BIOS-150: Human Biology - Week 2

BIOS-150: Human Biology - Week 2: Human Cells: Structure and Function

Lecture Objectives

• Describe the cell as the basic unit of life in the human body

• Identify key cell structures and explain their functions

• Explain the process and purpose of cell division (mitosis)

• Recognize how cellular imbalances can affect body systems and homeostasis

Introduction to the Cell

• The cell is the smallest living unit that carries out all life functions.

• The human body contains approximately 37 trillion cells.

• Key features common to all cells:

  • Plasma membrane

  • Cytoplasm

  • DNA

• Cells vary in shape and function to meet the body’s needs and maintain homeostasis.

Why It Matters in Healthcare:

• Sickle Cell Disease: Misshaped red blood cells can't carry oxygen effectively.

• Neuropathy: Damaged nerve cells disrupt signals, causing pain or weakness.

• Cancer: Characterized by abnormal cell growth that invades healthy tissue.

• Key Point: Understanding cells is vital for healthcare workers to diagnose and treat diseases at their source.

The Birth of Cytology - The Study of the Cell

• Cytology is the study of cell structure, function, and behavior.

• Critical for fields such as pathology, oncology, and genetics.

• Cytology helps explain disease processes, medication effects, and cell repair.

A Brief History:

• 1665: Robert Hooke was the first to use the term "cell" after observing cork under a microscope, identifying empty compartments that were remnants of dead plant cells.

• 1800s: Schwann & Schleiden proposed the Cell Theory: all living things are made of cells.

• Rudolf Virchow: Demonstrated that cells arise only from other cells and is known as the Father of Modern Pathology.

The Cell Theory

The 3 Principles of Cell Theory:

1. All living things are made of cells: Life, from the simplest bacteria to complex humans, is cellular.

2. Cells are the basic units of structure and function: They perform vital tasks such as energy production and growth.

3. All cells come from pre-existing cells: New cells arise through cell division, not spontaneously.

Why It Matters in Healthcare:

• Cancer: Uncontrolled cell division leads to tumors.

• Infection: Pathogens can hijack healthy cells.

• Wound Healing: Relies on cell regeneration.

• Key Point: Understanding cell theory is crucial for explaining how diseases start and how the body heals.

Essential Parts of Human Cells

1. Plasma Membrane – The Protective Barrier

   • Selectively permeable: Regulates what enters and exits the cell.

   • Functions in communication, fluid balance, and immune defense.

   • Important for understanding medication delivery and infection control.

2. Cytoplasm – The Cellular Workspace

   • Gel-like fluid filling the cell, containing organelles such as mitochondria and ribosomes that perform specific functions.

   • Cytosol contains water, nutrients, ions, and enzymes for biochemical reactions.

3. Nucleus – The Control Center

   • Houses DNA, directing cell function, growth, repair, and protein production.

   • Key to understanding genetic disorders, cancer, and the effects of medications.

The Plasma Membrane: Composition of Our Barriers

• The plasma membrane is a thin, flexible barrier separating the cell's interior from the external environment, controlling what enters and exits, thus helping maintain homeostasis.

Key Components:

• Phospholipid Bilayer:

  • Made up of two layers of phospholipids.

  • Hydrophilic heads face outward; hydrophobic tails face inward.

• Proteins:

  • Channel proteins: Allow specific substances to pass through.

  • Carrier proteins: Transport molecules across the membrane.

  • Receptor proteins: Detect and respond to external signals.

• Cholesterol:

  • Maintains membrane flexibility and stability, preventing it from becoming too rigid or too fluid.

• Carbohydrates:

  • Located on the outer surface, acting as ID tags to help cells recognize each other.

Cellular Transport - Overview

• Passive Transport (requires no energy):

  • Moves substances down their concentration gradient (high to low).

  • Includes:

  • Diffusion: Movement of small molecules (e.g., oxygen, CO₂).

  • Osmosis: Diffusion of water through a membrane.

  • Facilitated diffusion: Utilizes channel or carrier proteins for larger or charged molecules.

• Active Transport (requires energy/ATP):

  • Moves substances against their concentration gradient (low to high).

  • Requires protein pumps or vesicles.

• Key Point: Cells use passive transport to conserve energy and active transport when precise control is necessary.

Cellular Transport - Passive Transport Mechanisms

• Diffusion:

  • Molecules move from an area of high concentration to low concentration until equilibrium is achieved.

  • Example: Oxygen entering cells and CO₂ exiting cells.

• Osmosis:

  • Movement of water across a semi-permeable membrane towards a higher solute concentration.

  • Effects:

  • Cell swelling (in hypotonic solutions).

  • Cell shrinking (in hypertonic solutions).

  • No change (in isotonic solutions).

  • Key Point: Water follows solutes!

