1. Absorption and Secretion

2. Learning Outcomes • Identify the structure and properties of the cell wall. • Identify the structure of the plasma membrane with reference to the fluid mosaic model.

3. Absorption means the uptake of materials by a cell from its external environment. Secretion means the discharge of useful intracellular molecules into the surrounding medium by a cell. All living cells are surrounded by a cell membrane (plasma membrane). In addition, plant cells have a cell wall. Absorption and Secretion

4. The cell wall • The cell wall is a non-living layer composed mainly of cellulose. • Cellulose is a complex carbohydrate that is composed of unbranched chains of glucose molecules grouped together as fibers. Tip- Always describe cellulose as fibers.

5. The cell wall • The layers of cellulose fibers make the cell wall: • Strong • Elastic • Hydrophilic – water is attracted to cellulose • Non-living • Freely permeable – presents no barrier to the movement of molecules. • Provides a continuous water conducting channel. (allows water to move easily from tissue to tissue without having to enter and leave every living cell on the way)

6. The plasma membrane • The plasma membrane is composed of protein and phospholipid in an arrangement called a fluid mosaic. • The plasma membrane is composed of a fluid bilayer of constantly moving phospholipid molecules. • The term ‘mosaic’ describes the proteins which are embedded into the lipids in a random fashion.

7. Plasma Membrane • The plasma membrane is made of: • A phospholipid bilayer • With proteins embedded in it The Fluid Mosaic Model Phospholipid bilayer Protein

8. The Plasma Membrane • One end of a phospholipid molecule is hydrophilic (water loving), this is called the tail. The other end is hydrophobic (water hating), this is called the head. • The hydrophillic heads are water soluble and make up the 2 outer surfaces, where they form bonds with hydrogen molecules. • The hydrophobic tails are water-insoluble and point inwards to the centre of the bilayer since they are attracted to those in the opposite layer.

9. Plasma Membrane video

10. The Plasma Membrane • The arrangement of phospholipid molecules is fluid but at the same time forms a stable and effective boundary around the cell. • This allows tiny molecules like water to pass through it rapidly. • Larger molecules such as glucose depend upon the protein molecules for entry or exit to the cell.

11. Proteins • The protein molecules in the plasma membrane vary in size and structure. • Some of the proteins have narrow channels which make the membrane porous.

12. Proteins • The protein molecules vary in function: • Provides structural support ( strength ) • Enzymes – to speed up reaction • Receptors – for cell signalling • Channels – for movement of larger molecules such as glucose • Antigenic markers for cell recognition • Carriers – by active transport

13. Summary - The Fluid Mosaic Model Antigen marker Receptor Carrier Channel forming protein Structural protein

14. Beetroot Experiment • Aim: To investigate the chemical nature of the cell membrane. • Results: Test tube A is a control. • In test tube B and D there was bleeding as low pH and high temperature denatured the proteins in the plasma membrane. • In test tube C there was bleeding because lipids are soluble in alcohol, so the phospholipids in the plasma membrane dissolved.

15. Questions • What is the main component of a plant cell wall? • How does the fibrous nature of this substance make the wall suited to its function? • Name 2 types of molecule present in the plasma membrane. • Which of these would be denatured at 700C? • State 3 functions carried out by proteins in the cell membrane.

16. Learning Outcomes • Explain the mechanisms of osmosis and diffusion as basic cell processes. • Define the terms hypertonic, hypotonic and isotonic in relation to water content of cell contents and their surroundings. • Describe the fate of both animal and plant cells in hypertonic, hypotonic and isotonic solutions.

17. high concentration low concentration Diffusion • Diffusion is the movement of molecules or ions from a region of high concentration to a region of low concentration. • The difference in concentration that exists before diffusion occurs is called the concentration gradient. • During diffusion, molecules or ions always move down a concentration gradient from high to low.

18. Diffusion • Diffusion is important as it is the way that useful substances like oxygen enter a cell and waste substances like carbon dioxide leave a cell. • Diffusion is a passive process and doesn’t require any energy. • The cell wall is freely permeable to all molecules in solution, so it does not act as a barrier to diffusion.

19. The plasma membrane + diffusion • The plasma membrane is freely permeable to tiny molecules like oxygen that are small enough to diffuse rapidly through the bilayer. • Molecules like urea are able to diffuse slowly through the bilayer. • Larger molecules like glucose depend upon protein molecules to help them across. • Even larger molecules like starch are unable to pass through the membrane. The plasma membrane is selectively permeable.

20. Osmosis • Osmosis is the movement of water from a region of high water concentration to low water concentration through a selectively permeable membrane. • Osmosis is a passive process and does not require energy.

