The Cellular Level of Organization: Structure and Function of Cells

The Cellular Level of Organization

Introduction

The cell is the fundamental unit of life in all living organisms. Understanding the structure and function of cells is essential for comprehending how the human body operates at both microscopic and macroscopic levels. This section covers the cell theory, major cell components, membrane structure and function, transport mechanisms, organelles, and the central dogma of biology.

Cell Theory

Principles of Cell Theory

• All living things are composed of cells: Cells are the building blocks of all plants and animals.

• All new cells arise from preexisting cells: Cells reproduce by division, resulting in growth and the formation of new cells.

• Cells are the smallest units that carry out vital physiological functions: They perform essential processes such as nutrient uptake, waste removal, and energy production.

Major Components of the Cell

Overview of Cell Structure

• Plasma (Cell) Membrane: The outer boundary that separates the cell from its environment.

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

• Cytoplasm: The region between the plasma membrane and nucleus, containing cytosol and organelles.

The cytoplasm can be further subdivided into:

• Cytosol: The fluid component.

• Organelles: Specialized structures performing distinct cellular functions.

Classification of Organelles

Nonmembranous Organelles	Membranous Organelles
Cytoskeleton, Cilia, Flagella, Ribosomes, Microvilli	Mitochondria, Nucleus, Endoplasmic Reticulum (ER), Golgi Apparatus, Lysosomes, Peroxisomes

Plasma Membrane Structure and Function

Functions of the Plasma Membrane

• Physical Separation: Maintains the internal environment of the cell.

• Environmental Sensitivity: Contains receptors to monitor extracellular fluid (ECF).

• Regulation of Exchange: Selectively allows substances in and out via active and passive processes.

• Electrical Sensitivity: Generates membrane potential for signal transmission.

• Support: Provides structural support and connects cells to each other.

Phospholipid Bilayer and Fluid Mosaic Model

• Phospholipids: Form a bilayer with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Cholesterol: Maintains membrane fluidity and stability.

• Glycocalyx: Composed of glycoproteins and glycolipids; functions in cell recognition, protection, and adhesion.

• Membrane Proteins: Embedded or attached proteins with various functions (see below).

Types of Membrane Proteins

• Anchoring Proteins: Attach the cell to other cells or structures.

• Recognition Proteins: Identify the cell to the immune system (e.g., MHC proteins).

• Enzymes: Catalyze chemical reactions.

• Receptor Proteins: Bind ligands (e.g., hormones) from the ECF.

• Carrier Proteins: Transport specific substances across the membrane.

• Channel Proteins: Form pores for specific molecules to pass through.

• Peripheral Proteins: Usually attached to integral proteins; regulate channel and carrier activity.

Membrane Transport Mechanisms

Selective Permeability

• The plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

Types of Membrane Transport

Passive Processes	Active Processes
No energy (ATP) required - Simple diffusion - Facilitated diffusion (channels/carriers) - Osmosis	Require energy (usually ATP) - Primary active transport - Secondary active transport - Vesicular transport (endocytosis, exocytosis, pinocytosis, phagocytosis)

Passive Transport

• Simple Diffusion: Movement of small, non-polar, lipid-soluble molecules (e.g., O2, CO2) down their concentration gradient.

• Facilitated Diffusion: Movement of larger or polar molecules via channel or carrier proteins (e.g., ions such as Na+, K+, Cl-).

• Osmosis: Diffusion of water across a selectively permeable membrane from low solute concentration to high solute concentration.

Osmotic Pressure: The force exerted by solutes that pulls water across the membrane.

Osmolarity: The total solute concentration in a solution.

Tonicity and Clinical Relevance

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, causing swelling or lysis.

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

Active Transport

• Primary Active Transport: Direct use of ATP to move substances against their concentration gradient (e.g., Na+/K+ ATPase pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., sodium-glucose co-transport).

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

Na+/K+ Pump

• Maintains electrochemical gradients essential for nerve impulse transmission and muscle contraction.

• Moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

$3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Cellular Organelles and Their Functions

Nonmembranous Organelles

• Cytoskeleton: Provides structural support; composed of microfilaments, intermediate filaments, and microtubules.

• Microvilli: Increase surface area for absorption.

• Cilia and Flagella: Motility structures; move substances across cell surfaces or propel cells.

• Ribosomes: Sites of protein synthesis; can be free or attached to rough ER.

Membranous Organelles

• Mitochondria: Powerhouse of the cell; site of ATP production via aerobic respiration. Contains its own DNA (mtDNA).

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for export or membrane insertion.

  • Smooth ER: Synthesizes lipids, detoxifies drugs, stores calcium.

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

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

• Peroxisomes: Break down fatty acids and detoxify harmful substances; produce hydrogen peroxide.

• Nucleus: Contains DNA; controls cellular activities and protein synthesis.

The Central Dogma of Biology

Gene Expression: From DNA to Protein

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA) in the nucleus.

• RNA Processing: Introns are removed and exons are spliced together to form mature mRNA.

• Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble amino acids into a polypeptide (protein).

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Relationship Between Genes and Chromosomes

• Genes: Segments of DNA that code for specific proteins.

• Chromosomes: Structures composed of DNA and proteins (histones); contain many genes.

• During cell division, chromatin condenses to form visible chromosomes.

Cell Growth and Division

Cell Cycle

• Interphase: Period of cell growth and DNA replication.

• Mitotic Phase (Mitosis): Division of the nucleus and cytoplasm to form two identical daughter cells.

Stages of Mitosis

1. Prophase: Chromatin condenses into chromosomes; nuclear envelope dissolves; spindle forms.

2. Metaphase: Chromosomes align at the cell equator.

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

4. Telophase: Nuclear envelopes reform; chromosomes decondense.

5. Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Karyotype

• Karyotype: An organized profile of an individual's chromosomes.

• Used to detect chromosomal abnormalities, determine sex, and diagnose genetic diseases.

• Performed prenatally or for diagnosing genetic disorders at any age.

• Ethical Considerations: Issues may arise regarding privacy, discrimination, and reproductive choices.