Qureshi University, Advanced courses, via cutting edge technology, News, Breaking News | Latest News And Media | Current News

Admissions | Accreditation | Booksellers | Catalog | Colleges | Contact Us | Continents/States/Districts | Contracts | Examinations | Forms | Grants | Hostels | Honorary Doctorate degree | Instructors | Lecture | Librarians | Membership | Professional Examinations | Programs | Recommendations | Research Grants | Researchers | Students login | Schools | Search | Seminar | Study Center/Centre | Thesis | Universities | Work counseling

Q. What is a living thing?
Q. What is an animal?
Q. What is a plant?
Q. How are plants classified?
Q. What should you know about a plant?
Q. What is its scientific classification of the plant?
Q. Does it belong to bryophytes, pteridophytes, gymnosperms, or angiosperms?
Q. Is the plant utilized for humans' food?
Q. What is the kingdom classification of plants?
Q. What is plant classification according to how they reproduce?
Q. What are bryophytes, pteridophytes, gymnosperms, and angiosperms?
Q. What should you be able to describe about a flowering plant?
Q. What types of plants should you focus on?
Q. What are the main cellular features of the beings of the plant kingdom?
Q. How different are animal cells from plant cells?
Q. Do plants have tissue organization and specialized organs?
Q. What are the subkingdoms into which the plant kingdom is divided?
Q. What is the difference between bryophytes and tracheophytes?
Q. What are the four main groups into which the study of the plants is divided?
Q. What is the difference between cryptogamic and phanerogamic plants?
Q. What are the two divisions of the angiosperms?
Q. What are the three basic sexual life cycles studied in Biology? Which of them corresponds to metagenesis? Which of them is the human life cycle?
Q. What respectively are zygotic meiosis, gametic meiosis and sporic meiosis?
Q. Are gametes always made by meiosis?
Q. Why is the plant life cycle known as alternation of generations?
Q. How do you define perennial plants?
Q. What are examples of perennial fruits?
Q. What are examples of perennial food grains?
Q. What are examples of perennial vegetables?
Q. What are examples of perennial herbs?
Q. What other plants or crops can you grow alongside apple trees?
Q. Are there any perennial food grain plants?
Q. How many perennial food grain plants do you know?
Q. How do you evaluate the usefulness of fruit for humans?
Q. What is the nutritional value of various fruits?
Q. How do scientists calculate the nutritional value of food for humans?
Q. What are various ways plants are classified?
Q. There are various classifications; why do we need various classifications?
Only one classification does not solve real-world problems.
Q. What type of apple tree yields the maximum number of apples?
How Plants and Animals Differ
Various Plant Classification
Perennial plant
Q. What is a living thing?
* Living things are made of cells.
* Living things obtain and use energy.
* Living things grow and develop.
* Living things reproduce.
* Living things respond to their environment.
* Living things adapt to their environment.

If something follows one or just a few of the rules listed above, it does not necessarily mean that it is living. To be considered alive, an object must exhibit all of the characteristics of living things. Sugar crystals growing on the bottom of a syrup container is a good example of a nonliving object that displays at least one criteria for living organisms.

Can you think of some other examples of nonliving objects displaying living characteristics?

Q. What is a plant?
Plants are living things that usually make their own food, reproduce, but cannot move around. Many are green and have a substance called chlorophyll. Examples are vines, shrubs, flowers, trees, and bushes.

Most plants make their own food by a process called photosynthesis.

All green plants make their own food because they have a substance in them called chlorophyll. Chlorophyll is a key part of the process of photosynthesis.

Photosynthesis is the process by which plants use sunlight to produce the food they need to live and grow.


All living things have to have food for fuel and energy.

Animals and plants are different because animals cannot produce their own food; plants can produce their own food.

Animals and plants are different because animals can move around; plants cannot move around from one place to another.

Q. How are plants classified?

Plant Kingdom is mainly classified into two . This type of plant classification is done according to how they reproduce.

1) Spore bearing plants ( Algae, mosses, ferns and their relatives)
2) Seed bearing plants. (Conifers and flowering plants)

Plants can be classified as either vascular or nonvascular. Vascular plants have a specialized conductive system known as vascular bundles, a group of specialized cells made up of xylem and phloem. Nonvascular plants lack these conducting tissues.

