Lecture Notes: What are cells?

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University of Waikato
Dr Ryan Martinus

Lecture Notes from Dr. Ryan Martinus on the 10/7/13. What are cells and how do we study them? Learning Objectives:  Know the defining features of a cell  Know the difference between prokaryotic and eukaryotic cells and plant and animal cells  Know how we study the structure and function of cells LEVELS Organism – organ system – organ – tissue – cellular – chemical/molecular Biologists classify cells into three domains: Bacteria (Eubacteria), Archaea (Archaebacteria) and Eucarya (Eucaryotes). Qs to consider: How did this complexity arise? How did cells evolve? Prokaryotes = “before” nucleus Eukaryotes= “true” nucleus Viruses Prokaryotes Eukaryotes Size ~50-500nm ~0.3-5um >5um What are the characteristic features of cells?  Highly organized  Homeostasis (constant internal environment)  Reproduce, grow and develop  Take energy from the environment  Respond to stimuli  Adapt to the environment What do cells look like?  There is no “typical” cell.  They vary in size and shape. What features distinguish prokaryotic and eukaryotic cells? Prokaryotic Cells Eukaryotic Cells  Small cells (<5um)  Larger cells (>10um)  Always unicellular  Often multicellular  No nucleus or any membrane-  Always have nucleus and other bound organelles membrane-bound organelles  Circular DNA without proteins  Linear DNA, associated with  Ribosomes are small proteins to form chromatin  No cytoskeleton  Ribosomes are large  Motility by rigid rotating  Always has a cytoskeleton flagellum  Motility by flexible waving cilia  Cell division by binary fission or flagellae  Asexual reproduction  Cell division by mitosis/meiosis  Huge variety of metabolic  Reproduction either asexual or pathways sexual  Common metabolic pathways Q. How do we study cells? Microscopes provide a window into the world of cells The first light microscopes were used in the 17 century. Basic Terminology used in microscopy:  Magnification is the ratio of an objects image to its real size  Resolution is the ability to discriminate two points close together Best resolution with a light microscope is about 2.0um (small bacterium). Maximum magnification achievable with a light microscope is about 1,000 times the size of the actual specimen. Cells can be viewed stained – dyes enhance contrast and preserve cells – or unstained – light passes directly through specimen and the image has little contrast. The confocal microscope  Confocal microscopy yields more detail especially with thick preparations. While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles. To resolve smaller structures we use an electron microscope (EM), which focuses a bean of electrons through the specimen or onto its surface. The resolution of a modern EM is ~2nm. Transmission electron microscopy (TEM)  Uses electromagnets to focus electron beam through thin section of the specimen  Image is focused onto a screen  Contrast of the image is enhanced by staining sections with heavy metals (they attach to cellular structures).  These are used to study internal structures of cells  Resolves cellular details as small as 1-2nm Scanning electron microscopy (SEM)  Electron beams
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