Respiration in Plants

BREATHING AND EXCHANGE OF GASES

**Definition**: Breathing is the process of exchanging **O₂ from the atmosphere** with **CO₂ produced by cells**. It involves inhalation (inspiration) and exhalation (expiration) to maintain continuous supply of oxygen for cellular metabolism and remove harmful carbon dioxide.

**Importance**: Cells utilise oxygen to break down glucose, amino acids, and fatty acids through catabolism to derive energy (ATP). This process simultaneously produces CO₂ as a harmful byproduct that must be eliminated. Without continuous O₂ supply and CO₂ removal, cells cannot maintain metabolic activities.

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RESPIRATORY ORGANS

**Definition**: Structures specialised for exchange of gases between organism and environment, varying based on habitat and evolutionary level.

Respiratory Organs in Different Animals

**Lower Invertebrates** (sponges, coelenterates, flatworms):

Exchange O₂ and CO₂ by **simple diffusion over entire body surface**

No specialised respiratory organs needed due to small body size

**Earthworms**:

Use **moist cuticle** for gas exchange

Requires environment to remain damp

**Insects**:

Possess **network of tracheal tubes** (trachea, tracheoles) that transport atmospheric air directly within the body

Allows direct delivery to tissues without blood involvement

**Aquatic Animals**:

**Gills** (branchial respiration) in most aquatic arthropods and molluscs

Gill filaments are highly vascularised (rich blood supply) for efficient gas diffusion

Water flows over gills, oxygen dissolved in water diffuses into blood

**Terrestrial Vertebrates**:

**Lungs** (pulmonary respiration) in amphibians, reptiles, birds, mammals

Vascularised bags allowing efficient O₂-CO₂ exchange

Amphibians (frogs) additionally use **cutaneous respiration** (gas exchange through moist skin)

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HUMAN RESPIRATORY SYSTEM: ANATOMY AND STRUCTURE

External and Conducting Passages

**Nostrils and Nasal Chamber**:

Paired **external nostrils** open above upper lip

Lead to **nasal chamber** via nasal passage

Functions: warms, moistens, and filters air; removes foreign particles

**Pharynx**:

Chamber where nasal cavity opens

Acts as **common passage for food and air**

Continues into larynx

**Larynx**:

**Cartilaginous box** located at entrance to trachea

Contains **vocal cords** for sound production; called the **sound box**

**Epiglottis**: thin elastic cartilaginous flap that covers glottis during swallowing to prevent food entry into respiratory tract

**Trachea**:

Straight tube extending from larynx into thoracic cavity

**Divides at 5th thoracic vertebra** into right and left primary bronchi

Supported by **incomplete cartilaginous rings** maintaining its patency

Allows air to pass directly to lungs

**Primary, Secondary, and Tertiary Bronchi**:

Formed by repeated divisions of primary bronchi

Continue branching pattern within lungs

Supported by incomplete cartilaginous rings that decrease in size

**Bronchioles and Terminal Bronchioles**:

Thin tubes resulting from further bronchial divisions

Initial portions still supported by cartilage

Terminal bronchioles represent end of purely conducting pathway

Respiratory/Exchange Part

**Alveoli**:

**Thin, irregular-walled, vascularised, bag-like structures**

Arise from terminal bronchioles

**Primary sites of gas exchange** between air and blood

Each alveolus surrounded by network of pulmonary capillaries

Extremely thin walls (single cell layer) allow efficient diffusion

Enormous total surface area (approximately 70 m²) for exchange

**Bronchiolar-Alveolar Network**:

Branching network of bronchi, bronchioles, and alveoli comprises the **lungs**

Humans have **pair of lungs** (right lung has 3 lobes, left has 2 lobes)

Pleural Membranes

**Structure**:

Lungs enclosed in **double-layered pleura** (serous membrane)

**Outer pleural membrane**: in contact with thoracic cavity lining

**Inner pleural membrane**: in contact with lung surface

**Pleural fluid** between layers reduces friction

**Function**: Enables smooth lung expansion and contraction without friction during breathing

Conducting vs. Respiratory Parts

**Conducting Part**:

Extends from nostrils to terminal bronchioles

Functions: transports atmospheric air, clears foreign particles, humidifies air, brings it to body temperature

