Universal Features of Cells: The Building Blocks of Life
- Despite the enormous diversity of life on Earth, all cells share three fundamental structures:
- DNA as genetic material
- Cytoplasm composed mainly of water
- Plasma membrane composed of lipids
- These structures solve three universal problems every cell must address:
- Storing information
- Enabling metabolism
- Creating boundaries.
DNA: Storing And Transmitting Genetic Information
The problem: Cells need a stable molecule to store instructions, replicate accurately, and allow evolution.
Definition
DNA (Deoxyribonucleic Acid)
A double-stranded molecule that stores and transmits genetic information in living organisms.
- Stable storage:
- Double helix structure provides long-term stability.
- Complementary base pairing (A-T, C-G) allows repair—if one strand is damaged, the other serves as a template.
- Accurate replication: DNA can be copied with high fidelity, ensuring genetic information passes to daughter cells.
- Encodes instructions: Genes (DNA segments) code for proteins and RNAs that carry out cellular functions.
- Allows evolution: Mutations create genetic variation, providing raw material for natural selection.
- Why not RNA or proteins?
- RNA is less chemically stable (degrades more easily).
- Proteins cannot be replicated in a template-based manner.
- DNA is organized differently in:
- Prokaryotes: Single circular chromosome in nucleoid; may have plasmids.
- Eukaryotes: Linear chromosomes in nucleus; additional DNA in mitochondria and chloroplasts.
Aqueous Cytoplasm: Providing a Medium for Metabolism
The problem: Cells need an environment where enzymes and substrates can interact for metabolism.
Definition
Cytoplasm
The cytoplasm is the jelly-like substance inside a cell, excluding the nucleus, where various cellular activities occur.
- Water is the universal solvent:
- Dissolves ions, glucose, amino acids, and other polar/charged molecules (covered in A1.1.5).
- Dissolved enzymes and substrates can diffuse and collide, enabling reactions.
- Medium for metabolism: Most metabolic reactions occur in aqueous solution.
- Transport and communication: Dissolved molecules (nutrients, waste, signals) move through cytoplasm, connecting the cell.
- Structural support: Maintains cell shape and supports organelles.
Glycolysis occurs in the aqueous cytoplasm where glucose and enzymes are dissolved.
Lipid Plasma Membrane: Separating the Cell from Its Environment
The problem: Cells need to create a controlled internal environment while exchanging materials with surroundings.
- Creates a selective barrier:
- Hydrophobic core prevents most water-soluble molecules from crossing freely.
- Allows the cell to maintain different internal concentrations than the surroundings.
- Selective permeability:
- Small nonpolar molecules (O₂, CO₂) diffuse through easily.
- Polar and charged molecules (glucose, ions) require transport proteins.
- The cell controls what enters and exits.
- Flexibility and fluidity:
- Lipids and proteins move laterally within the membrane.
- Enables cell division, endocytosis, exocytosis, and protein clustering.
- Self-assembly and repair:
- Phospholipids spontaneously form bilayers in water.
- Membrane can self-seal if damaged.
- Communication: Membrane proteins act as receptors for signals, allowing cells to respond to their environment.
Because lipids are hydrophobic, they create an effective barrier while remaining fluid that proteins and carbohydrates (both hydriphilic) otherwise wouldn't be able to.
Why These Three Structures Are Universal
| Structure | Problem solved | Why? |
|---|---|---|
| DNA | Storing/transmitting information | Stable, replicable, evolvable |
| Aqueous cytoplasm | Enabling metabolism | Water dissolves reactants and enzymes |
| Lipid membrane | Creating boundaries | Hydrophobic barrier that's flexible |
- These three structures represent the minimum requirements for life.
- Any system lacking DNA, aqueous cytoplasm, or a lipid membrane cannot function as a living cell.
- What are the three structures common to all living cells?
- What three universal problems do these structures solve?
- Why is DNA more stable than RNA for genetic storage?
- Give an example of a metabolic process occurring in aqueous cytoplasm.
- Why are lipids ideal for forming the membrane?
- Why are these three structures considered universal?
