How are exchange surfaces adapted to absorb gases and nutrients efficiently?
The need for efficient exchange surfaces with a large surface area, and the adaptations of the alveoli for gas exchange in the lungs and of the villi for the absorption of nutrients in the small intestine.
An SQA National 5 Biology answer on the absorption of materials, covering the need for efficient exchange surfaces with a large surface area, and the adaptations of the alveoli for gas exchange in the lungs and of the villi for the absorption of nutrients in the small intestine.
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What this dot point is asking
The SQA wants you to explain why multicellular organisms need efficient exchange surfaces with a large surface area, and to describe how the alveoli in the lungs are adapted for gas exchange and how the villi in the small intestine are adapted for absorbing nutrients.
Why exchange surfaces are needed
The features that make any exchange surface efficient are a large surface area, a short diffusion distance (thin walls), and a way to keep a steep concentration gradient (usually a good blood supply).
Gas exchange in the alveoli
The alveoli are the tiny air sacs at the end of the airways in the lungs. Their adaptations:
Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out, each moving down its concentration gradient.
Absorption in the villi
The villi are the finger-like projections lining the small intestine. They absorb the products of digestion.
So the same three principles, large surface area, short distance and a maintained gradient, make both the alveoli and the villi efficient.
Examples in context
Example 1. Surface area in numbers. The human lungs contain hundreds of millions of alveoli, giving a total surface area roughly the size of a tennis court. This huge area, packed into the chest, is why the lungs can take in enough oxygen for the whole body, a clear case of surface area suiting function.
Example 2. Absorbing a meal. After digestion, glucose and amino acids are absorbed across the thin walls of the villi into the blood capillaries inside each villus. The blood carries them to the liver and on to the body's cells, where the glucose is used in respiration.
Try this
Q1. Name the structures in the lungs where gas exchange happens. [1 mark]
- Cue. The alveoli.
Q2. State two adaptations shared by the alveoli and the villi. [1 mark]
- Cue. Large surface area and thin walls (and a good blood supply).
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe three ways the alveoli are adapted for efficient gas exchange and explain how each helps.Show worked answer →
A 4-mark answer needs three adaptations, each linked to faster diffusion.
There are many alveoli, giving a very large surface area, so more gas can diffuse at once.
The alveoli have thin walls (one cell thick), giving a short diffusion distance for gases to cross.
They have a good blood supply, which keeps a steep concentration gradient by removing oxygen and bringing carbon dioxide.
A moist surface also helps the gases dissolve and diffuse.
Markers reward (1) large surface area from many alveoli, (2) thin walls for a short distance, and (3) good blood supply to maintain the gradient, each linked to gas exchange.
SQA N5 style3 marksExplain how the structure of villi makes the small intestine efficient at absorbing food.Show worked answer →
A 3-mark answer should link villi features to absorption.
There are many villi (with microvilli on them), giving a very large surface area for absorbing digested food.
Each villus has thin walls, so glucose and amino acids can diffuse quickly into the blood.
Each villus has a rich blood supply, which carries the absorbed food away and keeps a steep concentration gradient for fast absorption.
Markers reward (1) large surface area, (2) thin walls for quick diffusion, and (3) good blood supply to maintain the gradient.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)