The Cellular Level of Organization

Introduction to Cells

Cells are the fundamental units of life in the human body. All cells arise from preexisting cells and maintain homeostasis at the cellular level, which collectively supports homeostasis at higher levels of organization. The human body contains two main types of cells:

• Sex cells (germ cells or reproductive cells): Sperm in males and oocytes in females, responsible for reproduction.

• Somatic cells: All other body cells except sex cells.

The Plasma Membrane

Structure and Function

The plasma membrane forms the outer boundary of the cell, providing selective transport of substances. Its main components are lipids and proteins, which enable the membrane to perform several essential functions:

• Physical isolation: Separates the cytoplasm from the extracellular fluid.

• Regulation of exchange: Controls entry of ions/nutrients and elimination of wastes.

• Sensitivity: Contains receptors to respond to environmental changes.

• Structural support: Anchors cells and provides tissue stability.

Membrane Lipids

The plasma membrane is primarily a phospholipid bilayer:

• Hydrophilic heads: Face outward toward watery environments.

• Hydrophobic tails: Form the inner core, acting as a barrier to ions and water-soluble compounds.

• Cholesterol: Reduces membrane fluidity and permeability.

Membrane Proteins

Proteins in the membrane serve various roles:

• Integral proteins: Embedded within the membrane; may span the entire width (transmembrane proteins).

• Peripheral proteins: Bound to the inner or outer surface.

By function, membrane proteins include:

• Recognition proteins: Identify cells as normal or abnormal.

• Receptor proteins: Bind and respond to ligands (e.g., hormones).

• Carrier proteins: Transport specific solutes across the membrane.

• Channels: Permit water and small solutes to pass through.

Membrane Carbohydrates

Carbohydrates on the membrane surface form the glycocalyx (sugar coat), which functions in:

• Lubrication and protection

• Specificity in binding (receptors)

• Cell recognition (immune response)

Transport Across the Plasma Membrane

Types of Transport

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others. Transport mechanisms include:

• Passive transport: No energy required (e.g., diffusion, osmosis).

• Active transport: Requires energy (e.g., carrier-mediated, vesicular transport).

Diffusion and Osmosis

Diffusion is the net movement of a substance from an area of higher concentration to lower concentration, proceeding down a concentration gradient. Factors influencing diffusion rate include distance, molecule size, temperature, and gradient size.

Osmosis is the diffusion of water across a selectively permeable membrane toward a higher solute concentration. Osmotic pressure is the force with which water moves into a solution due to solute concentration, opposed by hydrostatic pressure.

Tonicity

Tonicity describes how a solution affects cell volume:

• Isotonic: Equal solute concentration; no net water movement.

• Hypotonic: Lower solute concentration outside; water enters cell, causing swelling or lysis.

• Hypertonic: Higher solute concentration outside; water leaves cell, causing shrinkage (crenation).

Carrier-Mediated Transport

Carrier proteins transport substances across the membrane. This can be:

• Active transport: Moves substances against their gradient using ATP (e.g., sodium-potassium pump).

• Secondary active transport: Uses gradients established by primary active transport.

• Facilitated diffusion: Passive movement of molecules too large or insoluble in lipids via carrier proteins.

Vesicular (Bulk) Transport

Vesicular transport moves materials in and out of cells via vesicles, requiring ATP. Types include:

• Endocytosis: Imports materials into the cell (receptor-mediated, pinocytosis, phagocytosis).

• Exocytosis: Exports materials out of the cell.

Cellular Organelles

Cytoplasm and Organelles

The cytoplasm includes all materials between the plasma membrane and the nucleus. It consists of:

• Cytosol: Intracellular fluid containing water, nutrients, ions, proteins, and wastes.

• Organelles: Specialized structures with specific functions.

Nonmembranous Organelles

• Cytoskeleton: Protein framework for shape, strength, and flexibility.

• Microvilli: Increase surface area for absorption.

• Centrioles: Form spindle apparatus during cell division.

• Cilia: Move fluids across cell surface.

• Flagella: Enable sperm cell movement.

• Ribosomes: Sites of protein synthesis.

• Proteasomes: Break down damaged proteins.

Membranous Organelles

• Endoplasmic Reticulum (ER):

  • Smooth ER (SER): Synthesizes lipids, steroids, and stores glycogen.

  • Rough ER (RER): Studded with ribosomes; synthesizes proteins.

• Golgi Apparatus: Modifies, packages, and delivers proteins and lipids.

• Lysosomes: Vesicles containing digestive enzymes for breakdown of materials.

• Peroxisomes: Break down fatty acids and neutralize hydrogen peroxide.

• Mitochondria: Produce ATP via aerobic metabolism (cellular respiration).

The Nucleus

Structure and Function

The nucleus is the largest organelle and serves as the control center for cellular operations. It stores and processes genetic information and controls protein synthesis.

• Nuclear envelope: Double membrane surrounding the nucleus.

• Nuclear pores: Allow chemical communication.

• Nucleolus: Synthesizes rRNA and assembles ribosomal subunits.

Genetic Code and Protein Synthesis

The genetic code is the sequence of DNA bases (A, T, C, G) that instructs protein synthesis. A gene is a DNA sequence coding for one protein.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide based on mRNA instructions at the ribosome.

Mutations are permanent changes in DNA that can alter protein structure and function.

The Cell Life Cycle

Phases of the Cell Cycle

The cell life cycle includes interphase (growth and DNA replication) and the M phase (mitosis and cytokinesis):

• Interphase: G1 (growth), S (DNA replication), G2 (preparation for division).

• Mitosis: Division of the nucleus (prophase, metaphase, anaphase, telophase).

• Cytokinesis: Division of the cytoplasm, producing two identical daughter cells.

Cells may undergo apoptosis (programmed cell death) at the end of their lifespan.

Regulation of the Cell Cycle

Cell division is regulated by internal and external factors:

• Stimulatory factors: M-phase promoting factor (MPF), growth factors.

• Inhibitory factors: Repressor genes, short telomeres.

Cell Division and Cancer

Cancer results from abnormal cell proliferation due to mutations in genes regulating growth and division. Tumors may be:

• Benign: Remain localized.

• Malignant: Invade surrounding tissues and may metastasize.

Mutagens and carcinogens are agents that cause mutations and cancer, respectively.

Cellular Differentiation

Definition and Importance

All somatic cells contain the same genetic material, but differentiation occurs by turning off specific genes, allowing cells to specialize (e.g., liver cells, neurons). This process is essential for the formation of diverse cell types in the body.