Matthias Schleiden was a German botanist who made significant contributions to the development of the cell theory. In 1838, he published a book called “Contributions to the Phytogenesis” in which he proposed that all plants are made up of cells or derivatives of cells. He also stated that the cell is the basic unit of plant structure and function.

Schleiden’s observations were based on his microscopic examination of plant tissues. He used a powerful microscope to study thin slices of plant material, and he was able to see the individual cells that make up these tissues. Schleiden also observed that the cells in different parts of a plant are often specialized for different functions. For example, the cells in the leaves of a plant are specialized for photosynthesis, while the cells in the roots of a plant are specialized for water and nutrient absorption.

Schleiden’s work on the cell theory was groundbreaking for its time. It helped to establish the cell as the basic unit of life, and it paved the way for future research on cell structure and function. Schleiden’s work was also influential in the development of the theory of evolution, as it showed that all plants and animals are made up of the same basic building blocks.

Here is a summary of the three main tenets of the cell theory, as originally proposed by Schleiden and his colleague Theodor Schwann:

1. All living things are composed of one or more cells.
2. The cell is the basic unit of structure and function in living things.
3. All cells come from pre-existing cells.

While the cell theory has been modified slightly over the years to account for new scientific discoveries, it remains one of the most fundamental theories in biology.

Schleiden’s contributions to the cell theory were significant, and he is considered to be one of the founding fathers of modern biology. His work helped to lay the foundation for our understanding of the cell as the basic unit of life.

Other Theories of Cell

Protoplasm theory

The protoplasm theory is an outdated scientific hypothesis that was prevalent in the 19th century. It proposed that living cells were composed of a substance called “protoplasm,” which was thought to be a homogeneous, jelly-like material responsible for all the functions of life. Protoplasm was believed to be a complex, yet amorphous substance that could give rise to various cellular structures and functions.

Key points of the protoplasm theory include:

1. Homogeneity: Protoplasm was thought to be a uniform substance throughout the cell, responsible for both the structure and function of living organisms.
2. Vitalism: The protoplasm theory was influenced by the vitalist idea that there was a mysterious life force inherent in protoplasm, which distinguished living matter from non-living matter.
3. Spontaneous Generation: Some proponents of the theory also believed that protoplasm could spontaneously generate life under the right conditions, which was consistent with the ideas of the time.
4. Lack of Cellular Organelles: The concept of cellular organelles, such as the nucleus, mitochondria, and endoplasmic reticulum, was not yet understood in detail during the era when the protoplasm theory was prevalent.

The protoplasm theory was eventually replaced by the cell theory, which was developed in the mid-19th century. The cell theory, proposed by scientists like Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, emphasized that cells are the basic structural and functional units of all living organisms. It also recognized the importance of cellular organelles and rejected the idea of spontaneous generation.

Modern cell biology has provided a much more detailed and accurate understanding of cells and their components, including the roles of cellular organelles, DNA, and proteins in life processes. As a result, the protoplasm theory is now considered obsolete and has been replaced by our current understanding of cell biology and molecular biology.

Plant cell theory

The plant cell theory is a fundamental concept in biology that states that plants are composed of cells. It is a fundamental component of the broader cell theory, which applies to all living organisms. The cell theory consists of three main principles:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. Cells arise from pre-existing cells through the process of cell division.

The plant cell theory, specifically, focuses on plant cells and extends the principles of the cell theory to plant life. Here are some key points related to the plant cell theory:

1. Plant cells are the basic building blocks of plants: Just as animal cells are the basic units of animals, plant cells are the basic units of plants. All parts of a plant, from leaves and stems to roots and flowers, are made up of plant cells.
2. Plant cells have unique features: Plant cells have some unique structures that set them apart from animal cells. These include a rigid cell wall made of cellulose, chloroplasts for photosynthesis, and large central vacuoles that store water and nutrients.
3. Plant cells can differentiate into various cell types: Similar to animal cells, plant cells can differentiate into specialized cell types, such as parenchyma cells, xylem cells, and phloem cells, to perform specific functions within the plant.
4. Plant cells can reproduce: Plant cells, like all cells, can divide through processes like mitosis and meiosis. This allows plants to grow, develop, and reproduce.

