The Cell

Basic Functions of the Cell

Cells are the fundamental units of life and must perform essential functions to sustain themselves and the organism. These functions ensure cellular survival and proper physiological activity.

• 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 can move or change shape, which is important for processes like muscle contraction and cell migration.

• Respond to stimulus: Cells detect and respond to changes in their environment, allowing adaptation and communication.

Major Features of the Eukaryotic Cell

Overview of Cell Structure

Eukaryotic cells possess distinct structural components, each with specialized functions. The main features include the nucleus, cytoplasm, and plasma membrane.

• Nucleus: Contains genetic material and acts as the control center.

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

• Plasma membrane: The outer boundary that regulates entry and exit of substances.

Nucleus

The nucleus is the largest organelle and is essential for storing genetic information and regulating cellular activities.

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

• Necessary for reproduction: DNA replication and cell division depend on the nucleus.

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

• Control center: The nucleus directs cellular activities by regulating gene expression.

• 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 dynamic structure that separates the cell from its environment and controls 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 homeostasis.

• Resting potential: The membrane helps establish an electrical potential difference across its surface (important in excitable 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 region between the plasma membrane and nucleus, containing organelles and other cellular components.

• Organelles: Specialized structures that perform distinct cellular functions.

• Other inclusions: May contain water vacuoles, food particles, and foreign material.

Cell Organelles

Ribosomes

Ribosomes are the sites of protein synthesis and can be found free in the cytoplasm or attached to the rough endoplasmic reticulum (ER).

• Composition: Made of ribosomal RNA (rRNA) and proteins.

• Function: Translate messenger RNA (mRNA) into polypeptides.

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; involved in lipid metabolism, steroid synthesis, and detoxification of drugs.

Golgi Apparatus

The Golgi apparatus is a stack of flattened sacs near the nucleus that modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

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

Lysosomes

Lysosomes are membrane-bound sacs containing digestive enzymes (hydrolases) that break down old organelles and foreign substances.

• Function: Intracellular digestion and recycling.

• Autolysis: Can rupture and destroy the cell if necessary.

Peroxisomes

Peroxisomes are small organelles found especially in the liver and kidneys, containing enzymes that detoxify harmful substances.

• Function: Breakdown of alcohol, hydrogen peroxide, and other toxins.

Mitochondria

Mitochondria are rod-shaped organelles with a double membrane; the inner membrane is highly folded (cristae).

• Function: Produce adenosine triphosphate (ATP), the cell's energy currency, through cellular respiration.

Centrioles

Centrioles are paired, cylindrical structures located near the nucleus.

• Function: Direct formation of the mitotic spindle during cell division; also found at the base of cilia and flagella.

Cytoskeletal Elements

The cytoskeleton provides structural support and facilitates intracellular transport.

• Microfilaments: Composed of actin; involved in cell movement and shape changes.

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

• Microtubules: Tubular structures that help maintain cell shape and serve as tracks for organelle movement.

Cell Division: Mitosis and Meiosis

Mitosis vs. Meiosis

Cell division is essential for growth, repair, and reproduction. There are two main types: mitosis and meiosis.

• Mitosis: Division of somatic (body) cells, resulting in two genetically identical daughter cells. Purpose: growth and tissue repair.

• Meiosis: Division of reproductive cells (gametes), producing four non-identical daughter cells with half the chromosome number. Purpose: sexual reproduction.

Cell Division Equation:

• Mitosis:

• Meiosis:

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.

Stages of Mitosis

Mitosis consists of several distinct phases that ensure accurate distribution of genetic material.

• Prophase (early and late): Chromatin condenses into chromosomes; nuclear envelope breaks down; spindle apparatus forms.

• Metaphase: Chromosomes align at the cell's equatorial plane.

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

• Telophase: Chromosomes decondense; nuclear envelope reforms; cytokinesis (division of cytoplasm) usually begins.

Summary Table: Comparison of Mitosis and Meiosis

Additional info: Some details about organelle function and cell cycle phases were expanded for academic completeness.