Transport in Animals and Plants

Adaptations for Fluid Transport in Animals and Plants

Introduction

• This section explores the adaptations that facilitate fluid transport in animals and plants, highlighting both the differences and similarities between these systems.

Capillaries: Exchange of Materials

• B3.2.1 Adaptations of capillaries for exchange: Capillaries facilitate the exchange of materials between blood and the internal or external environment.
• Large Surface Area: Branching and narrow diameters provide a large surface area for efficient diffusion.
• Thin Walls: The thin walls of capillaries reduce the diffusion distance.
• Fenestrations: Some capillaries have fenestrations (pores) for rapid exchange.
• Blood Circulatory System: (Cardiovascular system) delivers nutrients and oxygen to all body cells.
• Diameter: Narrowest blood vessels, about $10 \, μm$ in diameter.
• Capillary Network: Branch and rejoin repeatedly to form a network, transporting blood through almost all tissues.
• Surface Area: Many narrow capillaries have a greater total surface area than fewer wider vessels, enhancing diffusion.
• Density: The density of capillary networks varies depending on the metabolic needs of the cells, but all active cells are close to a capillary.
• Exceptions:
  • Lens and cornea of the eye: These tissues are transparent and lack blood vessels.
• Capillary Wall Composition:
  • Endothelium: A single layer of endothelium cells.
  • Basement Membrane: A coating of extracellular fibrous proteins (gel) that acts as a filter, allowing small to medium-sized particles to pass through but not macromolecules.
• Permeability: Pores between epithelium cells make the capillary wall very permeable.
  • Allows blood plasma (excluding red blood cells) to leak out through the basement membrane.
• Fenestrated Capillaries:
  • Large pores in the capillary walls.
  • Allow larger volumes of tissue fluid to be produced, speeding up exchange.
  • Example: Glomerulus of the kidney uses fenestrated capillaries to produce large volumes of filtrate in urine production.
• Tissue Fluid Formation:
  • Tissue fluid is similar in composition to blood plasma but lacks large protein molecules (too large to pass through the basement membrane).
  • Contains oxygen, glucose, and other substances from blood plasma.
  • Fluid flows between cells, allowing them to absorb nutrients and excrete waste products before re-entering the capillary network.

Arteries and Veins: Structure

• B3.2.2 Structure of arteries and veins: Arteries and veins have distinct structural features.
• Arteries:
  • Carry pulses of high-pressure blood away from the heart to the organs.
• Veins:
  • Carry a stream of low-pressure blood from the organs back to the heart.
• Distinguishing Features in Micrographs:
  • Arteries: Thicker walls, narrower lumen.
  • Veins: Thinner walls, wider lumen.

Table 1: Structural Differences Between Arteries and Veins

Arteries: Adaptations for Transport

• B3.2.3 Adaptations of arteries: Arteries are adapted to transport blood away from the heart at high pressure.
• Withstand and Maintain High Blood Pressure: Layers of muscle and elastic tissue help arteries withstand and maintain high blood pressure.
• Artery Wall Layers:
  • Tunica Externa: A tough outer layer of connective tissue with collagen fibres.
  • Tunica Media: A thick layer containing smooth muscle and elastic fibres (elastin protein).
  • Tunica Intima: A smooth endothelium lining the artery; some arteries also include a layer of elastic fibres.
• Lumen: Arteries have relatively narrow lumens to maintain high blood pressure and blood flow velocity.
• Wall Composition:
  • Elastic fibres (up to $50\%$ of dry mass) stretch and recoil.
  • Collagen fibres are tough and provide high tensile strength.
  • These features enable arteries to withstand high and variable blood pressures.
• Energy Storage:
  • Under high pressure, the artery wall widens, stretching elastic fibres.
  • This stores potential energy.
• Diastolic Pressure:
  • When pressure falls (diastolic pressure), elastic fibres recoil and squeeze blood forward.
  • This reduces the energy expended in transporting blood.
• Semilunar Valves:
  • Elastic fibre recoil occurs when semilunar valves at the exit of the ventricles are closed.
  • This prevents backflow and forces blood towards the organs.
  • Evens out blood flow in arteries.
• Smooth Muscle Cells: Artery walls contain smooth muscle cells, especially in arterioles.
  • Vasoconstriction: Circular muscle contraction narrows the lumen, reducing blood flow.
  • Vasodilation: Muscle relaxation widens the lumen, increasing blood flow.
  • Respond to hormone and neural signals to adjust blood flow rate to tissues based on need.

Pulse Rate Measurement

• B3.2.4 Measurement of pulse rates: Pulse rate reflects heart rate.
• Pressure Wave: Each heartbeat creates a wave of blood under high pressure along the arteries.
• Pulse: Where an artery is close to the body surface, this pressure wave is felt as a pulse due to artery wall stretching and recoiling.
• Heart Rate: One pulse per heartbeat, counted in beats per minute.
• Traditional Method:
  • Use fingertips (not thumb) to press lightly against the skin over an artery (e.g., wrist or neck).
• Modern Method:
  • Pulse Oximeters clipped to a fingertip shine red and infrared light through the finger.
  • Detectors measure light transmission, indicating blood volume changes with each heartbeat, thus calculating the heart rate.
  • Oxygen saturation is also deduced based on red and infrared light absorption differences (deoxygenated vs. oxygenated blood).

Veins: Adaptations for Blood Return

• B3.2.5 Adaptations of veins: Veins are adapted for the return of blood to the heart.
• Valves: Veins contain valves to prevent backflow and are designed to be compressed by muscle action.
• Low Pressure: Blood pressure in veins is much lower than in arteries.
• Thin Walls: Vein walls do not need to be thick to prevent bursting because of lower pressure.
• Backflow Prevention: Potential problem of backflow is addressed via valves.
• Collection and Conveyance: Veins collect blood from organs and return it to the heart continuously.
• No Pulse in Veins: Blood drains continuously, so there is no pulse.
• Fewer Elastic Fibres and Smooth Muscle: Veins have fewer elastic fibres and smooth muscle cells compared to arteries because they do not regulate blood flow to different areas.
• Venous Blood Flow Improvement: Walking, sitting, or fidgeting improves venous blood flow.
• Blood Distribution: About $80\%$ of blood is in the veins at rest, reduced during exercise.
• Pocket Valves: Maintain circulation:
  • Three cup-shaped flaps of tissue project into the vein in the direction of blood flow.
  • Blood flowing towards the heart pushes the flaps to the sides, opening the valve.
  • Backwards blood flow causes blood to get caught in the flaps, filling and closing the valve, blocking the lumen.
• Assisted Blood Flow: Blood flow in veins is assisted by gravity and skeletal muscle pressure.
  • Muscle contraction squeezes adjacent veins, acting as a pump.
  • Thin walls of veins allow them to be easily squeezed.
• Varicose Veins: Occur when pocket valves weaken or become damaged, causing backflow and blood accumulation, leading to swelling and enlargement.

Coronary Artery Occlusion

• B3.2.6 Causes and consequences of occlusion: Addresses the causes and consequences of coronary artery occlusion.
• Aorta: Carries blood pumped by the left side of the heart to all organs except the lungs.
• Coronary Arteries: Two arteries branch off from the aorta near its origin:
  * Supply oxygen to the right side of the heart.
  * Branch into two arteries that supply the left anterior and left posterior regions of the heart wall.
• Three Main Coronary Arteries: Each branches repeatedly to provide oxygenated blood to the heart's muscular wall.
• Narrowing or Blockage: Coronary arteries can be narrowed or blocked by fatty deposits (atheroma or plaque).
• Atheroma: Deposits build up in the artery wall and contain lipids (including cholesterol).
  * Restrict blood flow, causing chest pain (angina) or shortness of breath, especially during exercise.
• Arterial Hardening: Fatty deposits can become impregnated with calcium salts, hardening the artery and roughening the inner surface.
• Thrombosis: Rough surfaces prone to forming blood clots (thrombosis).
  • Hypertension (high blood pressure) increases the risk of thrombosis.
• Heart Attack:
  • Blood clots can block blood flow, depriving the heart muscle of oxygen and preventing normal contractions.
• Myocardial Infarction: Tissue death in heart muscle due to inadequate blood supply.

