How Plants Feed Themselves (Without a Mouth)
Every living thing needs food to survive — but plants figured out something remarkable. They don’t chase food or eat other organisms. They make their own food using sunlight, water, and a gas from the air. This is the central story of plant nutrition.
We call organisms that make their own food autotrophs (auto = self, troph = feed). Plants are the most important autotrophs on Earth — they sit at the base of every food chain. Animals (including us) are heterotrophs: we depend on other organisms for our food.
But here’s what makes plant nutrition genuinely interesting: the process plants use — photosynthesis — also produces the oxygen we breathe. So when you understand plant nutrition, you understand why life on Earth is even possible.
There are two main modes of nutrition in plants. Most plants photosynthesize. But a few have evolved special tricks — some are parasites, some trap insects, some partner with fungi. Class 7 covers all of these, and the CBSE board loves asking about the unusual ones.
Key Terms and Definitions
Autotrophic nutrition — Making food from simple inorganic substances (CO₂, water, minerals) using an energy source like sunlight. All green plants follow this mode.
Heterotrophic nutrition — Getting food by depending on other organisms. Fungi, animals, and some parasitic plants are heterotrophs.
Photosynthesis — The process by which green plants use sunlight to convert carbon dioxide and water into glucose and oxygen. Happens inside chloroplasts.
Chlorophyll — The green pigment in plant leaves that captures sunlight. Without chlorophyll, photosynthesis cannot happen — which is why non-green parts of a plant (like roots) cannot photosynthesize.
Stomata (singular: stoma) — Tiny pores on leaf surfaces that allow CO₂ to enter and O₂ to exit. Guard cells control whether stomata are open or closed.
Chloroplast — The organelle inside leaf cells where photosynthesis actually occurs. Think of it as the plant’s food factory.
Parasitic nutrition — A plant getting nutrition by living on or inside another plant (the host) and absorbing nutrients from it. Cuscuta (dodder plant) is the classic CBSE example.
Saprophytic nutrition — Breaking down dead organic matter for food. Fungi like mushrooms and bread mould use this.
Insectivorous nutrition — Some plants grow in nitrogen-poor soil and supplement their mineral diet by trapping and digesting insects. Venus flytrap and Pitcher plant are the textbook examples.
The Core Process: Photosynthesis
What Goes In, What Comes Out
Carbon dioxide + Water → Glucose + Oxygen
Read this equation carefully. The raw materials are carbon dioxide (from air, enters through stomata) and water (from soil, absorbed by roots). Sunlight provides the energy. Chlorophyll is not consumed — it acts as a catalyst. The products are glucose (food for the plant) and oxygen (released as a by-product).
Where Each Raw Material Comes From
| Raw Material | Source | How It Reaches the Leaf |
|---|---|---|
| Carbon dioxide | Atmosphere | Enters through stomata |
| Water | Soil | Absorbed by roots, transported through stem via xylem |
| Sunlight | Sun | Absorbed by chlorophyll in leaves |
The Role of Leaves
Leaves are shaped to maximise photosynthesis. They are broad and flat (large surface area to catch sunlight), thin (so CO₂ doesn’t have to travel far to reach cells), and arranged so they don’t shade each other much.
The upper surface of a leaf has more chloroplast-containing cells. The lower surface has more stomata. This is not random — it’s the plant’s efficient design.
When a CBSE question asks “why are leaves flat and thin?” — the answer has two parts: large surface area for sunlight capture, and short diffusion distance for CO₂.
Showing That Chlorophyll Is Needed
The classic experiment uses a variegated leaf — one that has green and white (non-green) patches. After exposing the plant to sunlight, you test the entire leaf for starch using iodine solution. Only the green parts turn blue-black (starch present). The white parts don’t — because they have no chlorophyll, they couldn’t photosynthesize.
This is a favourite CBSE practical question. Know the logic: no chlorophyll → no photosynthesis → no starch → no blue-black colour with iodine.
