Respiration and Nutrition in Plants

Respiration in Plants

When we think of respiration, we often imagine lungs. But plants challenge this idea completely.

How do plants breathe?

Plants require O₂ for respiration and release CO₂, just like animals. However, the mechanism is fundamentally different.

• Gas exchange occurs by diffusion
• There are no specialised organs like lungs

👉 The direction of diffusion depends on Environmental conditions and plant’s metabolic needs

🌗 Day vs Night Scenario

• During the day:
  • Photosynthesis dominates
  • CO₂ produced in respiration is reused
  • Net result → O₂ is released
• During the night:
  • No photosynthesis
  • Only respiration occurs
  • Net result → CO₂ is released

🌿 Structures for Gas Exchange

Plants use:

• Stomata → Present in leaves
• Lenticels → Present in woody stems

These are simple openings, not complex organs.

🤔 Why no complex respiratory system in plants?

This is a very important conceptual question.

1. Decentralised system

Every part (leaf, stem, root) performs its own gas exchange.

2. Lower metabolic demand

Plants are less active than animals. Generate O₂ during photosynthesis. So, their respiration needs are relatively lower.

3. Short diffusion distance

• Cells are close to the surface
• Presence of intercellular air spaces
  → Diffusion is sufficient

🔄 Source of Glucose for Respiration

Plants don’t “eat”—they produce.

• End product of photosynthesis → Sucrose
• Enzyme invertase converts: Sucrose → Glucose + Fructose
• These are used in cellular respiration

Nutrition in Plants

Plants require nutrients just like humans—but their system is more elegant and self-regulated.

Types of Nutrients

🟢 Macronutrients (Needed in large amounts)

(>10 millimoles per kg dry weight)

Key elements: C, H, O, N, P, K, Ca, Mg, S
👉 Among these: NPK (Nitrogen, Phosphorus, Potassium) are most critical for agriculture

📌 Source:

• C, H, O → Air & Water
• Others → Soil minerals

🔵 Micronutrients (Trace elements)

(<10 millimoles per kg dry weight)
Fe, Mn, Cu, Zn, B, Mo, Cl, Ni
👉 Required in small quantities, but absence = severe deficiency

🧩 Beneficial Elements

Not essential for all plants, but useful → Sodium (Na), Silicon (Si), Cobalt (Co), Selenium (Se)

🌍 Soil: The Nutrient Reservoir

Soil is not just dirt—it is a dynamic nutrient bank.

Functions:

• Provides minerals from weathered rocks
• Contains nitrogen-fixing bacteria
• Retains water and air
• Anchors plants

🌊 Hydroponics

A very important modern concept:
👉 Hydroponics = Growing plants in nutrient solution without soil

This shows → Soil is not essential, Nutrients are

⚙️ Role of Nutrients in Plants

🟢 Macronutrients (Functions)

Nutrient	Key Functions
Carbon (C), Hydrogen (H), Oxygen (O)	Fundamental components of biomolecules (proteins, carbohydrates like starch & cellulose); involved in photosynthesis (CO₂ → carbohydrates); oxygen required for cellular respiration
Nitrogen (N)	Essential for proteins, nucleic acids, vitamins, hormones, and chlorophyll; required most in growing tissues
Phosphorus (P)	Absorbed as phosphate ions; important for nucleic acids, proteins, cell membranes, and energy transfer (ATP, phosphorylation)
Potassium (K)	Maintains ionic balance; involved in protein synthesis, stomatal movement, enzyme activation, and cell turgidity
Calcium (Ca)	Provides cell wall stability (calcium pectate); important for cell division, membrane function, and enzyme activation; accumulates in older leaves
Magnesium (Mg)	Component of chlorophyll; activates enzymes in photosynthesis, respiration, and nucleic acid synthesis; part of ribosome structure
Sulphur (S)	Component of amino acids (cysteine, methionine), vitamins (thiamine, biotin), coenzymes (CoA), and proteins

🔵 Micronutrients (Functions)

