Lesson at a Glance | He Tirohanga Whakamua
Ngā Whāinga Ako | Learning Intentions
Students will know
- Why the cell membrane is termed a "Fluid Mosaic Model" (phospholipids flow laterally; proteins float like mosaic tiles).
- The chemical structure of amphipathic phospholipids (hydrophilic polar heads facing water; hydrophobic non-polar fatty acid tails facing inwards).
- How membrane composition dictates selective permeability: small non-polar molecules (O₂, CO₂) cross freely; polar solutes and ions generally require protein channels or carriers; and water crosses the bilayer slowly but often moves rapidly through aquaporins.
Students will demonstrate
- By sketching and annotating a fluid mosaic membrane diagram with 5 key components.
- By completing formative Section 2 in their class exercise book or revision folder.
Curriculum alignment
- NZC (2007) · Science · Level 7 · Living World: “Explore the diverse ways in which animals and plants carry out the life processes.”
Do Now | Tīmatanga Whakaaro (10 min)
Membrane Chemistry Prompt:
"Oil and water do not mix. How does a cell use this exact chemical rule to create a barrier between its cytoplasm and the outside world?"
Unpack: Cell membranes are made of phospholipids. Their hydrophobic (water-fearing) fatty acid tails point inward away from water, creating an impermeable fatty barrier in the middle of the membrane that stops water-soluble substances from leaking freely in or out.
Phospholipid Bilayer (15 min)
1. One molecule, two behaviours
A phospholipid has a phosphate head that is hydrophilic (attracted to water) and two fatty acid tails that are hydrophobic (repelled by it). A molecule with both properties is called amphipathic, and everything else in this lesson follows from that single fact.
2. Why it assembles itself
Put phospholipids in water and the bilayer forms on its own — heads out towards the water on both sides, tails tucked inward away from it. Nothing builds it. It is simply the lowest-energy arrangement available.
3. The barrier this creates
The middle of the membrane is a fatty, water-free core. Small non-polar molecules (O₂, CO₂) slip through it, and water crosses slowly. Ions and polar molecules such as glucose and amino acids are effectively blocked.
4. The consequence to carry forward
Anything water-soluble needs a protein to cross. That is not a detail — it is the reason transport proteins exist, and it sets up everything in Lessons 3 and 4.
Mahi | Do this: Sketch the bilayer and annotate one head and one tail with why it sits where it does. Then predict, for O₂, glucose, Na⁺, CO₂, water and a steroid hormone, which cross unaided — and check your predictions at the end of Lesson 3.
4 Key Components of the Fluid Mosaic Model (15 min)
1. Phospholipid Bilayer
Double layer of phospholipids. Hydrophilic phosphate heads face extra- and intra-cellular fluids; hydrophobic fatty acid tails face interior.
2. Transport Proteins
Channel Proteins: Water-filled pores for rapid ion diffusion.
Carrier Proteins: Change shape to bind and transport specific molecules.
3. Cholesterol
Steroid molecules embedded between tails. Maintains membrane fluidity at low temps and prevents excessive fluidity at high temps.
4. Glycoproteins & Glycolipids
Carbohydrate chains attached to proteins/lipids on outer surface. Act as cell recognition markers and hormone receptors.
📁 Formative Revision Exercise Book — Section 2: Membrane Structure & Permeability
Students complete Section 2 in their exercise book or class revision folder. This membrane model is formative evidence for kaiako feedback, not an NZQA submission. All required prompts are supplied below:
Section 2 Requirements:
1. Annotated Fluid Mosaic Diagram: Draw and label Phospholipid heads/tails, Channel Protein, Carrier Protein, Cholesterol, and Glycoprotein.
2. Selective Permeability Table: Categorise six cases taught above—O₂, CO₂, H₂O, glucose, Na⁺ moving down its gradient, and K⁺ moving down its gradient—by their main route across the membrane (direct lipid diffusion, aquaporin or ion channel, or carrier).
3. Excellence Rationale: 1 paragraph explaining why cells MUST regulate membrane transport to maintain internal homeostasis.
Exit Verification | Ka Mutu Hoki (5 min)
Exit Check:
"My Section 2 diagram correctly identifies hydrophobic tails facing inwards and labels channel/carrier proteins."
Teacher Planning & NCEA Alignment
NCEA Level 2 Biology Alignment (4 Credits External):
- Membrane Structure: Demonstrate understanding of the fluid mosaic model, phospholipid bilayer, and selective permeability.
- Cell Homeostasis: Explain how membrane architecture regulates movement of materials in and out of cells.
Vocabulary: Fluid mosaic model, phospholipid bilayer, hydrophilic, hydrophobic, amphipathic, channel protein, carrier protein, cholesterol, selective permeability.
Paired concept in Guided Media & Evidence: membrane structure & transport →