Other Modes of Nutrition in Plants
Parasitic Plants: Cuscuta (Amarbel)
Cuscuta is a yellow-orange twining plant you’ve probably seen wrapped around hedges or crop plants. It has almost no chlorophyll — so it cannot photosynthesize. Instead, it sends haustoria (thread-like structures that penetrate the host plant’s tissues) and directly absorbs water, minerals, and food from the host.
The host is harmed; Cuscuta benefits. This is a true parasitic relationship. In agriculture, Cuscuta is a serious weed problem — it attacks crops like pulses and tomatoes.
CBSE Class 7 frequently asks: “Name a parasitic plant and explain how it gets its nutrition.” Always name Cuscuta, mention haustoria, and state it is yellow-orange with no chlorophyll.
Saprophytic Plants: Fungi
Fungi (mushrooms, bread mould, Rhizopus) cannot photosynthesize. They secrete digestive enzymes onto dead organic matter — a rotting log, old bread, dead leaves — break it down externally, and then absorb the nutrients. This is called saprotrophic or saprophytic nutrition.
This is why fungi appear on damp, dead organic material. They are nature’s decomposers, recycling nutrients back into the soil.
Insectivorous Plants
These grow in swampy, nitrogen-deficient soils. They get their carbon through photosynthesis (they are still autotrophs for carbon) but supplement their nitrogen intake by trapping insects.
- Pitcher plant (Nepenthes): Has a modified leaf shaped like a pitcher, filled with digestive fluid. Insects fall in and are digested.
- Venus flytrap: Has hinged leaf-lobes that snap shut when trigger hairs are touched twice within 20 seconds (this prevents the plant from wasting energy on raindrops).
- Sundew (Drosera): Has sticky glandular hairs that trap insects.
Symbiotic Nutrition: Lichens
Lichen is a partnership between a fungus and an alga. The alga photosynthesizes and provides food. The fungus provides shelter, water retention, and minerals. Both benefit — this is mutualism. Lichens can grow on bare rocks where neither organism could survive alone.
Leguminous plants (beans, peas, groundnut) have Rhizobium bacteria in their root nodules. The bacteria fix atmospheric nitrogen into usable form for the plant; the plant provides food for the bacteria. Another classic mutualism example.
Solved Examples
Example 1 — Easy (CBSE Conceptual)
Q: Why do plants need nitrogen? They already make glucose through photosynthesis.
Solution: Glucose gives plants carbon, hydrogen, and oxygen — enough to make carbohydrates. But proteins and nucleic acids (DNA) also need nitrogen. Nitrogen is essential for building amino acids, which make proteins. Plants absorb nitrogen from soil in the form of nitrates and ammonium ions. That’s why nitrogen-deficient plants look stunted and pale — they can’t make enough proteins for growth.
Example 2 — Medium (CBSE Experiment-Based)
Q: A student kept a plant in a dark room for 48 hours, then covered one leaf with black paper (leaving the rest of the leaf exposed), and placed the plant in sunlight for 6 hours. She then tested the covered and uncovered parts with iodine. What results would she observe, and why?
Solution:
The 48-hour dark period destarchs the plant — it uses up all stored starch so we start with a clean slate. This is why this step is essential; if we skip it, the leaf already has starch from before the experiment.
After 6 hours of sunlight:
- Uncovered part: exposed to sunlight → photosynthesis occurs → starch is produced → iodine turns blue-black
- Covered part: no sunlight → no photosynthesis → no starch → iodine remains yellow-brown
This proves that sunlight is necessary for photosynthesis.
Students forget to explain the destarching step and lose marks. Always mention: “The plant was kept in darkness first to remove pre-existing starch, so results are not affected by previously stored starch.”
Example 3 — Hard (CBSE Application / HOTS)
Q: A student notices that the leaves of a plant placed near a window always turn towards the light, and the variegated leaves on the same plant show more growth on the green side than the white side. Give reasons for both observations.
