Biology HL Paper 1A (May 2025, TZ2) · Question 16
Mixed-topicMatching part: 16
Also tests: 7.1 Principles and Modes of Cell Signalling
Practice this question →6.4 Cell Size, Growth and Division Signals
Use geometry to explain cell-size limits and distinguish growth factors from mitogens.
Estimated time: 58 minutes
IB syllabus: B2.3 · D2.1 · SL and HL
Area and Volume Scale Differently
A cell's volume approximates the amount of metabolically active material requiring substrates and producing waste. Its surface area limits exchange across the plasma membrane. If every linear dimension increases by factor k, area increases by k² but volume by k³; the area-to-volume ratio therefore falls as size rises. Diffusion paths to the interior also lengthen.
For a cube of side L, area is 6L², volume is L³ and SA:V is 6/L. A 1-unit cube has ratio 6:1, a 2-unit cube 3:1 and a 3-unit cube 2:1. The cube is a model, but the scaling rule applies to similar shapes. It explains why one giant cell cannot simply replace many smaller cells with the same total volume.
HL extensionB2.3 AHL · D2.1 AHL
Cells Modify Shape or Internal Transport
Microvilli and membrane folds increase area without a proportional volume increase. Flattening shortens diffusion distance, as in gas-exchange surfaces; elongation can connect distant locations, as in axons. Internal cytoplasmic streaming, transport proteins and organelles improve distribution, but none abolishes geometric constraints.
Specialized size reflects function. Biconcave erythrocytes combine high area with a thin centre for gas exchange. Intestinal epithelial cells add microvilli for absorption and divide frequently because the surface is damaged. Adipocytes and oocytes can be large because storage dominates their role; an axon may be extremely long but remains narrow.
Growth and Cell-cycle Entry Are Distinct
Growth means increased mass or volume; division means production of daughter cells. Early embryos can divide repeatedly with little growth, partitioning one zygote into smaller cells. Muscle fibres, neurons and adipocytes can grow without dividing. Extracellular growth factors promote cell growth and survival, while mitogens bind receptors and stimulate entry into the cell cycle.
Division restores a higher area-to-volume ratio and creates new membrane, but it must be coordinated with DNA replication and organelle distribution. Cell size at division is therefore an outcome of nutrient supply, biosynthesis, growth signals, mitogens and checkpoint control—not a fixed geometric trigger acting alone.
Cell scale laboratory
Enlarge a cell, add folds and divide it to compare exchange area and metabolic volume.
Exchange · gradients · inheritance
Cell function laboratory
Test Yourself
What is the surface-area-to-volume ratio of a cube-shaped cell with side length 4 µm? Enter the numerical ratio to 1.
Exam questions on this topic
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