CELL THEORY

• Schleiden and Schwann together formulated the cell theory.
• Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells.
• Cell theory states that
• All living organisms are composed of cells and products of cells.
• All cells arise from pre-existing cells.

Flight adaptations found in birds
(i) Body is streamlined to reduce air resistance during flight.
(ii) Four limbs are modified into wings.
(iii) Wings have long quill feathers to increase the efficiency of heating of wings,
(iv) Pneumatic bones are present to reduce the body weight.
(v) High metabolic rate to provide energy.
(vi) Air sacs present in lungs help in double respiration.

Flight adaptations in birds
(i) Boat-shaped body helps to propel through the air easily.
(ii) Feathery covering of body to reduce the friction of air.
(iii) Holding the twigs automatically by hindlimbs.
(iv) Extremely powerful muscles that enables the wings to work during flight.
(v) Bones are light, hollow and provide more space for muscle attachment. Presence of pneumatic bones which reduce the weight of body and help in flight.

(vi) The first four thoracic vertebrae are fused to form a furculum for walking of the wings.

(vii) Lungs are solid and elastic and have associated air sacs.

(viii) The power of accommodation of eyes is well developed due to the presence of comb-like structure pecten. (ix) A single left ovary and oviduct to reduce the body weight.

REGULATION OF RESPIRATION

• A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation.
• Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre.
• Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
• A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO₂ and hydrogen ions.
• Receptors associated with aortic arch and carotid artery also can recognise changes in CO₂ and H⁺ concentration and send necessary signals to the rhythm centre for remedial actions.

Blood transports CO₂ from the tissue cells to the lungs in three ways:

1. Dissolved in plasma : About 7 – 10% of CO₂ is transported in a dissolved form in the plasma.

2. Bound to haemoglobin : About 20 – 25% of dissolved CO is bound and carried in the RBCs as carbaminohaemoglobin (Hb CO₂ ) CO₂ + Hb → Hb CO₂ .

3. As bicarbonate ions in plasma about 70% of CO₂ is transported as bicarbonate ions. This is influenced by pC0₂ and the degree of haemoglobin oxygenation. RBCs contain a high concentration of the enzyme, carbonic anhydrase, whereas small amounts of carbonic anhydrase is present in the plasma.

→ At the tissues the pCO₂ is high due to catabolism and diffuses into the blood to form HCO₂ and H ions. When CO₂ diffuses into the RBCs, it combines with water forming carbonic acid (H₂ CO₂ ) catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions.

Carbonic anhydrase facilitates the reaction in both directions.

The HCO₃^- moves quickly from the RBCs into the plasma, where it is carried to the lungs. At the alveolar site where pCO₂ is low, the reaction is reversed leading to the formation of CO₂ and water. Thus CO₂ trapped as HCO₃^- at the tissue level is transported to the alveoli and released out as CO₂ . Every 100 mL of deoxygenated blood delivers 4 mL of CO₂ to the alveoli for elimination.

Transport of oxygen

• Haemoglobin is a red coloured iron containing pigment present in the RBCs.
• O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.
• Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂ and partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
• A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against the pO₂ and the curve is called the oxygen dissociation curve.
• pCO2, H⁺ concentration have effect on binding of O₂ with haemoglobin.
• In the alveoli, where there is high pO₂, low pCO₂, lesser H⁺ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, and where low pO₂, high pCO₂, high H⁺ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.
• Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ and each haemoglobin molecule can carry a maximum of four molecules of O₂.

1. The exchange of gases between air and blood takes place across the walls of the alveoli.
2. The human respiratory system:

• Tidal Volume (TV): Volume of air inspired or expired during a normal respiration, which is approx. 500 mL.
• Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration, which averages 2500 mL to 3000 mL.
• Expiratory Reserve Volume (ERV ): Additional volume of air, a person can expire by a forcible expiration, which averages 1000 mL to 1100 mL.
• Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration, which averages 1100 mL to 1200 mL.
• Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration, which includes tidal volume and inspiratory reserve volume ( TV+IRV).
• Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration, which includes tidal volume and expiratory reserve volume (TV+ERV).
• Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration, which includes ERV+RV.
• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration, which includes ERV, TV and IRV.
• Total Lung Capacity: Total volume of air accommodated in the lungs at the end of a forced inspiration, which includes RV, ERV, TV and IRV.