Coronary Heart Disease (CHD) Risk Factors:

• Hypertension: Raised blood pressure increases clot formation.
• Smoking: Raises blood pressure because nicotine causes vasoconstriction.
• Saturated Fat and Cholesterol: Promotes plaque formation.
• Obesity: Associated with raised blood pressure and high blood cholesterol.
• High Salt Intake: Raises blood pressure.
• Excessive Alcohol: Associated with raised blood pressure and obesity.
• Sedentary Lifestyles: Lack of exercise is correlated with obesity and prevents venous blood return, increasing clot risk.
• Genetic Predisposition: Some genes increase hypertension and thrombosis risk.
• Old Age: Blood vessels become less flexible.

Water Transport in Plants: Transpiration

• B3.2.7 Transport of water from roots to leaves: Focuses on water transport from roots to leaves during transpiration.
• Process:
  • Water loss from leaf cell walls via transpiration causes water to be drawn out of xylem vessels.
  • Water moves through cell walls by capillary action, generating tension (negative pressure potentials).
  • This tension pulls water up the xylem.
  • Cohesion ensures a continuous water column.
• Suction Limitation: Atmospheric pressure can only push water up to $10.4 \,m$ in an air-filled tube. Trees can grow much taller, indicating a different mechanism in plants.
• Cellulose and Adhesion:
  • Cell walls contain hydrophilic cellulose molecules that form hydrogen bonds with water.
  • Adhesion occurs between water and cellulose.
• Capillary Action in Leaves:
  • Water loss causes water to be drawn through interconnected leaf cell walls in pores between cellulose molecules.
  • Similar to how water is drawn through filter paper (mostly cellulose).
• Xylem Sap: The source of water in leaf cell walls is the xylem vessel in the nearest vein.
• Transpiration Pull:
  • Water is lost by evaporation from spongy mesophyll cell walls and diffusion of water vapour through stomata.
  • Leaf cell walls drawing water from xylem generate tensions (pulling forces).
  • With a continuous water column in xylem, these tensions are transmitted from leaves down to the roots (transpiration pull).
  • Strong enough to move water upwards against gravity to the top of the tallest tree.
  • It is a passive process; energy comes from thermal energy (heat) that causes transpiration.
• Cohesion: The pulling of water upwards depends on the cohesion between water molecules.
• Cavitation: Some liquids would be unable to resist in the very low pressures in xylem vessels, which would cause the column of liquid to break.

Xylem Vessels: Adaptations for Water Transport

• B3.2.8 Adaptations of xylem vessels: Adaptations include lack of cell contents, incomplete or absent end walls, lignified walls, and pits.
• Efficient Water Transport: Structure facilitates efficient water transport.
• Formation: Formed from columns of cells arranged end-to-end.
• Structure:
  * Cell wall material between cells is largely removed.
  * Plasma membranes and cell contents break down.
  * Creates long, continuous tubes with minimal resistance to flow.
  * Mature xylem vessels are non-living, making water flow a passive process.
• Lignified Walls: Walls are thickened and impregnated with a polymer called lignin.
  • Provide strength, preventing vessel collapse under low pressure/tension.
• Gaps in Wall Thickening
  • The lignified wall thickenings are impermeable to water but there are always gaps in the thickening through which water can enter and exit.

Dicot Stem: Tissue Distribution

• B3.2.9 Distribution of tissues: Covers tissue distribution in a transverse section of a dicotyledonous plant stem.
• Structure: Dicots have two seed leaves - sunflowers, peas, and oaks.
• Epidermis: The outer layer of cells in young plants.
• Vascular Bundles: Dicot stems typically have vascular bundles near the epidermis.
• Tissue Arrangement:
  • Xylem is usually on the inner side of a vascular bundle.
  • Phloem is on the outer side.
  • Cambium (stem cells) lies between the xylem and phloem.
  • Pith is in the centre and cortex near the epidermis.

Table 2: Plant Tissues and Their Functions in Stems

Dicot Root: Tissue Distribution

• B3.2.10 Distribution of tissues: Discusses tissue distribution in a transverse section of a dicot root.
• Vascular Tissue Arrangement: All vascular tissue is grouped in the center of the root:
  • Xylem is in a star-shaped area.
  • Phloem is between the points of the star.
• Identifiable Features:
  • Xylem vessels: Large size, thick walls, and rounded shape (lignified walls may be stained red).
  • Phloem cells: Smaller than xylem with thinner walls (unlignified cells usually stained blue).
• Outer Layers:
  • Epidermis: Small cells with root hairs protruding.
  • Cortex: Between vascular tissue and epidermis, large and thin-walled cells.