MCAT Biology Essentials: Cellular Structures and Functions

Prokaryotes are characterized by being unicellular with simple structures, lacking a true nucleus and instead having a nucleoid with DNA, containing plasmids, having 30S and 50S Ribosomes, and lacking membrane-bound organelles.

Eukaryotes are defined by the presence of a nucleus containing DNA organized into chromosomes, ribosomes of 40S and 60S, and membrane-bound organelles. Prokaryotic cells lack a true nucleus, unicellular organisms can be either eukaryotic or prokaryotic, and the absence of a nucleus is a characteristic of prokaryotic cells.

The cell membrane is primarily composed of a phospholipid bilayer, which provides structure and regulates the passage of substances in and out of the cell.

Ribosomes are the cellular structures responsible for protein production in both free form in the cytoplasm and bound to the endoplasmic reticulum.

The Endoplasmic Reticulum is a network of membrane-enclosed spaces connected with the nuclear membrane. It can be further divided into Rough ER and Smooth ER with distinct functions related to protein synthesis, lipid synthesis, and detoxification.

The Golgi Apparatus is responsible for receiving vesicles and their contents from the Smooth Endoplasmic Reticulum (SER), modifying and packaging them into vesicles, and distributing them to their final destinations within the cell.

Lysosomes are known for their role in cellular waste management and recycling rather than energy production, protein synthesis, or lipid transport.

Microbodies are specialized organelles within cells that serve various metabolic functions. Peroxisomes and glyoxysomes are examples of microbodies with specific roles involving oxidation and conversion of fats into sugars, respectively.

Mitochondria are known as the powerhouse of the cell and are responsible for aerobic respiration, not protein synthesis, photosynthesis, or cell division.

Centrioles are specialized structures composed of microtubules that play a crucial role in organizing the spindle fibers during cell division. They are not exclusive to plant cells, nor are they involved in protein synthesis or photosynthesis.

Microtubules are hollow rods made of tubulin protein subunits that provide a dynamic framework for various cellular processes, including organelle movement. The other incorrect options do not accurately describe the structure or function of microtubules.

Microfilaments, made of actin, are indeed solid rods that provide structural support to the cell and are involved in cell movement through interactions with myosin. The other options do not accurately describe the function of microfilaments in the cytoskeleton.

Intermediate filaments are primarily responsible for providing structural support and maintaining the integrity of the cytoskeleton.

Simple diffusion involves the passive movement of particles down their concentration gradients without the need for energy input.

In a hypertonic medium, the cytoplasm has a lower solute concentration than the surrounding medium, causing water to move out of the cell, resulting in the cell shrinking.

Facilitated diffusion is a passive process that does not require cellular energy and involves the help of carrier molecules to move particles down their concentration gradient.

Endocytosis is the process by which cells absorb molecules such as proteins by engulfing them. It is a vital process for the functioning of cells and can be categorized into different types based on the size and nature of particles being absorbed.

Exocytosis is a fundamental cellular process where a vesicle within the cell merges with the cell membrane and releases its contents outside of the cell. This is crucial for the transportation of molecules out of cells and the secretion of various substances.

Epithelial tissue has specific functions related to covering, protection, absorption, secretion, and sensation, which differentiate it from roles such as muscle contraction, storage of nutrients, or transmission of electrical signals.

Connective tissue is primarily involved in providing structural support and connectivity in the body through various forms like bones, cartilage, tendons, and blood. It does not play a direct role in nerve transmission, energy production, or hormone production.

Nervous tissue is specifically composed of neurons which are responsible for perception, processing, and storage of information in the nervous system.

Muscle Tissue is known for its contractile capability and role in body movement. It is categorized into three types: Skeletal, Cardiac, and Smooth.

A virus is an acellular structure made up of genetic material (RNA/DNA) enclosed in a protein coat. It is an obligate intracellular parasite, meaning it requires a host cell to replicate and cause infection. Unlike bacteria, viruses are not considered living organisms and cannot reproduce on their own outside of a host cell.

Enzymes are indeed protein catalysts that aid in accelerating reactions, but they are not carbohydrates, inorganic substances, or molecules that store genetic information. They do not change equilibrium or delta G, are not consumed, are specific, and are influenced by pH and temperature.

The Lock and Key Theory in enzymology suggests that the spatial structure of an enzyme's active site perfectly matches the spatial structure of its substrate, much like a lock and key mechanism.

Induced fit refers to the conformational changes in the active site of an enzyme that occur upon substrate binding. This allows the active site to better accommodate the substrate for catalysis.

Cofactors are molecules required for enzymes to become catalytically active. Apoenzyme refers to the enzyme without the cofactor, while holoenzyme is the enzyme with the cofactor. Prosthetic groups are tightly bound cofactors essential for enzyme function.

The Michaelis Constant (Km) is a key parameter in enzyme kinetics that defines the substrate concentration at which the reaction rate is half of the Vmax. It is calculated using the equation (k2+k3)/(k1) Km = [S] when the rate is equal to half of the maximum velocity (Vmax) of the reaction.