Biology

If the theory of natural selection speaks only of local adaptation and not of general progress in it's "bare bones" mechanics, then how did Darwin justify his belief in progress, as expressed in such statements as "all mental and corporeal endowments will tend to progress towards perfection." In other words, what subsidiary arguments did Darwin advance to support the idea that general progress would mark the history of life?

When Darwin laid out his four postulates for natural selection, he was primarily speaking of the change within a population. Over time, the population would show gradual changes. This in turn would lead to differentiation of different populations and thus to speciation. Darwin was a firm believer that evolution was a gradual process (i.e. he writes ìNatura non facit saltumî ñ nature does not make jumps). As Darwin was aware of geologic evidence of the ancient age of the earth, he argued that there was sufficient time for species to come about in this way. ìThe mind cannot possibly grasp the full meaning of the term of a hundred million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations.î Throughout the book, however, Darwin did focus more on the bare bones of natural selection and less on the origin of species.

Many of the adaptations that Darwin talks about, such as the nectaries of flowers for attracting insects, are perfected for their environment. This perfection has come about because of many generations of gradual changes. Consequently, Darwin viewed this succession of forms as progress towards the perfected forms that we see today.

As an aside, modern evolutionary biology has done away with the terms progress and perfection in evolution. Evolution is blind ñ no progress ñ and no adaptation is perfect. These terms may reflect liberal English thought in the 19th century rather than current science.

Describe the role of G proteins in peptide hormone signalling.

Many hormone receptors are G-protein coupled receptors. That means that the receptor is attached to a protein the binds GTP and GDP. Let's look at the specific example of the beta-adrenergic receptor. It is a seven trans-membrane protein that has a cytoplasmic loop that binds a trimeric g-protein (3 subunits alpha, beta and gamma, the alpha subunit binds GTP and can hydrolize it to GDP). When adrenaline binds the receptor there is a conformational change in the receptor. This lead to the separation of the alpha-GTP subunit from the beta-gamma. This subunits can then bind to effector proteins turning them on or off. In our example the alpha-GTP activated adenylyl cyclase that in turn produces cAMP. cAMP is a second messenger that activates PKA (protein kinases A). PKA then phosphorylates other protein turning them on or off. In our example PKA will activate proteins involved in turning glycogen into glucose and glucose into ATP. This is all in effort to get the cellular response of increased energy. So G-proteins transmit the extracellular hormone signal into a cellular response throughout the creation of a second messenger.

Discuss the processes and outline the pathways that ultimately lead to the formation of a blood clot in response to injury to a blood vessel.

Neccessarily it is very important to seal any cuts in the blood vessel. The process of sealing injuries is called hemotasis. Upon injury, the smooth muscles contract to decrease and slow blood flow. Platelets are attracted to the site of injury. Once platelets bind the injured area they release factors to attract other platelets. These platelets then start to form a clot by activating thrombin that converts soluble plasma protein fibrinogen into larger, sticky strands of insoluble fibrin. The clot then tightens trapping the thrombin in the clot while repair of the injury takes place. Thrombin activates plasmin that will digest fibrin to dissolve the clot once the injury has been repaired. So the main steps are vascular spasm, platelet plug formation, coagulation and dissolution.

Which hormones increase intracellular production of AMP and how?

AMP or cAMP is produced by the enzyme adenylyl cyclase, which converts ATP into cAMP. Adenylyl cyclase is activated by the alpha subunit of a trimeric G-protein that is released when a G-protein coupled receptor is activated by binding of specific hormones. In adipose cells epinepherine, glucagon and ATCH stimulate adenylyl cyclase where as prostaglandin and adenosine inhibit it. So when these hormones are produced and released into the blood they will bind to their specific receptors that a G-protein coupled receptors. In the case of norepinepherine the receptor is called the beta-adrenergic receptor. The binding of the hormone causes a conformational change in the receptor that effects the trimeric (ie. three subunits alpha, beta and gamma) G-protein (meaning this protein binds GTP and GDP). The alpha subunit exchanges it GDP for a GTP and detaches from the beta and gamma subunits. It then activates adenylyl cyclase that converts ATP into cAMP. The cAMP activates other proteins in order to get a cellular response.

What are epilithic algae? Why are they important in lakes and streams?

Algae that grow attached to substrata, and are a component of the periphyton, are termed periphytic algae (see Crumpton, 1989). Periphytic algae can be further divided into those that grow within bottom sediments (epipelic algae), those that grow on plants (epiphytic algae), and those that grow on rocks (epilithic algae)

Epilithic alage are important for two main reasons:

1. They act as a food source for several organisms higher on the food chain.
2. They play a role in dissolved atmospheric N2 fixation via symbiotic bacteria.

As a result they serve as a "base" for the food web of lakes and streams. Carbon and Nitrogen fixation starts with them allowing the C and N molecules to become incorporated into organisms living further up the food web.

See Peterson et al., 1993. Ecology 74: 653-672. It should have a good description of a study where epilithic algae contributed to insect and fish growth after epilithic algae growth.