Chapter 3: Cells – The Living Units (Marieb HAP 12e) Study Notes

Cells: The Living Units

Overview of Cells (The Cell Theory)

The cell is the fundamental unit of life, responsible for all basic biological functions. The cell theory provides the foundational principles for understanding cellular structure and function.

• Definition: The cell is the smallest unit of life, capable of performing all vital physiological processes.

• Cell Theory:

  1. The cell is the basic structural and functional unit of living organisms.

  2. The activity of an organism depends on the collective activities of its cells.

  3. Biochemical activities of cells are dictated by their shapes and subcellular structures.

  4. Continuity of life has a cellular basis; all cells arise from pre-existing cells.

• Three Main Parts of a Cell:

  • Plasma Membrane

  • Cytoplasm

  • Nucleus

The Plasma Membrane (Fluid Mosaic Model)

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates the movement of substances in and out of the cell.

• Structure: Composed of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails inward, interspersed with proteins and cholesterol.

• Functions:

  • Acts as a selective barrier (selective permeability)

  • Facilitates communication with other cells

  • Enables cell recognition

• Membrane Proteins:

  • Integral Proteins: Span the membrane; function as transport channels/carriers, receptors, or enzymes.

  • Peripheral Proteins: Loosely attached to the membrane; provide support and anchor the cytoskeleton.

• Glycocalyx: A carbohydrate-rich "sugar coating" on the cell surface that acts as a molecular name tag for cell recognition.

• Cell Junctions:

  • Tight Junctions: Form impermeable seals between cells, preventing passage of molecules.

  • Desmosomes: Anchoring junctions that reduce tearing under stress; function like rivets or zippers.

  • Gap Junctions: Communicating junctions that allow ions and small molecules to pass between cells.

Membrane Transport (Moving In and Out of Cells)

Cells regulate their internal environment by controlling the movement of substances across the plasma membrane, using both passive and active mechanisms.

• Passive Transport (No ATP, Down Gradient):

  • Simple Diffusion: Movement of fat-soluble or small molecules directly through the membrane.

  • Facilitated Diffusion: Movement of larger or charged molecules via helper proteins (channels or carriers).

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

  • Tonicity: The ability of a solution to change cell volume.

    • Hypertonic: Solution causes cell to shrink (crenation).

    • Hypotonic: Solution causes cell to swell or burst (lysis).

• Active Transport (Requires ATP, Against Gradient):

  • Primary Active Transport: Direct use of ATP to pump substances (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from primary active transport to move other substances.

• Vesicular (Bulk) Transport:

  • Endocytosis: Cell takes in substances.

    • Phagocytosis: "Cell eating" of large particles.

    • Pinocytosis: "Cell drinking" of fluids.

    • Receptor-mediated Endocytosis: Specific uptake using receptors.

  • Exocytosis: Secretion or release of substances out of the cell.

Example: The sodium-potassium pump ($Na^+$-$K^+$ pump) moves 3 $Na^+$ ions out of the cell and 2 $K^+$ ions into the cell per ATP molecule hydrolyzed.

Cytoplasm and Organelles

The cytoplasm is the cellular material between the plasma membrane and the nucleus, containing the cytosol, organelles, and inclusions.

• Cytosol: The fluid component in which organelles are suspended.

• Organelles (Membrane-bound):

  • Mitochondria: Site of ATP production; known as the "powerhouse" of the cell.

  • Ribosomes: Sites of protein synthesis; can be free in cytosol or attached to rough ER.

  • Rough Endoplasmic Reticulum (Rough ER): Studded with ribosomes; modifies and folds proteins.

  • Smooth Endoplasmic Reticulum (Smooth ER): Involved in lipid and steroid synthesis, and detoxification.

  • Golgi Apparatus: Modifies, packages, and ships proteins; acts as the cell's "traffic director."

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

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support and facilitate movement.

Nucleus (Control Center)

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

• Structure:

  • Nuclear Envelope: Double membrane with nuclear pores for molecular exchange.

  • Nucleoli: Sites of ribosome assembly.

  • Chromatin: DNA complexed with histone proteins; condenses to form chromosomes during cell division.

• Gene: A segment of DNA that contains the instructions for making a specific protein.

Protein Synthesis (Transcription & Translation)

Protein synthesis is a two-step process involving transcription (in the nucleus) and translation (in the cytoplasm).

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA) in the nucleus. The enzyme RNA polymerase facilitates this process.

• Translation: mRNA is decoded by ribosomes in the cytoplasm, and transfer RNA (tRNA) brings the appropriate amino acids to form a polypeptide chain.

• Triplet/Codon: A sequence of three nucleotide bases in mRNA that codes for a specific amino acid.

Example: The DNA triplet ATG is transcribed to the mRNA codon AUG, which codes for the amino acid methionine.

Cell Cycle and Division

The cell cycle describes the sequence of events that a cell undergoes from its formation to its division into two daughter cells.

• Interphase: Period of cell growth and DNA replication; consists of $G_1$, S, and $G_2$ phases.

  • $G_1$ phase: Cell grows and carries out normal functions.

  • S phase: DNA replication occurs.

  • $G_2$ phase: Preparation for mitosis.

• Mitosis (Nuclear Division):

  1. Prophase: Chromatin condenses into chromosomes; spindle apparatus forms.

  2. Metaphase: Chromosomes align at the cell's equator.

  3. Anaphase: Sister chromatids are pulled apart to opposite poles.

  4. Telophase: Chromosomes decondense; nuclear envelopes reform.

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

Instructor Cues and Common Pitfalls

• Hydrophilic heads of phospholipids face the aqueous environments inside and outside the cell; hydrophobic tails face inward.

• Desmosomes function like zippers or rivets; tight junctions are like sealed Ziploc bags.

• Do not confuse transcription (occurs in the nucleus) with translation (occurs in the cytoplasm).

• Remember: The sodium-potassium pump moves 3 $Na^+$ out and 2 $K^+$ in per ATP.