B2.1  Membranes and Membrane Transport

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• How do molecules of lipid and protein assemble into biological membranes?
• What determines whether a substance can pass through a biological membrane?

​​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• What processes depend on active transport in biological systems?
• What are the roles of cell membranes in the interaction of a cell with its environment?

Resources:

• At SHS, Topic B2.1 is taught in the Membrane Structure, Membrane Transport, Signaling Within Neurons and Signaling Between Neurons units. 
• Quizlet study set for this topic.  Coming soon!
• View the general sequence of the SHS course
• View the SHS units mapped to the IB Biology curriculum roadmap

B2.1.1— Lipid bilayers as the basis of cell membranes.

• State that phospholipids naturally form continuous sheet-like bilayers in water.
• List locations of lipid bilayers in cells. ​

B2.1.2— Lipid bilayers as barriers.

• State the primary function of the cell membrane.
• Outline the location of aqueous solutions in relation to the lipid bilayer. 
• Explain why the hydrophobic core of a lipid bilayer has low permeability to large molecules and hydrophobic particles. 

B2.1.3—  Simple diffusion across membranes.

• Describe simple diffusion.
• Outline the impact of concentration gradient, particle size and polarity or charge of molecules on the rate of diffusion across a lipid membrane. 
• Explain two examples of simple diffusion of molecules into and out of cells.​

B2.1.4— Integral and peripheral proteins in membranes.

• Compare the location of integral and peripheral proteins in the membrane. 
• Outline how the hydrophobic and hydrophilic structures of proteins impact their anchoring to the membrane.  
• List at least four functions (with example) of membrane bound proteins.​

B2.1.5— Movement of water molecules across membranes by osmosis and the role of aquaporins.

• Define osmosis.
• Predict the direction of water movement based upon differences in solute concentration.
• Outline the structure and function of aquaporin proteins.​

B2.1..6-  Channel proteins for facilitated diffusion.

• Describe the structure and function of channel proteins. 
• Define facilitated diffusion. 
• Outline the specificity of channel proteins for ions.
• List types of gates on channel proteins.
  
• Describe one example of facilitated diffusion through a protein channel.

B2.1.7— Pump proteins for active transport.

• Describe the structure and function of pump proteins, including the role of specificity, conformational change and ATP.  
• Compare active transport using a pump protein to facilitate diffusion using a channel protein.
• Explain one example of active transport of molecules into and out of cells through a protein pump.

B2.1.8- Selectivity in membrane permeability.

• Define selective permeability. 
• Outline how channels and pumps in the membrane allow for selective permeability.
• State the simple diffusion is not selective and depends only on the concentration gradient, particle size and polarity or charge of molecules.

B2.1.9- Structure and function of glycoproteins and glycolipids.

• Outline the structure of glycoproteins and glycolipids.  
• Describe the role of glycoproteins and glycolipids in cell adhesion and cell recognition.

B2.1.10- Fluid mosaic model of membrane structure.
Draw and label a two-dimensional representation of the fluid mosaic model of membrane structure.  Include:

• Phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outwards
• Integral proteins shown embedded  the membrane
• Peripheral proteins on membrane surface or anchored to an integral protein
• A transmembrane protein channel with a pore
• Glycoproteins with a carbohydrate side chain facing the outside of the cell
• Glycolipid with a carbohydrate side chain facing the outside of the cell
• Cholesterol embedded between phospholipids in the hydrophobic region
• An indication of thickness (8-10nm)

AHL ​B2.1.11- Relationships between fatty acid composition of lipid bilayers and their fluidity.

• Compare and contrast the structure, melting point and relative fluidity of saturated and unsaturated fatty acids in lipid bilayers. 
• Outline an example of adaptation in membrane composition in relation to habitat.

AHL B2.1.12- Cholesterol and membrane fluidity in animal cells.

• Identify the structure of cholesterol in molecular diagrams.
• Describe the structural placement of cholesterol within the cell membrane.
• Outline how temperature affects cell membrane fluidity. 
• Describe the function of cholesterol as a modulator of membrane fluidity.

AHL B2.1.13- Membrane fluidity and the fusion and formation of vesicle.

• Outline the structure and function of vesicles in cells.
• State that the fluid properties of the cell membrane allow for the formation of vesicles. 
• Outline the formation of a vesicle within the cell.
• Describe the transport of materials within a cell using vesicles. 
• Describe the formation of a vesicle via endocytosis.
• Outline two examples of materials brought into the cell via endocytosis.
• Describe the release of materials from cells via exocytosis.
• Outline two examples of materials released from a cell via exocytosis.
• State the effect of endocytosis and exocytosis on the area of the cell membrane. 
• Contrast secretion with excretion. 

AHL B2.1.14- Gated ion channels in neurons.

• Outline the structure, function and specificity of voltage-gated channels. ​
• Describe the function of the voltage-gated potassium channel. 
• Outline the structure, function and specificity of ligand-gated channels.
• Describe the function of the ligand-gated sodium channel in the postsynaptic cell membrane (AKA the nicotinic acetylcholine receptor).

AHL B2.1.15- Sodium–potassium pumps as an example of exchange transporters.

• State the function of an exchange-transporter proteins. 
• Describe the structure and function of the sodium-potassium pump.
• Outline the role of the sodium-potassium pump in maintaining neuronal resting potential.

AHL B2.1.16- Sodium-dependent glucose cotransporters as an example of indirect active transport.

• Outline the function of cotransporters in indirect active transport.
• Describe the role of glucose cotransport proteins in the small intestine and kidney nephron.​

AHL B2.1.17- Adhesion of cells to form tissues.

• Outline the role of CAMs in the formation of tissues. ​