1. Ch 7: Membrane Structure
and Function
2016

2. Ch 7: Membrane From Topic 1.3
Essential idea: Membranes control the composition of cells by active and passive transport.
Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential (3.1).
Understandings:
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Applications and skills:
• 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.
• Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
• Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions (Practical 2).
Guidance:
• Osmosis experiments are a useful opportunity to stress the need for accurate mass and volume measurements in scientific experiments.
Utilization:
• Kidney dialysis artificially mimics the function of the human kidney by using appropriate membranes and diffusion gradients.
Aims:
• Aim 8: Organ donation raises some interesting ethical issues, including the altruistic nature of organ donation and concerns about sale of human organs.
• Aim 6: Dialysis tubing experiments can act as a model of membrane action. Experiments with potato, beetroot or single-celled algae can be used to investigate real membranes.
From Topic 1.3
Essential idea: The structure of biological membranes makes them fluid and dynamic.
Nature of science:
• Using models as representations of the real world—there are alternative models of membrane structure (1.11).
• Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model (1.9).
Understandings:
• 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.
• Cholesterol is a component of animal cell membranes.
Applications and skills:
• 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-Daniellimodel.
• Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
Guidance:
• Amphipathic phospholipids have hydrophilic and hydrophobic properties.
• Drawings of the fluid mosaic model of membrane structure can be two dimensional rather than three dimensional. Individual phospholipid molecules should be shown using the symbol of a circle with two parallel lines attached. A range of membrane proteins should be shown including glycoproteins.

3. Ch 7: Membrane From Topic 6.1 (introduced in HL 1 but covered in HL 2)
Understandings:
• Different methods of membrane transport are required to absorb different nutrients.
From Topic 6.5
• Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
From Topic 9.1
• Active uptake of mineral ions in the roots causes absorption of water by osmosis.
From Topic 9.2
• High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
• Active transport is used to load organic compounds into phloem sieve tubes at the source.

4. Biological Membranes
• Essential idea: The structure of biological membranes makes them fluid and dynamic
• 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.
• Amphipathic phospholipids have hydrophilic and hydrophobic properties.
Plasma membrane: a boundary that separates the living cell from it’s non-living surroundings; made of amphipathic phospholipids and proteins
@ 8 nm thick
Controls chemical traffic
Unique structure based on the different types of phospholipids and proteins found in the PM
Selectively permeable: allows some substance to cross more easily than others

5. Different Models of PM
• Using models as representations of the real world—there are alternative models of membrane structure
• 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. Membrane proteins are diverse in terms of structure, position in the membrane and function Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model.
Davson-Danielli Model “Sandwich” Model: In 1935, Hugh Davson and James Danielli suggest that the plasma layer is made of two layers of phospholipids that are each surrounded by a layer of protein
Pro’s and Con’s of this model?

6. Different Models of PM
Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
Using models as representations of the real world—there are alternative models of membrane structure
Essential idea: The structure of biological membranes makes them fluid and dynamic.
Singer-Nicolson “Fluid Mosaic” Model: In 1972, S.J. Singer and G. Nicolson proposed that the proteins are dispersed and inserted in the phospholipid bilayer with their hydrophilic regions facing the water
- This model was supported by freeze-fracture:

7. Fluidity of the Plasma Membrane
• Essential idea: The structure of biological membranes makes them fluid and dynamic.
• 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 • Amphipathic phospholipids have hydrophilic and hydrophobic properties• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
The fluidity of PM comes from the movement of the phospholipids and the proteins.
Lipids and proteins can drift laterally switching places, but it rare to switch between phospholipid layers.

8. Fluidity of the Plasma Membrane
• Membrane proteins are diverse in terms of structure, position in the membrane and function.
• Cholesterol is a component of animal cell membranes.
• Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
Unsaturated (kink) tails enhance fluidity
More saturated phospholipids makes it easier for it to solidify.
Cholesterol in eukaryotes modulates/stabilizes the fluidity of PM:
Less fluid in warmer temp by restraining phospholipid movement
More fluid in colder temp by preventing close packing of phospholipids.

