Comprehensive IB Biology Study Guide: Cell Biology, Molecular Genetics, and Ecology

Comprehensive review flashcards covering Cell Theory, Ultrastructure, Membrane Transport, Cell Specialization, Ecology, Cell Division, Biomolecules, Enzymes, and Classical/Molecular Genetics.

How does the amphipathic structure of phospholipid molecules cause them to form bilayers in water?

Phospholipids have hydrophilic (water-loving) polar phosphate heads and hydrophobic (water-fearing) nonpolar fatty acid tails. When placed in water, the hydrophobic tails orient away from the aqueous environment toward each other, while the hydrophilic heads face outward toward the surrounding water, spontaneously forming a stable bilayer.

What is the function of cholesterol as a modulator of membrane fluidity in animal cells?

Cholesterol disrupts the regular packing of hydrocarbon tails to prevent crystallization at low temperatures, while simultaneously restricting molecular motion to reduce membrane fluidity and permeability to hydrophilic solutes at high temperatures.

What are four functions of membrane-bound proteins?

Four functions are: channel proteins for facilitated diffusion, pump proteins for active transport, receptor sites for hormone or neurotransmitter binding, and cell-to-cell adhesion to form tissues.

How do channel proteins and pump proteins differ in terms of transport mechanism and energy requirements?

Channel proteins allow specific ions or polar molecules to cross passively down their concentration gradient via facilitated diffusion without energy expenditure, whereas protein pumps move specific substances against their concentration gradient using energy from $ATP$ hydrolysis via active transport.

How do the sodium-potassium ($\text{Na}^+/\text{K}^+$) pump and voltage-gated potassium channels operate in a nerve axon?

The sodium-potassium pump actively pumps $3\,\text{Na}^+$ ions out of the axon and $2\,\text{K}^+$ ions into the axon per cycle using $ATP$ to maintain resting potential. Voltage-gated potassium channels open during the repolarization phase of an action potential, allowing $\text{K}^+$ ions to rapidly diffuse out of the axon down their electrochemical gradient.

How does the sodium-dependent glucose cotransporter function as an example of indirect active transport?

It couples the downhill movement of $\text{Na}^+$ ions into the cell (driven by the concentration gradient established by the sodium-potassium pump) with the simultaneous uphill transport of glucose molecules against their concentration gradient into the cell without directly hydrolyzing $ATP$.

What role does membrane fluidity play in endocytosis and exocytosis?

Membrane fluidity allows the lipid bilayer to dynamically deform, pinch off into intracellular vesicles to engulf extracellular materials during endocytosis, and fuse with secretory vesicles to expel luminal contents into the extracellular space during exocytosis.

What are the structural roles and cellular functions of glycoproteins and glycolipids in the cell membrane?

Glycoproteins (proteins with attached carbohydrate chains) and glycolipids (phospholipids with attached carbohydrate chains) face the extracellular environment where they function in cell recognition, immune identification, and cell adhesion to form tissues.

A red blood cell has a diameter of $8\,\mu\text{m}$. If drawn $100$ times larger than its actual size, what is the diameter of the drawing in millimeters ($\text{mm}$)?

$0.8\,\text{mm}$. Calculation: $\text{Drawing size} = 8\,\mu\text{m} \times 100 = 800\,\mu\text{m}$. Since $1\,\text{mm} = 1000\,\mu\text{m}$, $800\,\mu\text{m} \div 1000\,\mu\text{m/mm} = 0.8\,\text{mm}$.

If a mitochondrion has an actual length of $5\,\mu\text{m}$ and a student's drawing of it is $10\,\text{mm}$, what is the magnification of the drawing?

$2000\times$. Calculation: convert $10\,\text{mm}$ to $\mu\text{m}$: $10\,\text{mm} \times 1000 = 10,000\,\mu\text{m}$. Magnification $= \frac{\text{Image size}}{\text{Actual size}} = \frac{10,000\,\mu\text{m}}{5\,\mu\text{m}} = 2000\times$.