**Does not participate in gas exchange**

**Respiratory/Exchange Part**:

Alveoli and their ducts

**Actual site of O₂ and CO₂ diffusion** between air and blood

Thoracic Cavity Anatomy

**Boundaries**:

**Dorsal**: vertebral column

**Ventral**: sternum (breastbone)

**Lateral**: ribs

**Inferior**: dome-shaped diaphragm

**Key Feature**: **Air-tight chamber** - any change in thoracic volume directly reflected in pulmonary volume; essential for breathing mechanism since we cannot directly alter lung volume

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STEPS IN RESPIRATION

**Respiration involves five coordinated steps**:

1. **Breathing (Pulmonary Ventilation)**: Atmospheric air drawn in, CO₂-rich alveolar air released out

2. **Diffusion of Gases**: O₂ and CO₂ diffuse across alveolar membrane between air and blood

3. **Transport of Gases**: Blood carries gases throughout body

4. **Diffusion at Tissues**: O₂ and CO₂ exchange between blood and tissue cells

5. **Cellular Respiration**: Cells utilise O₂ for catabolic reactions; CO₂ released as byproduct

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MECHANISM OF BREATHING

**Definition**: Two-stage process (inspiration and expiration) involving pressure gradient creation between lungs and atmosphere.

**Fundamental Principle**: **Pressure Gradient Movement**

**Inspiration occurs**: when intra-pulmonary pressure < atmospheric pressure (negative pressure)

**Expiration occurs**: when intra-pulmonary pressure > atmospheric pressure

Inspiration (Inhalation)

**Process**:

**Diaphragm contracts** (moves downward/inferiorly), increasing thoracic volume in antero-posterior axis

**External intercostal muscles contract**, lifting ribs and sternum upward and outward, increasing dorso-ventral axis volume

**Combined effect**: overall thoracic cavity volume increases

**Result**:

Pulmonary volume increases proportionally

Intra-pulmonary pressure decreases below atmospheric pressure (negative pressure)

Air pressure gradient forces **atmospheric air into lungs**

Alveoli fill with fresh air

Expiration (Exhalation)

**Process**:

**Diaphragm relaxes** (moves upward), returning to normal position

**External intercostal muscles relax**, ribs and sternum return to normal position

**Thoracic volume decreases**, pulmonary volume decreases

**Result**:

Intra-pulmonary pressure increases above atmospheric pressure

**Air expelled from lungs** passively

CO₂-rich alveolar air released

**Enhanced Breathing**:

**Abdominal muscles** provide additional force during forced inspiration and expiration

Allows increased tidal volumes during exercise

**Breathing Rate**: Average healthy human breathes **12-16 times per minute** (frequency varies with activity, age, health status)

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RESPIRATORY VOLUMES AND CAPACITIES

**Measurement**: **Spirometer** - instrument that measures volumes of air moved during breathing; clinically important for assessing pulmonary function

