4

Animations
(Animation Requirements)

• Four Animations

Art Labeling Activities

• Thirteen Art Labeling Activities

Explorations

Cell Size
This interactive exercise allows you to explore the architecture of a cell, observing how changes in cell size and shape affect ease of access to the cell interior.

Active Transport
This interactive exercise allows you to alter ATP concentrations and relative concentrations of substances and then explore transport across a membrane.

Cytoskeleton Interactions. One of the most exciting developments in cell biology toward the close of the last century was the revelation that a cell's interior is highly organized by a complex of fibers and tubes called the cytoskeleton. Some of the fibers serve structural roles, like the steel girders of a building, while others guide the movement of materials through the cell, much as a system of rails guides the movements of railway trains around the country. This transport function of the cytoplasm plays a key role in the development of a fruit fly, as it allows molecular signals to pass from the nucleus across the cytoplasm to the back end of the egg. The positioning of these molecular signals there determines the future axis of the body. How is this positioning achieved?

1. How does a fruit fly label its rear end, and why should we care?
2. Nobel Prize winner figured out how a cell's proteins decide where they will go.

Cystic Fibrosis: A Membrane Disorder
Cystic fibrosis is the most common fatal gene disorder of Caucasians. Every year about 1 in 2,500 American infants is born with the disorder. Their body cells secrete a thick mucus that clogs the airways of the lungs and blocks the ducts of the pancreas and liver. Cystic fibrosis is usually thought of as a children's disease because until recently few affected individuals lived long enough to become adults. Even today half die before their mid-twenties. There is no known cure.

Cystic fibrosis results from a defect in a gene encoding a plasma membrane protein. Called CFTR (cystic fibrosis transmembrane conductance regulator), this protein channel through the membrane regulates passage of chloride ions into and out of the body's cells. A defective CFTR channel leads to a buildup of chloride ions within lung, pancreas, and liver cells, causing water to move into the cells by osmosis. Removing water from the surrounding mucus causes it to thicken, clogging the passageways. Attempts are underway to cure cystic fibrosis with gene therapy, using a virus to ferry healthy CFTR genes into patients lacking them. While much remains to be done, the future is bright.

How Do the Cells of a Growing Plant Know in Which Direction to Elongate?
Sometimes questions that seem simple can be devilishly difficult to answer. For example, did you ever wonder how the individual cells within a blade of grass know in what direction to grow to keep the blade growing up and not out?

This question has been addressed experimentally in a simple and direct way in the laboratory of Richard Cyr at Pennsylvania State University. Plant cells conduct mechanical force well from one cell to another, and Carol Wymer (then a graduate student in the Cyr lab) suspected some sort of mechanical force was the signal guiding cortical microtubule alignment. Cortical microtubules lie just beneath the cell membrane and control the direction of cell elongation by forming a template for building the cell walls. Wymer set out to test this hypothesis using centrifugation. If cortical microtubules are obtaining their positional information from an applied force, then their realignment should be affected by centrifugal force. Also, chemicals that disrupt microtubule formation would interfere with the realignment of microtubules by the centrifugal force.