Cell Structure, Membrane, and Cell Division: Study Notes for Anatomy & Physiology

Cell Structure and Function

Basic Parts of a Cell

The cell is the fundamental structural and functional unit of living organisms. Its three basic parts are the plasma membrane, cytoplasm, and nucleus.

• Plasma Membrane: A selectively permeable barrier that regulates entry and exit of substances.

• Cytoplasm: The jelly-like substance within the cell containing organelles, where various cellular processes occur.

• Nucleus: The control center of the cell, containing genetic material (DNA) that regulates cellular activities.

Related Diseases:

• Cancer: Uncontrolled cell division due to mutations in genes regulating the cell cycle.

• Cystic Fibrosis: Genetic disorder affecting the plasma membrane's transport proteins.

• Muscular Dystrophy: Genetic mutations affecting cell membrane stability in muscle cells.

Structure of the Typical Cell Membrane

The cell membrane is a double layer of phospholipids with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Forms a semi-permeable membrane.

• Proteins: Serve as channels, carriers, receptors, enzymes, and identity markers.

• Cholesterol: Adds fluidity and stability to the membrane.

• Carbohydrates: Attached to proteins and lipids, functioning in cell recognition and adhesion.

Functions of the Membrane

• Phospholipid Barrier: Prevents free passage of water-soluble substances.

• Carriers and Channels: Facilitate movement of ions and molecules across the membrane.

• Enzymes: Catalyze reactions at the membrane surface.

• Receptors: Bind signaling molecules, initiating cellular responses.

• Surface Identity Markers: Enable cells to recognize each other.

Related Diseases:

• Diabetes Mellitus: Malfunctioning insulin receptors affect glucose uptake.

• Hypercholesterolemia: Defective LDL receptor causes high cholesterol levels.

Membrane Specializations

• Microvilli: Finger-like extensions of the plasma membrane that increase surface area for absorption (e.g., intestinal cells).

• Tight Junctions: Seal adjacent cells to prevent leakage of extracellular fluid (e.g., epithelial cells in the stomach).

• Desmosomes: Anchoring junctions that hold cells together (e.g., skin cells).

• Gap Junctions: Channels that allow direct communication between cells (e.g., cardiac cells for coordinated contractions).

Selective Permeability of the Membrane

Selective permeability allows certain molecules to pass through the membrane while blocking others. This is essential for maintaining homeostasis, nutrient uptake, and waste removal.

Active and Passive Processes of Movement Across the Membrane

• Active Processes: Require energy (ATP) to move substances against their concentration gradient.

• Passive Processes: Do not require energy and rely on the concentration gradient.

Passive Processes: Diffusion, Osmosis, Facilitated Diffusion

• Diffusion: Movement of molecules from an area of high concentration to low concentration.

• Facilitated Diffusion: Movement of molecules across a membrane via carrier proteins or channels.

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

Osmotic Pressure, Isotonic, Hypotonic, and Hypertonic Solutions

• Osmotic Pressure: The pressure required to prevent water movement across a membrane.

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

• Hypotonic Solution: Lower solute concentration outside the cell, causing water to enter the cell (may lead to lysis).

• Hypertonic Solution: Higher solute concentration outside the cell, causing water to leave the cell (may lead to crenation).

Active Transport, Endocytosis, and Exocytosis

• Active Transport: Moves molecules against their gradient using energy (e.g., sodium-potassium pumps).

• Endocytosis: The process of taking substances into the cell by engulfing them in a vesicle.

• Phagocytosis: "Cell eating," where large particles are engulfed (e.g., macrophages).

• Pinocytosis: "Cell drinking," where small particles and fluids are taken in.

• Exocytosis: The process of expelling substances from the cell using vesicles.

Cytoplasm, Organelles, and Inclusions

Components of the Cytosol

• Water

• Enzymes

• Wastes

• Proteins

Major Organelles: Structure and Function

• Nucleus: Contains genetic material; controls cellular activities.

• Mitochondria: Double membrane; produces ATP through cellular respiration.

• Endoplasmic Reticulum (Rough and Smooth): Network of membranes; Rough ER synthesizes proteins; Smooth ER synthesizes lipids.

• Golgi Apparatus: Stacked membranes; modifies, sorts, and packages proteins and lipids.

• Lysosomes: Membrane-bound vesicles containing enzymes; digest cellular waste.

