Biology: The Study of Life – Chapter 1 Study Notes

Biology: The Study of Life

1.1 What Does It Mean to Say That Something is Alive?

Biology is the scientific study of life, and all living organisms share five fundamental characteristics that distinguish them from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: Organisms process hereditary information encoded in genes and respond to environmental signals.

• Energy: All organisms acquire and use energy to maintain their internal order and survive.

• Evolution: Populations of organisms are continually evolving, adapting to their environments over time.

Theories in Biology

A theory in science is an explanation for a broad class of phenomena, supported by extensive evidence. This differs from the everyday use of "theory" as mere speculation.

• Cell Theory: Explains what organisms are made of and their origins.

• Theory of Evolution by Natural Selection: Explains how organisms are related and how they change over time.

• Chromosome Theory of Inheritance: Explains how hereditary information is transmitted across generations.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory was developed after early scientists observed cells using microscopes. Robert Hooke and Anton van Leeuwenhoek were pioneers in this field.

• Robert Hooke: Used a microscope with 30x magnification to observe and name "cells".

• Anton van Leeuwenhoek: Improved magnification to 300x and observed single-celled organisms.

• By the 1800s, biologists recognized that all organisms consist of cells.

Cell Theory and Spontaneous Generation

Cell theory states that all organisms are made up of cells and all cells come from preexisting cells. This challenged the idea of spontaneous generation, which suggested that life could arise from non-living matter.

• All-cells-from-cells hypothesis: Cells are produced when pre-existing cells grow and divide.

• Spontaneous generation hypothesis: Organisms could arise spontaneously under certain conditions.

Louis Pasteur’s Experiment

Louis Pasteur tested whether cells arise from pre-existing cells or by spontaneous generation. He used two flasks with nutrient broth, one with a swan neck open to air. Only the flask exposed to air developed life, supporting the cell theory.

Life Replicates through Cell Division

Cell division is essential for life. All cells in multicellular organisms descend from preexisting cells, sharing a common lineage. Evidence suggests life originated from non-life via chemical evolution.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

The chromosome theory of inheritance, proposed by Sutton and Boveri, states that hereditary information is encoded in genes located on chromosomes. In the 1950s, it was discovered that chromosomes are made of deoxyribonucleic acid (DNA).

• Genes: Segments of DNA that code for cell products.

• DNA: The hereditary material passed from cell to cell and from parent to offspring.

The Central Dogma

The central dogma describes the flow of genetic information in cells: DNA codes for RNA, which codes for proteins. This framework is essential for understanding how genetic information is expressed.

• DNA → RNA → Protein

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Genetic Variation and Heredity

DNA is copied accurately during cell division, but mistakes (mutations) can occur. These changes may alter proteins and lead to heritable variations, which are the basis for the diversity of life.

• Mutations: Changes in DNA sequence that can affect protein function and organism traits.

• Heritable variation: Variations passed to offspring, driving evolution.

Energy and Metabolism

Chemical reactions within cells require energy. Organisms must acquire energy (often in the form of ATP) and building blocks for macromolecules. The method of energy acquisition is central to the diversification of life.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Metabolism: The sum of all chemical reactions in an organism.

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of populations over time. Darwin and Wallace proposed that species are related by common ancestry and can be modified across generations (descent with modification).

• Population: Group of individuals of the same species living in the same area.

• Evolution: Change in population characteristics over time.

Natural Selection

Natural selection is the mechanism by which evolution occurs. It requires:

• Heritable variation among individuals.

• Certain traits that increase reproductive success in specific environments.

Traits that enhance fitness become more common, leading to evolutionary change and speciation.

Fitness and Adaptation

Fitness is the ability to produce offspring. Adaptation is a trait that increases fitness in a particular environment. For example, finches with small, pointed beaks had higher fitness when small seeds were abundant, leading to an increase in this adaptation in the population.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life is a family tree depicting evolutionary relationships among organisms. Phylogeny refers to the actual genealogical relationships. Genetic variation is analyzed by comparing DNA and RNA sequences; fewer differences indicate closer relationships.

• Example: Land plants and green algae have similar DNA sequences, indicating close relationship.

Phylogenetic Tree

A phylogenetic tree shows relationships between species. Branches sharing a recent common ancestor represent closely related species, while distant branches indicate more distant relationships. The tree of life is estimated from genetic data.

Major Groups of Organisms

The tree of life reveals three major groups:

• Eukaryotes: Organisms with a nucleus (Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

• Archaea: Prokaryotes lacking a nucleus, distinct from bacteria.

Additional info: Eukaryotic cells are typically larger and more complex than prokaryotic cells, which lack membrane-bound organelles.