Lecture 5 Cell Membrane Structure and Function (1) PDF

Structure of the Cell Membrane Lecture 8 BIOL 110 Successful learning relies on communication between brain cells Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. How Does Plasma Membrane Work? Which one of the st...

Structure of the Cell Membrane Lecture 8 BIOL 110 Successful learning relies on communication between brain cells Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. How Does Plasma Membrane Work? Which one of the statements do you agree the most: A. Cell membrane is like walls of a castle under siege: It does not let anything in or out. B. Cell membrane is like a door in your house: only “friendly” substances can come in, “unfriendly” ones will be expelled. C. Cell membrane is like a door in the Student center: anything can get in or out as long as it “fits” in the door. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Learning Objectives Discuss the properties of the phospholipid bilayer. Explain why biological membranes must stay fluid to function properly and list the factors important for maintaining membrane fluidity. Describe the structural organization of the extracellular matrix (ECM) Describe the functions of membrane proteins. List and describe the types of junctions in animal and plant cells. List and describe the components of cytoskeleton and their role in maintaining cell shape. Describe the role of membrane carbohydrates. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. How does the plasma membrane regulate inbound and outbound traffic? Some small molecules move across the cell membrane using passive transport—no input of energy—and may require transport proteins Some small molecules use active transport, which requires both energy and a transport protein Large molecules move in and out, using bulk transport; exocytosis or endocytosis Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Cellular membranes are fluid mosaics of lipids and proteins Lipids and proteins are the main components of membranes, but carbohydrates are also important Membranes are composed mainly of phospholipids Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions Phospholipids form a bilayer with hydrophobic tails inside the membrane, and hydrophilic heads exposed to water on either side Very effective barrier for majority of substances Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Current model of an animal cell’s plasma membrane The fluid mosaic model of membrane structure depicts the membrane as a mosaic of protein molecules moving in a fluid bilayer of phospholipids Proteins are not randomly distributed in the membrane; they often form groups that carry out common functions Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Fluidity of Membranes Lipids and proteins can move sideways within the membrane Rarely, a lipid may flip-flop across the membrane, from one phospholipid layer to the other Do membrane proteins move? Data from L. D. Frye and M. Edidin, The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons, Journal of Cell Science 7:319 (1970). Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Fluidity of Membranes Membranes must be fluid to work properly: – membrane permeability – movement of transport proteins Membranes that are too fluid cannot support protein function Organisms living in extreme temperatures adapt by changing membrane lipid composition – fish that live in extreme cold have a high proportion of unsaturated hydrocarbon tails in cell membranes Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Fluidity of Membranes As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Fluidity of Membranes Cholesterol is a membrane component in animal cells that acts as a fluidity buffer – At relatively high temperatures (such as 37°C) cholesterol restrains movement of phospholipids – At low temperatures, it maintains fluidity by preventing tight packing Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Membrane Proteins and Their Functions Two major types of membrane proteins: – Peripheral proteins are bound to the surface of the membrane – Integral proteins penetrate the hydrophobic core Transmembrane proteins are integral proteins that span the membrane Hydrophobic regions of an integral protein consist of nonpolar amino acids, often coiled into α helices Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Membrane Proteins and Their Functions Some membrane proteins are held in place by attachment to the cytoskeleton inside the cell Animal cells are covered by an extracellular matrix (ECM) made up of glycoproteins such as collagen, proteoglycans, and fibronectin Fibronectin and other ECM proteins bind to receptor proteins in the plasma membrane called integrins Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Membrane Proteins and Their Functions Proteins determine most of the membrane’s functions: Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Activation of Receptor Proteins and Fluid Mosaic Class of protein receptors known as receptor tyrosine kinases (RTK) are necessary for cellular growth and division Inactive protein receptor monomer RTK activation and deactivation requires lateral movement of proteins within phosphopholipid bi-layer – Single polypeptide chains are inactive Active protein receptor dimer – Activation requires binding of a signaling molecule (ligand), which brings two polypeptide chains together (dimerization). Dimerization brings two kinase domains in close contact for trans-autophosphorylation and allows binding to the next signaling protein (signal transduction) Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Membrane Proteins and Their Functions Cell-surface proteins are important in medicine – For example, HIV enters immune cells by binding to cell-surface protein CD4 and a “co-receptor” CCR5 – Individuals lacking CCR5 are immune to HIV infection – Drugs are in development to mask CCR5 and block HIV entrance in nonimmune individuals Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cell Junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact This contact is mediated by membrane proteins Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cell Junctions in Animal Cells Three types of cell junctions common in epithelial tissues – At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid – Desmosomes (anchoring junctions) fasten cells together into strong sheets – Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cell