B2.1  Membranes and Membrane Transport

Theme:  Form and Function

Membrane function emerges entirely from membrane structure - the amphipathic nature of lipids creates barriers, protein shapes determine transport specificity, and membrane fluidity enables dynamic processes, proving that cellular form determines cellular function.

• Amphipathic phospholipids form bilayers with hydrophobic cores that block polar molecules while allowing non-polar gases to pass through.
  
• Membrane fluidity from fatty acid composition and cholesterol determines flexibility and transport capability.
  
• Proteins in the cell membrane have specific shapes that create selective pathways for particular particles. 
  
• Glycoproteins and glycolipids extend carbohydrates from membrane surfaces that function as identification tags for cell recognition and adhesion.
  
• Fluid properties enable vesicle formation and fusion for endocytosis and exocytosis, showing how physical structure determines cellular capabilities.
  

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?

Key Terms to Know: * higher level only

Absorption*
Active Transport
Adenosine Triphosphate (ATP)
Adhesion
Amphipathic
Aquaporin
Aqueous
Bilayer
Carbohydrate
Carbon Dioxide (CO2)
Cell Junction*
Cell Membrane
Cell Recognition
Cell-Adhesion Molecules (CAMs)*
Channel Protein
Cholesterol
Concentration Gradient
Cotransporter*
Diffusion
Endocytosis*
Exchange Transporter*
Exocytosis*
Extracellular

Facilitated Diffusion
Fatty Acid*
Fluid Mosaic Model
Fluidity*
Fusion*
Gated Channel*
Glucose*
Glycolipid
Glycoprotein
Hydrocarbon
Hydrophilic
Hydrophobic
Impermeability
Indirect Active Transport*
Integral Protein
Ion
Lipid
Melting Point*
Membrane Potential*
Nephron*
Neuron*
Neurotransmitter*
Nicotinic Acetylcholine Receptor*

Osmosis
Oxygen (O2)
Peripheral Protein
Permeability
Phospholipid
Polar
Potassium Channel*
Protein
Pump Protein
Reabsorption*
Saturated*
Selective Permeability
Small Intestine*
Sodium Channel*
Sodium-Dependent Glucose Cotransporter*
Solute
Specific
Tissue*
Unsaturated*
Vesicle*
Voltage-Gated Channel*

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.
• Explain why the hydrophobic core of a lipid bilayer forms a barrier to hydrophilic particles. 
• List advantages of membranes forming barriers within and between cells.

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.  
• Outline six 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 facilitated 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. 
• State that simple diffusion through the membrane is not selective and depends only on the concentration gradient.
• Summarize the effect of particle size on ability to pass through the lipid bilayer.
• Summarize the effect of hydrophobic/hydrophilic properties of a particle on its ability to pass through the lipid bilayer.
• List examples of particles with low and high membrane permeability.
• Outline how channels and pumps in the membrane allow for selective permeability.

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

• Outline the structure of glycoproteins and glycolipids.  
• Outline 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
• 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​

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 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. ​

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.
• Outline the structure, function and specificity of ligand-gated channels.

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. ​
• State that the loss of CAM function can lead to metastasis of cancer cells.