General Biology: Foundations, Cell Structure, and Biomolecules

What is Biology?

Defining Biology and Characteristics of Life

Biology is the scientific study of life and living organisms. It seeks to answer fundamental questions such as: How do we characterize life? What features distinguish living things from non-living things?

• Order: Living things exhibit complex but ordered organization, from molecules to entire organisms.

• Evolutionary Adaptation: Populations adapt to their environments over generations.

• Regulation: Organisms maintain stable internal conditions (homeostasis) despite changes in their external environment.

• Energy Processing: Living things acquire and use energy for activities.

• Response to the Environment: All living things respond to environmental stimuli.

• Growth and Development: Organisms grow and develop according to specific instructions coded in their DNA.

• Reproduction: Organisms reproduce, passing genetic information to their offspring, ensuring the continuity of their species.

Example: A giraffe (order), a butterfly (energy processing), and a Venus flytrap (response to the environment) each illustrate different characteristics of life.

Levels of Biological Organization

Hierarchy and Complexity

Biology studies life at multiple levels of organization, from the largest scale of the biosphere to the smallest molecular components. Understanding these levels helps explain how complex living systems are structured and function.

• Biosphere: The global sum of all ecosystems, including all life and the environments in which they exist.

• Ecosystem: Communities of living organisms interacting with their physical environment.

• Community: All populations of different species living in a defined area.

• Population: Groups of individuals of the same species living in a defined area.

• Organism: Individual living entities performing vital functions.

• Organ System: Groups of organs that work together for specific functions.

• Organ: Structures composed of tissues working together.

• Tissue: Groups of similar cells with specific functions.

• Cell: Smallest unit of life.

• Organelle: Specialized structures within cells (e.g., nucleus, mitochondria).

• Molecule: Chemical structures consisting of two or more atoms bonded together.

Example: A flower (organism) is made up of organs (petals, leaves), which are composed of tissues, which in turn are made of cells containing organelles and molecules.

Cell Theory and Domains of Life

Cell Structure and Classification

Cells are the smallest units of life. All living things are made of cells, which are classified into three domains based on cell type:

• Domain Bacteria: Prokaryotic cells, lacking a nucleus and membrane-bound organelles.

• Domain Archaea: Also prokaryotic, but with unique genetic and biochemical characteristics.

• Domain Eukarya: Eukaryotic cells, containing a nucleus and membrane-bound organelles. Includes kingdoms such as Plantae, Animalia, Fungi, and Protista.

Additional info: Prokaryotes are generally unicellular, while eukaryotes can be unicellular or multicellular.

Genetic Information and Heredity

DNA and Genes

Organisms inherit genetic information in the form of DNA (deoxyribonucleic acid). DNA is organized into genes, which carry instructions for building proteins that determine the structure and function of cells.

• Reproduction: Ensures the transfer of genetic information from one generation to the next.

• Growth and development: Is directed by genetic instructions encoded in DNA.

Chemical Elements and Compounds in Biology

Elements and Compounds

All matter, including living organisms, is composed of chemical elements. Elements are substances that cannot be broken down into simpler substances by chemical means. When elements combine in fixed ratios, they form compounds.

• Example: Sodium (a reactive metal) and chlorine (a poisonous gas) combine to form sodium chloride (table salt), which is safe to eat.

Essential Elements for Life

Of the 92 naturally occurring elements, only a small number are essential for life. These elements are required in large or trace amounts by living organisms.

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N) make up about 96% of living matter.

• Other Essential Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), and Magnesium (Mg) are also important.

• Trace Elements: Elements required in very small amounts (e.g., iron, iodine).

Atoms: Structure and Properties

Atomic Structure

An atom is the smallest unit of an element that retains its chemical properties. Atoms are composed of subatomic particles:

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles also located in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus in energy levels (shells).

The atomic number is the number of protons in an atom and defines the element. The mass number is the sum of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons.

Electron Configuration and Chemical Behavior

The arrangement of electrons in an atom's electron shells determines how it interacts with other atoms. Atoms are most stable when their outer electron shell (valence shell) is full.

• Valence Electrons: Electrons in the outermost shell, involved in chemical bonding.