Vascular plants include club mosses, ferns, cycads, gymnosperms, and angiosperms.

* Club mosses : Primitive vascular plants also known as lycopods.

* Ferns and fern allies: Spore-bearing, vascular plants having leaves known as fronds.

* Cycads: Among the oldest plants, Cycads bear resemblence to palms and are native in South America, Africa, and Australia.

* Gymnosperms: Seed-bearing woody vascular plants, such as the conifers (pine, spruce, fir, etc.), whose seeds are not enclosed in an ovary or fruit, but are exposed.

* Angiosperms: Flowering plants that periodically produce flowers which have various parts including sepals, petals, stamens, and carpels.

Nonvascular plants include liverworts, hornworts, and mosses.

* Mosses: Simple green land plants, member of the phyla Bryophyta, along with liverworts and hornworts. They have leaves and a stem, but always lack roots.

* Liverworts and hornworts: Simple green land plants of the phyla Bryophyta with leaves and a stem and always without roots.

Plants are the fundamental building blocks of life on earth. Plants are life forms belonging to the kingdom Plantae. The scientific study has revealed at least 500,000 species of plants. The types of plants vary in size from microscopic algae, to huge sequoia trees more than 8m (26 ft) tall.

Plants are organisms which belong to the plant kingdom. Commonly multicellular, plants produce energy to grow and reproduce by converting light energy radiated from the sun into food through the process of photosynthesis.

What other plants or crops can you grow alongside apple trees?
Corn plants.
Apples have nutrients, including good carbohydrate and fiber.
Apples can be utilized for humans’ meals.
State food and supplies should encourage people to prepare various kinds of apple meals.
Plant classification according to this criterion, can be pictorially represented as follows:
Plant Classification
The plant kingdom can also be classified on the basis of the presence or absence of conductive (vascular) tissue.

Ferns (pteridophytes), gymnosperms and angiosperms have vascular tissue which transports the nutrients and water through the plant. They are collectively known as tracheophytes.

Mosses, liverworts, hornworts (bryophytes) are non-vascular i.e. they do not have conductive tissue to transport sugar, water and nutrients.

Spore Bearing Plants Algae, mosses, ferns and their genus all reproduce by means of spores. These are minute and are formed inside the sporangia that look like fine powder. Each spore contains a small quantity of vital genetic matter in a compact sheathe.


The simplest plants of this type is algae. They do not have leaves,stems or roots.Algae thrive in a moist or wet environment. Many are tiny single celled plants, but some seaweeds are huge.


Mosses and most liverworts have simple stems and tiny, slender leaves. They can be found growing on the plain land, on rocks, and on other plants. They habitually live in mild, damp regions, but some can live in very cold places.


Ferns are the most superior spore bearing type of plants. Many ferns grow in cool, dry places but the largest ones are found in the hot, damp tropic regions. Around 15,000 species of ferns are there in existence now according to scientific researches

Seed Bearing Plants

Plants that reproduce by means of seeds belong to this type of plants. Conifers or gymnosperms and flowering plants or gymnosperms reproduce by seeds. Each seed contains an embryo and a food supply. This is enclosed by a seed covering. A germinating seed is nourished by the food treasury until it can start to make its own.

Conifers or Gymnosperms

Gymnosperms or conifers are plants that have cones instead of flowers. Their seeds grow within female cones. The seeds develop on scales inside cones. The majority of gymnosperms are trees or shrubs. The cones are not as diverse as flowers but they can be brilliantly coloured and attractive.

Flowering plants or Angiosperms

Angiosperms or flowering plants are the most varied set of land vegetation. There are at least 250,000 kinds of flowering plants identified till now. The distinguishing trait of flowering plants or angiosperms is the flower. The chief role of the flower is to make certain that fertilization of the ovule occur and that result in the growth of fruit containing seeds.

Monocotyledons and Dicotyledons

Flowering plants or angiosperms have either one or two cotyledons. Monocotyledons (one seed leaf) have floral parts in multiple of three. Dicotyledons( two seed leaves) have floral parts usually in multiple of four or five. In the figure, left side plant is monocotyledon and right is dicotyledon.

Q. What are the main cellular features of the beings of the plant kingdom?
The typical plant cells are eukaryotic (have nucleus), autotrophic (produce their own food) and photosynthetic (use light to make food). Plant cells also have chloroplasts and a cell wall (a structure exterior to the plasma membrane) made of cellulose.

