IB Biology SL: Theme B - Form & Function
B3.2 - Transport
Moving Life's Essential Fluids: Blood in Animals, Sap in Plants
🫀 Introduction to Transport Systems
As organisms evolved and grew larger, simple diffusion became insufficient for transporting materials throughout the body. Multicellular organisms needed specialized transport systems to move oxygen, nutrients, hormones, and waste products efficiently over long distances.
In this topic, we explore two major transport systems:
🫀 ANIMAL TRANSPORT
Blood circulatory system with heart, blood vessels, and blood
🌱 PLANT TRANSPORT
Vascular tissue system with xylem and phloem
Both systems solve the same fundamental problem: how to deliver essential materials to every cell and remove waste products efficiently, despite the limitations imposed by diffusion distance and surface area-to-volume ratios.
PART 1: TRANSPORT IN ANIMALS - THE CIRCULATORY SYSTEM
🩸 Blood Vessels: The Transport Network
Overview of Blood Vessels
The circulatory system contains approximately 60,000 miles (100,000 km) of blood vessels! These vessels form a closed network that transports blood throughout the body.
There are THREE main types of blood vessels: Arteries, Veins, and Capillaries. Each has a unique structure perfectly adapted to its specific function.
🔴 ARTERIES: Carrying Blood Away from the Heart
Function
Arteries transport blood AWAY FROM the heart to the body's tissues and organs. Blood travels at HIGH PRESSURE in arteries.
Most arteries carry oxygenated blood (rich in O₂, low in CO₂). Exception: Pulmonary artery carries deoxygenated blood from the heart to the lungs for oxygenation.
Structure
Artery walls have THREE layers (like all blood vessels), but each layer is thicker and stronger than in veins:
1. Tunica Intima (Inner Layer)
- Single layer of endothelial cells (squamous epithelium)
- Provides smooth surface to reduce friction and blood clotting
- Minimizes resistance to blood flow
2. Tunica Media (Middle Layer) - THICKEST LAYER
- Thick layer of smooth muscle cells
- Elastic fibers (elastin)
- Functions:
- Smooth muscle contracts/relaxes to control blood flow (vasoconstriction/vasodilation)
- Elastic fibers allow arteries to stretch when blood surges from heart
- Elastic fibers recoil to maintain pressure between heartbeats
3. Tunica Externa (Outer Layer)
- Collagen fibers (tough connective tissue)
- Provides structural support and prevents rupture under high pressure
- Anchors artery to surrounding tissues
Key Structural Features
✓ Thick Walls
Much thicker than veins. Withstand and maintain high blood pressure (~120/80 mmHg at rest).
✓ Small Lumen (Central Space)
Relatively narrow compared to wall thickness. Maintains high blood pressure.
✓ NO Valves
High pressure ensures blood flows in one direction—no backflow risk. (Exception: semilunar valves at heart exit)
✓ Circular Cross-Section
Maintains round shape even when cut (due to thick walls).
✓ Pulse Present
Can feel rhythmic expansion and recoil with each heartbeat.
🔵 VEINS: Returning Blood to the Heart
Function
Veins transport blood TOWARD (into) the heart from the body's tissues. Blood travels at LOW PRESSURE in veins.
Most veins carry deoxygenated blood (low in O₂, high in CO₂). Exception: Pulmonary veins carry oxygenated blood from the lungs to the heart.
Structure
Vein walls also have the same THREE layers, but they're thinner and less muscular than arteries:
1. Tunica Intima (Inner Layer)
- Single layer of endothelial cells
- Smooth surface for blood flow
- Forms valves at intervals
2. Tunica Media (Middle Layer) - THIN
- Thin layer of smooth muscle (less than arteries)
- Few elastic fibers
- Less need to push blood (low pressure system)
- Less need for elastic recoil
3. Tunica Externa (Outer Layer)
- Collagen fibers
- Provides support (though less robust than arteries)
Key Structural Features
✓ Thin Walls
Much thinner than arteries. Only need to withstand low pressure (~0-10 mmHg).
