A LEVEL BIOLOGY: Body Transport in Plants AND Organic and Biomolecular Chemistry

Structural features of a phloem sieve tube element and their functional explanations

1. No nucleus or organelles: Creates an open path to maximize sap volume.

2. Sieve plates with pores: Allows fluid to pass with low resistance.

3. Peripheral cytoplasm: Stays clear of the central flow path to prevent drag.

4. Elongated cells: Form continuous transport lines.

5. Plasmodesmata: Connects to companion cells for sugar loading.

6. Transport proteins: Pump sucrose to draw water in by osmosis.

Role of sucrose unloading at the sink in maintaining mass flow from the source

1. Sucrose is unloaded from sieve tubes at the sink.

2. This increases the water potential inside the sieve tube element.

3. Water leaves the sieve tube element by osmosis.

4. The decrease in liquid volume lowers the hydrostatic pressure at the sink end.

5. This maintains a hydrostatic pressure gradient between the source and sink, driving mass flow.

Mechanism of semi-conservative DNA replication during interphase

1. Hydrogen bonds break between complementary bases to unzip the double helix.

2. Both strands act as templates.

3. Free activated nucleotides align by complementary base pairing (A to T, C to G).

4. DNA Polymerase links nucleotides together by forming phosphodiester bonds.

5. The lagging strand is synthesized discontinuously as Okazaki fragments, joined by DNA Ligase.

6. Each new DNA molecule contains one original strand and one newly synthesized strand.

Three major functions of antibodies in defending against pathogens

1. Agglutination: Antibodies bind to multiple pathogens at once, clumping them together to prevent spreading.

2. Opsonisation: The constant region marks the pathogen for recognition and phagocytosis by white blood cells.

3. Neutralisation: Antibodies bind to toxins or viral surface proteins, blocking entry into host cells.

Roles of the rER, Golgi apparatus, and mitochondria in protein synthesis and secretion

1. Rough Endoplasmic Reticulum (rER): Ribosomes on the surface translate mRNA into polypeptide chains, which fold inside the rER lumen.

2. Golgi Apparatus: Modifies proteins (e.g., adding carbohydrate chains to form glycoproteins) and packages them into secretory vesicles.

3. Mitochondria: Perform aerobic respiration to produce $ATP$, supplying energy for vesicle transport along the cytoskeleton.

Structural adaptations of xylem vessels for water transport

1. Lignin: Waterproofs walls and provides high tensile strength to prevent collapse under negative pressure.

2. Dead, hollow cells: Lack cytoplasm and end walls, creating a continuous hollow column.

3. Pits: Unlignified areas allowing lateral water movement between adjacent vessels.

4. Cohesion-tension mechanism: Water molecules stick to each other (cohesion) and to vessel walls (adhesion), pulled upward by transpiration.

Effects of competitive inhibitors, non-competitive inhibitors, and extreme heat on enzyme activity

1. Competitive Inhibitor: Complementary shape to active site; competes with substrate for binding (can be overcome by increasing substrate concentration).

2. Non-Competitive Inhibitor: Binds to an allosteric site, altering tertiary structure and active site shape so substrate cannot fit.

3. Extreme Heat: Increases kinetic energy, breaking hydrogen and ionic bonds to denature the active site.

Key cellular processes occurring during the G1, S, and G2 phases of Interphase

1. G1 Phase: Rapid cell growth, protein synthesis, and organelle replication.

2. S Phase: Nuclear semi-conservative DNA replication doubles the genetic material into sister chromatids.

3. G2 Phase: Error-checking of replicated DNA, tubulin synthesis for spindle fibers, and preparation for mitosis.

Pressure changes, valve operations during the cardiac cycle, and ventricular wall thickness

1. Atrial Systole: Atria contract, forcing remaining blood through open AV valves into ventricles.

2. Ventricular Systole: Ventricles contract; rising pressure closes AV valves, then opens semi-lunar valves when pressure exceeds aortic/pulmonary pressure.

3. Wall Thickness: The left ventricular wall is significantly thicker to generate the high hydrostatic pressure needed to pump blood through systemic circulation.

Key differences between facilitated diffusion and active transport across cell membranes

1. Facilitated Diffusion: Passive transport of large, polar, or charged molecules down a concentration gradient via channel or carrier proteins without $ATP$.

