LET Elementary Biological Science — Plants, Animals, Ecology and the EnvironmentSummary

Flowering plants possess three main vegetative organs: roots (absorb water and minerals, anchor the plant), stems (support and transport), and leaves (site of photosynthesis). Two transport tissues move materials: xylem carries water and minerals upward from roots via capillary action, while phloem transports food (dissolved sugars) from leaves to all other parts. The vascular bundle in the stem contains both tissues. Understanding this structure is foundational for explaining how plants obtain and distribute resources, a topic central to Grade 3–4 science lessons on plant life cycles.

Chlorophyll is the green pigment in chloroplasts (organelles in plant cells) that absorbs light energy, especially red and blue wavelengths. This light energy drives photosynthesis. Chlorophyll is located primarily in leaves, which is why leaves are green. The structure of the leaf (broad surface, networks of veins for transport) is adapted to maximize light absorption. Students often confuse chlorophyll (the pigment molecule) with chloroplast (the cellular structure), so clarity is vital.

Photosynthesis is the process by which plants make their own food (glucose) using carbon dioxide, water, and light energy. The balanced chemical equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂ (oxygen). This occurs in two main stages: the light-dependent reactions (in thylakoid membranes, producing ATP and NADPH) and the light-independent reactions or Calvin cycle (in the stroma, using ATP and NADPH to fix CO₂ into glucose). For LET purposes, emphasis is on the overall equation, the inputs and outputs, and that it requires light and occurs in chloroplasts. Photosynthesis is the origin of nearly all energy and oxygen on Earth.

Cellular respiration is the process by which all living cells release energy stored in glucose. It occurs in mitochondria and is essentially the reverse of photosynthesis: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). The process has three main stages: glycolysis (in cytoplasm), the Krebs cycle, and the electron transport chain (both in mitochondria). Aerobic respiration uses oxygen; anaerobic respiration (fermentation) does not and produces less ATP. All organisms—plants, animals, fungi, bacteria—respire continuously to power cellular functions. This process is the major source of ATP, the cell's energy currency.

These two processes are opposite in many ways: Photosynthesis stores energy (needs light, occurs in chloroplasts, uses CO₂ and water, produces glucose and O₂); respiration releases energy (occurs all the time, happens in mitochondria of all cells, uses glucose and O₂, produces CO₂ and water). Together, they drive the carbon and oxygen cycles. In plants, photosynthesis produces O₂ and glucose; respiration uses glucose and O₂. The balance between these two determines whether a plant grows or shrinks.

Flowering plants (angiosperms) reproduce sexually through the flower. The flower's male part is the stamen (which produces pollen in the anther), and the female part is the pistil (which contains the ovule in the ovary). Pollination is the transfer of pollen from stamen to pistil, mediated by insects (bees, butterflies), birds, wind, or water. Fertilization occurs when the pollen tube grows to the ovule and male gametes fuse with the female gamete. After fertilization, the ovule develops into a seed (containing an embryo and stored food wrapped in a seed coat), and the ovary ripens into a fruit that protects the seed. Seeds require adequate water, warmth, and air to germinate and grow into new plants.

Seeds are dispersed by multiple mechanisms: wind (light, winged seeds like mango and sampalok), water (coconuts and water lilies float), animals (fleshy fruits are eaten and seeds pass through digestive systems; burrs cling to fur), and ballistic dispersal (pods burst and eject seeds). This spreads the species and reduces competition among siblings. Many plants also reproduce asexually through vegetative propagation: runners (strawberries), cuttings (coleus), tubers (sweet potato, found widely in Philippine farms), bulbs (onions), and fragmentation. Asexual reproduction produces clones—genetically identical offspring—and is how farmers propagate crops such as sugarcane, banana, and cassava. This method is faster and requires no pollinator.

