Exchange Flashcards
What Is The Human Digestive System?
The human digestive system is made up of a long muscular tube and its associated glands.
These glands produce enzymes that hydrolyse large molecules into small molecules, ready for absorption.
The digestive system is, therefore, an exchange surface through which food substances are absorbed.
Major Parts Of The Digestive System?
- Oesophagus (carries food from mouth to stomach),
- Stomach,
- Ileum (small intestine),
- Large intestine,
- Rectum,
- Salivary glands,
- Pancreas.
Stomach?
Muscular sac with an inner layer that produces enzymes.
Its role is to store and digest food, especially proteins. It has a glance that produce enzymes which digests protein.
Ileum?
Another word for small intestine.
The ileum is a long, muscular tube.
Food is further digested in the ileum by:
- enzymes that are produced in the ileum walls and,
- by glands that pour secretions into ileum.
The inner walls of the ileum are folded into villi, which gives them a larger surface area.
The surface area of these villi is further increased by millions of tiny projections, called microvilli, on the epithelial cells of each villus.
The products of digestion are absorbed into bloodstream by ileum - the villus making this more efficient.
Large Intestine?
Absorbs water.
Most of the water that is absorbed is from the secretions of the many digestive glands.
Rectum?
Final section of the intestines.
The faeces is stored here before being removed by the anus in a process called egestion.
Salivary Glands?
The salivary glands are situated near the mouth.
They pass their secretions via a duct into the mouth.
These secretions contain the enzyme amylase, which hydrolyses starch into maltose.
Pancreas?
The pancreas is a large gland situated below the stomach.
It produces secretions called pancreatic juice.
This secretion contains:
- protease to hydrolyse proteins,
- lipase to hydrolyse lipids,
- and amylase to hydrolyse starch.
What Is Digestion?
- Physical breakdown,
- Chemical digestion.
Hint:
One purpose of digestion is to break down food into molecules that are small enough to pass across cell-surface membranes
Psychical Breakdown?
There are two types of physical breakdown in digestion: the teeth and stomach.
- Food is broken down by the teeth in the mouth. This makes it possible to ingest the food through the oesophagus and also provides a large surface area for chemical digestions.
- Food is churned by the muscles in the stomach wall and this also physically breaks it up.
Chemical Digestion?
Chemical digestion: hydrolyses large, insoluble molecules into smaller, soluble molecules.
All digestive enzymes function by hydrolysis (addition of water).
Enzymes are specific so more than one enzyme is needed to hydrolyse a large molecule.
Usually, one enzyme hydrolyses a large molecule into sections and the sections are then hydrolysed into smaller molecules other enzymes.
There are three different types of digestive enzymes which you should know:
- Carbohydrase,
- Lipase,
- Potease.
Carbohydrase?
A digestive enzyme.
Hydrolyses carbohydrates into monosaccharides.
Lipase?
A digestive enzyme.
Hydrolyses lipids (fat and oils) into glycerol and fatty acids.
Protease?
Digestive enzyme.
Hydrolyses proteins into amino acids.
Carbohydrate Digestion?
It usually takes more than one enzyme to completely hydrolyse a large molecule.
Usually, one enzyme hydrolyses the molecules into smaller sections and then other enzymes hydrolyse the sections further into their monomers.
These enzymes are usually produced in different parts of the digestive system. It is important that enzymes are added to the food in the correct sequence. This is true of starch (carbohydrate) digestion.
Steps Of Carbohydrate Digestion?
Carbohydrate = starch.
- Saliva enters the mouth from salivary glands and is mixed with food during chewing. Saliva contains amylase which hydrolyses any starch in the food to maltose (a disaccharide). Amylase does this by hydrolysing the alternate glycosidic bonds of the starch molecule to produce the disaccharide: maltose.
Saliva also contains mineral salts that help to maintain the pH at around neutral. This is the optimum pH for the salivary amylase to work. Amylase is produced by salivary glands in mouth and also by pancreas which releases amylase into small intestine.
- The food is swallowed and enters the stomach, where the conditions are acidic. This acidic conditions denatures the amylase and prevents further hydrolysis of the starch.
- After a while, the food is passed into the small intestine. Membrane-bound disacchardiase are enzymes attached to cell-membranes of epethilial cells in the ileum (in small intestine). They break down disaccharides into monosaccharides. Here, it mixes with the secretion from the pancreas called pancreatic juice.
- The pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch into maltose. Alkaline salts are produced by both the pancreas and the intestinal wall to maintain the pH at around neutral so that the amylase can function.
- Muscles in the small intestine wall push food along the ileum. The epithelial lining produces the disaccharidASE: maltase.
Maltase is not released into the lumen of the lieum but is part of the cell-surface membranes of the epithelial cells that line the ileum. It is therefore referred to as a membrane-bounce disaccharidASE. The maltase hydrolyses the maltose from starch breakdown into a-glucose (monosaccharide).
Monosaccharides are transported across cell membranes on the ileum epithelial cells via specific transporter proteins.
How Does Amylase Break Down Starch?
Amylase is produced in the mouth (salivary amylase) and pancreas (pancreatic amylase).
Amylase breaks down starch by hydrolysing the alternate glycosidic bonds of the starch molecule to produce the disaccharide: maltose.
The maltose is then hydrolysed by maltase into the monosaccharide: a-glucose.
Maltase (dissacharidASE) is produced by the lining of the ileum.
What Disaccharides In The Diet Are Broken Down?
- Maltose,
- Sucrose,
- Lactose.
Sucrose and lactose are also both hydrolysed by a membrane-bound disaccharidASE,
- SucrASE hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolyse hydrolysis produces the two monosaccharides: glucose and fructose.