Nutrient	Key Functions
Iron (Fe)	Important for electron transport proteins, chlorophyll synthesis, and enzyme activation
Manganese (Mn)	Activates enzymes in photosynthesis, respiration, nitrogen metabolism; involved in photolysis of water
Zinc (Zn)	Enzyme activation and synthesis of growth hormone (auxin)
Copper (Cu)	Involved in redox reactions and overall metabolic processes
Boron (B)	Aids calcium utilisation, cell function, pollen germination, and carbohydrate transport
Molybdenum (Mo)	Component of enzymes involved in nitrogen metabolism
Chlorine (Cl)	Maintains ionic balance; essential for oxygen evolution in photosynthesis
Nickel (Ni)	Required for nitrogen metabolism; prevents toxic accumulation of urea in plant tissues

🔻 Deficiency of Essential Elements

When a plant does not receive enough of a required nutrient, its growth doesn’t stop immediately—it slows down first.

👉 The minimum required level is called critical concentration.
Below this → deficiency symptoms appear

📍 Where do symptoms appear?

This depends on a very important concept: mobility of nutrients.

🔄 Mobile Elements

• Examples: Nitrogen (N), Potassium (K), Magnesium (Mg)
• These move from old leaves → young leaves

👉 So deficiency appears first in older leaves

🔒 Immobile Elements

• Examples: Calcium (Ca), Sulphur (S)
• Cannot move once fixed

👉 So deficiency appears in younger leaves

⚠️ Common Deficiency Symptoms

Symptom	Description	Deficient Nutrients
Chlorosis	Yellowing of leaves due to loss or reduced synthesis of chlorophyll	Nitrogen (N), Potassium (K), Magnesium (Mg), Sulphur (S), Iron (Fe), Manganese (Mn), Zinc (Zn), Molybdenum (Mo)
Necrosis	Death of plant tissue, especially at leaf tips and margins	Calcium (Ca), Magnesium (Mg), Copper (Cu), Potassium (K)
Stunted Growth & Inhibited Cell Division	Reduced plant growth due to impaired cell division and elongation	Nitrogen (N), Potassium (K), Sulphur (S), Molybdenum (Mo)
Delayed Flowering	Delay in reproductive phase and flowering	Nitrogen (N), Sulphur (S), Molybdenum (Mo)

☠️ Toxicity of Micronutrients

Here comes a paradox:

👉 Macronutrients → needed in large amounts
👉 Micronutrients → needed in tiny amounts

But…

⚠️ Even a slight excess of micronutrients can become toxic

• Their optimal range is very narrow
• If plant dry weight reduces by ~10% → toxicity condition

⚠️ Toxicity Symptoms

Aspect	Description	Example / Key Point
Reduced Growth	Excess micronutrients stunt plant growth or may even cause plant death	Overall decline in plant vigour
Discolouration	Leaves develop yellowing, spots, or blotches	Excess Manganese (Mn) → brown spots with chlorotic (yellow) veins
Abnormal Development	Distorted growth patterns and deformities in plant structure	Irregular leaf shape, abnormal tissue development

⚖️ Nutrient Interactions

This is where plant nutrition becomes dynamic, not static.

Type of Interaction	Description	Example
Synergism	One nutrient enhances the uptake or effectiveness of another, improving plant growth	Nitrogen (N) + Sulphur (S): Sulphur helps in amino acid and protein synthesis, enhancing nitrogen use
Zero Interaction	One nutrient has no effect on the uptake or activity of another	Nutrients function independently without influencing each other
Antagonism	Excess of one nutrient inhibits the uptake or function of another, causing deficiency-like symptoms	Excess Manganese (Mn) reduces uptake of Iron (Fe), Magnesium (Mg), and Calcium (Ca)

⚙️ Mechanism of Absorption of Elements

Absorption is not random—it is a two-phase process.

⚡ Phase 1: Passive Uptake

• Rapid | No energy required | Occurs in apoplast (outer region) | Via ion channels

🔋 Phase 2: Active Uptake

• Slower | Requires metabolic energy (ATP) | Occurs in symplast (inner region)

🔄 Ion Movement Terms

• Influx → Entry into cell
• Efflux → Exit from cell

🌍 Nitrogen Metabolism in Plants

Now we come to one of the most important biogeochemical cycles.

