My Science School

Living things are said to be animate. Inanimate things are not living. Metal, plastic and glass, for example, are inanimate. All animate things are able to do six things that inanimate things cannot.

Although seemingly diverse, living things, or organisms, share certain essential characteristics. The most recent classification system agreed upon by the scientific community places all living things into six kingdoms of life, ranging from the simplest bacteria to modern-day human beings. With recent innovations such as the electron microscope, scientists peered inside cells and began to understand the intracellular processes that defined life.

Composition

Cells compose all life, performing the functions necessary for an organism to survive in its environment; even the most primitive of life forms, bacteria, consists of a single cell. While peering through a microscope at slices of cork tissue in the late 17th century, scientist Robert Hooke discovered numerous tiny compartments which he coined “cells.” After several developments regarding cell structure and function, Robert Virchow compiled a book, “Cellular Pathology,” describing the nature of cells in relation to life. He formed three conclusions: cells form the basis of all life, cells beget other cells and cells can exist independent of other cells.

Energy Use

All processes occurring within organisms, whether single-celled or multicellular, expend energy. The method of procuring that energy, however, differs between organisms. Organisms called autotrophs make their own energy while heterotrophs must feed to obtain their energy needs. Autotrophs such as plants and some bacteria produce their own food by converting carbon dioxide and water into sugar with the aid of the sun’s energy via photosynthesis. Other autotrophic bacteria use chemicals such as sulfur to make energy in a process called chemosynthesis. The energy organisms need comes in the form of a molecule called ATP, or adenosine triphosphate. Living things make ATP by breaking down glucose.

Response

Organisms use their senses to obtain information from and have the capability of reacting to stimuli in their environments. Even unicellular organisms such as bacteria and seemingly immobile plants can respond to stimuli. Plants such as sunflowers can sense heat and light, so they turn toward the sun’s rays. Predators such as cats can track their prey with keen senses of vision, smell and hearing and then hunt them down with superior agility, speed and strength.

Growth

Living things grow and change through the process of cell division, or mitosis. In organisms composed of more than one cell, mitosis either repairs damaged cells or replace older ones that have died. Additionally, multicellular organisms grow larger in size by increasing the number of cells in their bodies. Unicellular organisms take in nutrients and enlarge. They grow to a certain point and then must divide into two new daughter cells. The process of mitosis takes place in four phases. Certain signals trigger cells to divide. The cell replicates its genetic information, resulting in two exact copies of the gene-bearing structures called chromosomes. Cellular structures separate the chromosome copies, moving them to different sides of the cell. The cell then pinches itself down the middle, creating a new barrier to separate the two new cells.

Reproduction

For a species or organism to continue existing, members of the species must reproduce, either asexually or sexually. Asexual reproduction produces offspring that exactly resemble the parent organism. Certain members in each of the kingdoms of life can reproduce asexually. Bacteria from Kingdoms Archaebacteria and Eubacteria, amoeba of the Kingdom Protista and yeast of Kingdom Fungi use binary fission to simply divide in two, resulting in two identical daughter cells. Worms called planaria can break off a segment that grows into a new organism. Plants such as potatoes form buds which, when cut off and planted, will produce a new potato plant. Sexual reproduction, which allows a mixing of genes from two individuals of a species, evolved from asexual reproduction because the benefits of sex outweigh its costs.

Adaptation

Since the beginning of life, organisms have adapted and evolved to survive according to their environments. Those individuals unable to adapt to changing conditions will die or be unable to pass on much of their genes to the next generation. Many times in the history of the earth, entire species, including many dinosaur groups, have died out when they failed to respond appropriately to environmental changes such as droughts or cooling climates. The environment selects for those individuals best acclimated to live under specific conditions; these creatures have the best selections of mates and will contribute to a greater percentage of descendants.

All cells have a cell wall, hut in plant cells this is made of a stiff, tough layer of cellulose. Cellulose is made of tiny fibres, layered together to form a strong sheet. Most plant cells also contain organelles called chloroplasts. It is in these that photo-synthesis takes place.

