Transportation

Most oxygen in human blood is transported by binding reversibly to haemoglobin, the iron-containing respiratory pigment present in red blood cells. In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin; in tissues, oxygen is released for respiration. Therefore option A is correct. Saliva and bile aid digestion, while urea is a nitrogenous waste, not an oxygen carrier.

Blood is the principal transport fluid in the human body. Its plasma carries dissolved nutrients, hormones, carbon dioxide, urea and other substances, while red blood cells transport most oxygen through haemoglobin. Blood also distributes heat and helps protect the body. Therefore option A is correct. Saliva and bile support digestion, whereas sweat mainly helps cooling and excretion rather than general internal transport.

The governing concept is transport in vascular plants. Roots absorb water and dissolved mineral ions from the soil, and xylem conducts them mainly upward to the stem and leaves. Therefore option A, xylem, is correct. Phloem transports sugars and other organic nutrients, while epidermis provides a protective covering and cork forms protective tissue; neither is the main water-conducting tissue.

The governing concept is water loss in plants. Transpiration is the loss of water vapour from the aerial parts of a plant, especially leaves, mainly through microscopic stomata. Therefore option A is correct. It is not the taking of food, the formation of soil by roots, or blood production by flowers; those choices describe unrelated or biologically incorrect processes.

The governing concept is transpiration pull. When water evaporates from leaf surfaces, especially through stomata, it creates tension in the continuous water column inside xylem. This helps draw water and dissolved minerals upward from roots toward leaves, so option A is correct. Digestion, oxygen binding in blood, and urine formation are animal processes and are unrelated to plant transpiration.

The governing concept is transportation, which distributes essential materials and removes wastes. Digested nutrients and oxygen must reach cells, while carbon dioxide and other wastes must be carried away to suitable organs. In plants, xylem and phloem transport water, minerals, and food. Therefore option A is correct; the other options describe no general life-process function.

Haemoglobin is the iron-containing respiratory pigment inside red blood cells. In the lungs, it combines reversibly with oxygen to form oxyhaemoglobin, allowing oxygen to be carried efficiently through the blood; in tissues, oxygen is released for cellular respiration. Bile helps digest fats, saliva begins digestion, and urea is a nitrogenous waste, so none of these transports oxygen. Therefore option A is correct.

The governing concept is transpiration pull in plant transport. When water evaporates from leaf surfaces through stomata, it creates negative pressure in the continuous water column inside xylem. Cohesion between water molecules helps transmit this pull downward, assisting the upward movement of water and dissolved minerals from roots to leaves. Therefore option A is correct. The other options refer to animal digestion, circulation or excretion, not transport through plant xylem.

The governing concept is transport in plants. Roots absorb water and mineral ions from the soil, and xylem tissue carries them mainly upward through vessels and tracheids to stems and leaves. Transpiration pull, along with root pressure and cohesion, supports this movement. Phloem transports food, stomata regulate gas exchange and chlorophyll absorbs light, so option A is correct.

The governing concept is transportation in living organisms. Cells in different parts of an organism need materials such as digested food, water, minerals, and oxygen, while carbon dioxide and other wastes must be carried away. Blood transports substances in animals, and vascular tissues such as xylem and phloem perform comparable functions in plants. Thus option A states the main importance; cooling, taste, and sleep are not the primary purposes of transport.

The governing concept is the comparison of forces involved in xylem water transport. Root pressure develops when roots actively absorb ions and water enters by osmosis, producing a positive push from below. Transpiration pull develops when evaporation from leaves creates negative pressure that pulls the continuous water column upward. Thus option A states the key difference; the other options concern digestion, excretion, or lungs and are irrelevant.

The governing concept is phloem translocation from a source to a sink. A mature leaf may act as a source by producing sugars, while roots, growing shoots, flowers, seeds, or fruits may act as sinks by using or storing them. Since different sinks may lie above or below a source at different times, phloem transport can occur in either direction in different sieve tubes. Therefore option A is correct; xylem and the remaining choices are unrelated.

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. As water evaporates from leaf surfaces, it creates tension or a transpiration pull in the continuous water column of the xylem. Cohesion between water molecules helps transmit this pull downward, drawing more water upward from the roots. Thus option A is correct; the remaining processes belong to animal physiology or digestion.

Phloem is the living conducting tissue that transports sugars and other organic nutrients made in photosynthetic leaves to growing or storage regions, such as roots, fruits, and young shoots. This movement is called translocation and can occur in different directions according to the source and sink. Damage to phloem therefore most directly disrupts food distribution. Water transport is mainly a xylem function, so option A is correct.

Transport is necessary because essential materials and wastes are not always located where they are needed. In animals, blood carries oxygen, digested nutrients, hormones, and wastes between organs. In plants, xylem transports water and minerals, while phloem distributes food. Therefore option A expresses the governing purpose. The other options would prevent, rather than support, cellular activities and do not describe transport.

The governing concept is phloem translocation from a source to a sink. Mature leaves commonly produce sugars, but growing roots, fruits, seeds, stems, and buds may require them at different times. Phloem can therefore move dissolved food from the producing region to the region of use or storage, either upward or downward according to need. Option A is correct; xylem mainly transports water and minerals, not food.

Haemoglobin is the iron-containing pigment in red blood cells that binds oxygen in the lungs and transports it to body tissues. If haemoglobin concentration falls, the blood carries less oxygen, so cells may receive insufficient oxygen for respiration; tiredness and weakness can result. Option A is correct. Taste, starch production and stomatal opening are unrelated to haemoglobin-dependent oxygen transport in humans.

The governing concept is transpiration pull. Water evaporates from moist cell surfaces inside leaves and exits through stomata as water vapour. This loss lowers water pressure in the leaf and creates tension in the continuous water column. Cohesion between water molecules and adhesion to xylem walls help pull water upward from the roots. Hence option A is correct; the other choices do not describe transpiration.

Food prepared in leaves is transported through phloem by translocation. Depending on the location of the source and the current sink, sugars may move toward roots, stems, fruits, seeds, or growing buds. Consequently, the direction is not permanently fixed: movement can be upward in one part and downward in another, or change with the plant’s needs. Thus option A is correct; the one-way alternatives incorrectly describe phloem transport.

Human blood circulation is driven primarily by the muscular heart, which creates pressure to move blood through vessels. In plants, xylem transports water and minerals upward without a heart; evaporation from leaves produces transpiration pull, assisted by cohesion and sometimes root pressure. Thus option A identifies the main difference. The other options wrongly give plants a heart or assign haemoglobin and photosynthesis to xylem or blood.