Transport
Transport in living organisms moves water, dissolved nutrients, hormones and gases between tissues. The PMDC MDCAT 2026 syllabus expects you to explain the ascent of sap and transpiration in plants, describe how stomata open and close, list the components of human blood, and predict outcomes in the ABO and Rh blood-group systems. Expect 4-6 MCQs — very high yield.
Ascent of Sap
The upward movement of water and dissolved minerals from roots through xylem to the leaves is called the ascent of sap. In a tall tree this involves lifting water more than 100 m against gravity.
The most accepted explanation:
- Transpiration pull — water evaporating from mesophyll cells creates negative pressure (tension) in the xylem, pulling the water column up.
- Cohesion — water molecules stick to each other through hydrogen bonds, so the column does not break.
- Adhesion — water molecules stick to the lignified xylem walls, helping to support the column.
- Continuous water column from roots to leaves through narrow xylem vessels and tracheids.
Active transport of mineral ions into root xylem lowers the water potential there; water enters by osmosis, generating a positive pressure that pushes sap upward. Root pressure can produce guttation (drops of water at leaf margins early morning). It contributes only modestly to ascent in tall trees — transpiration pull does most of the lifting.
Pathway of water in roots
Water enters root hairs by osmosis and travels to the xylem via three routes:
- Apoplast — through cell walls and intercellular spaces (no membranes crossed).
- Symplast — through plasmodesmata between cell cytoplasms.
- Vacuolar — from vacuole to vacuole.
The Casparian strip in the endodermis blocks the apoplastic path, forcing water through cell membranes — allowing the plant to control mineral entry.
Transpiration
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly from leaves through stomata. It powers the ascent of sap and helps cool leaves but a very high transpiration rate causes wilting.
- Stomatal — through stomata. Accounts for ~90-95% of total water loss.
- Cuticular — through the waxy cuticle of leaves and stems. ~5-10%; higher in non-xerophytes.
- Lenticular — through lenticels of woody stems. <1%.
Factors affecting transpiration
- Light — opens stomata; higher rate.
- Temperature — warmer air holds more water vapour; faster diffusion.
- Humidity — high humidity reduces the gradient; slower rate.
- Wind — removes humid air around leaves; faster rate.
- Soil water — low availability eventually closes stomata.
Opening and Closing of Stomata
A stoma (plural stomata) is a tiny pore on the leaf epidermis flanked by two specialised guard cells. Guard cells regulate the size of the pore, balancing CO2 entry against water loss.
In the light, guard cells actively pump K+ ions in from neighbouring cells. The increase in solute concentration lowers their water potential; water enters by osmosis. The guard cells become turgid, but because their inner walls are thicker (and their radial cellulose microfibrils unequal), they bow apart — opening the stoma.
In the dark or under water stress, K+ leaves the guard cells, water follows out by osmosis, the cells become flaccid and the stoma closes.
Under drought, the hormone abscisic acid is released and causes K+ efflux from guard cells → loss of turgor → stomatal closure. This conserves water during stress.
Blood Composition
Blood is a specialised connective tissue with two parts: a fluid plasma (~55%) and formed elements (~45%) suspended in it.
~90% water. Solutes:
- Plasma proteins (~7%): albumin (osmotic pressure), globulins (transport, immunity — including immunoglobulins), fibrinogen (clotting precursor).
- Inorganic ions: Na+, K+, Cl−, HCO3−, Ca2+.
- Nutrients (glucose, amino acids, lipids), wastes (urea, creatinine), hormones, dissolved gases.
- Plasma vs Serum
- Plasma = liquid part of unclotted blood; contains all clotting factors including fibrinogen. Serum = liquid part of clotted blood; lacks fibrinogen and other clotting factors.
- Erythrocytes (RBCs) — ~5 million/mm3. Biconcave, anucleate in mammals; carry haemoglobin. Lifespan 120 days. Made in red marrow; recycled by spleen.
- Leukocytes (WBCs) — ~7,000/mm3. Granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Defense.
- Platelets (thrombocytes) — ~250,000/mm3. Cell fragments from megakaryocytes. Clotting.
Blood clotting (haemostasis) — outline
Damaged vessel → vasoconstriction → platelet plug → coagulation cascade activated → prothrombin → thrombin (catalysed by factor X) → thrombin converts fibrinogen → fibrin, which forms a mesh trapping cells. Calcium ions and vitamin K are essential.