Animal cells and plant cells are similar in that they are both eukaryotic cells. These cells have a true nucleus, which houses DNA and is separated from other cellular structures by a nuclear membrane. Both of these cell types have similar processes for reproduction, which include mitosis and meiosis. Animal and plant cells obtain the energy they need to grow and maintain normal cellular function through the process of cellular respiration. Both of these cell types also contain cell structures known as organelles, which are specialized to perform functions necessary for normal cellular operation. Animal and plant cells have some of the same cell components in common including a nucleus, Golgi complex, endoplasmicreticulum, ribosomes, mitochondria, peroxisomes, cytoskeleton, and cell (plasma) membrane. While animal and plant cells have many common characteristics, they are also different.

Size

Animal cells are generally smaller than plant cells. Animal cells range from 10 to 30 micrometers in length, while plant cells range from 10 and 100 micrometers in length.

Shape

Animal cells come in various sizes and tend to have round or irregular shapes. Plant cells are more similar in size and are typically rectangular or cube shaped.

Energy Storage

Animal cells store energy in the form of the complex carbohydrate glycogen. Plant cells store energy as starch.

Proteins

Of the 20 amino acids needed to produce proteins, only 10 can be produced naturally in animal cells. The other so-called essential amino acids must be acquired through diet. Plants are capable of synthesizing all 20 amino acids.

Differentiation

In animal cells, only stem cells are capable of converting to other cell types. Most plant cell types are capable of differentiation.

Growth

Animal cells increase in size by increasing in cell numbers. Plant cells mainly increase cell size by becoming larger. They grow by absorbing more water into the central vacuole.

Cell Wall

Animal cells do not have a cell wall but have a cell membrane. Plant cells have a cell wall composed of cellulose as well as a cell membrane.

Centrioles

Animal cells contain these cylindrical structures that organize the assembly of microtubules during cell division. Plant cells do not typically contain centrioles.

Cilia

Cilia are found in animal cells but not usually in plant cells. Cilia are microtubules that aid in cellular locomotion.

Cytokinesis

Cytokinesis, the division of the cytoplasm during cell division, occurs in animal cells when a cleavage furrow forms that pinches the cell membrane in half. In plant cell cytokinesis, a cell plate is constructed that divides the cell.

Glyoxysomes

These structures are not found in animal cells but are present in plant cells. Glyoxysomes help to degrade lipids, particularly in germinating seeds, for the production of sugar.

Lysosomes

Animal cells possess lysosomes which contain enzymes that digest cellular macromolecules. Plant cells rarely contain lysosomes as the plant vacuole handles molecule degradation.

Plastids

Animal cells do not have plastids. Plant cells contain plastids such as chloroplasts, which are needed for photosynthesis.

Plasmodesmata

Animal cells do not have plasmodesmata. Plant cells have plasmodesmata, which are pores between plant cell walls that allow molecules and communication signals to pass between individual plant cells.

Vacuole

Animal cells may have many small vacuoles. Plant cells have a large central vacuole that can occupy up to 90% of the cell's volume.

Prokaryotic Cells

Animal and plant eukaryotic cells are also different from prokaryotic cells like bacteria. Prokaryotes are usually single-celled organisms, while animal and plant cells are generally multicellular. Eukaryotic cells are more complex and larger than prokaryotic cells. Animal and plant cells contain many organelles not found in prokaryotic cells. Prokaryotes have no true nucleus as the DNA is not contained within a membrane, but is coiled up in a region of the cytoplasm called the nucleoid. While animal and plant cells reproduce by mitosis or meiosis, prokaryotes propagate most commonly by binary fission.

Other Eukaryotic Organisms

Plant and animal cells are not the only types of eukaryotic cells. Protists and fungi are two other types of eukaryotic organisms. Examples of protists include algae, euglena, and amoebas. Examples of fungi include mushrooms, yeasts, and molds.

Everything in the universe is mare of atoms, arranged in different ways. But living things, unlike rocks or metal, have larger building blocks called cells. Some living things have only one cell, while others contain millions. Each cell has a job to do, but they all work together to make a living organism.

