Cells

Types of Cells

Cells are the fundamental units of life, classified into two main types: prokaryotic and eukaryotic cells. Prokaryotic cells lack a membrane-bound nucleus and organelles, while eukaryotic cells possess these structures, allowing for compartmentalization of cellular functions.

• Prokaryotic cells: Include Bacteria and Archaea; lack nucleus and organelles.

• Eukaryotic cells: Include plants, animals, fungi, and protists; have nucleus and organelles.

Three Domains of Life

The tree of life illustrates the evolutionary relationships among the three domains: Bacteria, Archaea, and Eukaryotes. All domains share a common ancestor, but diverged to form distinct lineages.

Membranous Organelles

Eukaryotic cells contain various organelles, some with single membranes (e.g., lysosome, endoplasmic reticulum, Golgi apparatus) and others with double membranes (e.g., nucleus, mitochondrion, chloroplast). These organelles perform specialized functions essential for cell survival.

Eukaryotic Cell Origin

The origin of eukaryotic cells is hypothesized to involve endosymbiosis, where ancestral prokaryotes engulfed other cells, leading to the formation of mitochondria and other organelles. Membrane fusion and compartmentalization were key steps in this evolutionary process.

Biochemistry

Chemical Bonds

Chemical bonds are essential for molecular structure and function. Covalent bonds involve electron sharing between atoms and can be classified as non-polar or polar based on electronegativity differences. If the difference exceeds 1.6 units, the bond becomes ionic.

• Non-polar covalent: Electronegativity difference < 0.4

• Polar covalent: Electronegativity difference 0.4–1.6

• Ionic: Electronegativity difference > 1.6

Macromolecules

Types of Macromolecules

Cells are built from four major types of macromolecules: polysaccharides, fats, proteins, and nucleic acids. These are synthesized from smaller organic building blocks: sugars, fatty acids, amino acids, and nucleotides.

• Polysaccharides: Energy storage and structural support

• Fats: Membrane structure and energy storage

• Proteins: Enzymatic and structural functions

• Nucleic acids: Genetic information storage and transfer

Nucleotides and Nucleic Acids

Nucleotides are the building blocks of nucleic acids (DNA and RNA). Each nucleotide consists of a nitrogenous base, a five-carbon sugar, and one or more phosphate groups.

• Bases: Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil)

• Sugar: Ribose (RNA) or deoxyribose (DNA)

• Phosphate: Links nucleotides together

DNA Structure

DNA is a double-stranded helix with a sugar-phosphate backbone and hydrogen-bonded base pairs. The strands are antiparallel, running 5' to 3' and 3' to 5'.

• Base pairing: Adenine pairs with thymine, guanine pairs with cytosine

• Helical structure: Provides stability and allows for replication

Amino Acids and Proteins

Proteins are polymers of amino acids. There are 20 standard amino acids, with 9 essential for humans. Amino acids are linked by peptide bonds to form polypeptides.

• Essential amino acids: Must be obtained from diet

• Non-essential amino acids: Synthesized by the body

Polysaccharides & Energy Storage

Polysaccharides such as starch (plants) and glycogen (animals) serve as energy reserves. Starch consists of amylose (unbranched) and amylopectin (branched), while glycogen is highly branched.

• Starch: α(1→4) and α(1→6) glycosidic bonds

• Glycogen: Similar structure, more branching

Cellulose and Chitin

Cellulose is a linear polymer of glucose with β1-4 glycosidic bonds, providing mechanical support in plant cell walls. Chitin is a linear polymer of N-acetylglucosamine, found in insect exoskeletons and fungal cell walls.

Genome Organization

Genome Definition and Size

The genome is the total genetic information carried by all chromosomes, including coding and non-coding DNA. Genome size is measured in nucleotides or base pairs and does not correlate directly with organism complexity.

Chromosome Number and Genes

The number of chromosomes does not equal the number of genes. Different organisms have varying chromosome counts and gene numbers.

Chromosomal DNA Organization During Cell Cycle

Chromosomal DNA undergoes structural changes during the cell cycle, including duplication and condensation for mitosis.

Duplicated Mitotic Chromosome

A duplicated mitotic chromosome consists of two sister chromatids joined at the centromere, ready for segregation during cell division.

Nucleosome Structure and Histones

Nucleosomes are the basic units of chromatin, consisting of DNA wrapped around histone proteins. Core histones (H2A, H2B, H3, H4) form the nucleosome, while linker histones (H1, H5) help compact chromatin.

Summary Table: Model Organisms and Their Genomes

This table compares genome sizes and gene numbers in several model organisms, illustrating the diversity in genome organization.

Additional info: Academic context was added to clarify cell types, macromolecule functions, genome organization, and the significance of histones and nucleosomes.