Edexcel IAS Biology Unit 1 Cell Membrane

Unit 1 Topic 2

1. Cell Membrane Prepared by Mohammed Sami

2. Specification know the structure and properties of cell membranes understand how models such as the fluid mosaic model of membrane structure are interpretations of data used to develop scientific explanations of the structure and properties of cell membranes. understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane, down a water potential gradient. understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis understand the involvement of carrier and channel proteins in membrane transport

3. Phospholipids form bilayer on cell membrane Phospholipids have polar / hydrophilic heads and non-polar / hydrophobic tails

4. Micelle formation – due to hydrophobic nature of lipid tails

5. Gorter and Grendel model Made up of only phospholipid bilayer. This bilayer is so weak and it cannot hold all the cell contents

6. Davson - Danielli Sandwich M odel Outer and inner layers of protein coat Middle = phospholipid bilayer Phospholipid bilayer is sandwiched between two layers of protein.

7. Drawbacks of Davson - Danielli Model

8. Fluid- mosaic model of Singer and Nicolson

9. Fluid Mosaic Model Membrane is made up of phospholipid bilayer and proteins are randomly embedded into this layer. Fluid - phospholipid molecules can change the places within the membrane. Proteins and phospholipids can move around in the plasma membrane. Proteins are scattered in the membrane Mosaic - proteins are randomly embedded

10. Functions of Membrane Components Component Function Phospholipid bilayer maintain the composition of the cytoplasm act as a selectively permeable membrane Glycolipid carbohydrate attached on phospholipid act as receptor site for chemical signals Glycoprotein carbohydrate attached on protein, act as name tag (antigens) Glycocalyx (glycolipid+ glycoprotein ) cell recognition chemical protection adhesion Cholesterol makes the membrane less fluid and more stable reduces the entry and escape of polar molecules It combines with fatty acid tails and holds the fatty acid chains together. This reduces the movement of phospholipid.

11. Cell Membrane Functions

12. Types of membrane protein Peripheral proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).

13. Transmembrane proteins Carrier proteins :- transport of molecules by facilitated diffusion Channel proteins :- selective transport of polar molecules

14. Factors affecting cell membrane permeability and membrane structure Temperature Cell Membrane Composition Solvent Concentration

15. Cell Membrane Permeability and Fluidity Cell Membrane Permeability Permeability of a membrane is the rate of passive diffusion of molecules across the membrane. It’s the ease with which molecules pass through the membrane barrier . The cell membrane is selectively permeable and only allows specific molecules to enter the cell. This function is essential for the normal functioning of organisms. Fluidity Fluidity is defined as the ability or ease of molecules to move in the membrane. It refers to the viscosity of the cell membrane and affects the diffusion of proteins and other molecules inside cells, thus, affecting their function. This property is affected by phospholipid structure, cholesterol composition in the membrane, and temperature .

16. Factors affecting cell membrane permeability and membrane structure Temperature Cell membranes are sensitive to temperature. Changes in temperature can affect the fluidity and permeability of cell membranes, which impacts cell structure and function .

17. Temperature 0°C – 45°C Membranes are fluid. Between 0-45°C, phospholipids can easily move (although their movement is naturally restricted by cholesterol). Membranes are semi-permeable. At these temperatures, the membrane is also semipermeable. As temperatures increases, the kinetic energy of the phospholipids also increases, which increases their movement. This increase in movement leads to an increase in permeability of the membrane. Above 45°C Phospholipid bilayer begins to break down. Increasing kinetic energy allows the phospholipids to move far away from each other, which destroys the structural integrity of the membrane, causing it to “melt”. Cell Membrane Structure becomes freely-permeable. Transport and channel proteins denature, thus making them unable to regulate what gets into and out of cells, which leads to increased membrane permeability. The membrane may burst. The heat causes water inside of the cells to expand which puts pressure on the membrane, causing it to burst.

18. Factors affecting cell membrane permeability and membrane structure 2) Cell Membrane Composition The predominance of saturated or unsaturated fatty acids in the membrane The amount of cholesterol in the membrane Shorter fatty acid chains and a higher amount of unsaturated bonds between carbon atoms of fatty acids increase the membrane fluidity. Also, an increase in cholesterol concentration decreases membrane fluidity by restricting the motions of membrane molecules.

19. Factors affecting cell membrane permeability and membrane structure 2) Solvent Concentration Solvents such as ethanol increase membrane permeability .   Lipids dissolve in alcohol, therefore, the phospholipids in a cell membrane will easily dissolve in solutions such as ethanol. As a result, the cell membrane becomes more fluid and permeable as it starts to break down. Increasing solvent concentration increases membrane permeability.  Solvent concentration and membrane permeability are directly correlated i.e. increasing solvent concentration increases membrane permeability. This is because as the solvent becomes more and more concentrated, it has a greater ability to dissolve phospholipids and disrupt the membrane structure, making it more permeable to external substances.

20. Simple Diffusion Diffusion is the tendency for molecules of any substance to spread out into available space. Movement of molecules from areas of higher concentrations to lower concentrations. Small non polar molecules can move through phospholipid bilayer by simple diffusion.

21. Facilitated Diffusion Movement of molecules down concentration gradient across the membrane using either channel protein or carrier protein. Nevertheless, what characterizes facilitated diffusion from the other types of passive transport is the need for assistance from a transport protein lodged in the plasma membrane .

22. Fick’s Law of Diffusion Fick's Law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area Proportionality  means the rate of diffusion will double if: The surface area or concentration difference  doubles The diffusion pathway  halves

23. Osmosis Osmosis is the net movement of water molecules from a solution with a lower concentration of solute to a solution with a higher concentration of solute through a partially permeable membrane

24. Types of Solutions

25. Cells in different solutions

26. Stages of Plasmolysis The complete process of Plasmolysis take place in three different stages: Incipient plasmolysis : It is the initial stage of the plasmolysis, during which, water starts flowing out of the cell; initially, the cell shrinks in volume and cell wall become detectable. Evident plasmolysis : It is the next stage of the plasmolysis, during which, the cell wall has reached its limit of contraction and cytoplasm gets detached from the cell wall attaining the spherical shape. Final plasmolysis : It is the third and the final stage of the plasmolysis, during which the cytoplasm will be completely free from the cell wall and remains in the centre of the cell.

27. Active Transport Movement of molecules occurs against a concentration gradient across the membrane The substance to be transported binds to the carrier protein. Energy from ATP changes the shape of carrier protein The substance is released on the other side of the membrane. Movement occurs in one direction.

28. Bulk Transport Transport of substances across the membrane by forming vesicles. Endocytosis: transport of substances in to the cell by forming vesicles. Exocytosis: transport of substances out of the cell by forming vesicles

29. Endocytosis Endocytosis is the process by which the cell surface membrane  engulfs material , forming a small sac (or ‘ endocytic vacuole ’) around it There are two forms of endocytosis: Phagocytosis : This is the bulk intake of solid material by a cell Cells that specialise in this process are called  phagocytes The vacuoles formed are called phagocytic  vacuoles An example is the engulfing of bacteria by phagocytic white blood cells Pinocytosis : This is the bulk intake of liquids If the vacuole (or  vesicle ) that is formed is extremely small then the process is called  micropinocytosis

30. Endocytosis

31. Exocytosis Exocytosis is the process by which materials are removed from, or  transported out of , cells (the  reverse of endocytosis ) The substances to be released (such as  enzymes, hormones or cell wall building materials ) are packaged into  secretory vesicles  formed from the Golgi body These vesicles then travel to the cell surface membrane Here they  fuse  with the cell membrane and  release their contents  outside of the cell

32. Example of exocytosis