unit 4 test prep bio

1. Endocytosis and Exocytosis

• Endocytosis:

  • Definition: The process by which cells internalize substances from their external environment by engulfing them in a membrane-bound vesicle.

  • Types: Phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

• Exocytosis:

  • Definition: The process by which cells expel materials to the extracellular environment when vesicles fuse with the plasma membrane.

• Key Difference: Endocytosis brings materials into the cell, while exocytosis sends materials out of the cell.

2. Fluid Mosaic Model of the Cell Membrane

• Description:

  • The model portrays the cell membrane as a flexible, dynamic structure composed of a phospholipid bilayer with proteins, cholesterol, and carbohydrate chains interspersed.

• Components & Functions:

  • Phospholipid Bilayer: Provides a semi-permeable barrier.

  • Proteins: Serve as channels, carriers, receptors, or enzymes.

  • Cholesterol: Maintains membrane fluidity and stability.

  • Carbohydrate Chains: Involved in cell-cell recognition and signaling.

• Importance: This model explains how the membrane can be both stable and fluid, allowing lateral movement of proteins and lipids.

3. Role of Cholesterol and Carbohydrate Chains

• Cholesterol:

  • Function: Acts as a fluidity buffer; at high temperatures, it stabilizes the membrane, and at low temperatures, it prevents tight packing of phospholipids.

• Carbohydrate Chains:

  • Function: Attached to proteins (glycoproteins) or lipids (glycolipids), they are crucial for cell recognition, adhesion, and communication.

4. Calvin Cycle (Light-Independent Reactions)

• Location: Occurs in the stroma of chloroplasts.

• Inputs: Carbon dioxide (CO₂), ATP, and NADPH.

• Process:

  • Carbon Fixation: CO₂ is fixed by the enzyme RuBisCO into an organic molecule.

  • Reduction Phase: ATP and NADPH are used to convert the fixed carbon into a carbohydrate (glucose).

• Output: Glucose (and other sugars) which can be used for energy or stored.

5. Glycolysis and the Krebs Cycle

• Glycolysis:

  • Location: Cytoplasm.

  • Process: Breaks down one molecule of glucose into 2 pyruvate molecules.

  • Energy: Uses 2 ATP in the early stages and produces 4 ATP (net gain of 2 ATP) along with 2 NADH.

• Krebs Cycle (Citric Acid Cycle):

  • Location: Mitochondrial matrix.

  • Process: Acetyl CoA (derived from pyruvate) enters the cycle and is oxidized.

  • Outputs: Produces NADH, FADH₂, a small amount of ATP (or GTP), and releases CO₂ as a waste product.

6. Positive Feedback Loops

• Definition: A process that amplifies a change in a system rather than reversing it.

• Characteristics:

  • Once initiated, the response causes further deviation from the starting condition.

• Examples:

  • Childbirth: Contractions cause the release of oxytocin, which intensifies contractions.

  • Blood Clotting: The initial clotting triggers a cascade that rapidly accelerates the process to seal a wound.

7. Cell Theory

• Three Main Tenets:

  1. All living organisms are composed of one or more cells.

  2. The cell is the basic unit of life.

  3. All cells arise from pre-existing cells.

8. Virus Structure

• Basic Components:

  • Genetic Material: DNA or RNA (can be single- or double-stranded).

  • Capsid: Protein coat that encases and protects the genetic material.

  • Envelope (in some viruses): A lipid membrane derived from the host cell, often studded with proteins important for infecting host cells.

• Note: Viruses lack the machinery for independent metabolism and reproduction; they must infect a host cell.

9. Lytic vs. Lysogenic Cycles (Viral Replication)

• Lytic Cycle:

  • Process: The virus commandeers the host cell’s machinery to produce new viral particles. This rapid replication culminates in the lysis (bursting) of the host cell, releasing new viruses.

• Lysogenic Cycle:

  • Process: The viral DNA integrates into the host cell’s genome and replicates along with it. The virus can remain dormant (prophage) until triggered to enter the lytic cycle.

• Key Difference: The lytic cycle results in immediate destruction of the host cell, while the lysogenic cycle allows the virus to persist in the host for a longer period.