Gink & Go Dialogues

Gink & Go at the Ski Resort

Setting: Gink & Go are riding on the chair life to the top of the mountain to do some snowboarding. It is a cold day with a stiff wind.

Go:		Brrrr…..I could sure use a little brown fat today.
Gink:		Huh?
Go:		Don’t you remember Saupe’s lecture this morning about how some vertebrates and newborn babies use brown fat for warmth?
Gink:		Oh no! Don’t spoil another beautiful day talking about Saupe’s stupid bio class.
Go:		You have to admit that the evolution of this specialized adipose tissue as a source of respiratory heat is pretty cool.
Gink:		Bad pun, dude. What’s wrong with shivering like I’m doing now? It does the same thing.
Go:		True, but brown fat is a great mechanism for heat production. It's located in the neck between the shoulder blades and it’s crammed full of mitochondria and serviced by lots of blood vessels.
Gink:		Yeah, yeah. And there’s a controversy over whether adult humans rely on brown fat for warmth. Tell me something I don’t know.
Go:		I'm impressed. Well, did you know that the membranes of the mitochondria in these adipose cells have a special protein in the inner membrane that provides a channel for protons.....
Gink:		....and that uncouples ATP production from electron flow in the mitochondrion making brown fat a real respiratory furnace – it produces lots of heat but no ATP. You're starting to bore me.
Go:		It’s similar to how dinitrophenol, or DNP, was once used to help people loose weight.
Gink:		You mean the stuff that poked holes in the inner mitochondrial membrane and uncoupled ATP production and electron flow. This caused the body to break down more fats to provide the missing ATP.
Go:		Right again! But the treatment was stopped when they realized that it generated too much heat and could be fatal.
Gink:		I hope you’re almost finished because we’re almost at the top of the hill.
Go:		I thought the most interesting part of Saupe’s lecture was when he talked about skunk cabbage and some related plants in the arum family that can generate heat.
Gink:		I was mad that he ruined a perfectly good class about animal physiology discussing dopey plants.
Go:		Still, it’s pretty neatl that skunk cabbage flowers can produce temperatures considerably above the ambient temperature. There are very few plants that can do that.
Gink:		Big deal. Why bother?
Go:		They need to warm up the volatile chemicals that will attract pollinators for reproduction. The plants are well-named because the odors they release smell like rotting meat which attracts carrion flies.
Gink:		I guess that's why the specialized leaf surrounding the flowers is maroon, just like a slab of beef. But what does this have to do with brown fat?
Go:		The mitochondria in the skunk cabbage flowering spike have a similar adaptation. Rather than channels in the inner membrane of the mitochondria, the mitochondria of these plants have an alternate electron transport pathway that bypasses the normal one.
Gink:		And the alternate system doesn't result in a proton gradient so that like brown fat and individuals treated with DNP, they burn sugars and release heat without the build-up of ATP.
Go:		Exactly. Biologists know that it involves a different path of electron flow because it is not inhibited by hydrogen cyanide like the normal electron transport system.
Gink:		Enough of this geek-talk about cyanide-insensitive respiration and brown fat. We’re ready to dismount.
Go:		I’ll see you at the bottom of the pipe.
Gink:		Only if you promise not say anything more about Saupe’s bio class - it stinks as much as his skunk cabbage.

Summary: This dialog explores the relationship between mitochondria and heat and ATP production:

Normal mitochondria are cyanide-sensitive. ATP is coupled to electron transport resulting in the production of both ATP (about 38 per glucose) and heat
Treatment of normal mitochondria with hydrogen cyanide blocks electron transport resulting in no ATP or heat (=death if severe enough).
Mitochondria in brown fat have channels that allow protons to bypass the ATPase. Thus, electron flow is uncoupled from ATP synthesis. Respiration results in the production of heat by no ATP.
Normal mitochondria treated with DNP cannot produce ATP because DNP punches holes in teh inner membrane, uncoupling ATP production from electron flow. These mitochondria continue to breakdown sugars resulting in heat production.
The mitochondria of skunk cabbage have an alternate pathway of electron transport that bypasses the normal one. The bypass results in the flow of electrons without movement of protons across the membrane resulting in heat production but no ATP.

Questions:

Explain how shivering warm us up.
Is brown fat found in both adult humans and infants?
Where is brown fat located?
Identify two characteristics of brown fat.
Recall normal respiration. In the inner mitochondrial membrane electrons are passed to oxygen. During this process, protons are moved across the membrane into the intermembrane space. The protons move back to the matrix through an ATPase in the membrane. Thus, ATP production is coupled to electron flow. Explain why ATP is not produced (not coupled) to electron transport in brown fat.
Dinitrophenol was once used a weight loss supplement. Explain how it worked. Explain why individuals who used DNP would get very hot.
Skunk cabbage flower spikes can generate enough heat to warm them several degrees above ambient temperatures. Explain how this occurs.
What is the difference between cyanide-sensitive and cyanide-insensitive respiration.
What is the pollinator of skunk cabbage? Identify at least two adaptations of the plant for this pollinator.
Explain the practical advantage to skunk cabbage flower spikes for producing heat.