Chapter 2 Basic Function of the Cell

1. Chapter 2 Basic Function of the Cell Shenyang Medical college www.themegallery.com

2. Cells are the structural and functional units of the body. Each type of cell is specially adapted to perform one particular function. Many cells of the body may differ from each other, but all of them have certain basic characteristics that are alike.

3. This chapter focuses on basic functions of cells: • The transport of molecules across cell membranes; • Cellular signal transduction; • Bioelectrical phenomena of cells; • Mechanisms of muscular excitation and contraction.

4. Section 1 Cell membrane structure and membrane transport of substances

5. Molecular structure of the Cell Membrane Compositions of cell membrane protein 55% phospholipids 25% cholesterol 13% other lipids 4% carbohydrates 3% 0% 20% 40% 60% 80% 100% Cell membrane serves as a permeability barrier that allows the cell to maintain the compositions of the cytoplasm.

6. Fluid mosaic model 1972, Singer and Nicholson Main point: a lipid bilayer as the frame of cell membrane; Interspersed in a lipid bilayer are larger globular protein molecules.

7. lipid bilayer Lipids are amphipathic Polar Nonpolar

8. Lipids spontaneously form structures A lipid bilayer is a stable, low energy structure Self sealing structure/eliminate free edge What drives this structured association? Exclusion of Lipids from Water… not lipid association

9. Membrane proteins a. Extrinsic protein (peripheral protein) -to attach only to one surface of the membrane, and do not penetrate. -associated by weak electrostatic bonds to membrane proteins or lipids, may be dynamic: transient, and regulated. Function: almost entirely as enzyme --- p.ser. +++ --- --- arg --- +++ +++ +++

10. Membrane proteins b. Intrinsic protein (integral protein) To penetrate all the way through membrane. Function: as enzyme and ionic channel, ionic pump, carrier, controller (G protein).

11. Membrane carbohydrates • Occur almost in combination with proteins or lipids to form glycoproteins or glycolipids. • They play a major role in immune reactions and often act as receptors for binding hormones.

12. Transport of substances through the membrane Different substances are transported through the cell membrane by different processes. These processes range from relatively simple process of diffusion to extremely complexmechanisms that require the presence of specialmolecules within cell membranes.

13. Simple diffusion

14. Simple diffusion • Small substances that are soluble in lipids can be transported through a cell membrane from high to low concentrations of them. • such as oxygen, carbon dioxide, alcohol, urea, etc • No energy source is required, so this is referred to as a passive transport mechanism

15. Simple diffusion Simple diffusion is mainly affected by the following factors: • Transmembrane solute concentration gradients, i.e. the difference in solute concentration across membrane. • Membrane permeability to the solute.

16. Simple diffusion

17. Protein-mediated Transport • The transport for slight large polar molecules or electrical charged ions is mediated by proteins within the membrane. • Two types of mediated transport can be distinguished as facilitated diffusion and active transport.

18. The facilitated diffusion uses a transporter called a channel or a carrier to move solute "downhill" from a higher to a lower concentration across a membrane, which belongs to passive transport. • The active transport uses a transporter that is coupled to an energy source to move solute "uphill' across a membrane against its electrochemical gradient. The active transport is divided into primary active transport and secondary active transport.

19. Facilitated diffusion Some small water-soluble substances pass through the cell membrane down their concentration or electrical gradients by aid of membrane proteins. Content Title Carrier-mediated diffusion. glucoses, amino acids Channel-mediated diffusion Na+, K+, Ca2+, etc

20. Carrier-mediated diffusion. a “ferry” or “shuttle” process

21. Carrier-mediated diffusion Carrier proteins in the cell membrane selectively bind to a specific conformational substrate. Substratespecificity Molecules with similar chemical structures compete for carrier site. Competitive inhibition Characteristics there is limitation of facilitated diffusion to Vmax level, relating to quantity of carrier proteins Saturation

22. Channel-mediated diffusion • Ions such as Na＋, K＋, Cl－and Ca2＋diffuse across plasma membranes at rates that are much faster than would be predicted from their very low solubility in membrane lipids. • The protein component of the membrane that is responsible for these permeability differences.

23. characteristics of these channels • have selectivity, less than carriers • opened or closed by gates

24. Channel-mediated diffusion mechanical – gated channel Types of channels chemical-gated channel Voltage-gated channel

25. mechanical – gated channel Physically deforming (stretching) the membrane may affect the conformation of some channel proteins. Voltage-gated channel

26. Voltage-gated channel Changes in themembrane potential can causemovement of the charged regions on a channelprotein, altering its shape.

27. chemical-gated ACh Channels Binding of specific molecules tochannel proteins may directly or indirectlyproduce a change in the shape of thechannel protein.

