Cells and Their Discovery

Cells and Their Discovery

• The cell is the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed.
• Cells are too small to be seen with the naked eye.
• In 1665, Robert Hooke used a basic microscope to examine thin slices of plant tissues, particularly cork. He observed empty chambers that resembled rooms in a monastery, which he named "cells."
• In 1676, Anton van Leeuwenhoek, using a microscope, was the first to view organisms in pond water, observe blood and sperm cells, and introduce the first unicellular organisms, which he called "animalcules," making detailed sketches.
• In 1838, Matthias Schleiden, a German botanist, concluded that all plants are composed of cells after observing various plant structures.
• In 1839, Theodore Schwann, a German zoologist, reached similar conclusions, stating that all animals are composed of cells.
• In 1858, Rudolf Virchow, a German physician, studied cell reproduction and proposed that all cells arise from pre-existing cells.

Cell Theory

• Cell theory consists of three fundamental principles:
  • All living organisms are composed of one or more cells.
  • The cell is the basic structural and functional unit of living organisms.
  • All cells arise from pre-existing cells.

Examples of Cell Theory

• Unicellular organisms, like amoeba, bacteria, euglena, and paramecium, perform all essential functions for growth, survival, and reproduction within a single cell.
• Multicellular organisms are structurally and functionally more complex, but still rely on the same fundamental mechanisms for life as unicellular organisms.

Early Forms of Life

• The first life is believed to have appeared about 3.5 billion years ago.
• The earliest evidence of life is found in microfossils, which are fossils containing remains of tiny plants and animals.
• Some early organisms lacked a nucleus, known as prokaryotes, and are considered the earliest forms of life.
• Prokaryotes have survived extreme conditions of the early environment.
• Photosynthetic organisms developed, able to use energy from the sun and carbon dioxide in the atmosphere to produce their own food. This process is called photosynthesis.
• Cyanobacteria were the first photosynthetic organisms to form, with easily identifiable microfossils. Their morphology remained relatively unchanged, leaving behind chemical fossils of pigment byproducts.
• Eukaryotes, organisms composed of cells with a nucleus and organelles, appeared later with more complex structures. Microfossils of these organisms with distinct structures date back about 1.5 billion years.

Theories about the Origin of Life

• Panspermia:* Life did not originate on Earth but was transported here from another celestial body.
• Special Creation Theory:* Life was created by a supreme being called God.
• Abiogenesis:* Life evolved from non-living matter through a process of spontaneous generation.
• Models of Abiogenesis:*
• Spontaneous Generation: Life arises from non-living matter. This theory was later disproven.
• Primordial Soup Theory: Proposed by Alexander Oparin, suggesting life originated in oceans or ponds. Supported by the Miller-Urey experiment.
• The Clay Hypothesis: Proposes that clay surfaces concentrated organic molecules and helped organize them into patterns, leading to the formation of life.
• Deep Hot Biosphere Hypothesis: Argues that life originated in the Earth's crust rather than on the surface.

Disproving Spontaneous Generation

• Francesco Redi (Italian physician) disproved spontaneous generation with experiments using open, closed, and gauze-covered flasks containing meat, showing that flies were not spontaneously generated from rotting meat.
• Lazzaro Spallanzani (Italian physiologist) further challenged the theory by boiling soups in sealed containers, showing that the microbes responsible for cloudiness entered after the seals were broken.
• Louis Pasteur (French chemist and microbiologist) used an S-shaped flask to demonstrate that microbes come from air-borne dust particles, not spontaneously generated from the air itself.

Evolution of Life

• Theory of Adaptation (Jean Baptiste de Lamarck): Changes an organism makes during life in order to adapt to its environment are passed onto its offspring. This theory was later disproven.
• Natural Selection (Charles Darwin): More individuals are produced each generation than can survive. Phenotypic variation exists within individuals and is heritable. Individuals with advantageous traits for their environment are more likely to survive and pass on those traits.

Mechanisms of Change

• Mutation: Small changes in DNA replication.
• Gene Flow: Migration of individuals with breeding in new populations resulting in genetic variation.
• Recombination: Through sexual reproduction, novel gene combinations are introduced in a population, adding to genetic variation.
• Genetic Drift: Random changes in allele frequencies within a population.

Selection in Humans

• Natural Selection: Environmental changes can favor different traits within a species. As humans significantly alter the environment, we may expect to see adaptation in other species.
• Artificial Selection: Human intervention, with specific goals, selects and controls traits in organisms.