Organelle in plant and algal cells where photosynthesis happens; contains chlorophyll to absorb light.

Mitochondrion (plural: mitochondria)

Organelle in most eukaryotic cells where cellular respiration releases energy from glucose to make ATP.

An organism (usually using photosynthesis) that makes its own food from simple substances; entry point of energy into most food chains.

Adenosine triphosphate; a small molecule that stores and transfers energy within cells.

An organism that obtains energy by eating other organisms or their remains.

A network of interconnected food chains showing all feeding relationships in an ecosystem.

Energy in Living Things: Photosynthesis, Respiration, and Food Chains — SHS Science Foundations: Integrated Chemistry, Biology, and Physics

Step 1: What Does “Energy in Living Things” Mean?

Energy in Living Things

In biology, energy usually means chemical energy stored in bonds of molecules like glucose and fats. Living things cannot create energy; they transform it from one form to another.

ATP: The Cell Battery

Cells use a small molecule called ATP as a kind of rechargeable battery. Making ATP stores energy; breaking ATP releases small packets of energy to power cell work.

Big Picture Flow

Sunlight → plants and microbes (photosynthesis) → chemical energy in glucose → all organisms (cellular respiration) → ATP → activities like moving muscles. Food chains show this energy passing along.

Step 2: Photosynthesis – Capturing Sunlight

Where Photosynthesis Happens

In plants, photosynthesis happens in chloroplasts, mainly in leaf cells. Chloroplasts contain green chlorophyll, which absorbs light energy from the sun.

Photosynthesis Word Equation

Basic word equation: carbon dioxide + water + light energy → glucose + oxygen. Plants use light to turn CO₂ and water into sugar (glucose) and release oxygen.

Why It Matters

Photosynthesis is the main way energy from the sun enters living systems and it produces most of the oxygen that animals, plants, and many microbes use for respiration.

Step 3: Trace a Photosynthesis Scenario

Use this thought exercise to make photosynthesis concrete.

Imagine a sunny day after rain:

A tree’s roots are sitting in wet soil. Its leaves are spread out in the sunlight.
Air around the leaves contains nitrogen, oxygen, and a small amount of carbon dioxide.
Sunlight hits the leaves.

Your task:

In your own words, list three inputs and two outputs of photosynthesis in this tree.
For each item, say where it comes from or goes:

Example: "Water – taken up by roots from the soil."

Decide which of the outputs you personally depend on most and explain why in 1–2 sentences.

Optional quick check (mentally or in notes):

Inputs should include: carbon dioxide (air), water (soil), light energy (sun).
Outputs should include: glucose (stored in plant), oxygen (released to air).

Step 4: Cellular Respiration – Releasing Energy From Food

What Is Cellular Respiration?

Cellular respiration is how cells release energy from glucose in a controlled way. Most of it happens in mitochondria, often called the cell’s powerhouses.

Respiration Word Equation

Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy (ATP). Glucose and oxygen are used; carbon dioxide, water, and usable energy are released.

Respiration in Your Muscles

When you move, your muscles use respiration. Blood delivers glucose and oxygen; mitochondria make ATP, which powers each muscle contraction and keeps you going.

Step 5: Compare Photosynthesis and Respiration

Check that you can distinguish the two key processes.

Which statement best describes the relationship between photosynthesis and aerobic respiration in plants?

Photosynthesis and respiration are the same process happening in chloroplasts.
Photosynthesis makes glucose and oxygen in chloroplasts; respiration uses glucose and oxygen in mitochondria to release energy.
Respiration makes glucose and oxygen in mitochondria; photosynthesis uses them in chloroplasts to release energy.
Plants only do photosynthesis, not respiration.

Show Answer

Answer: B) Photosynthesis makes glucose and oxygen in chloroplasts; respiration uses glucose and oxygen in mitochondria to release energy.

In plants, **photosynthesis** in chloroplasts uses light, carbon dioxide, and water to make **glucose and oxygen**. **Respiration** in mitochondria then uses glucose and oxygen to release energy (ATP), producing carbon dioxide and water.

Step 6: Producers, Consumers, and Decomposers

Producers

Producers (autotrophs) make their own food, usually by photosynthesis. Plants, algae, and many bacteria capture light and store its energy in glucose.

Consumers

Consumers (heterotrophs) must eat other organisms. Herbivores eat plants, carnivores eat animals, and omnivores eat both to obtain energy-rich molecules.

Decomposers

Decomposers like bacteria and fungi break down dead organisms and wastes, releasing nutrients back to the environment and also using respiration for energy.

Step 7: Building a Food Chain and Food Web

Food Chain Example

Grassland chain: Sun → grass (producer) → grasshopper (primary consumer) → frog (secondary) → snake (tertiary) → hawk (top) → decomposers. Energy moves in one direction.

Food Web Example

In a pond, algae and plants feed many herbivores, which feed several predators. Because organisms eat more than one thing, their feeding links form a complex food web.

Respiration in Every Link

At each step in a food chain or web, organisms use cellular respiration to convert chemical energy from food into ATP to power their life processes.

Step 8: Draw and Label Your Own Food Chain

Create a simple local food chain to connect these ideas.

Instructions:

Choose a real environment near you (for example: school field, garden, park, river, or coastline).
Identify at least one producer, two levels of consumers, and decomposers that could live there.
On paper or in a notes app, draw arrows:

From sun to the producer.
From producer to primary consumer.
From primary to secondary (and maybe tertiary) consumer.
From each organism to decomposers.

Next to each organism, write:

P (producer), C1 (primary consumer), C2 (secondary consumer), etc., or D (decomposer).
Whether it uses photosynthesis, respiration, or both.

Quick reflection:

Which organism in your chain performs photosynthesis?
Which organisms perform cellular respiration?

Step 9: Energy Flow and Energy Loss

Energy Loss at Each Step

When one organism eats another, only some energy becomes new body tissue. Much is lost as heat during respiration or remains in uneaten parts that decomposers later break down.

Energy Pyramid

An energy pyramid has a wide base of producers and a narrow top of predators. At each step up the food chain, there is less usable energy available.

Energy vs Matter

Atoms and nutrients are recycled by decomposers, but energy flows one way through the ecosystem and is eventually lost as heat to the surroundings.

Step 10: Quick Energy Flow Check

Test your understanding of energy loss in food chains.

Why do top predators (like hawks or sharks) usually have much smaller populations than producers?

They do not use cellular respiration.
They receive less energy because energy is lost at each step of the food chain.
They do not have decomposers.
They perform photosynthesis instead of eating.