Understanding(s)

• Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
• Membrane proteins are diverse in terms of structure, position in the membrane and function.

  Membrane Structure 

• Cholesterol is a component of animal cell membranes.

  Further Membrane Structure

• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

  Membrane transport models

  Membrane Permeability Experiment 

• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

Essential Question(s)

• Why do the lipid tails of phospholipids attract each other?
• How does a phospholipid bilayer prevent the loss of water from cells ?

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• How can proteins attach to the bilayer?
• What functions can membrane proteins have within cells?
• What other substances are found in the plasma membrane in addition to protein and phospholipid?

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• How can cells control the movement of substances through the plasma membrane using the membrane proteins?
• How can the cell take in or excrete substances using membrane fluidity?

ToK / NOS / IM

• Biologists use models to represent the real world, e.g. cell membranes.
• Evidence is used to support or to falsify theories.
• Theories are modified in the light of new evidence and gradually improve.
• In some circumstances evidence in support of an old theory which was once falsified comes to light.

Skills students will have

• Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
• Skill: Drawing of the fluid mosaic model.
• Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
• Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

  Falsification of membrane structures

• Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
• Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
• Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)

  Onion cell plasmolysis experiment (new guide)

Students will be able to demonstrate their factual understanding in the worksheets and question sheets in all the activities below.

In the Falsification of membrane structures there is an open ended task to link research evidence to the nature of science. This allows bright students to extend themselves.

The two experiments on membrane permeability and plasmolysis are useful in scaffolding student skills for the Analysis and Evaluation aspects of the IA as part of the PSOW.