🔄 Nitrogen Cycle

Nitrogen is essential—but paradoxically, it is abundant in air yet unavailable to plants.
👉 Reason: Atmospheric nitrogen (N₂) has a strong triple bond

🔁 Steps of Nitrogen Cycle

Step	Process	Key Transformation	Agents Involved	Important Points
1	Nitrogen Fixation	Nitrogen gas (N₂) → Ammonia (NH₃)	Lightning, UV radiation, industrial combustion, forest fires, automobile exhausts, power plants	Breaks strong triple bond of N₂; makes nitrogen biologically usable
2	Ammonification	Organic nitrogen → Ammonia (NH₃)	Decomposers (bacteria & fungi)	Occurs during decomposition of dead plants & animals; ammonia partly released to atmosphere, mostly remains in soil
3	Nitrification	NH₃ → NO₂⁻ → NO₃⁻	Nitrosomonas, Nitrococcus (NH₃ → NO₂⁻); Nitrobacter (NO₂⁻ → NO₃⁻)	These bacteria are chemoautotrophs; stepwise oxidation process
4	Absorption & Denitrification	NO₃⁻ → NH₃ (in plants) and NO₃⁻ → N₂ (denitrification)	Plants (absorption); Pseudomonas, Thiobacillus (denitrification)	Plants use nitrate for amino acid synthesis; denitrification returns nitrogen to atmosphere, completing cycle

You can read about Nitrogen Cycle here in detail.

🧫 Biological Nitrogen Fixation

👉 Only certain prokaryotes can fix nitrogen
👉 They possess enzyme nitrogenase

Types of Nitrogen-Fixing Microbes

🌍 Free-Living

• Aerobic: Azotobacter, Beijerinckia
• Anaerobic: Rhodospirillum
• Cyanobacteria: Anabaena, Nostoc

🌱 Symbiotic

Mutual relationship with plants:
Example 1: Rhizobium + Leguminous plants
Example 2: Frankia + non-leguminous plants

🌿 Nodule Formation (Rhizobium Case)

This is a stepwise biological process:

Bacteria attach to root hairs ➡️Root hairs curl ➡️ Infection thread forms ➡️ Bacteria enter cortex ➡️Nodules develop

⚡ Inside Nodules

• Enzyme: Nitrogenase
• Reaction: N₂ → NH₃
• Energy: ~8 ATP per NH₃

👉 Ammonia → NH₄⁺ → Amino acids

🩸 Special Component: Leg-Haemoglobin

• Gives pink colour to nodules
• Protects nitrogenase from oxygen

👉 Why?
Because nitrogenase works in anaerobic conditions

🚚 Transport of Nitrogen

Plants don’t just produce nitrogen compounds—they transport them smartly.

📦 Forms of Transport

Plants produce Amides like Asparagine and Glutamine → by adding extra NH2 to Amino Acids.
👉 These Nitrogen-rich compounds transported via xylem

🌱 Special Case

• Some plants (e.g., soybean) transport nitrogen as Ureides
  (High nitrogen-to-carbon ratio)

Photosynthesis in Higher Plants

Green plants are called autotrophs because they synthesise their own food.

👉 Photosynthesis is the process by which → Sunlight (light energy), CO₂ (from air), Water (from soil) are converted into → Glucose (chemical energy) and Oxygen (by-product)

🧠 Conceptual Insight

Think of photosynthesis as → Conversion of solar energy into chemical energy stored in glucose. This glucose becomes → Energy source for plants and base of all food chains

⚙️ How Photosynthesis Works

Photosynthesis is not a single step—it is a two-stage process inside chloroplasts.

Stage 1: Light Absorption

• Occurs in chloroplasts
• Pigment involved → chlorophyll
• Located mainly in leaves

👉 When sunlight strikes chlorophyll → process begins

Stage 2: Light Reactions

📍 Location: Thylakoid membranes

What happens?

