Digestion & Absorption

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Digestion: An Overview

Digestion is the hydrolysis of large biological molecules into smaller molecules that can be absorbed across cell membranes.
Proteins are hydrolysed into amino acids.
Carbohydrates are hydrolysed into simple sugars.
Lipids are hydrolysed into glycerol and fatty acids.
The small molecules produced are used by cells to release energy via respiration and to build new molecules for growth, repair and function.
The human digestive system is an organ system containing a series of organs that work together to digest and absorb food.

Organs of the Digestive System

Mouth and salivary glands: teeth break food into smaller pieces; saliva is secreted; amylase begins starch digestion into maltose.
Stomach: protease enzymes begin protein digestion; hydrochloric acid provides a suitable pH for enzymes and destroys pathogens.
Liver: produces bile salts, which aid lipid digestion and neutralise stomach acid as it exits the stomach.
Pancreas: produces amylase, protease and lipase, which are released into the duodenum.
Small intestine (duodenum): acidic stomach contents are neutralised by bile and become slightly alkaline; enzymes complete chemical digestion.
Small intestine (ileum): food and water are absorbed into the blood via villi in the lining.

Enzymes in Digestion

Digestive enzymes are extracellular, meaning they function outside body cells.
The three main types of digestive enzymes are carbohydrases, lipases and proteases.
Carbohydrases include amylase, maltase and lactase.
Amylase hydrolyses starch into the disaccharide maltose; it is made in the salivary glands, pancreas and small intestine.
Maltase hydrolyses maltose into the monosaccharide glucose; it is a membrane-bound disaccharidase attached to the cell-surface membranes of epithelial cells lining the small intestine.
Lipase enzymes in the lumen of the small intestine break down lipids to glycerol, monoglycerides and fatty acids.
Bile salts emulsify lipids by binding to large lipid droplets and breaking them into smaller droplets, increasing surface area for lipase action.
Bile is not an enzyme and does not carry out chemical breakdown of lipids.

Protein Digestion

Protein digestion involves endopeptidases, exopeptidases and dipeptidases.
Endopeptidases hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains.
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids.
Dipeptidases are a type of exopeptidase that break down dipeptides into individual amino acids.
Membrane-bound dipeptidases are attached to the cell-surface membrane of epithelial cells in the small intestine.
Remember: endo = within, exo = outside.

Investigating Digestive Enzymes

The effect of pH on amylase activity can be investigated using the iodine test.
A strong positive iodine test (blue-black) means starch is present at high concentration, so amylase activity is absent or very low.
A weak positive iodine test means starch is present at low concentration, so amylase activity is high.
A negative iodine test (orange-brown) means no starch remains, so amylase has converted all starch to maltose.
Control variables include: equal volume and concentration of enzyme solution, equal volume and concentration of substrate solution, equal volumes of buffer solution, and the same stirring method.
The point at which iodine no longer changes colour can be subjective; a colorimeter can measure the reaction objectively.
The effect of bile salts on lipase activity can be investigated using milk, phenolphthalein indicator and sodium carbonate solution.
Phenolphthalein is pink in alkaline conditions and colourless in acidic conditions; as lipase breaks down lipids, fatty acids lower the pH and the pink colour disappears.

Mechanisms of Absorption: Co-transport

Amino acids and monosaccharides are absorbed via co-transport.
Co-transporter proteins are found in the cell-surface membranes of epithelial cells in the small intestine.
Step 1: Sodium ions are actively transported from the epithelial cell into the blood via a sodium-potassium pump, decreasing sodium ion concentration in the epithelial cell.
Step 2: Sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying an amino acid or glucose at the same time via the co-transporter protein (a form of facilitated diffusion).
Step 3: The concentration of amino acids or glucose in the epithelial cell increases, and they diffuse down their concentration gradient into the blood.
While the co-transporter protein action is passive, energy is required to create the sodium ion gradient, so co-transport is considered overall active transport.

Mechanisms of Absorption: Lipids

The products of lipid digestion are fatty acids and monoglycerides.
Monoglycerides and fatty acids associate with bile salts to form micelles, which transport insoluble molecules to the cell-surface membranes of epithelial cells.
Micelles constantly break up and reform; when they break apart, their lipid-soluble contents can cross the membrane by diffusion.
The contents of micelles are non-polar and can diffuse through the phospholipid bilayer of the cell membrane.
Short fatty acid chains can move directly into the blood via diffusion.
Longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum.
Triglycerides are packaged into chylomicrons, which eventually enter the bloodstream.