Cellular Transport - Active Transport (Endocytosis)

• Endocytosis is the process cells use to engulf large substances using vesicles.

Types of Endocytosis:

• Phagocytosis (“cell eating”):

  • Engulfs large particles like bacteria or debris, utilized by immune cells.

• Pinocytosis (“cell drinking”):

  • Involves the intake of fluids and dissolved substances.

• Receptor-Mediated Endocytosis:

  • Targets specific molecules using cell surface receptors (e.g., cholesterol uptake by liver cells).

• Key Point: These forms of bulk transport are essential for nutrient absorption, immune defense, and cell signaling.

Inside the Cell: Cytoplasm & Cell Interactions

• Composition of Cytoplasm:

  • Cytosol: Watery fluid with dissolved nutrients, ions, and proteins.

  • Organelles: Tiny structures that perform specific cell functions.

  • Cytoskeleton: A network of proteins providing structural support and facilitating movement.

Importance of the Membrane and Cytoplasm in Healthcare:

• Fluid Imbalances:

  • Damage to the membrane may result in nutrient leakage or toxin entry.

• Medication Action:

  • Many drugs bind to membrane receptors to trigger internal biochemical changes.

• Infection Control:

  • Viruses and bacteria often attach to the plasma membrane.

Inside the Cell: Nucleus - Control Center of the Cell

• Contains DNA, which provides instructions for:

  • Growth, division, repair, and protein production.

• Surrounded by the nuclear envelope, regulating entry and exit of materials.

• Houses the nucleolus, which synthesizes ribosomes for protein production.

Importance in Healthcare:

• Genetic Disorders: Mutations in DNA can result in faulty proteins.

• Cancer: Damaged DNA may lead to uncontrolled division of cells.

• Aging & Disease: Nuclear damage affects cellular responses to stress over time.

• Genetic Testing: Relies on the analysis of nuclear DNA for diagnostic purposes.

Inside the Cell: Endoplasmic Reticulum and Ribosomes - Production Line of the Cell

• Endoplasmic Reticulum (ER):

  • A network of membranes located near the nucleus, involved in protein and lipid production.

  • Rough ER:

  • Studded with ribosomes.

  • Modifies and folds proteins for cellular use or secretion.

  • Smooth ER:

  • Lacks ribosomes.

  • Synthesizes lipids and detoxifies harmful substances.

• Ribosomes:

  • Synthesize proteins by assembling amino acids according to DNA instructions.

  • Present in the cytoplasm or attached to the rough ER.

Importance in Healthcare:

• Genetic disorders may arise from errors in protein synthesis.

• Antibiotics (e.g., tetracyclines) target bacterial ribosomes without affecting human ribosomes.

Inside the Cell: Golgi Complex - Shipping Center of the Cell

• The Golgi complex (apparatus) processes and packages proteins and lipids produced in the ER.

  • Packages materials into vesicles for:

  • Internal cellular use.

  • Export outside the cell (exocytosis).

  • Assists in quality assurance through lysosome manufacturing.

Importance in Healthcare:

• Errors in Golgi processing can lead to genetic diseases.

• Example: In cystic fibrosis, misfolded proteins fail to reach their destinations, disrupting cellular function.

Inside the Cell: Lysosome - Recycling Center of the Cell

• Lysosomes are small organelles loaded with digestive enzymes that break down waste, old cell parts, and foreign pathogens (like bacteria).

Importance in Healthcare:

• Malfunctioning lysosomes can lead to toxic accumulation of cellular components.

• Example: In Tay-Sachs disease, missing enzymes cause severe cell damage.

• White blood cells utilize lysosomes to eliminate bacteria through phagocytosis.

Inside the Cell: Mitochondria - Powerhouse of the Cell

• Mitochondria are double-membraned organelles responsible for producing ATP (adenosine triphosphate), the energy currency of cells.

  • ATP production occurs through cellular respiration, utilizing glucose and oxygen.

  • The inner mitochondrial membrane is folded into structures called cristae to maximize energy production.

  • Mitochondria contain their own DNA, inherited maternally.

  • High abundance in energy-demanding cells, such as muscle and nerve cells.

Importance in Healthcare:

• Dysfunctional mitochondria can lead to reduced energy availability, hampering cell survival.

• Associated conditions:

  • Mitochondrial diseases

  • Chronic fatigue syndrome

Cell Cycle: An Overview

• Every healthcare worker encounters cell division, whether directly or indirectly.

• Cells divide to support:

  • Growth: Increasing cell numbers during development.

  • Healing: Replacing cells after injury.

  • Routine Maintenance: Replacing aging or dead cells.

Importance in Healthcare:

• Normal cell division facilitates tissue repair and recovery.

