Cells: The Basic Units of the Body
Introduction to Cells as the Basic Units of the Body
The Cell Definition: The cell serves as the basic unit of structure and function within the human body.
Measurement: Cells are measured in units called micrometers.
Cell Differentiation: This is the process where cells develop specialized characteristics. Cells that have achieved these specific traits are referred to as differentiated.
Form and Function: Cells vary significantly in size and shape. The structure of a cell is fundamentally interrelated with its specific function.
Nerve Cells: Feature long extensions allowing them to conduct electrical impulses across different body parts.
Epithelial Cells: Organized in sheetlike structures that enable them to protect underlying tissues.
Muscle Cells: Contain aligned contractile proteins that allow them to contract and pull structures closer together.
Specific Cell Dimensions (Text Alternative Data):
Red Blood Cell: .
White Blood Cell: .
Circular Cell Example: .
Elongated Cell Example: .
The Composite Cell
Concept of a Composite Cell: While there is no such thing as a "typical" cell, a composite cell model is used to show the standard components found in most cells.
Three Major Parts of a Cell:
Nucleus: Contains genetic material and serves as the director of cellular activities.
Cytoplasm: Consists of specialized structures called organelles suspended in a liquid medium known as cytosol.
Cell Membrane: The outer boundary of the cell.
Cell Membrane Structure and Function
General Characteristics:
Acts as the outer boundary and maintains cellular integrity.
Separates the intracellular fluid (cytosol) from the extracellular fluid.
Selectively Permeable: It regulates exactly which substances enter and exit the cell.
Signal Transduction: Allows the cell to receive and respond to external messages.
Chemical Composition:
Phospholipid Bilayer: The framework of the membrane. It features water-soluble (hydrophilic) heads forming the inner and outer surfaces and water-insoluble (hydrophobic) tails forming the interior. It is permeable to lipid-soluble substances but impermeable to water-soluble substances.
Cholesterol: Functions to stabilize the membrane and assists in keeping it impermeable to water-soluble substances.
Fluid Mosaic: Term used to describe the mobility of some lipids and proteins within the membrane.
Proteins: Serve varied functions including pores, channels, receptors, enzymes, and cell adhesion molecules (CAMs).
Carbohydrates: Used for cell recognition, interaction, and acting as self-markers.
Types of Membrane Proteins
Receptors: Respond to extracellular signals.
Pores, Channels, and Carriers: Function to transport small molecules and ions; they also transduce signals.
Enzymes: Act as catalysts for chemical reactions.
Cell Surface Proteins: Establish the identity of the cell as "self."
Cellular Adhesion Molecules (CAMs): Enable cells to adhere to one another.
Clinical Application: Faulty Ion Channels
Sodium () Channels: Mutations can lead to either an inability to feel pain or various extreme pain conditions.
Potassium () Channels: Mutations may disrupt the heart's electrical activity, disturbing heart rhythm, or impairing hearing.
Chloride () Channels: Abnormalities are the root cause of Cystic Fibrosis, characterized by thick mucus production that clogs the lungs and pancreas and creates salty sweat.
Cellular Adhesion Molecules (CAMs)
Definition: Molecules that guide cells that are on the move.
Selectins: Coat white blood cells to anchor them to capillary walls via friction (rolling attachment).
Adhesion Receptor Proteins: Bind to integrins to assist white blood cells in leaving the capillary.
Integrins: Direct white blood cells through the capillary walls toward sites of infection.
Other CAM Roles:
Guide embryonic cells toward maternal cells to establish the placenta.
Establish vital connections between nerve cells.
Cytoplasm and Organelles
Cytoplasm Composition: Defined as the combination of cytosol (fluid) and organelles (solid structures). It contains the cytoskeleton, a framework of protein rods and tubules.
Ribosomes: Composed of protein and RNA. They are found free in the cytoplasm or attached to the Rough Endoplasmic Reticulum (RER). Their role is to provide structural support and enzyme activity to link amino acids during protein synthesis.
Endoplasmic Reticulum (ER):
Rough ER (RER): Studded with ribosomes; site of protein synthesis.
Smooth ER (SER): Lacks ribosomes; site of lipid synthesis and sugar synthesis.