MECHANISM OF BREATHING

• Breathing involves two stages :
• Inspiration- during which atmospheric air is drawn in.
• Expiration- during which the alveolar air is released out.
• Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure.
• Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
• The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs.
• The contraction of external inter-costal muscles lifts up the ribs and the sternum causes an increase in the volume of the thoracic chamber in the dorso-ventral axis.
• An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs.
• Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume, which leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs.
• A healthy human breathes 12-16 times/minute.
• The volume of air involved in breathing movements can be estimated by using a spirometer.

Larynx – “Adam‟s apple” 

Glottis – Aperture in laryngopharynx which opens into trachea. 

Epiglottis – A cartilaginous flap present at the anterior margin of glottis. It projects into the pharynx opposite the uvula. During swallowing, larynx moves upward to close the glottis to check the entry of food into it. 

Trachea/ Windpipe – thin walled tube extends downward through the neck. 

Bronchi – Trachea divides into two tubes called bronchi in the middle of the thorax. 

Bronchioles – Bronchi divide and re-divide into tertiary bronchi which divide into alveolar ducts which enter into alveolar sacs. 

Lungs – Human respiratory organ, located in the thoracic cavity.  

Alveolar sac – In the lung, each alveolar duct opens into a blind chamber, the alveolar sac which appears like a small bunch of grapes. 

Alveoli / Air sacs – The central passage of each alveolar sac gives off several small pouches on all sides, the alveoli or air sacs. 

Alveolar wall – is very thin (0.0001 mm) wall composed of simple moist, non-ciliated, squamous epithelium which easily recoil and expand during breathing.

• Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule.
• Equal number of amino and carboxyl groups makes it neutral
• More number of amino than carboxyl groups makes it basic
• More carboxyl groups as compared to amino groups makes it acidic

• Amino acids are further classified as essential & Non-essential amino acids depending upon their synthesis
• The amino acids, which can be synthesised in the body, are known as nonessential amino acids. Examples: Alanine, Cysteine, Glutamate

• Blood is red because of the hemoglobin inside our red blood cells.
• Hemoglobin is a protein that forms a complex with iron molecules and together they transport oxygen molecules throughout the body. Iron has the property of reflecting red light and because there is so much iron in our blood, blood looks red.
• Hence Haemoglobin makes the blood look red.

• Blood is red because of the hemoglobin inside our red blood cells.
• Hemoglobin is a protein that forms a complex with iron molecules and together they transport oxygen molecules throughout the body. Iron has the property of reflecting red light and because there is so much iron in our blood, blood looks red.
• Hence Haemoglobin makes the blood look red.

Blood supports growth by distributing nutrients and hormones, and by removing waste. Red blood cells contain hemoglobin, which binds oxygen. These cells deliver oxygen to the cells and remove carbon dioxide.
Blood plays a protective role by transporting clotting factors and platelets to prevent blood loss after injury. Blood also transports the disease-fighting agents white blood cells

Blood plasma is a yellowish liquid component of blood that holds the blood cells of whole blood in suspension. It is the liquid part of the blood that carries cells and proteins throughout the body. It makes up about 55% of the body's total blood volume. Plasma constitutes 55% of total blood volume. Composed of 90% water, salts, lipids and hormones, it is especially rich in proteins (including its main protein albumin), immunoglobulins, clotting factors and fibrinogen.

Histamine is an organic compound, which is involved in local immune responses and also acts as a neurotransmitter. It is also involved in the inflammatory response and is a mediator of pruritus. Histamine is produced by basophils and is found in tissues that are connected nearby.

Viroids are free RNA molecules of low molecular weight without any protein coat while viruses can have either RNA or DNA molecules encapsulated in a protein coat. Viroids are smaller in size than the viruses. Viroids infect only plants whereas virus infects all types of organisms. In viroids, protein coat is absent whereas in viruses a protein covering or a coat called as capsid is present around the genetic material.

Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane. It is quasifluid in nature. Membranes also occur inside the cells. They are collectively called biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast. It was replaced by the term plasmalemma by Plower (1931).

Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60% of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different substances. Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and organism involved (in humans, myelin 79%). The lipids of plasma membrane are of three types namely phospholipids, glycolipids and sterols. The sterol found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms).

Carbohydrates form 2%–10%. Oligosaccharides are the main carbohydrates present in plasma membrane. The carbohydrates of plasma membrane are covalently linked to both lipid and protein components.

Ultrastructure : Under electron microscope the plasma membrane appears three layered, i.e., trilaminar or tripartite. One optically light layer is of lipid and on both sides two optically dense protein layers are present.

Molecular structure and different models : Several models have been proposed to explain the structure and function of the plasma membrane.

(1) Overton’s model : It suggests that the plasma membrane is composed of a thin lipid single layer.

(2) Sandwitch model : It was proposed by Davson and Danielli (1935). According to this model the light biomolecular lipid layer is sandwitched between two dense protein layers (globular a type protein). This model was also said to be unit membrane hypothesis.

(3) Robertson’s unit membrane model : It states that all cytoplasmic membranes have a similar structure of three layers with and electron transparent phospholipid bilayer being sandwitched between two electron dense layer of proteins (extended or 

β

  type protein).

Its thickness is about 75 Å with a central lipid layer of 

35 AA

 thick and two peripheral protein layers of 

20 AA

 thick.

(4) Fluid mosaic model : The most important and widely accepted latest model for plasma membrane was given by Singer and Nicolson in 1972. According to them it is “protein iceberg in a sea of lipids.”

According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules. Protein molecules occur as separate particles asymmetrical arranged in a mosaic pattern.

Some of these are loosely bound at the polar surfaces of lipid layers, called peripheral or extrinsic proteins. Others penetrate deeply into the lipid layer called integral or intrinsic proteins. Some of the integral proteins penetrate through the phospholipid layers and project on both the surface. These are called trans membrane or tunnel proteins (glycophorins). Singly or in groups, they function as channels for passage of water ions and other solutes.

• The human digestive system consists of the alimentary canal and the associated glands.

Alimentary canal 

• The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the ****.
• The mouth leads to the buccal cavity or oral cavity, which has a number of teeth and a muscular tongue and each tooth is embedded in a socket of jaw bone with a type of attachment called as thecodont.
• A set of temporary milk or deciduous teeth is replaced by a set of permanent or adult teeth, and the type of dentition is called diphyodont.
• An adult human has 32 permanent teeth, which are of four different types
• incisors (I)
• canine (C)
• premolars (PM)
• and molars (M).
• Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123.
• The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum, and the upper surface of the tongue has small projections called papillae, some of which bear taste buds.
• The oesophagus and the trachea open into the pharynx.
• A cartilaginous flap called epiglottis prevents the entry of food into the glottis, which is the opening of the wind pipe.
• The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
• The stomach has three major parts
• a cardiac portion into which the oesophagus opens
• a fundic region
• a pyloric portion which opens into the first part of small intestine.
• Small intestine is distinguishable into three regions
• a ‘U’ shaped duodenum
• a long coiled middle portion jejunum
• a highly coiled ileum.
• The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
• Ileum consists of
• caecum
• colon
•  
• Caecum is a small blind sac from which a vestigial organ called vermiform appendix
• The colon is divided into three parts
• an ascending
• a transverse
• a descending part.
• The descending part opens into the rectum which opens out through the ****.
• The wall of alimentary canal from oesophagus to rectum possesses four layers
• Serosa
• muscularis
• sub-mucosa
• Serosa is the outermost layer and is made up of a thin mesothelium and some connective tissues.
• Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer.
• The submucosal layer is formed of loose connective tissues.
• Mucosa is the innermost layer which forms rugae in the stomach and small finger-like foldings called villi in the small intestine.
• The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.
• Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.
• Mucosal epithelium has goblet cells which secrete mucus that help in lubrication.
• Mucosa forms gastric glands in the stomach crypts of Lieberkuhn in different parts of the alimentary canal.