Living organisms are made up of cells. Cells are the structural and functional units of a living organism. In 1665, Robert Hooke discovered the existence of cells using a microscope, which further paved way for the discovery of various other microscopic organisms. Some organisms consist of a single cell, for example, the amoeba. Other organisms are multicellular, having millions of cells.

A single cell is able to produce many cells through a process known as cell division. Different organisms have different kinds of cells. A human body alone shows various kinds of cells such as – blood cells, nerve cell, fat cell etc. Shapes and sizes of cells depend upon the functions they perform. Amoeba has an ever-changing shape as it changes form to locomote. Some cells have a fixed shape and perform a specific function, such as nerve cells, which are usually shaped like trees.

An organism is any being that consists of a single cell or a group of cells, and exhibit properties of life. They have to eat, grow and reproduce to ensure the continuation of their species. Organ systems collectively work together for the proper functioning of a living organism, failure of even one of these systems has an impact on our lives.

A fast-flowing river sweeps soil from the riverbed so that plants cannot grow there. On the other hand, there is more oxygen dissolved in the water, so that fish such as salmon thrive. Rivers in areas where the soil is peaty often have very little wildlife, because acid from the soil washes into the water.

Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish.  However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.

Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked.  So, why is that a problem?

A bay or estuary that has the milky colour of pea soup is showing the result of high concentrations of algae.  Large amounts of algae can cause extreme fluctuations in dissolved oxygen.  Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants.

Eutrophication – “The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.

Anoxia is a lack of oxygen caused by excessive nutrients in waterways which triggers algae growth. When the plants die and decay, oxygen is stripped from the water, which then turns green or milky white and gives off a strong rotten egg odour.  The lack of oxygen is often deadly for invertebrates, fish and shellfish.

Beavers are rodents with very long, sharp front teeth. They use their teeth to gnaw down small trees for use in dam building or for food. Beavers build dams of sticks and mud across a river. This makes a calm pool the other side of the dam in which the beaver can build its home, or lodge. The inside of the lodge is reached by means of underwater tunnels. This keeps the beaver safe from predators such as wolves, even when the surface of the water is frozen in winter.

Dam-building is synonymous with beavers, the ultimate aquatic engineers. Using branches from trees they have felled, these large rodents dam lakes to create moat-like ponds of still water where they construct islands known as ‘conical lodges’ out of timber, mud and rocks. The body of water surrounding the lodges provides protection from predators – resident beavers enter and exit their sophisticated homes incognito via water-filled tunnels leading from the lodges to the pond. The largest lodge, found in Alberta, Canada, measures over 500m in length – though contrary to a widely circulated myth, it is not visible from space! In deep or fast-moving water areas, beavers simply excavate into river banks and set up home there instead.

Beaver dam building is a pretty fascinating topic. Unfortunately, no-one really knows how beavers evolved, let alone how dam building behaviour evolved. Beavers appear to build dams for two main reasons: protection from predators and to provide a stable source of food and easy access to it for themselves.

This offers some clues about how they evolved – almost certainly as a response to selection pressures for these two reasons avoid predation, obtain food. These dams are made of branches stuck down into the stream bed and then built up with a thick mortar of mud, gravel and interwoven branches.

The dam is constantly maintained to keep the water at the same level for beaver comfort and security. Beaver dams are sometimes maintained and expanded over many generations. They can be up to 1,000 feet long and 15-20 feet high.

Beavers are famous for their logging skills, chiseling down trees up to 3 feet in diameter. However, they are not clever enough to aim a tree’s fall and on rare occasions a beaver has been crushed by a tree trunk. The beaver is a very powerful animal, capable of dragging a heavy log through the woods and down into the water.

Although they often lay hundreds or even thousands of eggs, some fish do build nests to protect their young. The stickleback, found in European ponds and rivers, builds a nest of plant fibres in which the male guards the eggs until they have hatched, chasing away even the female that laid them.

The most interesting habits of fishes are their parental behavior in guarding the eggs and caring the young ones. In Chondrichthyes, young ones hatch out in a fully developed condition. But, in Osteichthyes larvae hatch out from eggs and then metamorphose to young adults. In most cases these larvae are quite numerous and so chance will favor at least few of them to tide over adverse condition.

