Exchange between Organisms - Digestion and Absorption Flashcards
What is the human digestive system made up of?
The human digestive system is made up of a long muscular tube and its associated glands. The glands produce enzymes that hydrolyse large molecules into small ones ready for absorption across cell membranes. The digestive system is therefore an exchange surface through which food substances are absorbed.
What is the oesophagus?
The oesophagus is a muscular tube that carries food from the mouth to the stomach using peristalsis. No digestion takes place here.
What is the stomach?
The stomach is a large, muscular sac that links the oesophagus and small intestine. It has an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands that secrete hydrochloric acid and produce enzymes which digest protein. Other glands in its wall produce mucus. The mucus prevents this organ being digested by its own enzymes.
What is the ileum (small intestine)?
- The ileum is a long muscular tube.
- Food is further digested in the ileum by enzymes (maltase, sucrase and lactase enzymes) that are produced by its walls and by glands that pour their secretions into it.
- The inner walls of the ileum are folded into villi, which gives them a large 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.
- This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream.
What is the colon (large intestine)?
The large intestine is a muscular tube that links the small intestine with the rectum and anus. It absorbs water and mineral ions. Most of the water that is absorbed is water from the secretions of the many digestive glands. The food within this part therefore becomes drier and thicker in consistency and forms faeces.
What is the rectum?
The rectum is the final secretion of the intestines of undigested material. The faeces are stored here before periodically being removed via the anus in a process called egestion.
Is digestion extracellular or intracellular?
Extracellular as it happens outside of cells. As it happens outside of your body, waste being removed is called egestion and no excretion.
This means that the contents of the intestines are not inside the body either. Molecules and ions only truly enter the body when they cross the cells and cell-surface membranes of the epithelial lining of the intestines.
What are the salivary glands?
The salivary glands are situated near the mouth. They pass their secretions (saliva, composed of water, sodium ions, mucus and amylase) via a duct into the mouth. These secretions contain the enzyme amylase (which is activated by the sodium ions), which hydrolyses starch into maltose. Water helps to dissolve soluble substances, and mucus is for lubrication.
What is the pancreas?
The pancreas is a large, glandular organ situated below the stomach. It produces a secretion called pancreatic juice, which is delivered to the small intestine (duodenum) by the pancreatic duct.
This secretion contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch. It also produces the hormones insulin and glucagon that are secreted into the blood.
What is the difference between endopeptidase and exopeptidase?
Endopeptidases break polypeptides into shorter polypeptide chains. Exopeptidases break polypeptides into dipeptides at the ends of the chain.
What is the importance of endopeptidases?
You need endopeptidases to shorten the polypeptides and get more ends for the exopeptidases to digest.
What two stages does digestion take place in in humans?
- physical breakdown
2. chemical digestion
What is physical breakdown in humans?
- If the food is large, it is broken down into smaller pieces by means of structures such as the teeth.
- This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion.
- Food is churned by the muscles in the stomach wall and this also physically breaks it up.
What is chemical digestion in humans?
- Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones.
- It is carried out by enzymes.
- All digestive enzymes function by hydrolysis.
- Hydrolysis is the splitting up of molecules by adding water to the chemical bonds that hold them together.
- Enzymes are specific and so it follows that more than one enzyme is needed to hydrolyse a large molecule.
- Usually one enzyme hydrolyses a large molecule into sections and these sections are then hydrolyses into smaller molecules by one or more additional enzymes.
What does amylase do?
enzyme that breaks the glycosidic (chemical) bonds in starch by hydrolysis to release maltose units
What does carbohydrase do?
hydrolyses carbohydrates, ultimately into monosaccharides
What does lipase do?
hydrolyses lipids (fats and oils) by breaking the ester bonds in triglycerides into glycerol and fatty acids
What does protease do?
hydrolyses proteins, ultimately to amino acids, by breaking peptide bonds
What do glandular tissues in the stomach secrete?
hydrochloric acid, Pepsin (a protease enzyme), and a thick layer of mucus for protection
How many enzymes are needed to hydrolyse a large molecule?
It usually takes more than one enzyme to completely hydrolyse a large molecule. Typically one enzyme hydrolyses the molecule into smaller sections and then other enzymes hydrolyse these 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.
Describe starch digestion.
Firstly, the enzyme amylase is produced in the mouth and the pancreas. Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose. The maltose is in turn hydrolysed into the monosaccharide alpha-glucose by a second enzyme called maltase.
Where is maltase produced?
the lining of ileum
Describe the process of starch digestion in humans.
- Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing.
- Saliva contains salivary amylase. This starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the pH at around neutral. This is the optimum pH for salivary amylase to work.
- The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of the starch.
- After a time the food is passed into the small intestine, where 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 to 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 intestine wall push the food along the ileum. Its epithelial lining produces the disaccharidase maltase. Maltase is not released into the lumen of the ileum but is to part of the cell-surface membranes of the epithelial cells that line the ileum. It is therefore referred to as a membrane-bound disaccharidase. The maltase hydrolyses the maltose from starch breakdown into alpha-glucose.
What other common disaccharides in the diet are hydrolysed?
Sucrose - found in many natural foods, especially fruits.
Lactose - found in milk, and hence in milk products, such as yoghurt and cheese.
What does sucrase do?
Sucrase hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolysis produces the two monosaccharides glucose and fructose.
What does lactase do?
Lactase hydrolyses the single glycosidic bond in the lactose molecule. This hydrolysis produces the two monosaccharides glucose and galactose.
Describe lipid digestion.
- Lipids are hydrolysed by enzymes called lipases.
- Lipases are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides.
- A monoglyceride is a glycerol molecule with a single fatty acid molecule attached.
- Lipids (fats and oils) are firstly split up into tiny droplets called micelles by bile salts (which stay attached), that are produced by the liver.
- This process is called emulsification and increases the surface area of the lipids so that the action of lipases is speeded up.
- The bile salts are released.
- The monoglycerides and fatty acids at the epithelial membrane are to be absorbed by simple diffusion.
- Triglycerides are reformed in the SER.
- The Golgi produces chylomicrons from triglycerides and lipoproteins.
- The chylomicrons are exported by exocytosis.
- Chylomicrons are absorbed into the lacteals in the villi or the hepatic portal vein (into the lymphatic system).
Short and medium-chain fatty acids and glycerol (small products) are absorbed into the blood via capillaries.
Long-chain fatty acids and monoglycerides (large products) form into triglycerides and are transported in chylomicrons (inside Golgi apparatus). Chylomicrons are extruded from the epithelial cell and enter a lacteal (lymph capillary). Lymph in the lacteal transports chylomicrons away from intestine.
What hydrolyses proteins?
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases).
What are endopeptidases?
Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of polypeptide molecules.
What are exopeptidases?
Exopeptidases hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids.
What are dipeptidases?
Dipeptidases hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
What is lactose intolerance?
Milk is the only food of human babies and so they produce a relatively large amount of lactase, the enzyme that hydrolyses lactose, the sugar in milk. As milk forms a less significant part of the diet in adults, the production of lactase diminishes and children get older. This reduction can be so great in some adults that they produce little, or no, lactase at all.
This was not a problem to our ancestors but can be to humans of today. Humans that produce no lactase cannot hydrolyse the lactose they consume. When the undigested lactose reaches the large intestines. microorganisms hydrolyse it. This gives rise to small soluble molecules and a large volume of gas. This can result in diarrhoea because the soluble molecules lower the water potential of the material in the colon. The condition is known as lactose intolerance. Some people with the condition cannot consume milk or milk products at all while other can consume them only in small amounts.
