the cell biology 1 How we investigate cells

the cell biology 1

• • • How we investigate cells Cell types and size A tour of the cell The nucleus The endomembrane system Specialized organelles

How we investigate cells • Microscopy – Light microscope (Hooke 1665) • Magnification x 1000 • Limited by resolution of light energy (0. 2 µm) – Electron microscope (since 1950 s) • Uses beam of electrons to resolve 0. 2 nm) – Transmission electron microscope (TEM) examines internal structure – Scanning electron microscope (SEM) examines surface features (3 d) • Other techniques include cell fractionation (via centrifuge)

Cell types and sizes • First cells observed in 1665 by Hooke were cork cells • Further work by Leeuwenhoek, Schleiden and Schwann led to proposal of cell theory, which states – Organisms are composed of one or more cells – Cells are the smallest unit of life – Cells arise only by division of previously existing cells

• Cell types include prokaryotic and eukaryotic cells

The importance of surface area: volume ratio • Let amount of metabolism that occurs within a cell be a function of cell volume • Let rate of metabolism (which is a function of supply of reactants, and removal of products) be a function of surface area • Therefore, surface area: volume ratio (SA: Vol) dictates the efficiency of a cell – Small cells have high SA: Vol, and are therefore efficient – Large cells tend towards a low SA: Vol ratio, and are therefore inefficient

How to maximize SA: Vol 1 5 1 Original Cell: SA = 1 x 1 x 6 = 6 units 2 Vol = 1 x 1 x 1 = 1 unit 3 SA: Vol = 6: 1 Grow large? NO SA = 5 x 5 x 6 = 150 units 2 Vol = 5 x 5 x 5 = 125 unit 3 SA: Vol = 1. 2: 1 1 1 5 Divide? YES SA = 5 x 5 x 5 x 6 x 1 x 1 = 750 units 2 Vol = 5 x 5 x 5 = 125 unit 3 SA: Vol = 6: 1 5 5

SA: Vol – the limiting factor in cell size • Prokaryotes, because of their smaller size, have optimum SA: Vol • Single-celled eukaryotes which are larger cells compensate by an internal membrane system that partitions cells into compartments – Folded surface maximizes surface area for reaction – Membranes incorporate some enzymes that participate in reactions – Compartments provide localized environment for reactions • For larger biomasses, eukaryotic strategy is instead to divide = multicellular organisms, with cell specialization

A tour of the cell • All cells have at least three components – Nucleus or nucleoid region – Plasma membrane – Cytoplasm • In addition, eukaryotic cells have membrane bound organelles, and endomembrane system • Prokaryotic cells have an additional boundary the cell wall, primarily a peptidoglycan matrix (review Prokaryotes, pp. 84 -85) • Some eukaryotic cells (e. g. , plants) also have a cell wall, which instead is cellulose based

Eukaryotes: the Nucleus • Most, but not all eukaryotic cells have a nucleus • Nucleus contains a majority of the genetic information of the cell • Contains a mixture of DNA and proteins complexes known as histones—this mixture is known as chromatin • Enclosed by a nuclear envelope—a double bilipid layer membrane – Outside membrane is continuous with the cell’s endomembrane system – Inside membrane may be bound to a nuclear matrix • Nucleolus is the site of r. RNA production

The endomembrane system • Divides cell into compartments • Includes – endoplasmic reticulum (ER) • Rough ER (with ribosomes) • Smooth ER (without ribosomes) – Golgi apparatus • Golgi bodies – (Lysosomes, vacuoles) • Responsible for production of various macromolecules, including proteins and some lipids

The ER’s role in protein production • m. RNA, transcribed from DNA, enters the cell’s cytoplasm from the nucleus through a nuclear pore • m. RNA binds with a ribosome, and migrates to the surface of the ER • As polypeptide chain is assembled, it is passed into the lumen of the ER • Polypeptide is passed along ER and budded off into vesicle that travels to the cis face of a golgi body • Final assembly of molecule occurs in Golgi body, vesicle budded off of trans face • Transport vesicle containing final assembled protein travels to destination

Smooth ER • Synthesis of lipids, phospholipids and steroids • Participates in cellular metabolism • Detoxifies drugs and poisons (drug tolerance? ) • Stores Ca 2+ ions necessary for muscle contraction

is an extension of Nuclear envelope is confluent with Rough ER Smooth ER Membrane and secretory proteins produced in ER are transported in Vesicles Fuse with the forming cis face of Golgi Apparatus Pinches off maturing (trans) face Gives rise to Lysosomes And Vacuoles Vesicles Fuse with and add to plasma membrane and may release cellular products to outside Plasma Membrane

Specialized organelles • Mitochondria are sites of cellular respiration – Number of mitochondria correlates with metabolic activity of cell – Enclosed by a double membrane – Features internal folds termed cristae – Internal fluid is termed mitochondrial matrix – Evidence for Endosymbiotic theory? • Size • Have own DNA