Q. What should you be able to describe about a flowering plant?
Common names, family, flower color, plant type, location, native.

Q. What types of plants should you focus on?
Perrenial plants that yield fruits, food grains, vegetables.

Q. How different are animal cells from plant cells?
While plant cells are eukaryotic, autotrophic, photosynthetic and have chloroplasts and cell wall, the animal cells are eukaryotic, heterotroph and do not present chloroplasts nor cell wall.

Q. Do plants have tissue organization and specialized organs?
Plants have specialized organs (like reproductive organs, roots, limbs, leaves) and differentiated tissues (vascular tissue in tracheophytes, support tissue, parenchyma, etc.)

Q. What are the subkingdoms into which the plant kingdom is divided?
The kingdom Plantae is divided into two big subkingdoms: the bryophytes and the tracheophytes (pteridophytes, gymnosperms and angiosperms). The criterion for the division is the presence or not of conductive (vascular) tissue.

Plant Classification - Image Diversity: bryophytes pteridophytes gymnosperms angiosperms

Q. What is the difference between bryophytes and tracheophytes?
Bryophytes are nonvascular plants (mosses, liverworts, hornworts), i.e., they do not have a conductive system for transport of sugar, water and nutrients. Tracheophyte plants are vascular plants, they have conductive structures.

Q. What are the four main groups into which the study of the plants is divided?
In Botany the plant kingdom is divided into bryophytes, pteridophytes, gymnosperms and angiosperms.

When identifying trees, you will need to determine whether they are conifers or deciduous trees.

--Gymnosperms are a taxonomic class that includes plants whose seeds are not enclosed in an ovule (like a pine cone). Gymnosperm means as "naked seed". This group is often referred to as softwoods. Gymnosperms usually have needles that stay green throughout the year. Examples are pines, cedars, spruces and firs. Some gymnosperms do drop their leaves - ginkgo, dawn redwood, and baldcypress, to name a few.

--Angiosperms are a taxonomic class of plants in which the mature seed is surrounded by the ovule (think of an apple). This group is often referred to as hardwoods. Angiosperms are trees have broad leaves that usually change color and die every autumn. Oaks, maples and dogwoods are examples of deciduous trees. Some angiosperms that hold their leaves include rhododendron, live oak, and sweetbay magnolia.

Basics of Tree ID Identifying and Classifying organisms is fundamental to Biological Sciences. All living things are divided up into groups. Each individual in the group has similar characteristics. The broadest group is the Kingdom and the most specific group is the species.

The first step in tree identification is knowing that there are always distinguishing characteristics that separate one tree species from another. By examining different tree parts you will be able to confidently identify the different trees around your school. This will require some careful detective work on your part, but it should be fun and easy.

Here are some clues that you will need to examine:
* TREE TYPE --Deciduous or Conifer? Tree or a shrub? Determining these things starts you off on your way to tree identification.
* LEAF --Leaves are often the easiest way to identify most trees. Are the leaves arranged in an opposite or alternate pattern?
* BARK --Bark can be helpful for identifying some types of trees.
* FRUIT --The wide variety of fruit shapes makes them useful when identifying trees.
* TWIG --You can actually tell a lot just by looking at the twig.
* FORM --The way a tree grows can tell you a great deal about a tree.

After collecting all of your clues, you should use our leaf key to verify the tree species you are identifying. It also contains links to detailed descriptions about different tree species.

Leaf Key

Woody Plants in North America

Q. Which taxonomic class of plants has a mature seed surrounded by an ovule?
Q. How do you know if a leaf is simple or compound?
Q. How can a twig help you to identify a tree?
Q. What is the difference between cryptogamic and phanerogamic plants?
Cryptogamic (hidden sex organs) plants are those that do not present flowers or seeds. They comprise the bryophytes and the pteridophytes.

Phanerogamic plants are those having seeds. They comprise the gymnosperms and the angiosperms.

Plant Classification - Image Diversity: seeds

Q. What are the two divisions of the angiosperms?
The angiosperms are divided into monocotyledonous and dicotyledonous. (These categories are explained later in this text.)

Plant Classification - Image Diversity: monocots dicots

Q. What are the three basic sexual life cycles studied in Biology? Which of them corresponds to metagenesis? Which of them is the human life cycle?