9. Fluidity of the Plasma Membrane
• Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Supported by the 1970 Human-Mouse Hybrid Experiment.
Labeled with two different fluorescent dyes.
After a couple of hours they were evenly distributed.
Human Mouse Hybrids

10. “Mosaic-ness” of the Plasma Membrane
Membrane proteins are diverse in terms of structure, position in the membrane and function.
• Drawings of the fluid mosaic model of membrane structure can be two dimensional rather than three dimensional. Individual phospholipid molecules should be shown using the symbol of a circle with two parallel lines attached. A range of membrane proteins should be shown including glycoproteins.
Integral proteins
- generally transmembrane
Peripheral proteins
- not embedded but attached to the membrane surface.
- may be attached to integral proteins or held by fibers of ECM (Extra Cellular Matrix).
- on cytoplasmic side may be
* 2 video set of videos
IB Biology Topic Phospholipid Properties
IBguides's channel
IB Biology Topic Draw and Label the Plasma Membrane

11. Function of Membrane Proteins
• Membrane proteins are diverse in terms of structure, position in the membrane and function.
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
1) Transport 2) Intercellular joining
3) Enzymatic activity 4) Signal Transduction
5) Cell-Cell recognition 6) Attachment to cytoskeleton

12. Membrane Transport Selectively Permeable Non-polar Molecules
Essential idea: Membranes control the composition of cells by active and passive transport.
Selectively Permeable
- Types of membrane proteins: channel, pump, carriers?
- Nature of the substance: small/big, hydrophobic/hydrophilic, charged/charged?
Non-polar Molecules
- Dissolve in the membrane
- Smaller move faster than larger molecules
Polar molecules
- small polar, uncharged go right between the phospholipids
- Larger molecules need a transport protein
- Ions also need transport help.

13. Transport Proteins
• Essential idea: Membranes control the composition of cells by active and passive transport.
Transport Proteins: Specific molecules or ions can pass through integral proteins
May have a tunnel ( formed from hydrophilic amino acids)
May bind and physically move it across membrane acting as a carrier (gated/ungated)
Are specific for the substance they transport.

14. Passive Transport
• Essential idea: Membranes control the composition of cells by active and passive transport.
Passive Transport: Movement of a substance across a biological membrane.
No energy required.
Driven by the concentration gradient (from high to low)
Rate regulated by permeability and concentration
Example: Diffusion and Osmosis
How do you get the most efficient diffusion?
Steep gradient
Short Distance

15. Diffusion vs. Osmosis
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
• 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.
• Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions
• Osmosis experiments are a useful opportunity to stress the need for accurate mass and volume measurements in scientific experiments.
Diffusion: the tendency for molecules of any substance to spread out evenly into the available space
Osmosis: the passive transport of water across a membrane (from high to low concentration).
Hypertonic – A solution with a greater concentration of solute.
Hypotonic – A solution with a lower concentration of solute
Isotonic – A solution with an equal amount of solute.

16. Diffusion vs. Osmosis 100% water 30% solute Hypotonic Hypertonic
• Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential
100% water
30% solute
Hypertonic
Hypotonic

17. Diffusion vs. Osmosis 100% water 30% solute
• Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential
100% water
30% solute
Direction Osmosis Will Occur
Direction Diffusion of Solute Will Occur

18. Facilitated Diffusion
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Facilitated Diffusion: Diffusion across a membrane with the help of transport proteins.
Passive
Helps polar molecules and ions that are slowed down by the membrane lipid’s nature.
They are like enzymes because
have active sites
Max rate can be reached.
Can be inhibited.

19. Active Transport
• Active transport is used to load organic compounds into phloem sieve tubes at the source.
Energy is required to go against the concentration gradient (from high to low).
Requires energy
Helps maintain steep gradients, which is necessary for the body to work (ex. Action potentials in neurons)
Transport proteins work with ATP, which provides the necessary energy
Examples:
- Sodium Potassium Pumps in Eukaryotes
- Proton Pumps in Prokaryotes

20. Sodium Potassium Pump
• Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axon.
• Neurons pump sodium and potassium ions across their membranes to generate a resting potential
Three sodium are pumped out for every two potassium pumped in. Each is being pumped against the concentration gradient.
Na/K ATPase: Main electrogenic pump (meaning it creates a voltage across the membrane) in animal cells

21. Proton Pump
• Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons
Proton pumps: main electrogenic pump in plants, bacteria, and fungi and Chloroplasts, Mitochondria.
By creating a voltage, it stores energy that can be used for cellular work

22. Bulk Transport
• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Endocytosis: the transport of large molecules inside the cell by forming a vesicle from the plasma membrane
Exocytosis: transport of large molecules out of the cell using a vesicle that has budded off the plasma membrane

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

24. Unused IB Standards
• 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.
• Kidney dialysis artificially mimics the function of the human kidney by using appropriate membranes and diffusion gradients.
Aims:
• Aim 8: Organ donation raises some interesting ethical issues, including the altruistic nature of organ donation and concerns about sale of human organs.
• Aim 6: Dialysis tubing experiments can act as a model of membrane action. Experiments with potato, beetroot or single-celled algae can be used to investigate real membranes.
• High concentrations of solutes in the phloem at the source lead to water uptake by osmosis