Animals are divided into two main groups: vertebrates, which possess a backbone (vertebral column) and an internal skeleton, and invertebrates, which lack a backbone. The five vertebrate classes commonly taught at the elementary level are: (1) Fish—gills for breathing, scales, live in water, cold-blooded (ectothermic); example: tilapia, lapu-lapu, bangus (milkfish). (2) Amphibians—moist, scaleless skin, live in water during larval stage and on land as adults, cold-blooded; example: frog, toad. (3) Reptiles—dry, scaly skin, lay eggs on land, cold-blooded; example: snake, turtle, crocodile, iguana. (4) Birds—feathers, wings, hollow bones, lay hard-shelled eggs, warm-blooded (endothermic); example: maya (sparrow), kalapati (pigeon), Philippine eagle. (5) Mammals—hair or fur, produce milk for young, have a diaphragm, warm-blooded; example: carabao, human, whale, bat. Invertebrates include insects, crustaceans, mollusks, worms, and others—they make up over 99% of animal species.

Taxonomy is the science of classifying and naming organisms. Carolus Linnaeus (18th century) is the father of modern taxonomy and developed binomial nomenclature, a two-part Latin naming system. The first part is the genus (capitalized); the second is the species (lowercase), and both are italicized or underlined. Example: Homo sapiens (humans), Panthera leo (lion), Canis familiaris (domestic dog), Oryza sativa (rice). The genus groups closely related species; the species is the smallest unit and typically consists of organisms that can interbreed and produce fertile offspring. The taxonomic hierarchy, from most general (broad) to most specific (narrow), is: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. A common mnemonic aid is 'Dear King Philip Came Over For Good Soup.' Modern classification uses six kingdoms: Animalia (animals), Plantae (plants), Fungi (fungi), Protista (protists), Archaebacteria, and Eubacteria (bacteria); older systems use five kingdoms.

Biodiversity (biological diversity) is the variety of living species in a region, measured by species richness (number of species) and genetic diversity. The Philippines is one of the world's most biodiverse countries and a global biodiversity hotspot, meaning it has a high percentage of endemic species—species found nowhere else on Earth. Endemic species in the Philippines include the Philippine eagle (monkey-eating eagle, the national bird), the tamaraw (a small buffalo found only in Mindoro), the Visayan spotted deer, and numerous species of orchids and ferns. The country's rich coral reefs (part of the Coral Triangle), tropical rainforests, and volcanic landscapes support this extraordinary diversity. However, this biodiversity is under severe threat from deforestation, habitat fragmentation, illegal wildlife trade, and climate change. Conservation of these species and their habitats is a national and global priority and is aligned with international agreements such as the Convention on Biological Diversity.

An ecosystem is a dynamic system consisting of a community of living organisms (biotic factors) and their non-living environment (abiotic factors). Biotic factors include all plants, animals, fungi, protists, and bacteria in the area and their interactions. Abiotic factors include sunlight, temperature, precipitation, humidity, soil composition, pH, oxygen, and carbon dioxide. Both types of factors together determine what organisms can survive in a given place. Related terms in a hierarchy of organization: organism (single individual) → population (all members of one species in an area) → community (all populations of different species together) → ecosystem (community plus abiotic environment) → biome (large region with similar climate and organisms) → biosphere (all life on Earth). The balance between biotic and abiotic factors determines ecosystem health and productivity.

Organisms are classified by their role in energy flow: Producers (autotrophs) are mostly green plants that capture sunlight and convert it into chemical energy (glucose) through photosynthesis. They form the base of every food chain and provide energy and matter for all other organisms. Consumers (heterotrophs) cannot make their own food and must eat other organisms. Primary consumers are herbivores that eat plants (grasshoppers, rabbits, carabao). Secondary consumers are carnivores that eat primary consumers (snakes, eagles). Tertiary consumers eat secondary consumers (hawks, sharks). Some animals eat both plants and animals and are omnivores (pigs, humans, bears). Decomposers (bacteria, fungi, some protists) break down dead organic matter and return nutrients to the soil, completing the cycle. Without decomposers, nutrients would be locked in dead bodies and waste, and ecosystems would fail. Each feeding level is a trophic level, and the number of trophic levels is usually 3–5.