• Plastids are specialized membrane bound organelles found in plants, including • Amyloplasts (store starch) • Chromoplasts (store pigments other than chlorophyll) • Choloroplasts (store chlorophyll, site of cellular photosynthesis) – – Enclosed by double membrane Contains stacks (grana) of thylakoids Fluid inside of chloroplast is termed stroma Evidence for Endosymbiotic theory? • Size • Have own DNA

Cell walls • Some cells have an additional, structural boundary outside of the plasma membrane • In plants, the cell wall is cellulose based • In bacteria, the cell wall is peptidoglycan based – Gram positive and negative bacteria • Structural rigidity of cell wall allows cells to take on more water than an animal cell could

The cytosol • Contains cytoplasm and cytoskeleton • Cytoskeleton = network of fibers throughout the cytoplasm that forms a dynamic framework for support and movement. Constructed from: – Microtubules (e. g. , cilia, flagella, centrioles) – Intermediate filaments (e. g. , the cellular scaffold – Microfilaments (actin: muscle contraction, localized contractions of portions of cell)

The plasma membrane • Just one example of a lipid bilayer membrane found in cells • Controls passage of molecules in and out of the cell • Like all membranes, represents a complex interactions of phospholipids, proteins and carbohydrates

Membrane theory • Davson-Danielli model suggested phospholipid bilayer sandwiched between two layers of globular protein – Suggests a symmetrical geometry - in fact not the case (inside and outside face) – Some membranes looked different, and had different functions – Since most proteins are hydrophobic, woul not be stable

The Fluid Mosaic Model • Proposed by Singer-Nicolson • Membrane is a mosaic of proteins ‘bobbing’ in a fluid phospholipid bilayer • Hydrophilic portions of phospholipid and proteins are maximally exposed to aqueous interface, ensuring stability • Most lipids and some proteins drift laterally across surface of membrane fluidity is aided by addition of kinked hydrocarbon tails

Molecules found in the phospholipid bilayer matrix • Integral proteins – Unilateral or transmembrane • Peripheral proteins • Carbohydrates (cell-cell recognition) – Oligosaccharides – Glycolipids – Glycoproteins

Permeability - the ultimate control of metabolism? • Membranes display selective permeability – Solubility characteristics of the phospholipid bilayer • • Nonpolar molecules dissolve in membrane Small polar molecules pass easily Large polar molecules do not pass easily Ions are usually pumped across – Presence of specific integral transport proteins • Hydrophilic tunnel through membrane? • May bind to substance and move it across

Types of transport • Passive transport systems – Diffusion - movement of molecules down a concentration gradient (high conc. to low conc. ). Relies on intrinsic kinetic energy of molecules – Osmosis (the diffusion of water) - the movement of water from a low concentration solute (hypertonic solution) to a high concentration solute (hypotonic solution) – Facilitated diffusion. Uses transport proteins • Active transport – Requires energy (ATP) to pump molecules against concentration gradient

Large molecule transport across a membrane • Exocytosis (out of) and endocytosis (into the cell) – Phagocytosis (engulfment of particles using pseudopodia) – Pinocytosis (engulfment of fluid using pseudopodia) – Receptor-mediated endocytosis

Cell Communication • How do cells talk to each other? There are four general methods (Raven and Johnson, Figure 7. 3: • Direct contact • Paracrine signalling (short-lived, local) • Endocrine signalling (hormones) • Synaptic signalling (neurotransmitters)

A general model of communication. . . • Signal production • Signal Transmission • Signal Reception How are signal received? Proteins play a key role - their 3 -dimensionality provides specificity to signal types – Intracellular – Cell surface

Intracellular receptors • Lipid soluble or small molecule signals pass directly through cell membrane to receptor • Receptors are almost always enzymes-addition of chemical signal either inhibits or activates enzyme activity by altering shape of protein • Effected enzymes may be in the cytoplasm or nucleus (DNA transcription) • For example, Nitric Oxide activates the enzyme Guanylyl cyclase, which catalyzes the production of cyclic guanosine monophosphate, causing relaxation of smooth muscles around vessels, increasing blood flow

Cell Surface receptors • Chemically gated ion channels • Enzymic receptors • G-protein linked receptors Raven and Johnson Figure 7. 6

Chemically gated ion channels • Multipass protein whose core provides passage to ions if gate binds with signal • Channels are specific to particular ions, depending on geometry • Influx of ions (e. g. , Ca 2+, K+, Na+) has variable effects according to cell type

Enzymic receptors • Binding of signal to exterior of transmembrane protein reconfigure enzyme to be active on inside of cell • In most cases the enzyme is a kinase, causing phosphorylation

G-Protein linked receptors • Guanosine triphosphate (GTP)-binding protein • Trans-membrane multipass protein (7 times) creates channel through membrane • Addition of signal causes protein to bind GTP, activating it. GTP-protein complex then diffuses out to target

How does the receptor effect a change in the cell? • Typically, the resultant change will effect secondary signal - commonly Ca 2+ or c. AMP (cyclic adenosine monophosphate) • An amplification system causes the signal to proliferate throughout the cell

How to cells stick together • Tight junctions • Anchoring junctions (cadherin-mediated and integrin mediated links) – Hemidesmosome – Desmosome • Communicating junctions – Gap junction – Plasmodesmata