Modelling Absorption with Visking Tubing

Visking tubing (dialysis tubing) is a non-living, partially permeable membrane made from cellulose.
Pores in the tubing prevent passage of large molecules (e.g. starch) but allow smaller molecules (e.g. glucose) to pass through by diffusion.
Method: tie one end of tubing, fill with starch and amylase mixture, suspend in a beaker of water, and test the outside liquid at intervals for starch and glucose.
Results show glucose is present outside the tubing while starch is absent; starch molecules are too large to pass through the pores, but amylase digests starch into glucose, which diffuses out.
The rate of absorption/diffusion can be measured quantitatively using colorimetry to determine glucose concentration.
The model can investigate the effect of pH (using buffer solutions) and temperature (using water baths).

Limitations of the Visking Tubing Model

Visking tubing does not contain biological membranes or membrane features such as channel proteins.
Active transport cannot occur across Visking tubing due to a lack of carrier proteins and energy from respiration.
The surface area of Visking tubing is less than that of intestinal epithelium due to the absence of villi.
The distilled water does not flow like blood, so it does not maintain the concentration gradient.

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1.Which of the following is the correct definition of a gland in the context of digestion?
Medium
- AA group of cells that secretes substances such as enzymes or bile
- BA muscular organ that churns food
- CA membrane that absorbs nutrients
- DA type of enzyme that breaks down food
2.Which statement correctly describes why the stomach is classified as part of the alimentary canal whereas the liver is not?
Medium
- AThe stomach is a hollow tube through which food passes, while the liver is an accessory organ that secretes bile.
- BThe stomach produces enzymes, while the liver does not.
- CThe stomach absorbs nutrients, while the liver stores them.
- DThe stomach is connected to the mouth, while the liver is connected to the pancreas.
3.Which row correctly completes the table for the digestion of the named molecules?
Medium
- AK = amino acids; L = lipids; M = sucrase
- BK = amino acids; L = fatty acids and glycerol; M = maltase
- CK = peptides; L = lipids; M = sucrase
- DK = amino acids; L = lipids; M = maltase
4.Which of the following are features of the environment inside the stomach that enable chemical and physical digestion? (select all that apply)
Medium
- APresence of hydrochloric acid
- BPresence of protease enzymes
- CPresence of bile salts
- DMuscular contractions
- EPresence of amylase
5.In an experiment examining the rate of reaction of an amylase-catalysed reaction, what would be the final colour observed from the iodine test in a test tube containing denatured amylase?
Medium
- ABlue-black
- BOrange-brown
- CBrick red
- DPurple
6.Which of the following is a suitable control variable in an experiment measuring the effect of temperature on the rate of a lipase-catalysed reaction?
Medium
- AVolume and concentration of lipase solution
- BThe temperature of the water bath
- CThe time taken for the pH to change
- DThe final pH of the solution
7.Which of the following correctly describes the role of structures 0.5–1.0 mm in length present on the epithelial lining of the small intestine?
Medium
- AThey increase surface area for absorption of digested food.
- BThey secrete digestive enzymes into the lumen.
- CThey produce bile salts to emulsify fats.
- DThey store faeces before egestion.
8.Which of the following is a property of Visking tubing that makes it a good choice for experiments that model the action of enzymes in the gut?
Medium
- AIt is partially permeable, allowing small molecules to pass through but not large ones.
- BIt contains carrier proteins for active transport.
- CIt has villi to increase surface area.
- DIt contains biological membranes that mimic epithelial cells.