• Water (H₂O) is split → Oxygen (O₂), Protons (H⁺), Electrons

👉 This is called photolysis of water

⚡ Energy Conversion

Light energy → stored as ATP (energy currency); NADPH (reducing power)
(ATP → adenosine triphosphate and NADPH → nicotinamide adenine dinucleotide phosphate)👉 Oxygen is released as a by-product

Stage 3: Dark Reactions (Calvin Cycle)

📍 Location: Stroma of chloroplast

This stage:

• Does not require direct sunlight
• Uses ATP & NADPH from light reactions

What happens?

• CO₂ is fixed and converted into Glucose (C₆H₁₂O₆)

👉 This glucose:

• Fuels plant metabolism
• Supports entire food chain

🌬️ Entry of CO₂ → through stomata in leaves

🎨 Chlorophyll and Accessory Pigments

Plants are not just green—they are multi-pigment systems.

Types of Pigments

1. Chlorophyll a → Blue-green (primary pigment)
2. Chlorophyll b → Yellow-green
3. Xanthophylls → Yellow
4. Carotenoids → Yellow-orange

🌿 Functional Understanding

⭐ Chlorophyll a

• Most important pigment
• Absorbs → Blue light, red light

👉 Maximum photosynthesis occurs here

🔁 Accessory Pigments

(Chlorophyll b, xanthophylls, carotenoids)

• Absorb other wavelengths
• Transfer energy to chlorophyll a
• Protect against photo-oxidation

👉 Insight:
Plants maximise sunlight usage by covering a broader spectrum.

🌱 Does Photosynthesis Occur Only in Green Leaves?

This is a classic conceptual trap—answer is NO.

Case	Category	Key Explanation	Important Insight
1	Non-Green Leaves	Leaves may appear red, purple, etc. due to pigments like anthocyanins masking chlorophyll	Chlorophyll is still present → photosynthesis continues
2	Stems & Other Green Parts	Green stems and parts like unripe fruits contain chlorophyll	These parts can capture light energy and perform photosynthesis
3	Cacti & Succulents	Leaves are reduced to spines; green stems take over function	Photosynthesis mainly occurs in stems as an adaptation to arid conditions
4	Algae & Other Organisms	Includes algae, some bacteria, and other photosynthetic organisms	Photosynthesis is not limited to plants; different pigments may be used

⚖️ Factors Affecting Photosynthesis

Photosynthesis depends on a balance of multiple factors.

Category	Factors Included	Key Explanation	Important Insight
Internal Factors	Number, size, age & orientation of leaves; mesophyll cells; chloroplasts; internal CO₂ levels; chlorophyll content	These factors determine the plant’s inherent capacity to perform photosynthesis	Controlled by genetics and growth stage of the plant
External Factors	Sunlight, temperature, CO₂ concentration, water	These environmental conditions influence the rate of photosynthesis	Act as limiting factors; variation directly affects photosynthetic efficiency

📉 Law of Limiting Factors

A very important principle: The factor in shortest supply limits the rate of photosynthesis
🧠 Example: Plenty of light + waterbut low CO₂. Photosynthesis will still be slow

🔑 Key Factors Explained

Factor	Key Explanation	Critical Threshold / Behavior	Important Insight
Light	Rate of photosynthesis increases with light intensity	Saturation occurs at ~10% of full sunlight; excess light can damage chlorophyll	Shows light saturation and photoinhibition at very high intensity
CO₂ Concentration	CO₂ is a major limiting factor due to its low atmospheric concentration (0.03–0.04%)	Increase to ~0.05% enhances photosynthesis; higher levels may become harmful	Directly influences rate of carbon fixation
Temperature	Affects enzyme-driven dark reactions more than light reactions	Optimal range required; too high or too low reduces enzyme activity	Indicates enzyme sensitivity of Calvin cycle
Water	Acts as a reactant in light reactions; major effect is indirect	Water stress leads to stomatal closure and reduced CO₂ intake	Also causes wilting, reducing leaf surface area and efficiency