Cell Organelles/Ultrastructure

• The nucleus is the most obvious of all of the organelles present in a eukaryotic cell. It is enclosed in a double membrane and communicates with its surroundings via nuclear pores.
• Within a nucleus is the DNA responsible for providing the cell with its characteristics. The DNA is similar in every cell in the body, but some genes may be turned on or off depending on the specific cell type.
• The site of transcription of DNA to mRNA

Nucleolus

• A dense body within the nucleus where ribosomes are made.

• Ribosomes are made of ribosomal RNA and protein.
• Produced by the nucleolus (giving the nucleolus its dark colour on electron micrographs)
• Found free in the cytoplasm or attached to endoplasmic reticulum.
• Ribosomes are the site of protein synthesis.

• Rough endoplasmic reticulum is a system of interconnected membrane-bound, flattened sacs.
• Ribosomes are attached to the outer surface.
• Appears pebbled by electron microscopy due to the presence of the numerous ribosomes on its surface.
• Proteins synthesized by these ribosomes are then threaded through into the endoplasmic reticulum space, where the protein is folded.
• Small vesicles, containing the proteins, emerge from the space and then transport the proteins to other parts of the cell. 

• The smooth endoplasmic reticulum is a system of interconnected membrane-bound, flattened sacs.
• Does not have ribosomes attached.
• It appears smooth on electron micrographs. 
• Plays different functions depending on the specific cell type including: 
  • lipid and steroid hormone synthesis, 
  • breakdown of lipid-soluble toxins in liver cells, 
  • control of calcium release in muscle cell contraction.

• Membrane-bound sacs with a single membrane.
• The Golgi apparatus is formed by the fusion of vesicles from the endoplasmic reticulum.
• Is important in packaging macromolecules for transport elsewhere in the cell.
• Proteins and enzymes found in lysosomes are modified and then packaged in vesicles for transport.

• Spherical sacs bound by a single membrane.
• Contain the digestive enzymes needed to breakdown unwanted structures.
  • In white blood cells, enzymes released to kill and digest bacteria.
  • Uncontrolled production of these enzymes results in cell necrosis.

• Hollow cylinders made up of a ring of nine protein microtubules (polymers or globular proteins arranged in a helix to form a hollow tube).
• Every animal cell has one pair of Centrioles.
• They are involved in the formation of the spindle in nuclear division and in transport with the cell cytoplasm

• The energy a cell needs to be able to function is provided by the mitochondria during the later stages of aerobic respiration.
• Like the nucleus, mitochondria have a double membrane, with the inner of the two membranes folded to form Cristae:
  • Cristae increase the inner membrane’s surface area.
  • It is on these cristae that sugars combine with oxygen to form ATP which is the primary energy source for the cell.

• Phospholipid bilayer containing proteins cholesterol and other molecules forming a partially permeable barrier.
  • Phospholipids consisting of hydrophobic tails and a hydrophilic heads.
  • Glycoproteins - protein molecules with polysaccharides attached.
  • Glycolipids - lipid molecules with polysaccharides attached.
• Hydrophilic molecules and ions that are larger than carbon dioxide cannot simply diffuse through the bilayer. Instead they cross the membrane via facilitated diffusion:
  • via channel proteins
  • via carrier proteins (also used for active transport)
• Sometimes very large molecules need to be transported across the membrane. This is achieved by endocytosis and exocytosis using vesicles as transport.

• Made of Cellulose.
  • Cellulose - long chains of beta-glucoses joined together with 1,4 glycosidic bonds (alternate glucose molecules rotate 180 degrees for this to be possible) in a condensation reaction.
  • Hydrogen bonds form between neighbouring cellulose chains between the -OH groups and the CH2OH side groups of each chain.
  • Lots of chains bonded together form bundles called microfibrils.
• Microfibrils arranged at different angles and held together by short, branched polysaccharide "glues" known as hemicelluloses and pectins.

• Chloroplasts are the site of photosynthesis, where energy from the sun is used to make storage molecules.
• Glucose produced in photosynthesis is stored as starch within Amyloplasts.
• They also have a large Vacuole surrounded by a membrane called the tonoplast.