Vesicles: Membranous sacs used for storing or transporting substances.
Golgi Apparatus: A stack of flattened membranous sacs that refines, packages, and delivers proteins synthesized on the RER.
Milk Secretion Example: An interactive process involving the RER, SER, Golgi apparatus, and transport vesicles.
Lipids are synthesized in the SER.
Proteins are synthesized on the RER.
Sugars are synthesized in the SER and Golgi.
Vesicles bud from the Golgi for secretion.
Mitochondria: Membrane-bound, fluid-filled sacs that house reactions for cellular respiration to produce Adenosine Triphosphate (ATP). Known as the "powerhouse of the cell."
Lysosomes: "Garbage disposals" containing enzymes to digest proteins, carbohydrates, nucleic acids, bacteria, and worn-out cell parts.
Peroxisomes: Similar to lysosomes; contain enzymes that digest lipids, alcohol, and hydrogen peroxide ().
The Cytoskeleton and Other Structures
Microfilaments: Tiny rods of the protein actin. They provide cellular movement, such as muscle contraction.
Microtubules: Larger tubes made of the protein tubulin. They maintain cell shape, make up centrioles, cilia, and flagella, and help move organelles.
Intermediate Filaments: Composed of several proteins; they support the nuclear envelope and provide structural frameworks.
Centrosome: The "central body" containing two centrioles. Centrioles are cylindrical microtubule structures that produce spindle fibers during cell division to distribute chromosomes.
Cilia: Short, abundant, motile extensions forming a "fringe." They beat in a coordinated manner (power and recovery strokes) to move substances like mucus or eggs.
Flagella: Much longer than cilia; a cell typically has only one. In humans, the only example is the sperm cell tail, which moves the entire cell.
Microvilli: Tiny extensions of the cell membrane composed of actin that increase surface area for absorption.
Inclusions: Non-essential chemicals present in specific cells, such as melanin granules (skin) or fat (adipocytes).
Clinical Application: Organelle-Level Diseases
MELAS: A mutant gene in mitochondrial DNA that prevents a person from extracting maximum energy from nutrients.
Krabbe Disease: An inability to produce a specific lysosomal enzyme, resulting in the inability to produce myelin and subsequent severe nervous system damage.
Adrenoleukodystrophy (ALD): Caused by a lack of a protein in the peroxisome membrane. This leads to fatty acid buildup which destroys myelin sheaths, slowing nerve impulse transmission.
The Cell Nucleus
Functions: Houses genetic material and controls cellular activities.
Nuclear Envelope: A double-layered membrane separating the nucleoplasm from the cytoplasm. It contains nuclear pores for substance passage.
Nucleolus: A dense body made of RNA and protein; it is the site of ribosome production.
Chromatin: Consists of chromosomes (DNA wound around proteins) and stores information for protein synthesis.
Passive Mechanisms of Movement
General Definition: Processes that do not require cellular energy (ATP).
Diffusion: Movement from high concentration to low concentration until equilibrium is reached. It applies to lipid-soluble substances like oxygen and carbon dioxide.
Facilitated Diffusion: Diffusion across the membrane using ion channels or carrier proteins (transporters). Used for water-soluble substances like glucose and amino acids.
Osmosis: The diffusion of water across a selectively permeable membrane toward a region of higher impermeant solute concentration. Water moves through specialized channels called aquaporins.
Osmotic Pressure: The ability of osmosis to generate pressure to lift water. Pressure increases as the concentration of impermeant solutes increases.
Tonicity:
Isotonic Solution: Same osmotic pressure as the cell; no net water movement.
Hypertonic Solution: Higher osmotic pressure than the cell; cells lose water and shrink (crenation).
Hypotonic Solution: Lower osmotic pressure than the cell; cells gain water and swell, potentially bursting (lysis).
Filtration: A process that forces molecules through membranes using pressure (e.g., blood pressure in capillaries). Separates small solutes from large proteins.
Active Mechanisms of Movement
General Definition: Processes that require cellular energy (ATP) to move substances against a concentration gradient.
