Which of the following statements most accurately describes the role of heterotrimeric G proteins in signal transduction? Activation of G proteins by agonist-bound G protein-coupled receptors is initiated by the conversion of GDP to GTPat the a-subunit nucleotide binding site.
O Activation of G proteins by agonist-bound G protein-coupled receptors is initiated by the unbinding of GTP and
binding of GDP at the a-subunit nucleotide binding site.
Activation of G proteins by agonist-bound G protein-coupled receptors is initiated by the unbinding of GDP and
binding of GTP at the a-subunit nucleotide binding site.
Agonists binding to G proteins trigger their activation.

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.