Junctions between Plant Cells Plasmodesmata are channels that connect plant cells Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell This function is analogous to gap junctions in animal cells Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Integrin Proteins in Plasma membrane attach cytoskeleton to ECM The cytoskeleton is a network of fibers extending throughout the cytoplasm It organizes the cell’s structures and activities, anchoring many organelles It is composed of three types of Integrins molecular structures – Microtubules – Microfilaments – Intermediate filaments Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The structure and function of the cytoskeleton Microtubules (Tubulin Microfilaments (Actin Intermediate Filaments Polymers) Filaments) Hollow tubes Two Intertwined strands of Fibrous proteins coiled into actin cables 25 nm with 15-nm lumen 7nm 8-12 nm Tubulin, a dimer consisting Actin One of several different of an  -tubulin and a - alpha beta proteins (including keratins) tubulin Maintenance of cell shape; Maintenance of cell shape; Maintenance of cell shape; cell motility; chromosome changes in cell shape; anchorage of nucleus and movements in cell division; muscle contraction; certain other organelles; organelle movements cytoplasmic streaming formation of nuclear lamina (plant cells); cell motility; cell division (animal cells) Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Microtubule Organizing Center (MTOC): Centrosomes and Centrioles In animal cells, microtubules grow out from a centrosome near the nucleus In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Other eukaryotic cells organize microtubules in the absence of centrosomes with centrioles Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cilia and Flagella Microtubules control the beating of flagella and cilia Many unicellular protists are propelled through water by cilia or flagella Motile cilia are found in large numbers on a cell surface, whereas flagella are limited to one or a few per cell Cilia and flagella differ in their beating patterns Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Cilia and Flagella Cilia and flagella share a common structure – A group of microtubules sheathed in an extension of the plasma membrane – Nine doublets of microtubules arranged in a ring with two single microtubules in the center – A basal body that anchors the cilium or flagellum – A motor protein called dynein, which drives the bending movements of a cilium or flagellum Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Outer pair of microtubules How do Cilia and Dynein Plasma membrane Flagella Beat? Central microtubules Dynein has two “feet” that change shape depending on phosphorylation status Change in shape of the feet cause the microtubules to bend Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Microfilaments (Actin Filaments) Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits A network of microfilaments helps support the cell’s shape They form a cortex just inside the plasma membrane to help support the cell’s shape Bundles of microfilaments make up the core of microvilli of intestinal cells Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Dynamic Nature of Microfilaments A twisted double chain of actin subunits. Very dynamic: assembles- disassembles quickly: Have (+) and (–) end, which require different minimal concentrations of actin monomers in order to elongate At a critical concentration, (-) end disassembles, while (+) end grows (treadmilling) Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Amoeboid Movement Generate movement by rapidly assembling and disassembling. Amoebas, human white blood cells, and cancer cells can creep along a surface by changing Cortex (outer cytoplasm): shape: gel with actin network 100 µm Actin filaments push the plasma Inner cytoplasm membrane outward, forming bulges (more fluid) (pseudopodia) that adhere to the surface. Contractions of microfilaments at the opposite end of the cell force the Extending cytoplasm forward in the direction of pseudopodium locomotion. Amoeboid movement Cells crawl along a surface by extending pseudopodia and moving toward them. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Intermediate Filaments Intermediate filaments range in diameter from 8 to 12 nanometers, larger than microfilaments but smaller than microtubules Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes They support cell shape and fix organelles in place Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to molecules on the surface of the membrane Many of these surface molecules are bonded to short, branched chains of carbohydrates – Glycolipids are carbohydrates bonded to lipids – Glycoproteins are carbohydrates bonded to proteins The diversity of surface carbohydrates enables them to function as markers for cell identification Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Learning Objectives Relate the polarity and molecular size of a substance to the rate at which it passes phospholipid bilayer Define diffusion in terms of the concentration gradient. Determine tonicity of solutions and direction of osmosis with relation of the solute concentration. Describe structure and function of plant cell wall. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Membrane structure results in selective permeability The plasma membrane controls the exchange of materials between the cell and its surroundings Membranes exhibit selective permeability; some substances cross more easily than others Hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through the membrane rapidly The hydrophobic interior of the membrane interferes with the passage of hydrophilic (polar) molecules Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Diffusion Of Solutes Across A Synthetic Membrane Diffusion is the movement of particles of any substance so that they spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules may be directional At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Concentration Gradient High concentration of This arrow points molecules down the concentration gradient since it starts at high concentration and Low concentration of ends at low molecules concentration of molecules. If particles in a liquid or gas are not evenly distributed, then regions exist that have a higher or lower concentration of particles, forming a concentration gradient Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Diffusion