the fundamental unit of life A TOUR OF THE CELL the fundamental unit of life

Microscopes - windows to the world of the cell The discovery and early study of cells progressed with the invention and improvement of microscopes in the 17th century. Robert Hooke Anton van Leeuwenhoek Microscopes are a major tool in cytology, the study of cell structures Cytology coupled with biochemistry, the study of molecules and chemical processes in metabolism, developed modern cell biology. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

From Monk’s Room to Unit of Life Robert Hooke thought what he was looking at resembled the rooms monk’s occupied….. Cells

The Instruments

Compound Light Microscope Uses visible light Has at least 2 sets of lenses Can achieve maximum 2000X magnification Resolution of objects as small as 0.2 m

Light Microscopy In a light microscope visible light passes through the specimen and then through glass lenses. The lenses refract light such that the image is magnified into the eye or a video screen.

Brightfield Illumination Usual operations Specimens must be stained for viewing Best magnification and resolution with the oil immersion objective Oil has same refractive index as glass

Light Microscopes Microscopes vary in magnification and resolving power. Magnification is the ratio of an object’s image to its real size. Resolving power is a measure of image clarity. It is the minimum distance two points can be separated and still viewed as two separate points. Resolution is limited by the shortest wavelength of the source, in this case light. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Resolution of Light Microscopes The minimum resolution of a light microscope is about 2 microns, the size of a small bacterium Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen. At higher magnifications, the image blurs. Fig. 7.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Electron Microscopy Beam of electrons has shorter  so gives better resolution than visible light Electromagnetic lenses rather than glass Done in a vacuum Can resolve to 0.5nm and magnify up to 100,000 times. Specimen must be dry….dead

Electron micrographs

Cell Fractionation The goal of cell fractionation is to separate the major organelles of the cells so that their individual functions can be studied.

The Procedure Fractionation begins with homogenization, gently disrupting the cell Then, the homogenate is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the supernatant. As the process is repeated at higher speeds and longer durations, smaller and smaller organelles can be collected in subsequent pellets ultracentrifuge can spin at up to 130,000 revolutions per minute and apply forces more than 1 million times gravity (1,000,000 g).

Why Look at the Parts rather than the Whole Cell fractionation prepares quantities of specific cell components. enables the functions of these organelles to be isolated, especially by the reactions or processes catalyzed by their proteins. For example, one cellular fraction is enriched in enzymes that function in cellular respiration. Electron microscopy reveals that this fraction is rich in the organelles called mitochondria. Cytology and biochemistry complement each other in connecting cellular structure and function.

Fluorescent stain of cell Cell Structure and Function Fluorescent stain of cell

Early Discoveries Mid 1600s - Robert Hooke observed and described cells in cork Late 1600s - Antony van Leeuwenhoek observed sperm, microorganisms 1820s - Robert Brown observed and named nucleus in plant cells

Cell Theory Schleiden and Schwann Virchow Every organism is composed of one or more cells Cell is smallest unit having properties of life Virchow All exisiting cells arise from pre-existing cells.

Cell Smallest unit of life Can survive on its own or has potential to do so Is highly organized for metabolism Senses and responds to environment Has potential to reproduce

Measuring

Cells Vary in Size

Why Are Cells So Small? Surface-to-volume ratio The bigger a cell is, the less surface area there is per unit volume Above a certain size, material cannot be moved in or out of cell fast enough

Size is Limited

Smaller objects have a greater ratio of surface area to volume. Metabolic requirements also set an upper limit to the size of a single cell. As a cell increases in size its volume increases faster than its surface area. Smaller objects have a greater ratio of surface area to volume. Fig. 7.5

Structure of Cells Two or Three types of cells Archeo - cell type There is much evidence that this is a third cell type Prokaryotic Eukaryotic All cells have: Plasma membrane Region where DNA is stored Cytoplasm ribosomes