Solution:
Observation 1 (leaves turning towards light): This is called phototropism. The plant hormone auxin accumulates on the shaded side of the stem, causing those cells to elongate more than the lit side. This differential growth bends the plant towards light — it maximises surface area for photosynthesis.
Observation 2 (unequal growth in variegated leaf): The green portions contain chlorophyll and can photosynthesize, producing glucose for energy and growth. White portions lack chlorophyll — they cannot produce food. Since the white cells receive less energy and organic molecules, cell division and growth are slower there. The leaf grows unevenly as a result.
Both observations point to the same underlying fact: photosynthesis drives plant growth.
Exam-Specific Tips
CBSE Class 7 Marking Patterns
- “What is photosynthesis?” — Write the definition + the balanced equation + name the site (chloroplast). 3-mark question typically.
- Experiments on photosynthesis (light, CO₂, chlorophyll necessity) — Know the procedure, observation, AND conclusion for each. 5-mark questions follow this structure.
- Distinguish between autotrophic and heterotrophic — Always give examples. Writing only the definition gets you 1/2 marks.
- Cuscuta is asked almost every year. Know: it’s a parasite, has haustoria, is yellow-orange, attacks crop plants.
For diagram questions: The cross-section of a leaf is a frequent diagram. Label: upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, stomata, guard cells. Practice drawing clean cross-sections.
For experiments: The three photosynthesis experiments (proving light is needed, CO₂ is needed, chlorophyll is needed) each have a specific experimental setup. Know which variable is being tested in each.
Common Mistakes to Avoid
Mistake 1: Saying plants get food from the soil
Roots absorb minerals and water from soil — not food. Food (glucose) is made in the leaves during photosynthesis. This confusion causes wrong answers in “how do plants get their nutrition” questions.
Mistake 2: Forgetting CO₂ enters through stomata, not through roots
Carbon dioxide is a gas. It enters leaves through stomata. Roots absorb water and dissolved minerals — not gases. Don’t mix these up.
Mistake 3: Saying oxygen is a raw material of photosynthesis
Oxygen is a product, not a raw material. The raw materials are CO₂ and water. Writing O₂ as an input will cost you marks.
Mistake 4: Calling fungi “autotrophs” because they don’t eat like animals
Fungi are heterotrophs — specifically saprotrophs. They don’t photosynthesize. They absorb pre-formed organic molecules from dead matter. Not an autotroph just because it doesn’t “eat” visibly.
Mistake 5: Saying insectivorous plants are not autotrophs
Insectivorous plants like pitcher plant are autotrophs — they still photosynthesize. They only trap insects to get nitrogen, which they can’t get from their poor soil. They don’t substitute insects for photosynthesis.
Practice Questions
Q1. Name the raw materials needed for photosynthesis.
Carbon dioxide (from air, enters through stomata), water (absorbed from soil through roots), and sunlight (captured by chlorophyll). Chlorophyll is not a raw material — it is a pigment that captures light energy.
Q2. What is the role of stomata in photosynthesis?
Stomata are tiny pores on the leaf surface (mainly the lower surface) that allow CO₂ to enter the leaf from the air. They also allow O₂ produced during photosynthesis to exit. Guard cells control the opening and closing of stomata — they open during the day (when photosynthesis occurs) and close at night.
Q3. Distinguish between autotrophic and heterotrophic nutrition with one example each.
Autotrophic nutrition: The organism synthesises its own food from simple inorganic substances using energy (usually sunlight). Example: green plants.
Heterotrophic nutrition: The organism cannot make its own food and depends on other organisms (directly or indirectly) for nutrition. Example: fungi, animals, Cuscuta.
Q4. Cuscuta is called a total parasite. Why?
Cuscuta has almost no chlorophyll, so it cannot photosynthesize at all. It gets 100% of its nutrition — water, minerals, and organic food — from its host plant through haustoria (specialised absorbing organs). Since it is completely dependent on the host for all nutrition, it is called a total parasite.
Q5. A plant was kept in a dark room for two days, and then a leaf was tested with iodine. The iodine remained yellow-brown. Explain.
In the dark, the plant cannot photosynthesize because there is no light. Without photosynthesis, no new starch is produced. Whatever starch was previously stored gets used up by the plant for its own energy needs during the dark period. So when tested with iodine, no starch is present — the iodine stays yellow-brown (negative test). This also explains why we keep plants in darkness before photosynthesis experiments: to remove pre-existing starch (destarching).
Q6. Why do pitcher plants trap insects? Are they non-autotrophic?
Pitcher plants grow in nitrogen-poor, swampy soils. They cannot get enough nitrogen from the soil to make proteins and nucleic acids. So they supplement their mineral nutrition by trapping and digesting insects, which are rich in nitrogen-containing compounds.
However, pitcher plants are still autotrophs — they have chlorophyll and photosynthesize to make their own glucose. They only use insect-trapping as a nitrogen supplement, not as a substitute for photosynthesis.
Q7. What happens to the glucose produced during photosynthesis?
The plant uses glucose in multiple ways:
- Energy source — broken down in cells during respiration to release energy for growth, repair, and other activities.
- Building block — converted to cellulose for cell walls.
- Storage — converted to starch and stored in leaves, stems, roots, seeds, and fruits.
- Protein synthesis — combined with nitrogen (from soil) to make amino acids and proteins.
- Fat synthesis — converted to fats and oils, especially in seeds.
Q8. Why do leaves turn yellow when a plant is not getting enough nitrogen from the soil?
Chlorophyll contains nitrogen in its molecular structure. When a plant is nitrogen-deficient, it cannot synthesise enough chlorophyll. Less chlorophyll means the green colour fades and leaves turn yellow. This condition is called chlorosis. The plant also grows poorly because nitrogen is needed for all proteins, including the enzymes that run every cellular process.
FAQs
Do plants eat food like animals?
No. Plants make food — they are autotrophs. Animals (including us) consume pre-made organic food from other organisms. The key process plants use is photosynthesis, which converts inorganic CO₂ and water into organic glucose using sunlight energy.
Can plants photosynthesize at night?
No. Photosynthesis requires light. At night, plants only carry out respiration (like animals do round the clock) — they consume oxygen and release CO₂. The popular belief that plants release only CO₂ at night is partially true: they release CO₂ from respiration, but they cannot photosynthesize (so no O₂ output either).
Why are plant leaves green?
Chlorophyll absorbs red and blue wavelengths of light for photosynthesis, but reflects green wavelength. Since green light is reflected back to our eyes, leaves appear green.
What is the difference between photosynthesis and respiration?
| Photosynthesis | Respiration | |
|---|---|---|
| Occurs in | Only green cells (chloroplasts) | All living cells |
| Time | Only in presence of light | Always (day and night) |
| Uses | CO₂ + H₂O | Glucose + O₂ |
| Produces | Glucose + O₂ | CO₂ + H₂O + energy |
| Net effect | Builds organic molecules | Breaks organic molecules |
Why can’t animals make their own food like plants?
Animals lack chlorophyll and chloroplasts — the essential tools for photosynthesis. We evolved as consumers because it was evolutionarily efficient to eat energy already packaged by plants rather than develop the elaborate machinery for photosynthesis from scratch.
Is Cuscuta always yellow-orange?
Almost always. Because it has very little chlorophyll, it cannot manufacture the green pigment. The yellow-orange colour you see is from other pigments (carotenoids) present in small amounts. This is actually a useful identification feature in the field.
What is the significance of photosynthesis beyond plant nutrition?
Photosynthesis is the primary source of all oxygen in Earth’s atmosphere. It also fixes atmospheric CO₂ into organic molecules, which forms the base of all food chains. Fossil fuels (coal, oil, gas) are essentially ancient photosynthesis — carbon captured by plants millions of years ago. Without photosynthesis, complex life on Earth would not exist.