Active Transport: Uses carrier molecules or "pumps." Example: The ATPase pump transports three sodium ions () out of the cell for every two potassium ions () brought in.
Secondary Active Transport: A carrier protein uses the gradient to transport another substance (like glucose) without direct ATP use.
Endocytosis: Movement into the cell via a vesicle.
Pinocytosis: "Cell drinking"; engulfing liquid droplets.
Phagocytosis: "Cell eating"; engulfing solid particles. Phagosomes fuse with lysosomes to digest the contents.
Receptor-Mediated Endocytosis: Engulfing specific substances (ligands) that have bound to membrane receptors.
Exocytosis: The release of substances from a cell as vesicles fuse with the cell membrane (e.g., neurotransmitter release).
Transcytosis: Combines receptor-mediated endocytosis and exocytosis to move a substance rapidly across a cell barrier (e.g., HIV moving across epithelial linings).
The Cell Cycle
Definition: The series of changes from the time a cell forms until it divides.
Stages:
Interphase: Active period of growth and replication.
and Phases: Growth phases; structures and molecules are duplicated.
Phase: Synthesis phase; DNA is replicated.
Mitosis: Division of the nucleus (karyokinesis).
Prophase: Chromatin condenses into chromosomes; centrioles move to poles; nuclear envelope and nucleolus disperse.
Metaphase: Spindle fibers attach to centromeres and align chromosomes midway.
Anaphase: Centromeres separate; sister chromatids move toward opposite centrioles.
Telophase: Chromosomes return to chromatin; nuclear envelopes and nucleoli reform.
Cytokinesis: Division of the cytoplasm.
Begins in anaphase and continues through telophase. A contractile ring of actin filaments creates a cleavage furrow to pinch the cell in half.
Control of Cell Division
Regulation: Division frequency varies by cell type (e.g., skin cells divide often; neurons eventually stop).
Telomeres: Chromosome tips that shorten with each division, acting as a "mitotic clock."
External Controls: Hormones and growth factors.
Surface Area to Volume Relationship: Cells divide to maintain a favorable ratio.
Contact Inhibition: Healthy cells stop dividing when they become crowded.
Tumors and Cancer
Tumor Types:
Benign: Remains localized and enlarged.
Malignant: Invasive, cancerous, and capable of metastasis (spreading).
Genetic Causes:
Oncogenes: Overexpressed genes that abnormally accelerate the cell cycle.
Tumor Suppressor Genes: Normally limit mitosis; cancer occurs if they are inactivated.
Characteristics of Cancer Cells: Loss of cell cycle control, heritability, transplantability, dedifferentiation (loss of specialization), loss of contact inhibition, ability to induce angiogenesis (blood vessel formation), invasiveness, and metastasis.
Stem and Progenitor Cells
Stem Cell: Undifferentiated cell that can divide to produce two new stem cells (self-renewal) or one stem cell and one progenitor cell.
Progenitor Cell: A partially specialized, "committed" daughter cell of a stem cell.
Potency:
Totipotent: Can become any cell type (e.g., fertilized egg).
Pluripotent: Can become a limited number of cell types (e.g., stem cells of later development).
Regenerative Medicine: Field using stem cells to treat disease and injury. Sources include donors (umbilical cord) or the patient (bone marrow or reprogrammed cells).
Cell Death
Apoptosis: Programmed cell death. A normal, stepwise process (e.g., removing fetal webbing, peeling sunburnt skin). Involves the activation of caspases that destroy cell components, leading to cell fragmentation and phagocytosis.
Necrosis: Cell death resulting from damage; not a normal physiological process.
Questions & Discussion
Q: How do white blood cells reach an infection site?
A: White blood cells utilize CAMs. Selectins provide friction to slow the cell down, then adhesion receptor proteins bind to integrins to help the cell exit the capillary and move toward the infection.
Q: What is the primary difference between passive and active transport?
A: Passive transport (diffusion, osmosis, filtration) moves substances along a concentration or pressure gradient without ATP. Active transport (pumps, endocytosis, exocytosis) requires ATP to move substances, often against a concentration gradient.
Q: What determines a cell's identity as "self"?
A: Cell surface proteins and carbohydrates act as self-markers for recognition and interaction.