The plant cell theory, along with the broader cell theory, laid the foundation for modern cell biology and our understanding of life at the cellular level. It was formulated based on the work of early scientists like Matthias Schleiden and Theodor Schwann, who made significant contributions to the development of cell theory in the 19th century. This theory continues to be a fundamental concept in biology and serves as a basis for studying the structure and function of plant cells and their role in the growth and development of plants.

Exceptions of cell theory

The cell theory is a fundamental concept in biology, but there are a few exceptions or special cases that do not strictly adhere to all aspects of the theory. While the cell theory is generally valid for the vast majority of living organisms, there are some exceptions and variations that have been discovered over time. Here are a few notable exceptions:

1. Giant Algae: Some unicellular algae, such as Acetabularia, are quite large and can reach sizes of several centimetres. These giant single cells have a complex internal structure and challenge the idea that cells are always small.
2. Aseptate Fungi: Most fungi have cells with well-defined cell walls and septa (dividing walls) that separate individual cells. However, some fungi, like coenocytic fungi, lack these septa, and their multinucleated, continuous cytoplasm challenges the concept of discrete cells.
3. Striated Muscle Cells: Muscle cells, particularly skeletal and cardiac muscle cells, are long, multinucleated cells with multiple nuclei scattered throughout the cell. These nuclei work together to control the cell’s functions, and this multinucleation contradicts the idea that cells typically have a single nucleus.
4. Syncytia: In some animals, such as certain types of insects, syncytia are formed when multiple cells fuse together during development, creating a large, multinucleated cell. This is different from the typical individual cell structure found in most animals.
5. Viruses: Viruses are not considered living organisms because they cannot carry out metabolic processes on their own. They do not have typical cell structures and are essentially bundles of genetic material (either DNA or RNA) surrounded by a protein coat. They challenge the idea that cells are the basic units of life.
6. Mitochondria and Chloroplasts: These organelles, which are found within eukaryotic cells, have their own DNA and can reproduce independently. This suggests that they were once free-living prokaryotic organisms that were engulfed by a host cell in a symbiotic relationship. While they are considered organelles within a cell, their unique origin sets them apart from typical cellular components.
7. Nucleated Red Blood Cells in Some Vertebrates: In most vertebrates, mature red blood cells lack nuclei. However, some vertebrates, such as birds and reptiles, have nucleated red blood cells. This contrasts with the idea that mature animal cells typically lack nuclei.

While these exceptions challenge some aspects of the cell theory, they do not invalidate the fundamental principles of the theory itself. The cell theory remains a robust framework for understanding the organization and function of living organisms in the vast majority of cases.

Animals are classified mainly into three groups namely – Oviparous, Viviparous and Ovoviviparous depends on the site of development of embryo and whether they lay eggs (unfertilized or fertilized) or give birth to young ones.

Oviparous

In Oviparous Ovum-egg-, Parere- to produce) animals (egg laying animals), the young hatch from eggs
laid outside the mother’s body. e.g. reptiles and birds (their eggs are covered by hard calcareous shells), invertebrates, fishes and amphibians (eggs are not covered by hard calcareous shells but covered by a membrane).

Viviparous

Viviparous (L., Vivus – alive, Parere to produce) animals give rise to young ones. Viviparity is a type of development in which the young ones are born alive after being nourished in the uterus through the placenta.
Majority of mammals including human beings are viviparous.

Ovoviviparous

In Ovoviviparous animals, the embryo develops inside the egg and remains in the mother’s body until they
are ready to hatch. This method of reproduction is similar to viviparity but the embryos have no placental connection with the mother and receive their nourishment from the egg yolk. Ovoviviparity is seen in
fishes like shark.

The Normal period of gestation or pregnancy is around 9 months and 40 weeks.

Human fetal development stages

First Trimester – It is from conception to 12 weeks. It is the first three months of pregnancy. During this period, the baby will change from a group of cells to a fetus and starts to have a baby’s characteristics.

Week 1 to 4 – During this period, the fertilized egg grows. These eggs would be filled with fluid and a watertight sac surrounds them. This sac is called Amniotic Sac and its purpose is to provide shock absorption to the developing embryo.

