Biology 9th Edition Glossary
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,5′ Cap - A Modified Form Of Guanine Nucleotide Added Onto The 5′ End Of A Pre-
Mrna Molecule.
A Site - One Of A Ribosome's Three Binding Sites For Trna During
Translation. The A Site Holds The Trna Carrying The Next Amino Acid To Be
Added To The Polypeptide Chain. (A
,Stands For Aminoacyl Trna.)
Abc Hypothesis - A Model Of Flower Formation Identifying Three Classes Of
Organ Identity Genes That Direct Formation Of The Four Types Of Floral
Organs.
Abiotic - Nonliving; Referring To The Physical And Chemical Properties Of An
Environment.
Abortion - The Termination Of A Pregnancy In Progress.
Abscisic Acid (Aba) - A Plant Hormone That Slows Growth, Often
Antagonizing The Actions Of Growth Hormones. Two Of Its Many Effects Are
To Promote Seed Dormancy And Facilitate Drought Tolerance.
Absorption - The Third Stage Of Food Processing In Animals: The Uptake Of
Small Nutrient Molecules By An Organism's Body.
Absorption Spectrum - The Range Of A Pigment's Ability To Absorb Various
Wavelengths Of Light; Also A Graph Of Such A Range.
Abyssal Zone - The Part Of The Ocean's Benthic Zone Between 2,000 And
6,000 M Deep.
Acanthodian - Any Of A Group Of Ancient Jawed Aquatic Vertebrates From
The Silurian And Devonian Periods.
Accessory Fruit - A Fruit, Or Assemblage Of Fruits, In Which The Fleshy Parts
Are Derived Largely Or Entirely From Tissues Other Than The Ovary.
Acclimatization - Physiological Adjustment To A Change In An Environmental
Factor.
Acetyl Coenzyme A (Acetyl Coa) - The Entry Compound For The Citric Acid
Cycle In Cellular Respiration, Formed From A Fragment Of Pyruvate
Attached To A Coenzyme.
Acetylcholine - One Of The Most Common Neurotransmitters; Functions By
Binding To Receptors And Altering The Permeability Of The Postsynaptic
Membrane To Specific Ions, Either Depolarizing Or Hyperpolarizing The
Membrane.
, Acid - A Substance That Increases The Hydrogen Ion Concentration Of A
Solution.
Acid Precipitation - Rain, Snow, Or Fog That Is More Acidic Than Ph 5.2.
Acoelomate - A Solid-Bodied Animal Lacking A Cavity Between The Gut And
Outer Body Wall.
Acrosomal Reaction - The Discharge Of Hydrolytic Enzymes From The
Acrosome, A Vesicle In The Tip Of A Sperm, When The Sperm Approaches Or
Contacts An Egg.
Acrosome - A Vesicle In The Tip Of A Sperm Containing Hydrolytic Enzymes
And Other Proteins That Help The Sperm Reach The Egg.
Actin - A Globular Protein That Links Into Chains, Two Of Which Twist
Helically About Each Other, Forming Microfilaments In Muscle And Other
Kinds Of Cells.
Action Potential - An Electrical Signal That Propagates (Travels) Along The
Membrane Of A Neuron Or Other Excitable Cell As A Nongraded (All-Or-
None) Depolarization.
Action Spectrum - A Graph That Profiles The Relative Effectiveness Of
Different Wavelengths Of Radiation In Driving A Particular Process.
Activation Energy - The Amount Of Energy That Reactants Must Absorb
Before A Chemical Reaction Will Start; Also Called Free Energy Of Activation.
Activator - A Protein That Binds To Dna And Stimulates Gene Transcription. In
Prokaryotes, They Bind In Or Near The Promoter; In Eukaryotes, They
Generally Bind To Control Elements In Enhancers.
Active Immunity - Long-Lasting Immunity Conferred By The Action Of B Cells
And T Cells And The Resulting B And T Memory Cells Specific For A
Pathogen. It Can Develop As A Result Of Natural Infection Or Immunization.
Active Site - The Specific Region Of An Enzyme That Binds The Substrate
And That Forms The Pocket In Which Catalysis Occurs.
Active Transport - The Movement Of A Substance Across A Cell Membrane
Against Its Concentration Or Electrochemical Gradient, Mediated By Specific
Transport Proteins And Requiring An Expenditure Of Energy.
Adaptation - Inherited Characteristic Of An Organism That Enhances Its
Survival And Reproduction In A Specific Environment.
Adaptive Immunity - A Vertebrate-Specific Defense That Is Mediated By B
Lymphocytes (B Cells) And T Lymphocytes (T Cells). It Exhibits Specificity,
Memory, And Self-Nonself Recognition. Also Called Acquired Immunity.