Respiration and Transport

What is Cellular Respiration?

Cellular respiration is the fundamental biochemical process by which living cells break down glucose molecules to release energy in the form of Adenosine Triphosphate (ATP). This vital process occurs in every living organism to fuel cellular activities, maintenance, and growth. It serves as the primary mechanism for transferring chemical energy stored in nutrients into a usable biological currency.

Aerobic vs. Anaerobic Respiration:

Respiration is broadly categorized into two types based on the presence or absence of oxygen:

• Aerobic Respiration: Occurs in the presence of oxygen. It involves the complete oxidation of glucose into carbon dioxide and water, yielding a high amount of energy.
• Anaerobic Respiration: Occurs in the complete absence of oxygen. It involves the partial breakdown of glucose, resulting in either lactic acid (in animal muscle cells) or ethanol and carbon dioxide (in yeast), along with a significantly lower energy yield.

ATP as the Energy Currency:

Adenosine Triphosphate (ATP) consists of an adenine base, a ribose sugar, and three phosphate groups. The bonds linking the phosphate groups are high-energy bonds. When the terminal phosphate bond is broken via hydrolysis, it releases roughly 30.5 kJ/mol of energy, which directly powers cellular processes like muscle contraction, active transport, and biosynthesis.

The Major Metabolic Pathways:

1. Glycolysis: This pathway occurs entirely within the cytoplasm of the cell. It is an anaerobic process (does not require oxygen) that splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, generating a net gain of 2 ATP and 2 NADH molecules.
2. Krebs Cycle (Citric Acid Cycle): Occurs inside the mitochondrial matrix. Pyruvate enters the mitochondria, is converted to Acetyl-CoA, and undergoes cyclic oxidation. This cycle produces carbon dioxide, ATP, NADH, and FADH2.
3. Oxidative Phosphorylation & The Electron Transport Chain (ETC): Located on the inner mitochondrial membrane (cristae). High-energy electrons carried by NADH and FADH2 are passed down a series of protein complexes. Oxygen acts as the final electron acceptor, combining with hydrogen ions to form water. This process drives protons across the membrane, establishing a gradient that powers ATP Synthase to manufacture the bulk of the cell's ATP.

The Pathway of Air:

The human respiratory system is specialized to facilitate the intake of oxygen and the expulsion of carbon dioxide. The anatomical pathway traversed by air during inhalation includes:

1. Nostrils / Nasal Cavity: Air enters, is filtered by mucus and hairs, warmed by blood capillaries, and moistened.
2. Pharynx: A common passageway for both air and food.
3. Larynx: The voice box, protected by the epiglottis which prevents food entry during swallowing.
4. Trachea: The windpipe, reinforced with C-shaped cartilaginous rings to prevent collapse.
5. Bronchi & Bronchioles: The trachea splits into right and left primary bronchi, branching further into smaller bronchioles.
6. Alveoli: Tiny, thin-walled, vascularized air sacs where external gas exchange occurs.

The Mechanism of Breathing:

Breathing (pulmonary ventilation) is a mechanical process driven by volume and pressure changes in the thoracic cavity, dictated by two major muscle sets:

• Inspiration (Inhalation): The diaphragm contracts and flattens downward, while the external intercostal muscles contract, pulling the ribcage upward and outward. This increases thoracic cavity volume, lowering internal lung pressure below atmospheric pressure, forcing air into the lungs.
• Expiration (Exhalation): The diaphragm and external intercostal muscles relax. The diaphragm moves upward into its dome shape, and the ribcage drops down and inward. This decreases thoracic volume, increasing pressure above atmospheric levels, forcing air out of the lungs.

Lung Volumes and Capacities:

• Tidal Volume (TV): The volume of air inspired or expired during normal, relaxed breathing (approx. 500 mL).
• Residual Volume (RV): The volume of air remaining in the lungs even after a forceful, maximum exhalation (approx. 1200 mL); it prevents lung collapse.
• Vital Capacity (VC): The maximum volume of air a person can exhale after a maximum, deepest inhalation ($\text{VC} = \text{TV} + \text{IRV} + \text{ERV}$).

Gas Exchange at Alveoli and Tissues:

Gas exchange occurs through simple diffusion across specialized respiratory surfaces. It happens at two primary locations:

1. External Respiration: Exchange of gases between the air in the alveoli and the blood flowing within the pulmonary capillaries.
2. Internal Respiration: Exchange of gases between systemic capillary blood and tissue cells.

