Organelles and Cellular structures

• What is the cell theory?

  The cell is the smallest, most basic unit of life. All living organisms are composed of cells, be it unicellular or multicellular. All cells arise from pre-existing cells by cell division, through mitosis and meiosis in eukaryotic cells and binary fission in prokaryotes.
• What are eukaryotes?

  Organisms made of cells that have a true, membrane bond nucleus, also processing membrane bound organelles.
• What does a typical eukaryotic cell contain?

  Cell surface membrane, nucleus, cytoplasm
• What does a cytoplasm contain?

  cytosol, organelles and cytoskeleton
• What is the cytosol?

  The aqueous solution of ions and organic compounds (excluding organelles)
• What is the Cytoskeleton?

  A network of microtubules, intermediate filaments and microfilaments made of protein.

  it has a network of fibres permeating the matrix of living eukaryotic cells that provide a supporting framework for the cell
• structure of the cell surface membrane

  It is a single membrane found at the periphery of the cell.

  It is made of biomolecules like phospholipids, cholesterol, glycolipids, proteins and glycoproteins.

  the membrane has the phospholipid bilayer, with the hydrophobic ‘tails’ facing away from the aqueous medium and the hydrophilic ‘heads’ facing towards the aqueous medium.
• Function of the cell surface membrane

  it regulates the movement of molecules into and out of the cell; selective permeability.
• Structure of the ribosomes

  It is made of ribosomal RNA and protein, and are 20-30nm in diameter.

  Each have one small subunit made of one molecule of rRNA with 21 molecules of proteins and one large subunit made of 2 molecules of rRNA with 34 molecules of proteins.

  Each have three binding sites for tRNA; A site, P site and E site.
• Cell membranes

  • made of mainly lipids and proteins
  • the proportions will vary depending on type, location an function of the membrane
• Hydrophilic (phosphorus containing) ‘head’

  electrically charged, associate with polar water molecules
• Hydrophobic non-polar fatty acid ‘tails‘

  does not dissolve in water
• purpose of the phospholipid (cell surface membrane)

  • forms the bulk of the membrane
  • controls permeability
  • allows passage of only polar substances
  • its composition can be altered to suit the temperatures, like having more saturated chains to increase the fluidity at low temperatures
• purpose of cholesterol (cell surface membrane)

  • maintain membrane fluidity
  • restrict movement of phospholipids at high temperatures
  • interferes with overly rigid packing of phospholipids at low temperatures
• purpose of proteins (cell surface membrane)

  • aid in the transport of substances like facilitating diffusion and active transport
  • act as enzymes
  • act as receptors
• two types of ribosomes

  70S and 80S
• 70S ribosomes

  • made up of a 30S small ribosomal subunit and a 60S large ribosomal subunit
  • found in prokaryotes, chloroplasts and mitochondria
• 80S ribosomes

  • made up of a 40S small ribosomal subunit and a 60S large ribosomal subunit
  • found in eukaryotes as free ribosomes in the cytoplasm or attached to the rough endoplasmic reticulum
• subunits

  • made of proteins and rRNA (ribosomal ribonucleic acid)
  • (40S and 60S) produced and partially assembled in nucleolus, only fully assembled in cytoplasm
• Three binding sites for tRNA (transfer RNA)

  A site; aminocyl-tRNA site

  • holds the tRNA carrying the next amino acid to be added to the chain

  P site; peptidyl-tRNA site

  • holds the tRNA carrying the growing polypeptide chain

  E site; exit site

  • discharged tRNAs leave through this site
• function of ribosomes

  • site of protein synthesis
  • RER ribosomes generally make proteins that are for the inclusion into the membranes or for packaging to be sent to other organelles for the eventual export out of the cell
  • free lying ribosomes synthesise proteins to be retained within the cell
• structure of centrioles

  • measure around 0.2 nanometers in diameter and 0.3 to 0.5 micrometers in length
  • from a tranverse cut of the centrioles, 9 triplets of microtubules can be seen fused together in a rod-like structure
  • centriole are positioned at 90 degrees from their partner
  • situated next to the nucleus
• function of the centrioles

  • help to guide movement of chromosomes during nuclear division by organising spindle fibers to guide the movement of chromosomes
  • act as an anchorage for cilia and flagella
• function of cytoskeleton

  • maintains cell shape and give structural support
  • provides for cell mobility
  • some of its fibres act as supports for motor proteins, which help move things within the cell
• endoplasmic reticulum

  it is an extensive network of membrous tubules and sacs, called the cisternae.

