Biology TRANSPORT IN PLANTS

### KEY TOPICS

star Translocation
star Diffusion

### TRANSLOCATION

● color{violet}("Plants") need to move molecules over very color{brown}("long distances,") much more than animals do; they also do not have a color{violet}("circulatory system") in place.

● Water taken up by the roots has to reach all parts of the plant, up to the very tip of the color{violet}("growing stem.")

● The color{brown}("photosynthates") or color{violet}("food synthesised") by the leaves have also to be moved to all parts including the root tips embedded deep inside the soil.

● Movement across short distances, say within the color{violet}("cell"), across the membranes and from color{violet}("cell to cell") within the tissue has also to take place.

● In a color{violet}("flowering plant") the substances that would need to be transported are color{violet}("water, mineral nutrients,") organic nutrients and color{violet}("plant growth regulators.")

● Over small distances substances move by color{brown}("diffusion") and by color{brown}("cytoplasmic streaming ") supplemented by color{violet}("active transport.")

● Transport over longer distances proceeds through the color{violet}("vascular system") (the xylem and the phloem) and is called color{brown}("translocation.")

● An important aspect that needs to be considered is the color{violet}("direction of transport.")

● In rooted plants, transport in color{brown}("xylem") (of water and minerals) is essentially unidirectional, from roots to the stems.

● color{brown}("Organic and mineral nutrients") however, undergo multidirectional transport.

● color{violet}("Organic compounds synthesised") in the color{violet}("photosynthetic leaves") are exported to all other parts of the color{violet}("plant including storage organs.")

● From the color{violet}("storage organs") they are later re-exported.

● The color{violet}("mineral nutrients"0 are taken up by the roots and transported upwards into the stem, leaves and the growing regions.

● When any plant part undergoes color{brown}("senescence"), nutrients may be withdrawn from such regions and moved to the color{violet}("growing parts.")

● color{violet}("Hormones or plant growth") regulators and other chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarised or unidirectional manner from where they are color{violet}("synthesised") to other parts.

● Hence, in a color{violet}("flowering plant") there is a complex traffic of compounds (but probably very orderly) moving in different directions, each organ receiving some substances and giving out some others

### DIFFUSION

● Movement by color{brown}("diffusion") is passive, and may be from one part of the color{violet}("cell") to the other, or from color{violet}("cell to cell"), or over short distances, say, from the intercellular spaces of the leaf to the outside.

● No color{brown}("energy expenditure") takes place.

● In color{violet}("diffusion, molecules") move in a random fashion, the net result being substances moving from regions of higher concentration to regions of color{violet}("lower concentration.")

● color{violet}("Diffusion") is a slow process and is not dependent on a color{brown}("‘living system’.")

● color{violet}("Diffusion") is obvious in gases and liquids, but diffusion color{violet}("𝘪𝘯 𝘴𝘰𝘭𝘪𝘥𝘴") rather than color{violet}("𝘰𝘧 𝘴𝘰𝘭𝘪𝘥𝘴") is more likely.

● color{violet}("Diffusion") is very important to plants since it the only means for color{brown}("gaseous movement") within the plant body.

● color{violet}("Diffusion rates") are affected by the gradient of concentration, the permeability of the membrane separating them, color{violet}("temperature and pressure.")