AQA A-Level Biology (7402)

## Learning Outcomes

- Explain why digestion is needed before absorption can take place
- Describe how carbohydrates, proteins, and lipids are digested into absorbable products
- Identify the structural adaptations of the ileum as an exchange surface
- Explain step by step how monosaccharides and amino acids are absorbed by Na+ co-transport
- Describe the role of bile, micelles, and chylomicrons in lipid absorption
- Distinguish between terms that are often confused, including villi and microvilli, and emulsification and micelle formation
- Apply this knowledge to exam questions on exchange surfaces, membrane transport, and enzyme action

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand digestion and absorption in mammals, with a focus on the following syllabus points:

- Know that digestion is the hydrolysis of large insoluble biological molecules into small soluble molecules
- Identify the roles of amylases and membrane-bound disaccharidases in carbohydrate digestion
- Explain the roles of endopeptidases, exopeptidases, and membrane-bound dipeptidases in protein digestion
- Understand that bile is not an enzyme and explain how it emulsifies lipids and provides suitable alkaline conditions
- Describe how the ileum is adapted for absorption, including villi, microvilli, blood supply, thin epithelium, transport proteins, and lacteals
- Explain the absorption of glucose and amino acids by Na+ co-transport
- Explain the role of micelles in lipid absorption and how chylomicrons enter lacteals

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why must large biological molecules be digested before they can be absorbed?
2. How do villi and microvilli each increase absorption in the ileum?
3. Describe how sodium ions are involved in the absorption of glucose or amino acids.
4. What is the difference between emulsification and micelle formation?
5. Why do lipid digestion products enter lacteals rather than blood capillaries directly?

## Introduction

Digestion and absorption are a classic AQA topic because they bring together several parts of the course: enzyme action, membrane transport, exchange surfaces, and the relationship between structure and function. In exams, students often know the names of the processes but lose marks by mixing up their order or using imprecise terms.

Digestion turns large, insoluble molecules into smaller, soluble ones. Absorption then moves these products across the epithelial cells of the ileum into blood or lymph. The key to high marks is not just naming molecules, but explaining exactly how each product crosses the epithelium and why the ileum is so effective as an exchange surface.

> **Key Term: digestion**
> The hydrolysis of large insoluble biological molecules into small soluble molecules that can be absorbed.

### Digestion of carbohydrates, proteins, and lipids

Physical digestion and chemical digestion work together. Physical breakdown by chewing makes food pieces smaller, which increases the surface area available for enzymes. Chemical digestion then uses **hydrolase** enzymes to break covalent bonds by adding water.

> **Key Term: hydrolysis**
> A reaction in which a molecule is split by the addition of water.

Carbohydrates must be reduced to monosaccharides before absorption. **Amylase** hydrolyses starch into maltose. This is not the final stage. Maltose is still a disaccharide, so it must be broken down further by a membrane-bound disaccharidase.

> **Key Term: membrane-bound disaccharidase**
> An enzyme attached to the cell-surface membrane of epithelial cells in the ileum that hydrolyses disaccharides into monosaccharides.

Three examples matter:

- maltase: maltose to glucose + glucose
- sucrase: sucrose to glucose + fructose
- lactase: lactose to glucose + galactose

This matters because final digestion happens at the epithelial surface, so the monosaccharides are produced right where absorption can occur. Examiners like this link between digestion and absorption.

Protein digestion also happens in stages. Proteins are too large to absorb, and even many peptides are still too large. Different peptidases act in sequence.

> **Key Term: endopeptidase**
> An enzyme that hydrolyses peptide bonds within the middle of a polypeptide chain, forming shorter polypeptides.
>
> **Key Term: exopeptidase**
> An enzyme that hydrolyses peptide bonds at the ends of polypeptide chains, releasing amino acids.
>
> **Key Term: membrane-bound dipeptidase**
> An enzyme on the epithelial cell membrane of the ileum that hydrolyses dipeptides into amino acids.

The order matters. Endopeptidases cut long chains into shorter ones. Exopeptidases then remove amino acids from the ends. Dipeptidases finish the process by splitting dipeptides into single amino acids. If you only say “proteins are broken down by protease into amino acids,” you may miss marks because AQA often wants the separate roles.

Lipids are handled differently because they are not polymers in the same way as starch or protein, and they do not mix well with water. Large lipid droplets have a small surface area compared with their volume, so lipase would act slowly if nothing changed.

Bile, made by the liver and released into the small intestine, does two key jobs. First, it neutralises acidic chyme from the stomach, giving a more alkaline pH suitable for intestinal enzymes. Second, bile salts **emulsify** lipids into smaller droplets.

> **Key Term: emulsification**
> The physical breakdown of large lipid droplets into smaller droplets by bile salts, increasing the surface area for lipase action.

Bile is not an enzyme. It does not hydrolyse lipids itself. Instead, it makes lipase more effective by increasing surface area. Lipase then hydrolyses triglycerides into monoglycerides and fatty acids.

> **Test Tip:** When a question asks about the role of bile, do not say that bile “digests fats” or “contains lipase.” State two clear points: it emulsifies lipids and it neutralises stomach acid to create suitable alkaline conditions.

### The ileum as a specialised exchange surface

Most absorption of digested products happens in the ileum. This makes the ileum a specialised exchange surface, so many of its features match the general rules for efficient exchange.

The ileum is long, folded, and lined with **villi**. Each villus is about 1 mm long, and each epithelial cell on a villus has **microvilli** on its surface. Together, these features create a very large surface area.

> **Key Term: villus**
> A finger-like projection of the ileum wall that increases the surface area for absorption.
>
> **Key Term: microvillus**
> A microscopic projection of the cell-surface membrane of an epithelial cell that further increases surface area for absorption.

Students often confuse these. A villus is a projection of the tissue. A microvillus is a projection of the membrane of a single epithelial cell. So microvilli increase membrane surface area directly, while villi increase the area of the intestinal lining as a whole.

The epithelium is only one cell thick, so the diffusion pathway is short. This is essential because the faster substances cross the epithelium, the faster they can be removed into blood or lymph.

Each villus has a rich capillary network. This brings blood close to the epithelium and carries away glucose, amino acids, and other water-soluble products. Removing these molecules helps maintain a steep concentration gradient between the lumen and the blood.

Each villus also contains a **lacteal**, a lymphatic capillary. This is important for lipid absorption because chylomicrons are too large to enter blood capillaries easily at this stage.

Epithelial cells contain many mitochondria. This is linked to absorption because active transport is needed to maintain the sodium ion gradient used in co-transport. A good answer links organelles to function: many mitochondria mean more aerobic respiration, so more ATP is available for active transport.

Some accounts also note that villi can move slightly because they contain muscle. This helps mix the contents of the ileum and replace material next to the epithelium, helping maintain concentration gradients.

> **Test Tip:** If asked how the ileum is adapted for absorption, do not just list features. Link each feature to its effect. For example, “microvilli increase membrane surface area, so more transport proteins can be present and the rate of absorption is higher.”

### Absorption of monosaccharides and amino acids by Na+ co-transport

AQA often tests this as a sequence. To score well, write it in order.

> **Key Term: co-transport**
> The transport of one substance into a cell linked to the movement of another substance, such as glucose or amino acids with sodium ions.

Glucose and amino acids cannot simply rely on diffusion from the lumen into epithelial cells. If absorption happened only by diffusion, it would stop once concentrations became equal on each side. Some nutrients would then remain in the gut and be lost from the body.

The system solves this by using the sodium ion gradient.

Step 1: A sodium-potassium pump in the epithelial cell membrane actively transports sodium ions out of the epithelial cell. This lowers the sodium ion concentration inside the cell.

Step 2: Sodium ions then move from the lumen into the epithelial cell down their concentration gradient through a co-transporter protein.

Step 3: Glucose or amino acids bind to the same co-transporter. As sodium ions enter, they carry glucose or amino acids into the cell as well, even if the nutrient is moving against its concentration gradient.

Step 4: Once inside the epithelial cell, glucose or amino acids move into the blood by facilitated diffusion through carrier proteins in the membrane facing the capillary.

The key idea is that ATP is used indirectly. The co-transporter itself does not hydrolyse ATP. Instead, ATP is used by the sodium-potassium pump to create the sodium ion gradient that makes co-transport possible.

This is why “glucose is absorbed by active transport” is not precise enough for AQA. The best wording is that glucose and amino acids are absorbed into epithelial cells by Na+ co-transport, driven by a sodium ion gradient maintained by active transport.

Fructose is different. It is absorbed by facilitated diffusion through a different transporter. That distinction can appear in multiple-choice questions.

A useful cause-and-effect chain is:

- active transport lowers sodium ion concentration in epithelial cell
- sodium ions diffuse in from lumen
- co-transporter carries glucose or amino acids with sodium ions
- nutrients then leave cell into blood by facilitated diffusion

If any one step is missing, the mechanism is incomplete.

### Absorption of lipids: micelles, chylomicrons, and lacteals

Lipid absorption is easy to confuse because it has more stages and several similar-sounding terms.

After lipase acts, the products are monoglycerides and fatty acids. These remain associated with bile salts, forming **micelles**.

> **Key Term: micelle**
> A tiny structure formed from bile salts with monoglycerides and fatty acids that transports lipid digestion products to the epithelial surface.