• Electron Orbitals: Regions of space where electrons are likely to be found. The shape and number of orbitals influence chemical bonding.

Chemical Bonds and Molecules

Types of Chemical Bonds

• Covalent Bonds: Atoms share pairs of electrons. Can be single, double, or triple bonds.

• Polar Covalent Bonds: Electrons are shared unequally between atoms (e.g., in H2O), resulting in partial charges (δ+ and δ−).

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms of similar electronegativity, resulting in neutral molecules (e.g., H2).

• Ionic Bonds: Electrons are transferred from one atom to another, creating oppositely charged ions (cations and anions) that attract each other (e.g., NaCl).

• Hydrogen Bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (such as oxygen or nitrogen) and another electronegative atom. Important in the structure of water and biological molecules.

Example: In sodium chloride (NaCl), sodium donates an electron to chlorine, forming Na+ and Cl− ions that are held together by ionic bonds.

Chemical Reactions

Chemical reactions make and break chemical bonds, transforming reactants into products. The properties and functions of molecules depend on their structure and the types of bonds they form.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of the reaction.

Water: Structure and Properties

Structure of Water

• Water (H2O) is a polar molecule with a bent shape.

• Oxygen is more electronegative than hydrogen, creating partial negative (O) and partial positive (H) charges.

• Hydrogen bonds form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

Properties of Water Due to Hydrogen Bonding

• Cohesion: Water molecules stick to each other, leading to surface tension.

• Adhesion: Water molecules stick to other substances.

• Capillary Action: Movement of water within narrow spaces due to cohesion and adhesion (important in plant transport).

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change.

• Solvent Properties: Water dissolves many substances, making it a universal solvent.

Acids, Bases, and pH

• Acid: A substance that increases the concentration of H+ ions in solution (pH < 7).

• Base: A substance that decreases the concentration of H+ ions (pH > 7).

• Neutral: pH = 7 (pure water).

The pH scale measures the concentration of hydrogen ions in solution.

Organic Molecules

Importance of Carbon

Carbon forms the backbone of organic molecules due to its ability to form four covalent bonds. Organic compounds contain carbon; inorganic compounds do not (e.g., NaCl).

Carbohydrates

• Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen (typically in a 1:2:1 ratio).

• Functions: Provide energy and structural support.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., maltose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, glycogen).

Key Reactions

• Dehydration synthesis: Joins two molecules by removing water.

• Hydrolysis: Breaks a bond by adding water.

Proteins

Amino Acids and Protein Structure

• Amino acids are the building blocks of proteins, containing an amino group, carboxyl group, hydrogen, and R group.

• Peptide bond: Covalent bond linking amino acids.

• Polypeptide: A chain of amino acids.

Levels of Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding (e.g., helix, β-sheet) stabilized by hydrogen bonds.

• Tertiary structure: Shape determined by interactions among R groups.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Functions

• Structural proteins: Provide support (e.g., collagen).

• Enzymes: Catalyze biochemical reactions.

• Transport proteins: Move substances (e.g., hemoglobin).

• Defense proteins: Protect against disease (e.g., antibodies).

Lipids

Types and Functions

• Fats: Store energy and insulate.

• Phospholipids: Form cell membranes; contain hydrophilic heads and hydrophobic tails.

• Steroids: Four fused carbon rings; function as hormones (e.g., cholesterol).

Phospholipids and Cell Membranes

• Phospholipid bilayer: Forms the basic structure of cell membranes, creating a selectively permeable barrier.

• Hydrophilic heads face outward; hydrophobic tails face inward.

• Provides embedded roles in the membrane: assists in transport and communication.

Nucleic Acids

Structure and Function

• Nucleic acids (DNA and RNA) store and transmit genetic information.

• Composed of monomers called nucleotides (sugar, nitrogenous base, phosphate group).

• DNA bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• RNA bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

Metabolism

Metabolism refers to all chemical reactions in an organism, including catabolism (breaking down molecules) and anabolism (building molecules).

Prokaryotic vs. Eukaryotic Cells

Major Features of Prokaryotic Cells

• Cell wall: Maintains cell shape, protects the cell, and prevents bursting in hypotonic environments.