✓ Large Lumen
Wide central space (larger than arteries). Reduces resistance to blood flow. Can hold more blood (~70% of blood volume is in veins!).
✓ VALVES Present
Semilunar (pocket) valves at intervals. Prevent backflow of blood (especially important in limbs where gravity opposes blood flow). Open when blood flows toward heart, close to prevent backflow.
✓ Irregular/Collapsed Shape
May appear flattened or oval when cut (thin walls collapse). Shape varies with blood volume.
✓ NO Pulse
Blood flows smoothly without pressure surges—no detectable pulse.
💪 How Blood Returns Against Gravity:
Low pressure in veins isn't enough to push blood upward from legs. Three mechanisms help:
- Skeletal muscle pump: Muscle contractions squeeze veins, pushing blood toward heart
- Valves: Prevent backflow between muscle contractions
- Respiratory pump: Pressure changes during breathing help draw blood toward heart
🟢 CAPILLARIES: The Exchange Vessels
Function
Capillaries are the sites of material exchange between blood and tissues. They connect arteries to veins, forming vast networks throughout tissues.
What's exchanged? Oxygen, carbon dioxide, nutrients (glucose, amino acids), hormones, waste products (urea), water. Exchange occurs by diffusion and pressure filtration.
Structure
Capillaries have the SIMPLEST structure—perfectly adapted for exchange:
✓ ONE Cell Thick
Wall consists of single layer of endothelial cells (flattened squamous epithelium). No smooth muscle, no elastic tissue, no collagen. Minimizes diffusion distance—extremely short (~ 0.5-1 μm).
✓ Very Small Diameter
Lumen diameter: 5-10 μm (micrometers). Just wide enough for red blood cells to pass single-file. Forces RBCs close to capillary walls for efficient gas exchange.
✓ Highly Permeable
Thin walls allow rapid diffusion. Small gaps between cells allow plasma to leak out (forming tissue fluid). Some capillaries have pores (fenestrations) for even faster exchange.
✓ Extensive Networks (Capillary Beds)
Billions of capillaries throughout the body. Every cell is within ~2-3 cells of a capillary. Creates enormous total surface area for exchange.
✓ Slow Blood Flow
Blood slows down in capillaries (network increases cross-sectional area). Provides more time for exchange to occur.
✓ NO Valves
Flow is unidirectional—no backflow risk due to continuous connection between arterioles and venules.
🔑 Key Principle:
Capillaries are optimized for EXCHANGE, not transport. Their structure (thin walls, small diameter, extensive networks, slow flow) maximizes the rate of material exchange with tissues.
📊 Comparison: Arteries vs. Veins vs. Capillaries
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Function | Carry blood AWAY from heart | Carry blood TO heart | Exchange materials with tissues |
| Blood Pressure | HIGH (~120/80 mmHg) | LOW (~0-10 mmHg) | MEDIUM (decreasing) |
| Wall Thickness | THICK (3 layers, thick muscle & elastic tissue) | THIN (3 layers, but thin) | VERY THIN (1 cell thick) |
| Lumen Size | SMALL (narrow) | LARGE (wide) | VERY SMALL (5-10 μm) |
| Valves | NO (except near heart) | YES (prevent backflow) | NO |
| Pulse | YES (can be felt) | NO | NO |
| Blood Type | Oxygenated (except pulmonary artery) | Deoxygenated (except pulmonary veins) | Both (transition from O₂-rich to O₂-poor) |
| Shape When Cut | Round/circular | Irregular/collapsed | Very small dots |
💓 Pulse: Measuring Heart Activity
What is Pulse?
The pulse is the rhythmic expansion and recoil of arteries caused by the surge of blood pumped from the heart with each heartbeat.
Pulse rate = Heart rate. Measuring pulse is an easy, non-invasive way to assess cardiovascular health and fitness!