- LactASE hydrolyses the single glycosidic bond in the lactose molecule. This hydrolysis produces the two monosaccharides: glucose and galactose.
Lipid Digestion?
Lipids are hydrolysed by enzymes called lipase. This involves the hydrolysis of ester bonds in lipids.
Lipase enzymes are produced in the pancreas and hydrolyse the ester bond found in triglycerides to from fatty acids and monoglycerides. They work in the pancreas.
A monoglyceride is a glycerol molecule with a single fatty acid molecule attached.
Steps:
1. Bile salts are produced in the liver and emulsify lipids - they cause the lipids to form small droplets.
- Bile salts are really important in the process of lipid digestion. Several small lipid droplets have a bigger surface area than a single large droplet (for the same volume of lipid). So the formation of small droplets greatly increased the SA of lipid that’s available for lipase to work on.
- The lipid is then digested (broken down) by lipase. Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles.
Bile is produced in the liver, then stored in the gallbladder until it is released into the small intestine where it binds to lipid droplets.
What Is A Monoglyceride?
A monoglyceride is a glycerol molecule with a single fatty acid molecule attached.
Examples of disaccharides and monosaccharides?
Disaccharides- maltose, sucrose and lactose.
Maltose is broken down by maltase —> monosaccharide: glucose + glucose
Sucrose is broken down by sucrase —> monosaccharide: glucose + fructose
Lactose is broken down by lactase —> monosaccharide: glucose + galactose
How To Know If Something Is An Enyzme Or A Substrate?
Enzymes end in -ase.
E.g. lipase is the enzyme for the substrate, lipids.
Protein Digestion?
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called protease (also known as peptides).
There are a number of different protease:
- Endopeptidase,
- Exopeptidase,
- Dipeptidase.
Endopeptidase?
It’s a protease that breaks down proteins.
Hydrolyses the peptide bonds between the amino acids in the protein molecule.
Forms a series of peptide molecules.
Trypsin and chymotrypsun are two example of endopeptidase. They’re synthesised in the pancreas and secreted into the small intestine.
Pepsin is another example. It’s released into the stomach by cells in the stomach lining. It only works in acid idc conditions - provided by hydrochloric acid in stomach.
Exopeptidase?
It’s a protease that breaks down proteins.
Hydrolyses the peptide bonds on the terminal amino acid of the peptide molecules formed by endopeptidase.
In this way, they progressively release dipeptides and single amino acids.
Dipeptidase are examples of these enzymes that work specifically on dipeptides. They act to separate two amino acids that make up a dipeptide by hydrolysing the peptide bonds between them. They are often located in cell-surface membrane of epithelial cells in the small intestine.
Dipeptidase?
It’s a protease that breaks down proteins.
Hydrolyses the bond between the two amino acids of a dipeptide.
Dipeptidase are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
Structure Of The Ileum?
The ileum’s structure is adapted to absorbs the products of digestion.
Villi allow the ileum to be adapted. What are villi?
The wall of the ileum is folded into fingerlike projections, about 1 mm long, called villi.
The villi is lined with epithelial cells with further microvilli on them. Behind these epithelial cells are lots of blood capillaries.
The villi and microvilli increases the surface area of the ileum and therefore accelerates the rate of absorption.
Villi are situated between the lumen (cavity - considered outside the body) of the intestines and the blood/tissues inside the body.
They are part of a specialised exchange surface adapt for the absorption of the products of digestions.
Their properties increase the efficiency of absorption.
How Do Villi Increase Efficiency Of Absorption?
- Increase the surface area for diffusion,
- They are thin walled so they reduce the distance over which diffusion takes place,
- They contain muscle and are able to move. This helps to maintain diffusion gradients because the villi can absorb material from food and then move that material away. New, rich in material food then takes the place and the diffusion gradient is maintained.
- They are well supplied with blood vessels so the blood can carry away absorbed molecules and hence maintain a diffusion gradient.
- The epithelial cells lining the villi possesses microvilli. These are fingerlike projections of the cell surface membrane that further increase the surface area for absorption, thus increasing rate of absorption.
Absorption Of Monosaccharides?
Glucose is absorbed by active transport with sodium ions via a co-transported protein.
Galactose is absorbed in the same way using the same co-transporter protein.
Fructose is absorbed via facilitated diffusion through a different transporter protein.
Absorption Of Triglycerides?
Triglycerides break down into monoglycerides and fatty acids.
- Monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets.
The monoglycerides and fatty acids in association with bile salts are called micelles.
Micelles are tiny, being around 4-7 nm in diameter.
- Through the movement of material within the lumen of ileum, the micelles come into contact with the epithelial cells lining the villi of the ileum.
- Here, the micelles breakdown, releasing the monoglycerides and fatty acids. As these molecules are non-polar, they easily diffuse across the cell-surface membrane into the epithelial cells.
- Once inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic rectum column where they are re-combined to form triglycerides.
- Starting in the endoplasmic reticulum and continuing in the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons. Chylomicrons are special particles adapted to the transport of lipids.
- Chylomicrons move out of the epithelial cells by exocytosis.
- Chylomicrons then enter the lymphatic capillaries (called lacteals) that are found at the bottom, centre of each villus (the whole cell).
- Chylomicrons then pass to lympathatic vessels then into the bloodstream.
- The triglycerides in the chylomicrons are hydrolysed by an enzyme in the ENDOthelial cells of blood capillaries from where they diffuse into cells.
Absorption of amino acids?
Amino acids are absorbed by co-transport.
Sodium ions are actively transported out of the ileum and epithetical cells into the blood.
This creates a sodium ion conc gradient.
Sodium ions can then diffuse from the lumen of the ileum into the epithelial cells through sodium-dependant transporter proteins, carrying the amino acids with them.
