OAT

Biology is the largest part of the Survey of the Natural Sciences: 40 of its 100 questions. The three science subtests share one 90-minute clock, so a steady pace of under a minute per question leaves time for the calculation-heavy chemistry items. Questions range from direct recall (organelle functions, hormone sources) to multi-step reasoning (pedigrees, Hardy-Weinberg, experimental outcomes).

The cell is the fundamental unit of life, and all living organisms are composed of one or more cells. Cells are classified as prokaryotic or eukaryotic according to their structural organization.

Prokaryoticvs.Eukaryoticcells

Feature	Prokaryotes (Bacteria, Archaea)	Eukaryotes (Animals, Plants, Fungi , Protists)
Nucleus	None;asinglecircularchromosomesitsinthenucleoid	Truenucleusenclosedbyanuclear
	region	envelope
Membrane-bound organelles	Absent	Present(ER Golgi mitochondria , , ,
		lysosomes andothers) ,
Ribosomes	70S	80Sinthecytoplasm(70Sinside
		mitochondriaandchloroplasts)
Otherstructures	Cellmembraneandcellwall;manyhaveflagellafor	Cytoskeleton;cellwallsonlyinplants
	motilityandpiliforattachmentorconjugation	(cellulose)andfungi(chitin)

The Plasma Membrane and Transport

The plasma membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. Phospholipid heads are hydrophilic and face outward; the hydrophobic tails face inward. Integral proteins span the membrane, and peripheral proteins attach to its surfaces. In animal cells, cholesterol buffers membrane fluidity.

Mechanism	Energy	Description
Simplediffusion	None	Small nonpolar molecules (O 2, CO 2) cross the bilayer directly , down their
		concentrationgradient
Facilitateddiffusion	None	Polarmoleculesandionsmovedowntheirgradientthroughchannelorcarrier
		proteins
Osmosis	None	Diffusionofwateracrossaselectivelypermeablemembranetowardthe	region
		ofhighersoluteconcentration
Primaryactivetransport	ATP	Movessubstancesagainsttheirgradient;theNa+/K+pumpmoves3Na+	outand
Secondaryactivetransport	Iongradient	2K+inperATPhydrolyzed Usestheenergystoredingradientscreatedbyprimaryactivetransport (symportorantiport)	1

Membranetransportmechanisms(Continued)

Mechanism	Energy	Description
Endocytosis	ATP	Bringsmaterialinbyvesicleformation:phagocytosis(largeparticles) ,
		pinocytosis (fluids) receptor-mediated endocytosis (specific ligands) ,
Exocytosis	ATP	Releasesmaterialwhenvesiclesfusewiththeplasmamembrane

Organelles

Organelle	StructureandFunction
Nucleus	Holdsthegeneticmaterial;surroundedbyanuclearenvelopewithnuclearpores Thenucleolus .
	synthesizesribosomalRNA ChromatinisthedispersedformofDNA;chromosomesarecondensed .
	chromatinduringdivision
RoughER	Studdedwithribosomes;synthesizesproteinsforsecretionormembraneinsertion
SmoothER	Lacksribosomes;synthesizeslipids metabolizescarbohydrates anddetoxifiesdrugs , ,
Golgiapparatus	Stackedcisternaethatmodify sort andpackageproteinsandlipids;thecisfacereceivesvesicles , ,
	fromtheERandthetransfaceshipsthemout
Mitochondria	Doublemembrane(innermembranefoldedintocristae);thematrixholdscitricacidcycleenzymes .
	Producemostofthecell'sATPandcarrytheirownDNAandribosomes
Lysosomes	Containhydrolyticenzymesthatdigestwornorganelles foodparticles andengulfedbacteria , ,
Peroxisomes	Containoxidativeenzymesthatbreakdownfattyacidsanddetoxifyharmfulsubstances
Cytoskeleton	Microfilaments(actin:movementandshape) intermediatefilaments(mechanicalstrength) and , ,
	microtubules(tubulin:spindleapparatus cilia flagella) , ,
Centrioles	Microtubule-organizing centers in animal cells that organize spindle fibers; plant cells lack centrioles
	butstillformspindles
Plant-specific structures	Cellulosecellwall(rigidsupport) centralvacuole(water ions andwastes) chloroplasts , , , ,
	(photosynthesis) andplasmodesmata(channelsbetweenadjacentcells) ,

Cellular Respiration

Cellular respiration breaks down glucose to produce ATP. In the presence of oxygen, pyruvate from glycolysis enters the mitochondria, where the remaining stages take place.

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP

➜

➜

➜

Cytoplasm · net 2 ATP, 2 NADH

Matrix · 1 NADH, 1 CO2 per

Matrix · per glucose 6 NADH, 2

Inner membrane · about 26–28

FADH2, 2 ATP

ATP

pyruvate

Glycolysis does not require oxygen. One glucose becomes two pyruvate; the investment phase uses 2 ATP and the payoff phase makes 4 ATP, for a net 2 ATP and 2 NADH.

Citric acid cycle: acetyl-CoA combines with oxaloacetate to form citrate. Each acetyl-CoA yields 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2; each glucose supplies two acetyl-CoA.

Electron transport chain: NADH donates electrons at complex I (path I → III → IV) and FADH2 at complex II (II → III → IV). The released energy pumps protons into the intermembrane space.

Chemiosmosis: protons flow back into the matrix through ATP synthase, driving ATP synthesis (oxidative phosphorylation). Oxygen is the final electron acceptor and forms water.

Total yield: about 30–32 ATP per glucose under optimal conditions.

Without oxygen, cells regenerate NAD+ so glycolysis can continue. Lactic acid fermentation (muscle cells during intense exercise) converts pyruvate to lactate; alcoholic fermentation (yeast and some bacteria) converts pyruvate to ethanol and CO2. Either way, the yield is only 2 ATP per glucose.

Photosynthesis

Photosynthesis converts light energy into chemical energy stored in glucose. Chloroplasts have a double membrane; the thylakoid membranes, stacked into grana, hold the photosystems and electron transport chain, and the surrounding fluid, the stroma, is where the Calvin cycle runs.

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Photosystem II (P680) absorbs light and splits water, releasing

Fixation: CO2 joins RuBP via rubisco. Reduction: 3-

O2, electrons, and protons. Electrons pass through the

phosphoglycerate is reduced to G3P using ATP and NADPH.

transport chain to Photosystem I (P700), where NADP+ is

Regeneration: RuBP is rebuilt using ATP. For every 3 CO2

reduced to NADPH. ATP forms by chemiosmosis through ATP

fixed, one G3P exits toward glucose.

synthase.

Adaptationsforhot,Dryclimates

Pathway	How CO Is Handled 2	Examples
C 4	CO 2 is first fixed into a 4 -carbon compound in mesophyll cells , then released in	Corn sugarcane ,
	bundle-sheath cells for the Calvin cycle; separating the steps in space reduces
	photorespiration
CAM	Stomata open at night; CO 2 is stored as organic acids and released during the day;	Cacti succulents ,
	separatingthestepsintimereduceswaterloss