⚙️ How Pulse is Generated
1. Ventricular Contraction (Systole)
Heart's ventricles contract, forcefully ejecting blood into arteries (aorta and pulmonary artery)
2. Pressure Wave
Sudden surge of blood creates a pressure wave that travels through arterial walls
3. Artery Expansion
Elastic fibers in artery walls stretch to accommodate increased blood volume
4. Elastic Recoil
Between heartbeats, elastic fibers recoil, maintaining blood pressure and flow
5. Wave Propagation
This expansion-recoil cycle travels as a wave through arterial system. Can be felt where arteries are close to skin surface
📍 Where to Measure Pulse
Pulse can be felt at various pulse points where arteries pass close to the skin surface:
🖐️ Radial Pulse (Wrist)
Most common site. Radial artery on thumb side of wrist. Easy to locate and measure.
🫱 Carotid Pulse (Neck)
Carotid artery on side of neck. Strong, easy to find during/after exercise.
💪 Brachial Pulse (Elbow)
Brachial artery inside elbow. Used for blood pressure measurement.
🦵 Femoral Pulse (Groin)
Femoral artery in groin area. Strong pulse.
📊 Normal Pulse Rates
| Population | Resting Pulse Rate (beats per minute) |
|---|---|
| Adults (at rest) | 60-100 bpm (average ~70-75 bpm) |
| Athletes (at rest) | 40-60 bpm (lower due to higher cardiac efficiency) |
| Children (6-15 years) | 70-100 bpm |
| Infants | 100-160 bpm |
| During Exercise | Can reach 120-200+ bpm (depends on intensity and fitness) |
📈 Factors Affecting Pulse Rate:
- Exercise/Physical activity: Increases pulse (muscles need more O₂)
- Emotional state: Stress, anxiety, excitement increase pulse
- Temperature: Heat increases pulse; cold decreases it
- Body position: Standing > sitting > lying down
- Fitness level: Fit individuals have lower resting pulse
- Age: Younger = generally higher pulse
- Caffeine/stimulants: Increase pulse rate
❤️ Coronary Arteries: Feeding the Heart
What are Coronary Arteries?
Coronary arteries are specialized blood vessels that supply oxygen-rich blood to the heart muscle (myocardium) itself.
The heart pumps blood to the entire body, but it also needs its own blood supply! Coronary arteries branch directly from the aorta (just above the heart) to nourish the heart muscle.
🫀 The Two Main Coronary Arteries
Left Coronary Artery
- Supplies blood to left side of heart
- Branches into:
- Left anterior descending (LAD): Supplies front/anterior wall of left ventricle
- Left circumflex: Supplies back/lateral wall of left ventricle and left atrium
- Critical: Supplies most powerful chamber (left ventricle)
Right Coronary Artery
- Supplies blood to right side of heart
- Branches to:
- Right ventricle
- Right atrium
- Part of interventricular septum
- SA node and AV node (heart's pacemakers) in many people
⚠️ Coronary Artery Disease (CAD)
What is CAD?
Coronary Artery Disease (also called coronary heart disease or ischemic heart disease) occurs when coronary arteries become narrowed or blocked, reducing blood flow to the heart muscle.
Cause: Atherosclerosis
Atherosclerosis is the buildup of plaque (fatty deposits) on the inner walls of arteries.
