Cells

• Units of measurement


  • millimetre (mm)
  • micrometre (μm)
  • nanometre (nm)
• millimetre (mm)


  10^-3 metres
• micrometre (μm)


  10^-6 metres
• nanometre (nm)


  10^-9 metres
• There are 1000 μm in every millimetre; therefore one million um in each metre
• Some very small cell structures are measured in nanometres (nm)
• There are 1000 nm in each um
• Electron microscope (EM)


  • Uses electrons rather than light to produce images
  • Key term is resolution, not magnification
  • Can resolve finer detail than light microscopes
• Electron microscope operation


  1. Electron source

  2. Condenser electromagnet

  3. Specimen on grid

  4. Objective electromagnet lens

  5. Projector electromagnet lens
• Magnification can be increased but at the cost of clarity and no further detail can be gained
• Resolution is the ability to see two adjacent (but separate) points as distinct entities following magnification
• Electron microscopes can resolve finer detail than light microscopes
• Electron microscope

  Can achieve magnifications in excess of 1 million, compared to light microscope which can only magnify meaningfully up to a few thousand
• Electrons have a shorter wavelength than light

  This is why electron microscopes can achieve higher magnifications
• Electron microscope


  • Can only be used on dead material (there must be a vacuum inside)
  • Artefacts (distortion due to preparation techniques) are often a feature of electron micrograph images
  • Electron micrograph images require a lot of preparation work
• Light microscope

  • Can be used to view living specimens and biological processes
  • Less preparation work required
• Transmission electron microscope (TEM)


  Involves electrons passing through a very thin specimen, produces images with very high resolution and can be used for very high magnifications
• Scanning electron microscope (SEM)


  Involves electrons reflecting off the surface of the image, resolution and magnification are not as high but useful for giving 3-D images of surface features
• The typical eukaryotic cell consists of a cell surface membrane, cytoplasm, nucleus, and a host of membrane systems and organelles
• Nucleus

  Largest and most obvious organelle, contains DNA in chromosomes, chromatin is more or less densely packed, contains nucleolus which makes ribosomes
• Nuclear envelope

  Double membrane with nuclear pores, outer membrane is site of origin of rough endoplasmic reticulum, allows transport of mRNA from nucleus to cytoplasm
• Endoplasmic reticulum (ER)


  Membrane system that extends throughout cytoplasm, some has ribosomes attached (rough ER) which provides scaffolding for protein synthesis, other parts don't have ribosomes (smooth ER) which has roles in lipid synthesis, detoxification, and carbohydrate metabolism
• Ribosomes


  Small organelles made of protein and ribosomal RNA, found free in cytoplasm or attached to ER, sites of protein synthesis
• Golgi apparatus

  Series of curved flattened sacs (cisternae), receives vesicles from ER containing newly synthesised proteins, modifies proteins, packages and sorts them for transport or export
• Lysosomes

  Tiny vesicles containing hydrolytic enzymes, fuse with other vesicles to digest their contents, important in phagocytes for digesting engulfed bacteria
• Mitochondria

  Relatively large organelles with a double membrane, inner membrane is folded into cristae to increase surface area, site of cellular respiration
• Protein-containing vesicles budding off from RER and moving to Golgi apparatus


  1. Budding off

  2. Moving to Golgi apparatus
• Lysosomes


  • Budded off from concave (maturing) face of Golgi
  • Contain concentrated supply of digestive enzymes
• Lysosomes fusing with vesicles


  Fusing with vesicles (e.g. phagosomes containing objects that need digesting such as worn out organelles or engulfed microbes in phagocytes)
• Mitochondria

  • Present in almost all types of animal cell
  • Relatively large organelles (up to 10 um in length)
  • Typically 'bean shaped' but can be very variable in shape
  • Enclosed within a double membrane, separated by an inter-membrane space
  • Inner membrane is folded to form cristae that extend into the matrix
  • Infolding gives the inner mitochondrial membrane a greater surface area, therefore increasing the number of enzymes that can be embedded within the membrane
• Mitochondrion

  The 'powerhouse' of the cell, the site of ATP synthesis during aerobic respiration
• Mitochondria are particularly common in cells that have high energy requirements, such as muscle cells
• Many of the enzymes involved in ATP synthesis are located within the inner mitochondrial membrane, so the cristae tend to be more numerous and more deeply infolded in highly active cells
• Microtubules

  • Hollow cylinders formed from proteins
  • Part of the cytoskeleton, the network of fibres that maintains cell shape and keeps organelles anchored in place
  • Provide a framework aiding the movement of substances within the cell
  • Spindle fibres, important in the movement of chromosomes during mitosis and meiosis, are formed of microtubules
  • Centrioles, involved in the assembly of spindle fibres during cell division, are formed of microtubules and are also important constituents of cilia and flagella
• Cytoplasm


  The general term that describes the part of the cell between the cell surface membrane and the nucleus, including all the organelles outside the nucleus
• Cytosol


  The part of the cell within the cell surface membrane but outside the nucleus and other membrane-bound organelles such as mitochondria
• Plant cell wall


  • Surrounds all plant cells, immediately outside the cell surface membrane
  • Around 1 μm thick
  • Main component is the polysaccharide cellulose
  • Cellulose is laid down as microfibrils, each microfibril containing many cellulose molecules cross-linked to each other
  • Primary cell wall made up of many microfibrils orientated in different and random directions, allowing the cell wall to expand as the cell grows
  • Secondary cell wall has additional layers of cellulose with the microfibrils orientated in the same direction, but each layer has a different orientation, giving great strength
• Middle lamella

  Polysaccharides called pectin that act as an adhesive, holding neighbouring cells together
• Function of plant cell walls

  • Provide support, restrict the outward expansion of the cell contents (protoplast) as the cell takes in water, thus providing the supporting force associated with turgor pressure
  • Unlike the cell surface membrane, the cell wall is fully permeable and plays no part in determining which substances can enter and leave cells
• Plasmodesmata

  • Strands of cytoplasm that extend between neighbouring plant cells
  • Provide 'pores' in the cell walls of adjacent cells that enable different kinds of molecules to pass through
  • The neighbouring cells are joined, physically (the cytoplasm is continuous between adjacent cells) and metabolically