Model organisms is your answer
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The heart’s electrical system is responsible for making and conducting signals that trigger the heart to beat. These signals cause the heart’s muscle to contract. With each contraction, blood is pumped throughout the body. This happens 24/7 without us having to monitor it all the time so that our heart does not stop.
A leech is an example of a parasite
Answer:
A Comparison of Cells Commonly Seen in Lab
Bacteria: prokaryotic. Very small. No nucleus, no chloroplasts, no mitochondria. Do have a cell wall.
Plant cells: eukaryotic. Relatively large. Have a nucleus, have mitochondria, sometimes have chloroplasts. Have a large central vacuole and a cell wall.
Animal cells: eukaryotic. Relatively large. Have a nucleus and mitochondria. Never have chloroplasts. Lack a cell wall, and have no central vacuole.
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This Elodea leaf cell exemplifies a typical plant cell. It has a nucleus, and a stiff cell wall which gives the cell its box-like shape. The numerous green chloroplasts allow the cell to make its own food (by photosynthesis).
The central vacuole takes up most of the volume of the cell. It is transparent, but you can see where it's pressing the chloroplasts up against the cell wall, especially at the ends of the cell.
Like animal cells, the cytoplasm of this plant cell is bordered by a cell membrane. The membrane is so thin and transparent that you can't see it, but it is pressed against the inside of the cell wall.
This cell was alive and at 1000x magnification when it was photographed.