Cell Structure and Function

Prokaryotes vs. Eukaryotes

• Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles. Their DNA is located in a region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotes have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Comparison Table:

Basic Features of Cells

• All cells have a plasma membrane, cytoplasm, DNA, and ribosomes.

• Cells carry out metabolism, respond to stimuli, and reproduce.

Parts of the Cell and Their Functions

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

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digestive organelles; break down macromolecules and waste.

• Vacuoles: Storage organelles; large in plant cells for water and nutrients.

• Plasma Membrane: Selectively permeable barrier; regulates entry/exit of substances.

• Detoxification: Mainly performed by smooth ER and peroxisomes.

The Endomembrane System

Definition and Function

• The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

Components and Functions

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pores for transport.

• Nuclear Pore Complex: Regulates movement of molecules between nucleus and cytoplasm.

• Rough ER (RER): Studded with ribosomes; synthesizes proteins for secretion or membranes.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

• Transitional ER (tER): Region where transport vesicles bud off from ER.

• Golgi Apparatus: Receives, modifies, and ships proteins and lipids.

• Lysosomes: Contain hydrolytic enzymes for digestion.

• Vacuoles: Storage and structural support (especially in plants).

• Plasma Membrane: Final destination for many secreted proteins and lipids.

RER vs. SER

• Rough ER: Protein synthesis (due to ribosomes), especially for secretion or membrane insertion.

• Smooth ER: Lipid synthesis, detoxification, calcium storage.

Protein Synthesis and Secretion Pathway

• Polypeptides destined for secretion are synthesized by ribosomes on the RER.

• Proteins enter the ER lumen, where they may be modified (e.g., glycosylation—addition of carbohydrates).

• Transport vesicles carry proteins to the Golgi apparatus for further modification and sorting.

• "Zip codes" (signal sequences) direct proteins to their correct destinations.

• Secretory vesicles bud from the Golgi and fuse with the plasma membrane, releasing proteins outside the cell (exocytosis).

• Membrane carbohydrates play roles in cell recognition and signaling.

Lysosomes and Vacuoles

• Lysosomes: Digest macromolecules, old organelles, and foreign substances.

• Vacuoles: Store nutrients, waste products, and help maintain turgor pressure in plant cells.

• Phagocytosis: Process by which cells engulf large particles or other cells, forming a phagosome that fuses with a lysosome for digestion.

Endosymbiont Theory

• Explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes similar to bacteria.

Cytoskeleton

Types of Cytoskeletal Fibers

• Microtubules: Hollow tubes made of tubulin; provide structural support, tracks for organelle movement, and form spindle fibers during cell division.

• Microfilaments (Actin Filaments): Thin fibers; involved in cell movement, shape, and muscle contraction.

• Intermediate Filaments: Rope-like; provide mechanical strength and maintain cell shape.

Motor Proteins and Functions

• Motor proteins (e.g., kinesin, dynein, myosin) move along cytoskeletal fibers, transporting vesicles and organelles.

• Polymerization: Assembly of tubulin or actin subunits into filaments.

• Support: Microtubules radiate from the centrosome; actin filaments are concentrated beneath the plasma membrane.

Cell Membrane Structure and Function

Fluid Mosaic Model

• The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer.

• Proteins are interspersed throughout, some spanning the membrane (integral), others attached to the surface (peripheral).

Membrane Proteins

• Two major populations: Integral (transmembrane) and peripheral proteins.

• Six major functions: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM.

Membrane Fluidity

• Fluidity is affected by temperature, fatty acid composition (saturated vs. unsaturated), and cholesterol content.

• Unsaturated fatty acids increase fluidity; cholesterol buffers fluidity against temperature changes.

Other Membrane Components

• Peripheral proteins: Loosely attached to membrane surface.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); important for cell recognition.

• Cytoskeleton: Provides structural support and anchors membrane proteins.

Membrane Transport

Transport Proteins

• Facilitate movement of molecules/ions across the membrane.

• Two types: Channel proteins (form pores) and carrier proteins (undergo shape change).

Passive and Active Transport

• Passive transport: Movement down concentration gradient; no energy required (e.g., diffusion, osmosis, facilitated diffusion).

• Active transport: Movement against gradient; requires energy (ATP). Example: sodium-potassium pump.

• Sodium-potassium pump: Moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Other mechanisms: cotransporters, endocytosis, exocytosis.

Osmosis, Diffusion, Tonicity

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

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

• Tonicity: Relative solute concentration; affects cell volume (isotonic, hypertonic, hypotonic).

Cell Communication and Signaling

Types of Cell Signaling

• Local signaling: Paracrine (nearby cells), synaptic (neurons).

• Long-distance signaling: Endocrine (hormones via bloodstream).

Signals and Receptors

• Cells respond to chemical signals (ligands) such as hormones, neurotransmitters, and growth factors.

• Receptors can be located on the cell surface, within the cytoplasm, or in the nucleus.

• Three major types of membrane receptors: G-protein coupled receptors (GPCRs), tyrosine kinase receptors (TKRs), ion channel receptors.

• GPCRs and TKRs initiate signal transduction pathways; TKRs are enzyme-linked.

• Ion channel receptors alter membrane potential.

Stages of Cell Signaling (Signal Transduction Pathway)

• Reception: Signal molecule binds to receptor.

• Transduction: Signal is relayed and amplified through a cascade of molecular events.

• Response: Specific cellular activity is triggered (e.g., gene expression, enzyme activation).

Ligands

• A ligand is a molecule that binds specifically to a receptor, triggering a response.

Cell Division

Basic Function

• Cell division enables growth, repair, and reproduction.

Binary Fission

• Prokaryotic cell division; DNA replicates, cell elongates, and divides into two identical cells.

Mitosis and Meiosis

• Mitosis: Produces two genetically identical diploid (2n) cells; for growth and repair.

• Meiosis: Produces four genetically unique haploid (n) gametes; for sexual reproduction.

• Mitotic spindle: Structure made of microtubules; separates chromosomes during mitosis/meiosis.

• Spindle fibers: Attach to kinetochores on chromosomes; non-kinetochore fibers help elongate the cell.

Centrosomes and Centrioles

• Centrosome: Microtubule-organizing center; duplicates before cell division.

• Centrioles: Found in animal cells; help organize spindle fibers.

Chromosome Structure and Crossing Over

• Homologous chromosomes: Pair of chromosomes with the same genes but possibly different alleles.

• Sister chromatids: Identical copies of a chromosome, joined at the centromere.

• Locus: Specific location of a gene on a chromosome.

• Centromere: Region where sister chromatids are joined.

• Kinetochore: Protein complex on centromere; attaches to spindle fibers.

• Cohesion: Proteins holding sister chromatids together.

• Non-sister chromatids: Chromatids from homologous chromosomes.

• Chiasmata: Sites where crossing over occurs between non-sister chromatids during meiosis.

Crossing Over

• Exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis; increases genetic diversity.

Genes, Locus, Gametes, Genome

• Gene: Unit of heredity; sequence of DNA encoding a protein or RNA.

• Locus: Physical location of a gene on a chromosome.

• Gametes: Haploid reproductive cells (sperm, egg).

• Genome: Complete set of genetic material in an organism.

Checkpoints of Mitosis

• Control points where the cell cycle is regulated (e.g., G1, G2, M checkpoints).

• Ensure proper division and prevent errors.

Purpose of Mitosis

• Growth, tissue repair, and asexual reproduction in multicellular organisms.

Endocytosis and Exocytosis

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

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

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Exocytosis: Secretion of materials out of the cell via vesicles fusing with the plasma membrane.

Summary Table: Key Cell Structures and Functions

Key Equations

• Osmosis (Water Potential):

Where = water potential, = solute potential, = pressure potential.

• Sodium-Potassium Pump:

• Cell Cycle Phases:

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard General Biology curriculum.