1. Membranes in cells
Chapter 2.3

2. Objectives of unit:
Understand the structure and properties of the plasma membrane
Investigate the properties of plasma membranes practically
Explain passive transport mechanisms of diffusion and facilitative diffusion, including the role of transporter and carrier proteins
Define the process of osmosis
Explain the process of active transport and the role of proteins and ATP
Explain the processes of endocytosis and exocytosis
Describe the properties of gas exchange surfaces in living organisms
Explain how the structure of the mammalian lung is adapted for rapid gaseous exchange

3. Cells have many membranes:
plasma membrane
tonoplast
outer mitochondrial membrane
inner mitochondrial membrane
outer chloroplast membrane
nuclear envelope

4. Membranes are flexible and able to break and fuse easily
Neutrophil engulfing anthrax bacteria.
Cover credit:
Micrograph by Volker Brinkmann, PLoS Pathogens Vol. 1(3) Nov
5 μm

5. Membranes allow cellular compartments to have different conditions
pH 4.8
Contains digestive enzymes, optimum pH
lysosome
Membrane acts as
a barrier
pH 7.2
cytosol

6. Membranes are mainly made of phospholipids
phosphate group
hydrophilic
head
phosphoester bond
glycerol
ester bond
fatty acid
hydrophobic
tail

7. The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble
Hydrophobic (water-hating) tail
air
aqueous solution
Hydrophilic (water-loving) head
Phospholipids form micelles when submerged in water

8. In 1925 Gorter and Grendel proposed that the unit membrane is formed from a phospholipid bilayer
Phosphate heads face aqueous solution
Extracellular space (aqueous)
phospholipid
bilayer
Cytosoplasm (aqueous)
Hydrophobic tails
face inwards

9. Click to reveal answers
Question: Explain why phospholipids form a bilayer in plasma membranes (4).
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Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions
The fatty acid tails are non-polar and will move away from an aqueous environment
As both tissue fluid and cytoplasm is aqueous
phospholipids form two layers with the hydrophobic tails facing inward
and phosphate groups outwards interacting with the aqueous environment
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10. Initial studies showed that the plasma membrane had layers:
Scientists also found that protein were present in membranes so Davson-Danielli proposed in 1935 the following model for membrane structure:
Phospholipid
bilayer
Protein

11. The development and use of electron microscopes showed that the Davson-Danielli model was incorrect
In the early 1970s Singer and Nicholson used techniques such as freeze-etching to confirm the lipid bilayer. They also showed that the proteins were distributed throughout the protein in a mosaic pattern. In addition they found that the membrane was fluid and had considerable sideways movement of molecules within it.
Hence they proposed the Fluid-Mosaic Model for Plasma Membrane Structure.

12. Freeze fracturing and etching
Using liquid nitrogen, biological tissue samples or cells are rapidly frozen to immobilize cell constituents.
Cell membranes are composed of two layers of phospholipids, called a bilayer, where the hydrophobic, or water-hating, lipid tails point to the inside of the membrane and the hydrophilic, or water-loving, ends of the lipid molecule point outward and toward the inside of the cell.
The frozen sample is cracked or fractured with a microtome, which is a knife-like instrument for cutting thin tissue slices.
This causes the cell membrane to split apart precisely between the two layers because the attraction between the hydrophobic lipid tails represents the weakest point.
Following fracturing, the sample undergoes a vacuum procedure, called "freeze etching".
The surface of the fractured sample is shadowed with carbon and platinum vapor to make a stable replica, which follows the contours of the fracture plane.
Acid is used to digest organic material adhering to the replica, leaving a thin platinum shell of the fractured membrane surface.
This shell is then analyzed by electron microscopy.

13. Activity: Read pages 100 – 103 of your textbook
Answer questions 1 – 3 on page 103

14. The fluid mosaic model of the plasma membrane:
The proteins can move freely through the lipid bilayer.
The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.

15. Fat-soluble organic molecules can diffuse through the bilayer but polar molecules require proteins
Fat-soluble molecules
Polar molecules
Extracellular
space
Cytosoplasm
(aqueous)
hydrophilic pore

16. Question 4: How can polar and non-polar molecules pass through the membrane (2).
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Polar molecules require proteins to enable them to pass through the membrane
Non-polar molecules can diffuse directly through the phospholipid bilayer Click here to hide answers

17. The membrane contains many types of protein:
carbohydrate chain
Glycocalyx: For cell recognition so cells group together to form tissues
Receptor: for recognition by hormones
glycoprotein
Enzyme or signalling protein
integral protein
carrier protein
peripheral protein
hydrophilic channel

18. Question: Label the diagram (11marks)
Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes. 1 4 5 6 3 7 2 10 9 11 8
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1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) peripheral protein; 5) carrier protein 6) hydrophilic channel; 7) phosphate group; 8) fatty acid; 9) phospholipid;
10) glycocalyx; 11) phospholipid bilayer click to cover answers

19. Click to reveal the answers
Question: Explain why the model for membrane structure is known as the fluid mosaic model (3).
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The phospholipid molecules can move freely laterally and makes the membrane fluid.
The proteins are distributed throughout the membrane un evenly and in a mosaic pattern.
The agreed structure is based upon experimental and chemical evidence and so is classed as a model.
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20. Question: Describe the structure and function of the glycocalyx (3)
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Consists of glycoproteins
Which are proteins with added carbohydrate chains
Used for cell recognition/receptors
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21. There are different types of carrier proteins in the membrane:
ATP
Carrier protein
(passive)
Gated-channel protein
Channel protein
Carrier protein
(active)

22. Membrane bound proteins allow chemical processes to occur on membranes in a sequential manner:
III I II IV
Cyt c
Enzyme and transporter proteins involved in aerobic respiration in the inner mitochondrial membrane
ATP synthase

23. Click to reveal answers
Question: Other than as carrier proteins state two functions of membrane bound proteins (2).
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Receptors
Enzymes
Structural (attached to microtubules)
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24. Practical Activity: Factors affecting membrane permeability
See practical sheets

25. Click to reveal answers
Question 3: Describe an experiment by which you could test to see whether alcohol concentration affected membrane permeability (5).
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Same volume discs of beetroot
Same volume of alcohol
Same temperature
Same time in alcohol
Range of alcohol concentrations
Use colourimeter to read amount of pigment in solution
Graph of colour intensity (% absorbance etc.) over alcohol concentration
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26. Summary The unit membrane consists of a phospholipid bilayer
Phospholipids consist of a polar, hydrophilic phosphate head and a non-polar, hydrophobic tail consisting of fatty acid chains.
Proteins also occur in the membrane and float freely throughout it.
The model for membrane structure is known as the fluid mosaic model.
Peripheral proteins occur on the inner or outer face of the membrane and integral proteins extend through both lipid layers.
Membrane bound enzymes occur allowing structured metabolic pathways.
Glycoproteins form the glycocalyx and allow cell to cell recognition.
Receptor proteins can act as binding sites for hormones and other substances and can transmit the information to the interior of the cell.
A variety of carrier proteins allow for the controlled movement of substance through the membrane using both passive diffusion or active transport.
Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer.
Ionic, polar molecules require carrier proteins to enable them to pass through the membrane.
Membrane structure loses integrity with high temperature or presence of organic solvents such as alcohol, thereby increasing permeability.