Cell Biology and Structure

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.

The nucleus contains the cell's genetic material (DNA) and controls all cellular activities by regulating gene expression. It is often called the cell's 'control center.' Mitochondria produce energy (ATP), ribosomes synthesize proteins, and endoplasmic reticulum processes proteins and lipids, but none direct overall cellular function like the nucleus.

All living things share three fundamental properties: synthesis (building complex molecules), growth (increasing in size), and replication (reproduction). Motility is not fundamental; many organisms, including plants and most microscopic life, are non-motile. Option C correctly identifies the three essential characteristics of life.

Mature mammalian RBCs lose their nucleus during development to maximize hemoglobin-carrying capacity, but they originate from nucleated eukaryotic progenitor cells. Being enucleated is a specialized adaptation, not evidence of being prokaryotic.

Cell size is regulated by the nuclear-cytoplasmic ratio (the nucleus can only control a limited volume) and metabolic demands. Oxygen needs affect mitochondrial content, material requirements determine cytoplasmic volume, and nuclear control mechanisms maintain size homeostasis.

Membrane pores and channels typically exhibit negative charge to repel anions while allowing cation passage. This charge selectivity is crucial for regulating ion flow across the membrane and maintaining electrochemical gradients.

Plasma membrane pores typically measure around 8 Ångströms, allowing small molecules and ions to pass while restricting larger molecules. This size is crucial for selective permeability and controlling molecular transport across membranes.

This is the correct option. ATP is synthesised by the cell in mitochondria and chloroplast.

Meristematic tissues contain undifferentiated cells with active division capabilities, enabling plant growth at root and shoot tips. These cells remain in a perpetual state of mitotic potential, allowing continuous plant development.

Tissue cells share embryological origin, similar structural characteristics, and perform collective specialized functions. This organizational principle enables complex organisms to develop specialized tissues with precise cellular arrangements supporting specific physiological roles.

Muscular tissue demands significant energy due to continuous contraction and relaxation processes. Even at rest, muscles maintain basal metabolic tone, actively consuming ATP through complex biochemical pathways for movement and posture maintenance.

Tissues are defined as groups of cells with similar structural characteristics and coordinated functional properties. This fundamental biological principle allows multicellular organisms to organize cells into specialized units performing specific physiological tasks.

Ribosomes are the cellular structures where protein synthesis occurs. They read mRNA and assemble amino acids into protein chains. Lysosomes break down waste materials. Mitochondria produce energy (ATP). Endoplasmic reticulum modifies proteins but the actual synthesis happens at ribosomes (which can be free or attached to the ER).