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Cell Biology (Binary fission (The bacterium splits into two, The circular…
Cell Biology
Binary fission
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The cytoplasm divides and two daughter cells are produced - each daughter cell has one copy of circular DNA, but a variable number of plasmids (copies of the original)
Bacteria can divide quickly if given the best conditions (a warm environment with lots of nutrients)
Some bacteria, such as E.Coli can replicate every 20 minutes
If conditions become unfavourable, the cells will stop dividing and eventually begin to die
Differentation
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Nerve cells
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Branched connections at their ends (dendrites) to connect to other nerve cells and form a network throughout the body
Axon covered with a myelin sheath to prevent loss of electrical impulse. It also speeds up the transmission of the impulse.
Muscle cells
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Come in antagonistic pairs - one muscle will contract in one direction, while another muscle will contract in the opposite direction
Root hair cells
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On the surface of plant roots, which grow into long 'hairs' that stick out into the soil
Gives big surface area to increase efficiency for absorbing mineral ions and water through osmosis and active transport
Phloem and Xylem
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Form phloem and xylem tubes, which transport substances like food and water around plants
To form the tubes, the cells are long and joined end to end
Xylem cells are hollow and phloem have very few sub-cellular structures so water and nutrients can flow through them
Phloem tubes have living "companion cells" with mitochondria to provide the energy needed to move food up and down the plant
Xylem tubes are made out of lignin when the living xylem cells die. The tubes are strong to withstand the pressure of water moving through the tubes
Exchange surfaces
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In humans, urea diffuses from the cells into the blood plasma for removal from the body by the kidneys
Multicellular organisms
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In single celled organisms, gases and dissolved substances diffuse directly into or out of the cell across the cell membrane
This is because they have a large surface area to volume ratio, so they can take in enough substances by diffusion
Multicellular organisms have a small surface area to volume ratio so not enough substances can diffuse into them
This means they need an exchange surface for efficient diffusion, to allow enough substances to pass through
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Active transport
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Root hair cells
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The concentration of minerals is normally higher in the root cells than in the soil - diffusion won't work
Active transport allows the plant to absorb mineral ions from a very dilute solution against the concentration gradient
This is essential for growth - but needs lots of energy produced through respiration for it to take place
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