Mehr Fragen

9.In a model gut experiment using Visking tubing containing starch and amylase suspended in a beaker of water at 37 °C for 30 minutes, what is the predicted result of an iodine test on the solution outside the tubing?
Medium
- ANegative, because starch molecules are too large to pass through the tubing.
- BPositive, because starch diffuses out of the tubing.
- CPositive, because amylase diffuses out and digests starch outside.
- DNegative, because amylase digests all starch inside and outside the tubing.
10.Which of the following is the correct definition of the term gland in the context of digestion?
Medium
- AA group of cells that secretes substances such as enzymes or bile
- BA muscular organ that churns food
- CA membrane that absorbs nutrients
- DA type of enzyme that breaks down food
11.Which of the following correctly describes the action of endopeptidases?
Medium
- AThey hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains.
- BThey hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids.
- CThey break down dipeptides into individual amino acids.
- DThey emulsify fats into smaller droplets.
12.Which of the following correctly describes the action of exopeptidases?
Medium
- AThey hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids.
- BThey hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains.
- CThey break down dipeptides into individual amino acids.
- DThey emulsify fats into smaller droplets.
13.Which of the following statements about the co-transport of glucose in the small intestine are correct? (select all that apply)
Medium
- ASodium ions are actively transported from the epithelial cell into the blood.
- BSodium ions move down their concentration gradient from the intestine into the epithelial cell.
- CGlucose is transported against its concentration gradient into the epithelial cell.
- DGlucose moves into the blood by facilitated diffusion.
- EThe co-transporter protein directly uses ATP to transport glucose.
14.Bile contains enzymes that chemically digest lipids.
Easy
True or false?
15.Maltase is a membrane-bound disaccharidase found on the cell-surface membranes of epithelial cells lining the small intestine.
Easy
True or false?
16.Endopeptidases hydrolyse peptide bonds at the ends of polypeptide chains.
Easy
True or false?
17.Visking tubing is a partially permeable membrane made from cellulose.
Easy
True or false?
18.Match each molecule with its digestion product.
Medium
- Starch
- Protein
- Lipid
- Sucrose
- Amino acids
- Glycerol and fatty acids
- Glucose and fructose
- Maltose
19.Place the steps of the co-transport of glucose in the small intestine in the correct order.
Medium
- Sodium ions are actively transported from the epithelial cell into the blood.
- Sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying glucose.
- Glucose diffuses into the blood by facilitated diffusion.
20.A student investigates the effect of bile salts on lipase activity using milk, phenolphthalein and sodium carbonate. The time taken for the pink colour to disappear is recorded. Which of the following is the most likely result if bile salts are added?
Medium
- AThe time taken decreases because emulsification increases the surface area for lipase.
- BThe time taken increases because bile salts inhibit lipase.
- CThe time taken stays the same because bile salts do not affect lipase activity.
- DThe time taken decreases because bile salts digest the lipids chemically.
21.In an experiment using Visking tubing to model absorption, a student places a starch and amylase mixture inside the tubing and suspends it in a beaker of water at 37 °C. After 30 minutes, which test results would be expected for the liquid outside the tubing?
Medium
- ANegative iodine test and positive Benedict's test
- BPositive iodine test and negative Benedict's test
- CPositive iodine test and positive Benedict's test
- DNegative iodine test and negative Benedict's test
22.Which of the following is a limitation of using Visking tubing to model absorption in the small intestine?
Medium
- AActive transport cannot occur because there are no carrier proteins or energy from respiration.
- BIt is not partially permeable, so no molecules can pass through.
- CIt contains villi, which increase the surface area too much.
- DIt contains biological membranes that interfere with diffusion.
23.Which of the following are products of lipid digestion? (select all that apply)
Medium
- AFatty acids
- BMonoglycerides
- CGlycerol
- DAmino acids
- EGlucose
24.The digestion of starch is a two-step process involving amylase and maltase.
