Consider statements about ecology: 1. A food web consists of interconnected food chains. 2. Decomposers recycle nutrients. 3. Biomass decreases at higher trophic levels. Which are correct? MCQ with Answer and Explanation

Explanation:
The sinoatrial (SA) node is a specialized cluster of cells in the right atrium near the opening of the superior vena cava. It generates electrical impulses spontaneously, setting the pace of the heartbeat, hence called the pacemaker. The impulse spreads to the atrioventricular (AV) node, then to bundle of His and Purkinje fibers. Normal SA node firing rate is 60-100/min.

Explanation:
The primary cell wall of plants consists of cellulose microfibrils embedded in a matrix of hemicellulose, pectin, and proteins. Secondary walls contain lignin. Chitin is in fungi and arthropods, peptidoglycan in bacteria. Cellulose is a polymer of β-glucose units and provides structural support.

Explanation:
The pineal gland, a small endocrine gland in the brain, secretes melatonin in response to darkness, regulating circadian rhythms and sleep patterns. The pituitary controls other glands, thyroid metabolism, thymus immune development.