Cell Structure and Function

Introduction to Cells

Cells are the basic structural and functional units of all living organisms. They perform essential functions such as energy production, protein synthesis, transport of materials, and waste disposal. Understanding cell anatomy and physiology is fundamental for students of Anatomy & Physiology.

• Definition: A cell is the smallest unit of life capable of independent existence and reproduction.

• Functions: Includes metabolism, growth, response to stimuli, and reproduction.

• Cell Diversity: Human bodies contain over 200 different cell types, varying in size, shape, and function.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells: Found in bacteria and archaea. Lack membrane-bound organelles and a nucleus. Possess a single circular chromosome. Reproduce asexually (cloning).

• Eukaryotic Cells: Found in animals, plants, fungi, and protists. Contain membrane-bound organelles and a nucleus. DNA is organized into multiple chromosomes. Reproduce sexually (genetic material from two parents).

Cell Anatomy

Common Cell Structures

Despite their diversity, most cells share several key structures:

• Plasma Membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Cytoplasm: The internal fluid containing organelles and inclusions.

• Nucleus: The control center housing genetic material (DNA).

Plasma Membrane

The plasma membrane is a selectively permeable barrier that maintains the cell's internal environment.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure composed of a double layer of phospholipids (75%), glycolipids (5%), and cholesterol (20%). Proteins are embedded within this lipid bilayer.

• Phospholipids: Have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Functions: Mechanical barrier, selective permeability, electrochemical gradient maintenance, communication, and cell recognition/signaling.

Membrane Proteins

• Transport Proteins: Facilitate movement of substances across the membrane.

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

• Structural Proteins: Provide support and maintain cell shape.

• Enzymes: Catalyze chemical reactions at the membrane surface.

Cell Junctions

• Tight Junctions: Fuse cells together, creating impermeable barriers.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow communication and passage of ions/small molecules between cells.

Cytoplasm and Organelles

The cytoplasm contains the cytosol, organelles, and inclusions.

• Cytosol: Fluid portion containing water, proteins, salts, and sugars.

• Organelles: Specialized structures performing distinct functions.

• Inclusions: Insoluble substances such as glycogen granules, lipid droplets, and pigments.

• Cytoskeleton: Network of filaments providing shape, support, and transport pathways.

Major Cytoplasmic Organelles

Additional info:

• Peroxisomes: Contain oxidases and catalases for detoxification.

• Centrioles: Involved in cell division and organization of microtubules.

Cell Physiology

Membrane Transport

Cells regulate the movement of substances across their plasma membranes to maintain homeostasis.

• Passive Transport: No energy required; substances move down their concentration gradient.

• Active Transport: Requires ATP; substances move against their concentration gradient.

Types of Passive Transport

• Simple Diffusion: Movement of lipid-soluble molecules directly through the membrane.

• Facilitated Diffusion: Movement of larger or charged molecules via carrier or channel proteins.

• Osmosis: Diffusion of water through a selectively permeable membrane.

• Filtration: Movement of substances across capillary walls due to pressure.

Types of Active Transport

• Primary Active Transport: Direct use of ATP to move ions (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses ion gradients created by primary transport to move other substances.

• Vesicular Transport: Movement of large particles via vesicles (endocytosis, exocytosis, phagocytosis, pinocytosis).

Key Equations

• Osmolarity:

• Sodium-Potassium Pump: per ATP hydrolyzed

Tonicity

• Isotonic: Same solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic: Lower solute concentration outside the cell; water moves in, cell swells.

Cell Cycle and Division

Phases of the Cell Cycle

• Interphase: Cell grows and performs normal functions; includes G1, S, and G2 phases.

• Mitotic Phase: Cell divides into two daughter cells; includes mitosis and cytokinesis.

Mitosis

• Prophase: Chromatin condenses into chromosomes.

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

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm.

Control of Cell Division

• Checkpoints: Ensure proper division and prevent errors.

• Signals: Growth factors, hormones, and contact inhibition regulate division.

• Repressor Genes: Inhibit cell division when necessary.

Protein Synthesis

Genetic Code and RNA Types

• DNA: Double-stranded molecule with nucleotides (A, T, C, G).

• RNA: Single-stranded; bases include A, U, C, G.

• Types of RNA:

  • Messenger RNA (mRNA): Carries instructions from DNA to ribosomes.

  • Ribosomal RNA (rRNA): Forms part of the ribosome and helps translate mRNA.

  • Transfer RNA (tRNA): Brings amino acids to the ribosome and decodes mRNA.

Steps of Protein Synthesis

• Transcription: Occurs in the nucleus; DNA is used to make mRNA.

• Translation: Occurs in the cytoplasm; mRNA is decoded to build a polypeptide chain.

Key Process

• Base Pairing: A pairs with T (DNA) or U (RNA); G pairs with C.

• Codon: Three-base sequence on mRNA corresponding to an amino acid.

• Anticodon: Three-base sequence on tRNA complementary to mRNA codon.

Summary Table: DNA to Protein

Cell Death and Growth

• Apoptosis: Programmed cell death.

• Hyperplasia: Increased cell production.

• Hypertrophy: Increase in cell size.

Example: "If Organelles Could Talk" Cartoon

The cartoon humorously illustrates the roles of major organelles:

• Nucleus: Commands the cell, acting as the control center.

• Mitochondria: Provides energy, often seen as lively or energetic.

• Ribosomes: Constantly working to synthesize proteins.

• Endoplasmic Reticulum: Overworked, involved in protein and lipid synthesis.

• Golgi Apparatus: Packages and ships cellular products.

• Lysosome: Destroys unwanted materials.

• Vacuole: Stores substances, feeling "fulfilled" when full.

Additional info: The cartoon provides a memorable way to associate organelle functions with their "personalities," aiding in retention and understanding for Anatomy & Physiology students.