28. Active transport Substances are transported through a cell membrane against their concentration or electrical gradients mediated by membrane protein carriers. Such carriers must use energy from some other source to do the necessary work According to the source of the energy used to transport, the active transport is divided into primary active transport and secondary active transport.

29. Primary active transport the energy used to cause the transport is derived directly from the hydrolysis of ATP or some other high-energy phosphate compound. The membrane protein carriers are called pumps. The pumps can hydrolyze ATP, so they are called ATPase , e.g. Always requires the input of energy. For example: Na+/K+pump or Na+/K+ATPase. Ca2+ pump proton pump

30. Na+/K+ ATPase -hydrolysis of 1 molecule ATP -pumps 3 Na+ ion outward -pumps 2 K+ ion from outside to inside -inhibitor of Na+ /K+ pump is Ouabain.

31. Importance of the Na+-K+ Pump • Maintain high intracellular K+ concentration gradients across the membrane. • Control the volume of cell. • Provides energy for secondary active transport. • Develop and maintain Na+ and K+ concentration gradients across the membrane which are essential for bioelectricity activity. • Electrogenic action influences membrane potential.

32. Intracellular vs extracellular ion concentrations Ion Intracellular Extracellular Na+ 5-15 mM 145 mM K+ 140 mM 5 mM Mg2+ 0.5 mM 1-2 mM Ca2+ 10-7 mM 1-2 mM H+ 10-7.2 M (pH 7.2) 10-7.4 M (pH 7.4) Cl- 5-15 mM 110 mM Fixed anions high 0 mM

33. Secondary active transport • The energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of the membrane, created in the first place by primarily active transport • Coupled transport. • Energy needed for “uphill” movement obtained from “downhill” transport of Na+. • Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.

34. Secondary active transport Na+-linked symporters import amino acids and glucose into many animal cells

35. Secondary active transport co-transport counter-transport (symport) (antiport) out in out in Na+ Na+ glucose H+ Glucose in the same direction as the Na+. H+ in the opposite direction to the Na+.

36. Exocytosis and endocytosis. • Exocytosis. Substances with large molecular size inside the cell are excreted from the cell in the form of membrane-bound vesicles, which is called exocytosis. • Endocytosis. Large membrane-impermeable substances outside the cell can be moved in cells in the form of membrane-bound vesicles, which is called endocytosis. Endocytosis is the reverse process of exocytosis.

37. Exocytosis and endocytosis Phagocytosis: solid material such as bacteria, dead tissue is engulfed by cells. Pinocytosis: fluid material is engulfed by cells in the body.

38. Key point Simple diffusion Facilitated diffusion Active transport characteristics of carrier-mediated difusion characteristics of channel-mediated difusion

39. Section 2 Signal transduction across cell membrane

40. The operation of control systems requires that cells be able to communicate with each other, often over long distances. • Much of this intercellular communication is mediated by chemical messengers. • These messengers include chemicals such as hormones, neurotransmitters, cytokines, and so on.

41. Signal transduction pathways differ between lipid soluble and lipid-insoluble messengers. • Most lipid-soluble messengers are hormones, they diffuse across the plasma membrane and bind to the intracellular receptors. • The very large members of lipid-insoluble combine with plasma membrane receptors, causing the changes in the conformation of receptors and subsequent cascade response.

42. Lipid-soluble messengers activate receptors that are located inside the cell. The signal transduction of lipid-insoluble messengers is mediated by three different pathways, which are G-protein-linked receptor, ionotropic receptor and enzyme-linked receptor.

44. ionotropic receptor Receptors may open or close membrane ion channels, leading to changes in the ion conductances of a cell. This is particularly a feature of neurotransmitter receptors in excitable tissue such as nerve and muscle. • The receptor may act directly on the channel. • Ion-channel responses can be very rapid (millisecondes).

45. Signal transduction mediated by G-proteinlinked receptor The signal transduction mediated by G-protein linked receptor is achieved by the cascade activities of the membranous receptors, G protein, G protein effector, second messenger and other molecules in the cell membrane and cytoplasm.

46. Pathways of signal transduction mediated by GProtein-Linked receptor A biological active substance which specifically binds membranous receptor is called ligand. The important pathways are as follows: • The receptor-G protein–cAMP pathway; • The receptor-G protein–Ca2+ pathway;

48. Signal transduction mediated by enzyme-linked receptor The receptors in this category of plasma membrane receptors have intrinsic enzyme activity. There are two most important receptors: tyrosine kinase receptor and guanylyl cyclase receptor.

49. Section 3 Bioelectrical phenomena of cell

50. Excitation and Excitable Cell • Excitation is used to describe responses of a cell to stimuli. • Excitability is the ability of excitable cells to response to stimulation or produce action potentials after stimulation. • Excitable cell means the cell which can produce action potentials after stimulation.