21. Water Concentration • A solution is made when a solute is dissolved in a solvent. • A hypertonic solution is one that has a lower concentration of water (solvent). • A hypotonic solution is one that has a higher concentration of water (solvent). • An isotonic solution is one where the concentration of solvent and solute are equal.

22. Water Concentration • If a cell is said to be in a hypertonic solution, this means that the water concentration of the cell contents is higher that the water concentration of the cell surroundings. • If a cell is placed in a hypertonic solution, water will move out of the cell, from high to low. • If water moves out of an animal cell, it will shrink.

23. Water Concentration • If a cell is said to be in a hypotonic solution, this means that the water concentration of the cell contents is lower that the water concentration of the cell surroundings. • If a cell is placed in a hypotonic solution, water will move into the cell, from high to low. • If an animal cell is placed in a hypotonic solution, it will take on water and burst (haemolysis).

24. Osmosis in Animal cells How does water move in these 3 cases? What are the relative water concentrations?

25. Osmosis in Plant cells • Osmosis in plant cells has slightly different results because plant cells have a cell wall. • If a plant cell is placed in a hypertonic solution, it will lose water by osmosis and the cell contents will shrink. The cell becomes plasmolysed. • If a plant cell is placed in a hypotonic solution, it will gain water by osmosis but the cell wall prevents it from bursting. Instead it becomes turgid.

26. Osmosis in Plant Cells

27. Learning Outcomes • Explain the basic cell process of active transport. • State the function of the plasma membrane in relation to selective ion uptake (active transport) and absorption and release of chemicals.

28. Active Transport • Active transport is the movement of molecules and ions across the plasma membrane from a low to a high concentration (against a concentration gradient). • Active transport requires energy. This energy is provided by ATP from respiration. video

29. Role of the plasma membrane • Certain protein molecules present in the plasma membrane act as carrier molecules. • These proteins ‘recognise’ specific ions and transfer them across the plasma membrane.

30. Sodium Potassium Pump • Active transport carriers are often called pumps. • Some carriers have two roles: they exchange one type of ion for another. An example of this is the sodium potassium pump. • The same carrier molecule actively pumps sodium ions out of the cell and potassium ions into the cell. video

31. Sodium Potassium Pump

32. Conditions Required • Factors such as temperature, availability of oxygen and concentration of respiratory substrate (e.g. Glucose) affect the rate of active transport. • Increase in temperature causes an increase in rate of ion uptake, until at high temperatures the enzymes needed for respiration becomes denatured.

33. Conditions Required • An increase in oxygen concentration results in increased rate of ion uptake until some other factor affecting the process becomes limiting (e.g. temperature).

34. 10 Mark Question • Write notes on: • Structure of the plasma membrane (4) • Function of the plasma membrane in active transport (3) • Structure and function of the cell wall (3)

35. Structure of the plasma membrane • Contains protein • Contains phospholipid • Bilayer/double layer/two layers of phospholipids • Phospholipids are fluid/constantly moving • Protein arranged as a mosaic/scattered • Fluid mosaic pattern/model (only awarded if 4 or 5 not scored) • Has channel forming proteins/pores

36. Function in active transport • Proteins/carriers pick up/bind ions/molecules AND move/carry/assist them across the membrane/into/out of the cell • Ion/molecule/substance uptake is selective • Low to high concentration OR against concentration gradient • Requires energy/ATP

37. Structure + function of cell wall • Made of cellulose fibers • Fully/freely permeable • Provides support/rigidity for cells/plants OR gives cells shapes (not strength or structure) • Stops cells bursting after water uptake/when turgid/when placed in hypotonic solution OR allows cells to become turgid.

38. Learning Outcomes • Describe the process of endocytosis.

39. Endocytosis • Endocytosis is the process by which a cell engulfs and takes inrelatively large particles or quantities of material. • This involves the plasma membrane folding inwards to form a ‘pouch’. The pouch then becomes closed off and detached from the cell membrane. • It is now called an intracellular vesicle.

40. Endocytosis • There are 2 types of endocytosis: • Phagocytosis (cell eating) • Pinocytosis (cell drinking)

41. Phagocytosis • Phagocytosis is the engulfing of large solid particles (eg bacteria by white blood cells). • The contents of the vesicle are then digested.

42. Pinocytosis • Pinocytosis involves the formation of small fluid-filled vesicles by the cell membrane. • This is how cells take in smaller particles. • These are then digested using enzymes.

43. Exocytosis • Exocytosis is the reverse of endocytosis. • Vesicles formed inside the cell fuse with the plasma membrane, allowing their contents to be expelled from the cell. • In this way, waste and intracellular products like enzymes and hormones can be secreted by the cell. video