Archeo-cell type and Prokaryotes No nucleus Nucleoid area where DNA resides No membrane bound organelles. 70s ribosomes Cell walls contain petidoglycan Prokaryotic Organisms Eubacteria Cyanobacteria Archeo-cell type Pseudomurein rather than peptidoglycan Organisms belong to the Archeobacter

A prokaryotic cell

E. coli

Eukaryotic Cells Have a nucleus and other organelles Eukaryotic organisms Protistans Fungi Plants Animals

Overview of a plant cell

Overview of an animal cell

The nucleus contains a eukaryotic cell’s genetic library contains most of the genes in a eukaryotic cell. Some genes are located in mitochondria and chloroplasts. The nucleus averages about 5 microns in diameter. The nucleus is separated from the cytoplasm by a double membrane. These are separated by 20-40 nm. Where the double membranes are fused, a pore allows large macromolecules and particles to pass through.

Nucleolus In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. ribosomal RNA (rRNA) is synthesized assembled with proteins from the cytoplasm to form ribosomal subunits. The subunits pass from the nuclear pores to the cytoplasm where they combine to form ribosomes.

The nucleus and its envelope

Functions of Nucleus Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells

The nucleus and its envelope

Cytomembrane System Group of related organelles in which lipids are assembled and new polypeptide chains are modified Products are sorted and shipped to various destinations Components of the cytomembrane system Endoplasmic reticulum Golgi apparatus vesicles

Endoplasmic Reticulum In animal cells, continuous with nuclear membrane Extends throughout cytoplasm Two regions - rough and smooth

Rough ER Arranged into flattened sacs Ribosomes on surface give it a rough appearance Some polypeptide chains enter rough ER and are modified Cells that specialize in secreting proteins have lots of rough ER

Smooth ER A series of interconnected tubules No ribosomes on surface Lipids assembled inside tubules Smooth ER of liver inactivates wastes, drugs Sarcoplasmic reticulum of muscle is a specialized form

Endoplasmic reticulum (ER)

Golgi Bodies Put finishing touches on proteins and lipids that arrive from ER Package finished material for shipment to final destinations Material arrives and leaves in vesicles

The Golgi apparatus

Vesicles Membranous sacs that move through the cytoplasm Lysosomes Peroxisomes

Lysosomes

Review: relationships among organelles of the endomembrane system

The mitochondrion, site of cellular respiration

Specialized Plant Organelles Plastids Central Vacuole

The chloroplast, site of photosynthesis

Other Plastids Chromoplasts Amyloplasts No chlorophyll Abundance of carotenoids Color fruits and flowers red-to-yellow Amyloplasts No pigments Store starch

The plant cell vacuole 

Organelles with no Membranes Ribosomes Function in protein synthesis Cytoskeleton Function in maintenance of cell shape and positioning of organelles Centrioles (animals only) Function during cell division

Figure 7.10 Ribosomes

Ribosomes build a cell’s proteins Ribosomes contain rRNA and protein. A ribosome is composed of two subunits that combine to carry out protein synthesis. Fig. 7.10 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cytoskeleton Present in all eukaryotic cells Basis for cell shape and internal organization Allows organelle movement within cells and, in some cases, cell motility

Cytoskeletal Elements intermediate filament microtubule microfilament

Microtubules Largest elements Composed of the protein tubulin Arise from microtubule organizing centers (MTOCs) Polar and dynamic Involved in shape, motility, cell division

Microfilaments Thinnest cytoskeletal elements Composed of the protein actin Polar and dynamic Take part in movement, formation and maintenance of cell shape

Intermediate Filaments Present only in animal cells of certain tissues Most stable cytoskeletal elements Six known groups Different cell types usually have 1-2 different kinds

Cell-to-Cell Junctions Plants Plasmodesmata Animals Tight junctions Adhering junctions Gap junctions plasmodesma

Animal Cell Junctions tight junctions gap junction adhering junction