• Blood coagulation or clotting is the mechanism to prevent excessive loss of blood from the body.
• Reddish brown scum formed at the site of a cut is due to clot formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.
• Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
• Thrombins are formed from another inactive substance present in the plasma called prothrombin by an enzyme complex known as thrombokinase.
• Calcium ions play a very important role in clotting.

The elemental analysis of a cell shows that it is composed of various elements such as carbon, hydrogen, oxygen, chlorine, etc.

But what do these elements constitute? What are the properties of the compounds formed by these elements?

Only a chemical analysis can answer these questions.

Analytical techniques provide information regarding different compounds (organic and inorganic), their molecular formula and structures. They also help us to isolate and purify one compound from another.

Here is a simple experiment to find the Chemical Composition Analysis in Biomolecules.

Take a piece of living tissue and crush and mix it with an acid. After its filtration, we obtain two portions. The fraction which is obtained as filtrate is acid-soluble portion while the other fraction is acid-insoluble, which retain on the filter membrane. This shows that within the tissues, there two or more compounds which have distinct properties.

Take another piece of tissue and burn it, once all the water in it is evaporated. All carbon compounds will be oxidized on burning. The left out ash gives us the inorganic compounds like calcium, magnesium, sulfate, phosphate, etc., in the tissue.

Glycolysis is the process of breakdown of glucose(6C) to produce 3C pyruvic acid and energy is glycolysis.

Fermentation is a process in which breakdown of sugar occurs by the microorganism in the absence of oxygen to produce ethanol.

ATP is produced by the oxidative decarboxylation

The respiratory pathway is an amphibolic pathway because it includes both catabolic and anabolic reaction. During respiratory reaction breaks down of organic substances like carbohydrates, fats, proteins occurs to release energy. Such a process is considered catabolic in nature

The tiny projections on the inner surface of the small intestine which help in absorbing the digested food are called villi. These helps to increase the surface area of intestinal walls.These are located in the inner walls of the small intestine.Their function is to increase the surface area of the small intestinal wall for absorption of the digested food.

Salient Features of DNA Double-Helix

• It consists of two polynucleotide chains where the sugar and phosphate group form the backbone and the nitrogenous bases project inside the helix.
• The two polynucleotide chains have anti-parallel polarity i.e. if one strand has 5′ → 3′ polarity, the other strand has 3′ → 5′ polarity.
• The bases on the opposite strands are connected through hydrogen bonds forming base pairs (bp). Adenine always forms two hydrogen bonds with thymine from the opposite strand and vice-versa. Guanine forms three hydrogen bonds with cytosine from the opposite strand and vice-versa. Therefore, a purine always pairs with a pyrimidine on the other strand, giving rise to a uniform distance between the two strands of the helix.
• The two strands coil in a right-handed fashion. Each turn of the helix is 3.4nm (or 34 Angstrom units) consisting of 10 nucleotides. These nucleotides are at a distance of 0.34nm (or 3.4 Angstrom units).
• The helix is stable because of the base pairs that stack over one another and hydrogen bonds that hold the bases together.

There are three types of root system, viz. tap root system, fibrous root system and adventitious root system.

1. Tap Root System: This type of root system is mainly present in dicotyledonous plants. Direct elongation of the radicle results in formation of primary root. The primary root bears secondary, tertiary, etc. roots. The primary root; along with its branches; makes the tap root system, e.g. mustard, banyan, etc.
2. Fibrous Root System: This type of root system is mainly present in monocotyledonous plants. The primary root is short lived after germination. This is replaced by a large number of roots and all of them emerge from the base of the stem. Such roots constitute the fibrous root system, e.g. wheat, paddy, grass, etc.
3. Adventitious Roots: Sometimes, roots arise from some other plant parts (other than radicle). Such roots are called adventitious roots. Adventitious roots are used for various purposes; like vegetative propagation, mechanical support, etc.

Proterospongia is a colonial choanoflagellate protozoan that closely resembles a sponge and act as connecting link between <i>Protozoa </i>and <i>Porifera</i>. It is a genus of single-celled aquatic organisms which form colonies. It consists of a number of cells embedded in a jelly-like matrix.