Download The Cell: A Review

Nucleus, Cytoplasm, Membrane
VIDEO: http://www.teachersdomain.org/asset/tdc02_vid_nucleus/
Cells are some of the most diverse structures in the world. A cell may exist as a lone organism, freefloating and independent (as in bacteria or algae), or a cell may be one small part of a complex,
integrated system (human cells). In a multicellular organism, cells can vary widely in size and structure.
That's because the structure of a cell is suited to its function, and different types of cells have different
functions. For example, nerve cells have long thread-like projections that receive and transmit electrical
signals throughout the body. Skin cells, on the other hand, look somewhat like roof shingles and
perform a similar protective function for the body.
For all their differences, however, most cells have three critical parts in common:
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the nucleus
the cytoplasm
and the membrane
The nucleus is arguably the most important structure for many cells. While some single-celled organisms
including bacteria have no nucleus (their single chromosome floats freely in the cytoplasm), nearly all
other cells do. The nucleus contains the cell's DNA. This genetic material provides the instructions for
building proteins and, thus, dictates the structure and function of the cell throughout its life. Even more
important, the DNA provides a means of passing genetic information to the next generation of cells.
Through mitosis, cells replicate their DNA and then pass these complete sets of genetic material to their
offspring, the daughter cells created when they divide.
The fluid-filled region between the nucleus and the cell membrane is called the cytoplasm. For many
years scientists thought that very little happened there; they couldn't have been more wrong. Despite
its modest appearance, cytoplasm provides the stage for some of the cell's most important functions,
including protein synthesis, energy production, and cell growth and replication. This is because
cytoplasm is packed with organelles that are critical to a cell's survival, including mitochondria,
ribosomes, Golgi bodies, and the endoplasmic reticulum.
In some types of cells, cytoplasm is also used to move the cell itself or to move organelles within the cell.
In plants this process is called cytoplasmic streaming. It allows plants to circulate their choroplasts and
position them for optimal exposure to sunlight. Amoebas and certain types of white blood cells use a
similar process, squeezing their cytoplasm in one direction or another as a means of locomotion.
Finally, the cell membrane gives the cell its structure. However, it has another function that is far more
complex. In order for a cell to survive, it needs to take in water, food, oxygen, and other materials. It
also needs to expel excess water, wastes, and the substances it produces that other cells in the body
may need. The membrane controls what materials pass into and out of the cell.
Review Questions:
1. Why do you think that cells producing large numbers of proteins have a greater number of rough
endoplasmic reticula (look it up) than those producing fewer proteins?
2. How does cell structure suggests cell function?
3. How are substances and structures transported within the cell?