Cell structure

• Cells:

  • building blocks of life
  • smallest unit of life
  • made from division of other cells
  • site of all chemical reactions of life
• magnification - the act of expanding something in apparent size
• resolution - the ability to see two distinct points separately
• Light microscope:

  • magnification - x1500
  • resolution - 200nm
• Specimens:

  • sectioning:
  • material can distort when cut into thin sections
  • specimens are embedded in wax
  • thin sections can be cut without changing structure
  • staining:
  • chemicals bind to the specimen allowing it to be seen
• Sample preparation:

  • Dry mount: solid specimens viewed whole or cut into thin slices, cover slip placed on top (e.g. pollen)
  • wet mount: specimens suspended in liquid, cover slip placed on from an angle (e.g. aquatic samples)
  • squash slides: wet mount squashed down with a lens tissue (e.g. root tip squashes)
  • smear slides: edge of slide smears sample, creating an even coating (e.g. blood)
• Staining:

  • allows cell to be seen better
  • some are positively charged so only stick to negatively charged materials
  • negative staining stains background to create contrast
  • differential staining can be used to distinguish two organisms/organelles
• Resolution:

  • limited by diffraction of light passing through sample
  • light can overlap, meaning structures are no longer visible
• magnification = image size/actual size
• transmission electron microscope:

  • electron beam passes through a thin sample
  • electrons pass through denser parts less easily
  • image is 2D
  • magnification - x500,000
  • resolution - 0.5 nm
• scanning electron microscope:

  • electrons don't pass through specimen
  • they bounce off specimen
  • image is 3D
  • magnification - x100,000
  • resolution - 3-10nm
• Preparation for EM:

  • specimen fixed in glutaraldehyde
  • dehydrated to replace water with ethanol
  • embedded in solid resin
  • sliced with diamond knife
  • stained using lead salts
  • mounted on copper grid
  • placed in vacuum
• artefacts - structures that are the result of the long and complicated preparation procedure. they are not true representations of the specimen's original structure
• ultrastructure: details of the inside of cells, as revealed by an electron microscope
• cytosol - aqueous part of the cytoplasm in which organelles are suspended
• metabolism: all the reactions of life
• Cytoplasm:

  • many reactions take place within the cytoplasm
  • cell organelles are found in the cytoplasm
• cell surface membrane:

  • extremely thin - 10nm
  • consists of a phospholipid bilayer
  • has 3 layers
  • retains the fluid of the cytosol
  • forms a barrier across which everything must pass
  • can be used to identify the cell for other cells
• nucleus:

  • largest organelle in eukaryotic cell - 10-20 micrometers
  • surrounded by nuclear envelope
  • outer membrane is continuous with ER
  • nuclear envelope contains nuclear pores - 100nm - which allow rapid movement out of nucleus
  • DNA associates with histones to form chromatin
  • nucleus manages cell
  • nucleolus: a non-membrane bound structure composed of protein and nucleic acids
  • mRNA is transcribed in the nucleolus
• Mitochondria:

  • rod-shaped
  • 0.5-1.5 micrometers wide, 3-10 micrometers long
  • very active cells contain thousands
  • double membrane, inner folded into cristae
  • matrix is inner part and contains metabolites and enzymes
  • site of aerobic respiration and ATP synthesis
  • contains small ribosomes (70s), also found in prokaryotes
  • contains circular DNA and can produce their own enzymes and replicate themselves
• Lysosomes:

  • single membrane
  • contain concentrated enzymes (hydrolytic)
  • many found in macrophages
  • destroy damaged organelles
  • involved in apoptosis
  • autolysis: when organism dies, lysosomes digest cell
• Cytoskeleton:

  • present in cytoplasm of all eukaryotic cells
  • holds organelles in place and controls cell movement
  • consists of:
  • microfilaments - contractile fibres, responsible for cell movement and contraction of cell during cytokinesis
  • microtubules: tubes that provides a scaffold for the cell, provides tracks to move organelles, separate chromosomes in cell division
  • intermediate fibres - give mechanical strength to cells
• Centrioles:

  • a component of cytoplasm
  • nine paired microtubules in the 9+2 arrangement
  • found in most eukaryotic cells except flowering plants and fungi
  • two centrioles occur at right angles forming the centrosome; they form spindle fibres in mitosis
  • microtubules are made of a globular protein tubulin. they shape and support the cytoplasm and move cell components + organelles
• Cilia and flagella:

  • known as undulipodia
  • found on some animal cells and a few types of plant cells
  • they are thin cytoplasmic threads projecting from the surface of the cell wall containing microtubules
  • flagella are longer
  • fewer flagella per cell
  • involved in cell locomotion/movement of substances across the surface of the cell
• ER:

  • folded membranes formed into sheets, tubes or sacs
  • buds off from the outer membrane of the nuclear envelope but may remain attached to it
  • metabolically active cells have a lot of it
• RER:

  • responsible for protein synthesis and transport
  • has ribosomes attached to the outer surface
  • vesicles are pinched off at the end
  • vesicles are used to store and transport substances around the cell
  • the vesicles fuse with the Golgi apparatus and are often discharged from the cell
• SER:

  • no ribosomes:
  • produces lipids, carbohydrates, steroids, oestrogen and testosterone - it will also transport them
  • involuntary muscle fibres have special SER stores of calcium ions
• Ribosomes:

  • 25nm in diameter
  • built of 2 subunits
  • consist of protein and RNA
  • found in the cytoplasm/on ER
  • site of protein synthesis
  • 80s in eukaryotes, 70s in prokaryotes
• Golgi Apparatus:

  • flattened membrane sacs
  • one side is formed by the fusion of vesicles from the RER and SER
  • opposite side is formed when vesicles are pinched off
  • lots found in metabolically active cells
  • site of synthesis of specific biochemicals e.g. hormones and enzymes
  • specific proteins may be activated by adding sugars, or removing amino acids; these are packaged into vesicles
  • in animal cells, vesicles may form lysosomes, in plants the may contain polysaccharides for cell wall formation
• Cell wall:

  • made of cellulose
  • freely permeable
  • give the cell shape
  • can defend against pathogens
  • secreted by the cell they enclose
• Vacuoles:

  • tonoplast lined sacs
  • maintain turgor pressure
  • selectively permeable
  • in animal cells, small and transient
  • 4-10 micrometers long
  • found in mesophyll cells
  • has a double membrane - inner folded to form thylakoids
  • stack of thylakoids make grana, where chlorophyll and other pigments are located
  • stroma fluid contains circular DNA, 70s ribosomes and enzymes
• Plasmodesmata:

  • cytoplasmic links between cells - run through cell wall
  • formed when a cell divides
• prokaryotic cells:

  • evidence that eukaryotic cells evolved from them
  • can be classified into archaea bacteria and bacteria
• Features:

  • bacteria
  • 1-5 micrometers
  • only have a cell surface membrane, no membrane-bound organelles
  • cell wall made of peptidoglycan
  • divide by binary fission
  • 70s ribosomes
  • DNA found in circular chromosome, may also contain plasmids
  • DNA is found in nucleoid (no membrane) and supercoiled
  • genes grouped into operons
• Operon:

  • functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter
  • the genes are transcfibed into an mRNA strand
  • numbers of genes can be switched on/off at the same time
• Prokaryotic flagella:

  • no 9+2 arrangement
  • energy comes from chemiosis not ATP
  • attached by a basal body and rotated by a molecular motor
  • spins in a whip-like movement