1.3 Membrane Structure

Essential idea: The structure of biological membranes makes them fluid and dynamic

Do Now

Challenge: what is it made of?

Support: what organelle allows substances to enter and exit?

1.3 Membrane Structure

Syllabus Reference

1.3 Membrane Structure

Vocabulary

1.3 Membrane Structure

Draw and label a prokaryotic cell

Starter

Challenge: what is the cell wall made of?

Support: how is a prokaryotic cell difference to eukaryotic

Cell Ultrastructure

Label the prokaryotic cell below

Task 1

Cell Ultrastructure

Cell Ultrastructure

1.3 Membrane Structure

How do components of the cell membrane work together to carry out the functions needed by the cell?

Guiding Question

What does a cell membrane need to be able to do?

• separate cell contents from outside
• separate organelles
• control what enters and exits
• be flexible (break apart and reform)
• transport substances in and out
• helps maintain concentration gradient
• maintain shape/support

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1.3 Membrane Structure

Fluid Mosaic Model of Cell Membranes

Task 1

1.3 Membrane Structure

Answer the below multiple choice question

Checkpoint

1.3 Membrane Structure

Answer the below multiple choice question

Checkpoint

1.3 Membrane Structure

Answer the below multiple choice question

Checkpoint

1.3 Membrane Structure

Answer the below multiple choice question

Checkpoint

1.3 Membrane Structure

Answer the below multiple choice question

1.3 Membrane Structure

Answer the questions

Checkpoint

1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

When put into water, an emergent property is that phospholipids will self-organise to keep their heads ‘wet’ and their tails ‘dry’

1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Amphipathic (am·fuh·pa·thick)

Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

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What happens when you put a drop of oil in water?

1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

• Phospholipids are the most abundant lipid in the plasma membrane
• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

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1.3 Membrane Structure

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

1.3 Membrane Structure

What does the fluid mosaic model tell us about cell membranes?

1.3.U1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Fluid

A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.

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Mosaic

A picture or decorative design made by setting small colored pieces, as of stone or tile, into a surface.

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1.3 Membrane Structure

1.3.S1 Drawing of the fluid mosaic model.

1.3 Membrane Structure

1.3.S1 Drawing of the fluid mosaic model.

1.3 Membrane Structure

Draw and label the fluid mosaic model

Task 2

http://www.youtube.com/watch?v=w9VBHGNoFrY

1.3 Membrane Structure

(a) Draw a labelled diagram to show the structure of a membrane (5)

Checkpoint

1.3 Membrane Structure

Be an IB Examiner!

Task

1.3 Membrane Structure

Label the diagram below

Support: What is the carbohydrate attached to?

1.3 Membrane Structure

Label the diagram below

Support: What is the carbohydrate attached to?

http://www.youtube.com/watch?v=n8ZvK0r9HIY

a. glycoprotein

b. glycolipid

c. carbohydrate

e. phospholipid bilayer

i. integral protein

h. cholesterol

g. peripheral protein

d. fatty acid tails (hydrophobic)

j. cytoskeleton

f. phosphate heads (hydrophilic)

1.3 Membrane Structure

1.3.S1 Drawing of the fluid mosaic model.

1.3 Membrane Structure

Answer the following

Checkpoint

Integral - span from one side of the bilayer to the other

Peripheral -sit on the surface of the membrane

Integral proteins are permanently embedded, many go all the way through and are polytopic (poly = many, topic = surface), integral proteins penetrating just one surface are monotopic.

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Peripheral proteins usually have a temporary association with the membrane, they can be monotopic or attach to the surface

1.3 Membrane Structure

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

Glycoproteins:

Are proteins with an oligosaccaride (oligo = few, saccharide = sugar) chain attached.

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They are important for cell recognition by the immune system and as hormone receptors

1.3 Membrane Structure

Key terminology

It makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane.

1.3 Membrane Structure

What is the role of Cholesterol?

1.3.U3 Cholesterol is a component of animal cell membranes.

1.3 Membrane Structure

1.3.U3 Cholesterol is a component of animal cell membranes.

Hydroxyl group makes the head polar and hydrophilic - attracted to the phosphate heads on the periphery of the membrane.

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Carbon rings – it’s not classed as a fat or an oil, cholesterol is a steroid

Non-polar (hydrophobic) tail –attracted to the hydrophobic tails of phospholipids in the centre of the membrane

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1.3 Membrane Structure

What are the functions of membrane proteins?

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

Transport: Protein channels (facilitated) and protein pumps (active)

Receptors: Peptide-based hormones (insulin, glucagon, etc.)

Anchorage: Cytoskeleton attachments and extracellular matrix

Cell recognition: MHC proteins and antigens

Intercellular joinings: Tight junctions and plasmodesmata

Enzymatic activity: Metabolic pathways

a. facilitated diffusion by channel proteins ✔

b. active transport by protein pumps

OR

protein pumps eg sodium-potassium ✔

c. cell recognition by glycoproteins/protein receptors ✔

d. communication/receptors for hormones/signal molecules ✔

e. cell adhesion ✔

f. allow up to one additional mark for AHL material ✔

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1.3 Membrane Structure

Describe the functions of proteins in cell membranes.(4 marks)

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

Fig. 7-9ac

(a) Transport

(b) Enzymatic activity

(c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

1.3 Membrane Structure

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

Fig. 7-9df

(d) Cell-cell recognition

Glyco-

protein

(e) Intercellular joining

(f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

1.3 Membrane Structure

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

1.3 Membrane Structure

How are cell membrane components arranged?

