Introduction to Eukaryotic Cells

Overview of Eukaryotic Cells

Eukaryotic cells are complex cells characterized by the presence of membrane-bound organelles and a true nucleus. They are found in animals, plants, fungi, and protists. This chapter explores the structure, function, and evolutionary origins of eukaryotic cells, with a focus on their organelles and cellular processes.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This symbiotic relationship led to the evolution of modern eukaryotic cells.

• Mitochondria are thought to have evolved from aerobic bacteria engulfed by an anaerobic host cell.

• Chloroplasts are believed to have evolved from photosynthetic cyanobacteria engulfed by a eukaryotic ancestor.

• Over time, these endosymbionts lost many genes and became integral organelles within the host cell.

Supporting Evidence:

• Both mitochondria and chloroplasts contain small, circular DNA similar to prokaryotes.

• They possess 70S ribosomes, like those found in bacteria.

• They replicate independently of the cell via binary fission.

• Both have double membranes, consistent with engulfment by a host cell.

Introduction to Eukaryotic Organelles

Membrane-Bound Organelles

Eukaryotic cells contain several membrane-bound organelles, each with specialized functions. Animal and plant cells share many organelles but also have unique structures.

• Common organelles: Nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and ribosomes.

• Plant-specific organelles: Chloroplasts, central vacuole, and cell wall.

• Animal-specific organelles: Lysosomes are more prominent.

Eukaryotic Ribosomes

Ribosomes are the sites of protein synthesis and are sometimes referred to as "non-membranous organelles." They can be free in the cytoplasm or attached to the rough ER.

• Eukaryotic ribosomes are 80S, composed of a large 60S and a small 40S subunit.

• The large subunit contains 28S, 5.8S, and 5S rRNAs; the small subunit contains 18S rRNA.

Map of Eukaryotic Organelles

The organization of eukaryotic organelles can be grouped by their roles in protein secretion, cellular digestion, energy production, cytoskeleton, and cell junctions.

Mitosis and Meiosis

Mitosis

Mitosis is the asexual process of dividing the nucleus and genetic material of a somatic cell, resulting in two genetically identical diploid daughter cells. It consists of five phases:

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

Mnemonic for Mitosis Phases:

Meiosis

Meiosis is the process by which diploid germ cells divide to produce four genetically diverse haploid gametes. It involves two rounds of cell division:

• Meiosis I (Reductional Division): Homologous chromosomes are separated, reducing the chromosome number by half.

• Meiosis II (Equational Division): Sister chromatids are separated, producing four haploid cells.

Endocytosis and Exocytosis

Bulk Transport Mechanisms

Large biomolecules are transported across cell membranes via endocytosis and exocytosis, as they are too large to diffuse through membranes or channels.

Types of Endocytosis

• Phagocytosis: Engulfment of large, solid material (cell "eating").

• Pinocytosis: Uptake of small, liquid material (cell "drinking").

• Receptor-Mediated Endocytosis: Specific uptake using receptor proteins.

Exocytosis

Exocytosis is the process by which vesicles fuse with the cell membrane to release their contents to the extracellular space. This is essential for secretion of hormones, neurotransmitters, and digestive enzymes.

Eukaryotic Cilia and Flagella

Structure and Function

Cilia and flagella are motile structures composed of microtubules, providing movement for cells or moving substances along cell surfaces.

• Cilia: Short, hair-like structures covering the cell surface.

• Flagella: Long, tail-like structures, usually fewer in number.

• Both have a 9 + 2 arrangement of microtubules: nine pairs surrounding two central microtubules.

Motility Differences

Cilia move with an oar-like motion, while flagella move in a whip-like fashion, both powered by ATP hydrolysis.

Endomembrane System: Protein Secretion

Components and Functions

The endomembrane system is a group of membrane-bound organelles in eukaryotic cells that work together for protein secretion and cellular transport. Organelles are interconnected by vesicles.

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Transport vesicles

• Lysosomes and peroxisomes

• Vacuoles

• Cell membrane

Protein Secretion Pathway

Protein secretion involves several organelles in a specific sequence:

1. Nucleus: Stores DNA and is the site of transcription.

2. Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes and folds proteins; smooth ER synthesizes lipids and detoxifies substances.

3. Golgi Apparatus: Modifies, sorts, and packages proteins for export.

4. Transport Vesicles: Carry proteins to their destinations, including the cell membrane for secretion.

Introduction to the Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein filaments that provides cell shape, structure, movement, intracellular transport, and signaling.

• Microfilaments: Smallest, made of actin proteins.

• Intermediate Filaments: Medium-sized, made of various proteins.

• Microtubules: Largest, made of tubulin proteins, form hollow tubes.

Mitochondria and Chloroplasts

Mitochondria

Mitochondria are the "powerhouse of the cell," synthesizing ATP through cellular respiration. They have their own DNA and ribosomes, supporting the endosymbiotic theory.

• Structure: Outer membrane, folded inner membrane (cristae), intermembrane space, and matrix (contains enzymes, ribosomes, and mitochondrial DNA).

Chloroplasts

Chloroplasts are the site of photosynthesis in plant cells, converting light energy into chemical energy (glucose). They also have their own DNA and ribosomes.

• Structure: Double membrane, thylakoids (disk-shaped sacs), grana (stacks of thylakoids), and stroma (innermost region containing enzymes, ribosomes, and DNA).