Respiratory Volumes

**Tidal Volume (TV)**:

**Definition**: Volume of air inspired or expired during normal, quiet breathing

**Value**: Approximately **500 mL**

**Clinical significance**: Average healthy person inspires/expires 6,000-8,000 mL air per minute

**Inspiratory Reserve Volume (IRV)**:

**Definition**: Additional volume of air that can be inspired after normal inspiration

**Measurement**: Achieved through forcible/deep inspiration beyond normal tidal volume

**Value**: **2,500-3,000 mL**

**Example**: Taking deepest possible breath after normal inhalation

**Expiratory Reserve Volume (ERV)**:

**Definition**: Additional volume of air that can be expired after normal expiration

**Measurement**: Achieved through forcible/deep expiration beyond normal tidal volume

**Value**: **1,000-1,100 mL**

**Residual Volume (RV)**:

**Definition**: Volume of air remaining in lungs even after forcible/maximal expiration

**Value**: **1,100-1,200 mL**

**Significance**: Prevents alveolar collapse; always present in lungs; cannot be measured by spirometry directly

Respiratory Capacities

**Capacities** are combinations of two or more respiratory volumes; used for clinical diagnosis

**Inspiratory Capacity (IC)**:

**Formula**: **IC = TV + IRV**

**Definition**: Total volume of air that can be inspired after normal expiration

**Value**: Approximately 3,000-3,500 mL

**Expiratory Capacity (EC)**:

**Formula**: **EC = TV + ERV**

**Definition**: Total volume of air that can be expired after normal inspiration

**Value**: Approximately 1,500-1,600 mL

**Functional Residual Capacity (FRC)**:

**Formula**: **FRC = ERV + RV**

**Definition**: Volume of air remaining in lungs after normal expiration

**Value**: Approximately 2,100-2,300 mL

**Significance**: Represents air available for gas exchange between breaths

**Vital Capacity (VC)**:

**Formula**: **VC = ERV + TV + IRV**

Alternative: **Maximum air exhaled after maximum inspiration**

**Definition**: Maximum volume of air a person can breathe after forced expiration

**Value**: Approximately 4,000-4,800 mL (varies with age, sex, body size)

**Clinical use**: Indicator of respiratory muscle strength and lung function

**Total Lung Capacity (TLC)**:

**Formula**: **TLC = RV + ERV + TV + IRV**

Alternative: **VC + RV**

**Definition**: Total volume of air accommodated in lungs after maximum inspiration

**Value**: Approximately 5,800-6,000 mL in adults

**Clinical importance**: Indicates overall lung size

**Exam Memory Aid**:

TV = 500 mL

IRV = 2,500-3,000 mL

ERV = 1,000-1,100 mL

RV = 1,100-1,200 mL

VC = TV + IRV + ERV (approximately 4,600 mL)

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EXCHANGE OF GASES

**Definition**: Diffusion of O₂ and CO₂ between alveoli and blood, and between blood and tissues based on pressure/concentration gradients.

**Primary Sites**: Alveoli (main), capillaries in tissues (secondary)

Partial Pressure Concept

**Definition**: **Partial pressure** - pressure contributed by individual gas in mixture of gases

**Notation**:

**pO₂** = partial pressure of oxygen

**pCO₂** = partial pressure of carbon dioxide

**Partial Pressures at Different Sites** (Table reference):

| Location | pO₂ (mm Hg) | pCO₂ (mm Hg) |

|----------|-------------|-------------|

| Atmospheric Air | 159 | 0.3 |

| Alveoli | 104 | 40 |

| Deoxygenated Blood | 40 | 45 |

| Oxygenated Blood | 95 | 40 |

| Tissues | 40 | 45 |

**Concentration Gradients**:

**O₂ gradient**: Alveoli (104) → Blood (40) → Tissues (40) - oxygen moves from high to low concentration

**CO₂ gradient**: Tissues (45) → Blood (45) → Alveoli (40) - opposite direction to oxygen

Factors Affecting Diffusion Rate

**Solubility of Gases**:

**CO₂ solubility 20-25 times higher than O₂**

More CO₂ can diffuse per unit partial pressure difference than O₂

Explains why CO₂ diffuses despite smaller pressure gradient

**Thickness of Diffusion Membrane**:

Composed of **three layers**: alveolar epithelium, capillary endothelium, basement membrane between them

**Total thickness**: much less than 1 millimetre

Thin membrane facilitates rapid diffusion

**Alveolar Structure**:

Thin squamous epithelium (single cell layer)

Extensive capillary network surrounding each alveolus

Enormous total surface area (≈70 m²) in both lungs combined

Diffusion Membrane Structure

**Three Major Layers** (from lumen outward):

1. **Squamous Epithelium of Alveolus**: Single layer of thin, flat cells lining alveolar interior

2. **Basement Substance**: Supporting tissue between epithelium and capillary endothelium

3. **Capillary Endothelium**: Single layer of endothelial cells forming capillary wall

**Integrated Function**:

Minimal thickness (< 1 mm) combined with large surface area creates ideal conditions for rapid gas diffusion