Sexual reproduction may take place through three different types of life cycles: the haplontic (the being is haploid) haplobiontic (a single type of being) cycle; the diplontic (the being is haploid) haplobiontic (a single type of being) cycle; and the diplobiontic cycle (two types of beings, one haploid and the other diploid). The diblobiontic cycle is known as alternation of generations, or metagenesis. In humans the cycle is diplontic haplobiotic (a single diploid being).

Q. What respectively are zygotic meiosis, gametic meiosis and sporic meiosis?

Zygotic meiosis is the one that occurs in the haplontic haplobiontic life cycle. Gametes from adult haploid individuals unite forming the diploid zygote. The zygote undergoes meiosis and generates four haploid cells that by mitosis develop into adult individuals. Therefore in the zygotic meiosis the cell that undergoes meiosis is the zygote and the gametes are formed by mitosis.

Gametic meiosis is that in which meiosis produces gametes, i.e., haploid cells that each of which can unite with another gamete forming the zygote. It occurs in the diplontic haplobiontic life cycle (e.g., in humans) in which the individual is diploid and meiosis forms gametes.

Sporic meiosis happens in metagenesis (alternation of generations, or diplobiontic life cycle). In this life cycle cells from the diploid individual (called sporophyte) undergo meiosis producing haploid spores that do not unite with others but instead develop by mitosis into haploid individuals (called gametophytes). In this life cycle the gametes are made by mitosis from cells of the gametophyte.

Q. Are gametes always made by meiosis?
In the plant life cycle (diplobiontic life cycle) and in the haplontic haplobiontic life cycle gametes are made by mitosis and not by meiosis. Obviously in some stage of these sexual life cycles meiosis must occur.

Q. Why is the plant life cycle known as alternation of generations?
The plant life cycle is known as alternation of generations because in this cycle there are two different forms of living beings that alternate each other, one haploid and the other diploid. Alternation of generations is also called the diplobiontic cycle, or metagenesis, and it does not occur only in plants, other living beings, like cnidarians, present the cycle.

Q. For each of the three types of life cycles what is the respective ploidy of the individual that represents the adult or lasting form?

In the haplontic haplobiontic life cycle the single and lasting form is haploid. In the diplontic haplobiontic life cycle it is diploid. In the diplobiontic life cycle the lasting individual that alternates with the intermediate form may be the haploid gametophyte (as in bryophytes) or the diploid sporophyte (as in pteridophytes).

Plant Classification - Image Diversity: haplontic haplobiontic life cycle diplontic haplobiontic life cycle diplobiontic life cycle

Q. Do plants present only sexual reproduction?
There are asexual forms of reproduction in plants. Some naturally detached pieces of root, limbs or leaves develop into another complete individual. Artificial asexual reproduction of plants can be obtained by means of grafting or cutting.

Anatomy of the Plant Cell

Like other eukaryotes, the plant cell is enclosed by a plasma membrane, which forms a selective barrier allowing nutrients to enter and waste products to leave. Unlike other eukaryotes, however, plant cells have retained a significant feature of their prokaryote ancestry, a rigid cell wall surrounding the plasma membrane. The cytoplasm contains specialized organelles, each of which is surrounded by a membrane. Plant cells differ from animal cells in that they lack centrioles and organelles for locomotion (cilia and flagella), but they do have additional specialized organelles. Chloroplasts convert light to chemical energy, a single large vacuole acts as a water reservoir, and plasmodesmata allow cytoplasmic substances to pass directly from one cell to another. There is only one nucleus and it contains all the genetic information necessary for cell growth and reproduction. The other organelles occur in multiple copies and carry out the various functions of the cell, allowing it to survive and participate in the functioning of the larger organism.

Like the fungi, another kingdom of eukaryotes, plant cells have retained the protective cell wall structure of their prokaryotic ancestors. The basic plant cell shares a similar construction motif with the typical eukaryote cell, but does not have centrioles, lysosomes, intermediate filaments, cilia, or flagella, as does the animal cell. Plant cells do, however, have a number of other specialized structures, including a rigid cell wall, central vacuole, plasmodesmata, and chloroplasts. Although plants (and their typical cells) are non-motile, some species produce gametes that do exhibit flagella and are, therefore, able to move about.