In this scanning electron micrograph of the nuclear envelope, what structure is indicated by the label X?

A nuclear pore, which regulates the exchange of macromolecules such as $RNA$ and proteins between the nucleus and the cytoplasm.

What main advantage did the development of electron microscopes provide over light microscopes, and what is a key disadvantage?

Electron microscopes achieved a much higher resolution and magnification due to the shorter wavelength of electron beams compared to visible light, allowing the observation of cellular ultrastructure. A key disadvantage is that specimens must be placed in a vacuum and cannot be viewed alive.

What four structures are common to cells in all living organisms?

All living cells possess a plasma membrane, cytoplasm, $DNA$ as genetic material, and ribosomes for protein synthesis.

How do prokaryotic cells differ from eukaryotic cells regarding their nucleus, ribosomes, and compartmentalization?

Prokaryotes lack a membrane-bound nucleus (having naked, circular $DNA$ in a nucleoid region), contain smaller $70\text{S}$ ribosomes, and lack membrane-bound cytoplasmic compartments. Eukaryotes have a true double-membrane-bound nucleus containing linear chromosomes, larger $80\text{S}$ ribosomes, and extensive membrane-bound compartmentalization.

What are the seven life processes carried out by unicellular organisms (HMNEGRR)?

Homeostasis, Metabolism, Nutrition, Excretion, Growth, Response, and Reproduction.

Why are skeletal muscle cells, aseptate fungal hyphae, and mature red blood cells considered atypical exceptions to the classical cell theory?

Skeletal muscle fibers are exceptionally long and multinucleated; aseptate fungal hyphae lack dividing septa and form continuous multinucleated cytoplasm; mature mammalian red blood cells lose their nucleus upon maturation to maximize space for hemoglobin.

What evidence supports the endosymbiotic theory for the origin of mitochondria and chloroplasts?

Both organelles contain their own naked circular $DNA$, possess $70\text{S}$ ribosomes similar to prokaryotes, are surrounded by a double membrane, and replicate independently via a process resembling binary fission.

What are two functional advantages of compartmentalization in eukaryotic cells?

Enzymes and substrates for specific metabolic pathways can be concentrated in localized regions for greater efficiency, and potentially damaging hydrolytic substances (such as digestive enzymes inside lysosomes) can be sequestered away from the rest of the cytoplasm.

What are the two defining properties of stem cells?

Stem cells can divide repeatedly through self-renewal without differentiating, and they retain the capacity to differentiate along different developmental pathways into specialized cell types.

What is the difference in potency between totipotent, pluripotent, and multipotent stem cells?

Totipotent stem cells can differentiate into any cell type, including embryonic and extra-embryonic tissues; pluripotent stem cells can differentiate into all body cells of the embryo; multipotent stem cells can only differentiate into a limited number of closely related cell lineages.

Why does an increase in cell size decrease the surface area-to-volume ($\text{SA:V}$) ratio, and how does this constrain cell size?

As a cell grows, its volume increases proportionally to the cube of its radius ($r^3$) while surface area increases only with the square ($r^2$). A lower $\text{SA:V}$ ratio impairs the cell's ability to take in sufficient nutrients and expel metabolic wastes fast enough to sustain metabolic activity, limiting maximum cell size.

How do the structural adaptations of type I and type II pneumocytes in the alveoli relate to their specific functions?

Type I pneumocytes are extremely thin and flattened with an expansive surface area to minimize the diffusion pathway for oxygen and carbon dioxide. Type II pneumocytes are cuboidal cells that secrete pulmonary surfactant, reducing alveolar surface tension to prevent alveolar collapse.

How do autotrophs and heterotrophs differ in their mode of nutrition?

Autotrophs synthesize their own organic molecules from simple inorganic substances using external energy (such as sunlight in photosynthesis), whereas heterotrophs obtain organic molecules by consuming other living or dead organisms.