During this week, the Placenta develops, an early face forms with dark circles for the eyes, the blood cells forms and begin their circulation, the heartbeats 65 per minute and also mouth, and throat develops.

By the end of the first month, the baby’s size would be 1/4 inch.

Week 5 to 8 – During this period, the ear begins to take shape, tiny buds grow to arms and legs, and also form the toes and fingers.

The eyes also form. The nervous system consists of the brain, spinal cord and other neural tissue are well grown, also the digestive system and sensory organs are formed.

Then the cartilage gets replaced by bones. After the 8th week, this embryo is called a Fetus. It is usually 1 inch long and weighs 0.00094 kg.

Fertilization of Embryo

The process of fusion of male and female gametes to form a diploid zygote is known as fertilization. The two types of fertilization are external fertilization and internal fertilization. The fusion of gametes that occurs outside the body of the animal is called external fertilization.

An example of external fertilization for frogs is echinoderms. The fusion of the gametes occurs within the body of the female known as internal fertilization. Examples of internal fertilization are reptiles, birds and mammals.

At the time of ovulation, the ovum is fully matured and enters the uterine tube called the infundibulum. Fertilization of the ovum occurs in the ampulla of the uterine tube. Only one spermatozoan pierces the membrane called zona pellucida. Polygamy, the entry of more sperms is prevented by the fertilization membrane around the ovum.

Human fetal development stages: Development of embryo

The fertilized ovum is called the zygote. The first phase of embryo development is called cleavage. As a result of cleavage, a ball of cells is formed known as a blastula. The outer surface forms the trophoblast and the embryo gets attached to the uterus wall, which is known as implantation.

The implanted embryo develops extra-embryonic membranes such as amnion, Allantois, Ohorion and yolk sac. The fluid medium of the developing embryo prevents desiccation called the amnion. The fusion of chorion and Allantois is called Placenta.

The exchange of gases between the mother and foetus and also their elimination of nitrogenous waste from the foetus are functions of the placenta. The embryo and the placenta are connected by an umbilical cord which is derived from the Allantois.

From the fertilization of the ovum to the birth of the baby of about 9 months during which the foetus remains inside the uterus is called the re-gestation period. the development of the foetus can be studied from the first, second and third trimesters.

The proliferation of cells takes place and gradually a single cell is transformed into the foetus is called the first trimester. Organogenesis takes place in the first trimester. Organogenesis means the formation of organs.

The foetus grows rapidly in the second trimester. The respiratory and circulatory systems become well developed and functional in the second trimester. The length and weight of the foetus increase very rapidly and development is completed in the third trimester.

The uterus begins to contract rhythmically under the influence of the oxytocin hormone. The contractions become stronger and more frequent called labour pain. With continued powerful contractions, the amnion ruptures and the fluid that flows out through the Female Reproductive System is amniotic.

The placenta breaks away from the uterus and is expelled after birth. The first milk that comes out from the mother’s mammary gland just after childbirth is Colestrum. Rich in proteins and nutrients and also containing antibodies to provide immunity for the newborn infant is Colestrum.

The secretion of the milk is stimulated by the pituitary hormone called prolactin. The best milk easily available clean, uncontaminated and sterile is mother’s milk. The milk contains antibodies that shield the body from external viral and bacterial infections in mother’s milk.

In rural areas, breast milk is used as eye drops for viral conjunctivitis. The calorific value of breastmilk is 70 per 100 ml of milk. The protein in breast milk that provides protection against intestinal and respiratory infections is lactoferrin.

The technique of in-vitro fertilization, fertilization and early development takes place in an artificial medium outside called test-tube babies. The invention of first test-tube baby is Steptoe and Edwards in the UK.

Early classification of organisms were based on only two criteria, beneficial or harmful animals. An ancient classification system recognized 5 animal groups – domestic, wild, creeping, flying and
sea animals.

Initially the classification was based on organism’s fundamental characteristics such as the habitat and morphology only.

