Transport moves water, nutrients, gases and wastes between parts of the organism. Plants use xylem and phloem; humans use a closed circulatory system. CBSE Class 11 and NEET test both.
Core Concepts
Xylem transport
Upward movement of water and minerals from roots to leaves. Tracheids and vessels form continuous tubes. Driven by transpiration pull and cohesion-tension. Root pressure contributes in short plants and at night.
The cohesion-tension theory (Dixon and Joly):
Water evaporates from mesophyll cells into leaf air spaces and exits through stomata. This creates a negative pressure (tension) in the leaf xylem.
Water molecules are strongly attracted to each other through hydrogen bonds (cohesion). This creates an unbroken column of water from leaves to roots.
Water molecules also adhere to the walls of xylem vessels (adhesion), preventing the column from pulling away from the walls. The narrow diameter of xylem vessels enhances this capillary effect.
The tension created by transpiration at the top pulls the entire water column upward. It is a passive process — no energy is spent by the plant on pumping. The energy comes from the sun (which drives evaporation).
| Force | Contribution | When Active |
|---|---|---|
| Transpiration pull | Major driving force | During the day (stomata open) |
| Root pressure | Minor (pushes water up) | At night, in short plants |
| Capillarity | Minor (in narrow tubes) | Always |
| Cohesion-adhesion | Maintains continuous column | Always |
Root pressure can be demonstrated by guttation — water droplets appearing on leaf margins in the early morning. This happens when root pressure pushes water upward at night (when transpiration is minimal), and excess water is expelled through hydathodes.
Phloem transport
Bidirectional movement of sugars from sources (mature leaves) to sinks (roots, fruits, growing tissues). Uses sieve tubes and companion cells. Pressure flow hypothesis (Munch) explains the mechanism.
Munch’s pressure flow hypothesis:
Sucrose is actively loaded into sieve tubes at the source (mature leaves) using ATP. This increases the osmotic concentration inside the sieve tube.
High solute concentration draws water from nearby xylem into the sieve tube by osmosis. This builds up turgor pressure.
The pressure gradient drives bulk flow of sap from high-pressure source to low-pressure sink through sieve plates.
At the sink (roots, fruits, growing regions), sucrose is unloaded (removed from sieve tubes). Water follows out by osmosis. Pressure drops.
Key difference: xylem transport is passive (driven by transpiration pull — physical forces). Phloem transport is active (requires ATP for loading sucrose at the source). This is a common NEET comparison question.
Transpiration
Loss of water vapour from leaves through stomata. Creates the pull for xylem transport. About 99% of water absorbed by roots is lost in transpiration — only 1% is used in photosynthesis.
Types of transpiration:
- Stomatal (90-95% of total) — through stomata on leaf surface
- Cuticular (5-10%) — through the cuticle on leaf epidermis
- Lenticular (very small) — through lenticels on bark of woody stems
Factors affecting transpiration:
- Temperature (higher temp → more evaporation → more transpiration)
- Humidity (lower humidity → steeper water potential gradient → more transpiration)
- Wind (moves moist air away from leaf surface → more transpiration)
- Light (opens stomata → more transpiration)
- Number and distribution of stomata
- A maize plant transpires about 200 litres of water during its growing season
- 99% of water absorbed is transpired; only 1% is used in photosynthesis
- Transpiration rate can be measured using a potometer (measures water uptake, which approximately equals transpiration)
Human circulation
Closed, double circulation. Four-chambered heart. Pulmonary circuit carries deoxygenated blood to lungs and back; systemic circuit carries oxygenated blood to body and back.
The cardiac cycle:
| Phase | Atria | Ventricles | Valves |
|---|---|---|---|
| Atrial systole | Contract | Relax | AV valves open, semilunar closed |
| Ventricular systole | Relax | Contract | AV valves close (lub), semilunar open |
| Joint diastole | Both relax | Both relax | AV valves open, semilunar close (dub) |
The cardiac cycle lasts about 0.8 seconds at a resting heart rate of 75 beats/min.
Blood pressure: Systolic (ventricle contracts, blood pushes against artery walls) / diastolic (ventricle relaxes). Normal: 120/80 mmHg. Hypertension: consistently above 140/90.
Blood vessels comparison:
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Wall thickness | Thick, elastic | Thin | One cell thick |
| Lumen | Narrow | Wide | Microscopic |
| Blood pressure | High | Low | Very low |
| Valves | No (except at heart) | Yes (prevent backflow) | No |
| Direction | Away from heart | Toward heart | Connect arteries to veins |
| Blood type | Usually oxygenated | Usually deoxygenated | Exchange site |
Lymphatic system
Complements blood circulation. Collects tissue fluid (lymph) and returns it to the bloodstream via the thoracic duct. Lymph nodes filter lymph and host immune cells.
Lymph differs from blood: it has no RBCs (so it is colourless/pale yellow), fewer proteins, and flows in only one direction (from tissues to veins). The lymphatic system also absorbs fats from the intestine through lacteals in villi.