Easy
True or false?
25.Dipeptidases are a type of endopeptidase.
Easy
True or false?
26.Match each enzyme with its substrate.
Medium
- Amylase
- Lipase
- Protease
- Maltase
- Starch
- Lipids
- Proteins
- Maltose
27.Place the steps of the iodine test method for investigating amylase activity in the correct order.
Medium
- Place single drops of iodine solution in the dips on a spotting tile.
- Add amylase, buffer solution and starch solution to a test tube and start the stopwatch.
- After 10 seconds, place one drop of the mixture on the first drop of iodine.
- Repeat every 10 seconds until the iodine solution remains orange-brown.
28.Which of the following correctly describes the absorption of short fatty acid chains after lipid digestion?
Medium
- AThey can move directly into the blood by diffusion.
- BThey are packaged into chylomicrons in the endoplasmic reticulum.
- CThey are recombined with monoglycerides to form triglycerides.
- DThey are actively transported into the blood by co-transport with sodium ions.
29.Which of the following correctly describes the formation of chylomicrons?
Medium
- ATriglycerides are packaged into chylomicrons in the endoplasmic reticulum.
- BFatty acids are packaged into chylomicrons in the nucleus.
- CMicelles are packaged into chylomicrons in the Golgi apparatus.
- DAmino acids are packaged into chylomicrons in the mitochondria.
30.Match each enzyme to the bond it hydrolyses or the substrate it acts on.
Medium
- Endopeptidase
- Exopeptidase
- Lipase
- Maltase
- Peptide bonds within a polypeptide chain
- Peptide bonds at the ends of a polypeptide chain
- Triglycerides to glycerol and fatty acids
- Maltose to glucose
31.Which of the following statements about the digestion of starch are correct? (select all that apply)
Hard
- AAmylase hydrolyses starch into maltose
- BMaltase is a membrane-bound disaccharidase
- CMaltase hydrolyses maltose into glucose
- DAmylase is found in the salivary glands, pancreas and small intestine
- EStarch is absorbed directly into the blood without digestion
32.Long fatty acid chains are recombined with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum.
Easy
True or false?
33.Which of the following are limitations of using Visking tubing to model absorption in the small intestine? (select all that apply)
Medium
- AIt does not contain carrier proteins for active transport
- BIt lacks villi so has a smaller surface area
- CIt is made of cellulose and is partially permeable
- DIt does not have a blood supply to maintain a concentration gradient
- EIt allows small molecules like glucose to pass through by diffusion
34.In the investigation of the effect of pH on amylase activity using iodine solution, what does a weak positive iodine test indicate?
Medium
- AStarch is present at a low concentration, so amylase activity is high
- BStarch is present at a high concentration, so amylase activity is low
- CNo starch remains, so amylase has been fully active
- DAmylase has been denatured and cannot break down starch
35.Match each enzyme to its site of production.
Medium
- Amylase
- Protease
- Lipase
- Maltase
- Salivary glands, pancreas and small intestine
- Stomach and small intestine
- Pancreas and small intestine
- Small intestine (membrane-bound)
36.Place the steps of the method for investigating the effect of pH on amylase activity using iodine in the correct order.
Medium
- Place single drops of iodine solution in the dips on a spotting tile
- Add amylase, buffer solution and starch solution to a test tube and start the stopwatch
- After 10 seconds, place one drop of the mixture on the first drop of iodine
- Wipe the glass rod and repeat every 10 seconds until the iodine remains orange-brown
37.Which of the following statements about the absorption of amino acids in the small intestine are correct? (select all that apply)
Hard
- ASodium ions are actively transported out of the epithelial cell into the blood
- BAmino acids enter the epithelial cell by facilitated diffusion via a co-transporter protein
- CAmino acids diffuse from the epithelial cell into the blood down their concentration gradient
- DThe co-transporter protein directly uses ATP to move amino acids
- EThe overall process is considered active transport because energy is used to create the sodium ion gradient
38.A model gut is set up with Visking tubing containing a mixture of starch and amylase, suspended in a beaker of water at 37 °C. After 30 minutes, samples are taken from inside the tubing and from the beaker. Which row correctly shows the expected results?
Medium