A food chain is a simple linear sequence showing energy transfer from one organism to the next. Example: grass → grasshopper → frog → snake → hawk. Each organism eats the one before it and is eaten by the one after. Energy always flows in one direction—from producer to consumer. A food web is a realistic representation of an ecosystem, showing many interconnected food chains. In a food web, an organism may eat multiple other species (e.g., a frog eats insects and worms; a snake eats frogs and mice). Food webs are more accurate than food chains because they reflect the complexity of real ecosystems. A key principle is that energy is not recycled—it flows one way. Energy enters the ecosystem from the Sun, is captured by producers, is passed through consumers, and is eventually lost as heat when organisms respire or die. This unidirectional flow has major implications: food chains are short (usually 3–5 trophic levels), and there are always fewer top predators than plants.

The 10 percent rule states that only about 10% of the energy available at one trophic level is transferred to the next trophic level; the remaining 90% is lost as heat during respiration, movement, and growth. This means that if plants capture 100,000 units of energy from the Sun, herbivores obtain about 10,000 units, carnivores about 1,000 units, and top carnivores about 100 units. An energy pyramid is a graphical representation of energy at each trophic level, with the base (producers) being the largest and each successive level smaller. This pyramid shape shows visually why there are many plants, fewer herbivores, and even fewer top predators. The 10 percent rule explains why it is more efficient for humans to eat plants than to eat meat (meat production loses much energy), and why there is a practical limit to the number of trophic levels in an ecosystem. If a food chain were too long, not enough energy would reach the top consumer to sustain a population.

The water cycle is the continuous circulation of water between the Earth's surface and the atmosphere. Key stages: (1) Evaporation—water from oceans, lakes, rivers, and soil surfaces turns into water vapor due to heat from the Sun. (2) Transpiration—plants release water vapor through their leaves (stomata). Evaporation and transpiration together are often called evapotranspiration. (3) Condensation—water vapor cools in the upper atmosphere and turns into liquid water droplets, forming clouds. (4) Precipitation—water falls to Earth as rain, snow, sleet, or hail. (5) Collection (infiltration and runoff)—water collects in oceans, lakes, aquifers, and groundwater. This cycle is powered by the Sun's energy and is essential for distributing fresh water across the planet. The cycle has no beginning or end; it is continuous. In the Philippines, the water cycle is visible through seasonal monsoons (southwest and northeast monsoons) that bring rain and influence agriculture.

The carbon cycle describes how carbon moves between the atmosphere (as CO₂), the biosphere (living things), and the geosphere (rocks, soil, fossil fuels). Key processes: (1) Photosynthesis—plants remove CO₂ from the air and convert it into organic compounds (glucose). (2) Respiration—plants, animals, and decomposers release CO₂ back into the atmosphere by breaking down glucose. (3) Combustion—burning of fossil fuels (coal, oil, natural gas) and wood releases stored carbon as CO₂. (4) Decomposition—dead organisms are broken down by decomposers, releasing CO₂. (5) Fossilization—over millions of years, dead organisms may become fossil fuels. The carbon cycle links directly to climate because CO₂ is a greenhouse gas: excess CO₂ in the atmosphere traps heat and causes global warming. Photosynthesis removes CO₂; respiration, combustion, and decay return it. The balance determines atmospheric CO₂ levels. Human activities—especially burning fossil fuels and deforestation—have increased atmospheric CO₂ by about 50% since the Industrial Revolution, causing climate change.

The oxygen cycle describes how oxygen moves between the atmosphere, the biosphere, and the geosphere. Key producers of O₂: (1) Photosynthesis by plants, algae, and cyanobacteria releases O₂ as a byproduct. (2) Photolysis of water by these organisms during the light reactions also releases O₂. Consumers of O₂: (1) Aerobic respiration in all organisms (plants, animals, fungi, aerobic bacteria) uses O₂ to break down glucose. (2) Combustion (burning) consumes O₂. (3) Weathering and oxidation of rocks and minerals can consume O₂. The oxygen cycle is intimately linked to the carbon cycle: photosynthesis produces O₂ and consumes CO₂; respiration produces CO₂ and consumes O₂. Photosynthetic organisms are the sole source of atmospheric O₂, which nearly all life depends on. The oxygen cycle is in rough balance under natural conditions, but disruptions in photosynthesis (e.g., deforestation) can lower oxygen availability.