Plaque consists of: cholesterol, fats, calcium, cellular waste, and fibrin (clotting material)
How Atherosclerosis Develops
1. Endothelial Damage
Inner lining of artery damaged by high blood pressure, smoking, high cholesterol, diabetes
2. Cholesterol Accumulation
LDL cholesterol ("bad" cholesterol) deposits in damaged area, forming fatty streaks
3. Inflammatory Response
White blood cells accumulate, trying to "clean up" cholesterol. Forms growing plaque
4. Plaque Growth & Calcification
Plaque grows larger, hardens with calcium deposits. Artery narrows, becomes less elastic
5. Reduced Blood Flow
Narrowed artery reduces oxygen supply to heart muscle, especially during exertion
6. Potential Rupture → Heart Attack
If plaque ruptures, blood clot forms → complete blockage → myocardial infarction (heart attack)
Symptoms of CAD
- Angina (chest pain): Pressure, tightness, or pain in chest during exertion or stress
- Shortness of breath: Especially during physical activity
- Fatigue: Due to reduced oxygen delivery to tissues
- Heart attack: Sudden, severe chest pain; radiating pain to arm, jaw, neck; nausea; cold sweat
🛡️ Risk Factors & Prevention
Risk Factors
- High cholesterol (high LDL, low HDL)
- High blood pressure (hypertension)
- Smoking
- Diabetes
- Obesity
- Physical inactivity
- Unhealthy diet (high in saturated/trans fats, salt, sugar)
- Chronic stress
- Family history
- Age (risk increases with age)
Prevention Strategies
- Healthy diet (fruits, vegetables, whole grains, lean protein)
- Regular exercise (150 min/week moderate activity)
- Maintain healthy weight
- Don't smoke
- Limit alcohol
- Manage stress
- Control blood pressure
- Control cholesterol
- Manage diabetes (if present)
- Regular check-ups
💊 Treatments for CAD:
Medications: Statins (lower cholesterol), blood thinners (prevent clots), beta-blockers (reduce heart workload). Procedures: Angioplasty with stent (opens blocked artery), coronary artery bypass grafting (CABG) - surgery to create new blood pathways.
PART 2: TRANSPORT IN PLANTS - VASCULAR TISSUE SYSTEM
🌱 Plant Transport System Overview
Why Do Plants Need Transport Systems?
Like animals, plants are too large for diffusion alone to move materials efficiently. They need specialized vascular tissues to transport substances over long distances.
Three Main Reasons:
- Size: Many plants are very tall (some trees > 100m!). Diffusion is too slow over such distances.
- Surface Area:Volume Ratio: As plants grow larger, their SA:V ratio decreases. Cannot rely on diffusion to supply all cells.
- Metabolic Demands: Roots don't photosynthesize (no glucose, no O₂ production). Leaves don't absorb water/minerals. Materials must be transported between organs.
🌿 The Two Vascular Tissues
XYLEM
Transports:
Water and dissolved minerals (ions)
Direction:
UPWARD ONLY
Roots → Stems → Leaves
Process:
Transpiration pull
PHLOEM
Transports:
Organic compounds (sugars/sucrose, amino acids)
Direction:
BIDIRECTIONAL
From source (leaves) → to sinks (roots, fruits, growing tissues)
Process:
Translocation (pressure flow)
💧 Xylem & Transpiration: Water Transport
🔬 Structure of Xylem
What is Xylem?
Xylem is a complex vascular tissue composed of several cell types. The main water-conducting cells are xylem vessels and tracheids.
Key Structural Features of Xylem Vessels:
1. Dead Cells
Xylem vessels are formed from dead cells. At maturity, the cell contents (cytoplasm, organelles, nucleus) die and are removed, leaving hollow tubes.
2. Hollow Tubes
The empty space allows unobstructed water flow. Continuous column of water from roots to leaves.
3. Lignified Walls
Cell walls contain lignin—a tough, waterproof polymer. Provides structural support (prevents collapse under negative pressure). Creates rigid tubes that help support plant height.
4. No End Walls
Multiple xylem cells stack vertically. End walls break down to form continuous tubes. Water flows freely without barriers.
5. Narrow Diameter
Small diameter (10-200 μm) allows capillary action and maintains water column through cohesion-tension.
6. Pits in Walls
Small pores allow lateral movement of water between adjacent xylem vessels and into surrounding cells.
💨 Transpiration: The Driving Force
What is Transpiration?
Transpiration is the evaporation of water from the aerial parts of plants, mainly through stomata in leaves. It creates the "pull" that draws water up through xylem.
