CELLS

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STUDYING CELLS

CELL FRACTIONATION

1. tissue placed in a cold, buffered, isotonic solution

- COLD to reduce enzyme activity that might break down organelles

- ISOTONIC to prevent osmotic movement of water

- BUFFERED to prevent fluctuations in pH that may damage the organelles

2. cells homogenised - broken up to release the organelles, producing a homogenate

3. homogenate filtered to remove debris

4. homogenate spun at low speed - heaviest organelles (nuclei) fall to bottom and form a pellet

5. remaining fluid (supernatant) removed, leaving the pellet

6. supernatant spun at increased speed - next heaviest organelles (mitochondria) form a pellet and are removed

7. process repeated

MICROSCOPY

  • TRANSMISSION ELECTRON MICROSCOPES

- electron gun produces a beam of electrons that is focussed onto the specimen using an electromagnet

- dense parts of the specimen absorb electrons + appear dark

- less dense parts don't absorb electrons + appear light

- high resolving power (though this cannot always be achieved - the specimen needs to be very thin and a higher energy electron beam is required which may destroy the organelles)

- system must be in a vacuum so no living specimens can be used

- specimen must be stained

- specimen must be extremely thin

- image may contain artefacts (appear on the photomicrograph but not the specimen, decrease the accuracy with which we can examine the specimen)

  • SCANNING ELECTRON MICROSCOPES

- electron beam directed onto surface of specimen and passed back and forth over its surface

- electrons scattered by the specimen and the scatter pattern depicts an image of the contours of the specimen's surface

- produces a 3D image

- specimens do not need to be thin

- vacuum = no living specimens

- lower resolving power

EUKARYOTIC CELL STRUCTURE

THE NUCLEUS

STRUCTURE

  • nuclear envelope - double membrane surrounding the nucleus whereby the outer membrane is continuous with the ER and has ribosomes on its surface. Controls the entry and exit of substances in the nucleus and the reactions within it.
  • nuclear pores - small holes that allow the passage of mRNA out of the nucleus
  • nucleoplasm - granular, jelly-like material forming the bulk of the nucleus
  • chromosomes - protein bound, linear DNA
  • nucleolus - small sperical region in the nucleoplasm that manufactures rRNA and assembles the ribosomes

FUNCTION

  • control centre - production of mRNA and tRNA and thus the control of protein synthesis
  • retain genetic material - DNA and chromosomes
  • manufacture rRNA and ribosomes

THE MITOCHONDRION

STRUCTURE

  • double membrane - controls the entry and exit of material
  • cristae - extensions of the inner membrane to increase surface area for attachment of enzymes + other proteins necessary for respiration
  • matrix - forms the remainder of the mitochondrion, contains proteins, lipids, ribosomes + DNA to allow production of mitochondrion's own proteins

FUNCTION

  • sites of aerobic respiration - production of ATP

THE CHLOROPLASTS

STRUCTURE

  • chloroplast envelope - double plasma membrane, highly selective in terms of entry + exit
  • grana - stacks of thylakoids containing chlorophyll where the first…

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