The nitrogen cycle describes how nitrogen moves between the atmosphere, the biosphere, and the soil. Nitrogen gas (N₂) makes up about 78% of the atmosphere, but most organisms cannot use it directly; it must be 'fixed' into usable forms. Key processes: (1) Nitrogen fixation—certain bacteria (free-living bacteria in soil and water, and symbiotic bacteria in root nodules of legumes such as beans and peas) convert atmospheric N₂ into ammonia (NH₃) and nitrates (NO₃⁻) that plants can absorb. (2) Assimilation—plants take up nitrates and use them to make amino acids and proteins. Animals eat plants and incorporate nitrogen into their own proteins. (3) Decomposition—when organisms die, decomposers break down proteins and release ammonia into the soil. (4) Nitrification—bacteria convert ammonia to nitrites and then nitrates, which plants can reuse. (5) Denitrification—some bacteria convert nitrates back to N₂, returning it to the atmosphere. Without nitrogen-fixing bacteria, life on land would be impossible because plants would have no source of usable nitrogen, and all amino acids and proteins would vanish. Legumes are grown in crop rotation specifically to replenish soil nitrogen through their symbiotic bacteria.

Symbiosis is a close, long-term relationship between two different species. The three main types are distinguished by the benefit or harm to each partner. (1) Mutualism (+/+): both species benefit. Example: bees and flowering plants (bees get nectar for food; flowers get pollinated). Another example: clownfish and sea anemone (clownfish get shelter; anemone gets food scraps and protection from predators). In the Philippines, this includes the relationship between farmers and nitrogen-fixing bacteria in legumes. (2) Commensalism (+/0): one species benefits, the other is unaffected. Example: orchids or ferns growing on tree trunks (they get light and moisture; the tree is unharmed). Another example: egrets feeding near grazing carabao (egrets eat insects stirred up by the carabao; carabao is unaffected). (3) Parasitism (+/-): one species (parasite) benefits, the other (host) is harmed. Example: ticks on a dog (tick feeds on blood; dog loses blood and may get disease). Another example: tapeworm in the intestine of a human or pig (tapeworm gets nutrition; host is malnourished). Other important relationships include predation (predator eats prey; +/-) and competition (two species compete for resources; usually -/-). These relationships structure communities and affect ecosystem stability.

Predation is a relationship in which one organism (predator) hunts and kills another (prey) for food. It is a +/- relationship (predator benefits, prey is harmed or killed). Examples: snakes eating rats, eagles catching fish, spiders catching insects. Predation controls prey populations and maintains ecological balance; without predators, prey populations can explode and overgraze vegetation. Competition is a relationship in which two or more organisms use the same limited resource (food, water, light, space). If resources are insufficient, one or both competitors may be harmed (usually -/- relationship), or one may outcompete the other and exclude it from the area. Examples: two plants competing for sunlight, two birds competing for nesting sites, herbivores competing for grass. Competitive exclusion principle states that two species cannot coexist indefinitely if they compete for the same resource; one will eventually exclude the other unless they partition the resource (eat different foods, occupy different spaces, etc.). These relationships shape community structure and drive natural selection.

An adaptation is an inherited trait that helps an organism survive and reproduce in its environment. Adaptations are the result of natural selection—the process by which organisms with traits suited to their environment tend to survive and pass those traits to offspring. Examples of adaptations: a cactus's thick, fleshy stem stores water in deserts; a fish's gills extract oxygen from water; thick fur insulates animals in cold climates; a hawk's keen eyesight helps it hunt; a plant's deep roots reach underground water. Adaptations can be structural (body shape, organs), physiological (internal processes, color for camouflage), or behavioral (migration, hibernation, courtship rituals). The Philippine context includes many adaptations: the Philippine eagle's powerful talons and keen eyesight for catching monkeys, the carabao's ability to wallow in mud to cool down, and Philippine rainforest plants' adaptations to high humidity and shade. Adaptations accumulate over many generations through natural selection, not purposefully designed.

