Homework #6 due Monday, October 25, 10:00 pm

Obtaining Energy Living organisms can obtain energy through “eating”, energy & nutrients from other organisms extraction from chemical reactions in the environment (black smokers - ocean vents) extraction from radiative energy (e.g., photosynthesis)

Metabolism

Metabolism: chemical reactions within living organisms Metabolism: chemical reactions within living organisms. It takes place within cells. Why in cells? Chemical reactions much faster than in the open Collects the raw materials for the chemical reactions Provides the energy for the reactions Provides enzymes to catalyze the reactions Instructions for enzymes encoded in DNA

Without enzymes, life as we know it would not exist. A specialized substance that acts as a catalyst to regulate the speed of the many chemical reactions involved in the metabolism of living organisms. Without enzymes, life as we know it would not exist.

Metabolism and Cells Metabolism: Cells: Four forms of metabolism defined by: Sources of carbon (direct or indirect) Sources of energy (light or chemical) The four forms of metabolism are quite general and should apply to life anywhere Cells: Needed environment for metabolism at acceptable rate Origin of Life (on Earth and elsewhere): Look for cells as sites of metabolism

Carbon and Energy Sources Heterotroph: eat other organisms Autotroph: self-feeding by converting atmospheric CO2 Energy: Photoautotrophs (plants): photosynthesis: CO2 + H2O + sunlight sugar Photoheterotrophs (rare prokaryotes): carbon from food but make ATP using sunlight Chemoheterotrophs (animals): energy from food Chemoautotrophs (extreme prokaryotes): energy from chemicals and not sunlight

Why Carbon based? Can bond to as many as 4 atoms at a time. Can form skeleton of long chains of atoms (polymers). The complexity of life requires complex molecules. Fire – actually has several of these characteristics; viruses have few All living entities have the ability to reproduce and perpetuate genetic variation

Silicon can also form 4 bonds and is relatively abundant, however… Bonds are weaker than those of carbon (fragile: complex Si-based molecules don’t last long in water) Does not normally form double-bonds like Carbon; this limits the range of chemical reactions and molecular structures. Carbon is more mobile in the environment - it can travel in gaseous form, e.g., CO2 Fire – actually has several of these characteristics; viruses have few All living entities have the ability to reproduce and perpetuate genetic variation

Environmental limits to life (as we know it) ? Is the planet of interest missing any of the key ingredients? (water, energy, nutrients) Are temperatures below –15 or beyond +115 C? Is it really cushy? – does it have an atmosphere >125 protein and carbohydrates and genetic materials break down Below freezing chemical reactions are too slow to support life Water can be intermittent – just has to be available as a liquid Atmosphere – thick enough to block radiation and incoming meteorites; moderate temperatures; gravity issue – planet or moon has to be large enough to hold an atmosphere Energy – light or chemical Nutrients – all planets and moons in the solar system have the same general chemical composition. Must have a tectonic or volcanic or water or atmospheric process to make these available.

Importance of liquid water Contact: organic chemicals float in the cell and find each other Transportation: bring chemicals in and out of cells Participant in reactions, e.g.,:ATP, photosynthesis Necessity: Life on Earth: all use water Dormant without water: for a limited time only Elsewhere: need a liquid (are there alternatives?)

Water Liquid water plays a fundamental role in life: Make chemicals available (dissolved) Transports chemicals Plays a role in many metabolic reactions

Cells All life on Earth is made of cells - microscopic units in which living matter is separated from the outside world by a membrane. All cells on Earth share common characteristics (e.g., use of ATP, DNA, …), leading to conclusion that they share a common ancestor All cellular life is carbon based (organic molecules)

Components of Cells Carbohydrates: energy needs and structures Lipids: Source of energy & major component of membranes. Lipids can spontaneously form membranes in water. Proteins: participate in a vast array of functions; structural, enzymes, catalysts. Built from long chains of amino acids. Nucleic acids: instructions for reproduction

70 amino acids known to exist; only 22 are found in life on Earth. Only left handed versions are found in living organisms Both of these traits suggest a common ancestor for life on Earth.

Based upon the cellular structure of an organism, living cells come in two types: Prokaryotes Eukaryotes

The prokaryotes simplest type of cell lack a cell nucleus Most are unicellular two domains: bacteria & archaea asexual reproduction many do not require free oxygen

Eucaryotes cells are organized into complex structures enclosed within membranes. Have a nucleus. typically much larger than prokaryotes May be unicellular, as in amoebae, or multi-cellular, as in plants and humans. both sexual and asexual reproduction

Extremophiles Life that exists under “extreme” conditions, conditions that until recently were thought to be inhospitable to life.

Extremophiles Volcanic vents: Water temperature reaches 400°C (750°F), possible because of the large pressure Black smokers: mixed with volcanic chemicals

Extremophiles Antarctic dry valleys: Microbes in small pockets of water in rocks

Extremophiles Lithophiles (rock lovers): Several kilometers below the surface Chemical energy from rocks Carbon from CO2 filtering down

Extremophiles Endospores (e.g., anthrax) Can lay dormant for long periods Can survive lack of water, extreme heat and cold, and poisons Some can survive in vacuum

Implications for Extraterrestrial Life Oxygen for eukarya on Earth for only ~10% of the age of the Earth What is the probability that eukarya-like organisms would develop? We are more likely to find extremophiles elsewhere Extremophiles may be the norm, not the exception

Searching for the Origin of Life When Did Life Begin? DNA Molecules as Living Fossils Where Did Life Begin? How Did Life Begin? Early Organic Chemistry Chemistry to Biology Migration of Life to Earth? Early Evolution and the Rise of Oxygen Early Microbial Evolution Photosynthesis and Oxygen Rise of Oxygen

Origins of Life

Certain chemical processes are energetically favored given specific circumstances, i.e., in the presence of specific elements, energy, liquid(s) Can the presence of specific elements and energy inevitably lead to the formation of life? We know it can lead to the building blocks

The Miller–Urey experiment Simulated conditions thought to be exist on the early Earth Tested for the occurrence of chemical evolution Considered the classic experiment on the origin of life After one week, Miller and Urey observed that 10–15% of the carbon was now in the form of organic compounds. Recent re-analysis of Miller's archived solutions found 22 amino acids

Conditions simulated in Miller-Urey experiment do not match conditions now thought to have existed. Experiment has been redone MANY times, with more realistic conditions. Results remain consistent: the presence of specific elements & energy inevitably lead to the building blocks of life

When Did Life Begin?

Stromatolites Fossil evidence Living: colonies of bacteria living in outer layer of sedimentary rocks 3.5 Byr old rocks: almost identical layered structure Inconclusive evidence: sedimentation layering may mimic stromatolites Fossil evidence 3.5 Byr old Australian rock shows “cells” Could this form naturally from minerals? Younger sites: at least two more (3.2-3.5 byr old) Older sites: sedimentary rock too altered to be useful

13C/12C ratio Sterilization Normal abundance ratio 1/89 Living tissue and fossils show less 13C Some rocks older than 3.85 byr show the low 13C abundance Sterilization Last sterilization: 3.9-4.2 byr ago (Late Heavy Bombardment)

The evidence indicates life formed quickly after the Earth formed. Within a few 100 million years, Perhaps as short as 100 million years