Biology Help UKAQA AS & A-level practice

3.3.1 Surface area to volume ratio

AQA A-level Biology 3.3.1 practice on Surface area to volume ratio, with free MCQs, clues and worked explanations drawn from the Organisms exchange substances with their environment section of specification 7402.

Specification route
3.3.1
Question bank
15 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.3.1 · LEVEL 2

As an organism increases in overall size, what happens to its surface area to volume ratio?

  • It decreases, making simple diffusion insufficient for exchange.
  • It increases, allowing the organism to rely entirely on simple diffusion.
  • It remains constant, regardless of the organism's dimensions.
  • It decreases initially but then increases as specialized systems develop.
Show clue

Consider the mathematical relationship: volume is cubed, while surface area is squared.

Show answer and explanation

Answer: It decreases, making simple diffusion insufficient for exchange.

The relationship between the size of an organism or structure and its surface area to volume ratio is inverse. A common misconception is confusing total surface area (which increases) with the ratio (which decreases). As the ratio reduces, specialized systems must develop.

QUESTION 2 · 3.3.1 · LEVEL 1

How do larger multicellular organisms generally adapt to facilitate exchange as their surface area to volume ratio reduces?

  • By changes to body shape and the development of specialized exchange systems.
  • By decreasing their overall metabolic rate to zero to prevent the need for exchange.
  • By relying completely on simple diffusion across their outer epidermal layer.
  • By actively transporting all necessary gases directly across their body surface.
Show clue

Think about why humans have lungs and a circulatory system, whereas amoebas do not.

Show answer and explanation

Answer: By changes to body shape and the development of specialized exchange systems.

Changes to body shape and the development of systems in larger organisms act as adaptations that facilitate exchange as this ratio reduces. Students often falsely believe simple diffusion scales infinitely.

QUESTION 3 · 3.3.1 · LEVEL 1

How does a single-celled organism, such as an amoeba, primarily exchange gases with its environment?

  • By simple diffusion directly across its body surface.
  • By using a specialized microscopic tracheal system.
  • By actively transporting oxygen and carbon dioxide using ATP.
  • Through microscopic gills extending from its cell membrane.
Show clue

Their large surface area to volume ratio makes complex systems unnecessary.

Show answer and explanation

Answer: By simple diffusion directly across its body surface.

Adaptations of gas exchange surfaces are shown by gas exchange across the body surface of a single-celled organism. Because they have a high surface area to volume ratio, simple diffusion is sufficient for their metabolic needs.

QUESTION 4 · 3.3.1 · LEVEL 1

What is the general mathematical relationship between the size of an organism and its surface area to volume (SA:V) ratio?

  • As the size of an organism increases, its surface area to volume ratio decreases.
  • As the size of an organism increases, its surface area to volume ratio increases proportionally.
  • The surface area to volume ratio remains constant regardless of the size of the organism.
  • As the volume increases, the surface area decreases.
Show clue

Think of a small cube versus a massive block. The massive block has a lot of 'inside' compared to its 'outside'.

Show answer and explanation

Answer: As the size of an organism increases, its surface area to volume ratio decreases.

The relationship between size and surface area to volume ratio. As an object gets larger, its volume (cubed) increases much faster than its surface area (squared), resulting in a smaller SA:V ratio for larger organisms.

QUESTION 5 · 3.3.1 · LEVEL 2

Why do large, multicellular organisms require specialised exchange surfaces and mass transport systems, whereas single-celled organisms do not?

  • Large organisms have a very small SA:V ratio and a long diffusion distance to their core cells, meaning simple diffusion across their outer surface is too slow to meet their metabolic needs.
  • Single-celled organisms do not require oxygen or nutrients to survive.
  • Large organisms have a massive SA:V ratio, which causes them to lose nutrients too rapidly without a transport system.
  • Specialised exchange surfaces are only required for thermoregulation, not for acquiring nutrients.
Show clue

Imagine oxygen trying to diffuse from your skin to the centre of your liver. It would take years.

Show answer and explanation

Answer: Large organisms have a very small SA:V ratio and a long diffusion distance to their core cells, meaning simple diffusion across their outer surface is too slow to meet their metabolic needs.

Single-celled organisms have a large SA:V ratio and short diffusion pathways. Large organisms have a small SA:V ratio, so they evolved specialised surfaces (like lungs) and transport systems (like blood) to move substances efficiently.

QUESTION 6 · 3.3.1 · LEVEL 4

A mouse and an elephant are both mammals. Why does a gram of mouse tissue require a significantly higher rate of oxygen consumption (respiration) than a gram of elephant tissue?

  • A mouse has a much larger SA:V ratio than an elephant, so it loses body heat to the environment much faster. It must maintain a higher metabolic rate to generate enough heat to maintain its core body temperature.
  • A mouse has a smaller SA:V ratio, so it traps heat and must respire quickly to burn off the excess energy.
  • Elephants are reptiles, so they do not need to generate metabolic heat to maintain their body temperature.
  • Mice lack lungs, so they have to force oxygen directly into their tissues using massive amounts of ATP.
Show clue

Small mammals lose heat very easily. How do endotherms replace lost heat?

Show answer and explanation

Answer: A mouse has a much larger SA:V ratio than an elephant, so it loses body heat to the environment much faster. It must maintain a higher metabolic rate to generate enough heat to maintain its core body temperature.

Metabolic rate correlates with SA:V ratio. Small endothermic mammals lose heat rapidly due to their large SA:V ratio. To compensate and maintain a constant core temperature, they must have a high rate of aerobic respiration, thus consuming more $O_2$ per gram of body mass.

QUESTION 7 · 3.3.1 · LEVEL 2

How is a flatworm structurally adapted to overcome the limitations of lacking a mass transport system?

  • It has a very flat, thin body shape, which massively increases its surface area to volume ratio and keeps all its cells very close to the external environment, ensuring a short diffusion pathway.
  • It has an incredibly high internal body temperature to speed up the kinetic energy of diffusing molecules.
  • It lives purely inside the blood of other animals, where it does not need to perform gas exchange.
  • It relies entirely on active transport across its epidermis to pull oxygen directly into its core.
Show clue

Its name gives away its physical shape, which is a common adaptation in leaves and simple animals.

Show answer and explanation

Answer: It has a very flat, thin body shape, which massively increases its surface area to volume ratio and keeps all its cells very close to the external environment, ensuring a short diffusion pathway.

Changes to body shape affect SA:V. A flattened body greatly increases the surface area relative to the volume and minimizes the diffusion distance from the outside to the deepest cells, allowing simple diffusion to suffice.

QUESTION 8 · 3.3.1 · LEVEL 2

According to Fick's Law, what three features are shared by almost all highly efficient, specialised biological exchange surfaces (like alveoli or villi)?

  • A very large surface area, a very thin exchange surface (short diffusion pathway), and the maintenance of a steep concentration gradient.
  • A small surface area, a thick protective barrier, and an equal concentration on both sides.
  • A high temperature, a low pH, and the continuous presence of ATP.
  • A massive volume, a constant supply of water, and a highly folded nucleus.
Show clue

Look for the mathematical variables that maximize the rate of diffusion.

Show answer and explanation

Answer: A very large surface area, a very thin exchange surface (short diffusion pathway), and the maintenance of a steep concentration gradient.

Fick's law dictates that the rate of diffusion is proportional to: (Surface Area $\times$ Difference in Concentration) / Thickness of exchange surface.

Practise this topic

15 questions are available. No sign-in required.

Start 20-question quizUse all available questions