Topic 6: Transport in Plants and Animals

1. The Need for Transport Systems

Single-celled organisms (like Amoeba) have a high surface area to volume ratio, allowing gases, nutrients, and waste products to exchange directly with their environment via simple diffusion.

However, complex multicellular organisms (like flowering plants and mammals) have a low surface area to volume ratio, and most cells are located deep within the body away from the external environment. Therefore, they require specialized internal transport systems to distribute nutrients and gases and to remove metabolic wastes efficiently.

2. Transport in Plants

Plants require transport systems to move water and mineral salts from the roots up to the leaves, and to distribute manufactured organic food (sugars) from the leaves to other parts of the plant.

Vascular Tissues

• Xylem:
  • Composed of dead, hollow, elongated cells (vessels and tracheids) whose walls are thickened with lignin to prevent collapse under tension.
  • Function: Transports water and mineral ions unidirectionally from roots to stems and leaves.
• Phloem:
  • Composed of living cells (sieve tube elements and companion cells).
  • Function: Transports manufactured food (sucrose and amino acids) bidirectionally between sources and sinks—a process known as translocation.

Mechanisms of Water Movement Upward in Plants

1. Root Pressure: Active transport of mineral ions into the root xylem creates a high solute concentration, drawing water inward by osmosis and pushing water a short distance up the stem.
2. Capillary Action (Adhesion and Cohesion): Water molecules stick to each other (cohesion) and to the hydrophilic walls of xylem vessels (adhesion), pulling water upward in narrow tubes.
3. Transpiration Pull (Cohesion-Tension Theory):
   • Water evaporates from the mesophyll cell walls into air spaces and diffuses out through stomata (transpiration).
   • This creates a low water potential (tension) in the leaf, pulling a continuous, unbroken column of water upward from the roots through the xylem.

Factors Affecting Transpiration Rate

• Temperature: Higher temperature increases the kinetic energy of water molecules, increasing evaporation rate.
• Humidity: High humidity in surrounding air decreases the concentration gradient, slowing down transpiration.
• Wind Speed (Air Movement): Wind blows away water vapor accumulating around stomata, maintaining a steep diffusion gradient and increasing transpiration.
• Light Intensity: High light intensity opens stomata for photosynthesis, facilitating water vapor diffusion.

3. Transport Systems in Animals

Animals possess circulatory systems to transport oxygen, carbon dioxide, nutrients, hormones, and waste products.

Types of Circulatory Systems

• Open Circulatory System: Blood (hemolymph) flows out of blood vessels into open body cavities (hemocoels) where it bathes tissues directly (e.g., arthropods, insects).
• Closed Circulatory System: Blood is entirely enclosed within a continuous network of blood vessels (arteries, capillaries, veins) and pumped by a heart (e.g., annelids, vertebrates).

Single vs. Double Circulation

• Single Circulation: Blood passes through the heart once in every complete circuit of the body (e.g., fish; heart $\rightarrow$ gills $\rightarrow$ body tissues $\rightarrow$ heart).
• Double Circulation: Blood passes through the heart twice in every complete circuit (e.g., mammals and birds):
  • Pulmonary Circulation: Deoxygenated blood flows from the heart to the lungs and back.
  • Systemic Circulation: Oxygenated blood flows from the heart to all body organs and back.

4. The Mammalian Heart and Blood Vessels

Structure of the Mammalian Heart

The heart is a muscular organ made of involuntary cardiac muscle, divided into four chambers: two thin-walled atria (receiving chambers) and two thick-walled ventricles (pumping chambers). [Vena Cava / Pulmonary Vein] │ ┌──────┴──────┐ │ Right Atrium│ ──(Tricuspid Valve)──┐ └──────┬──────┘ │ │ Right Ventric.| ▼ └──────┬──────┘ [Pulmonary Artery] │ │ [Interventricular Septum] ▼ │ [Lungs] ┌──────┴──────┐ │ Left Atrium │ ──(Bicuspid/Mitral)──┐ └──────┬──────┘ │ │ Left Ventric.│ ▼ └──────┬──────┘ [Aorta to Body] │

• Valves: Prevent backflow of blood.
  • Tricuspid Valve: Between right atrium and right ventricle.
  • Bicuspid (Mitral) Valve: Between left atrium and left ventricle.
  • Semi-lunar Valves: At the base of the aorta and pulmonary artery.
• Coronary Arteries: Supply oxygen and nutrients directly to the heart muscle tissue itself.

Comparison of Blood Vessels

5. Composition and Functions of Blood

Blood is a specialized fluid tissue consisting of cellular elements suspended in a liquid matrix called plasma.

Plasma (55% of blood volume)

• A pale yellow liquid composed of $90%$ water and $10%$ dissolved substances (glucose, amino acids, mineral ions, hormones, plasma proteins like fibrinogen and albumin, and urea).
• Function: Transports dissolved nutrients, waste products, hormones, and distributes heat across the body.

Cellular Components (Formed Elements - 45%)

1. Red Blood Cells (Erythrocytes):
   • Biconcave discs lacking a nucleus and mitochondria to maximize space for hemoglobin.
   • Function: Contain hemoglobin which reversibly binds with oxygen to form oxyhemoglobin, transporting oxygen from lungs to respiring tissues.
2. White Blood Cells (Leucocytes):
   • Nucleated cells capable of amoeboid movement.
   • Divided into phagocytes (engulf and digest pathogens) and lymphocytes (produce specific antibodies against foreign antigens).
3. Platelets (Thrombocytes):
   • Cell fragments derived from bone marrow megakaryocytes.
   • Function: Release thromboplastin to trigger the blood clotting cascade, preventing excessive blood loss and pathogen entry at injury sites.