Control and Regulation

Control and Regulation Patterns of Growth in Plants and Animals Mr G Davidson

Patterns of Growth • Growth is more than simply getting larger or heavier or having more cells. • If we compare ourselves now to when we were babies, many changes have occurred in shape and complexity , and these differences are the result of growth. G Davidson

Patterns of Growth • Growth is, however, more than simply getting larger or heavier or having more cells, we can measure height or weight at various stages to record growth patterns, but it is much more difficult to count cells. G Davidson

Patterns of Growth • We can measure height or weight at various stages to record growth patterns, but it is much more difficult to count cells. • This can be done for bacteria providing conditions that are right for growth. • In this way, we can work out how fast the population grows. G Davidson

Patterns of Growth • However, for larger organisms, measuring growth is not easy as our weight varies quite substantially depending of the amount of water in our bodies. • It is therefore more sensible to measure the dry mass of an organism by removing all the water. • In this way, organisms are killed at various stages of growth and their dry mass is measured. G Davidson

Patterns of Growth • This involves large numbers of organisms and is not a practical method for measuring human growth but it is often used to study plant growth. • “Growth is an irreversible increase in the dry mass of an organism along with an increase in cell numbers.” G Davidson

Growth Curves • Where the dry mass is impossible to measure, variable factors are used, e.g. height and weight. • If these are measured over a period of time, a growth curve can be drawn. • A growth curve normally takes the shape of a sigmoid curve. G Davidson

Accelerating growth Rapid steady growth Decelerating growth No growth Dry Mass Time Sigmoid Curve G Davidson

Growth Curves • The curve for the birch tree is different again but still follows the same pattern, just stacking up the curves. • Although each of the graphs is different, they all show an increase in growth as well as the sigmoid shape. G Davidson

Growth Time Birch Tree Growth Curve G Davidson

Growth Curves • It should be realised that one of the differences between the growth of plants and animals is that plants continue to show an increase in growth until they die, whereas in animals, growth stops on reaching adulthood. G Davidson

Dry Mass Time Growth Curve of a Sunflower G Davidson

Growth Curves • If we compare the growth curve of an insect with a human, the shape of the curve remains, but the insect loses weight every once in a while as it sheds its exoskeleton to allow further growth. • This process of moulting is called Ecdysis. G Davidson

Summary of Plant and animal growth G Davidson

Plant Meristems • A meristem is a specific area within an organism where a group of cells are found that can divide to form new tissue. • Cells in these regions divide by mitosis and make daughter cells identical to the parent cells. • Plants have quite specific meristematic regions. G Davidson

Plant Meristems • The cells in these regions are unspecialised (i.e. they have no set functions). • These cells can differentiate into a wide range of tissues • Animals have no meristems. G Davidson

Primary Growth • Primary growth only occurs at the root and shoot tips called apical meristems, which results in an increase in length of the plant by the production of new cells as well as elongation of these new cells. G Davidson

Primary Growth • Once the cells have elongated, they then differentiate, which is the process of an unspecialised cell being altered and adapted to become part of the permanent tissue of the plant, e.g. into root hair, xylem or phloem etc. G Davidson

Region of Differentiation Region of Elongation Root Cap Region of Mitosis G Davidson

Secondary Growth • Secondary growth is an increase in thickness or girth of a plant in order to support itself. • This is caused by lateral meristems – cambium and cork cambium. G Davidson

Transverse section through young tree twig Bark Cork Cambium Cortex Phloem Cambium Xylem Pith Secondary Growth • The cambium is a layer of cells found between the xylem and phloem in the vascular bundles. G Davidson

Secondary Growth • As the cambium divides, it forms a ring in the stem and makes new xylem and phloem tissue called secondary tissue. • The secondary xylem is formed inside the cambium and the secondary phloem on the outside. G Davidson

Secondary Growth • Each year of growth sees a new secondary xylem which results in annual rings in trees. • The secondary phloem just gets larger each year as the girth of the tree increases. • As the stem gets thicker, there is not only a need for water transport up the plant, but also, across the stem, and this is achieved by the medullary rays. G Davidson

Secondary Growth • These consist of rows of parenchyma (unspecialised cells) which come from the cambium. • The cork cambium is another ring of cells, this time, around the outside of the stem. • Its function is to make new cortex cells on the inside as well as dead cortex cells on the outside which make up the bark of the tree. G Davidson

Annual Rings • If we study the cross section of a fallen tree, we see a series of annual rings. • These result from the cambium producing large xylem vessels in Spring which transport large volumes of water, which are required for the new growth. G Davidson

Annual Rings G Davidson

This tree is 13 years old One year’s growth Annual Rings G Davidson

Autumn Wood Spring Wood Annual Rings • In the spring the cambium is very active and produced large xylem vessels , while in Autumn much smaller xylem vessels are produced. • In winter, there is no growth, and the following year, the pattern begins again. G Davidson

Annual Rings • A closer study of the rings reveals that some are wider than others and this can tell us a little about the weather of the year. • e.g. a wide ring indicates either warm or wet weather when growth conditions are optimum. G Davidson

Regeneration in Plants and Animals • Although an animal may stop growing, on reaching maturity, cell division doesn’t cease. • In plants and animals, new cells are needed to replace dead cells. • Regeneration is the replacement by growth of the parts of an organism which have been lost. G Davidson

Regeneration in Plants and Animals • Plants and animals have different powers of regeneration, • e.g. if you prune a hedge, it grows again in a matter of weeks, however, if you ‘prune’ a dog, it doesn’t grow a new head. • Market gardeners use this power of regeneration to increase their supplies of certain plants by taking cuttings, grafting etc. G Davidson

Regeneration in Plants and Animals • Tissue culturing is a fairly modern technique used to mass produce plants. • This involves the removal of a few cells and allowing them to reproduce in a tissue culture. G Davidson

Regeneration in Plants and Animals • This means thousands of plants can be grown from one. • The main disadvantage of this is that all the new plants are genetically identical and are called clones. G Davidson

Regeneration in Plants and Animals • Regeneration in animals is limited, but in some animals, it can be very extensive, • e.g. starfish. • In mammals, however, it is restricted to healing and replacement of damaged tissue (e.g. blood, bone, etc.) • Regeneration depends of the degree of specialisation of the cells. G Davidson

Regeneration in Plants and Animals • The more highly specialised, the lower the powers of regeneration. • e.g. nerve cells are highly specialised and cannot regenerate, however, liver cells are not very specialised which makes their regenerative powers much higher. G Davidson

Control and Regulation