Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 223–236 of the course textbook. Image credits are on slide 18.
A phospholipid has a hydrophilic phosphate head and two hydrophobic hydrocarbon tails, so it is amphipathic. In water, the heads face the water and the tails face one another. This forms a continuous double layer, the basis of every cell membrane.
The hydrophobic core has low permeability to large molecules and hydrophilic particles, including ions and polar molecules. The membrane therefore separates two watery solutions.
Diffusion is the net movement of particles from higher to lower concentration, and it uses no ATP. Oxygen and carbon dioxide are small and non-polar, so they pass between the phospholipids.
Integral proteins are embedded in one or both lipid layers, and the parts inside the core are hydrophobic. Peripheral proteins are attached to one surface. Membrane proteins vary widely in their structure and role, and more active membranes contain more of them.
Osmosis is the net movement of water across a membrane, from a lower to a higher solute concentration. Water molecules move at random, and the membrane does not let the solute cross. Aquaporins are channel proteins that greatly speed up water movement while blocking ions.
Watch out: describe direction using solute concentration. Water moves, and the solute does not.
A channel has a hydrophilic pore, and its size and charge decide which particles pass. Many channels open and close. Movement is passive and goes down the concentration gradient.
Pumps use energy from ATP to move specific particles against a concentration gradient. ATP makes the protein change shape, and pumps usually work in one direction.
| Route | ATP? | Protein | Direction |
|---|---|---|---|
| Simple diffusion | No | None | Down gradient |
| Facilitated diffusion | No | Channel | Down gradient |
| Active transport | Yes | Pump | Against gradient |
Facilitated diffusion and active transport are selective, because each protein handles particular particles. Simple diffusion is not selective, since it depends only on size and on hydrophobic or hydrophilic properties.
These have a carbohydrate attached to a protein or a lipid, and the carbohydrate is always on the extracellular side. They help with cell adhesion and cell recognition.
Molecules move sideways within the membrane, which makes it fluid, and proteins are scattered among the lipids like a mosaic. A drawing should show phospholipids with hydrophilic and hydrophobic regions, integral and peripheral proteins, glycoproteins and cholesterol.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all ten statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 7 marks in total. Your teacher has the mark scheme.
Red blood cells take up glucose through GLUT1 carrier proteins in the plasma membrane. The rate of glucose uptake was measured at different external glucose concentrations, in normal cells and in artificial lipid vesicles that have no carrier proteins.
You will need: Dialysis tubing, starch solution, glucose solution, beaker, iodine solution, glucose test strips
Safety: Eye protection when using iodine. Wipe up spills.
Carry this out with your teacher's supervision and follow your school's lab rules.
The Davson-Danielli protein sandwich model was replaced by the Singer-Nicolson fluid mosaic model in 1972, as new evidence contradicted the older one.
A model can be useful and later be shown to be wrong. Consider whether today's fluid mosaic model is true, or simply the best available.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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