A micelle is not the same as an emulsified droplet. Emulsification happens first and produces many small lipid droplets. Lipase then digests lipids. After that, the digestion products associate with bile salts to form micelles.

Micelles move through the watery contents of the ileum to the epithelial surface. At the membrane, the micelles break down and release monoglycerides and fatty acids. These then diffuse across the cell-surface membrane because they are non-polar and lipid-soluble. Whole micelles are not absorbed intact.

Inside the epithelial cell, monoglycerides and fatty acids are transported to the endoplasmic reticulum and recombined to form triglycerides again. These triglycerides then combine with cholesterol and proteins to form **chylomicrons**.

> **Key Term: chylomicron**
> A lipoprotein particle formed in epithelial cells from re-formed triglycerides and proteins for lipid transport.

Chylomicrons leave the epithelial cell by exocytosis and enter the lacteal. They pass into the lymphatic system and then later enter the blood.

This route matters. Glucose and amino acids enter blood capillaries. Lipid products leave as chylomicrons into lacteals. Students often lose marks by saying all products of digestion enter the blood directly from the villus.

A full sequence for lipid absorption is:

- bile salts emulsify lipids into small droplets
- lipase hydrolyses triglycerides to monoglycerides and fatty acids
- these associate with bile salts in micelles
- micelles deliver products to the epithelial surface
- monoglycerides and fatty acids diffuse into epithelial cells
- triglycerides are re-formed inside the cells
- triglycerides combine with proteins to form chylomicrons
- chylomicrons leave by exocytosis into lacteals

This pathway explains why the villus needs both blood capillaries and a lacteal. Different classes of digested products use different transport routes.

> **Test Tip:** In 3- or 4-mark questions on lipid absorption, keep the order exact. Examiners often penalise answers that confuse emulsification, micelles, and chylomicrons. Write them as three separate stages, not as interchangeable words.

### Practical and exam application: models, data, and common errors

Visking tubing can be used as a model of the ileum because it is partially permeable. Small molecules can pass through it, while larger ones cannot. In a model gut experiment, if sucrose and sucrase are placed inside the tubing, sucrase hydrolyses sucrose into glucose and fructose. These smaller molecules can then move out through the tubing, while the enzyme remains inside because it is too large.

> **Key Term: absorption**
> The movement of digested products across the epithelium of the ileum into blood or lymph.

This model helps test effects of pH or temperature on digestion, but it has limits. Real ileum epithelium contains membrane-bound enzymes, carrier proteins, mitochondria, capillaries, and lacteals. Visking tubing only models partial permeability, not the full biology of active transport or co-transport.

Common student errors include:

- confusing villi with microvilli
- saying bile digests lipids
- describing emulsification as hydrolysis
- saying micelles are absorbed whole
- forgetting that membrane-bound enzymes complete final digestion
- missing the role of the sodium-potassium pump in Na+ co-transport
- saying chylomicrons enter blood capillaries directly
- giving general digestion facts when the question asks specifically about protein digestion or lipid absorption

AQA mark schemes usually reward precise chains. For example, if asked why coeliac disease can reduce amino acid concentration in the blood, the key idea is that damage to villi reduces surface area for absorption, so fewer amino acids are absorbed into the blood even if proteins are still digested.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Digestion is the hydrolysis of large insoluble molecules into small soluble molecules that can be absorbed
- Amylase hydrolyses starch to maltose, and membrane-bound disaccharidases hydrolyse disaccharides to monosaccharides
- Endopeptidases cut within polypeptides, exopeptidases remove terminal amino acids, and membrane-bound dipeptidases hydrolyse dipeptides
- Bile is not an enzyme; it neutralises acidic chyme and emulsifies lipids into smaller droplets
- The ileum is adapted for absorption by its length, villi, microvilli, thin epithelium, rich blood supply, many mitochondria, transport proteins, and lacteals
- Villi are tissue projections, while microvilli are projections of the epithelial cell membrane
- Glucose and amino acids are absorbed into epithelial cells by Na+ co-transport, using a sodium ion gradient maintained by active transport
- Glucose and amino acids then move into blood capillaries by facilitated diffusion
- Lipase hydrolyses triglycerides into monoglycerides and fatty acids
- Micelles transport lipid digestion products to the epithelial surface, where the products diffuse into cells
- Inside epithelial cells, lipids are re-formed into triglycerides and packaged as chylomicrons
- Chylomicrons leave by exocytosis into lacteals, then pass through the lymphatic system before entering the blood

## Key Terms and Concepts

- digestion
- hydrolysis
- membrane-bound disaccharidase
- endopeptidase
- exopeptidase
- membrane-bound dipeptidase
- emulsification
- villus
- microvillus
- co-transport
- micelle
- chylomicron
- absorption


Exchange with the environment - Digestion and absorption