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Chapter 4: Cells - Membranes

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Energy Converting Organelles and Membranes

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Chloroplasts are where photosynthesis occurs--the conversion of light energy to energy stored in the chemical bonds of sugars. We generally associate photosynthesis with plants--but in fact, some single-celled organisms are capable of photosynthesis.


Mitochondria release energy stored in carbohydrates, fats, and other molecules.

The Cytoskeleton

The cytoskeleton is composed of all the pieces which provide structure to the cell. Don't be confused by the term skeleton--the structure is not nearly as firm as your bone-based skeleton.

If you are having trouble with the concepts in this section, you might find the Biology Project's Cytoskeleton Tutorial useful.

Cell Junctions

Some proteins exist only to hold cell walls together, like patches of velcro on your clothes. A demosome is an area on the inside of a cell wall, which has filaments that go through the cell wall and connect with the filaments of a demosome on the inside of another cell, like brass rivets. Tight junctions are made between rows of proteins embedded in the walls of two different cells. The cells connected by tight junctions form a barrier which seals off the area behind the cells, creating a division between body parts composed of the cells involved. Gap junctions let material pass between two cells through a channel, like hamsters passing from one cage to another through the habitat tunnels. Plants do not need demosomes for rigidity or tight junctions (since their cell walls are water-resistant already), and their plasmodemata perform the functions of gap junctons.

Membrane Junctions