Interesting fact: Plants lose ~90% of water absorbed by roots through transpiration! Only ~10% is used for photosynthesis and other processes. This seems wasteful, but transpiration is essential for water transport.
The Cohesion-Tension Theory
This theory explains how water moves from roots to leaves against gravity, without any pumping mechanism:
Step 1: Transpiration Creates Tension
Water evaporates from mesophyll cells in leaves through stomata. Creates negative pressure (tension) in leaf cells and xylem at top of plant.
Step 2: Cohesion Maintains Water Column
Water molecules are cohesive (stick together via hydrogen bonds). Form continuous column in xylem. As water is pulled up from top, entire column moves upward together.
Step 3: Adhesion to Xylem Walls
Water molecules adhere (stick) to lignified xylem walls. Prevents water column from falling and helps water climb walls.
Step 4: Water Uptake from Soil
As water is pulled up, more water enters root hair cells from soil by osmosis. Replaces water lost through transpiration.
🔑 Key Points:
- Water transport in xylem is PASSIVE—no ATP required!
- Driven by transpiration pull (evaporation at top)
- Relies on cohesion (water-water bonds) and adhesion (water-wall bonds)
- Xylem vessels under negative pressure (tension)
🌡️ Factors Affecting Transpiration Rate
1. Light Intensity
Higher light → Higher transpiration. Light causes stomata to open for photosynthesis, allowing more water loss.
2. Temperature
Higher temperature → Higher transpiration. Increases evaporation rate. Water molecules have more kinetic energy.
3. Humidity
Higher humidity → Lower transpiration. Reduces concentration gradient for water vapor diffusion from leaf to air.
4. Wind Speed
Higher wind → Higher transpiration (up to a point). Wind removes water vapor from leaf surface, maintaining steep gradient. Very strong wind causes stomata to close.
5. Stomatal Density & Size
More/larger stomata → Higher transpiration. More openings for water vapor to escape.
🍬 Phloem & Translocation: Sugar Transport
🔬 Structure of Phloem
What is Phloem?
Phloem is a complex living tissue that transports organic compounds (mainly sucrose) and amino acids. Main conducting cells are sieve tube elements supported by companion cells.
Key Structural Features:
1. Sieve Tube Elements (Living Cells)
Living cells (unlike xylem). Have cytoplasm but no nucleus at maturity (to maximize space for sap flow). Have few organelles. Elongated cells stacked end-to-end forming tubes.
2. Sieve Plates
Perforated end walls between sieve tube elements. Allow sap to flow between cells. Create continuous tubes like xylem, but with some barriers.
3. Companion Cells
Living cells with nucleus and many mitochondria. Connected to sieve tube elements by plasmodesmata (cytoplasmic connections). Provide metabolic support (produce ATP for active transport). Control loading/unloading of sugars.
4. Thin Cell Walls
Cellulose walls (NO lignin). Flexible, allowing expansion under pressure.
➡️ Translocation: Active Transport of Sugars
What is Translocation?
Translocation is the active transport of organic compounds (sucrose, amino acids) through phloem sieve tubes from sources (where made/stored) to sinks (where needed/stored).
Key Terms:
- Source: Regions where sugars are produced (photosynthesizing leaves) or mobilized (storage organs like tubers)
- Sink: Regions where sugars are used (roots, fruits, seeds, growing tips) or stored (tubers, bulbs)
- Direction: From source to sink—can be upward OR downward
⚡ Important Difference from Xylem:
Translocation requires ATP (energy)—it's an ACTIVE process! Sugars are actively loaded into phloem at source and unloaded at sink.
📐 Vascular Tissue Arrangement in Stems & Roots
🌿 Vascular Bundles
In plants, xylem and phloem are grouped together in structures called vascular bundles. The arrangement differs in different plant organs:
General Rule:
Xylem is typically positioned TOWARD THE CENTER (inside), while phloem is positioned TOWARD THE OUTSIDE. This arrangement is consistent across plant organs.
