cell structure

• technique that allows scientists to study the functions of the cells - cell fractionation
• cell fractionation - 1

  • take a sample of tissues containing the cells we are interested in
  • use a homogeniser to break up the tissue and open the cells. a homogeniser is a glass tube with a plunger
  • place the sample into the glass tube and cover with buffer solution. buffer solution keeps the pH constant and is important because if the pH changes, enzymes in the cells organelles could denature. the water potential of the buffer is the same as inside the cell, this prevents water moving into the organelles via osmosis
• cell fractionation - 2

  • homogeniser is then placed on ice, as cooling the sample means the enzymes work slower preventing destructive enzymes from damaging the organelles
  • push the plunger up and down to disrupt the tissue and break open the cells - produces a cell homogenate. a cell homogenate contains all the organelles that are found in a cell
  • in order to find out what these organelles do we need to separate them in size order. nucleus, mitochondria, lysosomes, ribosomes (in order biggest to smallest)
  • then place the tubes containing cell homogenate into the sample holder (centrifuge)
• cell fractionation - 3

  • centrifuge spins the sample and organelles are flung towards the bottom of the tube be the forces generated
  • starts with a low speed spin, large organelles are flung towards the bottom of the tube forming a pellet (band) and the remaining organelles stay in the liquid (supernatant)
  • supernatant is transferred into a new tube and spun at a higher speed, pellet now contains mitochondria
  • repeat over and over
• cell fractionation can be difficult as its hard to completely separate all the organelles, so there may be small amounts of other organelles in other samples
• resolution - minimum distance between 2 objects where they can still be seen as 2 separate objects
• light microscopes have one big advantage compared to other microscopes; this is that they can be used on living cells, this means we can explore processes such as cell division.

  however a stain can be needed which can kill the living cells
• high resolution - level of detail in the image is greater
• low resolution - when the objects are blurry and can't be distinguished
• standard light microscope the resolution is around 200 nm

  • if objects are closer than 200 nm we cannot see them as 2 separate objects using a light microscope so therefore would use an electron microscope
• magnification = image size / actual size
• light microscopy

  advantage - view living cells

  disadvantage - poor resolution, max is 200 nm
• scientists invented the electron microscope which uses electron beams instead of light

  • electrons have a shorter wavelength so resolution is 2000 x better than a light microscope
• electron microscope

  1. electron gun which is producing beams of electrons which pass down the microscope. inside the microscope there is a vacuum so electrons pass through without bouncing off air molecules
  2. the specimen is placed in the path of the electron beam, electrons can pass through some parts of the specimen more easily than others
  3. the final image is produced on a fluorescent screen
• electron microscope

  advantage - 2000 x better resolution, greater magnification before the image becomes blurred

  disadvantage - interior of electron microscope is a vacuum so cannot view living specimens, requires carful staining, specimen needs to be very thin and can lead to artefacts which are created by the staining process or conditions inside the microscope
• 2 different types of electron microscope

  • transmission electron microscope (TEM)
  • scanning electron microscope (SEM)
• TEM

  • electron beam passes through the specimen
  • produces 2D images
  • only works if the specimen is thin
  • has a very high resolution (0.1 nm)
• SEM

  • beam does not pass through the specimen, electrons are scattered on the surface of the specimen and detected
  • produces 3D images
  • specimen does not have to be thinly sliced
  • lower resolution than TEM
  • has to be coated with a metal (gold) which can lead to artefacts
• • prokaryotic cells are much smaller than eukaryotic cells
  • prokaryotic cells have no membrane bound organelles, DNA is found in the cytoplasm rather than the nucleus
  • prokaryotic cells have chromosomes arranged in circular shapes which are not bound to histone proteins
• some bacteria cells (prokaryotic cells) contain plasmids which are small loops of DNA. these contain a small number of genes; genes that make bacteria resistant to antibiotics. so plasmids are important for prokaryotic cells
• prokaryotic cells

  ribosomes - carry out protein synthesis (70s ribosomes)

  • 's' shows how quickly the organelles move in a centrifuge
• prokaryotic cells

  peptidoglycan cell wall - helps to maintain the structure of the cell. polymer between peptides and polysaccharide molecules
• prokaryotic cells

  slime capsule - protects the bacteria from phagocytosis from white blood cells
• prokaryotic cells

  flagellum - helps the cell to move
• prokaryotic cells

  pili - fine protein strands on the surface which help bacteria attach to other surfaces and to attach to other bacteria

  • when 2 bacterium are attached the DNA can be transferred from one bacterium to another
• prokaryotic cells

  lipid droplets and glycogen granules - act as a nutrient store for the bacterial cell
• bacteria cell

  A) pili

  B) flagellum

  C) bacterial chromosome

  D) plasmids

  E) ribosomes

  F) cytoplasm

  G) cell membrane

  H) cell wall

  I) slime capsule

  
• eukaryotic cells

  all DNA is contained in a nuclear bound membrane

  • nucleus has a double membrane
  • some eukaryotic cells can lose their nucleus as they develop (red blood cells)
• eukaryotic cells

  DNA is tightly wrapped around histone proteins; the DNA and histone proteins form chromosomes

  • means a lot of DNA can be tightly packed into the nucleus
  • DNA is linear in eukaryotic cells
• Animal cell

  A) cilia

  B) mitochondria

  C) cytoplasm

  D) ribosomes

  E) rough endoplasmic reticulum

  F) nucleolus

  G) nucleus

  H) Golgi apparatus

  I) cell membrane

  J) microfilament

  K) lysosome

  L) smooth endoplasmic reticulum

  M) vesicle

  N) peroxisome

  O) centrioles

  P) flagella

  
• plant cell

  A) vesicle

  B) ribosomes

  C) smooth endoplasmic reticulum

  D) nucleus

  E) nucleolus

  F) rough endoplasmic reticulum

  G) cytoskeleton

  H) cell wall

  I) peroxisome

  J) Golgi apparatus

  K) central vacuole

  L) chloroplast

  M) cytoplasm

  N) mitochondria

  O) cell membrane

  
• eukaryotic cells

  cells are surrounded with a cell surface membrane so the molecules that pass in and out of the cell can be controlled

  in plants and fungi the cell membrane is surrounded by a cell wall, which helps maintain the structure
• viruses

  • virus attaches to the host cell
  • virus enters the host cell
  • virus uses the host cell enzymes to produce copies of itself
  • virus particles leave the host cell, move onto infect new host cells and continue to reproduce and replicate
• viruses

  all viruses contain genetic material. either DNA or RNA

  • contained inside a capsid
• viruses

  attachment proteins allow the virus to attach and enter the host cell
• viruses

  some are surrounded with a lipid envelope, which is formed from the host cell membrane
• virus

  A) attachment proteins

  B) generic material

  C) capsid

  D) lipid envelope

  
• nucleus

  instructions for coding an amino acid sequence are contained within a gene for the desired protein

  • these genes are part of the chromosomes which are found in the nucleus
• nucleus

  • to synthesise a protein the genetic information is encoded by a gene which is converted into messenger RNA (mRNA) - transcription
  • mRNA then leaves the nucleus and a ribosome reads the information contained in the mRNA and synthesises the protein molecule - translation
• nucleus

  if the protein remains in the cytoplasm then translation will happen on a free ribosomes

  however, some proteins are secreted from cells (e.g. antibodies). secreted proteins are translated on a ribosome attached to the RER and these proteins make their way through the RER and golgi apparatus before leaving the cell