Nest and nursery building Nest building is the commonest method adopted by fishes to protect their eggs and young ones. It is exhibited mostly by fresh water fishes and also by marine fishes having demersal eggs. Nest building involves active participation of either males or females or both. The simplest form of nest building is exhibited by salmons, darters, sunfishes, cichlids, etc. Salmons select gravelly shallows of running streams as their spawning ground. Here they assemble in shoals. Female will make a nursery in the form of pit to lay eggs. After fertilization, she will cover them with layer of gravel. Similar method is adopted by Australian fresh water, Arius. In case of darters , sun fishes and cichlids males make shallow basin like dipressions on the bottom.

The male of N. American bowfin (Amia calva) constructs a crude circular nest of soft weeds and rootless amidst aquatic vegetation. Spawning takes place in the “weedy castle”. Sometimes later the young ones leave the nest in a swarm escorted by their watchful father. The male of N. American bowfin (Amia calva) constructs a crude circular nest of soft weeds and rootless amidst aquatic vegetation. Spawning takes place in the “weedy castle”. Sometimes later the young ones leave the nest in a swarm escorted by their watchful father.

The male of two-spinned stickle back builds an elaborate nest in fresh water, using twigs and weeds, fastened together by the threads of a sticky secretion from the kidney.

Nest building by female is rarely known. Female Heterotis has been shown to make nests in swamps. There are instances in which both the parents take part in nest building. Eg: Labrus Labrus.

Carrying the eggs on the body some fishes ensure protection to their eggs by carrying them safely either in the mouth or anywhere in the body. In Aspredo and Platystacus, the skin of the lower part of the female becomes smooth and spongy during breeding seasons. Fertilized eggs get attached to it.

 In some fishes like Arius and Tilapia mouth-brooding or buccal incubation is a characteristic property both the male and female carries eggs and young ones in their mouth. In oral incubation, the parent will not feed until young ones hatch out. male nursery fish krutus has a cephalic horn upon which female deposits grape like egg clusters.

Keeping the eggs in brood chambers in most species of Syngnathus, Hippocampus, siphonostoma, male has a brood pouch for the deposition of eggs. In Hippocampus, some sort of” placenta” may be formed for gas exchange between the father and the developing young ones. In syngnathus, brood pouch has a highly vascular spongy lining from which the developing young ones may draw nourishment. in unique pipe fish of Indian and pacific oceans, Salenostomus the inner side of the ventral fin of female coalesces with the integument forming a large pouch for keeping the eggs.

Coiling round the eggs The British gunnel or the butter fish has a peculiar way of parental care. The female roll the eggs to a ball and then curls around them. Often male may assist her in this process. Butter Fish.

Keeping the eggs in egg capsules Some chondrichthyes, gives maximum protection to eggs by enclosing them in an egg capsule. In Scyllium, Raja etc, fertilized eggs are kept in a specially designed horny egg capsule, popularly called “Mermaid’s purse”. Egg filled capsules get attached to aquatic weeds with the help of tendril like filaments. Development is completed inside the capsule, utilizing the yolk reserve. Young ones hatch out by breaking the capsule.

Oviposition means, act of laying or depositing eggs. It is mostly exhibited by central European bitterling. In this the genital papilla of the female serves as an ovipositor. With its help eggs may be introduced to the gill chamber of a pond mussel.  During this female takes to a vertical posture and spawning. Male swims around her and discharges sperms in to the mussel. Fertilization and development take place in the gill chamber and young ones leave the host later.

Several fishes provide maximum pre-natal protection to their embryos by adopting ovoviviparity. Here the development is internal and the special portion of the oviduct serves as an unspecialized uterus.  A true mammalian type of placenta is absent. Nutrition is given either by yolk reserve or the uterine milk which is secreted by uterine wall. Eg: for ovoviviparous chondrichthyes are scoliodon, sphyrna, pristis, stegastoma, squalus, mustelus, myliobatis, trygon pteroplatea,etc. Eg: for ovoviviparous osteichthyes are Gambusia,poicilia,blennis,allis,zoarces,cymogaster etc.