Aristotle (384 to 322 BC), was the first to classify all animals in his History of Animals (Historia Animalium in Latin). He attempted a basic classification of all living organisms into Plants and Animals. Animals were classified based on locomotion; walking (terrestrial), flying (aerial) and swimming (aquatic).

Based on the presence or absence of red blood he classified the animals into two as Enaima with blood and those without blood as Anaima.

Aristotle’s classification system had limitations and many organisms were not fitting into his classification. For example, the tadpoles of frogs are born in water and have gills but when they metamorphosed into adult frogs they have lungs and can live both in water and on land.

How to classify frogs and where to place them? Aristotle classified organisms based on locomotion, hence, birds, bats, and flying insects were grouped together just by observing one single characteristic feature, the flying ability.

On the contrary to the above said example, the ostrich, emu and penguin are all birds but cannot fly. So Aristotle would not have classified them as birds. In spite of these limitations Aristotle’s classification system was followed for more than 2000 years upto 1700.

After Aristotle, his student Theophrastus (372-287 BC) continued his research on the classification of plants, and he was known as the “Father of Botany.”

There was a huge gap till 16th century, then the English naturalist John Ray (1627–1705) wrote several important works through his life. His most important contribution was the establishment of species as the ultimate unit of taxonomy.

In 1682 he published the Methodus Plantarum Nova, which contained about 18,000 plant species, a result of a relatively narrow species concept.

His complicated classification was based on many combined characters, as opposed to earlier taxonomists. John Ray also aimed at publishing a complete system of nature, which included works
on mammals, reptiles, birds, fishes and insects.

The Swedish biologist Carolus Linnaeus (1707 – 1788) father of modern taxonomy and founder of modern systematics developed a scientific system of taxonomy and binomial nomenclature, which is still (with modifications) in use.

Aristotle to Linnaeus employed easily observable single to few traits for classification of organisms. With increased knowledge of the several biological domains, many characters were considered for classifying organisms.

This represented the phase of classical taxonomy which was based on overall similarities or affinities derived from morphology, anatomy and embryology of organisms. A modification of this system is the numerical taxonomy, which evolved in the 1950s.

This system evaluates the resemblances and differences through statistical methods followed by computer analyses to establish the numerical degree of relationship among individuals.

Later on biologists initiated studies on the evolutionary and genetic relationships among organisms, which led to the emerge of phylogenetic classification or cladistics. It is an evolutionary classification based on how a common ancestry was shared. Cladistic classification summarizes the genetic differences between all species in the ‘phylogenetic tree’.

Ernst Haeckal introduced the method of representing evolutionary relationships with the help of a tree diagram known as cladogram.

This system of classification takes into account ancestral characters (traits of basic body design which would be in the entire group) and derived characters (traits whose structure and functions differs from those of ancestral characters).

One or more derived characters which appeared during evolution resulted in the formation of new subspecies. In a cladogram each evolutionary step produces a branching and all the members of the branch would possess the derived character which will not be seen in organisms below the particular branch point.

Arranging organisms on the basis of their similar or derived characters which differ from the ancestral characters .produced a phylogenetic tree or cladogram.

Depending on the system of classification, organisms were classified into two or three kingdoms. Later into four, five, six and now into seven kingdoms.

R.H.Whittaker (1969) proposed the Five kingdom Classification, the Kingdoms defined by him were Monera, Protista, Fungi, Plantae, and Animalia based on the cell structure, mode of nutrition, mode of reproduction and phylogenetic relationships.

Below table gives a comparative account of different characteristics of the five kingdoms.

Classification has come a long way and now takes into an account even molecular level DNA and RNA identification. The advancement in molecular techniques and biochemical assays has led to a new classification – The “Three Domain” classification.

Three Domains of Life

Three domain classification was proposed by Carl Woese (1977) and his co- workers. They classified organisms based on the difference in 16S rRNA genes. The three domain system adds the taxon ‘domain’ higher than the kingdom.

This system emphasizes the separation of Prokaryotes into two domains, Bacteria and Arachaea, and all the eukaryotes are placed into the domain Eukarya.

Archaea appears to have more in common with the Eukarya than the Bacteria. Archaea differ from bacteria in cell wall composition and differs from bacteria and eukaryotes in membrane composition and rRNA types.

