General Science

Biology

Cell

Cell Theory

The term cell was coined by Robert Hooke in 1665. In 1838 matthias schleiden, German botanist proposed the idea that all plants consists of cells. In 1839, The Eodor Schwann a German zoologist asserted that all plant and animals are made up of cells. This finding forms the basis of cell theory.

Cell

All living organisms on earth are divided in pieces called cells. Cells are small compartments that hold all of the biolo-gical equipments necessary to keep organisms. Cells carry out all the basic functions of life: growth, metabolism and repro-duction. There are some organisms like amoeba consists of a single cell. This single cell increase in size and when it attains a certain size, it divides into two separates individuals. In case of multi cellular organisms, the cell also divides into two parts, but the two parts remaining joined this process is repeated crore of times so that body mass is built up. In this process some cells become specialized to perform specific functions but other retains their capacity for cell division throughout life.

Tissue

Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence a tissue is an ensemble of cells not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. The study of tissue is known histology.

Animal Tissue

The structure of animal tissue is directly related to its function. Tissue is groups of cells with a basic structure and function. There are four types of tissues.

Type of Animal Tissue

• Epithelial tissue

• Connective tissue

• Muscular tissue

• Nervous tissue.

Epithetical tissue

It is a tissue that is made up of tightly packed cells. Without much materials with in these cells. The reasons for the tightly packed cells are to act as a barrier against mechanical injury, invading micro-organisms and fluid loss. We can define epithetical tissue by considering two points in mind one is the number of cells layers and two the shape of the cells.

(i) On the basis of cell layers

Muscular and Skeletal System

The single-celled protozoan ancestors of animals had their weight supported by water and were able to move by cilia or other simple organelles. The evolution of large and more complex organisms (animals) necessitated the development of support and locomotion systems. Animals use their muscular and skeletal systems for support, locomotion, and maintaining their shape.
Skeletal Systems of Various Animals

(i) Movement is a major characteristic of animals. This movement is a result of contraction of muscles. The skeleton helps transmit that movement. Skeletons are either a fluid-filled body cavity, exoskeletons, or internal skeletons.

(ii) Hydrostatic skeletons consist of fluid-filled closed chambers. Internal pressures generated by muscle contractions cause movement as well as maintain the shape of the animals, such as the sea anemone and worms. The sea anemone has one set of longitudinal muscles in the outer layer of the body, and a layer of circular muscles in the inner layer of the body. The anemone can elongate or contract its body by contracting one or the other set of muscles.

(iii) Exoskeletons are characteristic of the Phylum Arthropoda. Exoskeletons are hard segments that cover the muscles and visceral organs. Muscles for movement attach to the inner surface of the exoskeleton
Exoskeletons restrict the growth of the animal, thus it must shed its exoskeleton (or molt) to form a new one that has room for growth. The bulk and weight of the exoskeleton and associated mechanical problems limits the size a animals can attain.

The Nervous System

Multicellular animals must monitor and maintain a constant internal environment as well as monitor and respond to an external environment. In many animals, these two functions are coordinated by two integrated and coordinated organ systems: the nervous system and the endocrine system. Three basic functions performed by nervous systems are:-

1. Receive sensory input from internal and external environments

2. Integrate the input

3. Respond to stimuli

Sensory Input

Receptors are parts of the nervous system that sense changes in the internal or external environments. Sensory input can be in many forms, including pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.

Integration and Output

In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc.

Endocrine Systems

Some animals have a second control system, the endocrine system. The nervous system coordinates rapid responses to external stimuli. The endocrine system controls slower, longer lasting responses to internal stimuli. Activity of both systems is integrated.

The Endocrine System

The nervous system coordinates rapid and precise responses to stimuli using action potentials. The endocrine system maintains homeostasis and long-term control using chemical signals. The endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis.

Hormones

The endocrine system is a collection of glands that secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands. Hormones are grouped into three classes based on their structure:

1. steroids

2. peptides

3. amines

Steroids

Steroids are lipids derived from cholesterol.
Testosterone is the male sex hormone.
Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics.
Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.

