Nurs 1002-Obj #3

Characteristics of a Typical Cell

Cells are the fundamental structural and functional units of all living organisms. Understanding their characteristics is essential for grasping how tissues and organs function in the human body.

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

• Cell Diversity: Human cells vary in size, shape, and subcellular components, which account for their specialized functions.

• Examples of Human Cells: Muscle cells, nerve cells, fat cells, blood cells, and reproductive cells each have unique roles.

• Cell Function: The function of an organism depends on the collective activity of its cells.

Additional info: The plasma membrane's properties can affect medical treatments, as different cell types respond uniquely to drugs and therapies.

Intracellular and Extracellular Materials

Cells interact with their environment through various fluids and substances found inside and outside the cell.

• Intracellular Fluid (ICF): The fluid within cells, containing water, ions, and dissolved molecules.

• Extracellular Fluid (ECF): Body fluids outside cells, including:

  • Interstitial Fluid: Surrounds and bathes cells.

  • Blood Plasma: Fluid component of blood.

  • Cerebrospinal Fluid: Surrounds the brain and spinal cord.

  • Cellular Secretions: Substances like saliva, mucus, and gastric fluids.

  • Extracellular Matrix: Network of proteins and polysaccharides that provide structural support and glue cells together.

Additional info: The balance between ICF and ECF is crucial for maintaining homeostasis and proper cell function.

Cell Structure

Common Structures of Human Cells

Despite their diversity, most human cells share three basic structural components:

• Plasma Membrane: A flexible outer boundary that separates the cell from its environment and regulates the movement of substances in and out.

• Cytoplasm: The intracellular fluid containing organelles and dissolved substances.

• Nucleus: The control center housing DNA, which directs cellular activities.

Additional info: The plasma membrane is composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol, contributing to its selective permeability.

Major Organelles and Their Functions

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival.

• Nucleus: Contains genetic material (DNA) and controls cell activities.

• Mitochondria: Powerhouse of the cell; site of ATP (energy) production.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

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

• Peroxisomes: Break down fatty acids and neutralize toxins.

• Cytoskeleton: Provides structural support and facilitates cell movement.

Additional info: Some cells have specialized extensions such as cilia (for movement of substances) and microvilli (to increase surface area for absorption).

Plasma Membrane Structure and Function

Phospholipid Bilayer and Membrane Proteins

The plasma membrane is a selectively permeable barrier that controls the entry and exit of substances.

• Phospholipid Bilayer: Composed of hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Membrane Proteins: Include channels, carriers, receptors, and enzymes that facilitate transport and communication.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx, which aids in cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Cell Junctions

Cells are connected to each other by specialized junctions that maintain tissue integrity and allow communication.

• Tight Junctions: Prevent substances from passing between cells.

• Desmosomes: Anchor cells together and prevent tearing.

• Gap Junctions: Allow ions and small molecules to pass directly between cells, facilitating electrical and metabolic communication.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require cellular energy (ATP) and relies on concentration gradients.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) directly through the membrane.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, amino acids) via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane through aquaporins.

Factors Affecting Diffusion Rate:

• Concentration gradient

• Molecule size

• Temperature

Additional info: Equilibrium is reached when there is no net movement of molecules in one direction.

Active Transport

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

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+-K+ pump).

• Secondary Active Transport: Uses gradients established by primary transport to move other substances (e.g., symporters and antiporters).

• Vesicular Transport: Movement of large particles or fluids via vesicles (endocytosis and exocytosis).

Na+-K+ Pump Equation:

Osmotic Pressure and Its Effects on Cells

Osmotic pressure refers to the force exerted by water movement across the membrane due to solute concentration differences.

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

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

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, causing cell swelling and possible lysis.

Cell Metabolism and Homeostasis

How Cell Metabolism Maintains Homeostasis

Cell metabolism encompasses all chemical reactions within a cell, enabling growth, energy production, and maintenance of homeostasis.

• Homeostasis: The ability of cells to maintain a stable internal environment despite external changes.

• Hyperplasia: Increased cell division in response to physiological demand (e.g., bone marrow producing more red blood cells).

• Atrophy: Decreased cell size or number due to reduced stimulation or disease (e.g., muscular dystrophy).

Additional info: Theories of aging include the wear and tear theory, mitochondrial theory, immune theory, and genetic theory.

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process by which somatic cells divide to produce two genetically identical daughter cells for growth and repair.

• Phases of Mitosis: Prophase, Metaphase, Anaphase, Telophase, followed by cytokinesis (division of cytoplasm).

• Purpose: Growth, tissue repair, and maintenance.

Meiosis

Meiosis is a specialized type of cell division that occurs in gametes (sex cells), resulting in four genetically unique daughter cells with half the chromosome number.

• Purpose: Sexual reproduction and genetic diversity.

DNA, Genes, and Protein Synthesis

DNA contains the genetic blueprint for protein synthesis. Genes are segments of DNA that code for specific polypeptides.

• Gene: Sequence of DNA that codes for one polypeptide.

• Nitrogen Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

• Transcription: DNA code is transferred to messenger RNA (mRNA).

• Translation: mRNA is decoded to assemble amino acids into proteins.

Central Dogma Equation:

Summary Table: Types of Transport Across the Plasma Membrane