OCR A-Level Biology Module 3: Exchange Surfaces, Transport in Animals and Plants - Complete Guide

Module Three - OCR Biology

1. Exchange Surfaces

Importance of Surface Area to Volume RatioExchange surfaces play a critical role in the efficient transport of substances, including gases like oxygen and carbon dioxide, throughout an organism. This efficiency is significantly influenced by the surface area to volume ratio (SA:V).

• Smaller Organisms: Smaller organisms such as amoebas and paramecia have a large surface area relative to their volume, which allows for efficient diffusion. Nutrients and gases can penetrate directly into and out of their cells without requiring specialized transport systems.

• Larger Organisms: As organisms increase in size, their surface area grows at a slower rate than their volume. This results in a lower SA:V ratio, which necessitates evolutionary adaptations, such as the development of specialized structures to optimize gas exchange and nutrient transport. Examples include:

  • Gills in Fish: Adapted for extracting oxygen from water.

  • Alveoli in Mammals: Adapted for gas exchange in the lungs.

  • Tracheal Systems in Insects: Allow for direct oxygen delivery to tissues.

Adaptations in Exchange SurfacesAdaptations that enhance the effectiveness of exchange surfaces include:

• Increased Surface Area: Structures like alveolar sacs in lungs and root hair cells in plants maximize surface area to facilitate greater exchange. In lungs, the numerous alveoli increase the surface area to approximately 70 square meters, which is critical for efficient gas exchange.

• Maintenance of Concentration Gradients: A continuous supply of oxygen and removal of carbon dioxide are maintained through:

  • Ventilation: In mammals, this involves the diaphragm and intercostal muscles to create pressure changes in the thoracic cavity, allowing fresh air to enter the lungs while expelling used air.

  • Blood Supply: A rich blood supply ensures the removed gases are quickly transported away, maintaining a concentration gradient favorable for diffusion.

• Reduced Diffusion Pathway: The presence of one cell thick squamous epithelial cells in alveoli minimizes the distance for diffusion, enhancing efficiency. The thickness of the diffusion pathway is thus reduced to 0.5 micrometers in human alveoli, promoting rapid gas exchange.

2. Transport in Animals

Circulatory Systems Overview:In animals, the circulatory system facilitates the transport of nutrients, gases, hormones, and waste products throughout the body. It is categorized into two main types:

• Open Circulatory System: Found primarily in invertebrates (e.g., arthropods and mollusks), where blood-like fluid called hemolymph is pumped into open spaces (hemocoils) at low pressure. This system allows for direct nutrient and gas exchange within body cavities, albeit less efficiently than closed systems.

• Closed Circulatory System: Blood is contained within vessels, allowing for more efficient transport of gases, nutrients, and waste products. This system is typical of vertebrates, including mammals, and allows for higher pressures and faster blood flow.

Single vs. Double Circulation:The circulatory system can be further classified based on the number of times blood passes through the heart during one complete circuit:

• Single Circulation: Blood passes through the heart only once per circuit. This is typical of fish, where deoxygenated blood is pumped to the gills for oxygenation, then directly to the rest of the body.

• Double Circulation: Blood flows through the heart twice for each circuit. In mammals, blood first passes through the pulmonary circuit (lungs) for gas exchange, then through the systemic circuit to deliver oxygenated blood to body tissues. This system ensures that tissue oxygenation is more efficient.

Blood Vessels and Capillary Dynamics:

• Functions of Different Blood Vessels:

  • Arteries: Carry oxygenated blood away from the heart (with the exception of the pulmonary arteries, which carry deoxygenated blood to the lungs). They possess thick, elastic walls that can withstand and maintain high pressure generated by the heart's contractions.

  • Arterioles: Smaller branches of arteries leading to capillaries; have muscular walls that can constrict or dilate, regulating blood flow to various tissues according to their needs.

  • Capillaries: Microscopic vessels that connect arterioles to venules; their narrow diameter slows blood flow, maximizing the time available for nutrient and gas exchange. Capillaries are comprised of a single layer of endothelial cells, minimizing diffusion distance and facilitating tissue fluid formation.

