The Cell: Structure, Function, and Processes

Cellular Organelles, Inclusions, and Structures

The cell is the basic structural and functional unit of life. Eukaryotic cells contain various organelles and structures, each with specialized roles essential for cellular function and survival.

• Plasma Membrane: A selectively permeable phospholipid bilayer that encloses the cell, regulates entry and exit of substances, and facilitates communication with the environment.

• Cytoplasm: The region between the plasma membrane and nucleus, containing cytosol (fluid) and organelles.

• Nucleus: The control center of the cell, housing DNA and coordinating activities like growth, metabolism, and reproduction.

• Mitochondria: Double-membraned organelles responsible for ATP production through aerobic respiration; often called the "powerhouse" of the cell.

• Peroxisomes: Small organelles containing enzymes that detoxify harmful substances and break down fatty acids.

• Ribosomes: Non-membranous structures composed of rRNA and proteins; sites of protein synthesis. Can be free in cytosol or bound to rough ER.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes and modifies proteins for secretion or membrane insertion.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Golgi Apparatus: Stacks of flattened membranes that modify, sort, and package proteins and lipids for delivery.

• Lysosomes: Membrane-bound vesicles containing digestive enzymes; break down waste, damaged organelles, and foreign substances.

• Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) providing structural support, shape, and facilitating movement.

• Microvilli: Fingerlike extensions of the plasma membrane that increase surface area for absorption.

• Cilia: Short, hairlike projections that move substances across the cell surface.

• Flagellum: Long, whip-like structure (e.g., sperm tail) enabling cell movement.

• Centrosome and Centrioles: Organize microtubules and play a key role in cell division.

Membrane Transport: Active and Passive Processes

Cells regulate the movement of substances across their membranes using passive and active transport mechanisms.

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2) through the lipid bilayer.

  • Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers (e.g., glucose, ions).

  • Osmosis: Diffusion of water across a selectively permeable membrane from low to high solute concentration.

• Active Transport: Movement of molecules against their concentration gradient, requiring ATP.

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

  • Secondary Active Transport: Uses energy from the movement of another substance down its gradient (co-transport).

• Vesicular Transport: Movement of large particles via vesicles.

  • Endocytosis: Uptake of materials into the cell by vesicle formation.

    • Phagocytosis: "Cell eating"; engulfment of large particles.

    • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

  • Exocytosis: Release of substances from the cell via vesicle fusion with the plasma membrane.

Predicting Molecular Movement: Molecules move from areas of high to low concentration (down the gradient) in passive transport; in active transport, movement is from low to high concentration (against the gradient).

Key Equation: Fick's Law of Diffusion

The rate of diffusion is described by Fick's Law:

Where: J = flux (rate of movement) D = diffusion coefficient dC/dx = concentration gradient

Protein Synthesis: Transcription and Translation

Protein synthesis is a two-step process involving transcription (in the nucleus) and translation (in the cytoplasm).

• Transcription: The process by which a gene's DNA sequence is copied into messenger RNA (mRNA).

  • Key Players: DNA template, RNA polymerase, transcription factors.

  • Triplet: A sequence of three DNA bases coding for one amino acid.

  • mRNA: Carries the genetic code from DNA to ribosomes.

• Translation: The process by which ribosomes synthesize proteins using the mRNA sequence.

  • rRNA: Ribosomal RNA, a structural component of ribosomes.

  • tRNA: Transfer RNA, brings amino acids to the ribosome; contains an anticodon complementary to mRNA codons.

  • Codon: Three-base sequence on mRNA specifying an amino acid.

  • Anticodon: Three-base sequence on tRNA complementary to the mRNA codon.

  • Ribosome Sites: A (aminoacyl), P (peptidyl), and E (exit) sites coordinate translation.

Example: The DNA triplet "TAC" is transcribed to mRNA codon "AUG," which codes for the amino acid methionine.

Key Equation: Central Dogma of Molecular Biology

The Cell Life Cycle and Its Phases

The cell life cycle includes periods of growth, DNA replication, and division, ensuring continuity of life.

• Interphase: The longest phase, consisting of G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Mitosis: Division of the nucleus into two genetically identical daughter nuclei.

  • Prophase: Chromatin condenses into chromosomes; nuclear envelope dissolves; spindle forms.

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

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

  • Telophase: Chromosomes decondense; nuclear envelopes reform.

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

• Semiconservative Replication: Each new DNA molecule consists of one old and one new strand.

  • Helicase: Unwinds the DNA double helix.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase: Adds nucleotides to form new DNA strands.

Summary Table: Cell Cycle Phases

Example: During the S phase, DNA is replicated so that each daughter cell receives an identical set of chromosomes.

Additional info: The above notes expand on the outline by providing definitions, examples, and context for each process, ensuring a comprehensive review for Anatomy & Physiology students.