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

Chapter 3: Cells – Structure, Function, and Processes

3.1 Introduction to Cells

Cells are the basic structural and functional units of all living organisms. Understanding their components and functions is essential for studying anatomy and physiology.

• Main Parts of a Cell:

  • Cytosol: The fluid portion of the cytoplasm where cellular processes occur.

  • Organelles: Specialized structures within the cell that perform distinct functions (e.g., mitochondria, ribosomes).

  • Chromosomes: Structures composed of DNA and proteins; carry genetic information.

  • Genes: Segments of DNA that code for proteins and determine hereditary traits.

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

• Example: Human somatic cells (e.g., skin cells, muscle cells).

3.2 The Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining homeostasis and mediating communication with the environment.

• Major Components:

  • Phospholipid bilayer: Provides structural integrity and fluidity.

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

  • Cholesterol: Stabilizes membrane fluidity.

  • Carbohydrates: Glycoproteins and glycolipids involved in cell recognition.

• Functions:

  • Regulates entry and exit of substances.

  • Facilitates cell signaling and adhesion.

  • Maintains cell shape.

• Selectively Permeable: Allows certain molecules to pass while restricting others.

• Major Functions of Membrane Proteins:

  • Transport (channels and carriers)

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to cytoskeleton and extracellular matrix

3.3 Transport across the Plasma Membrane

Transport mechanisms across the plasma membrane are essential for maintaining cellular homeostasis and function. These mechanisms can be passive or active.

• Concentration Gradient: The difference in concentration of a substance across a space or membrane.

• "Down its concentration gradient": Movement from high to low concentration (passive transport).

• "Against its concentration gradient": Movement from low to high concentration (requires energy; active transport).

• Types of Transport Mechanisms:

  • Passive Transport: Does not require energy.

    • Simple diffusion

    • Facilitated diffusion

    • Osmosis (diffusion of water)

  • Active Transport: Requires energy (ATP).

    • Primary active transport (e.g., sodium-potassium pump)

    • Secondary active transport (uses energy from another gradient)

    • Bulk processes: endocytosis (phagocytosis, pinocytosis), exocytosis

• Factors Affecting Diffusion:

  • Temperature

  • Size of molecules

  • Membrane permeability

  • Surface area

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

• Osmotic Pressure: Pressure required to prevent osmosis; depends on solute concentration.

• Effects of Solutions on Cells:

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

  • Hypotonic Solution: Water enters cell; cell may swell and burst.

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

3.4 Cell Organelles: Structure and Function

Organelles are specialized structures within cells that perform specific functions necessary for cell survival and activity.

• Cytoskeleton: Provides structural support; includes microfilaments, microtubules.

• Centrosome: Contains centrioles; important for cell division.

• Cilia and Flagella: Motile structures for movement.

• Microvilli: Increase surface area for absorption.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids; detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes; break down waste.

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

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

• Vesicles: Transport materials within the cell.

3.6 Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities such as growth, metabolism, and reproduction.

• Importance: Stores DNA; regulates gene expression.

• Structure: Surrounded by nuclear envelope; contains nucleolus and chromatin.

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

• Chromosomes: Humans have 46 chromosomes in somatic cells; gametes have 23.

• Nucleolus: Site of ribosome synthesis.

3.7 Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions. It involves transcription and translation.

• Location of DNA: DNA is located in the nucleus.

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

  • Occurs in the nucleus.

  • Base pairing: A-U, C-G in RNA.

• Translation: mRNA is decoded by ribosomes to build a protein.

  • Occurs in the cytoplasm (at ribosomes).

  • tRNA brings amino acids to the ribosome.

• Central Dogma:

  • DNA → RNA → Protein

• Example: Given a DNA sequence, transcribe to mRNA and translate to amino acids.

3.8 Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It includes interphase, mitosis, and cytokinesis.

• DNA Replication: Occurs during S phase of interphase; ensures each daughter cell receives identical genetic material.

• Mitosis: Division of the nucleus; produces two identical daughter cells.

  • Phases of Mitosis:

    • Prophase: Chromatin condenses; spindle forms.

    • Metaphase: Chromosomes align at the cell equator.

    • Anaphase: Sister chromatids separate.

    • Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm; occurs after mitosis.

• Meiosis: Cell division producing gametes; reduces chromosome number by half.

• Haploid Cell: Contains one set of chromosomes (n); gametes are haploid.

• Diploid Cell: Contains two sets of chromosomes (2n); somatic cells are diploid.

Key Table: Comparison of Transport Mechanisms

Transport Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Yes (indirect)	Against gradient	Glucose/Na+ cotransport
Endocytosis/Exocytosis	Yes	Bulk transport	Phagocytosis, secretion

Key Equations

• Osmotic Pressure: $\Pi = iMRT$ Where $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K).

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info: Some details (e.g., full organelle functions, phases of mitosis) have been expanded for academic completeness.