Absorption

1. Overview

Absorption is the critical process where the small, soluble molecules produced during digestion move from the lumen of the digestive system into the circulatory system. This process ensures that nutrients like glucose, amino acids, and fats reach the cells of the body to be used for energy, growth, and repair.

Key Definitions

• Absorption: The movement of small food molecules and ions through the wall of the intestine into the blood.
• Villi: Tiny, finger-like projections found on the inner wall of the small intestine that increase surface area.
• Microvilli: Even smaller projections on the cell membranes of the epithelial cells lining the villi.
• Epithelium: The thin layer of cells (one cell thick) forming the outer layer of a villus.
• Lacteal: A central vessel in the villus that absorbs fatty acids and glycerol into the lymphatic system.

Core Content

In the human digestive system, the majority of absorption occurs after chemical digestion has broken down large, insoluble polymers into small, soluble monomers.

• The Small Intestine: This is the primary region where nutrients are absorbed into the blood. By the time food reaches the end of the small intestine (the ileum), most digested food molecules, as well as mineral ions and vitamins, have been absorbed.
• Absorption of Water:
  • The small intestine absorbs the largest volume of water (approximately 5–8 liters per day).
  • The colon (part of the large intestine) absorbs most of the remaining water, helping to solidify the waste material into faeces.

Extended Content (Extended curriculum only)

To maximize the efficiency of absorption, the small intestine is highly adapted to increase the rate of diffusion and active transport.

The Significance of Surface Area

The rate of absorption is directly proportional to the surface area available. The small intestine achieves a massive internal surface area through three levels of folding:

1. Inner folding of the intestinal wall.
2. Villi: Millions of finger-like projections.
3. Microvilli: Microscopic folds on the surface of individual epithelial cells.

Structure of a Villus

Each villus is a functional unit of absorption with specific adaptations:

• One-cell thick wall (Epithelium): Provides a very short distance for molecules to travel, speeding up the rate of diffusion.
• Large Surface Area: Provided by the shape of the villus and the presence of microvilli on the epithelial cells.
• Network of Blood Capillaries: A rich blood supply that absorbs glucose and amino acids. The constant flow of blood maintains a steep concentration gradient between the lumen and the blood.
• Lacteal: A tube in the center of the villus that specifically absorbs fats (fatty acids and glycerol) and transports them into the lymphatic system.

Structure to Function Summary

Structure	Function
Microvilli	Further increases surface area for maximum absorption.
Capillaries	Transports glucose and amino acids away to the liver; maintains concentration gradient.
Lacteal	Transports fatty acids and glycerol away from the intestine.
Mitochondria	Epithelial cells contain many mitochondria to provide energy (ATP) for active transport of nutrients.

Key Equations

While there are no specific chemical equations for absorption, you must be able to calculate magnification for diagrams of villi:

• Magnification = Image size / Actual size
• $M = I / A$
  • I: The measurement of the diagram (usually in mm).
  • A: The real size of the villus (usually in $\mu m$).
  • Note: 1 mm = 1000 $\mu m$.

Common Mistakes to Avoid

• ❌ Wrong: Thinking that all water is absorbed in the colon.
• ✓ Right: Most water is absorbed in the small intestine; the colon absorbs the remainder.
• ❌ Wrong: Stating that the lacteal absorbs glucose.
• ✓ Right: The lacteal only absorbs lipids (fats); glucose and amino acids go directly into the blood capillaries.
• ❌ Wrong: Confusing "Villi" with "Villi" in the lungs.
• ✓ Right: Villi are in the small intestine; Alveoli are the structures in the lungs.

Exam Tips

• Command Word "Describe and Explain": If asked to "describe and explain how the small intestine is adapted for absorption," you must name the structure (e.g., villi) and then state the benefit (e.g., increases surface area for faster diffusion).
• Identify the Lacteal: In diagrams, the lacteal is the large central vessel. Capillaries are the smaller, branching network surrounding it.
• Contextual Questions: Exams often use "Celiac disease" as a context. This disease flattens villi. You would be expected to explain that this leads to poor growth/weight loss because there is less surface area for nutrient absorption.
• Typical Values: A human villus is roughly 0.5 to 1.6 mm long. If your calculation for "Actual size" results in 10cm, you have likely made a unit conversion error.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term absorption in the context of the digestive system. [2]

(b) State two ways in which the small intestine is adapted to maximise absorption. [2]

(c) State the substance absorbed into the lacteal of a villus. [1]

Worked Solution:

(a)

1. Absorption is the process of taking digested food molecules into the blood or lymph.
2. The molecules then pass into the cells of the body.

How to earn full marks:

• Mention the movement of digested food molecules.
• State the destination as the blood or lymph, and then the cells of the body.

(b)

1. The small intestine has a very large surface area due to the presence of villi.
2. The villi have a thin wall, one cell thick, providing a short diffusion pathway.

How to earn full marks:

• State the presence of villi and their role in increasing the surface area.
• State the thin walls of the villi as a short diffusion pathway.

(c)

1. Fats / Fatty acids and glycerol $\boxed{}$

How to earn full marks:

• Either state fats, fatty acids, or glycerol.

Common Pitfall: Many students confuse absorption with digestion. Remember, digestion is the breakdown of food, while absorption is the uptake of digested food molecules into the bloodstream or lymph. Also, be specific when describing adaptations – simply saying "large surface area" isn't enough; you need to mention how the small intestine achieves this (villi).