  • Venules and Veins: Carry deoxygenated blood back to the heart, featuring thinner walls than arteries and valves that prevent backflow, ensuring a unidirectional flow of blood.

Tissue Fluid Formation:The formation of tissue fluid from capillaries involves the interplay between hydrostatic pressure (which pushes liquid out of capillaries) and oncotic pressure (which draws liquid back into them due to plasma proteins). This process is crucial for regulating nutrient and waste distribution between blood and surrounding tissues.

The Heart:

• Structure and Function: The heart is a muscular organ composed of cardiac muscle, characterized as myogenic, indicating it can contract without external nervous stimulation. The heart consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle, each with distinct functions in the circulatory process.

• Cardiac Cycle: This cycle includes:

  • Diastole: The phase when the heart muscles relax and chambers fill with blood.

  • Atrial Systole: The phase when atria contract, pushing blood into the ventricles.

  • Ventricular Systole: The phase when ventricles contract, pumping blood to the lungs and systemic circulation.

• Cardiac Output: A vital measure defined as the amount of blood the heart pumps per minute, calculated as the product of heart rate (beats per minute) and stroke volume (volume of blood pumped per heartbeat), described by the equation CO = heart rate x stroke volume.

• Contraction Control: The sinoatrial (SA) node serves as the heart's natural pacemaker, generating electrical impulses that trigger heart contractions. These impulses travel through conduction pathways (e.g., the atrioventricular node and Purkinje fibers), ensuring coordinated contractions. Electrocardiograms (ECGs) provide a graphical representation of the electrical activity of the heart, allowing assessment of heart rhythms and identification of abnormalities or arrhythmias.

3. Transport in Plants

Main Transport Systems:Plants utilize specialized transport systems to move water, minerals, and organic compounds:

• Xylem: Responsible for the upward transport of water and dissolved minerals from the roots to the leaves. Water movement occurs primarily through capillary action, driven by transpiration (the loss of water vapor from aerial parts, mainly leaves). Xylem vessels consist of dead, hollow cells that allow for efficient transport under negative pressure generated by transpiration pull.

• Phloem: Facilitates the bidirectional transport of organic substances, particularly sucrose and amino acids, throughout the plant. This process occurs from sources (e.g., leaves where photosynthesis occurs) to sinks (e.g., roots or growing tissues). Phloem vessels consist of living cells and require energy for the active transport of nutrients.

Vascular Bundles in Roots, Stems, and Leaves:The arrangement of xylem and phloem in vascular bundles varies across different plant organs. In stems, vascular bundles are arranged in a ring to provide structural support, while in roots, they are located centrally. In leaves, they form a network that allows efficient distribution of water and nutrients for photosynthesis.

Water Transport:Water absorption occurs in root hair cells through osmosis, which can utilize two primary pathways:

• Symplastic Pathway: Water travels through the cytoplasm of root cells connected by plasmodesmata.

• Apoplastic Pathway: Water moves through the cell walls between root cells, reaching the endodermis where selective barriers regulate entry into the xylem.

Transpiration:Transpiration is the process of water vapor loss from aerial plant parts, primarily through stomata (small openings on leaves). This loss creates a negative pressure in the xylem, essential for drawing water upward from roots to shoots. Factors influencing transpiration rates include temperature, humidity, and wind speed.

Cohesion-Tension Theory:This theory explains how water moves upward through the xylem due to:

• Cohesive Forces: The attraction between water molecules helps maintain a continuous water column in the xylem.

• Adhesive Forces: Water molecules adhere to the vessel walls, supporting the rise against gravity. Root pressure, generated during water uptake, can also provide additional upward force.

Translocation:Translocation is the process of distributing organic nutrients throughout the plant, which requires energy. It involves the active transport of solutes into the phloem, creating a gradient that drives water from xylem into phloem, facilitating the flow of solutions (the mass flow hypothesis).Sources (where substances are produced or stored) and sinks (where they are utilized) play critical roles in this process, with seasonal changes affecting the direction of transport based on the