1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

How was the structure discovered?

1.3 Membrane Structure

Guiding Question

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

https://www.youtube.com/watch?v=YkPK3LjQkVM

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1.3 Membrane Structure

Guiding Question

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

1.3.S2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

The model:

• A protein-lipid sandwich
• Lipid bilayer composed of phospholipids (hydrophobic tails inside, hydrophilic heads outside)
• Proteins coat outer surface
• Proteins do not permeate the lipid bilayer

1.3 Membrane Structure

Before then the Davson-Danielli protein-lipid sandiwch (1935) model was widely accepted …

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1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

• It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio 
• It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
• It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores) 
• The temperatures at which membranes solidified did not correlate with those expected under the proposed model 

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1.3 Membrane Structure

There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli:

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

1.3 Membrane Structure

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Our current model of the cell membrane is called the Singer-Nicholson fluid mosaic model

Key features:

• Phospholipid molecules form a bilayer - phospholipids and proteins are fluid and move laterally
• Peripheral proteins are bound to either the inner or outer surface of the membrane
• Integral proteins - permeate the surface of the membrane
• The membrane is a fluid mosaic of phospholipids and proteins

1.3 Membrane Structure

Our current model of the cell membrane is called the Singer-Nicholson fluid mosaic model

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

1.3.S2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

This explains: Despite being very thin membranes are an effective barrier to the movement of certain substances.

The evidence: In high magnification electron micrographs membranes appeared as two dark parallel lines with a lighter coloured region in between.

Proteins appear dark in electron micrographs

phospholipids appear light - possibly indicating proteins layers either side of a phospholipid core.

1.3 Membrane Structure

Before then the Davson-Danielli protein-lipid sandiwch (1935) model was widely accepted …

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1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
• Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

1.3 Membrane Structure

What is the freeze-fracture method?

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Interpreting the image:

• The fracture occurs along lines of weakness, including the centre of membranes.
• The fracture reveals an irregular rough surface inside the phospholipid bilayer
• The globular structures were interpreted as trans-membrane proteins.

Conclusion: A new model is needed to explain the presence of as trans-membrane proteins.

1.3 Membrane Structure

1.3S2 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

• Fluorescent Antibody tagging
• Freeze fracturing revealed transmembrane proteins

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1.3 Membrane Structure

What is the evidence that falsified

1.3S2 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

The Davson-Danielli model of membrane structure was proposed in the 1930s. When electron micrographs of membranes were first produced, they were used as evidence for this model. The micrograph shows two adjacent membranes (indicated with arrows).

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Explain how the appearance of membranes in electron micrographs was used as evidence to support the Davson-Danielli model. (3 marks)

1. the black lines represent proteins;
2. forms a ‘sandwich’/2 layers;
3. there is a clear layer in the centre;
4. (the clear layer) is composed of phospholipids;
5. reference to both membranes being similar;

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The Davson–Danielli model of membrane structure proposed that membranes were composed of a phospholipid bilayer that lies between two layers of globular proteins, as shown in this diagram.

What evidence supported this model?

A. An electron micrograph that showed two dark lines with a lighter band in between

B. Freeze-fracture electron microscopy

C. Evidence that all membranes are identical

D. The hydrophobic regions of protein would be in contact with water

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1.3 Membrane Structure

Application: Cholesterol in animal cell membranes

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

1.3 Membrane Structure

How did the plasma membrane develop over time?

1.3S2 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

• What did you discover?
• What does this tell us about the membrane?
• What evidence is there to prove this or disprove previous theories?

1.3 Membrane Structure

1.3S2 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

VS

Can we damage the cell membrane with a simple heat treatment?�

At what temperature will proteins in the plasma membrane denature?

1.3 Membrane Structure

How do we investigate the damage to a cell membrane?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Quantitative Vs Qualitative

Set the filter to Blue (B)

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Set absorbance to 0 using a blank of distilled water

1.3 Membrane Structure

How do temperatures affect cell membranes?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

1.3 Membrane Structure

How do we investigate the damage to a cell membrane?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

How could you extend this experiment?

1.3 Membrane Structure

How could you extend this experiment?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Writing up the experiment

1. Present the data for the colorimeter readings for each heat treatment in a clear table.
2. Include a +/- uncertainty for each measurement and check that decimal points correspond.
3. Include your qualitative observations.
4. Plot a scatter graph of the data.
5. Make reference to uncertainties in the graph using error bars or the R2 value of a best fit line.
6. Use the graph to show the effect of the heat treatment on the diffusion of the purple pigment.
7. Identify the thermal death point (the point when integral proteins in the membrane denature).
8. Write a description of what trend the results show.

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Analysis

1.3 Membrane Structure

How do we write an analysis?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

1. Write a conclusion which refers to the research question of the experiment and
2. Conclude whether the conclusion supports your idea about the research question.
3. include a reference to the structure of membranes, protein structures and the process of diffusion.
4. Find something in a text book which either supports or disagrees with your conclusion.
5. Evaluate the limitations of the experiment method, the apparatus and your data
6. Suggest improvements to the limitations mentioned, giving specific details

1.3 Membrane Structure

How do we write an evaluation?

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

1.3 Membrane Structure

Bubble Play

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

1.3 Membrane Structure

Bubble Play Analysis

1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

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