Ch 7: Membrane
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• Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model (1.9).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.
From Topic 1.4
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.
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 Understandings:
• Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
From Topic 9.1 Understandings:
• Active uptake of mineral ions in the roots causes absorption of water by osmosis.
From Topic 9.2 Understandings:
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
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
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
Fluidity of the Plasma Membrane
•
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.
• 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.
Fluidity of the Plasma Membrane
•
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.
• Membrane proteins are diverse in terms of structure, position in the membrane and function. • Cholesterol is a component of animal cell membranes.
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.
“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 involved in reactions
http://www.susanahalpine.com/a
nim/Life/memb.htm
* 2 video set of videos
IB Biology Topic 2.4.2 Phospholipid Properties
https://www.youtube.com/watch?v=jrxnTgQD hrU
IBguides's channel
https://www.youtube.com/watch?v=Q_L3nylg mVY IB Biology Topic 2.4.1 Draw and Label the Plasma Membrane
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
Types of Transport
• 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.
Passive Transport – a type of transport that does NOT require energy
•
Examples
•
Diffusion
•
Facilitated Diffusion
•
Osmosis
All types of passive transport will follow a “concentration gradient” where
molecules move from an area of high concentration to an area of low
concentration
Active Transport – a type of transport that REQUIRES energy
•
Examples
•
Ion Pumps
•
Proton Pumps
•
Cotransport
Bulk Transport – Movement of large amounts of molecules or large objects
•
Endocytosis – substances enter the cell (cell eating or drinking)
Getting Through the Membrane
Some molecules can move through the membrane easier
than others, depending on their characteristics
Membrane Transport Rules
Characteristics that Help
Characteristics that don’t help
Being small
Being Non-Polar
Steep Concentration Gradient
Being Large
Being Polar
Being Charged
•
Non-polar Molecules
- Dissolve in the membrane
- Smaller move faster than larger molecules
•
Polar molecules
- small polar, uncharged go right between the phospholipids (but not as
easily as small non-polar can)
- Water is a bit of an exception
Types of Transport Proteins
•
Pump- Uses ATP to pump a specific molecules against it’s
concentration gradient
•
Protein Channel – Allows for the passive transport of a particular
molecule
Water Transport
• 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.
•
Water is small, but polar and
so usually uses a transport
channel called an aquaporin
Water does not need a channel (it just prefers one)
Essential idea: Membranes control the composition of cells by active and passive transport.
•
If the concentration gradient is steep enough some polar molecules can
slip through like non-polar molecules can.
•
In the image above the pressure from the high concentration of free
water molecules outside the cell, forces water molecules through the
hydrophobic part of the membrane even through they don’t like each
other.
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
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
Diffusion vs. Osmosis
100% water
30% solute
Hypotonic
Hypertonic
100% water
30% solute
Direction Osmosis
Will Occur
Direction Diffusion
of Solute Will Occur
Diffusion vs. Osmosis
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.
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 low to high).
•
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 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
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
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
Different Types of Endocytosis
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.
