Part of A2.1

Cells as the smallest unit of self-sustaining life

Cells are the smallest unit of self-sustaining life because they contain all the components necessary to carry out all eight processes of life at some point in their life cycle (Table 1).

Table 1. The eight processes of life.

Study skills

You can use the acronym MR HM GREN to help you to remember these eight life processes.

Viruses are considered non-living because they are unable to reproduce outside of the host cell. They rely on the host cell for many life processes, including nutrition and growth, and they do not possess a metabolism (see subtopic A2.3).

The spontaneous origin of cells

Scientists believe that there were necessary steps for the evolution of the first cells:

1. Simple organic molecules such as amino acids and hydrocarbon chains were formed.

2. Chemical reactions were accelerated in the process of 

   catalysis

   .

3. Larger organic molecules including RNA and phospholipids were assembled from smaller molecules.

4. Some of these molecules, including RNA were able to self-replicate.

5. Formation of a membrane-bound compartment (the cell surface membrane) allowed the internal chemistry of the cell to become different from that outside the compartment.

Evidence for the origins of carbon compounds

In 1952, two scientists, Stanley Miller and Harold Urey, simulated Earth’s ​

pre-biotic

 atmospheric conditions to show that spontaneous formation of organic molecules could occur.

Miller and Urey set up a closed system. The system contained:

• water, which simulated the ocean

• a gas inlet with which to add the reducing gases methane, ammonia and hydrogen

• electrical sparks to simulate the electrical storms present on pre-biotic Earth.

The water was vaporised and run through the gases and electrical sparks, after which a cooling jacket was used to condense the water. The condensed water droplets formed on the side of the apparatus and collected at the bottom, representing the primordial soup of the early oceans. After a week of running the experiment, they showed that the primordial soup contained basic organic monomers, including amino acids.

This experiment proved that non-living synthesis of organic molecules was possible in the conditions existing on early Earth. It is important to note that this experiment did not prove that carbon compounds originated in this way, only that it could have happened this way.

Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers

An important step in the formation of the first cell was the formation of a membrane-bound compartment. This would have occurred when fatty acids spontaneously coalesced (merged) to form a spherical 

bilayer

, a double layer of lipid molecules that encloses a space (Figure 1). Because of this physical separation, the interior of the vesicle would then have been able to provide a chemical environment with a different chemistry to the external environment.

Figure 1. A necessary step in the evolution of the first cells was the spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers.

This is an important step in the origins of cells, because this separation allows cells to control and maintain a precise set of conditions for proper cell functioning, including pH and solute concentration.

Membrane structure is covered in subtopic B2.1.

RNA as the first genetic material

The ‘RNA first’ hypothesis posits that RNA was the basis for the formation of the first cell-like structure, acting as both genetic material and as a 

catalyst

. This hypothesis suggests that:

1. RNA was formed from inorganic sources.

2. RNA was able to replicate using 

   ribozymes

   .

3. RNA was able to catalyse protein synthesis.

4. Membrane compartmentalisation occurred.

5. Inside the cell, RNA was able to produce both protein and DNA.

6. DNA took over as the main genetic material because it is more stable.

7. Proteins took over as the catalytic form (enzymes) because they are more capable of variability.

Evidence to support the RNA first hypothesis includes:

• Short RNA sequences have been shown to be able to duplicate other molecules of RNA, demonstrating that RNA can self-replicate.

• RNA has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells.

• Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis.

However, this is not the only hypothesis on how cells originated. Other theories, none of which has been definitively proved or disproved, include:

• The Miller–Urey hypothesis: spontaneous generation of simple organic molecules such as amino acids, carbohydrates and lipids occurred in the conditions on pre-biotic Earth.

• The ‘metabolism first’ hypothesis: life began with simple metabolic reactions that led to the formation of simple metabolic pathways, which then formed more complex molecules which formed the basis of cells.

• The ‘sulfur world’ hypothesis: the first forms of life were based on iron–sulfur chemistry.

• The ‘lipid world’ hypothesis: lipid bilayers evolved before RNA, providing a protective layer to encapsulate the RNA.

Each hypothesis has strengths and weaknesses and scientists continue to gather evidence and explore models to test these hypotheses.

Study skills

In the context of molecular biology, the terms ‘duplication’ and ‘replication’ both mean to make an exact copy of something, and are often used interchangeably; however, they do have slightly different meanings.

Duplication means to make an exact copy of something, meaning that the original is copied once to make a second identical version. In contrast, the term replication does not specify the number of copies produced.