Comprehensive Unit Review of Famous Scientists, Matter, Chemical Reactions, and Cell Biology

Notable Contributions to Global and Ethiopian Science

The history of science is marked by the foundational contributions of individuals who expanded human understanding of the physical world. Marie Curie is recognized for her pioneering work in radioactivity, specifically for the discovery of the elements Polonium and Radium. Isaac Newton formulated the essential laws of motion and universal gravitation, which provided a mathematical framework for classical mechanics. Michael Faraday's research into electromagnetism led to the discovery of magnetism and the construction of the first dynamo, a precursor to the modern electrical generator. Albert Einstein transformed theoretical physics with his development of the special theory of relativity.

Ethiopian scientists have also made significant contributions to various scientific fields, often addressing localize and global challenges. Dr. Aklilu Lemma is celebrated for his development of a treatment for snail fever, medically known as bilharzia. Yalemtsehay Mekonen conducted extensive research on the impact of pesticides on human health. Dr. Tewolde Birhan Gebre Egziabher's work focused on environmental and biological rights, specifically regarding communities' access to their genetic resources. Finally, Prof. Gebisa Ejeta is known for his work in agricultural science, where he developed improved plant varieties to enhance food security.

Classification of Matter and Chemical Substances

Matter is systematically classified into pure substances and mixtures based on its composition and properties. Pure substances are divided into elements and compounds. Elements are the simplest form of matter and cannot be broken down into simpler substances by ordinary chemical means; an example of an element is Iron ($Fe$). Compounds are substances formed from two or more elements chemically united in fixed proportions and can be broken down into simpler substances by chemical means. Examples of compounds include water ($H_2O$) and calcium nitrite ($Ca(NO_3)_2$).

Mixtures consist of two or more substances that are physically combined and can be separated by physical means. Homogeneous mixtures have a uniform composition throughout, meaning the individual components are not visible to the naked eye. Examples include saltwater and air. Heterogeneous mixtures have a non-uniform composition, where the different components are clearly visible. Typical examples of heterogeneous mixtures include soil and salad.

Chemical Notation, Valency, and Nomenclature

Chemical notation serves as a symbolic language to represent matter both qualitatively and quantitatively. Qualitative representation describes the types of atoms present in a substance. For instance, the formula for water, $H_2O$, indicates that the molecule is composed of Hydrogen and Oxygen atoms. Quantitative representation describes the exact number of atoms of each element in a chemical species using subscripts and coefficients. Subscripts indicate the number of atoms of a specific element within a molecule or formula unit. In $H_2O$, the subscript $2$ indicates two Hydrogen atoms, while the implied subscript $1$ for Oxygen indicates one Oxygen atom. Coefficients are numbers placed in front of a formula to indicate the number of molecules or formula units present. For example, $2Fe$ means two atoms of Iron, and $3H_2O$ means three molecules of water. In the case of $3H_2O$, since each molecule contains $2$ Hydrogen atoms and $1$ Oxygen atom, there are a total of $3 \times 2 = 6$ Hydrogen atoms and $3 \times 1 = 3$ Oxygen atoms.

Valence electrons are the electrons located in the outermost shell of an atom and are primarily responsible for chemical bonding. Atoms can be categorized by their valency. Atoms with one valence electron (Valence 1) can be remembered using the mnemonic: "Little Sam puts cats, snaker, Cour Behind Ice Fridges." Atoms with two valence electrons (Valence 2) use the mnemonic: "Monkeys Can Itch Zebrar Like Old Sock." Atoms with three valence electrons (Valence 3) use the mnemonic: "Always Ignore Moiser."

Chemical nomenclature utilizes specific suffixes to categorize substances. The suffix -ium often indicates a metal. The suffix -ide typically denotes a nonmetal in a binary compound. The suffix -ate indicates a polyatomic ion containing oxygen. The suffix -ite also indicates a polyatomic ion containing oxygen but with one less oxygen atom than an "-ate" ion. Finally, the suffix -gen often identifies an element that produces a gas.

