Cell Biology and Biomolecule Basics

Biomolecules: Nucleic Acids

Nucleoside: Union of a nitrogenous base with a pentose sugar.

Nucleotide: Union of a nucleoside with a phosphoric acid group.

Nucleic Acid: Polymer made of hundreds or thousands of nucleotides.

Functions: Genetic information storage, transmission, transcription, and translation.

Difference Between DNA and RNA

• Sugar: Deoxyribose (DNA) vs. Ribose (RNA)
• Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T) in DNA vs. Adenine (A), Guanine (G), Cytosine (C), Uracil (U) in RNA
• Structure: Generally long double-stranded chains in DNA vs. Generally shorter single-stranded chains in RNA (although RNA can have loops and folds, and some viruses have double-stranded RNA)
• Stability: DNA has no free OH group at the 2' carbon, which allows it to package more compactly and forms a very stable helix. RNA is chemically less stable due to the free OH group at the 2' carbon.
• Location: DNA is found in the nucleus, mitochondria, and chloroplasts. RNA is found in the nucleus (especially the nucleolus) and in the cytoplasm (on free ribosomes).

Cell Theory

The Cell Theory states:

• Unity of Life: The cell is the basic unit of life (the smallest living entity).
• Morphological Unity: All living beings are made of cells.
• Physiological Unity: All cells have the necessary biochemical mechanisms to stay alive.
• Genetic Unit: All cells originate from pre-existing cells.

Endosymbiotic Theory

According to this theory, billions of years ago, mitochondria evolved from aerobic bacteria that established a symbiotic relationship with primitive eukaryotes. Chloroplasts evolved from cyanobacteria, which established symbiosis with primitive eukaryotes. This explains the fact that both organelles have their own DNA and ribosomes.

Cell Transport

Passive Transport

Solutes move spontaneously from an area of higher concentration to an area of lower concentration (down the gradient) without requiring energy.

Two mechanisms:

• Simple Diffusion: Exchange of gases like O2 and CO2, and passage of compounds like water (osmosis), urea, hormones, ions. The concentration difference across the membrane determines the transport. No enzymatic system intervenes. Occurs directly across the membrane.
• Facilitated Diffusion: Allows the passage of polar substances (sugars, nucleotides, amino acids). Involves membrane proteins:
  • Channel Proteins: Form transient channels and allow certain ions to pass through the membrane down the gradient.
  • Carrier Proteins: Permeases are a type of carrier protein that binds to the substance and forms a complex that moves it across the membrane.

Active Transport

Requires membrane proteins and ATP expenditure because it acts against the gradient (from low to high concentration). These carrier proteins are also called pumps, and when they bind to the substance, they undergo a conformational change, transporting the substance inside.

Prokaryotic and Eukaryotic Cells

Prokaryotic cells are small and do not form tissues. They are unicellular and lack a nucleus (DNA is not enclosed by a membrane). They have circular DNA in the cytoplasm; RNA and protein synthesis occur in the same compartment. Cell division is direct (not mitosis), by binary fission. They can have aerobic or anaerobic metabolism.