General Biology: Introduction, Evolution, and Cell Structure Study Notes

Introduction and Evolution

Major Characteristics of Life

All living organisms share several fundamental characteristics that distinguish them from non-living matter.

• Life as Cellular: Cells are the basic units of life and are the smallest structures that exhibit all the properties of life. All living things are made of one or more cells.

• Life Processes: Information: Living things contain genetic information, mainly in the form of DNA. This information is used to control cellular activities and is passed from one generation to the next.

• Life Processes: Metabolism: All organisms must acquire and use energy from their environment. This energy powers all the processes necessary for life, like growth, movement, and repair.

• Life Processes: Reproduction: The capacity to produce new life is crucial for the continuation of species.

Hypothesis vs. Theory in Science

Scientific explanations are often classified as hypotheses or theories, each with distinct meanings and uses.

• Hypothesis: A proposed, testable explanation for a specific observation or question. It predicts what will happen and why it will happen for a particular case. Hypotheses must be testable and can be accepted or rejected, but never proven true.

• Theory: A broad, testable explanation for a wide range of observations, supported by a large body of evidence. Theories explain how and why phenomena occur on a much larger scale. Theories are never proven true, but are widely accepted because of strong supporting evidence.

• Key Differences: Hypotheses are narrow and specific; theories are broad and general. Hypotheses are often the starting point; theories are developed after repeated testing.

• Key Similarity: Both are testable and can be falsified (proven incorrect), but never absolutely proven true.

Unifying Theories in Biology

Two major unifying theories in biology are Cell Theory and Evolution.

• Cell Theory: States that all living organisms are made of cells and all cells come from pre-existing cells. This theory unifies biology by showing that the cell is the fundamental unit of life, and all life processes occur within cells.

• Theory of Evolution: States that living organisms evolved from a single common ancestor. Explains how populations of organisms change over time through processes like natural selection and genetic variation. This theory unifies biology by connecting all forms of life through shared ancestry and explaining the diversity of life.

Theory of Evolution

The theory of evolution states that all living organisms evolved from a single common ancestor and explains the diversity of life through mechanisms like natural selection and genetic variation.

• Unity of Life: All living organisms share a common ancestor and have fundamental similarities (like DNA, cells, and basic life processes).

• Diversity of Life: Populations change and adapt to different environments through natural selection and genetic variation, leading to the development of new traits and eventually new species.

• Summary: Evolution explains both the unity and diversity of life, and is supported by a large body of evidence.

Natural Selection

Natural selection is the process by which organisms with traits that help them survive and reproduce in their environment are more likely to pass those traits to the next generation. Over time, these beneficial traits become more common in the population.

• Key Point: Requires genetic diversity in a population.

• Biological Example: Giraffe Neck Length: In a population of giraffes, some have short necks, some have medium necks, and some have long necks. Giraffes with longer necks can reach more food in tall trees, survive better, and reproduce more. Over generations, long necks become more common in the population.

Steps in Natural Selection

Natural selection occurs through a series of steps:

1. Genetic Variation: Start with a population that has genetic differences. For example, crickets that are green or brown.

2. Selective Pressure: There is a pressure in the environment—like birds that eat crickets. The brown crickets blend in, but green crickets are harder for birds to see.

3. Differential Survival and Reproduction: Brown crickets survive and reproduce more often, so their traits are more likely to be passed on. Green crickets are eaten more and leave fewer offspring.

4. Change in Population Over Generations: Over many generations, the population shifts—more brown crickets, fewer green crickets. The DNA for brown color becomes more common.

Artificial Selection vs. Natural Selection

Artificial selection is driven by human intervention, while natural selection occurs without human intent.

• Artificial Selection: Humans choose which traits are desirable and selectively breed organisms to enhance those traits. Example: Breeding dogs for specific size or temperament, or selecting crops for higher yields.

• Natural Selection: Traits are inherited and controlled by nature. Example: Giraffes with longer necks survive better in the wild and pass on their genes.

Cells: Structure and Function

The Three Domains of Life

All living organisms are classified into three domains based on cellular structure and genetics.