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Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State the primary region of the alimentary canal where nutrient absorption occurs. [1]

(b) Explain the significance of microvilli in the absorption process. [2]

(c) Some individuals have a condition where their small intestine is shorter than normal. Suggest three possible consequences of this condition on their health. [3]

Worked Solution:

(a)

1. Small intestine $\boxed{}$

How to earn full marks:

• Simply stating "small intestine" is sufficient.

(b)

1. Microvilli are tiny projections on the surface of the epithelial cells lining the villi.
2. They further increase the surface area available for absorption.

How to earn full marks:

• Describe microvilli as projections on epithelial cells.
• Explain that they increase the surface area for absorption.

(c)

1. Reduced absorption of nutrients, leading to malnutrition.
2. Weight loss due to insufficient calorie intake.
3. Vitamin and mineral deficiencies, causing various health problems.

How to earn full marks:

• State reduced nutrient absorption and link it to malnutrition.
• State weight loss due to calorie deficiency.
• State vitamin/mineral deficiencies causing health problems.

Common Pitfall: Don't just say "poor absorption." You need to link the reduced absorption to specific health consequences. Also, remember that microvilli are on the villi themselves, further increasing the surface area already provided by the villi.

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Exam-Style Question 3 — Extended Response [9 marks]

Question:

(a) With reference to the diagram, describe the structure of a villus and its role in the absorption of digested food. [6]

(b) Explain why cells lining the small intestine have many mitochondria. [3]

Worked Solution:

(a)

1. The villus is a finger-like projection from the wall of the small intestine.
2. It is covered by a single layer of epithelial cells.
3. Inside the villus is a network of capillaries which absorb glucose and amino acids.
4. A lacteal is also present, which absorbs fatty acids and glycerol.
5. The epithelial cells have microvilli, further increasing the surface area.
6. The thin walls of the villus (one cell thick) provide a short diffusion pathway for absorption.

How to earn full marks:

• Describe the villus as a finger-like projection.
• State that it's covered by a single layer of epithelial cells.
• Mention capillaries and their role in absorbing glucose and amino acids.
• Mention the lacteal and its role in absorbing fatty acids and glycerol.
• Include the presence and function of microvilli.
• Include the thin walls and short diffusion pathway.

(b)

1. Mitochondria are the site of aerobic respiration.
2. Respiration releases energy needed for active transport.
3. Active transport is required to absorb some nutrients against their concentration gradient.

How to earn full marks:

• State that mitochondria are the site of aerobic respiration.
• Explain that respiration releases energy.
• Link the energy release to active transport for nutrient absorption against a concentration gradient.

Common Pitfall: When describing the villus, be sure to mention all the key structural features and their functions. Many students forget to mention the lacteal and its role in fat absorption. Also, remember that active transport requires energy, which is supplied by respiration in the mitochondria.

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Exam-Style Question 4 — Extended Response [10 marks]

Question:

A student investigates the effect of different animal diets on the length of their small intestine. The student dissects three animals with different diets: a herbivore (rabbit), an omnivore (rat), and a carnivore (cat). The student measures the length of the small intestine in each animal relative to its body length. The results are shown in the table.

Animal	Diet	Small Intestine Length (Relative to Body Length)
Rabbit	Herbivore	12
Rat	Omnivore	7
Cat	Carnivore	4

(a) Describe the trend shown in the table. [2]

(b) Explain why herbivores typically have longer small intestines than carnivores. [5]

(c) Suggest two ways that the student could improve the reliability of this investigation. [3]

Worked Solution:

(a)

1. As the diet changes from herbivore to omnivore to carnivore, the relative length of the small intestine decreases.
2. The rabbit (herbivore) has the longest small intestine relative to its body length, while the cat (carnivore) has the shortest.

How to earn full marks:

• State the inverse relationship between dietary type and relative intestine length.
• Give a specific example from the data to illustrate the trend.

(b)

1. Herbivores consume plant matter, which is difficult to digest due to the presence of cellulose.
2. Cellulose requires a longer time for digestion and absorption.
3. The longer small intestine provides a greater surface area and more time for the breakdown of cellulose by microorganisms (symbiotic bacteria).
4. Carnivores consume meat, which is relatively easier to digest.
5. Therefore, carnivores do not require as long a small intestine for efficient absorption.

How to earn full marks:

• State that plant matter (cellulose) is difficult to digest.
• Explain that a longer time is needed for digestion.
• Link the longer intestine to increased surface area and time for breakdown by microorganisms.
• State that meat is easier to digest.
• State that carnivores therefore do not need as long an intestine.

(c)

1. Increase the sample size (number of animals dissected for each diet type).
2. Repeat the measurements on each animal several times and calculate an average to reduce random errors.
3. Ensure the animals are of similar age/size to control for these variables.

How to earn full marks:

• State increasing the sample size to improve statistical significance.
• Mention repeating measurements and averaging to reduce random errors.
• Mention controlling for variables like age or size of the animals.

Common Pitfall: When explaining the difference in intestine length, be sure to focus on the digestibility of the food. Many students simply state that herbivores eat plants and carnivores eat meat, without explaining why plant matter is harder to digest (cellulose) and requires a longer digestive tract. Also, remember that reliability in experiments is improved by increasing sample size and repeating measurements.