Cellular Transport, Cell Cycle, Mitosis, and Cytokinesis

Movements Into and Out of the Cell

• Physical (Passive) Mechanisms:
  • Do not require cellular energy (ATP).
  • Include diffusion, osmosis, facilitated diffusion, and filtration.

Diffusion

• Movement of atoms, molecules, or ions from a region of higher concentration to a region of lower concentration.
• Occurs due to the constant motion of atoms, molecules, and ions.
• Occurs only with substances that the cell membrane is permeable to such as oxygen, carbon dioxide, and other lipid-soluble substances.
• Example: A sugar cube dissolving in water.
• Solutes and water can diffuse across a membrane permeable to both until equilibrium is reached, at which point the concentrations of water and solute are equal in both compartments.

Osmosis

• Movement of water across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
• Often called “diffusion of water”.
• Water moves into a region containing a higher impermeant solute concentration.
• This is a passive process; no ATP is required.

Osmotic Pressure and Tonicity

• Osmotic Pressure:
  • The ability of osmosis to generate enough pressure to lift a volume of water.
  • Osmotic pressure increases as the concentration of impermeant solutes increases in a solution.
  • Water moves toward solutions with higher osmotic pressure.
• Tonicity:
  • The ability of a solution outside a cell to alter water volume inside the cell.
  • Isotonic Solution: Same osmotic pressure; cells in an isotonic solution have no net gain or loss of water.
  • Hypertonic Solution: Higher osmotic pressure; cells in a hypertonic solution lose water.
  • Hypotonic Solution: Lower osmotic pressure; cells in a hypotonic solution gain water.

Active Mechanisms for Cellular Transport

• Require ATP to move substances across the cell membrane.
  • Active transport.
  • Endocytosis.
  • Exocytosis.
  • Transcytosis.

Active Transport

• Movement of substances across a membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient).
• Uses carrier molecules in the cell membrane, often called “pumps”.
• Active process; requires ATP energy.
• Examples: sugars, amino acids.
• In secondary active transport, a carrier protein uses a Na^+ gradient to transport another substance across a cell membrane; this process does not require ATP energy.

Cell Cycle

• Cell Cycle: Series of changes a cell undergoes from the time it forms until the time it divides.
• Stages of the Cell Cycle:
  • Interphase: Growth of cell, maintenance of normal functions.
  • Mitosis: Division of the nucleus.
  • Cytokinesis: Division of cytoplasm.

Interphase

• A very active period in the cell cycle.
• Cell grows and maintains normal functions.
• Cell replicates genetic material (DNA) to prepare for mitosis (nuclear division).
• Cell synthesizes organelles, membranes, and biochemicals to prepare for cytokinesis (division of cytoplasm).
• Phases of Interphase:
  • S (synthesis) phase: DNA is replicated during this phase.
  • G1 and G2 (growth or gap) phases: Structures and other molecules are duplicated.

Mitosis

• Somatic cell division produces two daughter cells from an original cell.
  • Mitosis: Division of the nucleus via karyokinesis.
  • Cytokinesis: Division of the cytoplasm.
• Phases of Mitosis:
  • Prophase: Chromatin condenses to form chromosomes, centrioles move to opposite sides of the cytoplasm, nuclear envelope and nucleolus disperse.
  • Metaphase: Spindle fibers from centrioles attach to chromosomes and align them midway between centrioles.
  • Anaphase: Chromosomes separate and move in opposite directions toward centrioles as the spindle fibers shorten.
  • Telophase: Chromosomes return to chromatin structure, the nuclear envelope forms around each chromosome set, and nucleoli become visible.

Major Events in Mitosis

Cytoplasmic Division

• Cytokinesis = cytoplasmic division.
• Begins during anaphase.
• Continues through telophase.
• A contractile ring of actin filaments pinches the cytoplasm in half.
• The constriction is called a cleavage furrow.
• Newly formed cells will have identical DNA and may have slightly different sizes and numbers of organelles.