1.5 The Origin of Cells Cells come from pre-existing cells The first cells arose from non-living materials Endosymbiosis

Cell Division & The Origin of Cells All cells are formed by the division of pre-existing cells. You are made of trillions of cells. Any cell that is produced by your body is the product of cell division from another cell. You originally started life as a single cell, a zygote, that underwent numerous cell divisions to produce the trillions of cells that comprise you today.

Even that single celled zygote came from other cells – the combination your father’s sperm cell and your mother’s egg cell. We can trace the origins of all the cells in our parents back to the zygotes from which they developed, to our human ancestors before them, to human’s pre-existing ancestral species, all the way back to the earliest cells on Earth.

We, and all other living things on this planet, are descendants from the “first cell”. But if all cells come from pre-existing cells, where did the first cell come from? How did life start on this planet?

Problems for starting life on Earth How could the lifeless ball of rock that the planet Earth was 3.5 billion years ago, become home to such lush vegetation and a wide variety of bacteria, fungi, protists, and animals that we see today? There are 4 problems which needed to be overcome for life on Earth to exist.

Requirements to start life Production of Simple Organic Compounds The assembly of organic compounds into polymers Development of a mechanism for inheritance. Formation of membranes

1. Production of Simple Organic Compounds Life as we know it is based on organic compounds (compounds containing carbon and hydrogen), such as amino acids (the building blocks of proteins) But early Earth only had inorganic matter: rocks, minerals, gases, water….

Abiogenesis had to occur. abiogenesis: the creation of organic matter from inorganic matter) It is believed that organic molecules were formed in the shallow waters of the oceans as the products of chemical reactions between compounds in the atmosphere and the water.

Miller and Urey Scientists Stanley Miller and Harold Urey performed a ground- breaking experiment in 1953 They recreated the conditions of early Earth and proved that organic compounds could be synthesized from inorganic compounds.

N2 = nitrogen gas H2O = water H2 = hydrogen gas NH3 = ammonia gas CO2 = carbon dioxide gas * CH4 = methane gas

The Experimental Design The apparatus included an “oceanic compartment” and an “atmospheric compartment” The H2O in the oceanic compartment was heated to evaporate and cooled to condense – thereby recreating the H2O cycle. Since early Earth did not have an ozone layer, they kept the system at a warm temperature and exposed it to UV radiation Generated electric sparks to simulate lightning

The Results After 1 week: 15% of the carbon was now found in organic form! 13 of the 20 amino acids had formed inside the primordial soup! Sugars had formed! The nitrogenous base adenine (a component of DNA and ATP) had formed!!!

2. Assembly of these molecules into polymers Organisms are organized! Simple organic molecules would have needed to undergo a process of polymerization to form the larger more complex organic chemicals required by cells.

Deep-Sea Vents Organic molecules could have first formed around hydrothermal vents – places where hot water emanated from beneath the ocean floor. Form when cracks in the crust of the seabed expose sea water to rocks below which are heated by magma

As the hot water rises it picks up countless minerals along the way. Hydrothermal vents are sometimes referred to as black smokers because the water coming out of them contains so many dark minerals it looks like smoke. The chemicals and source of energy in this environment could be suitable for the formation of biological polymers.

3. The development of a mechanism for inheritance Today, most organisms use DNA as its molecule for heredity. To replicate DNA and pass it on to the next generation, enzymes are required However, enzymes cannot be made without DNA. Therefore, it is unlikely that DNA was the early molecule for heredity

Ribozymes However, the small sequences of the molecule RNA can act as enzymes and replicate itself. These are called ribozymes Thus, RNA may be the early molecule for hereditary.

4. Formation of Membranes Water is important to life but tends to depolymerize (break down)molecules Many compounds dissolve in water, making it difficult to organize into polymers The formation of closed membranes is likely an early and important event in the origin of cellular life It allows for the development of an internal chemistry different from the external environment

Coacervates Coacervate - a microscopic sphere that forms from lipids in water. Forms spontaneously due to the hydrophobic forces between the water and lipid molecules. Can maintain an internal chemical environment different from the surrounding environments. Coacervates can be selectively permeable

Coacervates Coacervates (lipids)

Although they are not living organisms, coacervates are a significant step toward the formation of cells. They solve the problem of protecting polymers from their destructive environments. Could be primitive versions of the first cell membranes

(remember our cell membranes are lipid based) PROTOBIONTS – the first precursors to cells, were likely coacervate droplets which included polynucleotides (DNA or RNA) (remember our cell membranes are lipid based) Overtime, true cell membranes evolved and other characteristics of cells developed. Cellular respiration Asexual reproduction

Where did all the oxygen come from? 1/5 of the air you are breathing right now is oxygen. However, there was none at all present 4 billion years ago. The earliest life forms on Earth were bacteria and they lived in an environment with an atmosphere of mostly CO2 Thus, early life forms were anaerobic cells (did not require oxygen)

These single-celled organisms would consume organic molecules (i. e These single-celled organisms would consume organic molecules (i.e. simple sugars) that were forming from chemical reactions on Earth The more they reproduced, the more food that was consumed.

After million of years, their population would have reached such large numbers that food began to be scarce. In this food shortage, bacteria that could make their own food would have an advantage.

~3.5 billion years ago, bacteria (that is believed to be related to today’s cyanobacteria)developed the ability to photosynthesize. Must have contained a form of chlorophyll

Gives bacteria a source of energy (sunlight) to survive Development of photosynthesis was one of the most significant evens in the history of Earth Gives bacteria a source of energy (sunlight) to survive Created a mass pollution of the atmosphere Pollution of oxygen!!!

Oxygen gas is toxic to the kinds of bacteria which preceded photosynthetic ones, so this pollution would have eventually killed off large populations of anaerobes. Anaerobic bacteria that survived would live in mud of places protected from the new oxygen-rich atmosphere.

The ability of an organism to make its own food gives it a distinct advantage over those that cannot. As a result, photosynthetic bacteria proliferated and produced more and more oxygen

Evidence of Endosymbiosis Chloroplasts and mitochondria are surrounded by a double membrane (like the cell membrane) Mitochondria and bacteria (a prokaryote) have a similar size

Evidence of Endosymbiosis Mitochondrial and bacterial ribosomes are very similar in size and shape. Mitochondria and chloroplasts have their own DNA – which is circular like bacteria. Mitochondria divide in a process similar to binary fission like bacteria

Problems with Endosymbiosis The ability to engulf another cell and have it survive in the cytoplasm does not guarantee that the host cell can pass it on to its offspring the genetic code to synthesize the newly acquired organelle When chloroplasts or mitochondria are removed from a cell, they cannot survive on their own.