Cells

• Nucleus - contains genetic material in the form of protein bound linear chromosomes, acts as the control centre for the cell through the production of mRNA, tRNA and ribosomes in the nucleolus, has a double membrane which controls what enters and leaves the nucleus, pores in the nucleus membrane allow large molecules including mRNA.
• Mitochondria - the site of ATP synthesis by aerobic respiration, has a double membrane, the inner membrane is folded in to cristae which increases the surface area for the attachment enzymes and other proteins involved in respiration, the matrix is where the link reaction and Krebs cycle of respiration occur.
• Chloroplast - the site of photosynthesis, they have a double membrane, they contain dark areas called grana which are made of stacks of folded membranes called thylakoids containing chlorophyll, the stroma contains many enzymes in the LIS of photosynthesis along with large starch grains to store the products of photosynthesis.
• Rough Endoplasmic Reticulum - a system of ribosome covered, sheer like membranes running through the cytoplasm, RER provides a large surface area for the synthesis of proteins.
• Smooth Endoplasmic Reticulum - a system of sheet like membranes running through the cytoplasm, there are no ribosomes attached to SER, the site of lipid synthesis and transport.
• Golgi Apparatus - it receives proteins and lipids from the ER, it modifies proteins often attaching carbohydrates to form glycoproteins, molecules are labelled in the golgi apparatus so that they can be transported, It packages proteins and other materials into golgi vesicles which transport substances around the cell, golgi apparatus produces lysosomes.
• Lysosomes - larger vesicles produced by the golgi which contain lysozyme, they hydrolyse materials ingested by the phagocytes, they digest worn out organelles and dead cells (autolysis).
• Ribosomes - small granules made of rRNA and protein, ribosomes have two subunits which lock together, they are found in the cytoplasm or attached to the RER, ribosomes are larger in eukaryotic cells than in prokaryotic cells, ribosomes are involved in protein synthesis.
• Cell Wall - made of strong microfibrils of cellulose, the cell wall provides mechanical strength to stop cells bursting when water enters by osmosis, plant cells withstand water pressure (turgopressure) in an animal cell it would burst (osmotic lysis).
• Large Vacuole - a large fluid filled sac surrounded by a membrane called the tonoplast, acts as a temporary store of sugars and amino acids, helps to support the stems of non woody plants by making them turgid.
• All prokaryotic cells contain: a circular loop of DNA, cell membrane, cell wall (murein), cytoplasm, 70S ribosomes. Some contain: a flagellum, plasmids and a slime capsule.
• cell differentiation is the process by which cells become specialised to carry out a particular function.
• Palisade Cells

  • function - photosynthesis
  • they are long and thing forming a continuous layer to absorb sunlight
  • they are packed with chloroplasts that collect sunlight
  • they have a large vacuole that pushes the chloroplasts to the edge of the cell.
• Epithelial Cell

  • microvilli to increase surface area for absorption of nutrients
  • many mitochondria to make ATP for active transport
  • co-transport proteins in membrane
• Red Blood Cells (erythrocytes)

  • shape - biconcave disc to increase surface area making diffusion of oxygen into and out of cells more efficient.
  • cytoplasm full of haemoglobin
  • most organelles including the nucleus are removed to make more room for haemoglobin
  • flattened shape allows them to bend and quash against capillary wall reducing diffusion distance.
• Sperm Cell

  • tail for swimming
  • many mitochondria to produce ATP by aerobic respiration for movement of the tail
  • nucleus - carrying only one copy of each chromosome (23)
• Root Hair Cell

  function - exchange surface for absorption of water and minerals from soil

  • elongated to increase surface area
  • many dissolved solutes and minerals in vacuole and cytoplasm create low water potential so water enters by osmosis.
  • many mitochondria to provide ATP for active transport
  • many carrier proteins in the cell membrane to uptake minerals by active transport
• Xylem vessel

  function - transport water and minerals

  • small elongated cells
  • thick cell walls are reinforced and waterproof by deposits of lignin
  • ends of cells break down to form continuous tubes
  • cytoplasm and organelles are removed from the cell
• Atom --> Molecule --> Macromolecule --> Organelle --> Cell --> Tissue --> Organ --> Organism
• Tissue - a group of similar cells working together to preform a particular function
• Organ - a group of different tissues working together to preform a particular function
• System - a group of different organs working together to preform a particular function
• Viruses are non-living and acellular

  e.g. HIV

  P - capsid

  Q - reverse transcriptase

  
• Cell Fractionation

  Medium:

  • ice cold - to prevent enzymes hydrolysing organelles
  • isotonic - to prevent water entering or leaving organelles by osmosis causing them to shrivel or burst
  • pH buffer - to prevent the proteins in the organelles being denatured
• Cell Fractionation

  • blend the tissues into a homogenate. This breaks open the cells to release the organelles.
• Cell Fractionation - Filtration:

  • this removes any remaining whole cells and debris
• Ultracentrifugation:

  • place the homogenate in a centrifuge tube, spin in the centrifuge
  • a pellet of the densest organelle forms and a 'supernatant' is decanted into another tube
  • it is spun again but faster and longer
• Mitosis and the Cell Cycle:

  Growth and replacement of damaged cells, as well as asexual reproduction (cloning).
• Cell Cycle:

  • G1 - protein synthesis of enzymes for next stage
  • S - semi conservative replication of DNA
  • G2 - replication of DNA
  • Mitosis
• Mitosis:

  1. Prophase - chromosomes condense and become visible. They are composed of two identical, sister chromatids joined by a centromere. The centrioles move towards the 'poles of the cell'. The nuclear membrane begins to break down.
  2. Metaphase - spindle fibres have joined the centrioles to the centromeres of the chromosomes which enable them to move. The chromosomes are lined up on the equator of the cell.
• Mitosis:

  3. Anaphase - the spindle fibres contract causing the centromeres to divide. Each of the chromatids is drawn to opposite poles.

  4. Telophase - the chromatids have reached the poles and are now chromosomes. The cytoplasm divides (cytokinesis) and the cell nuclear membrane of the two cells forms. The chromosomes lengthen again and the nuclear membranes reform. Two 'clones' are formed.
• Cancer - cells that carry out uncontrolled cell division. Caused by a mutation of the genes that control the cell cycle.
• Division in prokaryotes - Binary Fission:

  • circular loops of DNA replicate by sc rep.
  • the plasmids replicated but the resulting no. are unpredictable.
  • the cell elongates
  • the cytoplasm divides and the murein wall reforms
  • the 2 new cell are not identical as the different number of plasmids renders them non identical.
• Viral Replication:

  • the HIV attachment proteins are complementary to the T Helper Cells receptors and so bind to them.
  • the capsid with the viral genome, is injected into the TH Cell.
  • reverse transcriptase converts RNA into DNA. This viral DNA is now integrated into the host DNA.
  • the viral DNA is transcribed and translated into viral proteins. It also contains the code to assemble the new viruses.
  • the new viruses burst out of the host cell - killing it and each virus obtains its phospholipid envelope from the cell membrane.