2.1 Life Science: Cell Biology, Human Anatomy & Genetics

Life Science: Cell Biology, Human Anatomy & Genetics

Life Science forms a cornerstone of the AFCT General Science subtest. Candidates are evaluated on fundamental biological principles ranging from micro-level cellular biochemistry to macro-level human anatomy, genetics, and ecological taxonomy.

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1. Cell Biology & Cellular Organization

All living organisms are composed of cells, the structural and functional units of life. Life is divided broadly into two cellular architecture types:

• Prokaryotes: Primitive single-celled organisms (such as bacteria and archaea) that lack a membrane-bound nucleus and membrane-bound organelles. Their genetic material floats freely in a nucleoid region.
• Eukaryotes: Complex single-celled or multicellular organisms (animals, plants, fungi, protists) possessing a membrane-bound nucleus containing DNA, as well as specialized internal structures called organelles.

Essential Cellular Organelles

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2. Cellular Energy & Metabolism

Cells require energy to perform work, maintain homeostasis, and grow. Two primary metabolic pathways govern energy conversion in living systems:

Photosynthesis

Photosynthesis is an anabolic, endergonic process occurring in plant chloroplasts. Autotrophs capture light energy to convert carbon dioxide and water into glucose and oxygen gas. $\text{Chemical Equation: } 6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• Light-Dependent Reactions: Occur in the thylakoid membranes; split water molecules ($H_2O$), release $O_2$, and produce ATP and NADPH.
• Calvin Cycle (Light-Independent Reactions): Occurs in the stroma; utilizes ATP and NADPH to fix $CO_2$ into glucose ($C_6H_{12}O_6$).

Cellular Respiration

Cellular respiration is a catabolic, exergonic process occurring in eukaryotic mitochondria. Heterotrophs and autotrophs break down glucose in the presence of oxygen to generate adenosine triphosphate (ATP). $\text{Chemical Equation: } C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36\text{--}38\text{ ATP}$

1. Glycolysis: Occurs in the cytoplasm; splits one 6-carbon glucose into two 3-carbon pyruvate molecules (Yields $2\text{ ATP} + 2\text{ NADH}$). Requires no oxygen (anaerobic).
2. Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; oxidizes acetyl-CoA to release $CO_2$ (Yields $2\text{ ATP}$, NADH, and $FADH_2$).
3. Electron Transport Chain (ETC): Occurs on the inner mitochondrial membrane (cristae); uses oxygen as the final electron acceptor to generate the bulk of cellular energy via oxidative phosphorylation (Yields $\sim 32\text{--}34\text{ ATP}$).

Anaerobic Respiration (Fermentation): In the absence of oxygen, cells undergo fermentation. Yeast perform alcoholic fermentation (producing ethanol and $CO_2$), while human muscle cells undergo lactic acid fermentation during intense effort, producing lactic acid and $2\text{ ATP}$.

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3. Cell Division & Reproduction: Mitosis vs. Meiosis

Organisms reproduce and repair tissues through coordinated nuclear division processes:

Mitosis:  [Parent 2n]  --->  [Daughter 2n] + [Daughter 2n]  (Identical Somatic Cells)

Meiosis:  [Parent 2n]  --->  [Gamete 1n] [Gamete 1n] [Gamete 1n] [Gamete 1n]  (Diverse Gametes)

Phases of Mitosis (PMAT)

1. Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form.
2. Metaphase: Chromosomes line up along the equator (metaphase plate) of the cell.
3. Anaphase: Sister chromatids are pulled apart by spindle fibers to opposite poles.
4. Telophase: Nuclear envelopes reform around separated chromatids; chromosomes uncoil back into chromatin.
5. Cytokinesis: Division of the cytoplasm, yielding two distinct daughter cells.

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4. Genetics & Molecular Biology

DNA and RNA Structure

Deoxyribonucleic acid (DNA) holds the master genetic code in a double-helix structure composed of nucleotide monomers (a sugar-phosphate backbone and nitrogenous bases).

• DNA Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). Base pairing rules: A-T and C-G.
• RNA Differences: Ribonucleic acid (RNA) is single-stranded, contains ribose sugar instead of deoxyribose, and replaces Thymine with Uracil (U) (pairing A-U and C-G).
• Central Dogma of Molecular Biology: $\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$.

Mendelian Inheritance

Gregor Mendel established fundamental inheritance laws using pea plants:

• Gene: A segment of DNA encoding a specific trait.
• Allele: Alternative forms of a gene (e.g., $B$ for brown eyes, $b$ for blue eyes).
• Genotype: The genetic makeup of an organism ($BB$ = homozygous dominant, $Bb$ = heterozygous, $bb$ = homozygous recessive).
• Phenotype: The observable physical characteristics resulting from the genotype.

Punnett Square Example ($Bb \times Bb$ Monohybrid Cross):

• Genotypic Ratio: $1,BB : 2,Bb : 1,bb$ (25% $BB$, 50% $Bb$, 25% $bb$).
• Phenotypic Ratio: 3 Brown eyes : 1 Blue eye (75% dominant, 25% recessive).

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5. Human Anatomy & Physiology: Major Body Systems

The human body contains 11 major organ systems working interdependently to sustain life:

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6. Biological Taxonomy & Classification

Taxonomy is the science of naming and classifying living organisms into hierarchical categories based on evolutionary relationships:

1. Domain (Archaea, Bacteria, Eukarya)
2. Kingdom (Animalia, Plantae, Fungi, Protista, Eubacteria, Archaebacteria)
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

Mnemonic for Taxonomy Hierarchy: Dear King Philip Came Over For Good Soup (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species).

Binomial Nomenclature: Organisms are formally named by their Genus (capitalized) and species (lowercase), written in italics (e.g., Homo sapiens for humans, Canis lupus for wolves).