Blood Groups
Blood groups are determined by the antigens on the surface of red blood cells. The two clinically most important systems are ABO and Rh.
Two antigens (A and B) on RBCs and corresponding antibodies in plasma:
- Group A — A antigen on RBCs; anti-B antibody in plasma.
- Group B — B antigen on RBCs; anti-A antibody in plasma.
- Group AB — both A and B antigens; no antibodies. Universal recipient.
- Group O — no antigens; both anti-A and anti-B antibodies. Universal donor.
Mismatched transfusion causes agglutination — antibodies in the recipient’s plasma cross-link incoming RBCs, leading to clumping and haemolysis.
The Rh (D) antigen is either present (Rh+) or absent (Rh−). About 85% of people are Rh+. Anti-Rh antibodies are not pre-formed; they appear only after exposure to Rh+ blood.
Erythroblastosis fetalis (haemolytic disease of the newborn) can occur when an Rh− mother carries an Rh+ foetus. During the first delivery, foetal RBCs leak into maternal blood; the mother makes anti-Rh IgG. In subsequent Rh+ pregnancies, those IgG antibodies cross the placenta and attack foetal RBCs. Prevented by giving anti-D immunoglobulin (RhoGAM) to Rh− mothers at 28 weeks and after delivery.
| Group | Antigen on RBC | Antibody in plasma | Can donate to | Can receive from | Approx. frequency (Pakistan) |
|---|---|---|---|---|---|
| A | A | anti-B | A, AB | A, O | ~22% |
| B | B | anti-A | B, AB | B, O | ~32% |
| AB | A and B | None | AB only | All groups (universal recipient) | ~10% |
| O | None | anti-A and anti-B | All groups (universal donor, esp. O−) | O only | ~36% |
Rh compatibility cheat sheet
- O− — universal donor (no A, B, or Rh antigens to provoke).
- AB+ — universal recipient (no antibodies in plasma; no anti-Rh).
- Rh− patients should receive only Rh− blood; first exposure sensitises, second causes haemolysis.
- Always cross-match before any transfusion in clinical practice.
Worked MCQs
Five MCQs covering the high-yield testing patterns for transport in plants and animals.
Q1. The most widely accepted explanation for the ascent of sap in tall trees is the:
Dixon and Joly’s cohesion-tension theory states that water lost from leaves by transpiration creates a negative pressure (tension) in xylem; cohesion among water molecules and adhesion to vessel walls maintain a continuous column from root to leaf. Root pressure helps but cannot account for the full lift in tall trees.
Q2. Stomata typically open when guard cells:
In light, K+ ions are pumped into guard cells. Water follows by osmosis, the cells become turgid, and because of unequal cell-wall thickening they bow apart, opening the stoma. ABA acts in the opposite direction during stress to close stomata.
Q3. Which of the following best describes an individual with blood group AB+?
An AB+ person expresses A, B and Rh(D) antigens on RBCs and therefore makes no antibodies against any of them — that is why AB+ is called the universal recipient. Group O− is the universal donor (no antigens to provoke a recipient).
Q4. Plasma differs from serum in that plasma:
Plasma is the fluid portion of unclotted blood and contains all clotting factors, including fibrinogen. Serum is the fluid that remains after blood has clotted — the fibrinogen has been converted to fibrin and trapped in the clot.
Q5. Which of the following increases the rate of transpiration?
Wind sweeps away the layer of saturated air that normally builds up around stomata, maintaining a steep water-vapour gradient and increasing the rate of transpiration. High humidity, darkness and cold all decrease transpiration.
Quick Recap
- Ascent of sap = cohesion-tension theory: transpiration pull + cohesion + adhesion + continuous water column.
- Root pressure causes guttation; significant in short plants only.
- Casparian strip in endodermis forces water across membranes.
- Transpiration: stomatal (~90%), cuticular, lenticular. Increased by light, heat, wind; decreased by humidity.
- Stomata open when guard cells gain K+, water enters → turgor → opening; ABA closes them in drought.
- Blood = ~55% plasma + ~45% formed elements. Plasma proteins: albumin, globulins, fibrinogen.
- Plasma has fibrinogen; serum does not.
- ABO: A makes anti-B; B makes anti-A; AB universal recipient; O universal donor.
- Rh− mother + Rh+ foetus → risk of erythroblastosis fetalis (prevented by anti-D Ig).