Lecture 9 Notes PDF - Cell Biology & Cell Structure

Introduction to Cell Biology & Cell structure [email protected] PHC62104 Principles of Life Sciences  At the end of this lesson you should be able to: name and describe the three main parts of a cel...

Introduction to Cell Biology & Cell structure [email protected] PHC62104 Principles of Life Sciences  At the end of this lesson you should be able to: name and describe the three main parts of a cell. Learning describe the concept of membrane outcomes fluidity. discuss the processes that transport substances across the plasma membrane.  Cellular Foundations Cells are the basic building blocks of life Smallest living unit of an organism A cell may be an entire organism (unicellular) or it may be one of billions of cells that make up the organism (multicellular). Grow, reproduce, use energy, adapt, respond to their environment Many cannot be seen with the naked eye a typical cell size is 10µm; a typical cell mass is 1 nanogram.) Cellular dimensions are limited by oxygen diffusion. n Animal and plant cells – 5-100 µm in diameter n Bacteria – 1-2 µm long What limits the dimension of a cell? n Lower limit – minimum number of each type of biomolecules required by the cell. n Upper limit – rate of diffusion of solute molecules in aqueous systems (ex: oxygen). Eukaryotic cells have a variety of membranous organelles, which can be isolated for study.  PARTS OF A CELL Most cells have many of the structures but can be divided into three main parts: plasma membrane, cytoplasm, and nucleus. Parts Description Plasma membrane forms the cell’s flexible outer surface, separating the cell’s internal from the external environment. selective barrier that regulates the flow of materials into and out of a cell to maintain the appropriate environment for normal cellular activities. The plasma membrane also plays a key role in communication among cells and between cells and their external environment. Cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus. has two components: cytosol and organelles. Cytosol contains water, dissolved solutes, and suspended particles. Within the cytosol are several different types of organelles. Each type of organelle has a characteristic shape and specific functions. Nucleus large organelle- contain cell’s DNA. Within the nucleus, each chromosome, a single molecule of DNA associated with several proteins, contains thousands of hereditary units called genes that control most aspects of cellular structure and function. Cytoplasm  The cytoplasm is organized by the cytoskeleton and is highly dynamic. The three types of cytoskeletal filaments Plasma Membrane  The plasma membrane, a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell, is best described by using a structural model called the fluid mosaic model. According to this model, the molecular arrangement of the plasma membrane resembles a continually moving sea of fluid lipids that Structure and contains a mosaic of many different proteins. Dynamics The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back layers made up of three types of lipid molecules— phospholipids, cholesterol, and glycolipids Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer but not absolute – can move  The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains Both proteins and lipids are free to move laterally in the plane of the bilayer, but movement of either from one face of the bilayer to the other is restricted The carbohydrate moieties attached to some proteins and lipids of the plasma membrane are exposed on the extracellular surface of the membrane The Lipid Bilayer The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back layers made up of three types of lipid molecules—phospholipids, cholesterol, and glycolipids The bilayer arrangement occurs because the lipids are amphipathic molecules, which means that they have both polar and nonpolar parts. In phospholipids, the polar part is the phosphate-containing “head,” (hydrophilic), the nonpolar parts are the two long fatty acid “tails” (hydrophobic). The heads face a watery fluid on either side—cytosol on the inside and extracellular fluid on the outside. The hydrophobic fatty acid tails in each half of the bilayer point toward one another, forming a nonpolar, hydrophobic region in the membrane’s interior. Cholesterol molecules are weakly amphipathic and are interspersed among the other lipids in both layers of the membrane. Glycolipids appear only in the membrane layer that faces the extracellular fluid, which is one reason the two sides of the bilayer are asymmetric, or different. Membrane Proteins Proteins are 20-80% of cell membrane Rest is lipid or carbohydrate; supramolecular assembly of lipid, protein and carbohydrate Proteins are also distributed asymmetrically TWO classes of Membrane Proteins: o Integral Membrane Proteins- firmly associated with the membrane o Peripheral Membrane Proteins - Interact weakly with the membrane lipid head groups or integral membrane proteins  Located WITHIN the lipid bilayer Usually span the bilayer one or more times – called transmembrane (TM) proteins Hydrophobic amino acids interact with fatty acid chains in the hydrophobic core of the membrane INTEGRAL Many integral proteins are glycoproteins, proteins with carbohydrate groups attached to the ends that MEMBRANE protrude into the extracellular fluid PROTEINS Can be removed from the membrane with detergents like SDS – need to disrupt the hydrophobic interactions The carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat called the glycocalyx which varies from one cell to another.  Types of integral proteins Types I and II have only one transmembrane helix; the amino-terminal domain is outside the cell in type I proteins and inside in type II. Type III proteins have multiple transmembrane helices in a single polypeptide Type IV proteins, transmembrane domains of several different polypeptides assemble to form a channel through the membrane. Type V proteins are held to the bilayer primarily by covalently linked lipids Type VI proteins have both transmembrane helices and lipid anchors. 