GAP Ch3: Cells

Overview of Cells

• Definition: The smallest structural and functional unit of an organism, typically microscopic.

• Composition: Consists of cytoplasm and a nucleus enclosed in a membrane.

• Focus: Importance of the cell membrane, organelles, cytoplasm, and processes such as DNA replication and protein synthesis.

• Context: Discussing somatic cells (diploid cells with 46 chromosomes).

Somatic Cells

• Definition: All body cells except for sperm and egg cells (germ cells).

• Chromosome Count: Humans have 46 chromosomes (23 pairs), one set from each parent.

Cell Membrane

• Function: Separates intracellular contents from the extracellular environment and regulates what enters or exits the cell.

• Structure: Composed primarily of a phospholipid bilayer.

  • Phospholipid: Contains a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails.

  • Polarity: Heads are polar; tails are nonpolar.

• Fluidity: The membrane remains fluid, with lipids and proteins able to move laterally.

• Types of Membrane Proteins:

  • Integral Proteins: Span the bilayer, e.g., channel proteins allow selective passage of ions.

  • Peripheral Proteins: Located on inner or outer surfaces; may serve as receptors or enzymes.

• Glycoproteins: Proteins with carbohydrate chains, important for cell recognition and immune response.

• Selective Permeability: Regulates movement of ions, nutrients, and wastes based on size and polarity.

Transport Mechanisms

• Passive Transport: Movement of substances without energy. Types include:

  • Diffusion: Movement from high to low concentration (e.g., oxygen entering cells).

  • Facilitated Diffusion: Uses channel or carrier proteins; facilitates movement of larger or charged molecules.

  • Osmosis: Diffusion of water across a semi-permeable membrane.

  • Filtration: Movement caused by hydrostatic pressure.

• Active Transport: Movement using energy (ATP) against concentration gradients.

  • Examples: Sodium-potassium pump.

  • Endocytosis: Process of bringing substances into the cell (phagocytosis - cell eating, pinocytosis - cell drinking, receptor-mediated endocytosis).

  • Exocytosis: Secretion of materials out of the cell using vesicles.

Internal Cell Components

• Cytosol: Jelly-like substance where biochemical reactions occur.

• Organelles: Specialized, membrane-bound structures within the cell, including:

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

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

  • Lysosomes: Digestive enzymes for breaking down waste.

  • Mitochondria: Powerhouse of the cell - site of cellular respiration.

  • Peroxisomes: Contain enzymes for lipid metabolism and detoxification.

  • Cytoskeleton: Structural support composed of microtubules, microfilaments, and intermediate filaments.

Nucleus and Genetic Material

• Function: Control center of the cell housing DNA.

• Components:

  • Nuclear Envelope: Double membrane with nuclear pores.

  • Nucleolus: Produces ribosomal RNA (rRNA).

  • Chromatin: DNA wrapped around histone proteins, condensing to chromosomes during cell division.

• DNA Structure: Double helix made of nucleotides (sugar, phosphate, nitrogenous bases).

  • Base Pairing: Adenine (A) with Thymine (T) and Guanine (G) with Cytosine (C).

DNA Replication and Protein Synthesis

• DNA Replication: Copying of DNA before cell division, involving:

  • Stages: Initiation, elongation, and termination.

  • Enzyme Roles: Helicase unzips DNA; DNA polymerase synthesizes new strands.

• Protein Synthesis: Process of producing proteins from DNA:

  • Transcription: Synthesis of mRNA from DNA.

  • Translation: Formation of proteins based on mRNA codons at ribosomes. Involves transfer RNA (tRNA).

Cell Growth and Division

• Cell Cycle: Sequence from cell creation to division, consisting of interphase and mitosis.

  • Interphase: Majority of cell life cycle; includes G1 (growth), S (synthesis/DNA replication), and G2 (preparation for mitosis).

  • Mitosis: Involves phases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis (division of cytoplasm).

• Chromosome Formation: After S phase, chromosomes consist of sister chromatids connected at the centromere.

• Homologous chromosomes: Pair encoding the same trait but may carry different alleles.