Cell structure & division

• Structure of chloroplasts :

  Small, flattened structure found in plant & algal cells. Surrounded by double membrane & also has membranes inside called thylakoid membranes. These membranes are stacked up in some parts of chloroplast to form grana. Grana are linked together by lamellae- thin, flat pieces of thylakoid membrane.
• Function of chloroplasts:

  Some parts of photosynthesis happen in grana, & other parts happen in stroma (thick fluid).
• Function of cell vacuole:

  Helps to maintain pressure inside cell & keep cell rigid. Stops plant wilting. Also involved in isolation of unwanted chemicals inside cell.
• Structure of cell vacuole:

  Membrane-bound organelle found in cytoplasm. Contains cell sap-a weak solution of sugar & salts. Surrounding membrane is called the tonoplast.
• Structure of cell surface membrane:

  Membrane found on surface of animal cells & just inside cell wall of other cells. Made mainly of lipids & protein.
• Function of cell surface membrane:

  Regulates movement of substances into & out of cell. Has receptor molecules on it, which allow it to respond to chemicals like hormones.
• Structure of nucleus:

  Large organelle surrounded by nuclear envelope (double membrane) containing many pores. Contains chromosomes & 1 or more nucleolus.
• Function of nucleus:

  Controls cell's activities by controlling transcription of DNA (DNA has instructions to make proteins). Pores allow substances (e.g, RNA) to move between nucleus & cytoplasm. Nucleolus makes ribosomes.
• Structure & function of mitochondrion :

  Oval-shaped. Has double membrane- inner membrane folded to form cristae- inside is matrix, has enzymes for respiration. Site of aerobic respiration- produces ATP.
• Structure & function of Golgi apparatus :

  Group of fluid-filled membrane-bound flattened sacs. Vesicles often seen at edges of the sacs. Processes & packages new lipids & proteins. Also makes lysosomes.
• Structure & function of lysosomes:

  Round organelle surrounded by a membrane, with no clear internal structure. Type of Golgi vesicle. Contains digestive enzymes called lysozymes-kept separate from cytoplasm by surrounding membrane. Digest invading cells or break down worn out components of cell.
• Structure & function of ribosomes:


  Very small organelle that floats free in cytoplasm or is attached to RER. Made up of proteins & RNA. Not surrounded by a membrane. Site of protein synthesis.
• Structure & function of rough endoplasmic reticulum:

  System of membranes enclosing a fluid-filled space. Surface covered with ribosomes. Folds & processes proteins that haven been made at ribosomes.
• Structure & function of smooth endoplasmic reticulum:

  System of membranes enclosing a fluid-filled space. No ribosomes. Synthesises & processes lipids.
• Structure & function of golgi vesicle:

  Small fluid-filled sac in cytoplasm, surrounded by a membrane & produced by golgi apparatus. Stores lipids & proteins made by golgi apparatus & transports them out of cell via cell-surface membrane.
• Structure & function of cell wall:

  Rigid structure that surrounds cells in plants, algae (made of cellulose) & fungi (made of chitin). Supports cells & prevents them from changing shape.
• Difference between eukaryotic & prokaryotic cells?

  Prokaryotic smaller & simpler.
• What is a tissue?

  A group of cells working together to perform a specific function.
• What special things do prokaryotic cells contain?

  Cell wall made of polymer murein, (slime) capsule that protects bacteria from attack by cells of immune system, plasmids (small loops of DNA), no nucleus, DNA free in cytoplasm (circular) & flagellum (helps cell move).
• Structure of viruses:

  Acellular (they're nucleic acids surrounded by protein). Invade& reproduce inside host cells of other organisms. Protein coat (capsid) with attachment proteins (lets virus cling to host cell). Core of genetic material. Virus is smaller than bacteria.
• How do prokaryotic cells replicate ?

  Binary fission- cell replicates genetic material & splits into 2 daughter cells.
• Explain binary fission:

  Circular DNA & plasmid(s) replicate. Main DNA loop replicated once, but plasmids can be replicated many times. Cell gets bigger & DNA loops move to opposite poles of cell. Cytoplasm begins to divide & new cell walls begin to form. Cytoplasm divides & 2 daughter cells produced. Each daughter cell has 1 copy of circular DNA but can have variable number of copies of plasmid(s).
• How do viruses replicate?

  Inject their DNA/RNA into host cell- uses its machinery (e.g, enzymes & ribosomes) to replicate virus particles. Attach to host cell surface via attachment proteins; bind to complementary receptor proteins on host cell membrane.
• How do you calculate magnification?

  Size of image / size of real object
• How do you convert from mm to micrometre to nm?

  Multiply by 1000
• What is resolution?

  How well microscope distinguishes between 2 points close together.
• What are optical (light) microscopes?

  Max resolution = 0.2 micrometres. Can't view ribosomes, lysosomes & endoplasmic reticulum. Can make out mitochondria. Can see nucleus. Max useful magnification = X 1500 Uses light to form image.
• What are electron microscopes?

  Uses electrons. Max resolution = 0.0002 micrometres Produce black & white images- often coloured by computer. Max useful magnification = x 1,500,000
• What are transmission electron microscopes (TEMs) ?

  Use electromagnets to focus beam of electrons which is then transmitted through specimen. Denser areas of specimen absorb more electrons- makes them look darker on image. Give high resolution images- can see internal structure of organelles like chloroplasts. Have to view specimen in a vacuum, so can't look at living organisms. Only can be used on thin specimens. Higher resolution images compared to SEMs.
• What are scanning electron microscopes (SEMs) ?

  Scan beam of electrons across specimen- knocks off electrons from specimen which are gathered in a cathode ray tube to form an image. Show specimen surface & can be 3D. Can be used on thick specimens, but give lower resolution images than TEMs. Only non-living specimen.
• Homogenisation in cell fractionation:

  Vibrate cells or grind cells in blender to break plasma membrane & release organelles into solution. Solution kept ice-cold, isotonic & buffer solution added to maintain pH.
• Why must solution in cell fractionation be isotonic?

  Should have same concentration of chemicals as cells being broken down to prevent damage to organelles through osmosis.
• Why must a buffer solution be added to the solution?

  To maintain pH.
• Why must the solution be ice-cold?

  To reduce the activity of enzymes that breakdown organelles.
• Describe the process of filtration in cell fractionation:

  Homogenised cell solution is filtered through a gauze to separate any large cell debris or tissue debris, like connective tissue, from the organelles. The organelles are much smaller than the debris, so they pass through the gauze.
• Describe ultracentrifugation:


  Separate organelles. Cell fragments poured into tube-tube put into centrifuge & spun at low speed. Heaviest organelles at bottom of tube-form pellet (thick sediment). Rest of organelles suspended in fluid above (supernatant). Supernatant drained off & spun at higher speed. Process repeats.
• Organelles heaviest to lightest:

  Nuclei, chloroplasts, mitochondria, lysosomes, ER & ribosomes.
• What is the cell cycle?

  Starts when cell produced by cell division & ends with cell dividing to produce 2 identical cells. Consists of interphase (cell growth & DNA replication) & mitosis (cell division). Interphase divided into 3 growth stages: G1, S & G2.
• What happens during interphase?


  Cell carries out normal functions & prepares to divide. Cell's DNA unravelled & replicated to double its genetic content. Organelles replicated so it has spare ones & ATP content increased (provides energy for cell division).
• What is mitosis?

  Cell division in cell cycle. Parent cell divides to produce 2 genetically identical daughter cells. Needed for growth & repairing damaged tissues. Consists of prophase, metaphase, anaphase & telophase.