The Cell

Basic Functions of the Cell

Cells are the fundamental units of life and must perform essential functions to maintain homeostasis and support the organism.

• Maintain boundaries: Cells separate their internal environment from the external environment, primarily through the plasma membrane.

• Digest nutrients: Cells break down nutrients to obtain energy and building blocks for growth and repair.

• Dispose of waste: Metabolic waste products are removed to prevent toxicity.

• Move: Some cells are motile, and all cells can move substances internally.

• Respond to stimulus: Cells detect and respond to changes in their environment.

Major Features of the Eukaryotic Cell

Overview of Cell Structure

Eukaryotic cells contain several key structures, each with specialized functions. The main components include the nucleus, cytoplasm, and plasma membrane.

• Nucleus: The control center of the cell, containing genetic material.

• Cytoplasm: The region between the plasma membrane and nucleus, containing organelles.

• Plasma membrane: The outer boundary of the cell, regulating entry and exit of substances.

Nucleus

The nucleus is a membrane-bound organelle that houses the cell's genetic material and coordinates activities such as growth, metabolism, and reproduction.

• Contains DNA: Genetic material is present as chromatin (thread-like, dispersed) or chromosomes (condensed, during cell division).

• Essential for reproduction: The nucleus directs cell division and inheritance.

• Nucleoli: Small structures within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Nuclear envelope: A double membrane with pores that allow RNA and proteins to move between the nucleus and cytoplasm.

Plasma Membrane

The plasma membrane is a selectively permeable barrier that separates the cell from its environment and regulates the movement of substances.

• Phospholipid bilayer: Composed of two layers of phospholipids with embedded proteins and cholesterol for stability.

• Selective permeability: Allows certain substances to pass while restricting others, maintaining cellular integrity.

• Resting potential: The membrane helps maintain electrical potential across its surface (important for nerve and muscle cells).

• Surface area: The membrane may be folded (e.g., microvilli) to increase surface area for absorption or secretion.

Cytoplasm

The cytoplasm is the semi-fluid matrix that fills the cell, providing a medium for organelles and molecular processes.

• Location: Between the plasma membrane and nucleus.

• Contents: Includes organelles, water, nutrients, and sometimes vacuoles or foreign particles.

Cell Organelles

Ribosomes

Ribosomes are the sites of protein synthesis, translating genetic instructions into functional proteins.

• Structure: Composed of RNA and protein.

• Location: Free in cytoplasm or attached to rough endoplasmic reticulum (ER).

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes and transports proteins.

• Smooth ER: Lacks ribosomes; synthesizes steroids and lipids, metabolizes lipids, and detoxifies drugs.

Lysosomes and Peroxisomes

These organelles contain enzymes for digestion and detoxification.

• Lysosomes: Sacs with digestive enzymes (hydrolases) that break down old organelles and foreign invaders; can cause cell death if ruptured.

• Peroxisomes: Sacs found mainly in liver and kidney cells; contain enzymes that detoxify alcohol and other harmful chemicals.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Structure: Stack of flattened sacs near the nucleus.

• Function: Packages proteins for export or incorporation into the plasma membrane.

Mitochondria

Mitochondria are the cell's powerhouses, generating ATP through cellular respiration.

• Structure: Rod-shaped with a double membrane; inner membrane is highly folded (cristae).

• Function: Produces ATP, the cell's energy currency.

Centrioles

Centrioles are cylindrical structures involved in cell division and the formation of cilia and flagella.

• Location: Found in pairs near the nucleus.

• Function: Direct formation of the mitotic spindle during cell division.

Cytoskeletal Elements

The cytoskeleton provides structural support and facilitates intracellular transport.

• Microfilaments: Composed of contractile proteins (e.g., actin); involved in cell movement and shape.

• Intermediate filaments: Provide mechanical strength and resistance to stress.

• Microtubules: Tubular structures that help maintain cell shape and enable transport within the cell.

Cell Division: Mitosis vs. Meiosis

Mitosis

Mitosis is the process by which somatic (body) cells divide to produce two genetically identical daughter cells. It is essential for growth and tissue repair.

• Purpose: Growth and repair of tissues.

• Result: Two identical daughter cells.

• Phases: Prophase, Metaphase, Anaphase, Telophase (followed by cytokinesis).

Meiosis

Meiosis is the process by which reproductive cells (gametes) are produced. It involves two consecutive divisions, resulting in four non-identical daughter cells with half the chromosome number.

• Purpose: Production of gametes (sperm and egg).

• Result: Four genetically unique daughter cells.

• Phases: Meiosis I and Meiosis II.

Interphase

Interphase is the period of the cell cycle when the cell is not dividing but is preparing for division. DNA is present as chromatin, and the nuclear envelope and nucleoli are intact.

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA replication occurs.

• G2 phase: Final preparations for mitosis.

Phases of Mitosis

Mitosis consists of several distinct phases:

• Prophase: Chromatin condenses into chromosomes; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform around the separated chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Summary Table: Comparison of Mitosis and Meiosis

Additional info: The notes have been expanded to include definitions, examples, and a summary table for comparison. All phases of mitosis and meiosis are described for clarity.