Biology Chapter: Characteristics of Life

Defining Life and the Chemistry of Life

• Life is defined by characteristics, including eating, surviving, and reproducing.
• Life is composed of cells.
• Key characteristics shared by all living things include a hierarchy, harnessing and utilizing energy (metabolism), sensing and responding to stimuli, regulating through homeostasis, and having genetic material that allows for growth, reproduction, and independent evolution.
• Viruses do not meet all the requirements of life, containing nucleic acids but lacking cellular machinery and metabolism, relying on living cells for reproduction.

Overview

• The study of life covers what life is, its chemical origins, the macromolecules of life, the earliest forms of life, eukaryotic cells, and multicellularity.

Properties of Life

• Life is composed of cells.
• Living organisms display a hierarchical organization and utilize energy (metabolism), sensing and responding to stimuli.
• They regulate through homeostasis and have genetic material to grow, reproduce, and evolve.

How Life Originated

• Earth formed 4.6 billion years ago.
• Life likely started around 3.8-4 billion years ago.
• The first clear fossil evidence of prokaryotic cells occurred 3.6 billion years ago.
• This involved the formation of biomolecules, their assembly into cell-like structures, and the acquisition of genetic capability by one or more biomolecules.
• Earth's habitable zone allows for liquid water and is crucial for life as we know it.

Origins of Life - Prokaryotes

• Early Earth had no cells, hence no life.
• At one point, there were no cells and thus no life. Prokaryotes, including bacteria and archaea emerged later.

Origins of Life - Eukaryotes

• Eukaryotes emerged about 2 billion years ago.
• Animals emerged 525 million years ago.
• Humans have existed for roughly 150,000 years.

Evolution of Life

• Evolution of life involved several key steps, including the formation of biomolecules, assembly into cell-like structures, and acquisition of genetic capabilities by one or more biomolecules.

Biologically Important Molecules

• Life is built on major macromolecules (biomolecules): nucleic acids, proteins, lipids, and carbohydrates.
• These can develop from simpler molecules.

How Did Biomolecules Originate?

• There are three main hypotheses for biomolecule origin: a reducing atmosphere, deep-sea vents, and extraterrestrial origins.

Reducing Atmosphere

• The primordial atmosphere (4 billion years ago) likely contained water vapor (H₂O), hydrogen (H₂), ammonia (NH₃), methane (CH₄), and no oxygen (O₂).
• Early molecules (H₂, CH₄, NH₃) have a high abundance of electrons.
• This atmosphere contained reactive molecules that facilitated the formation of more complex molecules.

UV Light and Lightning

• The lack of ozone in the early atmosphere allowed energetic UV light to reach the lower atmosphere.
• Lightning was also a common energy source.

Miller-Urey Experiment

• Stanley Miller (1953) simulated the early Earth's reducing atmosphere.
• He mixed gases (H₂, CH₄, NH₃, H₂O) in a closed chamber and sparked electrodes to provide energy.
• After a week, many organic molecules, including urea, amino acids (e.g., lactic acid, formic acid, acetic acid) were detected.
• These molecules are key components of living organisms.

More Complex Molecules

• When hydrogen cyanide (HCN) and formaldehyde were added to the Miller-Urey experiment, more complex molecules (such as purines, pyrimidines, components of nucleic acids) and other biomolecules were formed.
• Sugars like glyceraldehyde, ribose, glucose, and fructose were also detected.

Role of Deep Sea Vents

• Deep-sea vents also provided heat and reduced molecules (methane, ammonia, hydrogen sulfide) for the formation of biomolecules.

Extraterrestrial Origins

• Organic molecules may have come from space via carbonaceous chondrite meteorites.
• These meteorites are rich in organic molecules, including amino acids (glycine, glutamic acid, alanine) and purines and pyrimidines.

From Monomers to Polymers

• Polymers (macromolecules like proteins, DNA) form when monomers (simple molecules like amino acids) bond together.
• Solid surfaces (like clays) could have provided a friendly environment ("clay hypothesis") for polymerization.

Key Attribute of Life

• Life requires defining compartments (membranes) to separate reactions.
• It requires genetic information storage and translation into proteins.
• Pathways are needed to obtain and use energy.

Lipid Spheres (Liposomes)

• Lipid spheres (liposomes) are compartments that separate reactions within a space, a key step in early cell development.
• Probionts, or abiotically produced organic molecules, are surrounded by a membrane.

Probionts

• Probionts can spontaneously form bilayer structures, similar to lipid vesicles.
• These are selectively permeable, meaning only certain molecules can pass through.
• Clays can accelerate the formation of lipid vesicles.

The Central Dogma

• In modern life, DNA is copied into RNA, which then guides protein synthesis.
• Proteins carry out life's functions.

RNA World

• RNA may have initially been both the genetic material and the catalyst (ribozymes) responsible for life's functions.
• RNA molecules fold into specific shapes to perform these functions, including carrying information.

RNA Replaced by DNA & Proteins

• Specialized molecules emerged for stable information storage (DNA) and catalytic functions (proteins).
• Some RNA molecules may be able to direct small protein synthesis; proteins provide much more variability.
• DNA is chemically more stable than RNA.

Simple Oxidation Reduction Reactions

• Early reactions were simple steps, eventually evolving into more complex, efficient multistep ones.
• Adenosine triphosphate (ATP) acts as an energy carrier, combining energy-releasing and energy-requiring reactions.

Earliest Forms of Life

• Stromatolites are the earliest fossilized evidence of life.
• They are formed by photosynthetic bacteria (cyanobacteria).
• Oxygenic photosynthesis is crucial, using sunlight to oxidize water.

Common Ancestor

• Present-day organisms are categorized into three domains: Archaea, Bacteria, and Eukaryotes.
• They all share a common ancestor (LUCA—last universal common ancestor).

Eukaryotic Cells and Multicellularity

• Eukaryotes arose approximately 2.5 billion years ago.
• Key characteristics of eukaryotes include:
  • Separation of DNA and cytoplasm via a nuclear envelope.
  • Membrane-bound compartments, or organelles, with specialized functions.
• Other organelles include mitochondria, chloroplasts, endoplasmic reticulum, and Golgi complex.

Endosymbiotic Theory

• Chloroplasts and mitochondria evolved from free-living prokaryotic cells.
• Mitochondria originated from aerobic bacteria.
• Chloroplasts originated from photosynthetic cyanobacteria.
• These cells were engulfed by larger prokaryotic cells, forming an endosymbiotic relationship.

Endosymbiosis and Horizontal Gene Transfer

• Some proto-endosymbiotic genes became redundant and were lost.
• Other genes were relocated to the nucleus.
• Horizontal gene transfer also occurred.

Why Eukaryotes?

• Specialized mitochondria support greater energy production.
• Larger genomes and more complex cells are allowed.
• Evolution of specialization and multicellularity is enhanced.