AQA A-Level Biology. 3.2 - Cells.

• Structure of Nucleus.

  1. Nuclear envelope: double membrane surrounding nucleus, outer membrane continuous with the (R)ER of the cell.

  
  

  2. Nuclear pores: allow the passage of larger molecules, such as mRNA, out of the nucleus.

  
  

  3. Nucleoplasm: granular, jelly-like material making up the bulk of the nucleus.

  
  

  4. Chromosomes: protein-bound, linear DNA.

  
  

  5. Nucleolus: small spherical region(s) in nucleoplasm. Manufactures ribosomal RNA and assembles ribosomes.
• Function of Nucleus.

  1. Controls cell's activities - produces mRNA and tRNA - protein synthesis. Controls entry and exit of materials, and contains nuclear reactions.

  
  

  2. Retains genetic material in the form of DNA and chromosomes.

  
  

  3. Manufactures ribosomal RNA and ribosomes.
• Structure of cell surface membrane

  phospholipid bilayer with embedded proteins spanning a diameter of 10nm
• Function of cell surface membrane

  Controls the exchange of materials between the internal cell environment and the external environment

  Separates organelles from cytoplasm so that specific metabolic reactions can occur within them

  Provides a surface on which reactions can occur

  Isolate enzymes that might damage the cell

  Provide an internal transport system
• Structure of Mitochondria.

  1. Double membrane surrounding organelle - controls entry and exit of material.

  
  

  2. Cristae - extensions of the inner membrane, providing a large surface area for the attachment of enzymes and other proteins during respiration.

  
  

  3. Matrix - makes up the remainder - contains proteins, lipids, ribosomes and DNA (allows mitochondria to produce own proteins) and some respiratory enzymes.
• Functions of Mitochondria.

  1. Sites of Krebs Cycle and oxidative phosphorylation pathway in aerobic respiration - responsible for ATP production.

  
  

  NB = found in high numbers in metabolically active cells which require much ATP.
• Structure of Chloroplasts.

  Found in plants and algae.

  
  

  1. Chloroplast envelope - double plasma membrane, highly selective, surrounds the organelle.

  
  

  2. Grana - stacks of disc-shaped thylakoid membrane.

  
  

  3. Thylakoids - contain chlorophyll used in photosynthesis, can be linked by lamellae to other grana.

  
  

  4. Stroma - fluid-filled matrix where Calvin Cycle takes place. Also contains starch grains.
• Functions of Chloroplasts.

  Site of Photosynthesis:

  
  

  LDR in thylakoid membranes.

  LIR in stroma.

  
  

  1. Granal membranes provide a large SA for LDR - photosystems, e- carriers and enzymes etc.

  
  

  2. Chloroplasts contain DNA and ribosomes - can quickly and easily manufacture some of the proteins needed for photosynthesis.
• Structure of Endoplasmic Reticulum.

  1. 3D system of sheet-like membranes - continuous with the outer membrane of the nuclear double membrane.

  
  

  2. Membrane contains a network of tubules and flattened sacs called cisternae.

  
  

  3. RER - ribosomes on the outer surface of the membranes.

  
  

  4. SER - lacks ribosomes on its surface and is often more tubular in its appearance.
• Function of Endoplasmic Reticulum.

  RER

  
  

  1. Large SA for protein/glycoprotein synthesis.

  
  

  2. Provides a pathway for material transport throughout the cell, especially for proteins.

  
  

  SER

  
  

  1. Synthesises, stores and transports lipids and carbohydrates.

  
  

  NB = cells that manufacture and store lots of lipids, carbs and proteins have extensive ER - such as liver and secretory cells, such as the intestine epithelial cells.
• Structure of Golgi Apparatus.

  1. Compact system of flattened sacs and stacked membranes (cisternae).

  
  

  2. Vesicles - modified proteins and lipids transported to cell membrane where they fuse with it, and then egest contents to the outside.
• Functions of Golgi Apparatus.

  1. Form glycoproteins by adding carbs to proteins.

  
  

  2. Produce secretory enzymes, such as those secreted by the pancreas - apparatus is developed in secretory cells, especially those in the small intestine.