2. Active Transport: Movement of ions or molecules against a concentration gradient via carrier proteins, requiring energy from $ATP$ hydrolysis.

Cooperative defense mechanism of goblet cells and ciliated epithelial cells in the respiratory system

1. Goblet Cells: Synthesize and secrete sticky mucus onto the epithelial lining to trap inhaled dust, pollen, and pathogens.

2. Ciliated Epithelial Cells: Possess hair-like cilia that beat in synchronized waves to sweep mucus upward toward the throat to be swallowed.

Pathogens and modes of transmission for Malaria and Tuberculosis (TB)

1. Malaria: Caused by the protoctist parasite Plasmodium; transmitted via the bite of an infected female Anopheles mosquito vector.

2. Tuberculosis: Caused by the bacterium Mycobacterium tuberculosis; transmitted via aerosol droplets when an infected person coughs or sneezes.

Structural and bonding differences between starch and cellulose

1. Starch: Made of $\alpha$-glucose monomers with 1,4 and 1,6 glycosidic bonds; forms a coiled, branched, compact storage molecule.

2. Cellulose: Made of $\beta$-glucose monomers with 1,4 glycosidic bonds (alternate monomers rotated $180^\circ$); forms straight, unbranched chains cross-linked by hydrogen bonds into strong microfibrils.

How $CO_2$ transport in red blood cells causes the Bohr effect

1. $CO_2$ diffuses into red blood cells and reacts with water via carbonic anhydrase to form carbonic acid.

2. Carbonic acid dissociates into $H^+$ and $HCO_3^-$ ions.

3. $H^+$ binds hemoglobin to form hemoglobinic acid, inducing oxygen release.

4. Bohr Effect: High $pCO_2$ shifts the dissociation curve rightward, lowering hemoglobin's affinity for oxygen and promoting oxygen release to active tissues.

Reasons for energy loss between trophic levels in an ecosystem

1. Not all of the organism is consumed (e.g., uneaten roots, bones).

2. Energy is lost as undigested material in feces or urine.

3. Significant energy is lost as heat generated during metabolic respiration.

Chemical synthesis and structure of a triglyceride molecule

1. Formed by joining one glycerol molecule to three fatty acid chains.

2. Occurs via a condensation reaction, releasing 3 water molecules ($H_2O$).

3. Forms 3 covalent ester bonds between glycerol hydroxyl groups and fatty acid carboxyl groups.

Steps involved in producing monoclonal antibodies via hybridoma technology

1. An animal (e.g., mouse) is injected with the target antigen to stimulate an immune response.

2. Specific B-plasma cells producing the antibody are extracted from the spleen.

3. B-cells are fused with myeloma (cancer) cells using a fusogen (e.g., PEG) to form hybridoma cells.

4. Hybridomas are screened, cloned, and cultured in fermenters to continuously produce identical monoclonal antibodies.

Main features of the fluid mosaic model of cell surface membranes

1. Fluid: Phospholipids and embedded proteins move laterally and rotate within their monolayer.

2. Mosaic: Proteins are scattered throughout the phospholipid bilayer.

3. Phospholipids: Hydrophilic phosphate heads face outward; hydrophobic fatty acid tails face inward.

4. Cholesterol: Positioned between phospholipid tails to regulate membrane fluidity and stability.

Mechanism of carrier proteins during active transport

1. Target ion binds to a specific, complementary binding site on the transmembrane carrier protein.

2. $ATP$ binds and undergoes hydrolysis into $ADP$ and inorganic phosphate.

3. Energy released induces a conformational (shape) change in the carrier protein.

4. The protein flips open, moving the ion across the membrane against its concentration gradient.

Three chemical components of a standard DNA nucleotide

1. A pentose sugar (deoxyribose).

2. An inorganic phosphate group.

3. A nitrogen-containing organic base (Adenine, Thymine, Cytosine, or Guanine).

Structural adaptations of arteries, veins, and capillaries for their functions

1. Artery: Thick walls with elastic fibers to stretch and recoil under high pressure, plus smooth muscle to regulate lumen size.

2. Vein: Large, wide lumen to reduce friction under low pressure, with valves to prevent backflow.

3. Capillary: Wall consists of a single layer of squamous endothelial cells (one cell thick) to minimize diffusion distance for rapid exchange.