Cell Processes

Instructor's Guide

Explores the secret life of cells

Cell Processes

These notes follow the video through, commenting further on the cells and organisms seen and they suggest some simple practical demonstrations suitable for the lab or class room.


The opening introductory sequences put the cell into a familiar context and show why cells are difficult to study. The main problem is their size - the usual way to study cells is with the microscope. Most microscopic systems generate an image by shining light through a specimen and magnifying the image produced. Simply shining light on the surface of an opaque object is very disappointing, as generations of children discover on trying out their toy microscopes for the first time.

Slicing most tissues thin enough to see through them is usually impossible unless the material is dead and infiltrated with a substance like wax or an epoxy resin that is hard enough to slice thinly. Much larger cells are available if one knows where to look. The green alga Chara, common in ponds and streams, has single cells up to several centimetres in length, and even with a low power microscope these reveal their internal living, streaming cytoplasm (shown later in this film). Other algae with very large cells are common in many shallow marine environments such as rock pools. Very thin tissues of animals such as the tadpole tail reveal blood flow with low power microscopes, but some delicacy is needed in handling the animals.

The video introduces cell sectioning to emphasise how the appearance of cells changes with the plane and orientation of sectioning. The first pictures of tissues show largely empty cells, typical of older plant tissues which when sectioned show beautiful patterns formed by the cell walls. Even living plant cells often appear "empty", while being full of water - see the cells of celery stem later in the video.

The flatworm is Mesostoma..

LATER: it is best to peel the epidermis off leaves like those from Tradescantia under water. Leave the peels in water to recover before putting them on a slide under the microscope. The clusters of cells are stomatal complexes. The transparent (fragile!) skin from the surface of the layers in a red onion (see also section on vacuoles below) also gives beautiful specimens of living cells. The sequence after the amoeba contains many different euglenoid flagellates (from a particularly putrid pond!). The beautiful spheres of Volvox are visible to the naked eye, and are quite common in plankton tows of clean lake water.

CELL MEMBRANE: This section starts with a rare, unidentified amoeba, then follows cultured cells of newt lung tissue, favourites of the microscopist because they are very large and flatten against the slide. These cells star throughout the video. The smaller cells are cultured mouse epithelial cells. The very large pulsing cell, speeded up by time-lapse, is the slime mould Physarum. A little later is an antheridium and spiral oogonium of the alga Chara (note the streaming); then a Tradescantia stamen hair and a diatom (alga).

CYTOPLASM: cultured newt cells. The motion of the cell margin, called "ruffling", is an actin-based movement common in many animal cells.

MITOCHONDRIA: cultured newt cells and PtK (mammalian) cells.

CHLOROPLASTS: To see chloroplasts in living cells: place a small, pale green leaf of Impatiens (the common garden flower) in water and then aspirate under a vacuum; over about 30 mins. bubbles will appear as air expands through the stomata. Then remove the vacuum slowly. Water will then fill the air spaces in the mesophyll. Now mount the leaf under water: the tissue should be transparent while cells and cellular activity should be clear. The red alga releasing spores (carpospores) in Spyridia; the next (red filaments) is Erythrocladia and the bluish-green one is Chroodactylon.

ENDOPLASMIC RETICULUM: cells of epidermal peel from an onion prepared as above.

GOLGI BODIES: the first transmission electron micrograph is from frozen mouse epididymal cells; next is by a transmission electron micrograph of a golgi body in the green alga Oedogonium. The live green algal cell is Micrasterias followed by the diatom Surirella. During pinocytosis, in mouse macrophages and newt cells, watch carefully to see the vesicles forming at the cell margin. The feeding protozoan is probably Actinophrys. The alga secreting tine plates, called scales, is Pleurochrysis, and the scanning micrographs are of scales from the closely related Coccolithus.

VACUOLES: Celery stems are, of course, familiar and easy to slice up. The cells have huge vacuoles and like many other plants cells, can be likened to water-filled balloons. Healthy cells of Chara (mentioned earlier) are rigid because of turgor pressure inside their large vacuole and they collapse dramatically when punctured. The following simple experiment, shown in our video disc Living Cells, demonstrates the importance of turgor pressure. Place some filaments into fairly concentrated sugar in water. After about 5 mins, the cells completely lose their rigidity. Place them carefully in fresh water; as the sugar diffuses away, the cells inflate like miniature balloons.

NUCLEUS and MITOSIS are mostly in newt cells.

CYTOSKELETON: the actin and microtubule cytoskeletons were stained by fluorescently-labelled antibodies. The streaming cells are Tradescantia and Chara, followed by an amoeba. The brilliantly coloured pulsing cells are chromatophores in scales of fish. Then there are several cell division sequences, including the euglenoid flagellate Phacus, several desmids (complex, symmetrical green algae) and Volvox releasing baby colonies. A mammalian egg cell (mouse) is surrounded by sperm; this is followed by a cleaving embryo of a starfish; the final embryo is that of the zebrafish.

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