The Transport System of Plants:
Plants also require a regular supply of oxygen and nutrients, yet their requirements differ from those of animals in several ways. Instead of oxygen, carbon dioxide is required for photosynthesis while oxygen is the waste product. Moreover, organic nutrients are mostly produced rather than taken in. Even though their energy requirements are lower as they are less metabolically active, thus their transport system is slower compared to the mammalian blood system with no obvious pump like the heart. Plants have two transport systems compared to only one in mammal species. One of them is used to carry water and mineral ions from roots to aerial parts of the plant while the other is used to carry substances made by photosynthesis from leaves to other areas. Neither of them is used to carry respiratory gases as in a mammalian system, oxygen and carbon dioxide are travelled by diffusion through the stomata.
Vessels in a Mammal:
There are three main types of vessels in the blood system, known as arteries, veins and capillaries. All three of them have three layers of different cell types – Tunica intima, a layer of smooth squamous epithelia cells; Tunica media, a layer with elastic fibres and smooth muscles; Tunica externa, which contains collagen fibres.
The artery is to transport oxygenated blood from the heart (at a high pressure) to body tissues, therefore it has adaptations to maximize transport and prevent damage. Artery has a thick wall to withstand high pressure produced by the heart, which contains lots of smooth muscle and collagen fibres to prevent the vessel from damaging. It also contains elastic fibres to prevent the vessel from bursting.
The capillary takes oxygenated blood as close as possible to all cells for rapid transfer of substances. It is one cell thick (endothelial cell) to minimize diffusion distance with its diameter about the same size as a red blood cell. There are gaps in the endothelium layer to allow substance to seep back into the blood.
The vein brings deoxygenated blood (or oxygenated for pulmonary vein) back to the heart. Since blood in vein does not have a high pressure, less muscle and elastics are required, it also has a thin wall. Semilunar valves are present to prevent backflow of the blood.
Vessels in a Plant:
There are two main types of vessels in a plant transport system, known as xylem and phloem. Like vessels in mammals, xylem and phloem are both complex tissues containing more than one cell type. However, the cell types are different compared to mammals.
Xylem has the dual functions of support and transport of water & mineral ions. Vessel elements in the xylem are involved with the transport of water. Compared to all the vessels in mammals, xylem is different as it is consists of fibres of dead cells with no living components. Similar to arteries, it has no end walls. Both of these adaptations are designed so that nucleus and cell walls do not impede water flow. Xylem is narrow, which has a similar function as the capillary, to increase adhesion and the ‘sucking’ forces. It also has parenchyma, a layer of packing cells with pits in between to allow lateral water movement. Xylem is also lignified to strengthen the structure for support of the whole plant. Xylem allows transpiration to occur.
Phloem allows translocation to occur, which is the transport of soluble organic substances. Phloem consists of sieve tubes elements and companion cells, both of which contain living organelles. Sieve tubes are arranged end to end to create long tubes forming a vessel, yet possess no nucleus and ribosome so that transportation can occur more efficiently. The end walls of sieve tubes are perforated by large pores to allow materials to move from one cell to the next, this is similar to the capillary pores or vein valves in mammals and is known as sieve plates. Each sieve tube element has at least one companion cell associated with it joined together by plasmodesmata. Companion cell contains lots of mitochondria and ribosome, as well as a nucleus to carry out protein synthesis for sieve tubes, transfer cells then bring these substances into the phloem for the sieve tubes.
Heart in mammals:
Pump, i.e. the heart, is not present in plants. It creates a pressure for blood so that substance can be transferred to other parts of the body. Firstly deoxygenated blood from vena cava enters the right atrium during diastole, when both the atrium and ventricle relax. When the right atrium contracts, blood are pushed into the right ventricle. This is atrial systole, with tricuspid valves opening to allow blood flow. Valves in vena cava also close to prevent back flow. During ventricular systole, the right ventricle contracts which forces blood into the pulmonary artery. This pressure pushes open the semi-lunar valves while heart tendons prevent the tricuspid from opening to stop blood backflow into the atrium. Deoxygenated blood then collects blood from the lungs and return to the left side of the heart via the pulmonary vein, where heart undergoes diastole for blood to fill chamber, opening of bicuspid during atrial systole and blood pushes around to the body during ventricular systole through the aorta.
In a cardiac cycle, the sinoatrial node, which is patch of muscle in wall with innate rhythm, sends impulse to atria which contracts (atrial systole). Non-conducting tissue between atria and ventricles and the atrio-ventricular node delay impulses for a short period of time to ensure atria and ventricles do not contract at the same time. Signal then is sent by the AVN through the Bundle of His to the Purkyne tissue, which brings contraction to ventricular wall.