All factors favourable for O₂ movement from alveoli to tissues and CO₂ movement from tissues to alveoli

Gas Exchange Process

**At Alveoli**:

High pO₂ in alveoli (104) > pO₂ in deoxygenated blood (40)

**O₂ diffuses into blood**, binds with haemoglobin

High pCO₂ in blood (45) > pCO₂ in alveoli (40)

**CO₂ diffuses out of blood** into alveoli

Results in **oxygenated blood** leaving lungs

**At Tissues**:

High pO₂ in blood (95) > pO₂ in tissues (40)

**O₂ diffuses from blood into tissues**, used in cellular respiration

High pCO₂ in tissues (45) > pCO₂ in blood (40)

**CO₂ produced by metabolism diffuses into blood**

Results in **deoxygenated blood** returning to lungs

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TRANSPORT OF GASES

**Medium of Transport**: Blood (via RBCs and plasma)

Oxygen Transport Percentages

**97% of O₂**: transported by RBCs (as oxyhaemoglobin)

**3% of O₂**: dissolved in plasma

Carbon Dioxide Transport Percentages

**70% of CO₂**: transported as bicarbonate ions (HCO₃⁻)

**20-25% of CO₂**: transported by RBCs (as carbamino-haemoglobin)

**7% of CO₂**: dissolved in plasma

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TRANSPORT OF OXYGEN

**Carrier Molecule**: **Haemoglobin**

Red-coloured, iron-containing pigment in RBCs

Each molecule contains **4 iron atoms** (in 4 haem groups)

**Formation of Oxyhaemoglobin**:

**Reversible binding**: O₂ + Haemoglobin ⇌ Oxyhaemoglobin

Each haemoglobin molecule carries **maximum 4 O₂ molecules** (one per iron atom)

Binding is **non-covalent**, allowing easy dissociation

Factors Affecting Oxygen Binding

**Primary Factor**: **Partial Pressure of O₂ (pO₂)**

Direct relationship: higher pO₂ → more O₂ binding

**Secondary Factors**:

**pCO₂** (partial pressure of CO₂)

**H⁺ concentration** (pH)

**Temperature**

**Bohr Effect**: Increased pCO₂, H⁺ concentration, and temperature **decrease haemoglobin's affinity for oxygen**, promoting O₂ release in tissues

Oxygen Dissociation Curve

**Definition**: **Sigmoid (S-shaped) curve** obtained by plotting percentage saturation of haemoglobin with O₂ against pO₂

**Shape Significance**:

**Steep slope at low pO₂**: Small changes in pO₂ cause large changes in saturation (sensitive at tissue levels)

**Plateau at high pO₂**: Saturation approaches maximum (efficient loading in lungs)

**S-shape ensures efficient loading and unloading**

**Interpretation**:

**At Alveoli** (high pO₂=104, low pCO₂, low H⁺, low temperature): All factors favour **oxyhaemoglobin formation**, blood becomes well-oxygenated

**At Tissues** (low pO₂=40, high pCO₂, high H⁺, high temperature): All factors favour **O₂ dissociation**, oxygen released to tissues

Oxygen Delivery Capacity

**Normal Physiological Condition**:

**Every 100 mL of oxygenated blood delivers approximately 5 mL of O₂ to tissues**

Reflects efficient extraction of oxygen by metabolically active tissues

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TRANSPORT OF CARBON DIOXIDE

Carbamino-Haemoglobin Formation

**Definition**: **CO₂ binds with haemoglobin** to form carbamino-haemoglobin (or carbamate)

**20-25% of CO₂ transported** this way

**Reversible binding**: Similar to oxygen transport

**Factors Affecting Binding**:

**High pCO₂** (tissues): promotes CO₂ binding to haemoglobin

**High pO₂** (alveoli): promotes CO₂ release from haemoglobin

**Haldane Effect**: Deoxygenated haemoglobin binds CO₂ more readily than oxygenated haemoglobin

Bicarbonate Formation and Transport

**Enzyme**: **Carbonic Anhydrase**

Present in **very high concentration in RBCs**

Minute quantities in plasma

**Catalyses bidirectional reaction**

**Reaction Equation**:

**CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺** (carbonic anhydrase catalyst)

**At Tissue Level** (high pCO₂):

CO₂ produced by metabolism diffuses into blood

RBCs: CO₂ + H₂O → H₂CO₃ (carbonic acid) → HCO₃⁻ (bicarbonate) + H⁺

HCO₃⁻ moves out of RBC into plasma (transported via chloride shift)

