cell structure

• Eukaryotes

  - Plants, animals, algae and fungi

  - Cell contains a membrane-bound nucleus

  
• prokaryotes

  - does not contain a membrane-bound nucleus

  - bacteria

  - much smaller than eukaryotes

  
• nucleus - animal

  - contains chromatin = made of DNA coiled around histone proteins

  - nuclear envelope with nuclear pores = allows substances to move between the nucleus + cytoplasm

  - nucleolus = makes ribosomes

  - contains chromosomes

  
  

  - DNA replication occurs here

  - mRNA and tRNA made here

  - DNA codes for polypeptides
• ribosomes

  - made from rRNA and proteins

  - small subunit and large subunit

  - found in the cytoplasm/ attached to the rough endoplasmic reticulum

  - eukaryotes contain 80s ribosomes

  - prokaryotes contain 70s ribosomes

  - site of protein synthesis
• rough endoplasmic reticulum

  - made of flattened membrane sacs = cisternae

  - outer surface has ribosomes attached

  - synthesise proteins and transport them through the cell
• smooth endoplasmic reticulum

  - made of flattened membrane sacs = cisternae

  - synthesise and transport lipids
• golgi apparatus

  - made of flattened membrane sacs = cisternae

  - modifies + packages proteins from the RER (exocytosis) for secretion

  - modifies lipids from SER into vesicles for secretion
• lysosomes

  - vesicles containing lysozymes (digestive enzymes/hydrolytic) = cause hydrolysis (eg of invading pathogens + worn out organelles)
• cell membrane

  - main component of phospholipid bilayer

  - proteins, carbohydrates + cholesterol embedded

  
  

  - controls movement of substances in and out of the cell
• mitochondria

  - double membrane = inner membrane is folded to form cristae which increases surface area for attachment of proteins + enzymes involved in respiration

  - have their own DNA + ribosomes

  - matrix contains enzymes for aerobic respiration

  - carry out aerobic respiration to produce ATP

  
• plant cells

  - cellulose cell wall

  
  

  - vacuole

  
  

  - chloroplast:

  ->site of photosynthesis

  -> stroma = fluid which contains enzymes for photosynthesis

  -> thylakoids = membranes which contain the chlorophyll for photosynthesis

  -> granum (stack of thylakoids) = increase surface area for absorption of light

  -> starch grains = store products of photosynthesis as starch

  
• prokaryotic cells

  - smaller (70s) ribosomes

  - cell membrane

  - murein cell wall (glycoprotein)

  - cytoplasm with no membrane-bound organelles

  - no nucleus = single circular DNA molecule that is free in cytoplasm, not associated with histones

  
  

  - one or more flagella

  - one or more plasmids

  - a capsule surrounding the cell

  
• how is prokaryotic DNA different from eukaryotic DNA?

  - eukaryotic DNA is found in membrane-bound nucleus whereas prokaryotic DNA is free in cytoplasm

  - prokaryotic cells have circular DNA whereas eukaryotic DNA is linear

  - prokaryotic DNA is not associated with histone proteins but eukaryotic DNA is

  - in some prokaryotes, plasmids are present (may also be present in eukaryotes)

  - prokaryotic DNA does not contain introns but eukaryotic DNA does

  - prokaryotic DNA is shorter so contains fewer genes
• mitochondria and chloroplasts

  - have their own 70s ribosomes

  - have their own DNA which is different to eukaryotic DNA found in the nucleus;

  -> shorter + contains fewer genes

  -> circular

  -> not associated with histone proteins

  -> introns absent

  
  

  led to the theory that mitochondria and chloroplasts evolved from prokaryotic cells
• viruses

  - acellular + non-living

  - consist of genetic material surrounded by a capsid with attachment proteins sticking out

  - used to attach to host cells so virus can enter the cell + use cell to replicate

  - much smaller than prokaryotic cells

  
• what is cell fractionation?

  - where cells are broken open and the organelles can be separated out and studied in detail

  - organelles need to be separated so that they can be studied individually without other organelles affecting their function
• process of cell fractionation

  - tissue is homogenised in a ice cold, isotonic, buffered solution to break open the cells and release the organelles

  - mixture is then filtered to remove cell debris and unbroken cells

  - solution is then centrifuged at a slow speed to pellet the most dense organelles (nucleus)

  - supernatant is removed + placed in a new tube + centrifuged at a faster speed to pellet less dense organelles

  - this is called differential centrifugation

  
  

  order of pellets is:

  - nuclei

  - chloroplasts

  - mitochondria

  - ribosomes
• why ice cold?

  reduce enzyme activity to prevent damage to organelles
• why isotonic?

  prevent osmosis so organelles do not burst or shrink
• why buffered?

  prevents a change in pH so proteins within organelles do not denature
• what is magnification?

  how much bigger the image appears than the real object
• what is resolution?

  ability to see 2 objects close together as separate objects
• light microscopes


  - use light focused by glass lenses

  - lower resolution of 200nm

  - alive or dead specimen which often need to be stained

  - produces a 2D and colour image

  - large organelles can be seen eg nucleus, cytoplasm, cell wall, chloroplasts, cell membrane

  - easily prepared sample that must be thin so light can pass through
• Scanning electron microscope


  - uses a beam of electrons focused by electromagnets

  - specimen must be dead as they must be viewed in a vacuum

  - high resolution = electrons have a much shorter wavelength than light

  - produces a 3D + black and white image

  - allows you to see surface of specimens

  - time consuming + complex staining procedure to produce sample
• transmission electron microscope


  - uses a beam of electrons focused by electromagnets

  - specimen must be dead as they must be viewed in a vacuum

  - highest resolution = electrons have a much shorter wavelength than light

  - produces a 2D + black and white image

  - allows you to see smaller organelles + internal structures of organelles

  - time consuming + complex staining procedure to produce sample
• artefacts

  may be dust, air bubbles or fingerprints
• conversions in biology


  each time x1000

  nm

  um

  mm

  m