The respiratory membrane at the alveoli is exceptionally thin (less than 1 micrometer), consisting of alveolar epithelium, capillary endothelium, and an ultra-thin basement membrane, maximizing diffusion rates.

Partial Pressure Gradients:

Diffusion is driven entirely by differences in partial pressure ($p\text{O}_2$ and $p\text{CO}_2$). Gases spontaneously move from a region of higher partial pressure to a region of lower partial pressure:

• In the alveoli, $p\text{O}_2$ is high ($\sim 104\text{ mmHg}$) and capillary $p\text{O}_2$ is low ($\sim 40\text{ mmHg}$). Oxygen diffuses into the blood.
• In the tissues, oxygen is constantly consumed, so tissue $p\text{O}_2$ is low ($\sim 40\text{ mmHg}$), driving oxygen out of the systemic capillary blood ($p\text{O}_2 \sim 95\text{ mmHg}$) into cells.

Oxyhaemoglobin Dissociation Curve:

Haemoglobin (Hb) binds up to four oxygen molecules to form Oxyhaemoglobin ($\text{Hb}(\text{O}_2)_4$). The relationship between haemoglobin saturation and $p\text{O}_2$ is represented by a Sigmoid (S-shaped) curve.

• Shift to the Right: Decreased affinity for $O_2$, leading to easier unloading of oxygen into tissues. This is caused by high $p\text{CO}_2$, low pH (acidity / Bohr Effect), elevated temperature, and high 2,3-BPG—conditions typical of working muscles.
• Shift to the Left: Increased affinity for $O_2$, meaning haemoglobin binds oxygen tightly. This occurs in cooler, higher pH, and low $CO_2$ environments like the lungs.

What is Transpiration?

Transpiration is the physiological loss of water in the form of water vapor from the aerial parts of a plant, primarily through leaves. While it seems wasteful—99% of water absorbed by roots is lost via transpiration—it is a necessary process that drives nutrient transport and cools the plant.

Types of Transpiration:

• Stomatal Transpiration: The vast majority of water loss (approx. 85-90%) occurs through the stomata, microscopic pores found mostly on the lower epidermis of leaves.
• Cuticular Transpiration: Minor water loss occurring directly through the waxy cuticle covering leaf surfaces. Thicker cuticles in desert plants minimize this loss.

Factors Affecting Transpiration Rates:

• Light: Accelerates transpiration by triggering stomata opening for photosynthesis.
• Temperature: Higher temperatures increase water evaporation and reduce air humidity, speeding up transpiration.
• Humidity: High atmospheric humidity decreases the water vapor concentration gradient between the inner leaf spaces and the outside air, slowing down transpiration.
• Wind Speed: Moving air sweeps away saturated water vapor surrounding the leaf, maintaining a steep gradient and increasing transpiration.

The Transpiration Pull (Cohesion-Tension Theory):

Water moves upward through xylem vessels spanning hundreds of feet against gravity due to a continuous water column driven by:

1. Transpiration Pull: Evaporation of water from stomata creates a negative pressure (suction force) at the leaf top.
2. Cohesion: Strong mutual attraction between individual water molecules due to hydrogen bonding.
3. Adhesion: Attraction between water molecules and the hydrophilic cellulose walls of xylem vessels.

The Cardiac Cycle:

The cardiac cycle is the sequence of mechanical events occurring during a single complete heartbeat. It consists of two primary phases:

• Systole (Contraction): The heart muscle contracts, pumping blood out of the chambers. Atrial systole forces blood into ventricles; ventricular systole pumps blood into the pulmonary artery and aorta.
• Diastole (Relaxation): The heart muscle relaxes, allowing the chambers to fill with blood. During joint diastole, all four chambers are relaxed.

The standard cardiac cycle lasts approximately 0.8 seconds based on a normal heart rate of 72 beats per minute.

The Electrocardiogram (ECG):

An ECG is a graphic recording of the electrical electrical activity generated by the heart muscle during depolarization and repolarization. A standard wave contains:

• P Wave: Represents atrial depolarization (atrial contraction).
• QRS Complex: Represents ventricular depolarization (ventricular contraction). It masks atrial repolarization.
• T Wave: Represents ventricular repolarization (ventricular relaxation).

Blood Pressure, Heart Rate, and Pulse:

• Blood Pressure (BP): The hydrostatic force exerted by blood against the arterial walls. Recorded as $\frac{\text{Systolic Pressure}}{\text{Diastolic Pressure}}$. Normal adult value is 120/80 mmHg, measured using a sphygmomanometer.
• Heart Rate: The number of cardiac contractions per minute (average: 70–75 bpm).
• Pulse: The rhythmic expansion and elastic recoil of an artery felt near the body surface, matching the ventricular contraction rate.

