Cell structure and division

• Cell surface membrane (plasma membrane):

  • found on the surface of animal cells & in the cell wall of other cells.
  • It is mainly made of lipids and proteins.

  Function:

  • it regulates movement of substances into and out of the cell.
  • it also contains receptors that respond to hormones.
• Nucleus:

  • contains a nuclear envelope with nuclear pores.
  • inside is the nucleoplasm containing chromosomes and a nucleolus.

  Function:

  • Controls cell activities such as transcription of DNA.
  • The pores allow substances like RNA to move from the nucleus to the cytoplasm.
  • The nucleolus makes ribosomes.
• Mitochondrion:

  • rod shaped with an outer membrane and a highly folded inner membrane called the cristae.
  • inside is the matrix which contains enzymes involved in respiration.

  Function:

  • the site of aerobic respiration where ATP is produced.
• Chloroplast:

  • contains an outer and inner membrane containing a fluid stroma with stacks on thylakoid membranes called grana.
  • Grana are linked by lamellae.

  Function:

  • the site of photosynthesis.
  • some parts occur in the stroma (LIR) & some in the thylakoid membranes (LDR).
• Golgi apparatus:

  • flattened sacs of fluid filled membranes.
  • vesicles are often seen at the ends of these sacs.

  Function:

  • modifies & packages lipids and proteins.
  • it also makes lysosomes.
• Golgi vesicle:

  • a small fluid filled sac surrounded by a membrane.

  Function:

  • stores lipids & proteins and transports them out of the cell via the cell membrane.
• Lysosome:

  • a round organelle surrounded by a membrane, it is a type of golgi vesicle.

  Function:

  • contains hydrolytic enzymes called lysozymes.
  • this can be used to digest pathogens & break down worn out cells.
• Ribosome:

  • made up of a small subunit stacked onto a large subunit.
  • it is made up of RNA and proteins.

  Function:

  • the site of protein synthesis.
• Rough endoplasmic reticulum:

  • flat fluid filled membranes covered with ribosomes.

  Function:

  • folds and processes proteins that have been made as ribosomes.
  • a site for ribosomes to do protein synthesis.
• Smooth endoplasmic reticulum:

  • flat fluid filled membranes with a smooth surface.

  Function:

  • synthesis and processing of lipids.
• Cell wall:

  • only in plants, algae & fungi.
  • made up of cellulose (plants)
  • made up of chitin (fungi)

  Function:

  • supports cells and prevents them from changing shape or bursting.
• Vacuole:

  • only in plant cells.
  • a membrane bound organelle found in the cytoplasm.
  • it consists of a membrane called a tonoplast which contains cell sap.

  Function:

  • structurally supports the cell.
  • a storage area for substances e.g sugars, waste, etc
• Structures in prokaryotic cells:

  • plasma membrane
  • flagellum
  • circular DNA
  • Plasmids
  • cell wall
  • Slime capsule (some)
  • ribosomes (smaller 70s)
• Plasma membrane (prokaryotic):

  • made up of lipids and proteins
  • controls movement of substances into and out of the cell.
• Flagellum:

  • long hair like structures that allows the cell to move.
• slime capsule:

  • protects bacteria from attacks from the immune system.
• Cell wall (prokaryotic):

  • made of murein
  • supports the cells structure & stops it changing shape.
• viruses are acellular = they are non living
• virus structure:

  • capsid / protein coat.
  • attachment protein
  • genetic material of either DNA or RNA.
• prokaryotic cells replicate by binary fission.
• Binary fission:

  1. the circular DNA divides once and the plasmids divide many times.
  2. each circular DNA moves to opposite poles of the cell.
  3. the cytoplasm begins to divide & and new cell wall begins to form.
  4. the cytoplasm divides into two daughter cells.
• Viral replication:

  • viruses use a host to replicate.

  1. viruses use their attachment proteins to bind to complementary receptor proteins of the host cell.
  2. they inject their DNA or RNA into the host cell, using its 'machinery' like ribosomes & enzymes to replicate viral particles.
• magnification = image size / actual size
• resolution is how detailed an image is & how well a microscope can distinguish between two different points.
• what are the two main types of microscope?

  optical & electron
• optical microscope:

  • Use light to form an image.
  • Have a maximum resolution of 0.2 micrometres so cannot be used to view organelles smaller than that.
  • Cannot see ribosomes, endoplasmic reticulum & lysosomes.
  • Have a maximum magnification of x 1500
• Electron microscope:

  • Use electrons to form an image.
  • Have a higher resolution than light microscopes so give a more detailed image.
  • Have a maximum resolution of 0.0002 micrometres.
  • Have a maximum magnification of x 1500000.
• what are the two types of electron microscope?

  scanning & transmission
• Transmission electron microscope:

  • Use electromagnets to focus a beam of electrons through a specimen.
  • Denser parts of the specimen absorb more electrons so look darker.
  • They have high resolution so give detailed images of organelles.
  • But they can only be thin specimins.
  • Gives a 2D image.
• Scanning electron microscope:

  • Scan a beam of electrons across the specimen.
  • Can give a 3D image
  • Thicker specimens can be used.
  • Give a lower resolution than TEMs.
• prokaryotic cells replicate by binary fission
• what are the stages of cell fractionation?

  • homogenisation - breaking up the cells
  • filtration
  • ultracentrifugation - separating the organelles
• homogenisation:

  1. blend the sample to break open the cells & releases the organelles.
  2. the solution must be ice cold, isotonic & have a buffer solution.
• why must a solution be kept ice cold in cell fractionation?

  to reduce activity of enzymes that may break down organelles
• why must a solution be isotonic in cell fractionation?

  prevents osmosis which may damage the organelles.
• why must the solution be buffered during cell fractionation?

  to maintain pH to avoid damaging organelles
• filtration:

  1. filter the solution through gauze to remove any debris e.g connective tissue
• ultracentrifugation:

  1. the solution is poured into a tube which is then spun in a centrifuge.
  2. The first spin is a low speed this leaves the most dense organelles at the bottom as a pellet.
  3. the other organelles are still in the fluid supernatant.
  4. the supernatant is poured into another tube & spun again at higher speed.
  5. this is repeated until all the organelles have been separated out.
• which organelles are in the first pellet / densest ?

  nuclei
• what is the order of organelles collected during cell fractionation?

  nuclei, mitochondria, lysosomes, endoplasmic reticulum, ribosomes

  (heaviest -> lightest)