Cell Structure and Function: Study Notes for Anatomy & Physiology

Cells: Structure, Function, and Processes

Overview of a Typical Cell

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

• Definition: A cell is the smallest unit of life capable of carrying out all vital physiological functions.

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

• Common Features: Despite diversity, most cells share three basic parts:

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

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: Organelle containing DNA and controlling cellular activities.

• Cellular Environment: Materials inside and outside cells include:

  • Extracellular Fluids (ECF): Body fluids outside cells, such as interstitial fluid (surrounds cells), blood plasma, and cerebrospinal fluid.

  • Cellular Secretions: Substances like saliva, mucus, and gastric fluids.

  • Extracellular Matrix: A network of proteins and polysaccharides that provides structural support and acts as a "glue" to hold 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: The membrane is primarily composed of a double layer of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Membrane Proteins: Embedded proteins serve various functions, including transport (channels and carriers), enzymatic activity, signal transduction, cell recognition, and intercellular joining.

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

• Cell Junctions: Specialized structures that connect adjacent cells:

  • Tight Junctions: Prevent passage of molecules between cells.

  • Desmosomes: Anchor cells together, providing mechanical stability.

  • Gap Junctions: Allow direct communication between cells via channels.

Cell Organelles and Their Functions

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions.

• Cytosol: Gel-like fluid where organelles are suspended; contains water, ions, and soluble molecules.

• Inclusions: Non-living substances such as glycogen granules, lipid droplets, and pigments.

• Major Organelles:

  • Mitochondria: Site of ATP (energy) production via cellular respiration.

  • Ribosomes: Synthesize proteins; can be free in cytosol or attached to rough endoplasmic reticulum (ER).

  • Endoplasmic Reticulum (ER):

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

    • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

  • Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

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

  • Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) that maintain cell shape and enable movement.

  • Centrioles: Organize microtubules during cell division; form bases of cilia and flagella.

  • Cilia and Flagella: Extensions that move substances across cell surfaces (cilia) or propel cells (flagella).

  • Microvilli: Fingerlike projections that increase surface area for absorption (e.g., in intestinal cells).

Organization and Function of the Nucleus

The nucleus is the largest organelle and serves as the control center of the cell, housing genetic material (DNA).

• Nuclear Envelope: Double membrane that encloses the nucleus and separates it from the cytoplasm.

• Nucleolus: Site of ribosome synthesis.

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

• Cell Nucleation: Most cells are uninucleate (one nucleus), but some are multinucleate (e.g., skeletal muscle) or anucleate (e.g., mature red blood cells).

Passive and Active Transport Processes

Substances move across the plasma membrane by passive or active transport, depending on energy requirements.

• Passive Transport: Does not require energy (ATP); substances move down their concentration gradient.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

  • Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

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

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

  • Secondary Active Transport: Uses energy from the gradient created by primary active transport to move other substances (e.g., symporters and antiporters).

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

Key Equations:

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Osmotic Pressure and Its Effects on Cells

Osmosis can cause cells to change shape or volume depending on the relative concentration of solutes inside and outside the cell.

• Isotonic Solution: Equal solute concentration inside and outside; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell, causing it to shrink (crenation).

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, causing it to swell and possibly burst (lysis).

Cell Metabolism and Homeostasis

Cell metabolism includes all chemical reactions that occur within a cell, enabling it to maintain homeostasis.

• Homeostasis: The maintenance of a stable internal environment.

• Hyperplasia: Increased cell production (e.g., bone marrow during increased demand for red blood cells).

• Atrophy: Decrease in cell size or number, often due to reduced stimulation or disease (e.g., muscular dystrophy).

• Theories of Aging: Wear and tear, mitochondrial, immune, and genetic theories explain cellular aging and loss of homeostasis.

Cell Division: Mitosis and 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. Stages include:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

  • Cytokinesis: Division of the cytoplasm

• Meiosis: Special type of division producing gametes (sperm and egg), resulting in four genetically unique cells with half the chromosome number.

Protein Synthesis

Protein synthesis involves transcription (DNA to mRNA) and translation (mRNA to protein).

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

• Transcription: DNA sequence is copied into messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a polypeptide chain.

• Base Pairing: Adenine (A) pairs with Thymine (T) in DNA, and with Uracil (U) in RNA; Cytosine (C) pairs with Guanine (G).

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down water gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Yes (indirectly)	Against gradient	Symporters, antiporters
Vesicular Transport	Yes (ATP)	Varies	Endocytosis, exocytosis

Additional info: Some details, such as the full list of organelles and the stages of mitosis, were expanded for completeness and clarity based on standard Anatomy & Physiology curriculum.