Cell biology

• Prokaryotic cells


  Lack internal membrane-bound organelles
• Eukaryotic cells


  Have a prominent nucleus and numerous membrane-bound organelles

  
• Organisms


  • Prokaryotes
  • Eukaryotes
• Bacteria (kingdom Prokaryota) are prokaryotes
• Protista, Fungi, Plantae and Animalia are eukaryotes
• Cell size


  • Most cells fall within a range of 1- 100 µm in diameter
  • Prokaryotic cells are smaller than eukaryotic cells (prokaryotic cells 1-10 µm, eukaryotic cells 10 -100 µm)
  • Cells are so small because their function limits their size
• Exchange of materials across the cell membrane in single-celled organisms

  1. Happens mostly by diffusion

  2. Passive process (does not use energy)

  3. Relatively slow process
• As single cells grow larger

  The contents increase in volume, and it will take a longer time for substances to reach the cell membrane and pass to the outside of the cell
• For this reason, single-celled organisms must remain small so that substances will not take a long time to move into and out of the cell
• Surface Area to Volume ratio (SA:V)

  The greater the surface area relative to the volume, the faster diffusion across the surface of the cell takes place
• As a cell grows larger

  • It absorbs more nutrients, oxygen and water from outside
  • It produces more carbon dioxide and waste products
  • The distance from the centre of the cell to the cell membrane increases
• SA does not increase proportionally with volume and therefore, it will limit the size of the cell
• Cell shapes


  • Rod-shaped cells will have more surface area compared to their contents than spherical ones
  • Cells bearing microvilli
  • Having many small cells rather than a few large ones
• Organelle


  A biological structure that performs a distinctive function inside a cell

  
• Advantages of eukaryotic organization over prokaryotic organization

  • Having membrane-bound organelles allows each organelle to perform a different function efficiently - compartmentalization allows division of labour between organelles
  • Having DNA enclosed in a nucleus allows the DNA to be protected and so less likely to be damaged, and allows reproduction to be a lot more controlled
  • Having a cytoskeleton means that the cell is more supported, can support organelles and can allow the cell to move and take up specific shapes, and allows cell division to occur by mitosis and meiosis
• Nucleus


  Largest organelle (diameter 5µm), spherical or ovoid in shape, typically found in the centre of animal cells, present in all cells except red blood cells and phloem sieve tube cells, of prime importance as it stores genetic information in the form of DNA

  
• Components of the nucleus

  • Nuclear Envelope
  • Chromatin
  • Nucleolus

  
• Nuclear Envelope


  A double membrane that separates the nucleus from the rest of the cytoplasm, consisting of an outer membrane continuous with the endoplasmic reticulum and an inner membrane, perforated with nuclear pores that allow communication between the nucleus and the cytoplasm

  
• Chromatin


  Consists of coiled DNA bound to basic proteins called histones, can be found in two forms: heterochromatin (tightly coiled, stains deeply, contains inactive DNA) and euchromatin (loosely coiled, stains lightly, contains active DNA)
• Nucleolus


  Appears as a rounded, darkly stained structure inside the nucleus, its function is to synthesise ribosomes
• The nucleus contains DNA which consists of a large number of genes which together control the cell's activities, plays an important role in cell division and reproduction
• Endoplasmic Reticulum (ER)


  An interconnected network of membrane vesicles called cisternae in the cytoplasm, allows the passage of molecules through the interior of the cell
• Types of Endoplasmic Reticulum

  • Rough Endoplasmic Reticulum (RER)
  • Smooth Endoplasmic Reticulum (SER)

  
• Protein synthesis and transport in the RER

  1. The growing protein is bound to the ribosome until its synthesis is complete

  2. A receptor on the membrane of the RER provides a channel through which the protein can pass into the ER

  3. The protein is transported through the cisternae, undergoing structural and chemical modification

  4. After modification, the proteins are packaged into transport vesicles

  
• Functions of the RER

  • Supports ribosomes for protein synthesis
  • Transports proteins
  • Modifies proteins
  • Assembles lipids into bilayers to form membranes

  
• Smooth Endoplasmic Reticulum (SER)

  Regions of the ER with very few or no bound ribosomes, contains many embedded enzymes important for various cellular functions

  
• Proteins in the RER

  May be structurally and chemically modified, altering their function and destination

  
• Types of proteins synthesised by RER

  • Glycoproteins (proteins formed by the addition of a carbohydrate chain)
  • Secretory proteins (function outside the cell e.g. digestive enzymes, hormones)
  • Structural proteins (e.g. Membrane proteins)

  
• Formation of membranes


  1. The RER assembles lipids into bilayers from precursors found in the cytoplasm

  2. Protein molecules are embedded into these lipid bilayers
• Amount of RER in a cell


  Reflects how busy the cell is synthesising proteins for export
• Smooth Endoplasmic Reticulum (SER)

  Regions of the ER with very few or no bound ribosomes

  
• Enzymes in the SER

  • Catalyse synthesis of a variety of carbohydrates and lipids
  • Involved in detoxification in liver cells
  • Involved in the conversion of glycogen to free glucose in liver cells
  • Involved in chemical modification of proteins produced on the RER
  • Form a specialised structure called the sarcoplasmic reticulum which plays an important part in muscle contraction
• Ribosome


  • Found in large numbers throughout the cytoplasm, concentrated in the region around the RER
  • Their function is protein synthesis

  
• Types of ribosomes


  • Free ribosomes (suspended in the cytoplasm, synthesise proteins needed within cell)
  • Bound or attached ribosomes (bound to the RER membranes, synthesise proteins that are included in membranes or secreted out of cell)

  
• Structure of a ribosome

  • Consists of two subunits, one large and one small
  • Each sub-unit is composed of ribosomal RNA (rRNA) bound within a complex of several proteins
  • The two subunits only come together during protein synthesis, in the presence of a messenger RNA (mRNA) molecule

  
• Polyribosome or polysome


  Chain of ribosomes actively synthesizing a particular protein

  
• Golgi apparatus


  • Consists of a stack of flattened membrane-bound sacs (cisternae) together with a system of associated vesicles (small sacs) called golgi vesicles
  • Inside the golgi, proteins are processed, modified and sorted, then packaged into vesicles for transport to their destination

  
• Cis-face of the Golgi

  Originates close to the endoplasmic reticulum, receives the proteins released from the ER

  
• Trans-face of the Golgi

  Ends near the plasma membrane, vesicles pinch off from this end of the Golgi apparatus
• Mode of action of the Golgi apparatus

  1. Proteins synthesised on the RER are transported in vesicles to the cis-face

  2. Vesicles fuse with the membrane of the cis-face and their contents are released into the Golgi

  3. Proteins are moved in golgi vesicles between the cisternae as they are modified and/or packaged

  4. Vesicles leaving the trans-face either join to other organelles or to the plasma membrane, releasing their contents by exocytosis