  It has an internal space called the lumen.
• Structure of the rough endoplasmic reticulum

  • cisternae are flattened and interconnected with each other, and are continuous with the outer membrane of the nuclear envelope
  • ribosomes on the surface gives the RER a granular appearance
  • can be broken and revealed into vesicles known as microsomes during the homogenisation procedure
• Function of rough endoplasmic reticulum (ER):

  • Ribosomes associated with rough ER synthesise protein
  • Enzymes in the ER membrane modify polypeptides by attaching carbohydrates to proteins (to form glycoproteins); known as glycosylation
  • Increases surface area of the intracellular membranes for biochemical reactions
  • Forms a separate compartment allowing metabolic reactions to be localized
  • Stores secretions (like hormones)
  • Serves as an intracellular transport network between various parts of the cytoplasm and between cytoplasm and nucleus (e.g. transporting proteins to their parts of the cell by transport vesicles which are pinched off from the ER)
• structure of smooth endoplasmic reticulum


  • lacks ribosomes
  • consist only of tubular cisternae
  • has a different set of membrane-bound proteins as compared to the rough endoplasmic reticulum
  • membrane contains enzymes catalysis reactions involving lipids
• Functions of the smooth endoplasmic reticulum (SER):

  • Lipid and steroid synthesis and transport (e.g. phospholipids and steroid hormones)
  • Chemical modification of small molecules taken into the cell (e.g. detoxification of drugs and poisons by adding hydroxyl group to drugs, making them more soluble and easily flushed from bodies; drugs induce proliferation of SER and associated detoxification enzymes, increasing the rate of detoxification)
  • Secretion of chloride ions in the stomach
  • Regulates concentration of calcium ions in cells by storing and releasing calcium ions, especially in muscle cells
  • Uses the release of calcium ions as a signal to trigger a wide array of activities inside the cell, such as nerve impulse conduction
• structure of golgi apparatus

  • consist of flattened saucer-like membrane-bound stacks (cisternae) and a system of associated vesicles; continuously formed at one end and budding off at the other (golgi vesicle)
  • seen as a single large stack in animal cells and in separate stacks in plant cells
  • consist of a cis face and a trans face
• cis face of golgi apparatus

  • face nearest to the nucleus
  • new cisternae constantly formed by fusion of transport vesicles from the rough and smooth ER with golgi apparatus
  • membrane of ER vesicle becomes the membrane of the golgi apparatus
  • contents from the vesicles are passed into the lumen of the golgi apparatus
• trans face of golgi apparatus

  • face nearest to the cell membrane
  • golgi apparatus beaks up to form vesicles (like lysosomes, transport vesicles or secretory vesicles)
• function of golgi apparatus

  • chemcial modification of proteins and lipids
  • further chemical modification of existing glycoproteins and glycolipids
  • sorting and packaging
  • synthesis of macromolecules in plant cells (e.g. non-cellulose polysaccharide of cell wall)
• chemical modification of proteins and lipids (golgi apparatus)

  • enzymes of golgi apparatus attach short carbohydrate chains (oligosaccharides) to proteins and lipids to form glycoproteins and glycolipids respectively
• further chemcial modification of existing glycoproteins and glycolipids

  *carbohydrates are first added to proteins in the RER usually during the process of polypeptide synthesis.*

  the carbohydrate on the glycoprotein is then further modified as it passes through the ER and golgi apparatus, through the process of cleaving the carbohydrate chains or modifying the sugar on the carbohydrate chain
• Sorting and packaging (golgi apparatus)

  • sorted and packaged into vesicle respectively according to their identification tag
  • hydrolytic enzymes sorted and packaged together to form membrane- bound digestive vesicle
  • secretory proteins sorted and packaged into membrane of secretory vesicle to be secreted out of cell
  • membrane proteins sorted and embedded into membrane of secretory vesicle to be transported and fused with cell surface membrane
  • proteins for other organelles are sorted and packaged to be transported to target organelles
• structure of lysosomes

  • spherical sacs of 0.2-0.5 nanometer in diameter
  • contains hydrolytic enzymes (like proteases and lipases)
  • serves as a storage vesicle to keep degradation enzymes apart from the rest of the cell, preventing them from destroying the cell
  • acidic contents
  • two types of lysosomes: primary lysosome and secondary lysosome
• primary lysosomes

  • processed enzymes are contained in the golgi vesicles which later buds off from the golgi apparatus
  • enzymes are first syntheisised on the rough ER and then transported to golgi apparatus
• secondary lysosome

  • made from primary lysosomes after it fuses with certain vacuoles
• function of lysosomes

  • digestion of materials
  • autolysis
  • autophagy