Worked Examples
Transpiration pull plus the unbroken column of water held together by cohesion (via hydrogen bonds) lifts water hundreds of feet without any pump. This is the cohesion-tension theory.
Sucrose is actively loaded into sieve tubes at the source, raising osmotic concentration. Water follows, building pressure. At the sink, sucrose is unloaded, water leaves, pressure drops. The gradient drives bulk flow.
The left ventricle pumps blood through the systemic circuit — to the entire body, including extremities like feet and brain. This requires high pressure and a muscular wall. The right ventricle only pumps to the nearby lungs (pulmonary circuit), requiring less pressure and a thinner wall.
Girdling removes the phloem (which is in the bark). Without phloem, sugars from leaves cannot reach the roots. The roots starve and die. Without roots, the tree cannot absorb water, and it dies too. Xylem (deeper in the trunk) is not affected by bark removal, so water transport continues temporarily.
Common Mistakes
Saying xylem transport is active. It is passive — driven by transpiration pull.
Confusing xylem (water) and phloem (sugars).
Calling lymph a form of blood. It is derived from tissue fluid, with no RBCs.
Saying phloem transport is only downward. It is bidirectional — from source to sink. In spring, sugars stored in roots move upward to growing buds.
Writing that all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood from heart to lungs. The pulmonary vein carries oxygenated blood from lungs to heart. Definition is based on direction (away from/toward heart), not oxygen content.
Exam Weightage and Revision
Transport in Plants and Circulation carry 3-4 NEET questions per year combined. The most tested concepts: transpiration pull theory, phloem loading, double circulation, and blood vessel comparison. CBSE boards ask for diagrams (heart, xylem/phloem) and process descriptions (5-8 marks).
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Cohesion-tension theory | Every year | Medium |
| Phloem transport mechanism | Most years | Medium |
| Heart structure/function | Every year | Easy-Medium |
| Artery vs vein comparison | Most years | Easy |
| Transpiration factors | Every 2 years | Easy |
| Guttation/root pressure | Occasional | Easy |
Two guaranteed NEET topics from this chapter: (1) the mechanism of water transport in xylem (cohesion-tension), and (2) some aspect of the cardiac cycle or blood vessel comparison. Know both thoroughly.
Practice Questions
Q1. What is the role of companion cells in phloem transport?
Companion cells are metabolically active and provide energy (ATP) for the loading of sucrose into sieve tubes. Sieve tube elements lack nuclei and most organelles, so they depend on companion cells for metabolic support. Companion cells also help maintain sieve tube membrane integrity and assist in phloem loading through plasmodesmatal connections.
Q2. Why does transpiration rate increase on a windy day?
Wind removes the layer of humid air immediately surrounding the leaf surface (the boundary layer). This increases the water potential gradient between the inside of the leaf (high water content) and the surrounding air (now drier). A steeper gradient drives faster diffusion of water vapour out of the stomata, increasing transpiration rate.
Q3. What are the heart sounds “lub” and “dub”?
“Lub” (first heart sound) is produced by the closure of atrioventricular (AV) valves (bicuspid and tricuspid) at the start of ventricular systole. “Dub” (second heart sound) is produced by the closure of semilunar valves (aortic and pulmonary) at the end of ventricular systole. Abnormal sounds (murmurs) indicate valve defects.
Q4. A potometer measures water uptake by a plant. Is this exactly equal to transpiration? Why or why not?
Not exactly. Water uptake measured by a potometer slightly exceeds transpiration because a small fraction (about 1%) of absorbed water is used in photosynthesis, cell expansion, and other metabolic processes. However, since transpiration accounts for about 99% of water uptake, the potometer reading is a very close approximation of transpiration rate.
FAQs
Can water be pushed up a tall tree by root pressure alone?
No. Root pressure can push water only a few metres (it generates about 2 atm of pressure). A 100-metre tall redwood needs about 10 atm just to overcome gravity, plus more to overcome friction. Transpiration pull generates the much larger negative pressures (up to -20 atm) needed for tall trees.
Why do plants wilt on hot days even with adequate soil moisture?
On very hot, dry days, transpiration rate exceeds water absorption rate. Water leaves the leaves faster than the roots can replace it. Cell turgor drops, and the plant wilts. This is temporary wilting — the plant recovers in the evening when transpiration slows.
What happens if a person’s heart valves do not close properly?
Blood flows backward through the leaky valve (regurgitation). This reduces the efficiency of blood pumping and creates abnormal heart sounds (murmurs). In severe cases, the heart has to work harder to maintain circulation, leading to heart enlargement and eventually heart failure if untreated.
Draw two arrows — xylem upward, phloem bidirectional — with the main driving force labelled on each. Covers the chapter.
Transport is a logistics problem. Plants solve it with passive physics; humans solve it with an active pump. Same problem, different engineering.