- AInside tubing: starch present, glucose present; Beaker: starch absent, glucose present
- BInside tubing: starch absent, glucose absent; Beaker: starch present, glucose present
- CInside tubing: starch present, glucose absent; Beaker: starch present, glucose present
- DInside tubing: starch absent, glucose present; Beaker: starch absent, glucose absent
39.A student investigates the effect of pH on amylase activity using the iodine test. At pH 2, the iodine solution remains blue-black throughout the 5-minute experiment. At pH 7, the iodine solution turns orange-brown after 40 seconds. What is the most likely explanation for the result at pH 2?
Medium
- AAmylase is denatured at pH 2, so starch is not broken down.
- BAmylase works faster at pH 2, so starch is broken down immediately.
- CIodine does not react with starch at pH 2.
- DStarch is converted to maltose at pH 2 but maltose also gives a blue-black colour with iodine.
40.A student measures the rate of a lipase-catalysed reaction by recording the pH of a milk sample every minute after adding lipase. The pH decreases from 7.0 to 5.5 over 10 minutes. Which statement best explains this change?
Medium
- ALipase hydrolyses triglycerides to fatty acids and glycerol, and the fatty acids lower the pH.
- BLipase hydrolyses triglycerides to fatty acids and glycerol, and the glycerol lowers the pH.
- CLipase hydrolyses triglycerides to amino acids, which lower the pH.
- DLipase hydrolyses triglycerides to glucose, which lowers the pH.
41.Name the organ of the digestive system in which the catalytic activity of lipase takes place.
Easy
42.Name the substance that is also transported when glucose is absorbed.
Easy
43.Name a suitable biochemical test to demonstrate the presence of the products of starch digestion in the water surrounding the Visking tubing after 30 minutes.
Easy
44.Complete the sentence about the digestion of starch.
Medium
Amylase hydrolyses starch into the disaccharide ____, which is then hydrolysed into the monosaccharide ____ by maltase.
45.Complete the sentence about lipid digestion.
Medium
Bile salts ____ large lipid droplets into smaller droplets, increasing the ____ for lipase enzymes to act.
46.Complete the sentence about protein digestion.
Medium
____ hydrolyse peptide bonds within polypeptides, while ____ hydrolyse peptide bonds at the ends of polypeptide chains.
47.In an experiment investigating the effect of pH on amylase activity, a drop of the reaction mixture is placed on iodine solution every 10 seconds. If the iodine solution first remains orange-brown after 70 seconds, how many drops of the mixture have been tested?
Easy
48.Complete the sentence about the role of the liver and pancreas.
Medium
The ____ produces bile salts, while the ____ produces amylase, protease and lipase enzymes.
49.Name the process by which bile salts break large lipid droplets into smaller ones.
Medium
50.Complete the sentence about lipid absorption.
Medium
Short fatty acid chains can move directly into the blood by ____, while longer fatty acid chains are packaged into ____ which enter the bloodstream.
51.Complete the sentence about the role of bile salts in lipid digestion.
Medium
Bile salts bind to large lipid droplets and break them into smaller droplets, a process called ____. This increases the ____ available for lipase to act on.
52.A student measures the time taken for the iodine solution to remain orange-brown in an amylase-catalysed reaction at pH 7. The results for three repeats are 42 s, 38 s, and 40 s. Calculate the mean time in seconds.
Medium
53.Define the term absorption in the context of nutrition.
Medium
54.State the two main roles of bile in digestion.
Medium
55.Define the term emulsification.
Medium
56.Explain why pH is a suitable dependent variable for an experiment measuring the effect of temperature on the rate of a lipase-catalysed reaction.
Medium
57.Describe the role of a buffer solution and state why a buffer is needed in an investigation of the effect of pH on amylase activity using Visking tubing.
Medium
58.In an experiment investigating the effect of pH on amylase activity, a drop of the reaction mixture is placed on iodine solution every 10 seconds. If the iodine solution first remains orange-brown after 70 seconds, how many drops of the mixture have been tested?
Easy
- A9 drops
- B7 drops
- C8 drops
- D6 drops
59.A student measures the time taken for the iodine solution to remain orange-brown in an amylase-catalysed reaction at pH 7. The results for three repeats are 42 s, 38 s, and 40 s. Calculate the mean time in seconds.
Medium
- A40 s
- B48 s
- C44 s
- D36 s

