12.3 Ecology and Life Processes

Life processes keep organisms alive; ecology explains how organisms interact with each other and their environment. On USTET Science, expect equation-level photosynthesis/respiration questions plus food-web and energy-flow reasoning from Grade 11 Earth and Life Science.

Photosynthesis: Capturing Energy

Photosynthesis is the process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored in sugars.

Overall equation (memorize this form):

$6\mathrm{CO_2} + 6\mathrm{H_2O} \xrightarrow{\text{light, chlorophyll}} \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}$

Two Linked Stages

1. Light-dependent reactions (thylakoid membranes) — light energy splits water, releases oxygen, and forms ATP and NADPH.
2. Light-independent reactions / Calvin cycle (stroma) — ATP and NADPH help fix CO₂ into sugar.

You usually do not need enzyme names, but you should know that oxygen released in photosynthesis comes from water, and that sugar production depends on both light reactions and carbon fixation.

Where It Happens

In eukaryotic plants, photosynthesis occurs in chloroplasts. Leaves are specialized for gas exchange through stomata. Factors that affect rate include light intensity, CO₂ concentration, temperature, and water availability. Beyond an optimum, rates level off or fall (for example, enzymes denature at high temperature).

Cellular Respiration: Releasing Energy

Organisms cannot live on trapped sunlight alone; they must convert food energy into ATP.

Aerobic respiration overall:

$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{ATP (energy)}$

Notice that respiration is roughly the reverse of photosynthesis in terms of reactants and products, though the pathways and organelles differ.

Major stages of aerobic respiration:

1. Glycolysis (cytoplasm) — glucose → pyruvate; small ATP yield.
2. Krebs / citric acid cycle (mitochondrial matrix) — completes oxidation; produces CO₂.
3. Electron transport chain (inner mitochondrial membrane) — largest ATP yield; oxygen is the final electron acceptor.

Without oxygen, many cells use anaerobic pathways (fermentation) with much less ATP and products such as lactic acid or ethanol and CO₂, depending on the organism.

Exam tip: Photosynthesis stores energy; respiration releases usable ATP. Plants do both. Animals respire but do not photosynthesize.

Ecology Basics: Levels of Organization

Biotic factors are living or once-living influences: predators, prey, competitors, pathogens, decomposers. Abiotic factors are nonliving: sunlight, temperature, rainfall, soil pH, salinity, oxygen levels. Ecosystem questions often ask you to classify a factor as biotic or abiotic.

Food Chains, Food Webs, and Trophic Levels

A food chain is a linear sequence of who eats whom. A food web is an interconnected set of food chains and better reflects real ecosystems.

Energy Flow Rules

• Energy flows one way: sun → producers → consumers → (eventually) heat loss.
• At each transfer, only about 10% of energy typically moves to the next trophic level; the rest is lost as heat, used in metabolism, or left in uneaten/undigested material.
• That is why food chains are short and why biomass pyramids usually narrow toward the top.

If a question asks why there are fewer top predators than herbivores, energy loss between trophic levels is the standard explanation.

Matter Cycles While Energy Flows

Unlike energy, matter is recycled.

• Carbon cycle: photosynthesis removes atmospheric CO₂; respiration and combustion return CO₂; decomposition releases carbon from dead matter.
• Nitrogen cycle: atmospheric N₂ is fixed into usable forms by certain bacteria; plants absorb nitrates; consumers obtain nitrogen from food; denitrifying bacteria can return N₂ to air.
• Water cycle: evaporation, condensation, precipitation, runoff, and transpiration move water through ecosystems.

Decomposers are essential: without them, nutrients stay locked in corpses and wastes, and producer growth collapses.

Ecological Relationships

Symbiosis is a close long-term association; mutualism, commensalism, and parasitism are common categories tested by definition.

Human Impact and Philippine Context (Exam-Aware)

Entrance-exam ecology often links processes to environmental issues:

• Deforestation reduces photosynthetic carbon uptake and destroys habitats.
• Overfishing removes high trophic levels and destabilizes food webs.
• Pollution (including excess nutrients) can cause algal blooms and oxygen crashes that kill aquatic consumers.
• Climate change alters abiotic conditions and shifts species ranges.

You do not need local case-study memorization for every item, but you should connect cause → ecosystem effect using trophic and cycle language.

Integrating Life Processes with Ecology

Producers power ecosystems through photosynthesis. Consumers and producers alike depend on cellular respiration for ATP. Food webs map how chemical energy in organic matter moves between organisms, while decomposers return inorganic nutrients so producers can keep building new biomass. If you can write the photosynthesis and respiration equations, assign trophic roles in a short food chain, and explain the roughly 10% energy transfer rule, you are prepared for the core ecology and life-process questions on USTET Science.