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.