Lernnotizen

## Digestion & Absorption **Digestion** hydrolyses large biological molecules into smaller ones that can be absorbed across cell membranes: - Proteins → amino acids - Carbohydrates → simple sugars - Lipids → glycerol and fatty acids These products are used to release energy via respiration, and to build new molecules for growth, repair and function. ### Organs of the digestive system - **Mouth/salivary glands**: teeth break food into smaller pieces; saliva secreted; amylase begins starch digestion into maltose. - **Stomach**: protease enzymes begin protein digestion; hydrochloric acid provides suitable pH and destroys pathogens. - **Liver**: produces bile salts, which aid lipid digestion and neutralise stomach acid as it exits the stomach. - **Pancreas**: produces amylase, protease and lipase, released into the duodenum. - **Duodenum**: acidic stomach contents neutralised by bile, becoming slightly alkaline; enzymes complete chemical digestion. - **Ileum**: food and water absorbed into blood via villi. ### Enzymes in digestion Digestive enzymes are **extracellular** — they function outside body cells. Three main types: carbohydrases, lipases, proteases. **Carbohydrate digestion** (two-step): 1. Amylase hydrolyses starch → maltose (made in salivary glands, pancreas, small intestine). 2. Maltase hydrolyses maltose → glucose. Maltase is a **membrane-bound disaccharidase** attached to epithelial cell membranes of the small intestine. **Lipid digestion**: 1. **Emulsification** — bile salts bind to large lipid droplets, breaking them into smaller droplets, increasing surface area for lipase. 2. Lipase in the lumen breaks lipids down to glycerol, monoglycerides and fatty acids. **Protein digestion**: - **Endopeptidases** hydrolyse peptide bonds *within* polypeptides, creating shorter chains. - **Exopeptidases** hydrolyse peptide bonds at the *ends* of chains, producing single amino acids. - **Dipeptidases** (a type of exopeptidase) break dipeptides into individual amino acids; can be membrane-bound. ### Investigating digestive enzymes **Iodine test for amylase activity**: strong positive (blue-black) = starch present at high concentration, amylase absent/low; weak positive = low starch, high amylase activity; negative (orange-brown) = no starch remaining. **Bile salts and lipase**: phenolphthalein turns pink in alkaline, colourless in acid. As lipase breaks lipids into fatty acids, pH decreases; when phenolphthalein becomes colourless, white milk colour returns. ### Mechanisms of absorption **Co-transport** (amino acids and monosaccharides): 1. Sodium ions actively transported from epithelial cell into blood via sodium-potassium pump, decreasing Na⁺ concentration in cell. 2. Na⁺ moves down concentration gradient from intestine into epithelial cell, carrying amino acid/glucose via co-transporter protein (facilitated diffusion). 3. Amino acids/glucose diffuse down concentration gradient into blood. Overall considered **active transport** because energy is required to create the Na⁺ gradient. **Lipid absorption**: monoglycerides and fatty acids associate with bile salts to form **micelles**, transporting insoluble molecules to epithelial cell membranes. Micelles break up and reform; lipid-soluble contents diffuse through the phospholipid bilayer. Short fatty acid chains diffuse directly into blood; longer chains recombine into triglycerides in the endoplasmic reticulum, packaged into **chylomicrons** which enter the bloodstream. ### Modelling absorption **Visking tubing** (dialysis tubing) is a non-living, partially permeable membrane made from cellulose. Pores allow small molecules (e.g. glucose) to pass by diffusion but prevent large molecules (e.g. starch). In a model gut, starch and amylase mixture inside tubing suspended in water: glucose appears outside, starch remains inside (too large to pass through pores). **Limitations**: no biological membranes or channel proteins; no active transport (no carrier proteins or respiratory energy); less surface area than intestinal epithelium (no villi); distilled water does not flow like blood, so does not maintain concentration gradient.

Digestion: The hydrolysis of large biological molecules into smaller molecules that can be absorbed across cell membranes.
Absorption: The process by which the products of digestion pass through the intestinal lining into the blood.
Extracellular enzymes: Enzymes that function outside the body cells, such as digestive enzymes.
Carbohydrase: A group of enzymes involved in carbohydrate digestion, e.g. amylase, maltase, lactase.
Amylase: A carbohydrase enzyme that hydrolyses starch into the disaccharide maltose; made in the salivary glands, pancreas and small intestine.
Maltase: A membrane-bound disaccharidase enzyme that hydrolyses maltose into glucose; attached to the cell-surface membranes of epithelial cells lining the small intestine.
Membrane-bound disaccharidase: An enzyme attached to the cell-surface membrane of epithelial cells that breaks down disaccharides into monosaccharides.
Lipase: An enzyme that breaks down lipids into glycerol, monoglycerides and fatty acids.
Bile salts: Substances produced by the liver that aid lipid digestion by emulsifying fats and neutralising stomach acid.
Emulsification: The process by which bile salts bind to large lipid droplets and break them into smaller droplets, increasing surface area for lipase action.
Endopeptidase: A protease enzyme that hydrolyses peptide bonds within polypeptides, creating shorter polypeptide chains.
Exopeptidase: A protease enzyme that hydrolyses peptide bonds at the ends of polypeptide chains, producing single amino acids.
Dipeptidase: A type of exopeptidase that breaks down dipeptides into individual amino acids; can be membrane-bound.
Co-transport: The mechanism by which amino acids and monosaccharides are absorbed; sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying the nutrient at the same time via a co-transporter protein.
Micelles: Tiny, spherical structures formed when the products of lipid digestion associate with bile salts and phospholipids; they transport insoluble molecules to the epithelial cell membranes.
Chylomicrons: Structures formed when triglycerides are packaged in the endoplasmic reticulum; they eventually enter the bloodstream.
Visking tubing: A non-living, partially permeable membrane made from cellulose, used to model absorption in the small intestine.

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