lecture 4; major components of the cell

endosymbiosis theory

Lynn Margulis’ theory

• two different cell types combined (bacterial cell taken up inside an archaea cell)

  • chloroplasts

  • mitochondria

evidence of endosymbiosis theory

• Look inside the cases of endosymbiosis and check what the DNA looks like (linear or circular)

  • Chloroplasts and mitochondria have their own DNA and its a single circular chromosome

• Prokaryotes and eukaryotes have differences in their cell membranes;

  • If one cell takes up another, the cell that was taken up (will be present in membrane) within the cell that was doing the taking up

  • Chloroplasts and mitochondria both have double membranes (the composition of the intracellular membrane is much more like a prokaryote membrane)

  • The outer membrane has the characteristics of a eukaryotic cell membrane

endosymbionts; mitochondria

• different types of cells have different numbers of mitochondria; usually correlates with the energy requirement of the cell

• main function: production of ATP—> ‘powerhouse of the cell’

• outer membrane: tends to match in composition with eukaryotic cell

• inner membrane: prokaryotic-like membrane

  • contains special characteristics that are specific to mitochondria

• convoluted shape: increases the surface area of the inner membrane

  • important because the more SA the more space the mitochondria has to carry out the reactions to generate energy

  • different enzyme complexes in this membrane (e.g. ATP synthase)

• circular chromosome: genes that are encoded on this chromosome encode key requirements for energy production

• most of the mitochondria is constructed using genes that are in the nucleus

• over the period of evolution, the original cell that was endosymbiot would have had a complete genome with all the information needed to make mitochondria, but over the evolutionary time, many of these genes have been transferred to the nucleus

endosymbionts; chloroplasts

• main function: photosynthesis

• found in plant cells

• gives the cell green colour pigment chlorophyll

• within the chloroplasts there are enzymes which fucntion to produce sugar

  • sugar isn’t enough to produce energy by itself

  • It is a substrate that can be used by other processes such as by the mitochondria  to convert the sugar into energy sources such as ATP

• has an outer and inner membrane

• has its own circular DNA chromosome

• has thycoloid membranes where photosynthesis is carried out

nucleus

• Contains all of the genetic material except for the endosymbose organelles' (e.g. Mitochondria and chrloroplasts)

• Nucleolus; darker fragment

• Ribosomal RNA (rRNA) is made here (a component of ribosomes)

• Nuclear pore complex:

  • The nucleus is surrounded by a membrane that blocks access to the nucleus except through the nuclear pores

  • Controls transport of things into and out of the nucleus

ribosomes

• Closely associated with the nucleus

• mRNA is transcribed within the nucleus and then transported out of the nucleus to the ribosomes

• Job: synthesise proteins based on the information carried by the mRNA

• DNA -> mRNA -> protein

endoplasmic reticulum (ER)

• Proteins enter the ER using signal sequence of amino acids

• Wrapped around the nucleus

• Very closely associated with the nucleus because it is very closely associated with the production of proteins

• Inside of the ER: lumen

• Rough ER: has ribosomes attached

• Smooth ER: no ribosomes attached

• biosynthetic factory: lipids, glycoproteins etc.

smooth ER

functions

• metabolism of carbohydrates

• lipid production

• phospholipid production

rough ER

functions

• when the ribosomes translate mRNA to a protein, it can be directly transported into the cell

• processes result in the production of sugar molecules or lipids is protected from the rest of the cytoplasm

  • allows for the production of more

  • complicated products

the endomembrane system

• A system of internal, membrane-bound compartments within the cell that can form physical links to exchange components

  • Nuclear envelope (nucleus)

  • Endoplasmic reticulum

  • Golgi apparatus

  • Lysosome and vacuoles

• Continuous system

• Pieces of the endomembrane can break off into vesicles (little spherical compartments)

  • Can go to the plasma membrane and fuse and cause the contents to go outside the cell, or can travel to specific locations within the cell and deposit their material there, or go to the Golgi Apparatus

endomembrane system; golgi apparatus

• ‘The postal service’

• Broken into two areas:

  • Cis golgi; next to (same side) as the ER

  • Trans golgi: far side (opposite) as the ER

• Not continuous with the ER

  • Breaking off of materials is thus necessary into the vesicles so they can travel from the ER to the golgi apparatus and deposit the contents inside the luma

  • Contents can then be modified

  • Once the contents have matured, a new vesicle can then be formed to transport this material to other parts of the cell (fuse with another organelle or outer plasma membrane)

endomembrane system: lysosomes

• ‘Waste recycling centre’

• Vesicles full of hydrolytic (digestive) enzymes

• A highly acidic environment

• Phagocytosis: 'food' from outside the cell is engulfed

• Macrophages are white blood cells that engulf bacteria

• Autophagy: breaking down damaged organelles for recycling

• A human liver cell recycles half its macromolecules each week

endomembrane system: vacuoles

• can control large spaces within the cell

• Storage compartments

• Can contain food or water

• Provide good example of why eukaryote cells want compartments

  • E.g. acid hydrolase enzymes can digest cell components; it would not be good for the cell if these got out and moved freely throughout the cell, or these enzymes would digest the cell itself

endomembrane system: cytoskeleton

• unique to eukaryotes

• support and motility

• three different support structures summary:

endomembrane system; cytoskeleton; support structures; microtubules

• Formed by sets of a dimer (two proteins that always go together)

  • Dimer; each side of the dimer will have different properties

• The alpha side is able to grow in length

• The beta side is less prone to being able to group

• The side of the microtubule that can grow quickly is the + side

• The side of the microtubule that cannot grow as quickly is called the - side

  • The - side of the microtubule starts at the microtubule organising centre (centrosome--> type)

  • Then these molecules are able to rapidly grow or contract based on the local concentration of tubulents

endomembrane system; cytoskeleton; motility

• Microtubules in cilia and flagella

• More rigid structures that can be used for motility

• Flagella:

  • One or a few tails that propel the cell forward

• Cilia:

  • Many cilia (shorter than flagella) cover the cell, beat back and forth

  • 9 + 2 configuration

  • radio spokes that connect the fixed parts of the flagella

  • dynein; protein used for movement

  • One side of the dynein is fixed and the other side can move

  • One side will move up and then the other side will move up, creating a angulating motion