Questions for Stimulating Inquiry
Answers and Discussion
The differentiated cells are human blood (red blood cells and granulocytes), Elodea cells, and human cheek cells. Red blood cells carry oxygen to tissues; granulocytes are a category of white blood cells that fight infection; the Elodea cells are from the leaf of an aquatic plant and contain sites of photosynthesis; and human cheek cells are a type of epithelial cell that lines mucous membranes and protects underlying tissue. The amoeba is a single-celled protist that does not differentiate further; however, it is highly specialized for functions such as movement, engulfing food, and sensing environmental cues, so that it also can be considered as a differentiated cell.
Nuclei are visible in the sea urchin embryo, stem cells, and cheek cells. The nucleus of a eukaryotic cell is a membrane-bound structure containing nearly all the cell’s genetic material (DNA).
Microtubules, seen as faint striations radiating from the two nuclear regions in each of the two dividing cells in the sea urchin embryo, are hollow rods of the protein tubulin and are found in cilia, flagella, the cytoskeleton, and the mitotic spindle, visible here during the embryo’s second mitotic division.
Chloroplasts, visible as green inclusions in the Elodea cells, are the site of photosynthesis in plant cells and photosynthetic protists.
Cell walls, clearly delineating the outline of each cell in the Elodea image, are found in plant cells, fungi, and some protests, as well as in bacteria and archaeia. They are composed of polysaccharides and proteins. This rigid structure is located exteriorly to the plasma membrane and functions to protect the cell, maintain its shape, and aid in cellular homeostasis.
Food vacuoles, visible as red spots in the amoeba, are formed when the animal encounters prey, engulfs it by endocytosis and fusion, and creates a vacuole. Enzymes flood the vacuole and catalyze digestion of the food particle. The pH-sensitive dye used to stain the amoeba becomes dark red in the acidic environment of the food vacuoles.
Form and function relationships can be seen in a number of the images, including the following:
The donut-like shape of red blood cells maximizes their surface area for gas exchange. This shape also confers flexibility to the cells, which aids their passage through tiny capillaries.
Changes in a cell’s cytoskeleton of microtubules and microfilaments are the basis for cellular movement. The irregular shapes of granulocytes reflect their mobile search for invasive particles, which they will engulf and destroy. The amoeba moves by forming and retracting pseudopodia as it tests the environment for food and potential threats. Mouse embryonic stem cells move, grow, and divide in culture.
The brick-like shapes of the Elodea cells permit tight packing and provide rigidity to the leaf. The cells communicate with each other through junctions in adjacent cell walls (not visible here).
The cleavage furrows shown in the two dividing cells of the sea urchin embryo are formed by microfilaments, which act as “purse strings” to parcel the contents of each mother cell efficiently into its two daughter cells at the completion of mitosis.
The transparent exterior membrane of the sea urchin embryo is the fertilization membrane, which provides a physical barrier against polyspermy, the fertilization of an egg by more than one sperm.
To calculate the magnification of each displayed cell or organism, first measure its diameter shown on the poster, and then divide that measurement by its actual diameter (found in the caption). For example, if the human cheek cell image measures 30 mm in diameter, and its actual diameter is 0.03 mm: 30 mm (poster diameter) / 0.03 mm (actual diameter) = 1000x magnification