• Bacteria: Single-celled, prokaryotic organisms (no nucleus). Found in many environments—soil, water, and even inside other organisms.

• Archaea: Also single-celled and prokaryotic, but genetically and biochemically distinct from bacteria. Often live in extreme environments, like hot springs or salty lakes.

• Eukarya: Organisms with eukaryotic cells (cells have a nucleus and organelles). Includes plants, animals, fungi, and protists.

Common Components and Characteristics of All Cells

All cells share a set of basic structures and molecules that are essential for life.

• Cell Membrane: All cells have a plasma membrane made of a phospholipid bilayer. It controls what enters and leaves the cell.

• Cytoplasm: Gel-like fluid inside the cell where chemical reactions occur.

• Genetic Material (DNA): All cells contain DNA, which stores genetic information and instructions for making proteins.

• Ribosomes: Structures that read genetic instructions and build proteins from amino acids.

• Proteins & Protein Structure: Proteins are essential molecules made of amino acids. They perform many functions, like catalyzing reactions (enzymes), providing structure, and transporting materials.

  • Primary Structure: Sequence of amino acids.

  • Secondary Structure: Local folding (like alpha helices and beta sheets).

Metabolic Diversity in Bacteria and Archaea

Metabolic diversity refers to the wide variety of ways bacteria and archaea obtain energy, electrons, and carbon for growth and survival.

• Phototrophs: Use sunlight for energy. Example: Cyanobacteria perform photosynthesis like plants.

• Chemotrophs: Use chemical compounds for energy.

  • Chemoorganotrophs: Use organic molecules (like glucose). Example: Escherichia coli in the human gut.

  • Chemolithotrophs: Use inorganic molecules (like H2, Fe2+, or NH3) for energy and electrons. Example: Nitrosomonas bacteria oxidize ammonia (NH3).

• Autotrophs: Use CO2 as their carbon source (make their own food).

• Heterotrophs: Use organic molecules as their carbon source (consume food). Example: Most bacteria in the human gut.

Summary: Bacteria and archaea can survive in many environments because of their metabolic diversity—they can "eat" and "breathe" things that other organisms cannot.

Structure and Function of Bacterial and Archaeal Cells

Bacteria and archaea are prokaryotes—they lack a nucleus and membrane-bound organelles.

• Cell Membrane: Surrounds the cell, controls movement of substances in and out.

• Cell Wall: Provides shape and protection.

  • Bacteria: Made of peptidoglycan.

  • Archaea: Lack peptidoglycan; have other molecules for strength (e.g., proteins, polysaccharides).

• Cytoplasm: Gel-like fluid inside the cell where metabolic reactions occur.

• Nucleoid: Region where the circular DNA is located (not surrounded by a membrane).

• Ribosomes: Small structures (70S) that build proteins from amino acids. Archaeal ribosomes have unique RNA sequences.

• Other Structures:

  • Flagella: Used for movement.

  • Pili/Fimbriae: Used for attachment to surfaces or other cells.

Relating Structure to Function: The cell wall and membrane protect the cell and help it survive in different environments. Nucleoid allows quick access to genetic material for rapid cell division. Specialized structures (flagella, pili) help with movement and interaction with the environment.

Chromosomal vs. Plasmid Bacterial DNA

Bacteria can carry genetic information in both chromosomal DNA and plasmid DNA, each with distinct features.

Feature	Chromosomal DNA	Plasmid DNA
Main genetic material of the cell	Yes	No
Contains essential genes for survival and reproduction	Yes	No
Location	Nucleoid region	Cytoplasm
Size	Large	Small
Can carry non-essential genes (e.g., antibiotic resistance)	No	Yes
Transferable between bacteria (horizontal gene transfer)	No	Yes

Summary: Chromosomal DNA is the main genetic blueprint, while plasmids are smaller, accessory DNA pieces that can provide extra abilities (like antibiotic resistance).

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Scientific names have been italicized where appropriate.

• Table entries have been logically inferred and expanded for academic completeness.