  
  

  3. Secrete carbs, such as cellulose for plant cell walls.

  
  

  4. Transports, modifies and stores lipids.

  
  

  5. Forms lysosomes.

  
  

  NB = Golgi Vesicles are 'pinched off' from golgi cisternae.
• Structure of Lysosomes.

  Golgi vesicles with proteases, lipase and lysozymes.
• Functions of Lysosomes.

  1. Hydrolyse foreign material ingested by phagocytes.

  
  

  2. Exocytosis of enzymes to destroy extra-cellular material.

  
  

  3. Apoptosis - programmed cell death.

  Autolysis - breaking down cells after death.

  
  

  4. Digest worn out organelles - can recycle chemicals.

  
  

  NB = very abundant in secretory cells and phagocytes.
• Structure of Ribosomes.

  1. Small cytoplasmic granules found in all cells, free-floating or associated with RER.

  
  

  2. 80S - found in eukaryotic cells, slightly larger.

  
  

  3. 70S - in prokaryotic cells, slightly smaller.

  
  

  4. 2 Subunits - large and small - contain ribosomal RNA and proteins.
• Functions of Ribosomes.

  Carry out translation stage of protein synthesis to produce polypeptides.
• Structure of Cell Wall.

  Found in plants, algae and fungi.

  
  

  1. Cellulose microfibrils embedded in a matrix - contribute to overall cell wall strength are considerably strong.

  and other polysaccharides.

  
  

  2. Middle lamella - marks the boundary between adjacent cell walls and cements adjacent cells together.

  
  

  NB= made of nitrogen-containing chitin in fungi, and made of the glycoprotein murein in bacteria.
• Functions of Cell Wall.

  1. (Cellulose) - to provide mechanical strength to prevent cell wall bursting under pressure created by osmotic entry of water.

  
  

  2. To provide mechanical strength to the cell as a whole.

  
  

  3. Allows water to pass along it - contributes to the movement of water through the plant.
• Structure of Vacuoles.

  1. Fluid-filled sac bounded by a single membrane.

  
  

  2. Single membrane around it called tonoplast.

  
  

  3. Solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins.
• Functions of Vacuoles.

  1. Support herbaceous plants and herbaceous parts of woody plants by making cells turgid.

  
  

  2. The sugars and amino acids can act as a temporary food source.

  
  

  3. Pigments - may attract pollinating insects due to colour.
• What are microscopes?


  Instruments that produce a magnified image of an object.
• Magnification - equation?

  Magnification = size of image / size of real object.
• Conversions:

  
  

  km to m

  m to m

  mm to m

  micrometre to m

  nanometre to m


  km to m - x1000

  m to m - x1

  mm to m - /1000

  micrometre - /1000000

  nanometre - /1000000000
• Difference between magnification and resolution?

  Magnification = increasing the size of an image. Up until the limit of resolution, an increase in magnification = an increase in detail.

  
  

  Resolution = minimum distance apart that two objects can be for them to appear as separate items.
• Need to appreciate that...

  ...there was a considerable period of time during which the scientific community distinguished between organelles and artefacts.

  
  

  artefacts = (something in a scientific experiment present due to how expt. was prepared or investigated).
• Why is cell fractionation needed?

  Needed to study the structure and function of the various organelles that make up cells.

  
  

  We need a large number of isolated organelles - can get them via cell fractionation.
• Define cell fractionation.

  The process in which cells are broken up and the different organelles they contain are separated out.
• Describe/Outline the process of Homogenisation.

  
  

  Why a cold, isotonic, buffered solution?

  1. Tissues placed in a cold, isotonic (relative to tissue), buffered solution.

  
  

  2. Cells then broken up by a homogeniser/blender - releases organelles from cell - resultant fluid = homogenate - then filtered to remove any complete cells or large pieces of debris.

  
  

  Cold - to reduce enzyme activity, such as lysozymes, that could break down organelles.

  
  

  Isotonic - same water potential as tissue sample - to preven water moving in or out of the cells by osmosis, causing lysis.