Of Solutes Across A Synthetic Membrane Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases Each substance moves down its own concentration gradient, unaffected by the concentrations of other substances Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Passive transport is diffusion of a substance across a membrane with no energy investment The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell The concentration gradient represents potential energy that drives diffusion Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Osmosis Osmosis is the diffusion of water across a selectively permeable membrane Water molecules diffuse across a membrane from the region of lower solute concentration to the region of higher solute concentration Water keeps moving until the solute concentration is equal on both sides Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Water Balance of Cells Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water Tonicity depends on the concentration of solutes in the solution that cannot cross the membrane, relative to that inside the cell isotonic hypertonic hypotonic Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Water Balance of Animal Cells A solution is isotonic if its solute concentration is the same as that inside the cell Water diffuses across the membrane at the same rate in both directions; there is no net movement of water across the membrane The volume of a cell is stable in an isotonic solution Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Water Balance of Animal Cells A solution is hypertonic if the solute concentration is greater than that inside the cell Net diffusion of water is from inside the cell to the surrounding solution Cells will lose water, shrivel, and likely die in hypertonic solution Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Water Balance of Animal Cells A solution is hypotonic if the solute concentration is less than that inside the cell Net diffusion of water is from the surrounding solution to the inside of the cell Cells without cell wall will gain water, swell and lyse (burst) in a hypotonic solution Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Cell Walls of Plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells Prokaryotes, fungi, and some protists also have cell walls The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Water Balance of Plant Cells hypotonic isotonic hypertonic If a plant cell and its surroundings are isotonic, there is no net movement of water Plasma membrane Nucleus Vacuole into the cell Plant cells become Vacuole flaccid (limp) in an Vacuolar isotonic solution, membrane and the plant wilts (tonoplast) Cytoplasm Plasma membrane Plant cells lose water in a hypertonic environment The cell shrivels and the membrane pulls away from the cell wall in multiple locations, a phenomenon called plasmolysis Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Learning Objectives Contrast: – passive and active transport across plasma membrane – simple and facilitated diffusion – channel and carrier transport proteins – Exo- and endocytosis Describe transport of substances by the – Potassium channel – GLUT1 glucose transporter – Sodium-potassium and proton pumps – Sodium-glucose and proton-sucrose co-transporters. Phagocytosis, pinocytosis and receptor-mediated endocytosis. Identify the source of energy for each type of transport. Describe the structure and define the function of lysosomes Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Transport Proteins Transport proteins move only specific substances – For example, glucose carrier proteins only transport glucose; they will not transport fructose, a structural isomer of glucose The selective permeability of a membrane is dependent on both the lipid bilayer and the specific transport proteins it contains Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Facilitated Diffusion: Passive Transport Aided by Proteins Hydrophilic substances require transport proteins to cross the membrane; such transport is facilitated diffusion Transport proteins include channel proteins and carrier proteins Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel Carrier proteins, bind to molecules and change shape to shuttle them across the membrane Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Facilitated Diffusion: Passive Transport Aided by Proteins Ion channels facilitate the transport of ions Some ion channels, called gated channels, open or close in response to a stimulus – In nerve cells, potassium ion channels open in response to electrical stimulus – Other gated channels open in response to chemical stimulus— binding of a specific substance to the protein Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Why Do I Need To Know This?!!! Cystic fibrosis (CF) is a genetic disease caused by a dysfunctional Cl - channel Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Facilitated Diffusion: Passive Transport Aided by Proteins Outside cell Glucose High High concentration concentration of glucose of glucose Low concentration Glucose transporter How does Glucose of glucose (GLUT 1) the transport concentration continue? stays low 1 Cytosol 2 3 Transport continues Once inside the cell, glucose is driven by concentration modified by ATP into gradient phosphorylated glucose – Carrier proteins undergo a subtle change in shape that translocates the substance across the membrane. – Example: glucose transport – Glucose transporter 1 (GLUT 1) facilitates glucose diffusion into red blood cells – Red blood cells immediately convert glucose to glucose phosphates, which maintains the concentration gradient Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  The Need for Energy in Active Transport Active transport requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradients Enables cells to maintain solute concentrations that differ from the environment – Sodium-potassium pump is used to maintain K+ concentration higher and Na+ lower inside the cell compared to the surrounding environment All proteins involved in active transport are carrier proteins Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The sodium-potassium pump: a specific case of active transport Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Ion Pumps Maintain Membrane Potential Membrane potential is the voltage across a membrane created by differences in the distribution of positive and negative ions across a membrane The inside of the cell is