1.Domain Archaea

Th is domain includes single celled organisms, the prokaryotes which have the ability to grow in extreme conditions like volcano vents, hot springs and polar ice caps, hence are also called extremophiles.

Th ey are capable of synthesizing their food without sunlight and oxygen by utilizing hydrogen sulphide and other chemicals from the volcanic vents.

Some of the them produced methane (methanogens), few live in salty environments (Halophiles) and are thermoacidophiles which thrive in acidic environments and at high temperatures.

2.Domain Bacteria

Bacteria are prokaryotic, their cells have no definite nucleus and DNA exists as a circular chromosomes and do not have histones associated with it. They do not possess membrane bound organelles except for ribosome (70S type).

Their cell wall contains peptidoglycans. Many are decomposers, some are photo-synthesizers and few cause diseases. There are benefi cial probiotic bacteria and harmful pathogenic bacteria which are diversely populated.

Cyanobacteria are photosynthetic blue green algae which produce oxygen. Th ese had played a key role in the changes of atmospheric oxygen levels from anaerobic to aerobic during the early geologic periods.

3.Domain Eukarya (Eukaryotes)

Eukaryotes are animals which have true nucleus and membrane bound organelles. DNA in the nucleus is arranged as a linear chromosome with histone proteins, ribsosomes of 80S type in the cytosol and 70S type in the chloroplast and mitochondria.

Animals in this domain are classifi ed under kingdoms, namely, Protista, Fungi, Plantae and Animalia.

In 1987, Cavalier-Smith revised the six kingdom system to Seven Kingdom system. The concept of super kingdom was introduced and revised to seven kingdom classification.

The classification is divided into two Super Kingdoms (Prokaryota and Eukaryota) and seven kingdoms, two Prokaryotic Kingdoms (Eubacteria and Archaebacteria) and five Eukaryotic Kingdoms (Protozoa, Chromista, Fungi, Plantae and Animalia).

Taxonomic Hierarchy

In biological classification, the taxonomical hierarchy includes seven major categories namely kingdom, phylum, class, order, family, genus and species and other intermediate categories such as subkingdom, grade, division, subdivision, subphylum, superclass, subclass, superorder, suborder, superfamily, subfamily and subspecies.

Species

Species is the basic unit of classification in the taxonomic hierarchial system. It is a group of animals having similar morphological features (traits) and is reproductively isolated to produce fertile offspring.

There are some exceptional animals which can produce sterile offspring because of mating with closely related species.

Genus

It is a group of closely related species which have evolved from a common ancestor. It some genus there is only one species which is called as monotypic genus such as Red panda is the only species in the genus Ailurus: Ailurus fulgens.

It there are more than one species in the genus it is known as polytypic genus, for example ‘cats’ come under the Genus Felis, which has a number of closely related species, Felis domestica (domestic cat) ,Felis silvestris (wild cat).

Family

It is a taxonomic category which includes a group of related genera with less similarity as compared to genus and species. For example, the family Felidae includes the genus Felis (cats) and the genus Panthera (lions, tigers, leopards).

Order

This category includes an assemblage of one or more related families which show few common features. One or more similar families are grouped together to form an order. For example, family Canidae and Felidae are placed in the order Carnivora.

Class

This category includes one or more related orders with some common characters. For example order Primata comprising monkeys, apes and man is placed in the Class Mammalia, along with the order Carnivora which includes dogs and cats.

Phylum

The group of classes with similar distinctive characteristics constitute a phylum. The classes Pisces, Amphibia, Reptilia, Aves and Mammalia constitute the next higher category, phylum Chordata.

These classes share some common features like presence of a notochord and a dorsal tubular nerve cord hence included in the phylum Chordata.

Kingdom

The group of classes with similar distinctive characteristics constitute a phylum. The classes Pisces, Amphibia, Reptilia, Aves and Mammalia constitute the next higher category, phylum Chordata.

These classes share some common features like presence of a notochord and a dorsal tubular nerve cord hence included in the phylum Chordata.

Kingdom

All living animals belonging to various phyla are included in the Kingdom Animalia and it is the top most of the taxonomic hierarchy.

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