1.6

• Nucleus


  The largest organelle present in the cytoplasm of a eukaryotic cell, which contains DNA coding for protein synthesis
• Nucleus


  • The nucleoplasm contains chromatin, which condenses to form chromosomes during cell division
  • A double membrane surrounds it, the outer membrane of which is continuous with the endoplasmic reticulum
  • It has pores that allow mRNA to leave the nucleus
  • The nucleolus is a small spherical body found within the nucleus and is responsible for the production of rRNA and ribosomes
• Mitochondria


  Cylindrical in shape, typically 1-10μm long, and are the site of aerobic respiration, producing ATP
• Mitochondria


  • The inner membrane is folded into structures called cristae, which provide a large surface area for the attachment of enzymes (e.g. ATP synthetase)
  • A fluid-filled matrix contains lipids and proteins, 70S ribosomes and a small circle of DNA
  • They are present in all cells, but are found in much higher number in metabolically active cells, e.g. in muscles and liver
• A cylinder has a larger surface area than a sphere of the same volume, and has a shorter distance from the edge to the centre, reducing diffusion distance and increasing respiration efficiency
• Prokaryote


  A single-celled organism lacking membrane-bound organelles, such as a bacterium, with its DNA free in the cytoplasm
• Chloroplasts


  Found in photosynthetic plants, chloroplasts are the site of photosynthesis
• Chloroplasts


  • The organelle is surrounded by a double membrane, and contains a fluid-filled stroma with starch grains, 70S ribosomes and a circle of DNA
  • A thylakoid is a flattened membrane, and stacks of thylakoids contain the photosynthetic pigments, which include chlorophyll
  • Unlike the mitochondrion, the inner membrane is not folded
• Ribosomes


  Structures found in the cytoplasm where proteins are synthesized during translation
• Ribosomes


  • They differ in size between cells, with eukaryotic ribosomes being slightly larger (80S) than prokaryotic ribosomes (70S)
  • Consisting of two subunits, the small subunit has an mRNA attachment site, and the large subunit has two RNA attachment sites
• Golgi body


  Similar to endoplasmic reticulum, the Golgi is more compact in shape and is a stack of curved distemae
• Golgi body


  • Vesicles containing polypeptides bud off the rough endoplasmic reticulum, and fuse with the Golgi
  • Proteins are modified and packaged into vesicles by the Golgi body for export
  • Golgi are also involved in the transport and storage of lipids, and the production of glycoproteins and lysosomes
• Lysosomes


  Small, single membrane-bound vacuoles that are pinched off from the Golgi body containing the digestive enzyme lysozyme
• Lysosomes


  • Their role is to digest worn out organelles within the cell, and foreign material that has been engulfed by phagocytosis, e.g. bacteria engulfed by a white blood cell
• Centrioles


  Present in animal cells and protoctistans, they are noticeably absent from higher plants
• Centrioles


  • Consisting of two rings of microtubules at right angles to each other, they organise the microtubules that make the spindle during cell division
• Vacuole


  Within plant cells there is a large central vacuole, surrounded by the tonoplast
• Vacuole


  • The main function of the vacuole is in supporting soft plant tissues, but they also store chemicals such as glucose and amino acids in the cell sap
• Cell wall


  In plants the cell wall is made largely from cellulose
• Cell wall


  • Providing strength to the cell wall, which resists the expansion of the vacuole due to osmosis, thus creating turgor and support for non-woody plants
  • Transport of water and dissolved molecules and ions through gaps in the cellulose fibres (the apoplast pathway)
  • Communication between cells via pores in the cell wall which allow strands of cytoplasm called plasmodesmata to pass (the symplast pathway)