negative in charge relative to the outside, favoring passive transport of cations into and anions out of the cell Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. How Ion Pumps Maintain Membrane Potential Electrochemical gradient: – A chemical force (the ion’s concentration gradient) – An electrical force (the effect of the membrane potential on the ion’s movement) An electrogenic pump is a transport protein that generates voltage across a membrane, storing energy that can be used for cellular work The main electrogenic pump differs between plants and animals – In animals, it is the sodium-potassium pump – In plants, fungi, and bacteria, it is the proton pump, Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when facilitated diffusion of a solute indirectly drives transport of other substances against their concentration gradients The “downhill” diffusion of solute is coupled to the “uphill” transport of a second substance against its own concentration gradient Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cotransport: Coupled Transport by a Membrane Protein In animals, a co-transporter couples transport of glucose to passive diffusion of sodium ions into intestinal cells The gradient of sodium ions is created by active transport conducted by sodium-potassium pump Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Cotransport: Coupled Transport by a Membrane Protein Plant cells couple active transport of sucrose from leaf into the veins to the passive diffusion of hydrogen ions back into the vein The gradient of hydrogen ions is created by proton pumps This is how plants load sucrose into their veins for transport around the plant body Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. How does the plasma membrane regulate inbound and outbound traffic? Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Review: passive and active transport Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Various Types of Transporters Act Together to Deliver Glucose from the Gut 1. Sodium-potassium pump moves sodium out of the cell to create sodium gradient using the energy of ATP 2. Sodium-glucose co-transporter moves glucose into the gut epithelial cells against 2 concentration gradient using the. 3 energy of sodium gradient.. Sodium- 3. Glucose transporter moves glucose potassium pump from epithelial cell into the bloodstream down concentration gradient by facilitated diffusion. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large molecules, such as polysaccharides and proteins, cross the membrane in bulk inside vesicles Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell Many secretory cells use exocytosis to export their products – Cells in the pancreas secrete insulin by exocytosis Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  Endocytosis In endocytosis, macromolecules are taken into the cell in vesicles The membrane forms a pocket that deepens and pinches off forming a vesicle around the material for transport Three types of endocytosis: Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Endocytosis A cell engulfs a particle by extending pseudopodia around it and packing it in a membranous sac called a food vacuole The vacuole fuses with a lysosome to digest the particle Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Lysosomes A membranous sac filled with hydrolytic enzymes that can break up macromolecules. Hydrolytic enzymes: Are made in the endomembrane system (ER, Golgi). Functional at an acidic pH Some lysosomes digest food in food vacuole. Some recycle the cell’s own organelles and macromolecules. Primary Secondary lysosome lysosome Figure 4-16 p92 Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Pinocytosis Molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Nonspecific for the substances it transports; any and all solutes are taken into the cell Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Receptor-Mediated Endocytosis Vesicle formation is triggered by solute binding to receptors Receptor proteins bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles Emptied receptors are recycled to the plasma membrane by the same vesicle Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Receptor-Mediated Endocytosis Human cells use receptor- mediated endocytosis to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs) Individuals with familial hypercholesterolemia have missing or defective LDL receptor proteins Cholesterol accumulates in the blood, building up lipids and narrowing the space in the blood vessels, resulting in potential heart damage or stroke Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Why are cells small? Why there is no cell size of an elephant? Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Biologists Use Microscopes to Study Cells Mitochondrion Red blood cells Chloroplast Human Typical bacteria egg Virus Chicken Protein egg Amino Nucleus Atom acids Smallest Ribosomes bacteria Epithelial Frog egg Adult cell human Some nerve cells Electron microscope Light microscope Human eye Figure 4-1 p73 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Limits to Cell Size Cell Cell B A To maintain homeostasis and conduct metabolism efficiently cells need to transport molecules across plasma membrane. Assuming that the rate of transport (the number of molecules that pass through a certain area of the membrane per unit of time) is same for the cell A and cell B, which cell will be filled with the molecules faster? Which two factors will affect your answer? Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. Geometric relationships between surface area and volume To maintain homeostasis and conduct metabolism efficiently, a cell must have: Large surface area Small volume This means that the higher the ratio of surface area to volume, the more efficient is cell metabolism. Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved. The Limitations on Cell Size Can be Overcome by Cell’s Shape or by Compartmentalization Amoeba Gut Folding of the cell’s epithelium membrane increases the surface area Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.  The Limitations on Cell Size Can be Overcome by Compartmentalization A eukaryotic cell has internal membranes that divide the cell into compartments—the organelles The cell’s compartments provide different local environments so that incompatible processes can occur in a single cell Copyright © 2021, 2017, 2014 Pearson Education, Inc. All Rights Reserved.