How do detritivores and saprotrophs differ in their method of nutrient acquisition?

Detritivores ingest non-living organic matter and digest it internally (e.g., earthworms), whereas saprotrophs secrete digestive enzymes externally onto non-living organic matter and absorb the soluble products of digestion (e.g., fungi).

If an organism has $40,000\,\text{kJ}\,\text{m}^{-2}\,\text{y}^{-1}$ of available energy, approximately how much energy will be transferred to the next trophic level?

Approximately $4,000\,\text{kJ}\,\text{m}^{-2}\,\text{y}^{-1}$. Typically, only about $10\%$ of the available energy is passed from one trophic level to the next, with the remaining $90\%$ lost primarily as metabolic heat, unconsumed biomass, or undigested material.

What is the fundamental difference between the flow of energy and the flow of nutrients through an ecosystem?

Energy enters ecosystems continuously (mostly as sunlight) and flows in a linear, non-cyclic path before eventually dissipating as heat. In contrast, nutrients are limited and must be continuously recycled within ecosystems by decomposers.

What key events occur during the $G_1$, $S$, and $G_2$ phases of interphase?

During $G_1$, the cell grows, synthesizes proteins, and carries out normal metabolic functions; during $S$, the cell replicates its nuclear $DNA$; during $G_2$, the cell synthesizes organelles, microtubules, and materials required for mitosis.

What are the four successive stages of mitosis and the key event in each?

Prophase (chromatin condenses into chromosomes, nuclear envelope breaks down), Metaphase (chromosomes align at the equatorial metaphase plate), Anaphase (sister chromatids separate and move to opposite poles), and Telophase (nuclear membranes reform around decondensing chromosomes).

How do mitosis and meiosis compare in the number of divisions, daughter cell count, and ploidy?

Mitosis involves $1$ division, producing $2$ genetically identical diploid daughter cells. Meiosis involves $2$ successive nuclear divisions, producing $4$ genetically distinct haploid gametes.

What two events during meiosis generate genetic variation in daughter gametes?

Crossing over between non-sister chromatids of homologous chromosomes during Prophase I creates new combinations of alleles, and random orientation (independent assortment) of bivalents along the equatorial plate during Metaphase I creates varied chromosome assortments.

What are the three core properties of water that support life, and why do they occur?

Thermal properties (high specific heat and high latent heat of vaporization), cohesive properties (strong hydrogen bonding between water molecules causing surface tension), and solvent properties (ability to dissolve polar and charged solutes). These all arise from water's polar covalent bonds and extensive hydrogen bonding.

What biochemical reaction and specific bond formation are shown in the provided diagram?

A condensation reaction (dehydration synthesis) forming a peptide bond between the carboxyl group of one amino acid and the amino group of another, accompanied by the removal of a water molecule ($\text{H}_2\text{O}$).

What chemical reaction is depicted in this diagram, and what are its reactants and products?

Hydrolysis of lactose. The disaccharide lactose reacts with a molecule of water ($\text{H}_2\text{O}$) to break the $\beta 1\text{-}4$ glycosidic bond, producing the monosaccharides galactose and glucose.

Which major biomolecule class—carbohydrates, proteins, or lipids—is the most efficient long-term energy storage, and why?

Lipids (triglycerides). They contain more than twice the energy per gram compared to carbohydrates or proteins because of their higher proportion of carbon-hydrogen bonds and low water content (insolubility), yielding approximately $37\,\text{kJ/g}$ versus $17\,\text{kJ/g}$.

How do competitive and noncompetitive enzyme inhibitors differ in binding site and effect on enzyme action?

A competitive inhibitor binds to the active site, directly competing with the substrate and blocking its entry. A noncompetitive inhibitor binds to an allosteric site away from the active site, inducing a conformational change that prevents effective substrate binding or catalytic turnover.

How does the induced-fit model refine the earlier lock-and-key model of enzyme function?

