Cells: Structure, Function, and Transport

Characteristics of a Typical Cell

Cells are the basic structural and functional units of life. Understanding their characteristics is essential for grasping how the body functions and responds to treatment.

• Definition: A cell is the smallest unit of life capable of independent existence and performing all vital functions.

• Cell Diversity: The human body contains many cell types, each specialized for particular functions (e.g., muscle cells, nerve cells, fat cells, immune cells).

• Common Features: Most cells share three basic components:

  • Plasma Membrane: Flexible outer boundary that separates the cell from its environment.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: Contains DNA and controls cellular activities.

• Cellular Materials:

  • Intracellular Fluid (ICF): Fluid within cells.

  • Extracellular Fluid (ECF): Fluid outside cells, including interstitial fluid, blood plasma, and cerebrospinal fluid.

  • Cellular Secretions: Substances like saliva and mucus.

  • Extracellular Matrix: Material that holds cells together.

Structure and Function of the Plasma Membrane

The plasma membrane, also known as the cell membrane, is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Phospholipid Bilayer: Composed of hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails, forming a double layer.

• Membrane Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx, which aids in cell recognition.

• Cell Junctions: Specialized structures that connect adjacent cells:

  • Tight Junctions: Prevent passage of molecules between cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

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

Cell Organelles and Their Functions

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival.

• Nucleus: Contains genetic material (DNA) and directs cellular activities.

• Mitochondria: Site of ATP production through cellular respiration.

• Ribosomes: Synthesize proteins.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breaking down waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Provides structural support and facilitates movement.

• Centrioles: Organize spindle fibers during cell division.

• Cilia and Flagella: Aid in movement of substances or the cell itself.

• Microvilli: Increase surface area for absorption.

Organization and Function of the Nucleus

The nucleus is the largest organelle and serves as the control center of the cell.

• Nuclear Envelope: Double membrane that encloses the nucleus.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Cell Types by Nucleus Number:

  • Nucleate: One nucleus (most cells).

  • Multinucleate: Multiple nuclei (e.g., skeletal muscle cells).

  • Anucleate: No nucleus (e.g., mature red blood cells).

Passive and Active Transport Processes

Cells exchange materials with their environment through passive and active transport mechanisms across the plasma membrane.

• Passive Transport: Does not require energy (ATP).

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

  • Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

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

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

  • Primary Active Transport: Direct use of ATP (e.g., Na+-K+ pump).

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

  • Vesicular Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Key Equations:

• Diffusion Rate: Where J is the flux, D is the diffusion coefficient, and dC/dx is the concentration gradient.

• Osmotic Pressure: Where is osmotic pressure, i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature.

Effects of Osmotic Pressure on the Cell

Osmotic pressure influences the movement of water into and out of cells, affecting cell volume and shape.

• Isotonic Solution: No net movement of water; cell volume remains unchanged.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, causing cell shrinkage (crenation).

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, causing cell swelling and possible lysis.

How Cell Metabolism Maintains Homeostasis

Cell metabolism encompasses all chemical reactions within a cell, enabling growth, repair, and adaptation to maintain homeostasis.

• Hyperplasia: Increased cell production in response to demand (e.g., bone marrow producing more red blood cells).

• Atrophy: Decreased size or function due to reduced stimulation or disease (e.g., muscular dystrophy).

• Theories of Cellular Aging:

  • Wear and Tear Theory

  • Mitochondrial Theory

  • Immune Theory

  • Genetic Theory

Cell Mitosis and Cell Meiosis

Cell division is essential for growth, repair, and reproduction. Mitosis and meiosis are two distinct processes.

• Mitosis: Division of somatic cells resulting in two identical daughter cells.

  • Phases: Prophase, Metaphase, Anaphase, Telophase, followed by Cytokinesis.

  • Purpose: Growth and tissue repair.

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

  • Purpose: Sexual reproduction and genetic diversity.

DNA and Protein Synthesis:

• Gene: Segment of DNA coding for a specific polypeptide.

• Transcription: DNA code is transcribed to messenger RNA (mRNA).

• Translation: mRNA is decoded to assemble amino acids into proteins.

• Base Pairing: Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G).

Key Equation:

• Central Dogma:

Summary Table: Types of Cell Transport

Additional info: Some explanations and examples have been expanded for clarity and completeness.