Exploring Cell Structure and Function: A Comprehensive Overview

1. Chapter 3 The Cellular Level of Organization Lecture Outline

2. INTRODUCTION • A cell is the basic, living, structural, and functional unit of the body. • Cytology is the study of cell structure, and cell physiology is the study of cell function.

3. PARTS of a CELL • A generalized view of the cell is a composite of many different cells in the body (Figure 3.1.) • No single cell includes all of the features seen in the generalized cell.

4. PARTS of a CELL • The cell can be divided into three principal parts for ease of study. • Plasma (cell) membrane • Cytoplasm • Cytosol • Organelles (except for the nucleus) • Nucleus

5. THE PLASMA MEMBRANE • The plasma membrane is a flexible, sturdy barrier that surrounds and contains the cytoplasm of the cell. • The fluid mosaic model describes its structure (Figure 3.2). • The membrane consists of proteins in a sea of lipids.

6. Plasma Membrane • Flexible but sturdy barrier that surround cytoplasm of cell • Fluid mosaic model describes its structure • “sea of lipids in which proteins float like icebergs” • membrane is 50 % lipid & 50 % protein • held together by hydrogen bonds • lipid is barrier to entry or exit of polar substances • proteins are “gatekeepers” -- regulate traffic • 50 lipid molecules for each protein molecule

7. The Lipid Bilayer • The lipid bilayer is the basic framework of the plasma membrane and is made up of three types of lipid molecules: phospholipids, cholesterol, and glycolipids (Figure 3.2).

8. Lipid Bilayer of the Cell Membrane • Two back-to-back layers of 3 types of lipid molecules • Cholesterol and glycolipids scattered among a double row of phospholipid molecules

9. The Lipid Bilayer • The bilayer arrangement occurs because the lipids are amphipathic molecules. They have both polar (charged) and nonpolar (uncharged) parts with the polar “head” of the phospholipid pointing out and the nonpolar “tail” pointing toward the center of the membrane. • Cholesterol molecules are weakly amphiphotic and are interspersed among other lipids. • Glycolipids appear only in the membrane layer which faces the extracellular fluid.

10. Phospholipids • Comprises 75% of lipids • Phospholipid bilayer = 2 parallel layers of molecules • Each molecule is amphipathic (has both a polar & nonpolar region) • polar parts (heads) are hydophilic and face on both surfaces a watery environment • nonpolar parts (tails) are hydrophobic and line up next to each other in the interior

11. Cholesterol within the Cell Membrane • Comprises 20% of cell membrane lipids • Interspersed among the other lipids in both layers • Stiff steroid rings & hydrocarbon tail are nonpolar and hide in the middle of the cell membrane

12. Glycolipids within the Cell Membrane • Comprises 5% of the lipids of the cell membrane • Carbohydrate groups form a polar head only on the side of the membrane facing the extracellular fluid

13. Arrangement of Membrane Proteins • The membrane proteins are divided into integral and peripheral proteins. • Integral proteins extend into or across the entire lipid bilayer among the fatty acid tails of the phospholipid molecules. • Peripheral proteins are found at the inner or outer surface of the membrane and can be stripped away from the membrane without disturbing membrane integrity.

14. Membrane Proteins Integral versus Peripheral Proteins

15. Integral Proteins • Integral membrane proteins are amphipathic. • Those that stretch across the entire bilayer and project on both sides of the membrane are termed transmembrane proteins. • Many integral proteins are glycoproteins. • The combined glycoproteins and glycolipids form the glycocalyx which helps cells recognize one another, adhere to one another, and be protected from digestion by enzymes in the extracellular fluid.

16. Functions of Membrane Proteins • Membrane proteins vary in different cells and functions as channels (pores), transporters, receptors, enzymes, cell-identity markers, and linkers (Figure 3.3). • The different proteins help to determine many of the functions of the plasma membrane.

