3.3 Biology

Biology Foundations

Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development, and evolution. A strong grasp of cellular biology, genetics, and physiology is imperative for the DCAT.

Cell Biology and Organelles

The cell is the basic structural, functional, and biological unit of all known organisms. Cells can be broadly categorized into prokaryotes (lacking a nucleus, like bacteria) and eukaryotes (containing a nucleus and membrane-bound organelles, like animals and plants).

Key eukaryotic organelles:

• Nucleus: The control center of the cell, storing the genetic material (DNA) and coordinating cell activities like growth, metabolism, and reproduction.
• Mitochondria: Known as the powerhouse of the cell, these double-membrane organelles generate most of the cell's supply of adenosine triphosphate (ATP) through cellular respiration.
• Ribosomes: Complex molecular machines found in all living cells that serve as the site of biological protein synthesis (translation).
• Endoplasmic Reticulum (ER): A network of membranous tubules and sacs. Rough ER is studded with ribosomes and synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids received from the ER for transport to their final destinations.
• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.
• Chloroplasts: Found only in plant and algal cells, these organelles conduct photosynthesis.

Cellular transport across the plasma membrane can be passive (requiring no energy, like diffusion and osmosis) or active (requiring ATP to move substances against their concentration gradient).

Cellular Reproduction: Mitosis vs. Meiosis

Cell division is essential for growth, tissue repair, and reproduction.

• Mitosis: The process by which a somatic (body) cell divides to produce two genetically identical diploid daughter cells. The stages are Prophase (chromosomes condense), Metaphase (chromosomes align at the equator), Anaphase (sister chromatids are pulled apart), and Telophase (nuclear envelopes reform), often remembered by the acronym PMAT.
• Meiosis: A specialized type of cell division that reduces the chromosome number by half, creating four genetically distinct haploid cells (gametes: sperm or egg). It involves two successive divisions (Meiosis I and II) and includes crossing over during Prophase I, which increases genetic diversity.

Genetics and Inheritance

Genetics is the study of genes, genetic variation, and heredity in organisms. The physical basis of heredity is DNA, which dictates cellular function through the Central Dogma of Molecular Biology: DNA is transcribed into mRNA, which is then translated by ribosomes into proteins.

Gregor Mendel's principles form the basis of classical genetics:

• Alleles: Variant forms of a given gene.
• Dominant and Recessive: A dominant allele (represented by a capital letter, e.g., 'A') masks the expression of a recessive allele (lowercase letter, e.g., 'a'). Recessive traits are only expressed if an organism is homozygous recessive (aa).
• Genotype: The exact genetic makeup or allele combination of an organism (e.g., AA, Aa, or aa).
• Phenotype: The observable physical or biochemical characteristics determined by the genotype.

Beyond Mendelian genetics, traits can exhibit incomplete dominance (a blending of traits), codominance (both traits expressed equally, like AB blood type), or be sex-linked (located on the X or Y chromosome).

Exam Tip: You will likely need to use Punnett squares to predict offspring probabilities. For a standard monohybrid cross between two heterozygous parents (Aa x Aa), the resulting genotypic ratio is 1:2:1 (AA:Aa:aa), and the phenotypic ratio is 3:1 (dominant to recessive).

Human Physiology

The human body functions through complex, interconnected organ systems designed to maintain homeostasis—the state of steady internal, physical, and chemical conditions maintained by living systems.

• Circulatory System: Centers around the heart, pumping blood through arteries, veins, and capillaries to deliver oxygen and nutrients to tissues and remove carbon dioxide and waste.
• Respiratory System: Includes the lungs and trachea; facilitates gas exchange, bringing oxygen into the blood and expelling carbon dioxide.
• Digestive System: Involves the stomach, intestines, liver, and pancreas. It breaks down food mechanically and chemically, absorbs nutrients, and excretes solid waste.
• Nervous System: Comprises the brain, spinal cord, and peripheral nerves. It controls voluntary and involuntary actions by transmitting rapid electrical signals.
• Endocrine System: A network of glands (e.g., pituitary, thyroid, adrenal) that secrete hormones directly into the bloodstream to regulate metabolism, growth, and reproduction.
• Immune System: Defends the body against pathogens. Includes white blood cells, antibodies, and the lymphatic system.

Metabolism: Respiration and Photosynthesis

Cellular respiration and photosynthesis are complementary metabolic pathways that sustain life.

Cellular Respiration breaks down glucose in the presence of oxygen to produce ATP, carbon dioxide, and water. It occurs in the cytoplasm and mitochondria. Overall Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$ Stages: Glycolysis (yields 2 ATP) $\rightarrow$ Krebs Cycle (yields 2 ATP) $\rightarrow$ Electron Transport Chain (yields ~32-34 ATP).

Photosynthesis captures sunlight to synthesize glucose from carbon dioxide and water, releasing oxygen as a byproduct. It occurs in chloroplasts. Overall Equation: $6CO_2 + 6H_2O + Light \rightarrow C_6H_{12}O_6 + 6O_2$ Stages: Light-dependent reactions (capture light energy to make ATP and NADPH) $\rightarrow$ Calvin cycle (uses ATP and NADPH to fix carbon into glucose).

Notice that the waste products of cellular respiration are the necessary reactants for photosynthesis, creating a perfectly balanced biological cycle.