Cell Structure and Function

Organelles and Cell Components

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding the structure and function of these organelles is fundamental in biology.

• Plasma (cellular) membne: The boundary that separates the cell from its environment; composed of a phospholipid bilayer with polar hydrophilic heads and non-polar hydrophobic tails.

• Cytoplasm: The jelly-like substance within the cell, containing organelles and cytosol.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; involved in protein synthesis.

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

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

• Mitochondria: The "powerhouse" of the cell; site of cellular respiration and ATP production.

• Large central vacuole (in plant cells): Stores water, nutrients, and waste products; maintains turgor pressure.

• Semipermeable membrane: Allows selective passage of substances in and out of the cell.

Additional info: Ribosomes (not listed) are also essential organelles for protein synthesis.

Membrane Transport

Diffusion and Osmosis

Cells regulate the movement of substances across their membranes through various transport mechanisms.

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Equilibrium: State where the concentration of molecules is equal throughout a space.

• Concentration gradient: Difference in concentration of a substance across a space.

• Simple diffusion: Direct movement of small, nonpolar molecules across the membrane.

• Facilitated diffusion: Movement of molecules via transport proteins; does not require energy.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Active transport: Movement of substances against their concentration gradient using energy (ATP); e.g., sodium-potassium pump.

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic solution: Lower solute concentration outside the cell; water moves in, cell swells.

• Endocytosis: Process by which cells engulf substances into a pouch which becomes a vesicle.

• Exocytosis: Process by which cells expel materials in vesicles to the outside.

Example: The sodium-potassium pump moves Na+ out and K+ into the cell, maintaining electrochemical gradients.

Additional info: Facilitated diffusion is essential for glucose transport into cells.

Cellular Respiration, Energy & Metabolism

Overview of Cellular Respiration

Cellular respiration is the process by which cells convert biochemical energy from nutrients into ATP, the energy currency of the cell.

• Terminology:

  • Catalyst: Substance that speeds up chemical reactions without being consumed.

  • Enzyme: Biological catalyst, usually a protein, that increases reaction rates.

  • Activation energy: Minimum energy required to start a chemical reaction.

  • Active site: Region on an enzyme where substrate binds and reaction occurs.

• Cellular respiration stages:

  1. Glycolysis

  2. Pyruvate oxidation

  3. Citric acid cycle (Krebs cycle)

  4. Electron transport chain

• ATP (Adenosine Triphosphate): High-energy molecule used to power cellular processes.

Equation for cellular respiration:

• ATP Cycle: ATP is hydrolyzed to ADP and inorganic phosphate, releasing energy; ADP is recycled back to ATP.

• Substrates and products: Glucose and oxygen are substrates; carbon dioxide, water, and ATP are products.

• Cellular location: Glycolysis occurs in the cytoplasm; pyruvate oxidation, citric acid cycle, and electron transport chain occur in mitochondria.

• Aerobic vs. Anaerobic: Aerobic respiration requires oxygen; anaerobic does not and produces less ATP.

Example: Muscle cells use anaerobic respiration during intense exercise, producing lactic acid.

Additional info: The structure of ATP, with three phosphate groups, makes it a high-energy compound due to the repulsion between negatively charged phosphates.

Cell Reproduction: Mitosis & Meiosis

Terminology and Concepts

Cell reproduction ensures the continuity of life and genetic inheritance. Two main types are mitosis and meiosis.

• Gene: Segment of DNA that codes for a protein.

• Mitosis: Division of a somatic cell into two genetically identical daughter cells.

• Meiosis: Division of a germ cell into four genetically unique gametes.

• Diploid (2N): Cell with two sets of chromosomes.

• Haploid (N): Cell with one set of chromosomes.

• Human diploid number: 46 chromosomes.

• Human haploid number: 23 chromosomes.

• Karyotype: The number and appearance of chromosomes in the nucleus.

• Somatic (body) cells: All cells except gametes.

• Gametes: Sperm and egg cells.

• Fertilization: Fusion of gametes to form a zygote.

• Chromatin: DNA and protein complex in the nucleus.

• Chromosomes: Condensed chromatin visible during cell division.

• Sister chromatids: Identical copies of a chromosome connected by a centromere.

• Centromere: Region where sister chromatids are joined.

• Spindle fibers: Microtubules that separate chromosomes during cell division.

• Cytokinesis: Division of the cytoplasm after mitosis or meiosis.

• Cleavage furrow: Indentation that begins the process of cytokinesis in animal cells.

Additional info: Plant cells form a cell plate during cytokinesis instead of a cleavage furrow.

Eukaryotic Cell Cycle

The eukaryotic cell cycle consists of interphase (G1, S, G2) and mitotic phase (mitosis and cytokinesis). Each phase has distinct events and checkpoints.

• Interphase: Cell grows, replicates DNA, and prepares for division.

• Mitosis: Prophase, metaphase, anaphase, telophase.

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

Example: Skin cells undergo mitosis for tissue repair.

Mitosis vs. Meiosis

Mitosis and meiosis are two types of cell division with distinct purposes and outcomes.

Additional info: Crossing over during meiosis increases genetic diversity.