The World of Living - 2

Centrioles are cylindrical organelles composed of microtubules that organize spindle fibers during cell division. They are present in animal cells and play a crucial role in mitosis and meiosis. Most plant cells lack centrioles, using alternative mechanisms to organize their microtubules. Bacteria and viruses do not have centrioles.

Cell walls are extracellular structures found in plant cells, fungi, and some protists, but are absent in animal cells. Animal cells rely on their cytoskeleton and extracellular matrix for structural support instead. Mitochondria, nucleus, and centrosomes are all present in animal cells.

Mature mammalian red blood cells (erythrocytes) lose their nucleus during development to maximize space for hemoglobin, which carries oxygen. This enucleation is unique among human cells. White blood cells, nerve cells, and epithelial cells all maintain their nuclei throughout their lifespan.

When a cell is in an isotonic solution, the solute concentration is equal inside and outside the cell. Water moves in and out at equal rates, so the cell maintains its normal volume without shrinking or swelling. In hypertonic solutions, cells shrink; in hypotonic solutions, they swell. 'Hyalotonic' is not a valid biological term.

Chlorophyll contains magnesium at its center, which is essential for photosynthesis. The magnesium ion helps absorb light energy. Iron is found in hemoglobin, not chlorophyll. Manganese is a micronutrient for plants but not part of chlorophyll. Calcium is important for cell walls but not a chlorophyll component.

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.

Vitamins are essential micronutrients required for various metabolic processes, but they do not provide energy when broken down. Glucose and pulses (proteins) provide 4 kcal/g, while oil (fats) provides 9 kcal/g. Only the macronutrients - carbohydrates, proteins, and fats - supply calories.

Unicellular refers to organisms that consist of a single cell, such as bacteria, protozoa, and some algae. Viruses are not considered living organisms as they cannot reproduce independently without a host cell.

Animal cells contain lysosomes for waste digestion and centrosomes for organizing cell division, which are generally absent in plant cells. Both cell types share mitochondria for energy production and ribosomes for protein synthesis.

The trachea (windpipe) is reinforced with 16-20 C-shaped rings of hyaline cartilage that prevent it from collapsing during inhalation when intrathoracic pressure drops. The cartilage rings are incomplete posteriorly (C-shaped) to allow the esophagus to expand during swallowing. Bony rings do not exist in the trachea.

In the electron transport chain, each FADH2 molecule yields approximately 1.5 to 2 ATP molecules (modern estimates use 1.5, older textbooks used 2). FADH2 enters the chain at Complex II (coenzyme Q), bypassing Complex I, so it pumps fewer protons and generates less ATP than NADH, which yields 2.5 to 3 ATP.

Metabolism is the sum of all chemical reactions in an organism, including both anabolic (constructive, building complex molecules) and catabolic (destructive, breaking down molecules) pathways. Catabolism only refers to breakdown reactions, while anabolism refers only to building reactions. Physiology is the broader study of how organisms function.

Contractions of the external intercostal muscles bring about increase in the volume of thoracic cavity.

Cellular respiration occurs in all living cells (plant and animal) as it's essential for energy production. In plants, respiration happens continuously, day and night, in mitochondria. While plants produce oxygen during photosynthesis (which only occurs in light), they simultaneously consume oxygen for respiration. Respiration never produces vitamins.

Tricuspid valves are present in between right atrium and right ventricle.

The truncus arteriosus in frogs is guarded by three semilunar valves that prevent backflow of blood into the ventricle during ventricular relaxation. These valves are similar to the semilunar valves (aortic and pulmonary) in the human heart and ensure unidirectional blood flow from the single ventricle to the arterial system.

The bicuspid valve (also called the mitral valve) is located between the left atrium and left ventricle. It prevents backflow of blood from the left ventricle to the left atrium when the ventricle contracts. The tricuspid valve is between the right atrium and right ventricle. Atria are separated from each other by the interatrial septum, not a valve.

Rouleaux formation is the stacking of red blood cells (erythrocytes) like a pile of coins, which occurs due to the discoid shape of RBCs and increased plasma proteins (especially fibrinogen). This is distinct from agglutination (antibody-mediated clumping) and coagulation (clot formation). Rouleaux formation is a normal phenomenon that aids RBC transport through capillaries.

Sodium oxalate prevents blood clotting by binding calcium ions, which are essential cofactors in the coagulation cascade. By removing calcium, the anticoagulant disrupts the clotting process and keeps blood in liquid form for storage.

Basophils are granulocytes that release histamine and heparin during allergic responses but do not phagocytose pathogens. In contrast, neutrophils and eosinophils (acidophils) are actively phagocytic cells that engulf and destroy microorganisms.

Mitral (or bicuspid) valve in mammals guards the opening between left atrium and left ventricle.

Erythroblastosis fetalis is the type of anemia in which the red blood cells (erythrocytes) of a foetus are destroyed in a maternal immune reaction resulting from a blood group incompatibility between the foetus and its mother. There are two main causes of erythroblastosis fetalis: Rh incompatibility and ABO incompatibility.

Thymosin is the hormone secreted by the thymus gland that stimulates the maturation and differentiation of T-lymphocytes. The thymus is the primary lymphoid organ where T-cells develop their antigen-specific receptors during fetal development.

After birth, the ductus arteriosus (a fetal shunt between pulmonary artery and aorta) closes and becomes the ligamentum arteriosum, a fibrous connective tissue remnant. The fossa ovalis is the remnant of the foramen ovale (atrial septal defect).