PIGMENTS: door to our colorful world.

Aren’t we fascinated by the colorful world around us so what is it that makes our world so colorful and vibrant? The answer lies in the presence of “pigments,”, a coloring matter found in the cells of both plants and animals. These pigments are responsible for the diverse colors we see around us. The biological pigments produced by living organisms absorb certain wavelengths of light and reflect others. We are all familiar with the green pigment chlorophyll that plants possess to trap light energy to carry out photosynthesis. Besides photosynthesis, plant pigments also help to attract insects for pollination, ultimately helping in fruit formation. But why do animals produce pigments? Animals have dark pigments like melanin (in skin, eyes, and fur) and its main purpose is to prevent damage caused to the tissues by the UV (ultra-violet) light.

MAJOR PIGMENTS IN PLANTS

The principal pigments in plants are Chlorophyll, Carotenoids, Flavonoids, and Betalains.

Chlorophyll is one of the major pigments found in plants that imparts a green color to the foliage and leaves. Besides plants, cyanobacteria (blue-green algae) and algae possess chlorophyll. Chlorophyll mainly absorbs red light (long wavelength) and blue light (short wavelength) of the visible spectrum and reflects the green spectrum. Chlorophyll is of five main types Chlorophylls a, b, c, d and related molecule bacteriochlorophyll found in prokaryotes.

Carotenoids (carotenes and xanthophylls) are the plant pigments responsible for bright red, yellow and orange colors of fruits and vegetables (e.g., pumpkins, carrots, corn, tomatoes). It is produced not only by plants and algae and by bacteria and fungi. Carotenoids absorb light in the blue-green and violet region and reflect orange, red, and yellow wavelengths of light. Carotenoids perform an important function in photosynthesis and photoprotection. Carotenoids are beneficial antioxidants that protect us from various diseases. Provitamin A carotenoids can be converted into vitamin A and are important for eye health.

Flavonoids, its common types include anthocyanins, aurones, flavonols, and proanthocyanidins. Flavonoids are abundant in the plants and most important for flower coloration. They produce yellow or red/blue pigmentation in petals aiding in attracting insects for pollination. In higher plants, flavonoids filter UV light and are also involved in symbiotic nitrogen fixation and pigmentation in flowers. Flavonoids also play an important role in plant defense. Flavonoids are synthesized only in the plants and they exhibit antioxidant activity.

Betalains, its common type includes betacyanins, and betaxanthins are a group of reddish pigments. In the plant kingdom, we find these in the leaves, roots, stalks, flowers, fruits, seeds and grains. Red beetroot, amaranth, prickly pear, and red pitahaya contain betalains. Betalains also contain antioxidant properties.

PIGMENTS IN AMINALS

The main pigment found in most of the mammals is the melanin, present in special cells called chromatophores and is responsible for eyes, skin, hair and fur color. Melanin protects the skin from harmful ultraviolet rays. The integuments of birds contain two different forms of melanin: Eumelanin (responsible for black, brown or grey coloration) and Pheomelanin (produces lighter yellowish to reddish coloration). However, most pigments are not made in the body, animals consume different things in their diet that impart them a certain color, for example, flamingoes are not pink. They are born grey but their diet comprises blue-green algae and brine shrimp, which turn them pink because of natural pink dye called canthaxanthin. Similarly, blue-footed boobies have bright blue feet to attract mates. Their blue color feet results from carotenoid pigments of their fishy diet.

Blue pigment is the rarest pigment in nature, and it is considered tough for a bird's body to create through dietary pigments. Blue color birds result from an illusion, such birds have special structures in their feathers that break up the light and reflect only blue. This is considered true for blue and green birds (parrot). Parrots lack any green pigment but appear green because of light scattering (Tyndall effect).  Likewise, blue butterflies appear blue because of the microscopic structure of the scales and because of the interference of light. The only butterfly that produces a blue pigment is an “olive wing” butterfly. Many green snakes and frogs are not green but possess a yellow pigment and blue structural color and these two combined produce green effect.

There are various animals that protect themselves from their predators through their biological pigments i.e., they use their biological pigments to camouflage (e.g., chameleons), for warning coloration and for mimicry.

CELL:THE BASIC UNIT OF LIFE

What are organisms made up of?

 We all are familiar just like a building is made up of bricks similarly the body of all the living organisms is made up of cells defined as “the basic unit of life. It is called so as a cell is the smallest unit that can perform all the processes needed for life or it is also called “the structural and functional unit of life”. Cells are usually not visible to an unaided eye except for amoeba, a paramecium and a human egg that are visible to the human eye.

 Why cells were named so?

 Robert Hooke in 1665 coined the term cell ((L., Cella = hollow space) while observing the cork cells (bark of the tree) under the microscope. The box-like cells (honeycomb-like structures) reminded him of the cells (small rooms) in which monks lived and so he named them as cells (the word cell means a small room in a prison). Robert Hooke observed dead cell walls of plant cells, it was Anton van Leeuwenhoek who in 1674 first observed a single living cell (animalcules) under the microscope. In 1674 he likely observed a protozoan and later saw bacteria. Anton van Leeuwenhoek is also known as the “Father of Microbiology”.

Cells shape, size, and number.

 Cells differ considerably in their shape and size, and this difference exists according to the function they perform. The smallest known cell is PPLO-Pleuropneumonia-like organisms or mycoplasma which is only 10 micrometer in size (1 micrometer is equal to 0.001 mm) while the largest cell is the egg cell of an ostrich (170 mm x 130 mm). In humans, the largest cell is a female ovum, while the smallest is the male gamete (sperm). The longest cell in the human body is the nerve cell.

Animal cells are usually round (spherical) or irregular (e.g., cheek cells) while the plant cells are more rigid and rectangular due to the presence of a cell wall. The shape and size of a cell depend upon:

(a) surface-volume ratio (if the cell is large it has a small surface-area-to-volume ratio, thus diffusion of substances takes a longer time. Thus most cells are small to overcome issues related to the diffusion of substances.)

(b) nucleocytoplasmic ratio(the cytoplasmic area should be just enough which a nucleus can control)

(c)  rate of cellular activity (metabolically active cells are usually smaller)

(d)  cell association (it is important in multicellular organisms that provide cells some amount of rigidity).

 Last, based on the number of cells, organisms are classified as unicellular or multicellular. Unicellular organisms like an amoeba, bacteria, and paramecium comprise one cell, while multicellular organisms are those whose body is made up of over two cells. 

Cell theory

 Cell theory was formulated in the year 1839 by Matthias Schleiden (German botanist) and Theodor Schwann. (German physician and physiologist). Besides other scientists, Rudolf Virchow, the “father of modern pathology” also contributes to this theory. Cell theory proposed that:

1. All living organisms comprise one or more cells.

2. The cell is the structural and functional unit of all living organisms.

3. All cells arise from pre-existing cells.

 Rudolf Virchow (1855) first explained the concept that new cells arise from pre-existing cells (Omnis cellula-e-cellula). He modified the hypothesis given by Schleiden and Schwann that Cells form by free-cell formation, similar to the formation of crystals and this how the cell theory got its final shape.

Amazing facts

 1. Not all organisms comprise cells, like a virus, virions and viroids are commonly referred to as acellular microbes or infectious particles (these are exceptions to the cell theory)

2. The average human body contains approximately 37.2 trillion cells!

3. Some neurons are less than one millimeter, while others may be about a meter long.

4. The largest single-celled organism is Caulerpa taxifolia (an aquatic alga), the single cell is 6-12 inches long.

5. Amongst the plants, the ovule of a cycas is the largest cell.