Lymphatic System and Immunity

The Lymphatic System

• The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorbtion of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.

• Lymph vessels are closely associated with the circulatory system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are scatted throughout the body. Contraction of skeletal muscle causes movement of the lymph fluid through valves.

• Lymph organs include the bone marrow, lymph nodes, spleen, and thymus.

• Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow.

• T-lymphocytes (T-cells) mature in the thymus gland.

• Other blood cells such as monocytes and leukocytes are produced in the bone marrow.

• Lymph nodes are areas of concentrated lymphocytes and macrophages along the lymphatic veins.

• The spleen is similar to the lymph node except that it is larger and filled with blood.

• The spleen serves as a reservoir for blood, and filters or purifies the blood and lymph fluid that flows through it.

The Respiratory System

Cellular respiration involves the breakdown of organic molecules to produce ATP. A sufficient supply of oxygen is required for the aerobic respiratory machinery of Kreb’s Cycle and the Electron Transport System to efficiently convert stored organic energy into energy trapped in ATP. Carbon dioxide is also generated by cellular metabolism and must be removed from the cell. There must be an exchange of gases: carbon dioxide leaving the cell, oxygen entering. Animals have organ systems involved in facilitating this exchange as well as the transport of gases to and from exchange areas.

Respiration in Single Cell Animals

Single-celled organisms exchange gases directly across their cell membrane. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size of single-celled organisms. Simple animals that lack specialized exchange surfaces have flattened, tubular, or thin shaped body plans, which are the most efficient for gas exchange. However, these simple animals are rather small in size.

Respiration in multicellular animals

Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies. A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.

The Circulatory System

Living things must be capable of transporting nutrients, wastes and gases to and from cells. Single-celled organisms use their cell surface as a point of exchange with the outside environment. Multicellular organisms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes.

Circulatory Systems in Single-celled Organisms

Single-celled organisms use their cell surface as a point of exchange with the outside environment. Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by ciliary action. Simple animals, such as the hydra and planaria lack specialized organs such as hearts and blood vessels, instead using their skin as an exchange point for materials. This, however, limits the size an animal can attain. To become larger, they need specialized organs and organ systems.

Circulatory Systems in Multicellular Organisms

Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide and metabolic wastes. Components of the circulatory system include

1. Blood: a connective tissue of liquid plasma and cells

2. Heart: a muscular pump to move the blood

3. Blood vessels: arteries, capillaries and veins that deliver blood to all tissues

The Integumentary System

The integumentary system (From Latin integumentum, from integere ‘to cover’; from in- + tegere ‘to cover’ is the organ system that protects the body from damage, comprising the skin and its appendages (including hair, scales, feathers, and nails).

The integumentary system has a variety of functions; it may serve to waterproof, cushion, and protect the deeper tissues, excrete wastes, and regulate temperature, and is the attachment site for sensory receptors to detect pain, sensation, pressure, and temperature. In humans the integumentary system also provides vitamin D synthesis.

The skin is the largest organ in the body: 12-15% of body weight, with a surface area of 1-2 meters. Skin is continuous with, but structurally distinct from mucous membranes that line the mouth, anus, urethra, and vagina.

Two distinct layers occur in the skin: the dermis and epidermis. The basic cell type of the epidermis is the keratinocyte, which contain keratin, a fibrous protein.

Basal cells are the innermost layer of the epidermis. Melanocytes produce the pigment melanin, and are also in the inner layer of the epidermis. The dermis is a connective tissue layer under the epidermis, and contains nerve endings, sensory receptors, capillaries, and elastic fibers.

The integumentary system has multiple roles in homeostasis, including protection, temperature regulation, sensory reception, biochemical synthesis, and absorption. All body systems work in an interconnected manner to maintain the internal conditions essential to the function of the body.

The Reproductive System

The ability to reproduce is one of the unifying characteristics of all living things. Sexual reproduction produces offspring that are genetically different from their parents. Asexual reproduction produces offspring genetically identical to their parent.

