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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 3 Cells: The Living Units: Part A Copyright © 2010 Pearson Education, Inc. Cell Theory • The cell is the smallest structural and functional unit of life • Continuity of life has a cellular basis: all cells come from pre-existing cells • Organismal functions depend on individual and collective cell functions • Biochemical activities of cells are dictated by their specific subcellular structures Copyright © 2010 Pearson Education, Inc. Cell Diversity • • Over 200 different types of human cells Types differ in size, shape, subcellular components, and functions Erythrocytes Fibroblasts Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Skeletal Muscle cell Smooth muscle cells (b) Cells that move organs and body parts Macrophage Nerve cell (e) Cell that gathers information and control body functions (f) Cell of reproduction Sperm Fat cell (c) Cell that stores nutrients Copyright © 2010 Pearson Education, Inc. (d) Cell that fights disease Figure 3.1 Basic Features Of All Cells • All cells have: Plasma membrane Semifluid substance called cytosol Chromosomes (carry genes) Ribosomes (make proteins) • Human cells have plasma membrane, cytoplasm and nucleus Copyright © 2010 Pearson Education, Inc. Basic Features Of A Eukaryotic Cell Chromatin Nucleolus Nuclear envelope Nucleus Smooth endoplasmic reticulum Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Cytoskeletal elements • Microtubule • Intermediate filaments Copyright © 2010 Pearson Education, Inc. Plasma membrane Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Peroxisome Figure 3.2 Plasma Membrane Extracellular fluid (watery environment) • Biomolecular double-layered structures of lipids and proteins forming the outer envelope of the cell • Plays a dynamic role in cellular activity • Functions as a selective barrier between extracellular fluid (ECF) and intracellular fluid (ICF), allowing passage of oxygen, nutrients, and wastes for the whole volume of the cell Copyright © 2010 Pearson Education, Inc. Polar head of phospholipid molecule Cholesterol Glycolipid Glycoprotein Carbohydrate of glycocalyx Outwardfacing layer of phospholipids Integral proteins Filament of cytoskeleton Peripheral Bimolecular Inward-facing proteins lipid layer layer of containing phospholipids Nonpolar proteins tail of phospholipid Cytoplasm molecule (watery environment) Membrane Lipids And Lipid Rafts LIPIDS Extracellular fluid (watery environment) • 75% phospholipids (lipid bilayer) • Phosphate heads: polar and hydrophilic • Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b) • 5% glycolipids • Cholesterol Glycolipid Glycoprotein Carbohydrate of glycocalyx Lipids with polar sugar groups on outer membrane surface • 20% cholesterol • Polar head of phospholipid molecule Increases membrane stability and fluidity LIPID RAFTS • 20% of the outer membrane surface • Contain phospholipids, sphingolipids, and cholesterol • May function as stable platforms for cell-signaling molecules Copyright © 2010 Pearson Education, Inc. Outwardfacing layer of phospholipids Integral proteins Filament of cytoskeleton Peripheral Bimolecular Inward-facing proteins lipid layer layer of containing phospholipids Nonpolar proteins tail of phospholipid Cytoplasm molecule (watery environment) Membrane Proteins • Integral proteins: Extracellular fluid (watery environment) • Firmly inserted into the membrane (most are transmembrane) Functions: Transport proteins (channels and carriers), enzymes, or receptors Polar head of phospholipid molecule Cholesterol Glycolipid Glycoprotein Carbohydrate of glycocalyx • Peripheral proteins • Loosely attached to integral proteins • Include filaments on intracellular surface and glycoproteins on extracellular surface Functions: Enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of glycocalyx Copyright © 2010 Pearson Education, Inc. Outwardfacing layer of phospholipids Integral proteins Filament of cytoskeleton Peripheral Bimolecular Inward-facing proteins lipid layer layer of containing phospholipids Nonpolar proteins tail of phospholipid Cytoplasm molecule (watery environment) Functions of Membrane Proteins 1. Transport (a) Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Figure 3.4a Copyright © 2010 Pearson Education, Inc. Functions of Membrane Proteins 2. Receptors for Signal Transduction Signal Receptor Copyright © 2010 Pearson Education, Inc. (b) Receptors for signal transduction A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. Figure 3.4b Functions of Membrane Proteins 3. Attachment to Cytoskeleton and Extracellular Matrix (ECM) (c) Attachment to the cytoskeleton and extracellular matrix (ECM) Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Copyright © 2010 Pearson Education, Inc. Figure 3.4c Functions of Membrane Proteins 4. Enzymatic Activity (d) Enzymatic activity Enzymes Copyright © 2010 Pearson Education, Inc. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. Figure 3.4d Functions of Membrane Proteins 5. Intercellular Joining (e) Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. CAMs Copyright © 2010 Pearson Education, Inc. Figure 3.4e Functions of Membrane Proteins 6. Cell-cell Recognition (f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Copyright © 2010 Pearson Education, Inc. Figure 3.4f Signaling molecule Enzymes ATP (a) Transport Receptor Signal transduction (b) Enzymatic activity (c) Signal transduction Glycoprotein Major (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular Functions Of Membrane Proteins matrix (ECM) Copyright © 2010 Pearson Education, Inc. Membrane Junctions 1. Tight Junctions: Impermeable junctions that prevent fluids and most molecules from moving through the intercellular space (space between cells) Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Interlocking junctional proteins Intercellular space Copyright © 2010 Pearson Education, Inc. Figure 3.5a Membrane Junctions 2. Desmosomes: Anchoring junctions (like “rivets” or “spot welds”) bind adjacent cells together and help form an internal tension-reducing network of fibers. Example: Found in cardiac or smooth muscle cells for spread of ions Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Copyright © 2010 Pearson Education, Inc. Linker glycoproteins (cadherins) Figure 3.5b Membrane Junctions 3. Gap Junctions: Transmembrane proteins that form communicating junctions to allow ions and small molecules to pass from one cell to the next for intercellular communication Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (connexon) Copyright © 2010 Pearson Education, Inc. Figure 3.5c Membrane Transport • Plasma membranes are selectively permeable, allowing the passage of some, but not all molecules • Transport of molecules across cell membranes occurs by passive or active processes Passive processes No cellular energy (ATP) required Substance moves down its concentration gradient Active processes Energy (ATP) required Occurs only in living cell membranes Copyright © 2010 Pearson Education, Inc. Passive Processes Of Membrane Transport • Whether or not a substance can passively permeate a membrane is determined by: 1. Lipid solubility of the substance 2. Size of membrane channels 3. Carrier proteins in the membranes • Passive processes can be categorized as: 1. Simple diffusion 2. Carrier-mediated facilitated diffusion 3. Channel-mediated facilitated diffusion 4. Osmosis Copyright © 2010 Pearson Education, Inc. Passive Processes: Simple Diffusion • Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer Extracellular fluid Lipidsoluble solutes Cytoplasm Copyright © 2010 Pearson Education, Inc. Passive Processes: Facilitated Diffusion • Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which: • Exhibit specificity (selectivity) • Are saturable; rate is determined by number of carriers or channels • Can be regulated in terms of activity and quantity Copyright © 2010 Pearson Education, Inc. Facilitated Diffusion Using Carrier Proteins • Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) • Binding of substrate causes shape change in carrier Lipid-insoluble solutes (such as sugars or amino acids) Copyright © 2010 Pearson Education, Inc. Facilitated Diffusion Using Channel Proteins • Aqueous channels formed by transmembrane proteins selectively transport ions or water • Two types: (a) Leakage channels (always open); (b) Gated channels: controlled by chemical or electrical signals Small lipidinsoluble solutes Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Copyright © 2010 Pearson Education, Inc. Passive Processes: Osmosis • Movement of solvent (water) across a selectively permeable membrane Water molecules • Water diffuses through plasma membranes: Lipid billayer Through the lipid bilayer Through water channels called aquaporins (AQPs) Aquaporin Copyright © 2010 Pearson Education, Inc. Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Passive Processes: Osmosis • Water concentration is determined by solute concentration because solute particles displace water molecules • Osmolarity: The measure of total concentration of solute particles • When solutions of different osmolarity are separated by a semi-permeable membrane, osmosis occurs until equilibrium is reached Importance of Osmosis: • When osmosis occurs, water enters or leaves a cell • Change in cell volume disrupts cell function Copyright © 2010 Pearson Education, Inc. Passive Processes: Diffusion (a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Left compartment: Solution with lower osmolarity Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged H2O Solute Membrane Copyright © 2010 Pearson Education, Inc. Solute molecules (sugar) Figure 3.8a Passive Processes: Osmosis (b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Left compartment Right compartment Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move H2O Membrane Copyright © 2010 Pearson Education, Inc. Solute molecules (sugar) Figure 3.8b Tonicity • Tonicity: The ability of a solution to cause a cell to shrink or swell • Isotonic: A solution with the same solute concentration as that of the cytosol • Hypertonic: A solution having greater solute concentration than that of the cytosol • Hypotonic: A solution having lesser solute concentration than that of the cytosol Copyright © 2010 Pearson Education, Inc. Effect Of Tonicity On Red Blood Cells (a) Isotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Copyright © 2010 Pearson Education, Inc. (b) Hypertonic solutions Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). (c) Hypotonic solutions Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9 Summary of Passive Processes Process Simple diffusion Facilitated diffusion Osmosis Energy Source Kinetic energy Kinetic energy Kinetic energy • Also see Table 3.1 Copyright © 2010 Pearson Education, Inc. Example Movement of O2 through phospholipid bilayer Movement of glucose into cells Movement of H2O through phospholipid bilayer or AQPs