cells and microscopy

light microscope

  • an instrument used to magnify an object
  • first developed in the 16th to 17th century
  • allowed scientists to develop the cell theory
  • inexpensive to buy and operate
  • sample preparation is simple and does not usually lead to distortion
  • a vacuum is not required
  • the natural colour of the sample can be seen
  • up to 2000x magnification 
  • a resolving power of 200nm
  • specimens can be living or dead
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cell theory

  • both plant and animal tissues are comprised of cells 
  • cells are the basic unit of all life
  • cells only develop from existing cells
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how a light microscope works

  • had two lenses
  • the objective lens is placed near the specimen
  • the objective lens produces a magnified image
  • the eyepiece lens then magnifies that image
  • illumination is provided by light underneath the sample
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magnification

  • how many times larger the image is than the actual size of the object being viewed
  • simply magnifying an object does not increase the amount of detail that can be seen
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resolution

  • ability to see individual objects as separate entities
  • limited by the diffraction of light as it passes through the sample and lenses
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calculation for magnification

magnification = size of image / actual size of object

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electron microscopy

  • a beam of electrons (with a wavelength less than 1nm) is used to illuminate a specimen
  • produces images up to x500,000
  • very expensive
  • hard to use
  • specimens can be damaged
  • complex sample preparation
  • a vacuum is required
  • artifacts are common
  • black and white images are produced
  • specimens must be dead
  • a resolving power from 0.5 - 10nm, depending on type
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transmission electron microscope

  • an electron beam is transmitted through the specimen
  • the best resolving power is 0.5nm
  • similar to light microscopy
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scanning electron microscope

  • a beam of electrons is sent across the surface of the specimen and reflected electrons are collected
  • resolving power of 3 - 5nm
  • 3D images of surfaces of specimens
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creation of artifacts

  • artifact - visible structural detail caused by the processing of the sample that is not a feature of the specimen
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laser scanning confocal microscopy

  • moves a single spot of focused light across a specimen
  • causes fluorescence from components labelled with a dye
  • the emitted light is filtered through a pinhole
  • only light radiated from very close to the focal plane (distance giving the sharpest focus) is emitted
  • spot illuminating the specimen is moved to produce 2D images
  • 3D images can also be produced by using different focal planes to produce images
  • non-invasive
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eukaryotic cell structure

  • can make up multicellular organisms
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nucleus

  • contains coded genetic information in DNA form
  • DNA directs the synthesis of all proteins required by the cell 
  • often the biggest single organelle
  • contained within a double membrane (nuclear envelope)
  • nuclear pores in the nuclear envelope allow molecules to move in and out of the nucleus
  • DNA is associated with histones to form chromatin
  • chromatin coils and condenses to form chromosomes
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nucleolus

  • an area within the nucleus 
  • comprised of proteins and RNA
  • produces ribosomes
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mitochondria

  • the site of the final stages of respiration
  • the number of mitochondria usually indicates the amount of energy the cell uses
  • more mitochondria = more energy
  • have a double membrane structure
  • the inner membrane is tightly folded and called cristae
  • the fluid interior is called the matrix
  • they contain a small amount of their own DNA
  • the can produce their own enzymes
  • the can reproduce themselves
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vesicles and lysosomes

  • membranes are membranous sacs
  • they have storage and transport roles
  • they have a single membrane with fluid inside
  • lysosomes contain hydrolytic enzymes
  • they break down waste material in cells
  • they break down pathogens ingested by phagocytes
  • they have a role in apoptosis (cell death)
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cytoskeleton

  • is present throughout the cytoplasm
  • they have a network of fibres
  • they have microfilaments
  • they have contractible fibres
  • they are responsible for cell movement
  • they are also responsible for cell contraction during cytokinesis
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microtubules

  • a scaffold-like structure
  • they determine the shape of the cell
  • control the movement of organelles around the cell
  • as spindle fibres, they move chromosomes during cell division
  • they are intermediate fibres
  • they give mechanical strength to cells
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centrioles

  • they are a component of the cytoskeleton
  • they are comprised of microtubules
  • two associated centrioles make a centrosome
  • they are responsible for assembly and organisation of the spindle fibres during cell division
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flagella

  • they have a whip-like structure
  • they enable cell mobility
  • are used as a sensory organelle to detect chemical changes
  • longer than cilia
  • less abundant than cilia
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cilia

  • they are hair-like projections
  • they are present in greater numbers than flagella 
  • can be mobile or stationary
  • mobile cilia beat in a rhythmic manner to enable movement of substances near to the cells
  • each cilium has 2 central microtubules surrounded by 9 pairs or microtubules arranged as a wheel 
  • this is the 9 + 2 arrangement
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endoplasmic reticulum

  • a network of membranes enclosing flat sacs 
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smooth endoplasmic reticulum

  • lipid and carbohydrate synthesis and storage
  • no ribosomes
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rough endoplasmic reticulum

  • has ribosomes bound to the surface 
  • responsible for the synthesis and transport of proteins
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ribosomes

  • can be free-floating or attached to ER
  • non-membrane bound
  • constructed of RNA
  • made in the nucleolus
  • site of protein synthesis
  • mitochondria and chloroplasts contain ribosomes
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Golgi apparatus

  • a compact structure
  • formed of cisternae
  • modifies and packages proteins into vesicles
  • may package into secretory vesicles or lysosomes
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protein production

  • proteins are synthesised on ribosomes bound to rough ER
  • proteins pass into cisternae and are packaged into transport vesicles
  • vesicles move towards Golgi apparatus via the cytoskeleton
  • vesicles fuse with the cis face of the Golgi apparatus and the proteins enter
  • proteins are structurally modified
  • proteins leave the Golgi apparatus in vesicles from the trans face
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cellulose cell wall (plants)

  • freely permeable
  • gives the cell its shape
  • the contents of the cell push against it making it rigid
  • supports individual cells and plants themselves
  • a defense mechanism against invading pathogens
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vacuoles (plants)

  • a membrane-lined sac
  • contains cell sap
  • many plant cells have large permanent vacuoles
  • the membrane of the vacuole is the tonoplast
  • it is selectively permeable
  • helps maintain turgor
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chloroplasts (plants)

  • the organelle responsible for photosynthesis
  • found in the green parts of the plant
  • the fluid interior is stroma
  • an internal network of membranes for sacs called thylakoids
  • several thylakoids are called a granum
  • grana are joined by membranes called lamellae
  • grana contain chlorophyll for photosynthesis
  • contain DNA and ribosomes
  • starch produced in photosynthesis is present as starch grains
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prokaryotic cells

  • first appeared 3.5 billion years ago
  • always unicellular 
  • DNA is not contained within a nucleus
  • organelles are not membrane-bound 
  • reproduce by binary fission
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prokaryote DNA

  • single chromosome
  • supercoiled to be compact
  • genes are often grouped into operons so a number can be switched on or off at the same time
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ribosomes

  • smaller (70S)
  • size is determined by the rate they settle in a solution
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cell wall

  • made of peptidoglycan
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flagella

  • thinner than in eukaryotes
  • does not have a 9 + 2 structure
  • energy to rotate comes from chemiosmosis, not ATP
  • attached to the cell by the basal body and rotated by molecular motor
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