Endosymbiosis Lab
Plastids and Mitochondria are organelles in eukaryotic cells that provide scientists with some of the most compelling evidence for
Endosymbiotic theory…the idea that larger cells evolved from smaller cells through a process of engulfing and assimilating them. Under the
microscope, these organelles look very much like bacteria(especially mitochondria) in there size, shape, membrane structure, and process of
division. Unbeknownst to the naked eye, these endosymbionts(plastids/mitochondria) contain their own ribosomes and DNA that is separate
from the genome of the larger organism within which they dwell. This unique DNA is also stored as a single circular genome just like
bacteria…how interesting.
Mitochondria are the power plants of ALL eukaryotic organisms(even plants!!!). They have the ability to turn glucose into the energy currency
of life: ATP. Without mitochondria, almost every living thing you can see would not be able to survive. These organelles are matrilineal.
This means they are introduced to the zygote by the mother’s egg cell. Some scientists have used this phenomenon to create family trees based
on mitochondrial DNA. Mitochondria have the most in common with the Proteobacteria, a phylum of Eubacteria that are very difficult to
culture in a lab due to the fact that they are usually found in symbiotic relationships with other prokaryotes. In our cells, these literally look
like motile bacteria dancing/trembling within the cytoplasm.
Plastids are a group of endosymbionts, like mitochondria, that have a great deal in common with bacteria. In plants and algae, plastids start off
as proplastids that can differentiate into a number of different classes of plastids:
chloroplasts- site of photosynthesis
elaioplasts- store fat molecules
amyloplasts-store starches
chromoplasts- color pigment producers
proteinoplasts-store proteins
Photosynthetic plastids actually have separate evolutionary lineages: plants- chloroplasts red algae-rhodoplasts glaucophytes(type of
unicellular fresh water algae) -cyanelles. Presently, many complex algae will undergo secondary endosymbiosis when they engulf a red or
green algae or cyanobacteria and retain them to harness the photosynthetic byproducts. Eventually the kleptoplastid is digested after it has been
used for some time.
Potato(amyloplasts) labels: cytoplasm, cell wall, nucleus, cell membrane, amyloplasts
-Using a razor blade, cut a paper thin sliver of potato and wet mount it on a slide using iodine(not water)
-Examine the potato cells at 100X and 400X. Draw, measure, and label one potato cell.
-The iodine reacts with amylase starch to form a dark purple/black color. When you look at the potato cells you
should see clumps of round, larger bacteria sized organelles that are dark purple/black in color. These are amyloplasts.
-This is not easy. In particular, getting a thin enough sliver of potato…try, try again.
Tomato(chromoplasts) labels: cytoplasm, cell wall, nucleus, cell membrane, chromoplasts, mitochondria
-Carefully pull some skin off of the tomato. Do your best to make sure there is very little pulp attached to the skin.
Remove excess pulp by scraping with your fingernail or a razor blade.
-Place the skin with its outer surface facing down on the slide. Nearby, place a small sample of pulp.
-Using a single coverslip make a wet mount of the skin and pulp. You may have to squish the pulp with your coverslip.
-Compare the skin cells to the pulp cells. Draw, label, and measure a skin cell AND a pulp cell.
-Look for chromoplasts and mitochondria. Chromoplasts are larger and red colored. Mitochondria look like bacteria.
Onion(osmosis/plasmolysis) labels: cytoplasm, cell wall, nucleus, cell membrane,
-Using the method Mr. Holmes showed you, tease some onion skin off of your piece of onion.
-Place the onion skin with its outside surface facing down on your slide. Make a wet mount using tap water.
-Observe the onion cells at 100X and 400X. Pay particular attention to where the cell membrane and cell wall meet.
-Next, draw salt water under the cover slip using the method Mr. Holmes showed you.
-Observe the onion cells again to look for changes that have occurred.
-Draw, measure, and label an onion cell before AND after the addition of salt water.
Elodea(chloroplasts/ streaming) labels: cytoplasm, cell wall, nucleus, cell membrane, chloroplasts, mitochondria
-Pluck a fresh bright green(new growth) leaf from the Elodea stalk. Make a wet mount of the leaf.
- Observe the leaf at 100X and 400X. You should see cells with numerous chloroplasts within…maybe mitochondria.
-Draw, measure, and label one elodea cell. Also attempt to draw cytoplasmic streaming of the chloroplasts.
-Let your leaf sit on your light source for 5-10 minutes. The light and heat from the lamp(light source) will help get the
cytoplasmic streaming started. Ensure that your slide doesn’t dry out by adding water when necessary.
-After time has passed, observe the leaf again. You should see the chloroplasts moving in a circular motion around the
perimeter of the cell wall. The cells in the central vascular area tend to start streaming earlier.
Celery(mitochondria/stomata) labels: cytoplasm, cell wall, nucleus, cell membrane, chloroplasts, mitochondria
-Tease off some skin like you did for the onion.
-Place the skin with its outer surface face down on the slide. Make a wet mount using sugar solution.
-First look for stomata…they look like eyelids. These allow a plant to breathe and transpire water.
- Now, look for mitochondria(look like bacteria inside the cytoplasm of the celery cells…faint trembling)
-Draw, measure, and label one stomata and one celery cell.
-Draw Janus Green B stain under the cover slip. Observe the mitochondria again…note their color change.
-Over the next fifteen minutes, watch for another color change.