Ecological succession is the gradual change in species composition of a community over time. Primary succession occurs when a new habitat (e.g., bare rock, lava flow, abandoned quarry) is colonized by plants and animals, starting with pioneer species (hardy, fast-growing organisms like lichens and grasses) that can tolerate harsh conditions. Over time, pioneers modify the environment (e.g., by adding organic matter to soil), making it suitable for other species. Gradually, more species move in, soil develops, competition increases, and the community becomes more diverse and stable. This process can take centuries. Secondary succession occurs when an existing ecosystem is disturbed (e.g., by fire, logging, farming) but not completely destroyed. Because soil and seeds remain, recovery is faster than primary succession, typically decades. The final, stable community reached at the end of succession is called the climax community. Succession demonstrates that ecosystems are dynamic and resilient but can recover slowly, especially if damage is severe or soil is removed.

Ecological balance refers to the stable state of an ecosystem in which populations of different species are relatively stable and the ecosystem is productive and resilient. This balance is maintained by numerous interconnections: food webs link species through feeding relationships; nutrient cycles connect organisms to the physical environment; predation and competition regulate populations; and mutualistic relationships sustain species. If one species is removed or added, effects can cascade through the entire ecosystem. Example: if a top predator (like sharks) is removed, herbivore populations may explode, vegetation may be overgrazed, and the whole ecosystem may degrade. Similarly, introducing an invasive species (one with no natural predators in the new area) can crowd out native species and disrupt balance. In the Philippines, invasive species such as the janitor fish and golden apple snail in freshwater systems and tilapia in native fish habitats have harmed endemic species. This interconnectedness is why conservation of biodiversity is essential—the loss of one species affects many others and ultimately affects humans.

Climate change refers to long-term shifts in global temperature and weather patterns, primarily caused by human activity. The main driver is the build-up of greenhouse gases, especially carbon dioxide (CO₂) from burning fossil fuels (coal, oil, natural gas), deforestation, and agriculture. Other greenhouse gases include methane (CH₄) from livestock and rice paddies, nitrous oxide (N₂O) from fertilizers, and fluorinated gases (from air conditioning and refrigeration). These gases trap heat in the atmosphere (like a blanket), preventing it from escaping to space, causing global warming. The consequences include rising sea levels (from melting ice and thermal expansion), more intense storms, droughts, floods, ecosystem disruption, and threats to human health and food security. The Philippines is highly vulnerable: as an archipelagic nation with low-lying islands, it faces rising seas; as an agricultural nation, it is exposed to drought and flooding; and as a megadiverse nation, it faces biodiversity loss. Mitigation strategies include reducing emissions (renewable energy, energy efficiency), protecting forests (which absorb CO₂), and adapting to impacts (building resilience, disaster preparedness). The LET expects teachers to understand the basic science, recognize human causes, and know local impacts.

Deforestation is the large-scale clearing of forests for timber, agriculture, urban development, or infrastructure. The consequences are severe: (1) Biodiversity loss—forests, especially tropical rainforests like the Philippine rainforests, harbor the majority of Earth's species; removing forests drives species to extinction. Endemic species in the Philippines are at particular risk. (2) Carbon release—forests store enormous amounts of carbon; when burned or left to decay, this carbon is released as CO₂, accelerating climate change. (3) Soil erosion—tree roots hold soil in place; without trees, soil washes away, reducing fertility and causing sedimentation in rivers and coasts. (4) Watershed disruption—forests regulate water flow; deforestation increases flooding and drought. (5) Climate disruption—forests produce oxygen and absorb CO₂; loss of forests exacerbates climate change. (6) Indigenous and local livelihoods—many communities depend on forests for food, medicine, and livelihood; deforestation displaces them. In the Philippines, deforestation has reduced forest cover from about 70% of land area in the 1950s to about 20% today, driving endemic species toward extinction and causing severe environmental degradation.