The Components of Blood:

Blood is a specialized fluid connective tissue that circulates throughout the body, delivering nutrients and oxygen while removing metabolic wastes. It is composed of formed elements (cells) suspended in a liquid matrix called plasma.

1. Red Blood Cells (Erythrocytes - RBCs):

• Structure: Biconcave discs that lack a nucleus and mitochondria at maturity, maximizing space for internal proteins.
• Function: Specialized for gas transport. Packed with haemoglobin, an iron-rich protein that binds reversibly with oxygen.

2. White Blood Cells (Leucocytes - WBCs):

• Structure: Nucleated, larger than RBCs, but fewer in number. Divided into Granulocytes (Neutrophils, Eosinophils, Basophils) and Agranulocytes (Lymphocytes, Monocytes).
• Function: Form the core of the immune system. They engulf pathogens via phagocytosis, secrete histamines, and produce specific antibodies to fight infections.

3. Platelets (Thrombocytes):

• Structure: Tiny, non-nucleated cell fragments broken off from giant bone marrow cells called megakaryocytes.
• Function: Essential for blood clotting (hemostasis). They aggregate at injury sites, releasing clotting factors that convert soluble fibrinogen into an insoluble fibrin mesh to seal wounds.

4. Plasma:

• The liquid matrix, making up roughly 55% of blood volume. Composed of 90-92% water, alongside critical solutes:
• Proteins: Albumin (osmotic balance), Globulins (defense/antibodies), and Fibrinogen (clotting).
• Nutrients & Hormones: Glucose, amino acids, lipids, and chemical messengers.

The ABO and Rh Systems:

Blood typing is determined by specific antigens (glycoproteins) located on the surface of red blood cells, and corresponding antibodies circulating in the plasma.

• ABO System: Based on the presence or absence of Antigen A and Antigen B:

• Type A: Has Antigen A on RBCs; Anti-B antibodies in plasma.

• Type B: Has Antigen B on RBCs; Anti-A antibodies in plasma.

• Type AB: Has both Antigens A and B; no anti-A or anti-B antibodies in plasma.

• Type O: Has neither antigen; both anti-A and anti-B antibodies in plasma.

• Rh System: Based on the presence or absence of the Rh antigen (D factor). Individuals with the antigen are Rh positive (Rh+), while those lacking it are Rh negative (Rh-).

Transfusion Guidelines and Cross-Matching:

Safe blood transfusions require matching donor antigens with recipient antibodies to prevent agglutination (clumping of cells), which blocks blood vessels and can be fatal.

• Universal Donor: Type O negative (O-). Its RBCs lack A, B, and Rh antigens, so they will not trigger an immune reaction in any recipient's body.
• Universal Recipient: Type AB positive (AB+). These individuals lack anti-A, anti-B, and anti-Rh antibodies, allowing them to safely receive blood of any type.

Before any transfusion, a cross-match test is performed in the laboratory by mixing donor RBCs with recipient serum to verify compatibility.

Structure of the Lymphatic System:

The lymphatic system is a specialized secondary circulatory loop and a core part of the immune system. It consists of:

• Lymph Capillaries: Blind-ended, highly permeable microscopic tubes interwoven among tissue cells that collect excess interstitial fluid.
• Lymph Nodes: Small, bean-shaped structures along lymph vessels that filter lymph and house dense clusters of lymphocytes.
• Spleen: The largest lymphatic organ; it filters blood, destroys old red blood cells, and stores platelets.
• Thymus: A specialized gland where T-lymphocytes mature and develop immune self-tolerance.

Lymph Formation and Roles:

As blood flows through capillaries, high hydrostatic pressure forces water and small solutes out into tissue spaces. Most fluid returns via osmosis, but about 10% remains behind as interstitial fluid. Once this fluid enters lymph capillaries, it is called lymph. Lymph is a clear, watery fluid containing less protein than plasma, but rich in white blood cells.

Key Functions:

1. Fluid Balance: Collects excess tissue fluid and returns it to the bloodstream via the subclavian veins, preventing fluid buildup (edema).
2. Fat Transport: Specialized lymph capillaries in the small intestine, called lacteals, absorb digested fats and fat-soluble vitamins, bypassing direct blood entry.
3. Immune Defense: Lymph nodes act as filtration checkpoints. When lymph passes through, foreign pathogens are trapped and destroyed by macrophages and lymphocytes, initiating an active immune response.