OSAT Biological Sciences (110)

About 96% of the mass of living things is made of six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (often remembered as CHNOPS). Their roles in biomolecules explain much of biochemistry.

Water is a polar molecule: oxygen pulls electrons away from the two hydrogens, so the oxygen end is partially negative and the hydrogen ends are partially positive. Hydrogen bonds between neighboring water molecules give water the properties that make life possible.

Macromolecules are built and broken by two opposite reactions. In dehydration synthesis (condensation), monomers are joined and a water molecule is removed for each new bond. In hydrolysis, water is added to break the bond and release monomers, as in digestion.

Living organisms are composed primarily of organic compounds—molecules containing carbon atoms bonded to hydrogen, oxygen, nitrogen, and other elements. Carbon's unique ability to form four covalent bonds allows it to create diverse molecular structures including chains, branches, and rings. This bonding versatility underlies the complexity of biological molecules.

Functional groups determine the chemical properties and reactivity of organic molecules. Hydroxyl groups (-OH) characterize alcohols and contribute to water solubility. Carboxyl groups (-COOH) define organic acids and can donate protons. Amino groups (-NH2) act as bases and are essential components of amino acids. Carbonyl groups (C=O) appear in aldehydes and ketones. Phosphate groups (-PO4) store energy in molecules like ATP. Sulfhydryl groups (-SH) form disulfide bridges in proteins.

Carbohydrates serve as energy sources and structural components. They contain carbon, hydrogen, and oxygen typically in a 1:2:1 ratio. Monosaccharides are simple sugars including glucose, fructose, and galactose. Glucose (C6H12O6) is the primary energy currency of cells and exists in ring and linear forms.

Disaccharides form when two monosaccharides join through dehydration synthesis, creating a glycosidic bond. Sucrose consists of glucose and fructose. Maltose contains two glucose units. Lactose combines glucose and galactose.

Polysaccharides are complex carbohydrates containing hundreds to thousands of monosaccharide units. Starch serves as energy storage in plants and consists of amylose (unbranched) and amylopectin (branched) forms. Glycogen functions as animal energy storage, primarily in liver and muscle cells, and is more highly branched than starch. Cellulose provides structural support in plant cell walls through beta-glucose linkages that most organisms cannot digest. Chitin forms the exoskeletons of arthropods and fungal cell walls.

Lipids are hydrophobic molecules that include fats, oils, phospholipids, and steroids. They function in long-term energy storage, insulation, cell membrane structure, and hormone signaling.

Triglycerides consist of glycerol bonded to three fatty acid chains. Saturated fatty acids contain no carbon-carbon double bonds, remain solid at room temperature, and are typically derived from animal sources. Unsaturated fatty acids contain one or more double bonds, remain liquid at room temperature, and derive mainly from plant sources. Trans fats result from hydrogenation of unsaturated fats and pose health risks.

Phospholipids contain glycerol, two fatty acid chains, and a phosphate group. Their amphipathic nature—hydrophilic head and hydrophobic tails—enables spontaneous formation of bilayer membranes in aqueous environments.

Steroids share a four-ring carbon structure. Cholesterol maintains membrane fluidity and serves as a precursor for steroid hormones. Testosterone, estrogen, and cortisol regulate physiological processes.

Proteins perform diverse functions including catalysis, transport, structural support, immune defense, movement, and signaling. Amino acids are protein monomers containing an amino group, carboxyl group, hydrogen atom, and variable R group attached to a central carbon.

Twenty standard amino acids exist, classified by R group properties. Nonpolar amino acids include glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, and proline. Polar amino acids include serine, threonine, cysteine, tyrosine, asparagine, and glutamine. Acidic amino acids are aspartate and glutamate. Basic amino acids are lysine, arginine, and histidine.

Peptide bonds form through dehydration synthesis between amino acids, creating polypeptide chains. Protein structure exists at four levels. Primary structure is the linear amino acid sequence determined by genetic information. Secondary structure involves local folding patterns including alpha helices and beta pleated sheets stabilized by hydrogen bonds between backbone atoms. Tertiary structure is the overall three-dimensional shape resulting from R group interactions including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. Quaternary structure occurs when multiple polypeptide subunits associate to form functional protein complexes.

Denaturation occurs when proteins lose their three-dimensional structure due to heat, pH changes, or chemical agents. Denatured proteins typically lose function. Some proteins can renature when conditions normalize.

Enzymes are protein catalysts that accelerate biochemical reactions by lowering activation energy. The active site is a specific region where substrates bind. The induced fit model describes conformational changes in enzymes upon substrate binding. Enzyme activity depends on temperature, pH, substrate concentration, and the presence of inhibitors or activators.

Nucleic acids store and transmit genetic information. Nucleotides are monomers consisting of a five-carbon sugar, phosphate group, and nitrogenous base.

Deoxyribonucleic acid (DNA) contains deoxyribose sugar and the bases adenine, guanine, cytosine, and thymine. DNA forms a double helix with antiparallel strands connected by hydrogen bonds between complementary base pairs: adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds).

Ribonucleic acid (RNA) contains ribose sugar and the bases adenine, guanine, cytosine, and uracil. RNA is typically single-stranded. Types include messenger RNA (mRNA) carrying genetic instructions, transfer RNA (tRNA) delivering amino acids during translation, and ribosomal RNA (rRNA) forming ribosome structure.

Adenosine triphosphate (ATP) is a nucleotide serving as the primary energy currency of cells. Hydrolysis of the terminal phosphate bond releases energy for cellular work.

Cell theory states that all living organisms consist of cells, cells are the basic units of life, and new cells arise from existing cells. Modern additions recognize that cells contain hereditary information and that all cells have similar chemical composition.

