Cell Structure and Mitosis

• Ultrastructure of eukaryotic cells

  All organisms are divided into two different domains: eukaryotes and prokaryotes. Eukaryotes include any organism whose cells contain a nucleus, while prokaryotes lack a nucleus and any other membrane-bound organelles.
• Eukaryotic cells:

  
• Eukaryotic cells


  Cells of animals, plants and fungi
• Organelles found in eukaryotic cells

  • Nucleus
  • Nucleolus
  • Nuclear envelope
  • Rough endoplasmic reticulum (RER)
  • Smooth endoplasmic reticulum (SER)
  • Golgi apparatus
  • Ribosomes
  • Mitochondria
  • Lysosomes
  • Chloroplasts
  • Plasma membrane
  • Centrioles
  • Cell wall
  • Flagella
  • Cilia
  • Vacuole
• Nucleus


  • Contains DNA which controls the activities of the cell by containing the base sequences (the 'instructions' needed to make proteins)
  • DNA is associated with histone proteins and referred to as chromatin which is wound into structures called chromosomes
• Nucleolus


  Region within the nucleus where ribosomes are made
• Nuclear envelope


  • A double membrane which surrounds the nucleus
  • Contains pores which allows small molecules (like single stranded RNA) to pass into the cytoplasm but keeps hefty chromosomes safely inside its walls
• Rough endoplasmic reticulum (RER)


  • An extension of the nuclear envelope and is coated with ribosomes
  • Facilitates protein synthesis by providing a large surface area for ribosomes
  • Transports the newly synthesised proteins to the Golgi apparatus for modification
• Smooth endoplasmic reticulum (SER)


  • Synthesises lipids including cholesterol and steroid hormones (such as oestrogen)
• Golgi apparatus


  • Made up of a group of fluid-filled membrane-bound flattened sacs surrounded by vesicles
  • Receives proteins from the RER and lipids from the SER
  • Modifies the proteins and lipids and repackages them into vesicles
  • The site of lysosome synthesis
• Ribosomes


  • Responsible for the translation of RNA into protein (protein synthesis)
  • Either float freely in the cytoplasm or are stuck onto the rough endoplasmic reticulum
• Mitochondria


  • Site of ATP production during aerobic respiration
  • Self-replicating so can become numerous in cells with high energy requirements
  • Contains a double membrane with folds called cristae, which provides a large surface area for respiration
• Lysosomes


  • Phospholipid rings which contain digestive enzymes separate from the rest of the cytoplasm
  • Engulf and destroy old organelles or foreign material
• Chloroplasts


  • The site of photosynthesis
  • Enclosed by a double membrane and has internal thylakoid membranes arranged in stacks to form grana linked by lamellae
  • Found only in plants and certain types of photosynthesising bacteria or protoctists
• Plasma membrane


  • Consists of a phospholipid bilayer with additional proteins to serve as carriers
  • Contains cholesterol to regulate membrane fluidity
  • Holds the cell contents and holds the cell together, whilst controlling the movement of substances into and out of the cell
• Centrioles


  • Bundles of microtubules which form spindle fibres during mitosis in order to pull sister chromatids apart
  • Important for the formation of cilia and flagella
  • Not found in plant and bacterial cells
• Cell wall


  • A rigid structure made of cellulose (in plants), chitin (in fungi) and murein (in prokaryotes) which provide support to the cell
• Flagella


  • A tail-like structure which are made up of bundles of microtubules
  • The microtubules contract to make the flagellum move and propel the cell forward
  • Found on sperm cells, which use it to swim up the fallopian tubes to fertilise the egg cell
• Cilia


  • Finger-like projections found on the surface of some cells
  • Also contain bundles of microtubules which contract to make the cilia move
  • Found on epithelial cells lining the trachea and move to sweep mucus up the windpipe
• Vacuole


  • An organelle which stores cell sap and may also store nutrients and proteins
  • Helps to keep plant cells turgid
  • Some vacuoles can perform a similar function to lysosomes and digest large molecules
• Protein production


  1. Translation of mRNA into a polypeptide chain

  2. Polypeptide chain folded at the rough ER

  3. Polypeptide chain transported to the Golgi apparatus inside vesicles

  4. Protein modified and processed by enzymes at the Golgi

  5. Protein packaged inside a vesicle and transported to where it is needed
• Ribosomes


  • Floating alone in the cytoplasm or attached to the rough ER
• Carbohydrate chain


  Added to the surface of the protein
• Sulfate or phosphate group


  Added to the protein
• Delivery of carrier protein


  1. Vesicle delivers the protein to the plasma membrane

  2. Protein incorporated into the plasma membrane
• Prokaryotes and eukaryotes


  • Share some of the same organelles (cytoplasm, cell membrane, ribosomes)
  • Have some important differences
• Prokaryotes


  • Have no membrane-bound organelles (so no mitochondria, Golgi, endoplasmic reticulum, nucleus etc.)
  • Their DNA floats freely in the cytoplasm
• Prokaryotic DNA


  • Consists of a single circular chromosome
• Eukaryotic DNA


  • Is linear and wrapped around chromosomes
• Prokaryotes


  • Have extra bits of DNA in the form of small circular plasmids
• Prokaryotic ribosomes


  • Are smaller (70S) compared to eukaryotic ribosomes (80S)
• Eukaryotes like plants and fungi


  • Have cell walls made of cellulose and chitin
• Bacterial cell walls


  • Are made of murein (a type of glycoprotein)
• Prokaryotic cells


  • Are much smaller than eukaryotic cells
• Flagella


  • In prokaryotes are made of a protein called flagellin
  • In eukaryotes are formed from microtubules
• Prokaryotic cell:

  
• Prokaryotes


  • Have some organelles that are absent from eukaryotic cells
• Pili


  Hair-like structures which stick out from the plasma membrane, used to communicate with other cells (including the transfer of plasmids between bacteria)
• Mesosome


  A folded portion of the inner membrane, some scientists believe it plays a role in chemical reactions such as respiration, other scientists doubt whether it even exists and think it may just be an artefact produced during the preparation of bacterial samples for microscopy
• Plasmids


  Small, circular rings of DNA which are separate from the main chromosome, house genes which are not crucial for survival but might prove useful - such as antibiotic-resistance genes, can replicate independently from the main chromosomal DNA