Membranes in cells Chapter 2.3. Objectives of unit: Understand the structure and properties of the plasma membrane Investigate the properties of plasma.

Membranes in cells

Chapter 2.3

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

Cells have many membranes:

plasma membrane

tonoplast

outer mitochondrial membrane

inner mitochondrial membrane

outer chloroplast membrane

nuclear envelope

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. 2005.

5 μm

Membranes allow cellular compartments to have different conditions

pH 4.8Contains digestive enzymes, optimum pH 4.5 - 4.8

pH 7.2

lysosome

cytosol

Membrane acts as a barrier

Membranes are mainly made of phospholipids

phosphate group

glycerol

fatty acid

phosphoester bond

ester bond

hydrophilichead

hydrophobictail

The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble

aqueous solution

Hydrophilic (water-loving) head

Hydrophobic (water-hating) tail

Phospholipids form micelles when submerged in water

air

In 1925 Gorter and Grendel proposed that the unit membrane is formed from a phospholipid bilayer

Extracellular space (aqueous)

Cytosoplasm (aqueous)

phospholipid bilayer

Phosphate heads face aqueous solution

Hydrophobic tails face inwards

Question: Explain why phospholipids form a bilayer in plasma membranes (4).

• 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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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:

Protein Phospholipid bilayer

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.

Activity:

Read pages 100 – 103 of your textbook

Answer questions 1 – 3 on page 103

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.

Fat-soluble organic molecules can diffuse through the bilayer but polar molecules require proteins

Extracellular space

Cytosoplasm (aqueous)

Fat-soluble molecules Polar molecules

hydrophilic pore

Question 4: How can polar and non-polar molecules pass through the membrane (2).

•Polar molecules require proteins to enable them to pass through the membrane

•Non-polar molecules can diffuse directly through the phospholipid

bilayer

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The membrane contains many types of protein:

glycoprotein

carbohydrate chain

integral proteinperipheral protein

carrier protein

Glycocalyx: For cell recognition so cells group together to form tissues

Receptor: for recognition by hormones

Enzyme or signalling protein

hydrophilic channel

Question: Label the diagram (11marks)

1

2 10

3

4

5 6

8

9

11

Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes.

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

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7

Question: Explain why the model for membrane structure is known as the fluid mosaic model (3).

• 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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Question: Describe the structure and function of the glycocalyx (3)

• Consists of glycoproteins• Which are proteins with added carbohydrate chains• Used for cell recognition/receptors

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There are different types of carrier proteins in the membrane:

ATP

Channel proteinGated-channel proteinCarrier protein(passive)

Carrier protein(active)

Membrane bound proteins allow chemical processes to occur on membranes in a sequential manner:

ATP synthase

Enzyme and transporter proteins involved in aerobic respiration in the inner mitochondrial membrane

membrane

Q IIIIII

IV

Cyt c

proteins

Question: Other than as carrier proteins state two functions of membrane bound proteins (2).

• Receptors• Enzymes• Structural (attached to microtubules)

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Practical Activity: Factors affecting membrane permeability

See practical sheets

Question 3: Describe an experiment by which you could test to see whether alcohol concentration affected membrane permeability (5).

• 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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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.