Cell Structure and Function: Foundations of Anatomy & Physiology

Cell Theory and Cellular Composition

Overview of Cells

Cells are the fundamental building blocks of all living organisms. The human body contains approximately 50-100 trillion cells, with about 10% being human and the remainder consisting of microbial cells. This highlights the complexity and diversity of cellular life within the human body.

• Cell Theory: All living things are composed of cells; cells are the basic unit of structure and function in organisms; all cells arise from pre-existing cells.

• Human Cell Count: Estimated at 50-100 trillion, with microbial cells outnumbering human cells.

• Implication: The human body is a complex ecosystem, with non-human cells playing crucial roles in health and disease.

General Cell Structure

Major Components

Each cell consists of three primary regions: the plasma membrane, cytoplasm, and nucleus. These components work together to maintain cellular integrity and function.

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

• Cytoplasm: Contains cytosol (fluid), organelles (specialized structures), and inclusions (stored nutrients).

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

Extracellular Environment

Outside the Cell

The area outside the cell includes body fluids, cellular secretions, and the extracellular matrix (ECM). These components facilitate communication, transport, and structural support.

• Body Fluids: Interstitial fluid, blood plasma, and cerebrospinal fluid.

• Cellular Secretions: Substances such as mucus, enzymes, and hormones.

• Extracellular Matrix (ECM): A network of proteins and polysaccharides providing structural support.

Plasma Membrane: The Fluid Mosaic Model

Basic Functions

The plasma membrane is a dynamic structure that serves as a mechanical barrier, regulates selective permeability, maintains electrochemical gradients, and enables cell communication and signaling.

• Mechanical Barrier: Separates intracellular and extracellular environments.

• Selective Permeability: Controls movement of substances.

• Electrochemical Gradient: Maintains differences in ion concentrations.

• Communication & Signaling: Facilitates interaction with other cells and response to signals.

Membrane Lipids

• Phospholipids: Form a bilayer with hydrophilic heads and hydrophobic tails, providing structural integrity.

• Glycolipids: Lipids with carbohydrate groups, involved in cell recognition.

• Cholesterol: Stabilizes membrane fluidity and structure.

Membrane Proteins

• Integral Proteins: Span the membrane, involved in transport and signaling.

• Peripheral Proteins: Attached to membrane surfaces, involved in support and signaling.

• Glycocalyx: Carbohydrate-rich area on cell surface, important for cell recognition.

Cell Junctions

• Tight Junctions: Prevent passage of substances between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

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

Movement of Substances Across Membranes

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

• Diffusion: Movement of molecules from high to low concentration.

• Simple Diffusion: Direct movement through the lipid bilayer (e.g., O2, CO2).

• Facilitated Diffusion: Uses carrier or channel proteins for larger or charged molecules.

• Osmosis: Diffusion of water through aquaporins.

Tonicity

Tonicity describes the effect of solutions on cell volume.

• Isotonic: No net movement of water; cell size remains constant.

• Hypertonic: Water leaves the cell; cell shrinks.

• Hypotonic: Water enters the cell; cell swells and may burst.

Active Transport

Primary and Secondary Active Transport

Active transport requires energy (ATP) to move substances against their concentration gradients.

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

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

Sodium-Potassium Pump

• Function: Maintains high Na+ outside and high K+ inside the cell.

• Mechanism: Moves both ions against their gradients using ATP.

Antiport and Symport Systems

• Antiport: Two substances move in opposite directions.

• Symport: Two substances move in the same direction.

Vesicular Transport

Types of Vesicular Transport

• Exocytosis: Release of substances from the cell via vesicles.

• Endocytosis: Uptake of substances into the cell via vesicles.

• Phagocytosis: Engulfment of large particles by pseudopods.

• Pinocytosis: Uptake of fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Specific uptake of molecules via receptors.

• Transcytosis: Movement of substances into, across, and out of the cell.

• Vesicular Trafficking: Movement of substances within the cell.

Summary Table: Active Processes

Process	Energy Source	Example
Primary active transport	ATP	Pumping of ions across membranes
Secondary active transport	Ion gradient	Movement of polar or charged solutes across membranes
Exocytosis	ATP	Secretion of hormones and neurotransmitters
Phagocytosis	ATP	White blood cell phagocytosis
Pinocytosis	ATP	Absorption by intestinal cells
Receptor-mediated endocytosis	ATP	Hormone and cholesterol uptake

Membrane Potentials

Resting Membrane Potential

Cells maintain a voltage across their plasma membrane due to differences in ion concentrations and permeability.

• Polarization: The inside of the cell is negatively charged relative to the outside.

• Leaky Channels: Allow passive movement of ions, contributing to the resting potential.

• Key Ions: Sodium (Na+), Potassium (K+), and chloride (Cl-).

Internal Cell Structures

The Nucleus

• Nuclear Envelope: Double membrane with pores for transport.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins (histones); condenses into chromosomes during cell division.

The Cytoplasm

• Cytosol: Fluid portion containing dissolved substances.

• Inclusions: Stored nutrients and pigments.

• Organelles: Specialized structures performing distinct functions.

Major Organelles and Their Functions

Mitochondria

• Structure: Double membrane with inner folds called cristae.

• Function: Site of aerobic respiration, producing most cellular ATP.

Ribosomes

• Composition: Protein and rRNA granules.

• Function: Site of protein synthesis; free ribosomes synthesize cytosolic proteins, membrane-bound ribosomes synthesize proteins for membranes or export.

Endoplasmic Reticulum (ER)

• Rough ER (rER): Studded with ribosomes; synthesizes secreted proteins, integral membrane proteins, and phospholipids.

• Smooth ER (sER): Lacks ribosomes; involved in lipid metabolism, calcium storage, and detoxification.

Golgi Apparatus

• Structure: Stacked, flattened membranous sacs.

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

Vesicles, Lysosomes, and Peroxisomes

• Vesicles: Membrane-bound sacs for transport and storage.

• Lysosomes: Contain digestive enzymes; break down waste, pathogens, and nonfunctional organelles.

• Peroxisomes: Contain oxidases and catalases; neutralize free radicals and detoxify harmful substances.

Additional info:

• These notes cover foundational concepts from Chapter 3: Cells—The Living Units, relevant to Anatomy & Physiology college courses.