SA

Membrane Transport and Cell Cycle Vocabulary

Membrane Transport

Passive Transport

  • Definition: Movement of substances that requires no energy input from the cell. Processes occur naturally.
  • Direction: Substances move "down their concentration gradient," meaning from an area of high concentration to an area of low concentration until equilibrium is reached.
  • Examples: Applies to concentration, pressure, and electrical charge.
    • Analogy: A drop of food coloring in a glass of water will naturally spread out and eventually turn the entire glass a light blue without any intervention.

Types of Passive Transport:

  1. Diffusion

    • Simple Diffusion:
      • Substance passes directly across the cell membrane on its own.
      • Examples: Gases (e.g., oxygen (O2), carbon dioxide (CO2)) and smaller lipids (e.g., fatty acids, cholesterol, steroid hormones, carotenoids). These substances are lipid-soluble.
    • Factors Influencing Diffusion Speed (Applies to simple, facilitated, and osmosis):
      • Distance: Shorter distance of travel leads to faster diffusion.
      • Particle Size: Smaller and lighter particles diffuse faster (e.g., oxygen gas diffuses faster than glucose in water).
      • Temperature: Higher temperature leads to faster diffusion because molecules move more quickly.
      • Concentration Gradient: A larger difference in concentration between two areas results in faster diffusion (e.g., a gradient of 95 (from 100 to 5) is faster than a gradient of 5 (from 10 to 5)).
      • Electrical Forces:
        • Repulsion between like charges will slow down diffusion.
        • Attraction between opposite charges will speed up diffusion.

  2. Osmosis

    • Definition: A special case of diffusion specifically referring to the diffusion of water down its own concentration gradient.

    • Fundamentals: Water is the solvent in which other substances are dissolved, and its movement follows the general rules of diffusion.

    • Pressures in Liquids (relevant to osmosis):

      • Hydrostatic Pressure:
        • Definition: Pressure based on the amount (volume) of fluid. More fluid in a given space means more hydrostatic pressure.
        • Nature: A "pushing pressure" – the liquid attempts to push out of its container.
        • Analogy: A water balloon stretches more with more water, eventually bursting due to the increasing pressure on its walls.
        • Real-world connection: Blood pressure is a measure of the average hydrostatic pressure in the arterial side of the cardiovascular system.
      • Osmotic Pressure:
        • Definition: A "pulling force" exerted by solutes in a solution. The higher the concentration of particulate matter (solute) in a solution, the more it tends to pull water towards it.
        • Direction: Water moves towards regions of higher solute concentration (higher osmotic pressure).
        • Example: If a membrane is permeable to water but impermeable to glucose, and Side A has 10 \% glucose (90 \% water) while Side B has 50 \% glucose (50 \% water), water will move from Side A to Side B.

    • Tonicity (Relative Term for Osmotic Pressure):

      • Definition: A comparative term used to describe the osmotic pressure of one solution relative to another. You cannot describe a single solution as simply "hypertonic" or "hypotonic" without a reference.
      • Isotonic (Iso = Same):
        • Definition: Solutions with the same solute concentration (and thus, same osmotic pressure) relative to each other.
        • Effect on Cells: If a red blood cell is placed in an isotonic solution (e.g., \$0.9 \% sodium chloride solution, known as "normal saline"), water moves in and out of the cell at equal rates, and the cell maintains its normal shape. This is why normal saline is used for intravenous (IV) fluid replacement.
      • Hypertonic (Hyper = More):
        • Definition: A solution with a higher solute concentration (higher osmotic pressure) compared to another solution.
        • Effect on Cells: If a red blood cell is placed in a hypertonic solution (e.g., a \$9 \% salt solution IV), there is less water outside the cell and more water inside. Water will rush out of the cell to try and balance the tonicity.
        • Result: The cell will shrivel, a process called "crenation." Severe Crenation can damage or kill the cell.
      • Hypotonic (Hypo = Less):
        • Definition: A solution with a lower solute concentration (lower osmotic pressure) compared to another solution.
        • Effect on Cells: If a red blood cell is placed in a hypotonic solution (e.g., pure water IV), there is more water outside the cell and less water inside. Water will rush into the cell to balance the tonicity.
        • Result: The cell will swell and can eventually burst, a process called "hemolysis" (for red blood cells). This can be fatal if too many cells burst.
      • General Rule: Water always moves from a hypotonic solution to a hypertonic solution.

  3. Facilitated Diffusion

    • Definition: Passive transport that requires assistance from specific transport proteins in the cell membrane.
    • Energy Requirement: No ATP is needed.
    • Direction: Substances still move down their concentration gradient (from high to low concentration).
    • Mechanism: Substances that cannot cross the lipid bilayer (e.g., glucose) bind to specialized "carrier proteins" or "transport proteins." These proteins undergo a conformational change to allow the substance to pass through the membrane.
    • Specificity: Transport proteins are specific to the type of molecule they transport (e.g., a glucose carrier protein will not transport sodium ions).
    • Types of Proteins: Can be called "carrier proteins," or "channels" (e.g., aquaporins for water, leak channels for ions).
    • Bidirectional: These proteins can facilitate movement in either direction, always following the concentration gradient.

Active Transport

  • Definition: Movement of substances that requires the cell to expend energy, typically in the form of ATP.
  • Direction: Substances are moved "against their concentration gradient," meaning from an area of low concentration to an area of high concentration.
  • Analogy: Pushing a boulder up a hill requires energy, just as active transport requires ATP to move substances against their natural flow.
  • Requirement: Requires specialized membrane proteins, often referred to as "pumps."