🌳 IN STEMS
Arrangement:
Vascular bundles are arranged in a ring pattern around the outer part of the stem (in dicots).
Position in Each Bundle:
- Xylem: Located on the INSIDE (toward center of stem)
- Phloem: Located on the OUTSIDE (toward epidermis)
- Cambium: Layer of dividing cells between xylem and phloem (allows secondary growth)
Why This Arrangement?
- Structural support: Xylem's lignified walls provide rigidity to support upright growth
- Ring pattern: Distributes support evenly around stem, preventing bending
- Protection: Phloem is closer to outer protective layers
- Efficient transport: Shorter distances for materials to move in/out of vascular tissue
🌱 IN ROOTS
Arrangement:
Vascular tissue forms a central core in the middle of the root (called the vascular cylinder or stele).
Position in Core:
- Xylem: Forms a central STAR or CROSS shape in the very center of root
- Phloem: Located in the spaces/pockets BETWEEN the xylem arms (alternating pattern)
- NO cambium between them in young roots (different from stems)
Why This Arrangement?
- Anchoring strength: Central xylem core provides strength to resist pulling forces
- Water uptake: Water absorbed by root hairs can easily reach xylem in center
- Protection: Vascular tissue protected by outer cortex layers
- Efficient absorption: Short distance from epidermis to vascular tissue
📊 Comparison: Xylem vs. Phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Transports | Water and dissolved minerals | Organic compounds (sucrose, amino acids) |
| Direction | UPWARD only (roots → leaves) | BIDIRECTIONAL (source → sink) |
| Cell Type | Xylem vessels and tracheids | Sieve tube elements + companion cells |
| Living/Dead | DEAD cells at maturity | LIVING cells |
| Cell Contents | Empty (hollow tubes) | Cytoplasm present (no nucleus in sieve tubes) |
| End Walls | Completely broken down (NO end walls) | Perforated (sieve plates) |
| Cell Walls | Lignified (contains lignin) - thick & rigid | Cellulose only (NO lignin) - thin & flexible |
| Transport Mechanism | PASSIVE (transpiration pull, cohesion-tension) | ACTIVE (requires ATP for loading/unloading) |
| Process Name | Transpiration stream | Translocation |
| Position (General) | INSIDE/CENTRAL (toward center) | OUTSIDE (toward periphery) |
| Additional Function | Provides structural support | No structural role |
🎯 Key Concepts Summary
✓ Blood Vessel Types
Arteries (thick walls, small lumen, high pressure, carry blood away from heart), Veins (thin walls, large lumen, low pressure, valves, carry blood to heart), Capillaries (one cell thick, site of exchange between blood and tissues).
✓ Pulse & Coronary Arteries
Pulse is the rhythmic expansion of arteries with each heartbeat. Coronary arteries supply oxygen to heart muscle itself. Coronary artery disease (atherosclerosis) causes narrowing/blockage, leading to angina or heart attack.
✓ Xylem Structure & Function
Xylem vessels are dead, hollow, lignified tubes that transport water and minerals upward (roots → leaves). Transport is passive, driven by transpiration pull via cohesion-tension mechanism. No ATP required.
✓ Phloem Structure & Function
Phloem consists of living sieve tube elements (no nucleus) supported by companion cells. Transports organic compounds (sucrose, amino acids) bidirectionally from source to sink. Translocation is active transport requiring ATP.
✓ Transpiration
Evaporation of water from leaves through stomata. Creates tension (negative pressure) that pulls water up xylem. Affected by light, temperature, humidity, and wind. ~90% of water absorbed is lost through transpiration.
✓ Vascular Tissue Arrangement
Stems: Vascular bundles in ring pattern; xylem inside, phloem outside. Provides structural support. Roots: Central vascular cylinder; xylem forms star in center, phloem in pockets between xylem arms. Provides anchoring strength.
📚 About the Author
Adam
Co-Founder @RevisionTown
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