Carbohydrates are essential for life in both plants and animals. Name the carbohydrates that are used as stora

NCERT Exemplar · Q51

Q. Carbohydrates are essential for life in both plants and animals. Name the carbohydrates that are used as storage molecules in plants and animals. Also name the carbohydrate which is present in wood or in the fibre of cotton cloth.

Rajasthan RbseLong· 2mImportance★★★★★

76% · 84/110 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Carbohydrate Functions Plants

Carbohydrates in Plants: Why They Matter

Think of a plant as a factory that builds itself out of thin air and sunlight. The raw material is carbon dioxide from the air, the energy comes from sunlight, and the first product it makes is glucose — a simple sugar. That glucose is the starting point for everything else.

Now, glucose is useful, but it's also fragile and reactive. A plant can't just leave piles of glucose lying around. It needs to store that energy for later (like a battery), and it needs to build strong structures (like a skeleton). This is where carbohydrates come in — they are glucose molecules linked together in different ways to serve different purposes.

The Two Big Jobs of Carbohydrates in Plants

1. Energy Storage (The Battery)

Plants make glucose during the day when the sun shines. But they need energy at night too, and during cloudy days, and when they're growing new leaves or making seeds. So they pack glucose molecules into a long, branched chain called starch.

Note

Starch is to plants what glycogen is to animals — a compact, insoluble way to store glucose. It's stored in roots (potatoes), seeds (rice, wheat), and stems (sugarcane).

When the plant needs energy, it breaks starch back down into glucose, which it then burns (respires) to get ATP — the energy currency of cells.

2. Structural Support (The Skeleton)

Plants don't have bones. Instead, they build rigid cell walls from cellulose — a completely different arrangement of glucose molecules. Cellulose chains lie flat and hydrogen-bond to each other, forming incredibly strong, rope-like fibres.

Watch out

Humans cannot digest cellulose. We lack the enzyme (cellulase) to break the bonds between its glucose units. That's why grass and wood pass right through us — they're structural, not food for us.

Cellulose gives plant cells their shape, allows trees to grow tall, and makes leaves stiff enough to catch sunlight.

The Precise Statement

\alpha

There's also a third, less famous role: sucrose (table sugar) is the main form in which plants transport glucose from leaves to other parts. It's a disaccharide — two glucose-like units stuck together — that dissolves easily in sap and doesn't react as readily as pure glucose.

A Quick Comparison Table

Why this formula?

Carbohydrate Functions in Plants: Why They Matter

Carbohydrates are not just energy sources — they are the structural backbone and chemical currency of plant life. Let’s break down why each key function works the way it does.

1. Photosynthesis: The Source of All Carbohydrates

Key equation:

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

Why this holds:

Carbon fixation: Plants use light energy to split water ( $H_2O$ ) into protons, electrons, and oxygen. The electrons reduce $CO_2$ to form glucose ( $C_6H_{12}O_6$ ).
Energy storage: Glucose is the first stable carbohydrate — it stores chemical energy in its C–H bonds. The 6-carbon skeleton is ideal because it can be easily polymerised into starch or cellulose.
Oxygen as byproduct: The oxygen comes from water, not $CO_2$ — proven by isotope labelling ( $^{18}O$ in water appears in $O_2$ ).

Exam tip: Remember — the light reaction produces ATP and NADPH; the Calvin cycle uses them to reduce $CO_2$ to sugar.

2. Starch: Energy Reserve (Why Glucose is Stored as Starch)

Key formula:

\text{Glucose} \xrightarrow{\text{condensation}} \text{Amylose} + \text{Amylopectin} \ (\text{Starch})

Why starch, not free glucose?

Osmotic problem: Free glucose would draw water into cells via osmosis, causing swelling or bursting. Starch is insoluble — it doesn’t affect water potential.
Compact storage: Starch granules pack many glucose units in a small volume. Amylose is helical (tight), amylopectin is branched (even denser).
Quick mobilisation: Enzymes (amylases) can rapidly break starch back to glucose when energy is needed (e.g., at night, during germination).

Derivation insight: The $\alpha$ -(1→4) and $\alpha$ -(1→6) glycosidic bonds in starch are hydrolysable — this is why starch is a reserve, not a structural material.

3. Cellulose: Structural Support (Why Glucose is Polymerised Differently)

Key formula:

\text{Glucose} \xrightarrow{\beta\text{-(1→4) bonds}} \text{Cellulose} \ (\text{linear chains})

Why $\beta$ bonds instead of $\alpha$ ?

$\beta$ -(1→4) linkage flips every alternate glucose molecule 180°. This allows hydrogen bonding between parallel chains, forming strong microfibrils.
Rigidity: Cellulose is crystalline — it resists tensile stress. This is why plant cell walls can withstand turgor pressure.
Indigestibility: Most animals (including humans) lack cellulase enzymes. Only ruminants and termites (with microbial symbionts) can break $\beta$ bonds.

Key contrast: Starch = $\alpha$ bonds (flexible, digestible). Cellulose = $\beta$ bonds (rigid, indigestible). This is a classic exam comparison.

4. Sucrose: Transport Sugar (Why Not Glucose?)

Key formula:

\text{Glucose} + \text{Fructose} \xrightarrow{\text{glycosidic bond}} \text{Sucrose} + H_2O

Why sucrose for transport?

