Ch 7: Membrane Structure
and Function
2016
Ch 7: Membrane
From Topic 1.4
From Topic 1.3
Essential idea: Membranes control the composition of cells by active and
Essential idea: The structure of biological membranes makes them
passive transport.
fluid and dynamic.
Nature of science: Experimental design—accurate quantitative
Nature of science:
measurement in osmosis experiments are essential (3.1).
• Using models as representations of the real world—there are
Understandings:
alternative models of membrane structure (1.11).
• Particles move across membranes by simple diffusion, facilitated
• Falsification of theories with one theory being superseded by
diffusion, osmosis and active transport.
another—evidence falsified the Davson-Danielli model (1.9).
• The fluidity of membranes allows materials to be taken into cells by
Understandings:
endocytosis or released by exocytosis. Vesicles move materials within
• Phospholipids form bilayers in water due to the amphipathic
cells.
properties of phospholipid molecules.
Applications and skills:
• Membrane proteins are diverse in terms of structure, position in the • Application: Tissues or organs to be used in medical procedures must
membrane and function.
be bathed in a solution with the same osmolarity as the cytoplasm to
• Cholesterol is a component of animal cell membranes.
prevent osmosis.
Applications and skills:
• Application: Structure and function of sodium–potassium pumps for
active transport and potassium channels for facilitated diffusion in axons.
• Application: Cholesterol in mammalian membranes reduces
• Skill: Estimation of osmolarity in tissues by bathing samples in
membrane fluidity and permeability to some solutes.
hypotonic and hypertonic solutions (Practical 2).
• Skill: Drawing of the fluid mosaic model.
• Skill: Analysis of evidence from electron microscopy that led to the Guidance:
• Osmosis experiments are a useful opportunity to stress the need for
proposal of the Davson-Daniellimodel.
• Skill: Analysis of the falsification of the Davson-Danielli model that led accurate mass and volume measurements in scientific experiments.
Utilization:
to the Singer-Nicolson model.
• Kidney dialysis artificially mimics the function of the human kidney by
Guidance:
using appropriate membranes and diffusion gradients.
• Amphipathic phospholipids have hydrophilic and hydrophobic
Aims:
properties.
• Aim 8: Organ donation raises some interesting ethical issues, including
• Drawings of the fluid mosaic model of membrane structure can be
the altruistic nature of organ donation and concerns about sale of human
two dimensional rather than three dimensional. Individual
organs.
phospholipid molecules should be shown using the symbol of a circle
• Aim 6: Dialysis tubing experiments can act as a model of membrane
with two parallel lines attached. A range of membrane proteins should action. Experiments with potato, beetroot or single-celled algae can be
be shown including glycoproteins.
used to investigate real membranes.
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:
• 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.
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 nonliving 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
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?
Different Models of PM
Skill: Analysis of the falsification of the Davson-Danielli model that led to the SingerNicolson 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 freezefracture:
http://www.sciencephoto.com/media/530082/view
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.
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.
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 HumanMouse Hybrid Experiment.
• Labeled with two different
fluorescent dyes.
• After a couple of hours they
were evenly distributed.
Human Mouse
Hybrids
“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 2.4.2 Phospholipid
Properties
https://www.youtube.com/watch?v=jrxnTgQ
DhrU
IBguides's channel
https://www.youtube.com/watch?v=Q_L3nylg
mVY IB Biology Topic 2.4.1 Draw and
Label the Plasma Membrane
IBguides's channel
http://www.susanahalpine.com/an
im/Life/memb.htm
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
3) Enzymatic activity
5) Cell-Cell recognition
2) Intercellular joining
4) Signal Transduction
6) Attachment to cytoskeleton
Membrane Transport
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.
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.
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
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.
Diffusion vs. Osmosis
• Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential
100% water
30% solute
Hypotonic
Hypertonic
Diffusion vs. Osmosis
• 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
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.
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
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
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
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
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.
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