Plants can be broadly categorized into two basic types: vascular and nonvascular. Vascular plants are considered to be more advanced than nonvascular plants because they have evolved specialized tissues, namely xylem, which is involved in structural support and water conduction, and phloem, which functions in food conduction. Consequently, they also possess roots, stems, and leaves, representing a higher form of organization that is characteristically absent in plants lacking vascular tissues. The nonvascular plants, members of the division Bryophyta, are usually no more than an inch or two in height because they do not have adequate support, which is provided by vascular tissues to other plants, to grow bigger. They also are more dependent on the environment that surrounds them to maintain appropriate amounts of moisture and, therefore, tend to inhabit damp, shady areas.

It is estimated that there are at least 260,000 species of plants in the world today. They range in size and complexity from small, nonvascular mosses to giant sequoia trees, the largest living organisms, growing as tall as 330 feet (100 meters). Only a tiny percentage of those species are directly used by people for food, shelter, fiber, and medicine. Nonetheless, plants are the basis for the Earth's ecosystem and food web, and without them complex animal life forms (such as humans) could never have evolved. Indeed, all living organisms are dependent either directly or indirectly on the energy produced by photosynthesis, and the byproduct of this process, oxygen, is essential to animals. Plants also reduce the amount of carbon dioxide present in the atmosphere, hinder soil erosion, and influence water levels and quality.

Plants exhibit life cycles that involve alternating generations of diploid forms, which contain paired chromosome sets in their cell nuclei, and haploid forms, which only possess a single set. Generally these two forms of a plant are very dissimilar in appearance. In higher plants, the diploid generation, the members of which are known as sporophytes due to their ability to produce spores, is usually dominant and more recognizable than the haploid gametophyte generation. In Bryophytes, however, the gametophyte form is dominant and physiologically necessary to the sporophyte form.

Animals are required to consume protein in order to obtain nitrogen, but plants are able to utilize inorganic forms of the element and, therefore, do not need an outside source of protein. Plants do, however, usually require significant amounts of water, which is needed for the photosynthetic process, to maintain cell structure and facilitate growth, and as a means of bringing nutrients to plant cells. The amount of nutrients needed by plant species varies significantly, but nine elements are generally considered to be necessary in relatively large amounts. Termed macroelements, these nutrients include calcium, carbon, hydrogen, magnesium, nitrogen, oxygen, phosphorus, potassium, and sulfur. Seven microelements, which are required by plants in smaller quantities, have also been identified: boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

Thought to have evolved from the green algae, plants have been around since the early Paleozoic era, more than 500 million years ago. The earliest fossil evidence of land plants dates to the Ordovician Period (505 to 438 million years ago). By the Carboniferous Period, about 355 million years ago, most of the Earth was covered by forests of primitive vascular plants, such as lycopods (scale trees) and gymnosperms (pine trees, ginkgos). Angiosperms, the flowering plants, didn't develop until the end of the Cretaceous Period, about 65 million years ago—just as the dinosaurs became extinct.

Cell Wall - Like their prokaryotic ancestors, plant cells have a rigid wall surrounding the plasma membrane. It is a far more complex structure, however, and serves a variety of functions, from protecting the cell to regulating the life cycle of the plant organism.

Chloroplasts - The most important characteristic of plants is their ability to photosynthesize, in effect, to make their own food by converting light energy into chemical energy. This process is carried out in specialized organelles called chloroplasts.

Endoplasmic Reticulum - The endoplasmic reticulum is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm. In plants, the endoplasmic reticulum also connects between cells via the plasmodesmata.

Golgi Apparatus - The Golgi apparatus is the distribution and shipping department for the cell's chemical products. It modifies proteins and fats built in the endoplasmic reticulum and prepares them for export as outside of the cell.

Microfilaments - Microfilaments are solid rods made of globular proteins called actin. These filaments are primarily structural in function and are an important component of the cytoskeleton.

Microtubules - These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and carry out a variety of functions, ranging from transport to structural support.

Mitochondria - Mitochondria are oblong shaped organelles found in the cytoplasm of all eukaryotic cells. In plant cells, they break down carbohydrate and sugar molecules to provide energy, particularly when light isn't available for the chloroplasts to produce energy.

Nucleus - The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).

Peroxisomes - Microbodies are a diverse group of organelles that are found in the cytoplasm, roughly spherical and bound by a single membrane. There are several types of microbodies but peroxisomes are the most common.