Types of Active Transport:

  1. Primary Active Transport

    • Energy Source: Directly uses ATP. The transport protein itself hydrolyzes ATP to obtain the energy for transport.
    • Key Example: Sodium-Potassium Pump (Na+/K+ ATPase or Sodium-Potassium Exchange Pump).
      • Mechanism: This protein simultaneously pumps \$3 sodium ions (Na+) out of the cell and \$2 potassium ions (K+) into the cell for each ATP molecule consumed.
      • Function: Crucial for maintaining specific ion concentration gradients: high Na+ outside, low Na+ inside; high K+ inside, low K+ outside.
      • Energy Cost: Approximately \$15 \% of all ATP in the body is used to power Na+/K+ pumps.

  2. Secondary Active Transport

    • Energy Source: Indirectly uses ATP. It does not directly consume ATP but relies on the concentration gradient established by a primary active transporter (e.g., the Na+ gradient created by the Na+/K+ pump).
    • Mechanism: It harnesses the potential energy of one substance moving down its concentration gradient (e.g., Na+ rushing into the cell) to power the movement of a second substance against its own concentration gradient.
    • Dependence: If primary active transport (e.g., Na+/K+ pump) stops due to lack of ATP, secondary active transport will also cease.
    • Directional Types:
      • Cotransporter (Symporter): Moves both substances in the same direction across the membrane (e.g., Na+ and glucose both moving into the cell).
      • Counter-transporter (Antiporter): Moves the two substances in opposite directions across the membrane.

Vesicular Transport (Bulk Transport)

  • Definition: A form of active transport used for substances that are too large to pass through the membrane or transport proteins, or for moving large quantities of substances at once.
  • Energy Requirement: Always uses ATP, regardless of concentration gradients, as it requires energy to build, move, and fuse vesicles.
  • Mechanism: Involves the formation and movement of membrane-bound sacs called "vesicles."

Types of Vesicular Transport:

  1. Exocytosis (Exo = Outside)

    • Mechanism: Vesicles containing intracellular material fuse with the plasma membrane, releasing their contents to the extracellular fluid.
    • Purpose: Used for secreting substances like hormones, neurotransmitters, or waste products.

  2. Endocytosis (Endo = Inside)

    • Mechanism: The plasma membrane invaginates (folds inward), forming a vesicle that encloses extracellular material and brings it into the cell.
    • Types of Endocytosis:
      • Pinocytosis (Cell Drinking):
        • Mechanism: The cell extends projections of its membrane to engulf a large volume of extracellular fluid, along with any solutes dissolved within it.
        • Analogy: "Pinot" (as in wine) sounds like "drink," helping to remember cell drinking.
      • Phagocytosis (Cell Eating):
        • Mechanism: The cell extends pseudopods (false feet) to engulf large solid particles, such as bacteria, cellular debris, or large proteins.
        • Distinction from Pinocytosis: The action is similar, but phagocytosis involves ingesting solids, whereas pinocytosis involves ingesting liquids.
      • Receptor-Mediated Endocytosis:
        • Mechanism: Specific external substances (ligands) bind to specific receptors on the cell surface. When enough receptors are bound in an area, a protein called "clathrin" (located beneath the membrane) becomes activated. Clathrin then causes the membrane to invaginate, forming a coated pit that pinches off to create a clathrin-coated vesicle containing the specific ligands.
        • Specificity: Allows the cell to selectively take up specific molecules in bulk quantities.
        • Recycling: The clathrin proteins are recycled after vesicle formation.

Cell Cycle and Cell Division

The Cell Cycle

  • Definition: The entire life cycle of a cell.
  • Main Phases:
    1. Interphase: The period when the cell is not actively dividing.
    2. M Phase (Mitotic Phase): The period of cell division.

Interphase Subdivisions:

  1. G1 Phase (Gap 1):

    • Activities: The cell performs its normal metabolic functions, grows, and synthesizes proteins and organelles needed for eventual division.
    • Duration: Highly variable, from hours to days or even weeks.
    • G0 Phase (Gap 0):
      • A quiescent state where cells exit the cell cycle and cease to divide. This can be temporary or permanent.
      • Permanent G0: Mature neurons, skeletal muscle cells, and cardiac muscle cells typically enter G0 permanently and do not divide again.
      • Temporary G0: Some cells (e.g., certain bone cells) can enter G0 but may re-enter the cell cycle to divide if stimulated (e.g., during tissue repair).

  2. S Phase (Synthesis):

    • Activity: DNA replication occurs during this phase. Each chromosome is duplicated to ensure that each daughter cell receives a complete set of genetic material.

  3. G2 Phase (Gap 2):

    • Activity: Final preparations for cell division are made. The cell synthesizes proteins required for mitosis and cytokinesis, and checks for any DNA damage from S phase.
    • Duration: Typically a relatively short phase.

M Phase Subdivisions:

  • Definition: The actual process of cell division, comprising Mitosis and Cytokinesis.
  • Mitosis: The process of nuclear division, ensuring that DNA (in the form of chromosomes) is accurately separated into two new nuclei.
  • Cytokinesis: The process of cytoplasmic division, where the entire cell divides into two distinct daughter cells.

DNA Replication (Occurs during S Phase of Interphase)

  • Purpose: To produce two identical copies of the cell's DNA before cell division, ensuring each daughter cell receives a full and functional genome.
  • Key Enzymes/Proteins:
    • DNA Helicase: Unwinds the DNA double helix, separating the two strands at specific