While the lock-and-key model proposed a rigid, pre-formed active site matching the substrate, the induced-fit model states that the active site is flexible and molds around the substrate upon binding, stressing bonds to lower activation energy.

What is complementary base pairing in $DNA$, and why is it essential?

Adenine (A) pairs specifically with Thymine (T) via two hydrogen bonds, and Cytosine (C) pairs with Guanine (G) via three hydrogen bonds. This ensures precise, semi-conservative replication of the genetic code and accurate transcription into $mRNA$.

What specific base substitution mutation causes sickle cell anemia, and how does it change the resulting protein?

In the $\beta$-globin gene, the base adenine (A) is replaced by thymine (T) on the coding strand ($\text{GAG} \rightarrow \text{GTG}$), transcribing an $\text{mRNA}$ codon of $\text{GUG}$ instead of $\text{GAG}$. This causes valine to be incorporated instead of glutamic acid at the sixth position of the polypeptide.

Gregor Mendel crossed heterozygous yellow-seeded pea plants ($Yy \times Yy$) and obtained $1500$ total seeds. How many seeds are expected to be yellow, and how many green?

$1125$ yellow seeds and $375$ green seeds. In a monohybrid cross between two heterozygotes, the expected phenotypic ratio is $3\,\text{yellow} : 1\,\text{green}$. Calculation: $\frac{3}{4} \times 1500 = 1125$ yellow, and $\frac{1}{4} \times 1500 = 375$ green.

Based on the breeding data in the table, which trait is recessive and which cross proves this conclusion?

The plain pattern is recessive. Cross 1 proves this because two checkered parents produce plain offspring in an approximate $3:1$ ratio ($27\,\text{checkered} : 9\,\text{plain}$), demonstrating that both checkered parents were heterozygous ($Cc$) carrying a hidden plain allele ($c$).

A tall plant of unknown genotype is test-crossed with a homozygous recessive dwarf plant. The offspring consist of $869$ dwarf plants and $912$ tall plants. What is the genotype of the unknown parent plant?

Heterozygous ($Tt$). A $1:1$ phenotypic ratio of tall to dwarf offspring ($912 : 869 \approx 1:1$) resulting from a cross with a homozygous recessive ($tt$) tester proves the unknown parent possesses one dominant allele and one recessive allele ($Tt \times tt \rightarrow \frac{1}{2}\,Tt, \frac{1}{2}\,tt$).

Achondroplasia is inherited as an autosomal dominant trait. Two dwarf parents have a normal child. What are the parents' genotypes, and what is the probability their next child will be an achondroplastic dwarf?

Both parents are heterozygous ($Aa$) because they produced an unaffected homozygous recessive ($aa$) child. The probability that their next child will be a dwarf is $75\%$ or $\frac{3}{4}$ (genotypes $AA$ or $Aa$).

According to the blood type table, Larry has blood type O and has a son with type B blood. Which female(s) from the table could be the biological mother?

Wendy (Type B) or Susan (Type AB). Larry has genotype $ii$ and can only donate an $i$ allele. To produce a child with type B blood ($I^B i$), the mother must contribute an $I^B$ allele, which only Wendy or Susan possess.

Anne has blood type O ($ii$) and is pregnant with Scott's child, who has blood type AB ($I^A I^B$). What is the probability that their baby will have blood type A?

$50\%$ ($\frac{1}{2}$). Anne donates an $i$ allele with $100\%$ probability. Scott donates either an $I^A$ allele ($50\%$) or an $I^B$ allele ($50\%$), yielding a $50\%$ chance of genotype $I^A i$ (type A).

If a red-green colorblind female ($X^b X^b$) mates with a normal-vision male ($X^B Y$), what percentage of their male offspring are expected to be colorblind?

$100\%$. Male offspring inherit their single X chromosome from their mother. Since the mother is homozygous recessive ($X^b X^b$), all sons receive an $X^b$ allele and a Y chromosome from the father ($X^b Y$), exhibiting colorblindness.