Grade 8 Comprehensive Biology Reviewer PDF

GRADE 8 Comprehensive Biology Reviewer Digestive System Functions of the Digestive System: ○ Ingestion: The process of taking in food through the mouth. This step involves both voluntary actions (like chewing) and involuntary actions (like swallow...

GRADE 8 Comprehensive Biology Reviewer Digestive System Functions of the Digestive System: ○ Ingestion: The process of taking in food through the mouth. This step involves both voluntary actions (like chewing) and involuntary actions (like swallowing). ○ Digestion: Mechanical Digestion: This includes actions such as chewing (mastication) in the mouth and the churning of food in the stomach. The physical breakdown increases the surface area for enzymes to act. Chemical Digestion: Involves the breakdown of food by enzymes. Salivary amylase in the mouth starts the digestion of carbohydrates, while gastric juice (containing hydrochloric acid and pepsin) in the stomach begins protein digestion. ○ Absorption: Primarily occurs in the small intestine (approximately 6 meters long), where nutrients are absorbed into the bloodstream through the intestinal walls, aided by structures called villi and microvilli that increase surface area. Water and some minerals are absorbed in the large intestine, highlighting its role in maintaining hydration and electrolyte balance. ○ Excretion: The final stage where undigested material is expelled from the body as feces. The large intestine compacts waste and the rectum stores it until it is eliminated through the anus. Digestive Process Breakdown Stages of Digestion: ○ Mouth: Mechanical digestion occurs as teeth chew food, breaking it into smaller pieces, increasing surface area for enzymes. Saliva, containing enzymes like salivary amylase, begins the breakdown of carbohydrates. Saliva also lubricates food, making it easier to swallow. ○ Esophagus: This muscular tube transports food from the mouth to the stomach via peristalsis, a series of coordinated contractions that push the food down.  The lower esophageal sphincter prevents backflow of stomach contents into the esophagus. ○ Stomach: Food is mixed with gastric juices (hydrochloric acid and digestive enzymes) to create chyme. Gastric acid activates pepsinogen into pepsin, which begins protein digestion. The acidic environment also helps kill pathogens. The pyloric sphincter regulates the passage of chyme into the small intestine. ○ Small Intestine: It Comprises three segments: duodenum, jejunum, and ileum. In the duodenum, bile (from the liver) emulsifies fats, and pancreatic enzymes digest carbohydrates, proteins, and fats. Nutrient absorption occurs primarily in the jejunum and ileum, where villi and microvilli increase the surface area for efficient nutrient uptake. ○ Large Intestine: Absorbs water and electrolytes, converting liquid chyme into solid waste. Houses beneficial bacteria that aid in digestion and synthesize certain vitamins (like vitamin K). Waste is stored in the rectum until excretion through the anus. Accessory Organs and Their Functions Liver: ○ Produces bile, essential for the digestion and absorption of fats. ○ Metabolizes nutrients absorbed from the small intestine and detoxifies harmful substances. ○ Stores glycogen, vitamins, and minerals, releasing them as needed to maintain homeostasis. Pancreas: ○ An exocrine gland producing digestive enzymes (lipase, proteases, and amylase) that are secreted into the small intestine. ○ An endocrine gland regulating blood sugar levels by releasing insulin and glucagon. Gallbladder: ○ Stores and concentrates bile until needed in the small intestine. ○ Bile aids in the emulsification of fats, improving their digestibility. Nutrients and Their Functions Macronutrients: ○ Carbohydrates: Simple Carbohydrates: Composed of one or two sugar molecules (monosaccharides and disaccharides). Found in fruits (fructose), dairy (lactose), and table sugar (sucrose). They provide quick energy but should be consumed in moderation due to potential spikes in blood sugar levels. Complex Carbohydrates: Made up of longer chains of sugar molecules (polysaccharides). Found in whole grains, legumes, and vegetables, they offer sustained energy and dietary fiber, which aids digestion and prevents constipation. ○ Proteins: Composed of amino acids; 20 different amino acids exist, with 9 being essential (must be obtained through diet). Sources include meat, fish, eggs, dairy, legumes, and nuts. Proteins are essential for building and repairing tissues, making enzymes and hormones, and supporting immune function. ○ Fats: Saturated Fats: Typically solid at room temperature and found in animal products (like meat and dairy) and some plant oils (like coconut oil). Excessive intake