Prokaryotic cells lack membrane-bound nuclei and organelles. They include bacteria and archaea. Prokaryotes have nucleoid regions containing circular DNA, ribosomes, cell membranes, and cell walls. Many possess flagella, pili, and capsules. Prokaryotes reproduce by binary fission.

Eukaryotic cells possess membrane-bound nuclei and organelles. They include protists, fungi, plants, and animals. Eukaryotic cells are typically larger and more structurally complex than prokaryotes.

Viruses are not cells. A virus is genetic material (DNA or RNA) inside a protein coat (capsid), sometimes surrounded by a lipid envelope taken from the host. Viruses have no ribosomes or metabolism of their own, so they can reproduce only inside a host cell. This is why most biologists do not classify them as living organisms, and why antibiotics, which target bacterial structures, do not work against them.

Retroviruses such as HIV carry RNA and use the enzyme reverse transcriptase to make DNA that inserts into the host genome. Because each virus binds specific receptor molecules, viruses have narrow host ranges and often infect only certain cell types.

The plasma membrane is a phospholipid bilayer with embedded proteins. The fluid mosaic model describes membrane structure with proteins floating in a fluid lipid matrix. Integral proteins span the membrane, while peripheral proteins attach to surfaces. Membrane carbohydrates function in cell recognition. Cholesterol regulates membrane fluidity in animal cells.

The nucleus contains genetic material enclosed by a double membrane nuclear envelope perforated by nuclear pores. The nucleolus within the nucleus produces ribosomal RNA and assembles ribosome subunits. Chromatin consists of DNA wrapped around histone proteins and condenses into chromosomes during cell division.

Ribosomes are sites of protein synthesis. Free ribosomes produce cytoplasmic proteins, while bound ribosomes on rough endoplasmic reticulum produce membrane and secreted proteins.

The endoplasmic reticulum is a membrane network continuous with the nuclear envelope. Rough endoplasmic reticulum has attached ribosomes and functions in protein synthesis and modification. Smooth endoplasmic reticulum lacks ribosomes and functions in lipid synthesis, carbohydrate metabolism, and detoxification.

The Golgi apparatus consists of flattened membrane sacs called cisternae. It modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. Vesicles transport materials between the endoplasmic reticulum, Golgi apparatus, and cell membrane.

Lysosomes are vesicles containing digestive enzymes that break down macromolecules, damaged organelles, and engulfed materials. They function in autophagy and apoptosis.

Mitochondria are double-membrane organelles that produce ATP through cellular respiration. The outer membrane is smooth, while the inner membrane is highly folded into cristae. The matrix contains enzymes for the citric acid cycle. Mitochondria possess their own circular DNA and ribosomes, supporting endosymbiotic theory.

Chloroplasts are double-membrane organelles in plant cells and algae that perform photosynthesis. Thylakoids are internal membrane sacs containing chlorophyll. Grana are stacks of thylakoids. The stroma is the fluid surrounding thylakoids where carbon fixation occurs. Chloroplasts also possess their own DNA and ribosomes.

The cytoskeleton is a network of protein filaments providing structural support, cell shape, and intracellular transport. Microfilaments are thin actin filaments involved in cell movement and division. Intermediate filaments provide tensile strength. Microtubules are thick hollow tubes of tubulin involved in chromosome movement, organelle transport, and forming cilia and flagella.

Centrioles are paired cylindrical structures in animal cells that organize microtubules during cell division. They form the centrosome, which serves as the microtubule organizing center.

Vacuoles are membrane-bound sacs for storage and transport. Plant cells contain large central vacuoles maintaining turgor pressure. Contractile vacuoles in freshwater protists expel excess water.

Plant cells possess cell walls composed primarily of cellulose, providing rigidity and protection. Cell walls have primary layers in growing cells and secondary layers in mature cells. Plasmodesmata are channels connecting adjacent plant cells.

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others. Passive transport requires no energy input and moves substances down concentration gradients.

Diffusion is the net movement of particles from high to low concentration. Simple diffusion occurs directly through the membrane for small nonpolar molecules. Facilitated diffusion uses transport proteins for polar molecules and ions. Channel proteins form hydrophilic pores. Carrier proteins bind and undergo conformational changes.

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from regions of lower solute concentration to higher solute concentration. In hypotonic solutions, cells gain water; animal cells may lyse while plant cells become turgid. In hypertonic solutions, cells lose water; animal cells shrivel while plant cells plasmolyze. In isotonic solutions, water movement is balanced.

Active transport requires energy, typically ATP, to move substances against concentration gradients. The sodium-potassium pump moves three sodium ions out and two potassium ions into cells, maintaining electrochemical gradients essential for nerve impulses.

Bulk transport moves large particles or quantities of material. Endocytosis brings materials into cells through membrane invagination. Phagocytosis engulfs solid particles. Pinocytosis takes in dissolved substances. Receptor-mediated endocytosis uses specific protein receptors. Exocytosis releases materials from cells through vesicle fusion with the plasma membrane.

Cells coordinate their activities by chemical signals. In every pathway a signal molecule (ligand) binds a specific receptor, the signal is transduced inside the cell, and the cell responds, for example by activating enzymes or changing gene expression.

Nonpolar signals such as steroid hormones cross the membrane and bind receptors inside the cell, often acting as transcription factors. Polar signals such as peptide hormones bind surface receptors (for example G protein–coupled receptors) and work through second messengers. One signal can trigger a huge response because each step activates many molecules in the next step.

Every cell in a multicellular organism carries the same genome, yet a neuron and a muscle cell look and act completely differently. Cell differentiation is the process by which cells become specialized by expressing different sets of genes. Signals from neighboring cells and from the cell's position in the embryo turn on specific transcription factors, which activate tissue-specific genes and silence others.