H⁺ buffered by haemoglobin (deoxygenated haemoglobin is better buffer)

**Result**: CO₂ "trapped" as bicarbonate in blood plasma

**At Alveolar Level** (low pCO₂):

HCO₃⁻ in plasma enters RBCs

Reaction reverses: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O

**CO₂ released as gas, expelled from lungs**

H₂O either remains in blood or exhaled as vapour

Carbon Dioxide Delivery Capacity

**Normal Physiological Condition**:

**Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to alveoli**

Reflects efficient removal of metabolic waste

Chloride Shift (Hamburger's Phenomenon)

**Mechanism**:

As HCO₃⁻ exits RBC into plasma, Cl⁻ ions enter RBC

Maintains electrical neutrality

**Important for CO₂ transport efficiency**

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REGULATION OF RESPIRATION

**Mechanism**: **Neural control** via specialized brain centres

Respiratory Centres in Brain

**Respiratory Rhythm Centre**:

Located in **medulla region** of brainstem

**Primary regulator** of respiratory rhythm

Generates automatic breathing pattern

Sends motor signals to respiratory muscles via spinal nerves

**Pneumotaxic Centre**:

Located in **pons region** of brainstem

**Moderates** respiratory rhythm centre function

Neural signals **reduce duration of inspiration**

**Alters respiratory rate** - increases frequency of breathing

**Chemosensitive Area**:

Located adjacent to **rhythm centre** in medulla

**Highly sensitive to CO₂ and H⁺ ions**

When CO₂ or H⁺ increases: activates centre

Signals rhythm centre to **adjust respiratory process** for elimination of these substances

**Increases ventilation** to blow off CO₂ and correct acidosis

Peripheral Chemoreceptors

**Locations**:

**Aortic arch** - aortic body chemoreceptors

**Carotid artery** - carotid body chemoreceptors

**Function**:

Monitor changes in **pO₂, pCO₂, and H⁺ concentration**

Send sensory signals via cranial nerves to rhythm centre

**Trigger remedial adjustments** in respiratory rate and depth

**Example**: During intense exercise, increased tissue metabolism produces more CO₂ → elevated pCO₂ detected by chemoreceptors → respiratory centre increases ventilation

Role of Individual Gases in Regulation

**CO₂ and H⁺ Ions**: **Major regulators**

Small changes in pCO₂ have **significant effect on ventilation**

Drives rhythmic breathing at rest

**O₂**: **Minor regulator**

Significant role only when **pO₂ drops severely** (below 60 mm Hg)

Normal atmospheric O₂ levels are adequate for regulation

Not primary control factor under physiological conditions

Reflex Control

**Stretch Receptors in Lungs**:

Proprioceptors in lung tissue

Stimulated by lung inflation

Send signals to rhythm centre via vagal nerve

Limit maximum inspiration depth (Hering-Breuer reflex)

**Irritant Receptors**:

Sensitive to irritants, dust, smoke

Trigger cough and sneeze reflexes

Protect airways

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DISORDERS OF RESPIRATORY SYSTEM

Asthma

**Definition**: Respiratory disorder characterized by difficulty in breathing and wheezing

**Pathophysiology**:

**Inflammation of bronchi and bronchioles**

**Smooth muscle spasm** in airways

Increased mucus secretion

Results in **airway narrowing and obstruction**

**Symptoms**:

**Wheezing** (whistling sound during breathing)

**Shortness of breath** (dyspnoea)

Chest tightness

Coughing (especially at night)

