The Cell: Structure, Membrane, and Division

Concept Areas

• Animal Cell Structure

• Plasma Membrane and Movement of Materials

• Cytoplasm, Organelles, and Inclusions

• Nuclear Activities: Protein Synthesis and Cell Division

Cell Structure and Basic Parts

A1: Define Cell and Identify Its Three Basic Parts

A cell is the basic structural and functional unit of living organisms. The three basic parts of a cell 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: A genetic disorder affecting the plasma membrane's transport proteins.

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

B1: Structure of a 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.

B2: Functions of the Membrane and Structural Components

The membrane controls movement of substances, maintains homeostasis, and facilitates communication.

• 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.

B3: Cell Junctions and Their Importance

• 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).

B4: 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.

B5: Active vs. Passive 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.

B6: 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.

B7: 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).

B8: Active Processes: Active Transport, Endocytosis, 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).

  • Caveolae: Small invaginations in the cell membrane involved in endocytosis and signal transduction.

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

Cytoplasm, Organelles, and Inclusions

C1: Cytoplasm, Cytosol, Organelle, Inclusion

• Cytoplasm: The material within a cell, excluding the nucleus, containing cytosol and organelles.

• Cytosol: The fluid component of cytoplasm where metabolic reactions occur.

• Organelle: Specialized structures within a cell performing distinct functions (e.g., mitochondria).

• Inclusion: Non-living substances within the cytoplasm (e.g., lipid droplets).

C2: Components of the Cytosol

• Water

• Enzymes

• Wastes

• Proteins

C3: 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; digests 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.

C4: Chromatin vs. Chromosome, Nucleolus, Histone

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

• Chromosome: Condensed DNA structure 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.

C5: 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.

C6: 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.

Genetic Material and Protein Synthesis

D1: Define Gene

A gene is a segment of DNA that contains the instructions for synthesizing a specific protein.

D2: DNA Structure and 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.

D3: Transcription vs. 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.

D4: 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.

D5: Types of RNA in Protein Synthesis

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

D6: 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

D7: Cell Cycle and Its Phases

• 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.

D8: Cell Division, Mitosis, Cytokinesis

• Cell Division: The process by which a parent cell divides into two or more daughter cells.

C9: Phases of Cell Division

• 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 re-form around the separated chromosomes.

C10: 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.

C11: 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.

Key Table: Comparison of Passive and Active Transport

Key Equations

• Osmosis: (where J is flux, D is diffusion coefficient, C is concentration, x is position)

• ATP Hydrolysis: