Cells: Structure, Function, and Diversity

Comparing Bacteria, Plant, and Animal Cells

Cells are the fundamental units of life, and their structure reflects their function. Bacteria (prokaryotes), plant cells, and animal cells (both eukaryotes) have distinct and shared features.

• Bacteria (Prokaryotic Cells): Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region.

• Plant Cells: Have a nucleus, cell wall, chloroplasts, and a large central vacuole.

• Animal Cells: Have a nucleus, lack a cell wall and chloroplasts, and contain lysosomes and centrioles.

Example: Only plant cells have chloroplasts for photosynthesis, while only animal cells have lysosomes for intracellular digestion.

Cellular Structures and Their Functions

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

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Ribosomes: Synthesize proteins; found free in cytoplasm or on rough ER.

• Rough Endoplasmic Reticulum (Rough ER): Studded with ribosomes; synthesizes and processes proteins.

• Smooth Endoplasmic Reticulum (Smooth ER): Synthesizes lipids, detoxifies chemicals, stores calcium ions.

• Golgi Body (Apparatus): Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Central Vacuole (plants): Stores water, nutrients, and waste; maintains turgor pressure.

• Cell Wall (plants, bacteria): Provides structural support and protection; made of cellulose in plants, peptidoglycan in bacteria.

• Cell Membrane: Phospholipid bilayer that controls movement of substances in and out of the cell.

• Lysosomes (mainly animals): Contain digestive enzymes to break down waste.

• Secretory Vesicles: Transport materials from Golgi to cell membrane for exocytosis.

• Vesicles: Small membrane-bound sacs for transport and storage.

Mitochondria and Chloroplasts: Structure, Function, and Evolution

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP (energy).

• Chloroplasts (plants): Site of photosynthesis; converts light energy into chemical energy (glucose).

• Endosymbiotic Theory: Suggests mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes their own DNA, double membranes, and similarity to bacteria.

The Cytoskeleton: Structure and Function

The cytoskeleton provides structural support, enables cell movement, and facilitates intracellular transport.

• Microtubules: Hollow tubes made of tubulin; maintain cell shape, form spindle fibers, and serve as tracks for motor proteins.

• Microfilaments (Actin Filaments): Thin fibers made of actin; support cell shape, enable muscle contraction, and drive cell movement.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength and anchor organelles.

Motor Proteins: Such as kinesin and dynein, move along microtubules to transport vesicles and organelles within the cell.

Cell Membranes and Cell-to-Cell Communication

The Fluid Mosaic Model of Membranes

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

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluidity: Maintained by unsaturated fatty acids (prevent tight packing) and cholesterol (stabilizes membrane).

• Movement: Phospholipids and proteins can move laterally within the layer.

Membrane Components and Permeability

• Integral Proteins: Span the membrane; involved in transport and cell signaling.

• Peripheral Proteins: Attached to membrane surface; involved in signaling and structure.

• Selective Permeability: Small nonpolar molecules (e.g., O2, CO2) pass easily; ions and large polar molecules require transport proteins.

Comparison: Integral proteins, like phospholipids, have hydrophobic and hydrophilic regions, allowing them to embed in the membrane.

Transport Across Membranes

• Passive Transport: Movement down a concentration gradient; no energy required.

  • Simple Diffusion: Direct movement of small, nonpolar molecules.

  • Facilitated Diffusion: Movement via channel or carrier proteins.

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

• Active Transport: Movement against a concentration gradient; requires energy (ATP).

  • Examples: Sodium-potassium pump, proton pump.

Tonicity and Its Effects on Cells

• Hypotonic Solution: Lower solute concentration outside; water enters cell; animal cells may burst, plant cells become turgid.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves cell; cells shrink.

Bulk Transport Mechanisms

• Endocytosis: Uptake of materials via vesicle formation.

• Phagocytosis: "Cell eating"; engulfing large particles.

• Exocytosis: Secretion of materials via vesicle fusion with membrane.

Cell Communication: Ligands, Receptors, and Signal Transduction

• Ligands: Signaling molecules (e.g., hormones) that bind to receptors.

• Receptors: Proteins that receive signals; can be intracellular or membrane-bound.

• Specificity: Only cells with the correct receptor respond to a ligand.

• Signal Transduction: Ligand binding changes receptor shape, triggering a cellular response.

Example: Insulin (a hormone) binds to its receptor on muscle cells, triggering glucose uptake.

The Cell Cycle and Cell Division

Asexual Reproduction and Cell Division

Asexual reproduction produces genetically identical offspring from a single parent. In eukaryotes, this occurs via mitosis.

Phases of the Cell Cycle

• Interphase: Cell grows, performs normal functions, and duplicates DNA.

• Mitosis (M phase): Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

DNA Replication: Occurs during the S phase of interphase.

Key Terms and Structures

• Chromatin: DNA-protein complex; condenses to form chromosomes.

• Chromosome: Condensed DNA structure visible during cell division.

• Chromatids: Identical halves of a duplicated chromosome.

• Centromere: Region where sister chromatids are joined.

• Centrosome: Microtubule-organizing center; forms spindle fibers.

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

Phases of Mitosis

• Prophase: Chromosomes condense; spindle forms.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis: Cleavage furrow (animals) or cell plate (plants) divides cytoplasm.

Diploid vs. Haploid Cells

• Diploid (2n): Two sets of chromosomes; typical of somatic cells.

• Haploid (n): One set of chromosomes; typical of gametes (sperm, egg).

Cell Cycle Regulation and Cancer

• Proteins: Cyclins and cyclin-dependent kinases (Cdks) regulate cell cycle progression.

• Benign vs. Malignant Masses: Benign tumors do not invade other tissues; malignant tumors (cancers) can invade and metastasize.

• Metastasis: Spread of cancer cells to distant body sites.

Viruses: Structure, Classification, and Life Cycles

Characteristics of Viruses

• Viruses: Non-cellular infectious agents; consist of genetic material (DNA or RNA) and a protein coat (capsid).

• Enveloped Viruses: Surrounded by a lipid membrane derived from host cell.

• Naked (Non-enveloped) Viruses: Lack a lipid envelope; only protein capsid surrounds genetic material.

Animal Virus Infection and Exit

• Attachment/Adsorption: Virus binds to host cell surface.

• Entry/Penetration: Virus or genetic material enters host cell.

• Synthesis: Viral genome replicated; viral proteins synthesized.

• Assembly: New viral particles assembled.

• Release: Viruses exit cell by lysis (breaking cell open) or budding (enveloped viruses).

Bacteriophage Life Cycles: Lytic vs. Lysogenic

• Virulent Phages: Only use the lytic cycle.

• Temperate Phages: Can use both cycles.

Key Vocabulary

• Virulent: Causes disease via lytic cycle.

• Lytic: Viral replication resulting in host cell lysis.

• Lysogenic: Viral DNA integrates into host genome; can become lytic.

• Bacteriophage (Phage): Virus that infects bacteria.

Additional info: The above notes expand on the provided outline with definitions, examples, and comparisons for clarity and completeness.