Non-reducing sugar: Sucrose has no free aldehyde/ketone group — it doesn’t react with proteins or other molecules during transport. Glucose would.
Energy efficiency: Sucrose carries two hexoses per molecule — twice the energy per transport event.
Phloem loading: Sucrose is actively loaded into sieve tubes via SUT transporters (sucrose uptake transporters). This creates osmotic flow (pressure flow hypothesis). …

Start your 14-day free trial to unlock the full solution →

Plants store energy as starch, animals store it as glycogen, and the structural carbohydrate in wood and cotton fibre is cellulose.

Carbohydrates serve two broad roles in living organisms: energy storage and structural support. The type of carbohydrate used for each job depends on the organism’s needs — plants and animals have evolved different molecules for storage, but they share a common structural polymer for building strong cell walls.

Let’s break this down.

Storage carbohydrate in plants: Starch

Plants make glucose during photosynthesis. They don’t need all of it immediately, so they link glucose units into a long, branched polymer called starch. Starch is insoluble in water, so it doesn’t affect the plant’s water balance, and it can be packed densely in roots, tubers, and seeds. When the plant needs energy (e.g., at night or during germination), enzymes break starch back into glucose.
Storage carbohydrate in animals: Glycogen

Animals also store glucose, but as glycogen. Glycogen is similar to starch but more highly branched — this allows rapid release of glucose when blood sugar drops. It’s stored mainly in the liver and muscles. The branching is key: more ends mean enzymes can work faster to free glucose.

Watch out

A common mistake is to say “cellulose” is the storage carbohydrate in plants. Cellulose is structural, not storage — plants never break it down for energy. Starch is the storage form.
Structural carbohydrate in wood and cotton: Cellulose …

Start your 14-day free trial to unlock the full solution →

Here are the common mistakes students make on this question, along with how to avoid each one.

Mistake 1: Confusing the storage carbohydrates in plants and animals

The Mistake:

Students often swap the answers, saying glycogen is stored in plants and starch is stored in animals. Another common error is to say cellulose is a storage molecule.

Why it happens:

Both starch and glycogen are made of glucose, so they look similar on paper. Students memorize the names without linking them to the function or the organism.

How to Avoid It (The Concept):

Think about energy demand.

Animals (including humans) need to release energy very quickly (e.g., for running, thinking). Glycogen is highly branched, allowing rapid glucose release. → Glycogen in animals.
Plants are sessile (don't move). They store energy for longer periods (overnight, winter). Starch is less branched, more compact, and stable for long-term storage. → Starch in plants.

Memory Trick: "Glycogen is for Go (animals move). Starch is for Stay (plants stay put)."

Correct Answer:

Plants: Starch
Animals: Glycogen

Mistake 2: Naming the wrong carbohydrate for wood/cotton fibre

The Mistake:

Students answer starch or sucrose because they know plants make these. Or they say "fibre" without naming the specific carbohydrate.

Why it happens:

Students don't connect the physical property (rigidity, strength) to the chemical structure. They think "plant = starch" without considering structure vs. storage.

How to Avoid It (The Concept):

Ask yourself: What gives a tree trunk its strength? What makes cotton fibres tough?

Cellulose is a structural polysaccharide. It forms long, straight chains that bundle together into strong fibres (microfibrils). This is what makes cell walls rigid.
Starch is for storage; it is not strong. If wood were made of starch, trees would be soft and mushy.

Correct Answer:

Wood / Cotton fibre: Cellulose

Mistake 3: Forgetting to specify "polysaccharide" or giving a monosaccharide

The Mistake:

Students write glucose as the storage molecule. "Plants store glucose."

Why it happens: …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ50 — On the basis of which evidences D-glucose was assigned the following s…→Next questionQ52 — Explain the terms primary and secondary structure of proteins. What is…

Q.Carbohydrates are essential for life in both plants and animals. Name the carbohydrates that are used as storage molecules in plants and animals. Also name the carbohydrate which is present in wood or in the fibre of cotton cloth.

Concept understanding — Carbohydrate Functions Plants

Carbohydrates in Plants: Why They Matter

The Two Big Jobs of Carbohydrates in Plants

1. Energy Storage (The Battery)

2. Structural Support (The Skeleton)

The Precise Statement

A Quick Comparison Table

Why this formula?

Carbohydrate Functions in Plants: Why They Matter

1. Photosynthesis: The Source of All Carbohydrates

Why this holds:

2. Starch: Energy Reserve (Why Glucose is Stored as Starch)

Why starch, not free glucose?

3. Cellulose: Structural Support (Why Glucose is Polymerised Differently)

Why $\beta$ bonds instead of $\alpha$ ?

4. Sucrose: Transport Sugar (Why Not Glucose?)

Why sucrose for transport?

Concept: Storage and Structural Polysaccharides

Method: Structure-Function Mapping

Mistake 1: Confusing the storage carbohydrates in plants and animals

Mistake 2: Naming the wrong carbohydrate for wood/cotton fibre

Mistake 3: Forgetting to specify "polysaccharide" or giving a monosaccharide

Q.Carbohydrates are essential for life in both plants and animals. Name the carbohydrates that are used as storage molecules in plants and animals. Also name the carbohydrate which is present in wood or in the fibre of cotton cloth.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days