Plant Transport and Human Excretory Systems

Transport of Materials in Phloem

• Energy-Dependent Loading: Materials such as sucrose are actively transferred into the phloem tissue. This process requires the expenditure of energy in the form of $ATP$ (Adenosine Triphosphate).

• Osmotic Pressure Mechanism: The active loading of sucrose increases the osmotic pressure within the phloem tissue.

• Water Movement: Following the increase in osmotic pressure, water moves into the phloem tissue.

• Pressure-Driven Bulk Flow: This increased pressure forces the material in the phloem to move toward tissues that have lower pressure.

• Physiological Adaptation: This mechanism allows the plant to relocate materials based on its immediate requirements.

• Secondary Application Example: During the spring season, sugar that has been stored in the root or stem tissues is mobilized and transported to the buds, which require significant energy to facilitate new growth.

Review Questions: Transport Systems

• Question $1$: What are the components of the transport system in human beings?

• Question $2$: What are the functions of these components?

• Question $3$: Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?

• Question $4$: What are the components of the transport system in highly organised plants?

• Question $5$: How are water and minerals transported in plants?

• Question $6$: How is food transported in plants?

Fundamental Concepts of Excretion

• Definition: Excretion is the biological process involved in the removal of harmful nitrogenous metabolic wastes from the body.

• Context of Waste: Organisms generate different types of waste. Gaseous wastes from photosynthesis or respiration (such as $CO_2$) are handled separately. Other metabolic activities produce nitrogenous materials that must be eliminated.

• Strategies Across Species:

  • Unicellular Organisms: These move wastes out of the body by simple diffusion from the body surface directly into the surrounding water.

  • Complex Multicellular Organisms: These utilize specialized organs to perform the excretory function.

Anatomy of the Human Excretory System

• Major Components (Figure $5.13$): The system comprises:

  • A pair of Kidneys.

  • A pair of Ureters.

  • A Urinary Bladder.

  • A Urethra.

• Kidney Location: The kidneys are situated in the abdomen, with one positioned on either side of the backbone.

• Associated Vasculature: Key vessels include the Left renal artery, Left renal vein, Aorta, and Vena cava.

• Pathway of Urine: Urine is produced in the kidneys, travels through the ureters to the urinary bladder (where it is stored), and is eventually released through the urethra.

The Mechanism of Urine Formation and Filtration

• Primary Purpose: The fundamental objective of making urine is to filter out nitrogenous waste products, such as urea or uric acid, from the blood.

• Comparison to Lung Function: Just as the lungs remove $CO_2$ from the blood, the kidneys remove nitrogenous filtration through capillary clusters.

• The Nephron: Each kidney contains a large number of filtration units known as nephrons, packed closely together.

• Structural Components of Filtration:

  • Capillary Cluster: A grouping of thin-walled capillaries.

  • Bowman’s Capsule: A cup-shaped end of a coiled tube that is associated with each capillary cluster and collects the filtrate.

• Selective Reabsorption: As the filtrate flows along the tube, several substances are selectively re-absorbed into the blood. These include:

  • Glucose

  • Amino acids

  • Salts

  • Water (reabsorbed in varying amounts).

• Factors Governing Water Reabsorption: The volume of water re-absorbed is dependent on two main factors:

  • The quantity of excess water currently in the body.

  • The amount of dissolved waste that needs to be excreted.

• Storage and Control:

  • Urine enters a long tube called the ureter which connects the kidney to the urinary bladder.

  • The urinary bladder is a muscular organ.

  • Storage capacity is managed by the expansion of the bladder.

  • Nervous Control: Because the bladder is muscular and under nervous system control, humans can usually control the urge to urinate (micturition).