**Triggers**: Allergies, exercise, cold air, stress, air pollution

**Nature**: Chronic inflammatory disease with recurrent episodes; reversible airflow obstruction

Emphysema

**Definition**: Chronic disorder in which **alveolar walls are damaged**

**Pathophysiology**:

**Destruction of alveolar septa**

**Loss of elastic recoil** in lungs

**Reduced respiratory surface area** (fewer alveoli available for gas exchange)

Air trapping in lungs

**Increased work of breathing**

**Major Cause**: **Cigarette smoking**

Smoke damages elastic fibres

Causes chronic inflammation

Leads to alveolar wall degradation

**Other Causes**: Long-term air pollution exposure, occupational hazards

**Symptoms**:

**Shortness of breath** during exertion (initially) or at rest (advanced)

Chronic cough

Blue discoloration of skin (cyanosis) in severe cases

Barrel-shaped chest

**Consequences**:

Reduced oxygen absorption

Difficulty expelling air (air trapping)

Increased effort to breathe

Chronic hypoxaemia (low blood oxygen)

**Irreversibility**: Unlike asthma, emphysema causes **permanent structural damage** to alveoli; damage cannot be reversed

Other Common Disorders

**Bronchitis**:

Inflammation of bronchi

Excessive mucus production

Chronic cough with sputum

**Pneumonia**:

Infection (bacterial, viral, fungal)

Alveoli fill with fluid/pus

Impaired gas exchange

**Tuberculosis (TB)**:

Bacterial infection (*Mycobacterium tuberculosis*)

Granuloma formation

Can be fatal if untreated

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SUMMARY OF KEY CONCEPTS

**Breathing Process**:

Two-stage process: inspiration (air drawn in) and expiration (air expelled)

Driven by pressure gradients created by diaphragm and intercostal muscle contraction

Average breathing rate: 12-16 times/minute

**Gas Exchange**:

Occurs at **alveoli** (lungs) and **tissues**

Driven by partial pressure differences (pO₂ and pCO₂ gradients)

Diffusion across thin alveolar-capillary membrane (< 1 mm)

Large alveolar surface area (≈70 m²) ensures efficient exchange

**Gas Transport**:

**O₂**: 97% by RBC haemoglobin, 3% dissolved in plasma

**CO₂**: 70% as bicarbonate, 20-25% as carbamino-haemoglobin, 7% dissolved

Haemoglobin crucial for efficient oxygen loading and unloading

**Oxygen Dissociation Curve**:

S-shaped sigmoid curve

Shows relationship between pO₂ and haemoglobin saturation

Allows efficient loading at lungs and unloading at tissues

**Regulation**:

**Neural control** via medullary respiratory centre

**CO₂ and H⁺** are major regulatory factors

Chemoreceptors monitor blood gas composition

Respiratory rate adjusts to meet metabolic demands

**Pulmonary Capacities** (Clinical Importance):

**VC** (vital capacity) = TV + IRV + ERV ≈ 4,600 mL

**TLC** (total lung capacity) = VC + RV ≈ 6,000 mL

Used for diagnosing respiratory dysfunction

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EXAM QUICK REFERENCE

**Important Values to Memorize**:

Atmospheric pO₂ = 159 mm Hg; pCO₂ = 0.3 mm Hg

Alveolar pO₂ = 104 mm Hg; pCO₂ = 40 mm Hg

Tissue pO₂ = 40 mm Hg; pCO₂ = 45 mm Hg

TV = 500 mL; IRV = 2,500 mL; ERV = 1,100 mL; RV = 1,200 mL

O₂ delivery = 5 mL per 100 mL blood

CO₂ delivery = 4 mL per 100 mL blood

**Key Definitions to State Precisely**:

Haemoglobin, Oxyhaemoglobin, Carbamino-haemoglobin

Partial pressure, Oxygen dissociation curve

Inspiratory, expiratory, and residual volumes

Vital capacity, total lung capacity

Bohr effect, Haldane effect

**Process Equations**:

**CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺**

**IC = TV + IRV; EC = TV + ERV**

**VC = TV + IRV + ERV; TLC = VC + RV**