Are We Alone in the Universe?


What Does Life Require?

The Definition of Life

- There is no simple definition of life!

- Life is “defined” by the possession of a number of complex characteristics.


- All living Earth organisms…

- have a common set of biological molecules

- can maintain homeostasis

- can evolve

- require liquid water


The Properties of Water

- Elements: fundamental forms of matter

- Atoms: the smallest units of an element

- Atoms are composed of protons, neutrons, and electrons


- Protons (positive charge) + neutrons = nucleus

- Electrons (negative charge) are outside the nucleus.


- Molecule: two or more atoms held together by chemical bonds

- Water molecule: two hydrogen atoms bonded to one oxygen atom

- A bond made by sharing of electrons is called a covalent bond


- Water is a polar molecule:

- Oxygen side is slightly negative

- Hydrogen side is slightly positive

- When molecules have no charges, they are nonpolar


- Water is a good solvent (a liquid that helps chemical reactions)

- Solute: what is being dissolved (a solid)

- Solution: the solute in the solvent (a mixture)


- Hydrogen bond: the weak attraction between the hydrogen atom of one water molecule and the oxygen atom of another

- Hydrogen bonds are the weakest form of chemical bond

- Water molecules tend to stick together: cohesion

- Water molecules tend to stick to charged molecules: adhesion


- Water can dissolve salts and hydrophilic (water–loving) molecules because it is polar (partially charged).


- Water can dissolve acids and bases.

- The pH scale is a measure of the relative amounts of acids and bases in a solution.

- pH greater than 7 = basic (more base)

- pH lower than 7 = acidic (more acid)

- Pure water has a pH of 7 = neutral


- Nonpolar molecules, such as oil, do not contain charged atoms.

- These atoms are called hydrophobic (water–hating).


Organic Chemistry

- All life on Earth is based on organic chemistry: the chemistry of the element carbon.

- Carbon makes up most of the mass of living organisms.


- Carbon: a molecular “TinkerToy”

- Can bond to up to 4 different atoms at once

- Covalent bonds: strong bonds from sharing electrons

- Carbon can make macromolecules, or molecules that are large and usually comprised of smaller subunit molecules


Structure and Function of Macromolecules

- Carbohydrates: molecules of carbon, oxygen, and hydrogen

- Major source of energy for cells

- Hydrophilic


- Proteins: polymers of amino acids; joined by peptide bonds

- Often have a mix of hydrophilic and hydrophobic portions


- Proteins are made up of carbon, oxygen, hydrogen, and nitrogen.

- There are 20 different amino acids, with different chemical properties.

- Different combinations of amino acids give proteins different properties.

- The amino acid sequence in a protein determines the shape of the protein


- Lipids: hydrophobic; composed mostly of carbon and hydrogen

- Three types:


- Nucleic acids: polymers of nucleotides

- Nucleotide: sugar + a phosphate + a nitrogenous base


- Nucleotides are of two types: RNA and DNA, depending on the sugar.

- DNA is the hereditary material in nearly all organisms.

- RNA is heavily involved with protein synthesis - converting the DNA code into a functioning protein.


- The structure of a DNA molecule is a double helix.

- Two strands of DNA are held together by hydrogen bonds


- Bonding between bases on opposite strands follows strict base-pairing rules:

- A with T

- G with C


Life on Earth

Prokaryotic and Eukaryotic Cells

Cell Structures

- All cells have the following:

- Plasma membrane surrounding the cell (this is a phospholipid bilayer)

- Cytoplasm (cytosol) inside the membrane; chemical reactions take place here

- Genetic information in the form of DNA

- Ribosomes for the production of proteins, made up of RNA and proteins


- All cells on Earth are either prokaryotic or eukaryotic.

- Prokaryotic cells are smaller and simpler in structure.

- They probably resemble the earliest cells to arise on Earth.

- Some structures in the Martian meteorite resemble them.


- All cells are surrounded by a plasma membrane.

- Made of a phospholipids bilayer: hydrophobic tails orient inside the membrane, away from water

- Fluid mosaic: lipids and proteins can move about within the membrane

- Semipermeable: some molecules can cross and some can’t


- Prokaryotes have DNA but do not have a nucleus (membrane-bound structure that contains the DNA).


- Eukaryotic cells are much more complex.

- Have true nuclei surrounded by a membrane

- Also have membrane-bound organelles with specialized jobs


The Tree of Life and Evolutionary Theory

- All Earth organisms share many similarities:

- Same basic biochemistry, with same types of macromolecules

- All organisms consist of cells

- Cells always have phospholipids bilayer plasma membrane

- Eukaryotes share most of the same organelles.


- This unity of life is best explained by a tree of life, with modern species having evolved from common ancestors.


- Early life.

- Evolutionary history of Humans.