can raise cholesterol levels and increase the risk of heart disease. Unsaturated Fats: Liquid at room temperature, primarily found in plant oils (like olive and canola oil), avocados, nuts, and fish. They are beneficial for heart health and should be included in a balanced diet. Trans Fats: Created through hydrogenation and found in some processed foods. They are harmful and should be avoided. Micronutrients: ○ Vitamins: Water-Soluble Vitamins: Includes Vitamin C and the B vitamins (like B12, B6, niacin, riboflavin). These are important for energy metabolism, red blood cell formation, and immune function. Since they are not stored in the body, regular intake is necessary. Fat-Soluble Vitamins: Includes Vitamins A, D, E, and K. These are stored in the liver and fatty tissues, and are essential for vision, bone health, antioxidant function, and blood clotting, respectively. ○ Minerals: Calcium: Essential for strong bones and teeth, muscle function, and nerve transmission. Sources include dairy products, leafy greens, and fortified foods.  Iron: Important for hemoglobin production and oxygen transport in the blood. Found in red meat, poultry, fish, beans, and fortified cereals. Potassium: Vital for heart health and muscle function, found in bananas, oranges, and potatoes. Water: Constitutes about 60% of the human body and is critical for hydration, nutrient transport, temperature regulation, and waste removal. Cell Biology (Cell Cycle and Division) Phases of the Cell Cycle 1. Interphase (Approximately 90% of the Cell Cycle) ○ G1 Phase (Gap 1): Role: The cell grows in size, synthesizes mRNA and proteins required for DNA replication, and produces organelles. Key Events: Nutrient uptake and metabolic activity increase. The cell assesses its environment and determines whether conditions are favorable for division. ○ S Phase (Synthesis): Role: The cell replicates its DNA, resulting in two identical sets of chromosomes. Key Events: DNA unwinds and each strand serves as a template for synthesizing a new complementary strand, facilitated by enzymes like DNA polymerase. Histone proteins are synthesized to package the new DNA. ○ G2 Phase (Gap 2): Role: The cell continues to grow and prepares for mitosis. Key Events: The cell checks for DNA replication errors and makes repairs if necessary. Production of proteins and organelles that are essential for cell division increases. 2. M Phase (Mitosis) ○ Definition: Mitosis is the process of nuclear division, followed by cytokinesis, resulting in two genetically identical daughter cells. ○ Stages of Mitosis:  Prophase: Chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and spindle fibers begin to form. Metaphase: Chromosomes align at the metaphase plate, ensuring proper attachment to spindle fibers. Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell. Telophase: Chromatids reach the poles, the nuclear envelope re-forms around each set, and chromosomes decondense back into chromatin. ○ Cytokinesis: The division of the cytoplasm occurs, resulting in two separate cells. In animal cells, a cleavage furrow forms; in plant cells, a cell plate develops. Regulation of the Cell Cycle Checkpoints: ○ G1 Checkpoint: Checks cell size, nutrient status, and DNA integrity before committing to DNA synthesis. ○ G2 Checkpoint: Ensures that all DNA has been replicated accurately and that there are no damages before entering mitosis. ○ M Checkpoint: Confirms that all chromosomes are properly attached to the spindle apparatus, preventing unequal chromosome distribution during division. Significance: These checkpoints prevent the division of cells that are damaged or not fully prepared, reducing the risk of mutations and cancer. Types of Cell Division 1. Mitosis ○ Purpose: Responsible for growth, tissue repair, and asexual reproduction in multicellular organisms. ○ Outcome: Produces two genetically identical daughter cells, each with the same diploid chromosome number as the parent cell. 2. Meiosis ○ Purpose: Reduces the chromosome number by half to produce haploid gametes (sperm and eggs). ○ Key Processes: Reduction Division: Involves two rounds of division (Meiosis I and Meiosis II).  Crossing Over: Occurs during prophase I, where homologous chromosomes exchange genetic material, leading to genetic variation. Independent Assortment: The random distribution of homologous chromosomes during metaphase I contributes to genetic diversity. Genetics 1. Chromosomes ○ Structure: Chromosomes consist of chromatin (DNA wrapped around histone proteins) and are visible during cell division. ○ Types: Diploid (2n): Cells containing two sets of chromosomes (e.g., human somatic cells have 46 chromosomes). Haploid (n): Cells containing one set of chromosomes (e.g., human gametes have 23 chromosomes). 