Pollution is the introduction of harmful substances or energy into the environment. Major types: (1) Air pollution—from vehicle exhaust, industrial emissions, burning of coal and biomass, and open dumpsites; causes respiratory disease, acid rain, and ozone depletion. (2) Water pollution—from industrial discharge, agricultural runoff (fertilizers and pesticides), sewage, and plastic waste; harms aquatic life and makes water unsafe for drinking and agriculture. (3) Land pollution—from solid waste, hazardous waste, pesticides, and heavy metals; contaminates soil and groundwater. (4) Noise and light pollution—from traffic, industry, and urban lights; disrupt wildlife behavior. Plastic pollution is a growing concern: plastics persist for decades, break into microplastics that enter food chains, and have been found in all environments from oceans to mountains. The Philippines is a major source of ocean plastic pollution, partly due to poor waste management. Pollution harms human health (respiratory disease, cancer, poisoning), damages ecosystems, and reduces quality of life. Sources include industry, vehicles, agriculture, and household waste. Prevention and reduction require regulation, technology, and behavior change.

Biodiversity loss is the decline in the variety of species and genetic diversity in ecosystems and across the planet. Causes include habitat destruction (deforestation, urban sprawl, dam building), overexploitation (overfishing, hunting), pollution, invasive species, and climate change. Species are lost at rates far exceeding natural background rates, and scientists speak of a 'sixth mass extinction' caused by human activity. Extinction is irreversible; once a species is gone, it is gone forever, taking with it unique genes, evolutionary history, and ecological roles. The Philippines, as a megadiverse nation, is losing species rapidly. Endemic species are at highest risk because they are found only in the Philippines and nowhere else; if they are lost here, they are lost globally. Examples of Philippine species at risk include the Philippine eagle, tamaraw, and Visayan spotted deer. Biodiversity loss has cascading effects: loss of pollinators affects food crops; loss of predators allows prey to overgraze; loss of decomposers disrupts nutrient cycling. Humans also benefit from biodiversity: medicines come from plants and animals; ecosystem services (water purification, climate regulation, pollination) depend on diverse species; and food security depends on maintaining genetic diversity in crops and livestock.

The ozone layer is a region of the stratosphere (10–50 km altitude) where ozone (O₃) gas is concentrated. Ozone absorbs ultraviolet (UV) radiation from the Sun, protecting life on Earth from harmful UV-B and UV-C radiation that can cause skin cancer, cataracts, immune suppression, and damage to phytoplankton and crops. In the 1970s, scientists discovered that the ozone layer was thinning, especially over Antarctica, due to chemicals called chlorofluorocarbons (CFCs) used in air conditioning, refrigeration, foam production, and aerosols. CFCs release chlorine atoms in the stratosphere, and chlorine catalytically destroys ozone molecules. The 'ozone hole' over Antarctica forms seasonally and has grown. International response was swift: the 1987 Montreal Protocol phased out CFCs and other ozone-depleting substances, and the ozone layer is slowly recovering. However, the recovery will take decades. The Philippines, lying near the equator, receives high UV radiation; ozone depletion increases risks of skin cancer and cataracts, especially in outdoor workers and in children who spend long hours outdoors. Sunscreen use and protective clothing are recommended.

The 3Rs are a hierarchy of actions to manage waste and reduce environmental impact: (1) Reduce—buy and use less stuff, reducing the amount of waste generated and the resources consumed. (2) Reuse—use items multiple times in their original form (e.g., glass jars, cloth bags, old clothes) or repurpose them for new uses, extending their life and reducing waste. (3) Recycle—collect used materials and process them into new materials (e.g., plastic bottles into new plastic products, paper into cardboard, glass into new glass). Recycling is energy-intensive and imperfect—some materials are 'downcycled' into lower-grade products—so it is less preferable than reduce and reuse. The Philippines has a waste segregation program under the Ecological Solid Waste Management Act (ESWMA), which sorts waste into biodegradable, recyclable, and residual categories. However, waste management infrastructure is often inadequate, and much waste ends up in landfills or oceans. Teachers can model the 3Rs in classrooms and help students develop environmental responsibility. Other waste management strategies include composting (of food and garden waste), industrial symbiosis (where one industry's waste is another's raw material), and extended producer responsibility (holding manufacturers accountable for their products' end-of-life).