Bring on the tough stuff
- The amount of DNA in the nuclei of the human egg and sperm cells is the same. Despite this fact, everyone received more total DNA from his or her mother than from the father. How can this be?
Hint: Compared to the human sperm cell, the human egg cell is enormous. The number of organelles in the egg cell also vastly outnumbers that of the sperm cell.
- Normally, when cells in the human body come in contact with one another, they stop dividing, a phenomenon called contact inhibition. Cells that do not stop dividing when in contact with other cells are considered cancerous. What specific structures do you think might not be working in cancer cells?
- If given only water, a plant and an animal placed in an airtight, clear, plastic box can survive for quite a while. What do these two organisms provide for each that allows them to survive for so long?
- The flagella of human sperm cells are lined with thousands of mitochondria. Why do you suppose this is the case?
- Explain how the leaves of a plant can be thought of as "eating sunlight."
- Explain why biologists claim there are more prokaryotic cells inside the human body than there are eukaryotic cells.
- Answer: Mitochondria have their own DNA, and we get a lot more mitochondria from Mom through the egg cell than from Dad through the sperm cell.
- Answer: Membrane proteins responsible for communicating and sensing the outside environment may not be working. If the nucleus does not know the cell has come into contact with another cell, it will not send out the message to stop dividing. The only way the nucleus can learn this information is from the messages it receives from the membrane proteins.
- Answer: The plant provides oxygen through photosynthesis to the animal. The animal provides CO2 through cellular respiration to the plant. With water and sunlight (the plastic box is clear), these organisms can survive for quite some time—at least until the animal gets hungry and eats the plant.
- Answer: A sperm cell must travel an incredible distance to reach the egg. The enormous amount of energy (ATP) needed to travel to the egg is generated by the many mitochondria lining the flagella. The more mitochondria, the more ATP.
- Answer: In the chloroplasts of plant cells, energy from the sun, or photon energy, is converted into sugars, which plant cell mitochondria can convert into ATP, or chemical energy.
- Answer: The human body is composed of 10–100 trillion eukaryotic cells. However, biologists estimate that there are at least 20 times this number of bacterial cells living inside the human body, especially in the digestive tract. So, many bacterial cells can live inside the body because they are so small compared to human cells.
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