Cell Structure, Function, and Physiology: Study Notes for Anatomy & Physiology

Cells: Structure, Function, and Physiology

Overview

Cells are the fundamental units of life in all living organisms. Understanding their structure and function is essential for comprehending how the human body operates at the microscopic level. This guide covers the characteristics of a typical cell, the plasma membrane, organelles, the nucleus, transport processes, osmotic pressure, cell metabolism, and cell division.

Characteristics of a Typical Cell

Definition and Diversity

• Cell: The smallest structural and functional unit of life.

• All body functions depend on the activities of individual cells.

• Human cells vary in size, shape, and subcellular components, leading to specialized functions (e.g., muscle cells, nerve cells, fat cells).

Example: Red blood cells transport oxygen, while neurons transmit electrical signals.

Intracellular and Extracellular Materials

• Intracellular fluid (ICF): Fluid within cells.

• Extracellular fluid (ECF): Fluid outside cells, including:

  • Interstitial fluid: Surrounds and bathes cells.

  • Blood plasma: Fluid component of blood.

  • Cerebrospinal fluid: Surrounds nervous system organs.

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

• Extracellular matrix: Network of proteins and polysaccharides that provide structural support and glue cells together.

Basic Parts of a Cell

• Plasma membrane: Flexible outer boundary.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: Control center containing DNA.

Structure and Function of the Plasma Membrane

Plasma Membrane Composition and Role

• Acts as a selective barrier between the intracellular fluid (ICF) and extracellular fluid (ECF).

• Controls entry and exit of substances, maintaining cellular homeostasis.

• Composed mainly of phospholipid bilayer with embedded proteins and carbohydrates.

• Contains cell junctions that connect cells together.

Membrane Permeability

• The plasma membrane is selectively permeable, allowing some substances to pass while restricting others.

• Small, nonpolar molecules (e.g., oxygen, carbon dioxide) can cross easily by simple diffusion.

• Larger or polar molecules require assistance via facilitated diffusion or active transport.

Membrane Proteins and Junctions

• Integral proteins: Span the membrane and function as channels or carriers.

• Peripheral proteins: Attached to the membrane surface; involved in signaling or maintaining cell shape.

• Cell junctions:

  • 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 adjacent cells.

Glycocalyx

• Consists of carbohydrates on the cell surface.

• Functions in cell recognition, immune response, and adhesion.

Cell Organelles and Their Functions

Cytoplasm and Its Components

• Cytosol: Gel-like solution containing water, ions, and soluble molecules.

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

• Organelles: Specialized structures performing specific cellular functions.

Major Organelles

• Mitochondria: Site of ATP (energy) production; the "powerhouse" of the cell.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER 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 neutralize toxins.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

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

Cell Surface Extensions

• Cilia: Short, hair-like structures that move substances across the cell surface (e.g., respiratory tract).

• Flagella: Longer extensions that propel the entire cell (e.g., sperm cell tail).

• Microvilli: Finger-like projections that increase surface area for absorption (e.g., intestinal cells).

Organization and Function of the Nucleus

Nucleus Structure and Role

• The largest organelle; contains the cell's genetic material (DNA).

• Controls synthesis of nearly all cellular proteins.

• 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

Overview of Membrane Transport

• The plasma membrane is selectively permeable, allowing only certain molecules to cross.

• Two main types of transport:

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

  • Active transport: Requires energy (ATP); substances move against their concentration gradient.

Passive Transport Mechanisms

• 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 protein carriers or channels.

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

Factors influencing diffusion:

• Concentration gradient (greater difference = faster diffusion)

• Molecule size (smaller = faster)

• Temperature (higher = faster)

Active Transport Mechanisms

• Primary active transport: Direct use of ATP to move substances (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy from the gradient created by primary active transport to move other substances.

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

Na+/K+ Pump Equation

• Pumps 3 Na+ out and 2 K+ into the cell for each ATP hydrolyzed:

$3\,\mathrm{Na}^+_{\text{in}} + 2\,\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{out}} + 2\,\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P_i}$

Vesicular Transport Types

• Endocytosis: Transport into the cell (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Transport out of the cell (e.g., secretion of hormones, neurotransmitters).

Effects of Osmotic Pressure on the Cell

Osmosis and Tonicity

• Osmosis: Movement of water across a membrane from low solute concentration to high solute concentration.

• Tonicity: The ability of a solution to change the shape or tone of cells by altering their internal water volume.

• Isotonic solution: No net water movement; cell shape unchanged.

• Hypertonic solution: Water leaves the cell; cell shrinks (crenation).

• Hypotonic solution: Water enters the cell; cell swells and may burst (lysis).

How Cell Metabolism Maintains Homeostasis

Metabolic Regulation and Homeostasis

• Cell metabolism includes all chemical reactions that occur within a cell to maintain life.

• Regulation of cell division and metabolic rates ensures tissue and organ homeostasis.

• Imbalances can lead to diseases (e.g., muscular dystrophy, osteoporosis).

Examples: Bone marrow increases cell division during blood loss; muscle atrophy occurs with disuse.

Cell Mitosis and Cell Meiosis

Cell Cycle and Division

• The cell cycle consists of interphase (growth and DNA replication) and the mitotic phase (cell division).

• Mitosis: Division of the nucleus, resulting in two identical daughter cells; essential for growth and repair.

• Meiosis: Specialized division producing gametes (sperm and egg) with half the chromosome number.

Phases of Mitosis

• Prophase: Chromosomes condense, nuclear envelope dissolves.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm.

Genetic Information and Protein Synthesis

• DNA: Double helix containing genetic instructions for protein synthesis.

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

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

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

Example: The sequence of nitrogen bases (adenine, guanine, thymine, cytosine) determines the amino acid sequence in proteins.

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	Indirect (uses gradient)	Against gradient	Glucose/Na+ symport
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Additional info: Some details, such as the specific mechanisms of protein synthesis and the regulation of cell division, have been expanded for academic completeness.