Chemical Reactions and Equations

A chemical reaction is a process in which substances undergo changes to form new substances. The materials that undergo the chemical change are called reactants, and the new substances formed as a result of that change are called products. The standard format for a chemical equation is represented as:

$\text{Reactant} \rightarrow \text{Product}$

The Evolution of Microscopy and Cell Discovery

The development of the microscope was essential for the advancement of cell biology. Zacharias Janssen and Hans Lipperhey are credited with developing the concept of the compound microscope. Robert Hooke created a magnifier with less than $30\times$ magnification; he is famously the first person to use the term "cell" after observing the structure of cork through his lenses. Later, Antonie van Leeuwenhoek developed improved magnifying lenses and constructed his own simple microscopes capable of up to $300\times$ magnification. He was the first to observe living organisms, which he termed "animalcules," a group we now recognize as protists.

Today, microscopes are categorized into two primary types: light microscopes and electron microscopes. Light microscopes use a beam of light to illuminate specimens. A simple light microscope consists of a single convex lens, similar to a hand lens, and provides magnification from $10\times$ to $20\times$. A compound light microscope uses a multiple lens system including various objectives: Low ($4\times$), Middle ($10\times$), High ($40\times$), and Oil Immersion ($100\times$). Electron microscopes utilize a beam of electrons rather than light, providing much higher resolution and magnification.

Cell Structure, Organelles, and Types

Cells are the basic units of life and are broadly categorized as eukaryotic (possessing a true nucleus) or prokaryotic (lacking a nucleus). Within eukaryotic life, plant and animal cells exhibit distinct structural differences. Plant cells feature a rigid cell wall that provides a regular, rectangular shape, contain chloroplasts for photosynthesis, and possess large central vacuoles. Animal cells lack a cell wall, resulting in an irregular shape, do not contain chloroplasts, and have much smaller vacuoles.

The functions of a cell are carried out by specialized structures called organelles. The Cell Wall (in plants) provides protection and support, while the Cell Membrane controls the passage of substances in and out of the cell. The Nucleus acts as the control center, storing genetic information. The Cytoplasm is the jelly-like substance that fills the cell and surrounds other organelles. Ribosomes are the sites of protein synthesis. Mitochondria are known as the "powerhouse" of the cell because they are the site of cellular respiration. Chloroplasts, found in plant cells, are the site of photosynthesis.

The Endoplasmic Reticulum (ER) exists in two forms: Rough ER, which is studded with ribosomes and involved in protein synthesis/modification, and Smooth ER, which synthesizes lipids and hormones while detoxifying the cell. The Golgi Apparatus sorts and packages proteins and lipids for transport. Lysosomes, often called "suicide bags," contain digestive enzymes to break down waste and debris. Vacuoles serve as storage areas for water, salts, nutrients, and waste products.

Cellular Metabolism and the Kingdoms of Life

Cellular respiration is the process by which cells derive energy. Aerobic respiration requires oxygen and is expressed by the equation:

$\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy}$

Anaerobic respiration occurs in the absence of oxygen. An example of this is yeast fermentation:

$\text{Glucose} \rightarrow \text{Ethanol} + CO_2 + 2\text{ATP}$

Photosynthesis is the process used by plants to convert light energy into chemical energy (glucose). The chemical equation for photosynthesis is:

$6CO_2 + 6H_2O \xrightarrow{\text{Light Energy}} C_6H_{12}O_6 + 6O_2$

Biological life is supported by essential molecules, including lipids (fats and hormones) and proteins. Life is organized into kingdoms, such as Kingdom Plantae (organisms performing photosynthesis) and Kingdom Animalia. Kingdom Animalia is divided into Invertebrates (lacking a backbone, such as worms and insects) and Vertebrates (possessing a backbone). Vertebrates are further categorized into Fish (cold-blooded), Amphibians (cold-blooded), Reptiles (cold-blooded), Birds (warm-blooded), and Mammals (warm-blooded).