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Membranes in cells Chapter 2.3.

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Presentation on theme: "Membranes in cells Chapter 2.3."— Presentation transcript:

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). Click to reveal answers 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 Click here to hide answers

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

13 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.

14 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

15 Question 4: How can polar and non-polar molecules pass through the membrane (2).
Click to reveal answer 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

16 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

17 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 Click to reveal answers 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

18 Click to reveal the answers
Question: Explain why the model for membrane structure is known as the fluid mosaic model (3). Click to reveal the answers 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. Click here to hide answers

19 Question: Describe the structure and function of the glycocalyx (3)
Click to reveal answers Consists of glycoproteins Which are proteins with added carbohydrate chains Used for cell recognition/receptors Click here to hide answers

20 There are different types of carrier proteins in the membrane:
ATP Carrier protein (passive) Gated-channel protein Channel protein Carrier protein (active)

21 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

22 Click to reveal answers
Question: Other than as carrier proteins state two functions of membrane bound proteins (2). Click to reveal answers Receptors Enzymes Structural (attached to microtubules) Click here to hide answers

23 Practical Activity: Factors affecting membrane permeability
See practical sheets

24 Click to reveal answers
Question 3: Describe an experiment by which you could test to see whether alcohol concentration affected membrane permeability (5). Click to reveal answers 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 Click here to hide answers

25 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.

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