Asexual reproduction

• Asexual reproduction allows an organism to rapidly produce many offspring without the time and resources committed to courtship, finding a mate, and mating.

• Fission, budding, fragmentation, and the formation of rhizomes and stolons are some of the mechanisms that allow organisms to reproduce asexually.

• The hydra produces buds;

• Starfish can regenerate an entire body from a fragment of the original body.

Plant Reproduction

Animal life cycles have meiosis followed immediately by gametogenesis. Gametes are produced directly by meiosis. Male gametes are sperm. Female gametes are eggs or ova.
The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into the gametophyte phase. Gametophyte size ranges from three cells (in pollen) to several million (in a “lower plant” such as moss). Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase.

The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing eggs). Within the plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns and “fern allies” to angiosperms.

Flowering plants

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing over 100 million years ago during the middle of the Age of Dinosaurs (late Jurassic). All flowering plants produce flowers and if they are sexually reproductive, they produce a diploid zygote and triploid endosperm.

The Digestive System

Animals, for the most part, ingest their food as large, complex molecules that must be broken down into smaller molecules (monomers) that can then be distributed throughout the body of every cell. This vital function is accpomplished by a series of specialized organs that comprise the digestive system.

Digestive system in various organism:

Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from direct contact with the outside environment, have developed specialized structures for obtaining and breaking down their food.

Animals Depend on Two Processes: Feeding and Digestion

• Animals are heterotrophs, they must absorb nutrients or ingest food sources.

• Ingestive eaters, majority of animals, use a mouth to ingest food.

• Absorptive feeders, such as tapeworms, live in a digestive system of another animal and absorb nutrients from that animal directly through their body wall.

• Filter feeders, such as oysters and mussels, collect small organisms and particles from the surrounding water

• Substrate feeders, such as earthworms and termites, eat the material (dirt or wood) they burrow through.

• Fluid feeders, such as aphids, pierce the body of a plant or animal and withdraw fluids.

The Excretory System

Cells produce water and carbon dioxide as by-products of metabolic breakdown of sugars, fats, and proteins. Chemical groups such as nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to which they were formerly attached, as part of preparing them for energy conversion. The continuous production of metabolic wastes establishes a steep concentration gradient across the plasma membrane, causing wastes to diffuse out of cells and into the extracellular fluid.

Single-celled organisms have most of their wastes diffuse out into the outside environment. Multicellular organisms, and animals in particular, must have a specialized organ system to concentrate and remove wastes from the interstinal fluid into the blood capillaries and eventually deposit that material at a collection point for removal entirely from the body.

Excretory systems in various animals

Excretory systems regulate the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. Components of this system in vertebrates include the kidneys, liver, lungs, and skin.

Not all animals use the same routes or excrete their wastes the same way as humans do. Excretion applies to metabolic waste products that cross a plasma membrane. Elimination is the removal of feces.

Photosynthesis

Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the “fuel” used by all living things. The conversion of unusable sunlight energy (solar energy) into usable chemical energy, is associated with the actions of the green pigment chlorophyll. Most of the time, the photosynthetic process uses water and releases the oxygen .We can write the overall reaction of this process as :

The above chemical equation translates as:

Six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

Structure of leaf

• Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.

• The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

• Water enters the root and is transported up to the leaves through specialized plant cells known as xylem.

Diversity in Living Organisms

Biologists such as Ernst Haeckel, Robert Whittaker and Carl woese have tried to classify all living organism into broad categories called kingdom. The classification which Whittaker proposed has five kingdoms: monera, protista, fungi, plantae and animals.

1. Monera : These organisms do not have a defined nucleus nor do any of them show multi- cellular body designs. This group includes bacteria, blue green algae, or cyanobacteria and mycoplasma.

2. Protista : These groups include many kinds of unicellular eukaryotic organisms some of these organisms use appendages, such as hair like cilia or whip like flagella for moving around. e.g. : unicellular algae, diatoms, protozoans.

3. Fungi : there are heterotrophic eukaryotic organisms. They are decaying organic materials as food and are therefore called saprophytes. They have a cell wall made up of a tough complex sugar called chitin.

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