• Errors in division can result in:

  • Cancer: Uncontrolled growth of abnormal cells.

  • Genetic Disorders: Inaccurate DNA replication leading to mutations.

  • Delayed Healing: Slow or stalled cell division hampers recovery processes.

Cell Cycle: Interphase - Preparation for Cell Division

• Interphase is the longest phase of the cell cycle, during which cells prepare for division.

• Major tasks in interphase:

  • Expand in size (grow larger).

  • Duplicate DNA.

  • Synthesize additional organelles and proteins.

Sub-Phases of Interphase:

• G1 (Growth Stage 1): Normal growth and cellular functions occur.

• S (Synthesis Stage): DNA replication takes place.

• G2 (Growth Stage 2): Continued growth, DNA error checks, and preparations for mitosis.

• Key Point: Healthy cell division is contingent upon thorough preparation during interphase.

Cell Cycle: Mitosis and Cytokinesis - Division of Nucleus and Cytoplasm

• Following interphase, the cell enters mitosis to split the nucleus and yield two identical daughter cells.

Stages of Mitosis:

1. Prophase: Chromosomes condense and nuclear envelope breaks down.

2. Metaphase: Chromosomes align in the center of the cell.

3. Anaphase: Sister chromatids are separated and pulled apart.

4. Telophase: Formation of new nuclear envelopes for each set of chromosomes.

5. Cytokinesis: Cytoplasm divides, resulting in two identical daughter cells.

Connection to Healthcare:

• Wound Healing: Increased cell division rate aids in tissue repair.

• Cancer: Cells may bypass regulatory checkpoints, resulting in uncontrolled division.

• Chemotherapy: Many cancer treatments target cells during mitosis to inhibit growth.

Cell Cycle: Meiosis - Division of Gametes

• Meiosis is a specialized type of cell division that produces four genetically unique gametes (sperm and egg), each with half the chromosome number (haploid), to ensure the proper chromosome count in fertilized cells.

Key Features of Meiosis:

• Produces genetic variation through crossing over and independent assortment, explaining why siblings may differ genetically.

Two Rounds of Division:

1. Meiosis I:

   • Homologous chromosomes pair and exchange genetic material (swapping genes).

   • Chromosome pairs separate into two cells.

2. Meiosis II:

   • Sister chromatids split, resulting in four non-identical haploid gametes.

Cell Cycle: When Cell Division Goes Wrong

What Can Go Wrong:

• Uncontrolled division can lead to tumors and cancer.

• Insufficient division hampers wound healing and can lead to tissue breakdown.

• Faulty DNA replication may create mutations that are transmitted to daughter cells.

• Errors in chromosome separation can result in genetic disorders.

Examples of Disorders from Division Errors:

• Cancer: Arises from damaged DNA leading to unchecked mitosis.

• Down Syndrome: Resulting from an extra chromosome 21 due to meiosis errors.

• Turner Syndrome: Characterized by the absence of one X chromosome.

• Klinefelter Syndrome: Presence of an extra X chromosome in individuals assigned male at birth (XXY).

Common Causes of Irregular Division:

• Genetic mutations.

• Radiation exposure (UV light, X-rays).

• Chemical damage (toxins, side effects of chemotherapy).

• Viral infections (e.g., HPV interfering with cell regulation).

Cancer: An Overview

• Cancer is not a singleton disorder but a group of conditions resulting from uncontrolled cell division.

How Cancer Starts:

• DNA damage or mutations disrupt normal cell cycle control, caused by:

  • Inherited genetic predispositions (family history).

  • Exposure to carcinogens (radiation, tobacco, chemicals).

  • Chronic infections (e.g., HPV).

  • Random errors during cell division.

What Goes Wrong in Cancer:

• Cancer cells ignore cellular checkpoints and do not initiate apoptosis (programmed cell death).

• Damaged cells continue to proliferate, forming tumors.

• These cells pass on faulty DNA to their progeny.

Comparison: Normal Cells vs. Cancer Cells

Cancer: Tumors - Benign Versus Malignant

• Not all tumors are cancerous. Tumors are abnormal cell masses that behave differently based on their type.

Benign Tumors:

• Non-cancerous.

• Grow slowly and stay localized.

• Often encapsulated; can typically be surgically removed.

• May exert pressure on adjacent organs or structures (e.g., brain, airway).

Malignant Tumors:

• Cancerous.

• Grow rapidly and invade nearby tissues.

• Can spread through blood or lymph (metastasize).

• Disrupt normal organ function.

• Require more aggressive interventions: surgery, chemotherapy, radiation.

End of Week Review

• Describe the cell as the basic unit of life in the human body.

• Identify key cell structures and explain their functions.

• Explain the process and purpose of cell division (mitosis).

• Recognize how cellular imbalances can affect body systems and homeostasis.