes act like chemical ‘scissors’.• They break down large molecules into small
molecules to speed up the process of digestion.• Substrates – substances that are transformed
with the help of enzymes
“Lock and key” hypothesis
active sites
Enzyme molecule
Food molecule
Enzyme-food complex
Enzyme free to take part in next reaction.
Two products leave the enzyme.
Product X
Product Y
“Lock-and-key” model of enzyme action
• Emil Fischer – a nobel laureate in Organic Chemistry in 1894
• “both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly to each other.”
• enzymes are never wasted, but are always recycled
Digestion in Man
Digestion in Man
• mechanical phase• chemical phase• digested food is absorbed by the body in
coordination with the circulatory and lymphatic systems
• undigested materials pass through the anus into the external environment
Mechanical Phase of Digestion
• involves the change in the physical properties of food.
• facilitated by peristalsis
Steps during mechanical digestion:1.Mastication – cutting and chewing of food with the use of the teeth•four kinds of teeth
Steps during mechanical digestion:2.Saliva produced from three pairs of salivary glands moistens the food
Steps during mechanical digestion:3.The tongue mixes food with saliva. The back of the tongue secretes mucus which makes the food easier to swallow•Deglutition – the act of swallowing
Steps during mechanical digestion:4.The food tube churns and mixes food with digestive juices in the stomach and small intestine (kneading).
Chemical Phase
• there is a change in the chemical composition of food
Types of digestive enzymes
• A particular enzyme can only bring about one type of chemical reaction.
• For example, enzymes that break down proteins cannot break down starch or fats.
Class of enzyme
Acts on Digested products(s)
Amylase Starch Maltose (a complex sugar)Maltase Maltose Glucose (simple sugar)Protease Proteins Amino acids
Lipase Fats Fatty acids and glycerol
Main Enzymes
• Carbohydrases – digest carbohydrates (such as starches and double sugars) that produces are simple sugars
• Proteases – digest proteins that produces amino acids
• Lipase – digests fats which are also called lipids that produces fatty acids and glycerol
Chemical Digestion of Carbohydrates
Organs, Glands and Enzymes
Carbohydrates Digestion Products
Salivary glands Amylase or ptyalin
Starch (amylum) Maltose
Pancreas Amylase or amylopsin
Starch (amylum) Maltose
Intestinal glands Maltase Sucrase Lactase
MaltoseSucroseLactose
GlucoseGlucose, fructoseGlucose, galactose
Chemical Digestion of ProteinsSite of
DigestionDigestive
JuicesSubstrate Products
Stomach Gastric juicesHydrochloric acid pepsin rennin (in infants)
PepsinogenProteinMilk protein
PepsinPolypeptidesPolypeptides
Intestine Pancreatic and Intestinal juice trypsin peptidases
ProteinPolypeptides
PolypeptidesAmino acids
Chemical Digestion of Fat• Liver – largest gland in the body; secretes bile
which is stored in the gallbladder• Bile is secreted by the time food is present in the
duodenum.• Bile has no enzyme.• Bile changes fat into tiny droplets through the
process of emulsification.• This increases the surface area of the oil and
allows the fats to be digested quickly by the lipases in the pancreatic and intestinal juices.
Emulsification
Bile
Large oil drop
Small oil droplets
Absorption of Digested Food• Absorption – the process by which
substances are taken in by the cells of the food tube
• final digestion of food takes place in the small intestine covered by very small projects call villu, absorb digested food
• Two kinds of vessels: capillaries & lacteals
Structure of a Villi
• Digested food in the form of molecules of amino acids, simple sugars, fatty acids and glycerol diffuse into the capillaries and reach the blood.
• Molecules of fatty acids diffuse into the lacteals and reach another circulating fluid, the lymph.
• food tube villu lymph & blood (circulatory system)
• circulating liquids distribute the digested food to all cells of the body.
Absorption of Digested Food
• Stomach – absorbs alcohol, water and minerals
• Large intestine – absorbs excess water
Absorption of Digested Food
Synthesis
• How does the digestive system interact with the excretory, respiratory and circulatory system?
• What is the importance of food and digestion in humans?
AssignmentA. What are the diseases of the digestive system?
B. Study for a quiz on Tuesday, Feb. 16.Topic: Digestion in Human BeingsC. Long Test on Thursday, Feb. 18. Topics: Modern Periodic Table – Digestive System
Disease Symptoms Cause Treatment
26.5 The Human Digestive System
• Food is digested in our body through the digestive system.
• The digestive system is made up a long tube called the gut (or alimentary canal).
