Meditaliano IMAT Prep

Part 1: The Cell - The Fundamental Unit

1.1 Defining the Cell and Core Structures

The cell is the basic structural, functional, and biological unit of all known organisms. It is the smallest entity that can be considered "alive". Despite their diversity, all cells share four core components:

• Plasma Membrane: A selective barrier that encloses the cell. It is made of a phospholipid bilayer, which is flexible and fluid. This structure is described by the fluid mosaic model. Embedded proteins act as channels and pumps to control what enters and exits the cell.
• Cytosol/Cytoplasm: The jelly-like substance filling the cell, composed mostly of water, salts, and organic molecules. It provides a medium for the chemical reactions of metabolism to occur.
• Genetic Material (DNA): The hereditary information. In every cell, DNA provides the instructions for building proteins, which carry out most cellular tasks.
• Ribosomes: The molecular machines responsible for protein synthesis (a process called translation). They read the instructions from the DNA and build proteins accordingly. Importantly, ribosomes are not membrane-bound organelles.

1.2 Prokaryotes vs. Eukaryotes: A Crucial Distinction

Cells are classified into two major groups based on their internal complexity: prokaryotes and eukaryotes. This distinction is one of the most fundamental in biology.

1.3 Overview of Key Eukaryotic Organelles

Eukaryotic cells contain specialized, membrane-bound compartments called organelles, each performing a distinct task vital for cell survival. Think of them as the "organs" of the cell.

• Nucleus: The "control center". It houses the cell's DNA and is the site of DNA replication and transcription (making RNA). The nucleolus inside is where ribosomal RNA (rRNA) is made.
• Mitochondria: The "powerhouse". This is the main site of cellular respiration, the process that generates the majority of the cell's ATP (energy currency).
• Endoplasmic Reticulum (ER): A network of membranes for synthesis and transport.
  • Rough ER (RER): Studded with ribosomes, it synthesizes proteins that will be exported from the cell or embedded in membranes.
  • Smooth ER (SER): Lacks ribosomes. It is involved in lipid synthesis, carbohydrate metabolism, and detoxification.
• Golgi Apparatus: The "cellular post office". It modifies, sorts, and packages proteins and lipids from the ER into vesicles for transport to their final destinations.
• Lysosomes: The "recycling center". These are sacs containing hydrolytic enzymes used to break down waste, damaged organelles, and foreign material.

1.4 Special Structures in Plant Cells

Plant cells are also eukaryotic but have three key structures not typically found in animal cells.

• Cell Wall: A rigid, external layer made primarily of cellulose. It provides structural support and protection to the cell.
• Chloroplasts: The site of photosynthesis. They contain chlorophyll and convert light energy into chemical energy (glucose).
• Large Central Vacuole: A large, membrane-bound sac that stores water, nutrients, and waste products. It also helps maintain turgor pressure against the cell wall.

Part 2: The Code of Life - DNA, RNA, and Protein Synthesis

2.1 DNA Structure and the Nucleotide

The DNA (Deoxyribonucleic Acid) molecule is the master blueprint for all life. It is a polymer made of repeating units called nucleotides. The structure of a single nucleotide is fundamental:

1. A Deoxyribose sugar (a five-carbon sugar).
2. A Phosphate group.
3. One of four Nitrogenous Bases: Adenine (A), Thymine (T), Guanine (G), or Cytosine (C).

These nucleotides link together to form a long chain. The DNA molecule itself consists of two of these chains running in opposite directions, forming a double helix. The two strands are held together by hydrogen bonds between complementary bases: A always pairs with T (forming 2 bonds), and G always pairs with C (forming 3 bonds). The specific sequence of these bases is the genetic code.

2.2 RNA: The Intermediate Messenger

RNA (Ribonucleic Acid) is another crucial nucleic acid that acts as a temporary copy and working molecule. It differs from DNA in three key ways:

• Sugar: Uses ribose instead of deoxyribose.
• Bases: Replaces Thymine (T) with Uracil (U). So, in RNA, A pairs with U.
• Structure: Typically single-stranded, not a double helix.

There are three main types of RNA involved in protein synthesis: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

2.3 The Central Dogma of Molecular Biology

The flow of genetic information in a cell is described by the Central Dogma. This is the core process of how the genetic code in DNA is used to create functional proteins, which in turn determine an organism's traits.

2.4 Chromosomes and Genetic Packaging

In eukaryotes, DNA molecules are extremely long. To fit inside the tiny nucleus, the DNA is tightly wound around proteins called histones. This DNA-protein complex is called chromatin. During cell division, chromatin condenses even further to form visible, distinct structures called chromosomes. Humans have 46 chromosomes (23 pairs) in their somatic (non-sex) cells.

Part 3: The Energy Engine - Metabolism

3.1 Catabolism, Anabolism, and ATP

Metabolism is the sum of all chemical reactions that occur within a living organism to maintain life. It is divided into two types of processes:

• Catabolism: The process of breaking down complex molecules (like glucose) into simpler ones. This process releases energy (it is exergonic). Cellular respiration is a prime example.
• Anabolism: The process of building up complex molecules (like proteins from amino acids) from simpler ones. This process requires energy (it is endergonic). Photosynthesis is a prime example.

3.2 Core Metabolic Pathways: Respiration and Photosynthesis

Two of the most important metabolic processes on Earth are cellular respiration and photosynthesis. They are complementary processes.

• Cellular Respiration: A catabolic process that occurs in the mitochondria of eukaryotes. It breaks down glucose and other organic fuels in the presence of oxygen to produce a large amount of ATP. Carbon dioxide and water are released as waste products.
• Photosynthesis: An anabolic process that occurs in the chloroplasts of plants and algae. It uses light energy to convert carbon dioxide and water into glucose (chemical energy) and releases oxygen as a byproduct.

Part 4: Practice Problems and Review