Plasmodesmata - Plasmodesmata are small tubes that connect plant cells to each other, providing living bridges between cells.

Plasma Membrane - All living cells have a plasma membrane that encloses their contents. In prokaryotes and plants, the membrane is the inner layer of protection surrounded by a rigid cell wall. These membranes also regulate the passage of molecules in and out of the cells.

Ribosomes - All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA.

Vacuole - Each plant cell has a large, single vacuole that stores compounds, helps in plant growth, and plays an important structural role for the plant.

Leaf Tissue Organization - The plant body is divided into several organs: roots, stems, and leaves. The leaves are the primary photosynthetic organs of plants, serving as key sites where energy from light is converted into chemical energy. Similar to the other organs of a plant, a leaf is comprised of three basic tissue systems, including the dermal, vascular, and ground tissue systems. These three motifs are continuous throughout an entire plant, but their properties vary significantly based upon the organ type in which they are located. All three tissue systems are discussed in this section.

What distinguishes kingdom plantae from all the other kingdoms, is that the cells of kingdom plantae have cell walls made of cellulose that are used to support the plant. This cell wall is not a semi-permeable membrane and the cell cannot transport material and nutrients in and out of the cell walls. For this function there is the large central vacuole that stores water and chemicals for use inside of the cell. Another characteristic belonging only to kingdom plantae is their chloroplasts, the organelle that converts light energy into chemical energy inside the plant where the energy is stored as sugar. Their ability to convert inorganic matter (atmospheric CO2) to organic matter using photosynthesis keeps us humans in kingdom animalia alive.

Introduction to Cell and Virus Structure

At first glance, the petal of a flower or the skin on the back of a human hand may seem smooth and seamless, as if they were composed of a single, indistinct substance. In reality, however, many tiny individual units called cells make up these objects and almost all other components of plants and animals. The average human body contains over 75 trillion cells, but many life forms exist as single cells that perform all the functions necessary for independent existence. Most cells are far too small to be seen with the naked eye and require the use of high-power optical and electron microscopes for careful examination.

The relative scale of biological organisms as well as the useful range of several different detection devices are illustrated in Figure 1. The most basic image sensor, the eye, was the only means humans had of visually observing the world around them for thousands of years. Though excellent for viewing a wide variety of objects, the power of the eye has its limits, anything smaller than the width of a single human hair being able to pass unnoticed by the organ. Therefore, when light microscopes of sufficient magnifying capability were developed in the late 1600s, a whole new world of tiny wonders was discovered. Electron microscopes, invented in the mid-twentieth century, made it possible to detect even tinier objects than light microscopes, including smaller molecules, viruses, and DNA. The detection power of most electron microscopes used today, however, stops just short of being able to visualize such incredibly small structures as the electron orbital systems of individual atoms. Atoms are considered the smallest units of an element that have the characteristics of that element, but cells are the smallest structural units of an organism capable of functioning independently.

Yet, until the mid-seventeenth century, scientists were unaware that cells even existed. It wasn't until 1665 that biologist Robert Hooke observed through his microscope that plant tissues were divided into tiny compartments, which he termed "cellulae" or cells. It took another 175 years, however, before scientists began to understand the true importance of cells. In their studies of plant and animal cells during the early nineteenth century, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between the two cell types. In 1839, they proposed that all living things are made up of cells, the theory that gave rise to modern biology.

Since that time, biologists have learned a great deal about the cell and its parts; what it is made of, how it functions, how it grows, and how it reproduces. The lingering question that is still being actively investigated is how cells evolved, i.e., how living cells originated from nonliving chemicals.

Numerous scientific disciplines—physics, geology, chemistry, and evolutionary biology—are being used to explore the question of cellular evolution. One theory speculates that substances vented into the air by volcanic eruptions were bombarded by lightning and ultraviolet radiation, producing larger, more stable molecules such as amino acids and nucleic acids. Rain carried these molecules to the Earth's surface where they formed a primordial soup of cellular building blocks.

A second theory proposes that cellular building blocks were formed in deep-water hydrothermal vents rather than in puddles or lakes on the Earth's surface. A third theory speculates that these key chemicals fell to earth on meteorites from outer space.