17  Interact weakly with the membrane lipid head groups or integral membrane proteins Found associated with the inner or outer leaflet or integral membrane proteins protruding from PERIPHERAL the inner or outer leaflet MEMBRANE Interactions are mainly hydrogen bonds or electrostatic interactions PROTEINS Removed from the membrane with MILD agents to distrupt electrostatic interactions Salt – raise the salt concentration pH – raise the Ph Functions of Membrane Proteins  Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase,→ movement constraint Membrane Fluidity At high temperatures, fluid state → hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon–carbon bonds of the long acyl side chains At intermediate temperatures, the lipids exist in a liquid-ordered state; there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement still takes place.  Fluidity of biological membranes Sat’d FA Unsat’d FA Cholesterol Temperature Decide the membrane fluidity. 21  Bulky rigid molecule Moderates fluidity of membranes – both EFFECT OF increases and decreases CHOLESTEROL Cholesterol in membranes DECREASES ON fluidity because it is rigid MEMBRANES: Prevents crystallization (making solid) of fatty acyl side chains by fitting between them. Disrupts close packing of fatty acyl chains. Therefore, INCREASED fluidity 22  Cells need materials from surroundings for energy and biosynthesis TRANSPORT Cells need to get rid of wastes and toxins ACROSS THE PLASMA Most transport occurs through proteins (pumps and MEMBRANE channels) at the membrane Substances generally move across cellular membranes via transport processes that can be classified as passive or active, depending on whether they require cellular energy. 23  Diffusion is a passive process in which the random mixing of particles in a solution occurs because of the particles’ kinetic energy. If a particular solute is present in high concentration in one area of a solution and in low concentration in another area, solute molecules will diffuse toward the area of lower concentration—they move down their concentration Passive gradient. Processes Simple diffusion: substances move freely through the lipid bilayer of the plasma membranes of cells without the help of membrane transport proteins Facilitated diffusion is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembrane integral proteins (channels or carriers).  Carrier proteins – move the solute across the membrane by binding it on one side and transporting it to the other side 2 Major Requires a conformation change bind their substrates with high stereospecificity Classes of Channel protein – small hydrophilic Integral pores that allow for solutes to pass through Proteins Use diffusion to move across Also called ion channels when only ions moving show less stereospecificity than carriers and are usually not saturable Simple diffusion, channel-mediated facilitated diffusion, and carrier-mediated facilitated diffusion Channel Vs Carrier-mediated facilitated diffusion Channel-mediated facilitated Carrier-mediated facilitated diffusion of potassium ions (K+) diffusion of glucose across a plasma through a gated K+ channel. A gated membrane. The carrier protein binds channel is one in which a portion of the to glucose in the extracellular fluid and channel protein acts as a gate to open releases it into the cytosol. or close the channel’s pore to the passage of ions.  Osmosis is a type of diffusion in which there is net movement of a solvent through a selectively permeable membrane. In osmosis, water moves through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. Osmosis During osmosis, water molecules pass through a plasma membrane in two ways: (1) by moving between neighboring phospholipid molecules in the lipid bilayer via simple diffusion, as previously described, (2) by moving through aquaporins, integral membrane proteins that function as water channels.  In active transport energy is required for carrier proteins to move solutes across the membrane against a concentration gradient. Active Two sources of cellular energy can be used to drive active transport: transport 1.energy obtained from hydrolysis of adenosine triphosphate (ATP) (primary active transport); 2.energy stored in an ionic concentration gradient (secondary active transport). Na+-K+ Ion Pump (Primary active transport) Sodium-potassium pumps maintain a low intracellular concentration of sodium ions. 31 Secondary active transport mechanisms. (a) Antiporters carry two substances across the membrane in opposite directions. (b) Symporters carry two substances across the membrane in the same direction.  A variety of substances are transported in vesicles from one structure to another within cells. Vesicles also import materials from and release materials into extracellular fluid. During endocytosis, materials move into a Transport in cell in a vesicle formed from the plasma membrane. Vesicles In exocytosis, materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell. Both endocytosis and exocytosis require energy supplied by ATP. Thus, transport in vesicles is an active process. Nucleus  NUCLEUS Most cells have a single nucleus, although some, such as mature red blood cells, have none while skeletal muscle cells and a few other types of cells have multiple nuclei. Nuclear envelope (double membrane) separates the nucleus from the cytoplasm. Many openings called nuclear pores extend through the nuclear envelope control the movement of substances between the nucleus and the cytoplasm. Inside the nucleus are one or more spherical bodies called nucleoli that function in producing ribosomes. Each nucleolus is simply a cluster of protein, DNA, and RNA; it is not enclosed by a membrane. Within the nucleus are most of the cell’s hereditary units, called genes, which control cellular structure and direct cellular activities Thank you… 38