17. Functions of Membrane Proteins • Formation of Channel • passageway to allow specific substance to pass through • Transporter Proteins • bind a specific substance, change their shape & move it across membrane • Receptor Proteins • cellular recognition site -- bind to substance

18. Functions of Membrane Proteins • Cell Identity Marker • allow cell to recognize other similar cells • Linker • anchor proteins in cell membrane or to other cells • allow cell movement • cell shape & structure • Act as Enzyme • speed up reactions

19. Membrane Fluidity • Membranes are fluid structures, rather like cooking oil, because most of the membrane lipids and many of the membrane proteins easily move in the bilayer. • Membrane lipids and proteins are mobile in their own half of the bilayer. • Cholesterol serves to stabilize the membrane and reduce membrane fluidity.

20. Membrane Permeability • Plasma membranes are selectively permeable. • Some things can pass through and others cannot. • The lipid bilayer portion of the membrane is permeable to small, nonpolar, uncharged molecules but impermeable to ions and charged or polar molecules. • The membrane is also permeable to water. • Transmembrane proteins that act as channels or transporters increase the permeability of the membrane to molecules that cannot cross the lipid bilayer. • Macromolecules are unable to pass through the plasma membrane except by vesicular transport.

21. Gradients Across the Plasma Membrane • A concentration gradient is the difference in the concentration of a chemical between one side of the plasma membrane and the other. • Oxygen and sodium ions are more concentrated outside the cell membrane with carbon dioxide and potassium ions more concentrated inside the cell membrane (Figure 3.4a).

22. Gradients Across Membrane • Concentration gradient • Electrical gradient

23. Gradients Across the Plasma Membrane • The inner surface of the membrane is more negatively charged and the outer surface is more positively charged (Figure 3.4b). This sets up an electrical gradient, also called the membrane potential. • Maintaining the concentration and electrical gradients are important to the life of the cell. • The combined concentration and electrical gradients are called the electrochemical gradient.

24. TRANSPORT ACROSS THE PLASMA MEMBRANE • Processes to move substances across the cell membrane are essential to the life of the cell. • Some substances cross the lipid bilayer while others cross through ion channels. • Transport processes that mover substances across the cell membrane are either active or passive.

25. TRANSPORT ACROSS THE PLASMA MEMBRANE • Three types of passive processes are • diffusion through the lipid bilayer • diffusion through ion channels • facilitated diffusion • Active transport requires cellular energy. • Materials can also enter or leave the cell through vesicle transport.

26. Transport Across the Plasma Membrane • Substances cross membranes by a variety of processes: • mediated transport movesmaterials with the help of atransporter protein • nonmediated transport doesnot use a transporter protein • active transport uses ATP todrive substances against theirconcentration gradients • passive transport moves substances down their concentration gradient with only their kinetic energy • vesicular transport move materials across membranes in small vesicles -- either by exocytosis or endocytosis

27. Principles of Diffusion • Diffusion is the random mixing of particles that occurs in a solution as a result of the kinetic energy of the particles. (Figure 3.6) • Diffusion rate across plasma membranes is influenced by several factors: • Steepness of the concentration gradient • Temperature • Size or mass of the diffusing substance • Surface area • Diffusion distance.

28. Diffusion • Crystal of dye placed in a cylinder of water • Net diffusion from the higher dye concentration to the region of lower dye • Equilibrium has been reached in the far right cylinder

29. Diffusion Through the Lipid Bilayer • Nonpolar, hydrophobic molecules such as respiratory gases, some lipids, small alcohols, and ammonia can diffuse across the lipid bilayer. • It is important for gas exchange, absorption of some nutrients, and excretion of some wastes.

30. Diffusion Through Membrane Channels • Most membrane channels are ion channels, allowing passage of small, inorganic ions which are hydrophilic. • Ion channels are selective and specific and may be gated or open all the time (Figure 3.6).

31. Osmosis • Osmosis is the net movement of a solvent through a selectively permeable membrane, or in living systems, the movement of water (the solute) from an area of higher concentration to an area of lower concentration across the membrane (Figure 3.7).

32. Osmosis • Water molecules penetrate the membrane by diffusion through the lipid bilayer or through aquaporins, transmembrane proteins that function as water channels. • Water moves from an area of lower solute concentration to an area of higher solute concentration. • Osmosis occurs only when the membrane is permeable to water but not to certain solutes.

