All Free Biology MCQs with Answers

• A. As energy converting organelles
• B. In post-translational modification of proteins and glycosylation of lipids
• C. In capturing light energy and converting it into chemical energy
• D. In the intracellular digestion of proteins and carbohydrates

Explanation: The Golgi complex is integral to the post-translational modification of proteins and the glycosylation of lipids. Proteins synthesized in the rough endoplasmic reticulum and lipids from the smooth endoplasmic reticulum are further processed in the Golgi apparatus. Here, proteins undergo modifications such as folding and cutting, and both proteins and lipids are glycosylated, meaning they are combined with carbohydrates to form glycoproteins and glycolipids, respectively. This is essential for their proper function and transport within the cell.

• A. Golgi apparatus, formation of glycolipids
• B. Rough endoplasmic reticulum, protein synthesis
• C. Rough endoplasmic reticulum, formation of glycoproteins
• D. Golgi apparatus, protein synthesis

Explanation: The correct answer is Option B: Rough endoplasmic reticulum, protein synthesis. The rough endoplasmic reticulum (RER) is clearly depicted in the figure, characterized by its ribosome-studded surface, which is crucial for protein synthesis. The RER plays a significant role in synthesizing proteins intended for secretion or for use within the cell.

• A. Mitochondria - Produces cellular energy in the form of ATP
• B. Golgi apparatus - Modifies, sorts, and packages proteins and lipids
• C. Lysosomes - Contains digestive enzymes for breaking down waste
• D. Endoplasmic reticulum - Synthesizes proteins and lipids

Explanation: The labelled organelle is the mitochondria, which is the site of cellular respiration and ATP production, making it the 'powerhouse' of the cell. It converts energy stored in food molecules into ATP, which is used to power various cellular processes. The other options describe organelles with different functions: the Golgi apparatus is involved in processing and packaging proteins and lipids, lysosomes contain enzymes for digestion, and the endoplasmic reticulum is involved in protein and lipid synthesis.

• A. Na+ and K+ ions move across the cell membrane by passive transport
• B. Proteins make up 60 to 70% of the cell membrane
• C. Lipids are arranged in a bilayer with polar heads towards the inner part
• D. Fluid mosaic model of cell membrane was proposed by Singer and Nicolson

Explanation: The correct answer is that the fluid mosaic model of the cell membrane was proposed by Singer and Nicolson. This model illustrates the cell membrane as a flexible and dynamic structure, comprised of a lipid bilayer with proteins embedded within it, which allows for the fluidity necessary for various cellular functions. Option A is incorrect because the transport of Na+ and K+ ions primarily involves active transport, not passive transport. Option B is misleading since proteins constitute about 50 to 60% of the membrane, not 60 to 70%. Option C is also incorrect because the arrangement of lipids in a bilayer has polar heads facing outward, towards the extracellular and cytoplasmic environments, not inward.

• A. The prokaryotic ribosomes are 80S, where "S" stands for sedimentation coefficient
• B. These are composed of ribonucleic acid and proteins
• C. These are found only in eukaryotic cells
• D. These are self-splicing introns of some RNAs

Explanation: Ribosomes are cellular organelles that play a crucial role in protein synthesis. They are composed of ribosomal RNA (rRNA) and proteins, forming two subunits: a small subunit and a large subunit. These subunits come together during protein translation to facilitate the assembly of amino acids into polypeptides, based on the sequence of codons in mRNA. Ribosomes are found in both prokaryotic and eukaryotic cells, although their size differs (70S in prokaryotes and 80S in eukaryotes).

• A. Ribosome
• B. Peroxisome
• C. ER (Endoplasmic Reticulum)
• D. Mesosome

Explanation: Ribosomes are critical for protein synthesis and are unique because they can be found within mitochondria and chloroplasts, both of which have their own DNA and protein synthesis machinery, thus qualifying ribosomes as 'organelles within an organelle.' Peroxisomes, endoplasmic reticulum, and mesosomes do not have this characteristic, as they do not reside within other organelles.

• A. Chloroplast
• B. Mitochondria
• C. Chromoplast
• D. Ribosomes

Explanation: Peptide synthesis inside a cell takes place in the ribosomes. Ribosomes are the sites of protein synthesis, where amino acids are assembled into proteins based on the sequence coded in messenger RNA. They are found in all living cells, both prokaryotic and eukaryotic.

• A. Vacuole
• B. Golgi apparatus
• C. Plastid
• D. Lysosome

Explanation: The correct answer is the Golgi apparatus. This organelle is key in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell. It is specifically involved in the formation of glycoproteins and glycolipids by attaching carbohydrate and lipid moieties to proteins and lipids. In contrast, vacuoles are for storage, plastids are primarily for photosynthesis and pigment storage, and lysosomes are involved in the breakdown of cellular waste.

• A. Plasma membrane
• B. Mitochondrion
• C. Cytoplasm
• D. Nucleus

Explanation: The correct answer is Cytoplasm. The cytoplasm is the main area of various cellular activities because it houses the organelles and provides the medium where many metabolic processes occur, such as glycolysis, protein synthesis, and intracellular transport. The plasma membrane is involved in protecting the cell and regulating what enters and exits, but it is not the site of intracellular activities. The mitochondrion is crucial for energy production, but it is just one of the many organelles within the cytoplasm. The nucleus stores genetic information and controls activities through gene expression, but it is not the site where most cellular processes take place.

• A. Nucleus only
• B. Nucleus & nucleolus only
• C. Nucleolus only
• D. None of the above

Explanation: The nucleolus, found within the nucleus, is the specific structure responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosome subunits. Option C is correct because it identifies the nucleolus as the only structure responsible for these functions. The other options include structures that do not specifically perform this role or incorrectly suggest that no structure is responsible.