What do human infants and philodendrons have in common?

One answer is that they both smell bad at times. Philodendrons are members of the Araceae, a family that includes skunk cabbage and even stinkier species such as the dead horse arum. If we look deeper into the odoriferousness of philodendrons and their relatives, what we see is that they heat their male flower parts to help disperse their odor. We often see skunk cabbage flowers that have melted the snow around them.

Localization of thermogenic cells in the spadix of skunk cabbage (Onda et al., 2008). A, An intact inflorescence. B, The spathe has been cut to reveal the heat-producing spadix. C, Longitudinal sectioning of the spadix. D, Thermal image of the plant shown in C using a high-resolution infrared thermal camera. Bar, 1 cm. Image from Roger Seymour, Plant Physiology Online Essay 11.6 Respiration by thermogenic flowers http://5e.plantphys.net/article.php?ch=e&id=503

Some species are able to thermoregulate; they can maintain nearly uniform set temperature over a wide range of ambient air temperatures.

Rate of oxygen consumption and heat production (top) and temperature of the central receptacle (bottom) in the sacred lotus Nelumbo nucifera, in relation to ambient temperature (Seymour and Schultze-Motel, 1998). The dashed line is isothermal, showing that evaporative heat loss predominates at high ambient temperatures, but metabolic heat production prevails at low ambient temperature. The means were derived from intact flowers in the field, during the thermoregulatory period associated with female receptivity. Image from Roger Seymour, Plant Physiology Online Essay 11.6 Respiration by thermogenic flowers http://5e.plantphys.net/article.php?ch=e&id=503

How these plants generate heat is by uncoupling respiration from ATP synthesis in their mitochondria. The flower cell mitochondria transfer electrons from food to oxygen, but make very little ATP. Instead, all of the energy of respiration is released as heat.

Uncoupling respiration from ATP synthesis is the second, and more significant, way that thermogenic flowers resemble human babies. Newborn mammals have a special organ, called brown adipose tissue, or brown fat, that helps warm the newborns via non-shivering thermogenesis. Brown fat differs markedly from ordinary adipose tissue (white fat) that we see in adult mammals. White fat cells are storage tissues with large lipid droplets, little cytoplasm, few mitochondria, and low oxygen consumption. Brown fat cells have smaller lipid droplets, lots of mitochondria, and high levels of oxygen consumption. The brown color of brown fat results from the numerous blood vessels that supply oxygen to these cells.

Brown adipocytes (left panel) are polygonal in shape, have a considerable volume of cytoplasm and contain multiple lipid droplets of varying size. Their nuclei are round and almost centrally located. White adipocytes (right panel) have a scant ring of cytoplasm surrounding a single large lipid droplet. Their nuclei are flattened and eccentric within the cell. from Laura Austgen and R. Bowen http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/brownfat.html

Both mammalian brown fat and thermogenic skunk cabbage cells make an uncoupling protein (UCP) in their mitochondria. UCP is a gated proton channel in the mitochondrial inner membrane, that when opened facilitates diffusion of protons across the membrane.

Questions: Why does uncoupled respiration generate more heat per molecule of sugar or fat burned than respiration that generates ATP via oxidative phosphorylation?

When UCP is open, what will happen to the proton gradient in the mitochondria? Will the rate of ATP synthesis increase, decrease, or stay the same? What will happen to oxygen consumption (if there is no change in the rate of the citric acid cycle)?

Question: What part of a fat molecule contains most of the energy available to cells: glycerol or fatty acids?

Fats are broken down to glycerol and fatty acids. Glycerol is converted to pyruvate. Fatty acid chains are cleaved two carbons at a time to generate acetyl-CoA.

Question: in what part of the cell does fat metabolism take place?

Question: which form of exercise will burn more fat: moderate, aerobic exercise or strenuous exercise leading to anaerobiosis?

I like this as a way to stimulate students to explore fat metabolism (aerobic respiration), the relationship between oxygen consumption and heat generation, and coupling of respiration to oxidative phosphorylation. I hope that it will also help students to fully embrace the concept that plants have mitochondria and respire in the same way as animals.

References and sources:

Roger Seymour, Respiration by thermogenic flowers. Plant Physiology Online Essay 11.6 http://5e.plantphys.net/article.php?ch=e&id=503

Laura Austgen and R. Bowen, Brown Adipose Tissue. http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/brownfat.html



About jchoigt

I'm an Associate Professor in the School of Biology at Georgia Tech, and Faculty Coordinator of the Professional MS Bioinformatics degree program.
This entry was posted in Teaching and learning biology and tagged , , , . Bookmark the permalink.

3 Responses to What do human infants and philodendrons have in common?

  1. jchoigt says:

    A story on CNN about a couple whose child has a mitochondrial disease:

  2. gjhdfhvcghhh says:

    Skunk cabbage and some others you mention are NOT Philodendrons. However they all are AROIDS of the family Araceae. Philodendrons are a genus within that family.

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