Cell+Structure+and+Function+I


 * 21 August 2006**
 * Cell Structure and Function I**
 * Dr. Joana Chakraborty, Ph.D.**

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=**The Cell**=

**Definition**

 * Basic functional unit
 * has [|plasma membrane], fluid suspension ([|cytosol]), nucleus, organelles (mitochondria, lysosomes, golgi apparatus, endoplasmic reticulum), inclusions and cytoskeleton.
 * Cell contains 2 compartments
 * Nucleus and cytoplasm compartments
 * Cytoplasm compartment contains organelles and cytosol
 * Cytosol – made up of micromolecules that perform different cellular functons.

**Cellular functions**

 * Summary of cell function: motility, contractility, conductivity, absorption, synthesis/assimulation, respiration, secretion, excretion, growth, reproduction, death.
 * Movement – phagocytes (contains lysosomes and bacteria) and muscle cell
 * Contraction – Muscle cell
 * Conductivity – Nerve cell
 * Synthesis/Secretion of enzymes – pancreatic acinar cells (exocrine)
 * Synthesis/Secretion of mucous – mucous-gland cells
 * Steroids – adrenal, testes and hormones (secretory cells have lots of smooth ER)
 * Ion transport – kidney and salivary gland ducts
 * Intracellular digestion – macrophages, WBC
 * Tranform stimuli to nerve impulse – sensory cell
 * Metabolite absorption – intestinal and kidney cells

**Dimensions**

 * 1 mm = 1,000 μm
 * 1 μm = 1,000 nm
 * 100 μm – visible via human eye.
 * 0.5-0.22 μm – visible via light microscope, need to cut sections
 * 1 nm – visible via electron microscope, requires very thin sections
 * Approximate sizes:
 * Cells – μm to cm
 * Organelles – nm to μm
 * Macromolecules – 50-500 nm
 * Small molecules (amino acids) – 1nm
 * Atoms – Å (angstrom)

**Location of Organelles**

 * Centrioles are usually closer to nucleus, however, in motile cells, the centrioles can be found near the edge.
 * Microenvironment of cell is compartmentalized

=**Selected Techniques**=


 * Resolving power – smallest distance between 2 particles at which they can be seen as separate particles

**Light microscopy**

 * max resolving power = 0.22 μm
 * Lens – Objective lens (4x, 10x, 40x), and Ocular lense (10x) condenser lens
 * Magnification: 40-1500x
 * Tissue Prep:
 * Tissue fixed, placed in parafilm blocks, and sectioned before staining
 * Longitudinal sections
 * Transverse (cross) sections
 * Oblique sections
 * Grazing sections
 * Tangential sections

**Electron microscopy**

 * max resolution power = 1 nm (0.1 nm, theoretical)
 * 1000-100,000x magnification
 * Tissue Prep:
 * Thin sectioning (~100 nm)
 * Stain w/ heavy metal
 * Uses electromagnetic lenses
 * Like an upside-down light microscope

**Differential Centrifugation**

 * Different fractions from each centrifugation precipitate different types of cellular organelles for study
 * Homogenize tissue and centrifuge precipitate
 * Take supernatant and centrifuge, and repeat
 * Ribosomes are too small to see on light microscope -- **must use electron microscope**

**Cell culture**

 * for study of animal cells growth, differentiation, gene structure and functions
 * Primary culture – homogenize tissue and plate culture
 * Passage of primary culture to produce secondary cultures
 * Stem cells – idea is to control the direction of stem cell development.

=**Plasma Membrane**=


 * Boundary of cell membrane
 * Components are in fluid condition:
 * Phospholipids:protein ratio can vary depending on the type of cell
 * Carbohydrates
 * Lipids and many proteins are laterally mobile
 * Organized into a lipid-bilayer with imbedded proteins
 * 5 nm in thickness
 * Organelles in eurkaryotic cells are membrane-bound – internal membrane surface area >>> plasma membrane
 * **Nuclear membrane (nuclear envelope) and mitochondria has a double membrane**
 * **Other organelles have a single membrane**

**Proteins in plasma membrane**

 * Trans-membrane proteins (integral proteins) can extend from cytosolic and ectoplasmic faces
 * Peripheral proteins (do not cross membrane)
 * Used for receptors, recognition and binding units, pores and channels, and enzymes
 * Freeze fracture technique can separate membrane bilayer
 * Two sides of membranes are called inner (E-face) and outer (P-face) leaflets

**Function of Plasma Membrane**

 * Structural integrity
 * Interface cytoplasm and external environment
 * Control movement of substances – **control internal environment**
 * Cell-cell interactions (tight junction, etc.)
 * Recognition and cellular identity (via carbohydrates and glycolipids)

**Endocytosis**

 * Two types:
 * [|Phagocytosis] – cell eating via evagination
 * Occurs in macrophage and neutrophil
 * When microbe in extracellular fluid contacts macrophage, macrophage will extend plasma membrane to engulf microbe.
 * Plasma membrane pinches off to form vesticle and combines with lysosome to become phagolysome
 * Microbe becomes digested and end-products either taken up by the cell or excreted
 * [|Pinocytosis] – cell drinking via invagination
 * Plasma membrane forms a depression to intake macromolecules
 * Fluid phase endocytosis – plasma membrane invaginates and takes in macromolecules
 * Receptor-mediated endocytosis – specific receptors on plasma membrane binds to specific macromolecules and internalize macromolecules (example: LDL binding). Very specific macromolecular uptake
 * Pinched off plasma membrane called “endosome.”