2. Punnett Squares ○ Definition: A tool used to predict the probability of offspring inheriting specific traits based on parental genotypes. ○ Example Scenario: Let’s say we are studying flower color in pea plants, where purple (P) is dominant and white (p) is recessive. If we cross two heterozygous plants (Pp): P p P PP Pp p Pp pp Results: PP: 1 (25%) - Homozygous dominant (Purple) Pp: 2 (50%) - Heterozygous (Purple) pp: 1 (25%) - Homozygous recessive (White) Classification and Taxonomy Organism Classification: Levels of Biological Classification: ○ Domain: The highest taxonomic rank; includes three domains: Archaea, Bacteria, and Eukarya. ○ Kingdom: Subdivisions within domains. For example, Eukarya includes kingdoms like Animalia, Plantae, Fungi, and Protista. ○ Phylum: Groups organisms based on major body plans or structural features (e.g., Chordata for vertebrates). ○ Class: Further divides phyla (e.g., Mammalia for mammals). ○ Order: Groups families with similar characteristics (e.g., Carnivora for carnivorous mammals). ○ Family: A grouping of related genera (e.g., Felidae for cats). ○ Genus: A grouping of species that are closely related and very similar (e.g., Panthera for big cats). ○ Species: The most specific level; a group of individuals that can interbreed and produce fertile offspring (e.g., Panthera leo for lions). Grouping Organisms: Organisms are classified based on shared characteristics such as morphology, genetic information, and evolutionary history. This system helps scientists communicate about species and understand relationships in the tree of life. Scientific Naming: Binomial Nomenclature: ○ Developed by Carl Linnaeus, this system gives each species a two-part name: the genus name (capitalized) followed by the species name (lowercase), both italicized or underlined (e.g., Homo sapiens). Importance: ○ Provides a universal naming system that avoids confusion caused by common names. ○ Allows for easier identification and classification. ○ Reflects evolutionary relationships. Examples of Organisms, Their Scientific Names, and Classifications: 1. Human ○ Scientific Name: Homo sapiens ○ Classification: Domain: Eukarya Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primates Family: Hominidae Genus: Homo Species: sapiens 2. Dog ○ Scientific Name: Canis lupus familiaris ○ Classification: Domain: Eukarya Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Canidae Genus: Canis Species: lupus familiaris 3. Tapioca ○ Scientific Name: Manihot esculenta ○ Classification: Domain: Eukarya Kingdom: Plantae Phylum: Angiosperms Class: Eudicots Order: Euphorbiales Family: Euphorbiaceae Genus: Manihot Species: esculenta 4. Sunflower ○ Scientific Name: Helianthus annuus ○ Classification: Domain: Eukarya Kingdom: Plantae Phylum: Angiosperms  Class: Eudicots Order: Asterales Family: Asteraceae Genus: Helianthus Species: annuus 5. Cat ○ Scientific Name: Felis catus ○ Classification: Domain: Eukarya Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Family: Felidae Genus: Felis Species: catus 6. Rice ○ Scientific Name: Oryza sativa ○ Classification: Domain: Eukarya Kingdom: Plantae Phylum: Angiosperms Class: Monocots Order: Poales Family: Poaceae Genus: Oryza Species: sativa 7. Corn ○ Scientific Name: Zea mays ○ Classification: Domain: Eukarya Kingdom: Plantae Phylum: Angiosperms Class: Monocots Order: Poales Family: Poaceae Genus: Zea Species: mays Food Webs and Ecosystems Roles in Ecosystems: Organisms within an ecosystem can be classified based on their roles: ○ Producers: Organisms that create their own food through photosynthesis (e.g., plants like sunflowers). ○ Consumers: Organisms that consume other organisms for energy. These include: Primary Consumers: Herbivores that eat producers (e.g., rabbits). Secondary Consumers: Carnivores that eat herbivores (e.g., snakes). Tertiary Consumers: Predators that eat other carnivores (e.g., hawks). ○ Decomposers: Organisms like fungi and bacteria that break down dead organic matter, returning nutrients to the soil. Energy Flow: Energy flows through an ecosystem in a one-way direction, typically starting from producers to various levels of consumers. Key points include: ○ Trophic Levels: Each level represents a step in the food chain. Producers are at the first trophic level, primary consumers at the second, and so on. ○ Energy Transfer: Only about 10% of the energy at one trophic level is passed to the next. Therefore, organisms at higher trophic levels receive less energy. ○ Food Webs: Illustrate the interconnected relationships and energy flow between different organisms in an ecosystem, emphasizing that real ecosystems are more complex than simple food chains. Approved by: MS. FLORENCE QUERO Biology Teacher