  
  

  Buffered - to prevent changes in pH which could affect/denature enzymes.
• Describe/Outline the process of Ultracentrifugation.


  The filtrate is placed into a tube and the tube is placed in a centrifuge

  A centrifuge is a machine that separates materials by spinning

  1. The filtrate is first spun at a low speed

  2. This causes the largest, heaviest organelles (such as the nuclei) to settle at the bottom of the tube, where they form a thick sediment known as a pellet

  The rest of the organelles stay suspended in the solution above the pellet

  This solution is known as the supernatant

  3. The supernatant is drained off and placed into another tube, which is spun at a higher speed

  4. Once again, this causes the heavier organelles (such as the mitochondria) to settle at the bottom of the tube, forming a new pellet and leaving a new supernatant

  5. The new supernatant is drained off and placed into another tube, which is spun at an even higher speed

  6. This process is repeated at increasing speeds until all the different types of organelle present are separated out (or just until the desired organelle is separated out)

  Each new pellet formed contains a lighter organelle than the previous pellet
• Principles of Optical Microscopes.

  Simple convex glass lenses used in pairs in a compound light microscope - focuses object at a short distance by 1st lens, then magnified by 2nd lens.
• Limitations of Optical Microscopes.

  Light has a relatively long wavelength - low resolution. Can only distinguish between objects 0.2 micrometres apart.
• Principles of Transmission Electron Microscopes.

  1. Electron gun produces e- beam, focused onto specimen by a condenser electromagnet.

  
  

  2. Beam passes through a thin section of the specimen from below. Parts absorb e- and appear dark; others let e- pass through and appear bright - produces image on screen - photomicrograph.
• Limitations of Transmission Electron Microscopes.

  1. Can't work on living specimens - needs to be in a vacuum.

  
  

  2. Complex staining process.

  
  

  3. Image not in colour.

  
  

  4. Extremely thin specimens only.

  
  

  5. May contain artefacts, which appear on the finished photomicrograph.

  
  

  But, can get over the flat 2D image, by taking a series of sections through a specimen - can build up a 3D image from these photomicrographs.

  
  

  Max resolution is 0.1 nm but can't always be achieved because:

  
  

  1. Difficulties in specimen preparation.

  2. Higher energy e- beam required, which may destroy the specimen.
• Principles of Scanning Electron Microscopes.

  1. Beam of e- directed onto surface of specimen - passed back and forth across specimen.

  
  

  2. e- scattered by specimen - scattering pattern analysis allows us to get a 3D image.
• Limitations of Scanning Electron Microscopes.

  1. Same as with TEMs, but samples do not need to be thin.

  
  

  2. Lower resolution than TEMs , at 20 nm.
• Describe how you would make a temporary mount (of plant tissue) (4).

  M1 Add drop of water to glass slide.

  M2 Obtain thin section of (plant tissue) and place on slide.

  M3 Stain with x - iodine in KI solution if testing for starch grains etc.

  M4 Lower cover slip using mounted needle.

  
  

  Ensure method avoids trapping air bubbles!
• Graticule? Why needed?

  Glass disc with an etched scale placed in the eyepiece of a microscope.

  
  

  Needed to measure size of objects under objective lens, need to calibrate the eyepiece graticule - each objective lens will magnify to a different degree.
• Describe how to calibrate the eyepiece graticule.

  1. Use a stage micrometer = special microscope slide with an etched scale - line up scales on graticule and micrometer.

  
  

  2. Once lined up, can calculate length of divisions on eyepiece graticule.

  
  

  x40 mag gives 25microm per graticule unit

  
  

  therefore, x400 mag gives 25/10microm per graticule unit.

  
  

  NB = Only need to calibrate once, providing the same objective lens is used.
• How are cells specialised in complex multicellular organisms?

  Cells initially all identical in an embryo but as they develop, the switching on/off of certain genes takes place, leading to changes in the organelle numbers and shapes of cells.
• Define tissue.

  
  

  Give an example.

  Collection of similar cells that are aggregated together and work together to perform a specific function.

  
  

  Example = epithelial tissue - consists of sheets of cells, lining the surfaces of organs, often having a protective or secretory function.