• Surface structures: Many have a capsule (sticky outer layer), fimbriae (attachment structures), and flagella (motility).

• Internal organization: Simpler than eukaryotes, but some have specialized membranes for metabolic functions.

Eukaryotes: Compartmentalization and Complexity

• Domains: Eukarya (includes protists, fungi, animals, and plants).

• Nucleus: Contains most of the cell's DNA, surrounded by a double membrane (nuclear envelope).

• Organelles: Specialized structures with unique functions (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Cytoskeleton: Network of protein fibers that maintain cell shape and assist in movement.

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	0.5–5 μm	10–100 μm
DNA location	Nucleoid	Nucleus
Examples	Bacteria, Archaea	Protists, fungi, plants, animals

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement of the cell and its components.

• Microfilaments (Actin Filaments): Thin filaments involved in cell movement and muscle contraction.

• Intermediate Filaments: Provide mechanical strength to cells.

• Microtubules: Act as tracks for processes such as mitosis, vesicle transport, and cell motility.

Example: The cytoskeleton is essential for processes such as mitosis, vesicle transport, and cell motility.

Antibiotics and Cell Structure

How Antibiotics Target Bacterial Cells

Antibiotics are chemicals that kill or inhibit the growth of bacteria by targeting specific features of prokaryotic cells that are absent in eukaryotic cells.

• Peptidoglycan Cell Wall: Many antibiotics, such as penicillin, inhibit the synthesis of peptidoglycan, weakening the bacterial cell wall and causing cell lysis.

• Ribosomes: Some antibiotics target bacterial ribosomes, interfering with protein synthesis.

• DNA Replication: Other antibiotics disrupt DNA replication or repair mechanisms in bacteria.

Example: Penicillin is effective against gram-positive bacteria by preventing cross-linking in the peptidoglycan cell wall.

Mitochondria and Chloroplasts: Change Energy from One Form to Another

Mitochondria: Cellular Respiration

• Mitochondria: Sites of cellular respiration, generating ATP by extracting energy from sugars, fats, and other fuels.

• Structure: Double membrane, with inner membrane folded into cristae to increase surface area.

• Matrix: Innermost compartment containing enzymes, mitochondrial DNA, and ribosomes.

Chloroplasts: Photosynthesis

• Chloroplasts: Found in plants and algae; sites of photosynthesis, converting solar energy to chemical energy in the form of glucose.

• Structure: Double membrane, internal system of thylakoids (flattened sacs) stacked in grana, surrounded by stroma (fluid).

Endosymbiont Theory

• Suggests that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence includes their own DNA, double membranes, and similarities to certain prokaryotes.

Key Biological Terms

Term	Definition
Cohesion	Attraction between molecules of the same substance
Adhesion	Attraction between molecules of different substances
Capillary action	Movement of liquid within narrow spaces due to cohesion and adhesion
Heat capacity	Amount of heat needed to change temperature of a substance
Carbohydrate	Organic molecule for energy and structure
Hydrolysis	Chemical breakdown by adding water
Starch	Polysaccharide for energy storage in plants
Cellulose	Polysaccharide for plant cell wall structure
Amino acids	Building blocks of proteins
Dehydration synthesis	Joining molecules by removing water
Peptide bond	Covalent bond between amino acids
Protein	Polymer of amino acids with diverse functions
Lipids	Hydrophobic molecules for energy storage and membranes
Nucleic acids	DNA and RNA; store genetic information
Phospholipids	Major component of cell membranes
Hydrophilic head	Water-attracting part of phospholipid
Phosphate	PO4 group in nucleic acids and phospholipids
Hydrophobic tails	Water-repelling part of phospholipid
Lipid bilayer	Double layer of phospholipids in membranes
Unsaturated	Fatty acid with double bonds
Permeability	Ability of substances to pass through a membrane
Metabolism	All chemical reactions in an organism

Additional info: This study guide covers foundational topics in general biology, including the characteristics of life, levels of biological organization, cell structure, biomolecules, and key biological terms. It is suitable for exam preparation and provides a concise overview of essential concepts.