• The gut is about nine metres long. The wall of the gut is muscular, which allows food to move along its length.
• Food enters the body through the mouth, and undigested food leaves the body through the anus.
26.5 The Human Digestive System
• Glands connected to the gut are organs which produce special juices containing enzymes.
• These glands are the salivary glands, liver and pancreas.
26.5 The Human Digestive System
salivary glandmouth cavity
oesophagus
stomach
salivary glands
pancreas
colon
rectumanus
large intestine
liver
gall bladder
small intestine
The mouth
food
oesophaguswindpipe
The mouth
• Food is chewed in the mouth with the teeth.
• Chewing helps to cut and grind the food the smaller pieces.
• This increases the surface area and allows the food to be digested faster.
• As food is chewed, salivary glands in the mouth secrete saliva.
The mouth
• Saliva serves two purposes:o Wets the food, so that it is easier to swallow.o Digests starch into sugars with the help of an
enzyme called salivary amylase. • While chewing food, the tongue rolls the
food into small balls when are pushed to the back of the mouth and squeezed into the oesophagus. This is known as swallowing.
The oesophagus
Muscles contract to push the food ball down.
Muscles relax, and the tube widens for food to move.
The oesophagus
• The oesophagus is a long muscular tube leading to the stomach.
• By contracting and relaxing, the muscles help to push the food down to the stomach.
• This is how food moves along the rest of the gut too.
• No digestion occurs in the oesophagus. • However, the digestion of starch by amylase
may continue as the food moves to the stomach.
The stomach
stomach
The stomach• The stomach is a muscular bag that lies in the upper
part of the abdomen.• Its muscles contract and relax, causing food to break
up into even smaller pieces.• This movement also mixes the food well with gastric
juice for better digestion.• Gastric juice is secreted by glands in the stomach
walls, into the stomach cavity. It contains:o proteases which digest proteins; ando hydrochloric acid, which helps proteases to work.
• Hydrochloric acid kills any bacteria in the food.• Food stays in the stomach for a few hours before
passing into the small intestine, bit by bit.
The small intestine, liver and pancreas
gall bladder
liver
small intestine
pancreas
The small intestine
• The small intestine is a long muscular tube, which is about 6 m long.
• The liver and the pancreas are connected to the small intestine.
• Food is mixed with 3 fluids in the small intestine to aid digestion:o Intestinal juice from the walls of the intestine. It
contains the enzymes maltase, proteases and lipases.
o Pancreatic juice from the pancreas.o Bile from the liver.
Pancreas
• The pancreas produces alkaline pancreatic juice.
• The juice contains the enzymes amylase, protease and lipase.
• The digestion of food in the small intestine are as follows:o Digestion of fats:
• Fat Fatty acids and glycerol lipase
Pancreas
• The digestion of food in the small intestine are as follows:o Digestion of starch:
• Starch Maltose
• Maltose Glucoseo Digestion of small protein molecules
• Protein molecules amino acids
amylase in pancreatic juice
maltase
protease
Liver
• The liver produces a yellowish-green fluid called bile.• Bile is stored in the gall bladder.• The gall bladder has a duct (a small tube) that carries
bile into the small intestine.• Bile does not contain digestive enzymes, but helps to
break up fast into smaller oil droplets in a process known as emulsification.
• This increases the surface area of the oil and allows the fats to be digested quickly by the lipases in the pancreatic and intestinal juices.
Emulsification
Bile
Large oil drop
Small oil droplets
Absorption in the small intestine
• The small intestine allows only small molecules like sugar and amino acids to pass through its wall and into the bloodstream.
• Large molecules like starch and proteins cannot pass through the walls of the small intestine.
• Digestion ends in the small intestine. • The final products of digestion are glucose,
amino acids, fatty acids and glycerol.
Absorption in the small intestine
• After digestion, the smaller digested food molecules can pass through the wall of the small intestine and into the bloodstream.
• Undigested matter is mostly made up of fibre. • Together with water and mineral salts, the
undigested food passes into the large intestine.• Fibre can be found in vegetables. It is important
for the gut muscles to move the food along the gut.
The large intestine
large intestine
rectumanus
The large intestine
• The large intestine is about 1.5 m long. • Its function is to absorb water and mineral
salts.• This takes place in the colon.• What is left now is a nearly solid waste
called faeces. • This is temporarily stored in the rectum,
before being expelled through the anus is a process called egestion.
Miscellaneous• http://kidshealth.org/kid/body/digest_noSW.html• http://www.ahealthyme.com/topic/digestivesyste
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