• Peroxisomes: Contain oxidative enzymes; detoxify harmful substances.

• Ribosomes: Protein synthesis machinery; can be free or attached to Rough ER.

• Cytoskeleton: Network of fibers (microfilaments, intermediate filaments, microtubules); provides structural support and enables movement.

Chromatin, Chromosomes, Nucleolus, and Histone

• Chromatin: Uncondensed DNA and protein complexes found in the nucleus during interphase.

• Chromosomes: Condensed DNA structures visible during cell division.

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized.

• Histone: Proteins around which DNA wraps, forming nucleosomes for DNA packaging.

Rough vs. Smooth Endoplasmic Reticulum

• Rough ER: Studded with ribosomes; involved in protein synthesis and processing.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

Cell Inclusions

• Glycogen Granules: Stored form of glucose in liver and muscle cells.

• Lipid Droplets: Stored fat in adipose cells.

• Pigment Granules: Melanin in skin cells.

Nuclear Activities: Protein Synthesis and Cell Division

Gene and Genetic Code

• Gene: A segment of DNA that contains instructions for synthesizing a specific protein.

• Genetic Code: DNA contains sequences of nucleotides (adenine, thymine, cytosine, guanine) arranged in triplets (codons), each coding for a specific amino acid in a protein.

Transcription and Translation

• Transcription: The process where a gene's DNA sequence is copied into mRNA.

• Translation: The process where the mRNA sequence is used to assemble amino acids into a polypeptide chain at the ribosome.

Process of Protein Synthesis in Human Cells

1. Transcription: DNA to mRNA in the nucleus.

2. mRNA Processing: Splicing and modification.

3. Translation: mRNA to protein in the cytoplasm at ribosomes.

Types of RNA in Protein Synthesis

• mRNA (Messenger RNA): Carries the genetic code from DNA to the ribosome.

• tRNA (Transfer RNA): Brings the correct amino acid to the ribosome based on the mRNA codon.

• rRNA (Ribosomal RNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

Codon and Anticodon

• Codon: A three-nucleotide sequence on mRNA that codes for a specific amino acid.

• Anticodon: A three-nucleotide sequence on tRNA that is complementary to an mRNA codon.

Cell Division: Mitosis and Cytokinesis

Phases of the Cell Cycle

• Interphase: Cell grows (G1), DNA replicates (S), and prepares for division (G2).

• Mitosis: Division of the nucleus (prophase, metaphase, anaphase, telophase).

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

Phases of Mitosis

• Prophase: Chromosomes condense, spindle fibers form, nuclear envelope dissolves.

• Metaphase: Chromosomes align at the cell's equator.

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

• Telophase: Nuclear envelopes reform around the separated chromosomes.

Importance of Cell Division

• Growth: Increasing the number of cells during development.

• Tissue Repair: Replacing damaged or lost cells.

• Replacement: Continuously renewing cells, such as skin and blood cells.

Control of Cell Division

Cell division is controlled by a complex interplay of signals, including growth factors, hormones, and regulatory proteins (e.g., cyclins and cyclin-dependent kinases) that ensure proper timing and progression through the cell cycle.

Related Diseases:

• Cancer: Caused by mutations leading to uncontrolled cell division.

• Progeria: A genetic disorder causing accelerated aging due to defective cell division.

Summary Table: Cell Organelles and Functions

Organelle	Structure	Function
Nucleus	Double membrane, contains DNA	Controls cellular activities
Mitochondria	Double membrane, inner folds (cristae)	Produces ATP (energy)
Rough ER	Membrane network with ribosomes	Protein synthesis and processing
Smooth ER	Membrane network without ribosomes	Lipid synthesis and detoxification
Golgi Apparatus	Stacked membranes	Modifies, sorts, packages proteins/lipids
Lysosomes	Membrane-bound vesicles	Digest cellular waste
Peroxisomes	Membrane-bound vesicles	Detoxify harmful substances
Ribosomes	Protein and rRNA complexes	Protein synthesis
Cytoskeleton	Microfilaments, microtubules, intermediate filaments	Structural support, movement

Key Equations and Definitions

• Diffusion Rate: $J = -D \frac{dC}{dx}$

• Osmotic Pressure: $\Pi = iMRT$

• Genetic Code: Codon: Three-nucleotide sequence on mRNA coding for an amino acid.

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Equations and table entries have been inferred and formatted for academic study.