33. Osmosis • Osmotic pressure of a solution is proportional to the concentration of the solute particles that cannot cross the membrane (Figure 3.7c).

34. Tonicity • Tonicity is a measure of a solution’s ability to change the volume of cells by altering their water concentration. • In an isotonic solution, red blood cells maintain their normal shape (Figure 3.8a). • In a hypotonic solution, red blood cells undergo hemolysis (Figure 3.8b). • In a hypertonic solution, red blood cells undergo cremation (Figure 3.8c). • There are important medical uses of isotonic, hypotonic, and hypertonic solutions.

36. Facilitated Diffusion • In facilitated diffusion, a solute binds to a specific transporter on one side of the membrane and is released on the other side after the transporter undergoes a conformational change. • Solutes that move across membranes by facilitated diffusion include glucose, urea, fructose, galactose, and some vitamins (Figure 3.6a). • Rate of movement depends upon • steepness of concentration gradient • number of transporter proteins (transport maximum)

38. Facilitated Diffusion • Transport maximum is the upper limit on the rate at which facilitated diffusion can occur. If all the transporters are occupied, then the rate of facilitated diffusion does not increase. • Glucose enters the cell by facilitated diffusion (Figure 3.9.)

39. Facilitated Diffusion of Glucose • Glucose binds to transportprotein • Transport protein changes shape • Glucose moves across cell membrane (but only downthe concentration gradient) • Kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose-6-phosphate • Transporter proteins always bring glucose into cell

40. Active Transport • Active transport is an energy-requiring process that moves solutes such as ions, amino acids, and monosaccharides against a concentration gradient. • In primary active transport, energy derived from ATP changes the shape of a transporter protein, which pumps a substance across a plasma membrane against its concentration gradient.

41. Primary Active Transport • The most prevalent primary active transport mechanism is the sodium ion/potassium ion pump (Figure 3.10). • requires 40% of cellular ATP • all cells have 1000s of them • maintains low concentration of Na+and a high concentration of K+ in the cytosol • operates continually

42. Clinical Application: • Cystic fibrosis is caused by a defective gene that produces an abnormal chloride ion transported. The disease affects the respiratory, digestive, urinary, and reproductive systems

43. Secondary Active Transport • In secondary active transport, the energy stored in the form of a sodium or hydrogen ion concentration gradient is used to drive other substances against their own concentration gradients. • Plasma membranes contain several antiporters and symporters powered by the sodium ion gradient (Figure 3.11a).

44. One in & one out. vs. Both going in Secondary Active Transport

45. Digitalis • Digitalis slows the sodium ion-calcium ion antiporters, allowing more calcium to stay inside heart muscle cells, which increases the force of their contraction and thus strengthens the heartbeat.

46. Transport in Vesicles • A vesicle is a small membranous sac formed by budding off from an existing membrane. • endocytosis • exocytosis

47. Overview: Vesicular Transport of Particles • Endocytosis = bringing something into cell • phagocytosis = cell eating by macrophages & WBCs • particle binds to receptor protein • whole bacteria or viruses are engulfed & later digested • pinocytosis = cell drinking • no receptor proteins • Exocytosis = release something from cell • Vesicles form inside cell, fuse to cell membrane • Release their contents • digestive enzymes, hormones, neurotransmitters or waste products • replace cell membrane lost by endocytosis

48. Endocytosis • In endocytosis, materials move into a cell in a vesicle formed from the plasma membrane. • Receptor-mediated endocytosis is the selective uptake of large molecules and particles by cells (Figure 3.12).

49. Receptor-mediated endocytosis

50. Endocytosis • The steps of receptor-mediated endocytosis includes binding, vesicle formation, uncoating, fusion with endosome , recycling of receptors, and degradation in lysosomes.. • Viruses can take advantage of this mechanism to enter cells. • Phagocytosis is the ingestion of solid particles (Figure 3.13). • Pinocytosis is the ingestion of extracellular fluid (Figure 3.14).