=**Intracellular Organelles**=


 * [|Rough ER]
 * [|Smooth ER]
 * [|Golgi Apparatus]
 * [|Nuclear Envelope]
 * [|Mitochondria]
 * [|Peroxisome]
 * [|Lysosome]

**Endoplasmic Reticulum (ER)**

 * Largest membrane bound system in cell
 * Rough and smooth sides
 * Site of protein (rough ER) and lipid (smooth ER) synthesis

**Smooth ER**

 * No ribosomes attached (hence, smooth)
 * Active synthesis of [|steroids], [|cholesterol], and [|triglycerides]
 * Detoxification of drugs
 * Cells synthesizing steroid hormones have lots of smooth ER

**Rough ER**

 * Studded with ribosomes on outer surface
 * Site of protein synthesis
 * Protein secretory cells have more rough ER
 * Continuous with nuclear envelope

**Golgi Apparatus**

 * Protiens synthesized and packaged in rough ER are transported to Golgi for modification and final packaging for secretion
 * Flattened, slightly curved membrane-bound cisternae
 * Cis-face (close to rough ER; convex in shape, entry phase)
 * ERGIC – Endoplasmic reticulum golgi interface cisternae
 * Medial Face
 * Trans-face (concave, exit phase)
 * Secreted granules can be smooth or coated vesticles

**Mitochondria**

 * 0.5-1 μm long (sometimes 7 μm long)
 * Produces ATP
 * Smooth outer membrane and folded inner membrane (cistae)
 * Has inner membrane space and matrix space
 * Matrix contains mitochondrial DNA and enzymes
 * ATP synthase on inner surface of inner membrane
 * Contains granules in matrix, but function unknown
 * [|Mitochondrial DNA]:
 * have own separate DNA
 * Circular DNA with multiple copies
 * Human mitochondrial genome is 16,569 base pairs
 * **Germline mutations in mitochondrial DNA are transmitted by the mother**
 * Recent research suggests that mitochondrial DNA from father may be integrated somehow to the genome of children

**Peroxisomes**

 * 0.1 to 1 μm
 * Contains 40-50 oxidative enzymes
 * urate oxidase
 * **catalase – destroys hydrogen peroxide generated by peroxisome**
 * Catabolism of long chan fatty acids

**Lysosome**

 * 0.3-0.8 μm diameter
 * Contains 40 types of enzymes
 * acid hydrolases such as sulfatases, proteases, nucleases, etc.
 * Actively transports H+ ions to lysosome
 * Lumen is acidic (pH = 0.5) for enzymes to function
 * Rough ER produces lysosomal enzymes
 * Golgi produces vesticles containing lysosomal enzymes - primary lysosome
 * Endocytosed vesticle combines with primary lysosome, producing secondary lysosome
 * After phagocytosed material is digested, secondary lysosome becomes a residual body and contents are either taken up by the cell or excreted to the external environment
 * Lysosomes can also digest old cellular components such as secretory granules from the Golgi

**Transport of Macromolecules**

 * Serveral methods of macromolecule transport
 * **Gated Transport**
 * Cytosol to nucleus
 * **Transmembrane Transport**
 * Cytosol to mitochondria, ER, membrane, and peroxisomes
 * **Vesicular Transport**
 * ER to Golgi
 * Golgi to lysosome, endosome, secretory vesticles
 * Secretory vesticles to cell surface
 * Cell surface to endosome

**Cytoplasmic Incusions**

 * [|Glycogen]
 * Store glucose
 * Mostly in liver and muscle cells
 * Varies from 10-40 nm
 * Lipid
 * [|Triacylglycerols] – stored fatty acids in membrane-bound lipid droplets
 * Pigments
 * possess some color
 * Hemoglobin, melanin, lipfuscin
 * Crystals
 * Crystal of Charcot Bottcher in Sertoli cells of testes

=**Abnormal Functions**=


 * Abnormalities:
 * Changes in morphology
 * Defects in storage of macromolecules
 * Vacuolization - cell produces many vacuoles as a result of recent cell injury
 * Cell death characterized by:
 * typical nuclear changes:
 * Condensed Nucleus
 * Fractured Nucleus
 * Nuclear materal coated on nuclear membrane
 * cell membrane rupture
 * [|Apoptosis]
 * Apoptosis is required for cell function
 * Inability for cell to correctly undergo apoptosis at a certain time may be abnormality.

=**Objectives**=