Edexcel IAL Biology · Unit 2
Gas Exchange, Cell Membranes & Transport
The big idea: living things need to move stuff (gases, water, nutrients) in and out of cells across membranes —
and the whole chapter is really just different answers to "how does that movement happen, and how do organisms make it efficient?"
Summary — the whole chapter in one scan
Read this first. If every line makes sense, you're in great shape. If any line confuses you, jump to that section below.
- Exchange surfaces (like alveoli, gills, leaves) are built to maximise diffusion — big surface area, thin/short diffusion pathway, steep concentration gradient.
- As organisms get bigger, their surface area : volume (SA:V) ratio drops, so simple diffusion alone stops being enough — hence specialised exchange organs evolved.
- Fick's Law gives the maths: rate of diffusion depends on surface area, concentration difference, and (inversely) the thickness of the membrane.
- The mammalian lung is a real-world example of an exchange surface — trachea → bronchi → bronchioles → alveoli, all adapted for efficient gas exchange.
- Cell membranes are built from a phospholipid bilayer — hydrophilic heads out, hydrophobic tails in — plus proteins, cholesterol, and glycoproteins/glycolipids. This whole structure is called the fluid mosaic model.
- Scientific models of the membrane evolved over time (Gorter-Grendel → Davson-Danielli → Singer-Nicolson) as new evidence and technology appeared — a great example of how science actually works.
- Osmosis is just a special, water-only case of diffusion — water moves from high to low water potential through a partially permeable membrane.
- Substances cross membranes by four main routes: simple diffusion (small/non-polar, passive), facilitated diffusion (large/polar/charged, passive, via channel or carrier proteins), active transport (against the gradient, needs ATP, uses carrier proteins), and endocytosis/exocytosis (bulk transport of huge molecules via vesicles).
- The Core Practical uses beetroot pigment leakage to investigate how temperature (and alcohol) affects membrane permeability.
Topic 1
Properties of Gas Exchange Surfaces
Why every exchange surface in biology "looks" similar, no matter which organism you're looking at.
Surface Area : Volume (SA:V) Ratio
Every organism needs to exchange gases (O₂ in, CO₂ out for respiration; CO₂ in, O₂ out for photosynthesis). This exchange happens by diffusion across an exchange surface.
Think of it like a warehouse and its loading dock. The "volume" is how much stuff is stored inside (how much oxygen the organism's cells need). The "surface area" is the loading dock — the only place goods can get in or out. A tiny warehouse has a huge dock relative to its stock, so deliveries are effortless. A giant warehouse with the same-shaped dock gets overwhelmed — not enough door space for how much needs to move in and out. That's exactly the SA:V problem organisms face as they grow.
As an organism's linear size increases, volume increases much faster than surface area (volume scales with length³, surface area only scales with length²). So SA:V ratio always falls as size increases:
Topic 2
Fick's Law of Diffusion
The maths behind everything you just read.
Fick's Law formalises the three factors above into one relationship: