2.3.1Inquiry Process
Lipid Bilayers — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 48 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Why a Bilayer Forms
Cell membranes are built on a sheet two phospholipids thick.
Amphipathic molecules
- Amphipathic molecules Each phospholipid has a polar phosphate head attracted to water and two non-polar fatty acid tails repelled by it.
Hydrophobic effect
- Hydrophobic effect In water the tails are pushed together and shielded from water, which is the most stable arrangement.
Two layers
- Two layers Heads face the watery cytoplasm on one side and the watery extracellular fluid on the other.
Spontaneous
- Spontaneous No energy or enzyme is required for the bilayer to assemble, because the arrangement is favoured by the properties of the molecules.
| Region | Chemistry | Faces |
|---|---|---|
| Head | Polar, hydrophilic | Water on both surfaces |
| Tails | Non-polar, hydrophobic | Interior of the bilayer |
Section 2 of 6Barrier Properties
Different substances cross the bilayer at very different rates.
Barrier Properties
Hydrophobic core
- Hydrophobic core The oily centre blocks movement of ions and polar molecules, so most such substances cannot pass freely.
Small non-polar molecules
- Small non-polar molecules Oxygen, carbon dioxide and steroid hormones dissolve in the core and diffuse across.
Water
- Water Small water molecules cross slowly between phospholipids, and faster through aquaporins.
Selective permeability
- Selective permeability Membrane proteins add controlled routes, so the cell decides what enters and leaves.
Section 3 of 6Fluidity and Flexibility
1
Phospholipids are held together by weak hydrophobic interactions rather than covalent bonds.
2
Individual molecules move sideways within their own layer very rapidly.
3
Occasionally a phospholipid flips between layers, which is much rarer and needs enzymes.
4
Weak bonding allows the membrane to change shape and to reseal if it is punctured.
5
Flexibility enables vesicles to bud off and fuse with other membranes.
Section 4 of 6Roles of Membranes
Cell boundary
- Cell boundary The plasma membrane separates the cell contents from the outside environment.
Compartments
- Compartments Membranes inside the cell separate organelles so reactions with different needs can happen side by side.
Surface for reactions
- Surface for reactions Membranes hold enzymes and electron carriers in organised arrangements.
Communication
- Communication Receptors and recognition molecules in the membrane allow cells to respond to signals.
Section 5 of 6Worked Example
Question Explain why oxygen can cross a phospholipid bilayer easily but sodium ions cannot, using the structure of the bilayer.
- The centre of the bilayer is made of hydrophobic fatty acid tails, so it is a non-polar environment.
- Oxygen is a small non-polar molecule, so it dissolves in this region and diffuses down its concentration gradient.
- A sodium ion is charged and surrounded by water molecules, so it is strongly repelled by the non-polar core and cannot cross without help.
Answer Oxygen passes through the hydrophobic core freely, whereas sodium ions need a channel or carrier protein.
Section 6 of 6Exam Checklist & Tips
2.3.1 – Key Points for the Exam
- Phospholipid bilayer A double layer of phospholipids with hydrophobic tails inside and hydrophilic heads outside.
- Amphipathic Containing both hydrophilic and hydrophobic parts.
- Partially permeable Allowing some substances to cross but not others.
- Hydrophobic Repelled by water and attracted to other non-polar molecules.
Common Mistakes
- Drawing a membrane with the tails facing the water on both sides.
- Saying the membrane is solid; it is fluid.
- Thinking all molecules cross the bilayer equally easily.
- Claiming that water cannot cross at all.
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