Given the basic building blocks and the right conditions, it would seem to be just a matter of time before cells begin to form. In the laboratory, lipid (fat) molecules have been observed joining together to produce spheres that are similar to a cell's plasma membrane. Over millions of years, perhaps it is inevitable that random collisions of lipid spheres with simple nucleic acids, such as RNA, would result in the first primitive cells capable of self-replication.

For all that has been learned about cells in over 300 years, hardly the least of which is the discovery of genetic inheritance and DNA, cell biology is still an exciting field of investigation. One recent addition is the study of how physical forces within the cell interact to form a stable biomechanical architecture. This is called "tensegrity" (a contraction of "tensional integrity"), a concept and word originally coined by Buckminster Fuller. The word refers to structures that are mechanically stable because stresses are distributed and balanced throughout the entire structure, not because the individual components have great strength.

In the realm of living cells, tensegrity is helping to explain how cells withstand physical stresses, how they are affected by the movements of organelles, and how a change in the cytoskeleton initiates biochemical reactions or even influences the action of genes. Some day, tensegrity may even explain the mechanical rules that caused molecules to assemble themselves into the first cells.

Animal Cells - Animal cells are typical of the eukaryotic cell type, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles.

Bacteria - One of the earliest prokaryotic cells to have evolved, bacteria have been around for at least 3.5 billion years and live in almost every imaginable environment.

Plant Cells - The basic plant cell has a similar construction to the animal cell, but does not have centrioles, lysosomes, cilia, or flagella. It does have additional structures, including a rigid cell wall, central vacuole, plasmodesmata, and chloroplasts.

Virus Structure - Viruses are not alive in the strict sense of the word, but reproduce and have an intimate, if parasitic, relationship with all living organisms.

Cells in Motion - In multicellular tissues, such as those found in animals and humans, individual cells employ a variety of locomotion mechanisms to maneuver through spaces in the extracellular matrix and over the surfaces of other cells. Examples are the rapid movement of cells in developing embryos, organ-to-organ spreading of malignant cancer cells, and the migration of neural axons to synaptic targets. Unlike single-celled swimming organisms, crawling cells in culture do not possess cilia or flagella, but tend to move by coordinated projection of the cytoplasm in repeating cycles of extension and retraction that deform the entire cell. The digital videos presented in this gallery investigate animal cell motility patterns in a wide variety of morphologically different specimens.

Fluorescence Microscopy of Cells in Culture - Serious attempts at the culture of whole tissues and isolated cells were first undertaken in the early 1900s as a technique for investigating the behavior of animal cells in an isolated and highly controlled environment. The term tissue culture arose because most of the early cells were derived from primary tissue explants, a technique that dominated the field for over 50 years. As established cell lines emerged, the application of well-defined normal and transformed cells in biomedical investigations has become an important staple in the development of cellular and molecular biology. This fluorescence image gallery explores over 30 of the most common cell lines, labeled with a variety of fluorophores using both traditional staining methods as well as immunofluorescence techniques.

Observing Mitosis with Fluorescence Microscopy - Mitosis, a phenomenon observed in all higher eukaryotes, is the mechanism that allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division. This, coupled with cytokinesis (division of the cytoplasm), occurs in all multicellular plants and animals to permit growth of the organism. Digital imaging with fluorescence microscopy is becoming a powerful tool to assist scientists in understanding the complex process of mitosis on both a structural and functional level.

Mitosis Java Tutorial - Explore the stages of mitosis in eukaryotic cells with this interactive Java tutorial. Step through prophase, metaphase, anaphase, and telophase as the chromosomes slowly condense, align, and divide before being segregated into daughter cells.

Cell Digestion and the Secretory Pathway - The primary sites of intracellular digestion are organelles known as the lysosomes, which are membrane-bounded compartments containing a variety of hydrolytic enzymes. Lysosomes maintain an internal acidic environment through the use of a hydrogen ion pump in the lysosomal membrane that drives ions from the cytoplasm into the lumenal space of the organelles. The high internal acidity is necessary for the enzymes contained in lysosomes to exhibit their optimum activity. Hence, if the integrity of a lysosomal membrane is compromised and the enzymatic contents are leaked into the cell, little damage is done due to the neutral pH of the cytoplasm. If numerous lysosomes rupture simultaneously, however, the cumulative action of their enzymes can result in autodigestion and the death of the cell.
Perennial Plant
Soil test