Lesson 3: Surface Area to Volume Ratio & Cell Size Limits
Use calculation and simulated diffusion evidence to explain why cell size and shape matter.
60-minute plan: retrieval 5 · media 10 · data task 25 · portfolio 15 · exit check 5
🎯 Ngā Whāinga Akoranga | Learning Intentions
How cell size and shape govern the surface area to volume (SA:V) ratio and dictate diffusion efficiency across membranes.
Calculate SA:V ratios for different cube/cell sizes, analyse agar block diffusion data, and justify why large organisms must be multicellular.
🎥 Media Anchor & Pedagogical Scaffold
Surface area to volume ratio of cells | Cell structure and function | AP Biology
Video: Khan Academy (full runtime: 5m 12s).
🧠 1. Before Viewing (Activate & Predict)
Why are most cells microscopic? Predict how surface-area-to-volume ratio constrains exchange as a cell grows.
👁️ 2. During Viewing (Watch With a Job)
- Watch for: Observe how volume increases faster than surface area as dimensions increase.
- Watch for: Record the relationship between SA:V ratio and exchange with the environment.
- Watch for: Note shapes or structures that increase effective surface area.
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako move: Model one cube calculation, then release learners to the simulated dataset.
Immediate task: Open Learner Evidence Pack · Sheet 3.
⚡ Whakaoho | Do Now: Cell Recall Challenge (5 mins)
Why most cells stay small. Surface-area-to-volume ratio is one important constraint on cell size, alongside biological regulation and resource supply.
Two minutes: a cube cell of side 1 cm has surface area 6 cm² and volume 1 cm³. Work out both for a 3 cm cube. Now state what happened to the ratio — and what that means for a molecule trying to diffuse from the surface to the centre.
📖 Activity 1: Core Process Investigation & Diagram Analysis (25 mins)
Calculate and interpret (12 min). Complete the cube table and show the formula for surface area, volume and SA:V. Describe the numerical pattern.
Use simulated evidence (13 min). Analyse the supplied agar-cube dataset. Connect penetration distance, exchange area and diffusion distance to the proportion reached. Then explain one specialised exchange surface.
Supplied inputs
- Learner Evidence Pack · Sheet 3 — formula support and explicitly simulated agar dataset
- Kaiako Guide · Lesson 3 — calculations and extension cues
📝 Activity 2: Portfolio evidence (15 mins)
Complete Portfolio Section 3. For the 5-minute exit check, explain why a tenfold increase in cell length does not preserve SA:V.
🏫 Kaiako Planning & Pedagogy Notes
Level 2 Biology focus: calculation supports the explanation; the expected endpoint is a biological consequence for exchange and cell function. The agar data are a labelled classroom model. See Kaiako Guide · Lesson 3.
Paired concept in Mechanism & Exam Synthesis: surface-area-to-volume ratio →