Breathing and Exchange of Gases

 

Breathing and Exchange of Gases

Table of Content

• What is Breathing and Respiration?
• Respiratory organs
• Human Respiratory System
• Mechanism of Breathing
• Respiratory volumes and capacities
• Exchange of gases
• Transport of gases
• Regulation of Respiration

What is Breathing and Respiration?

Breathing is the process of taking in oxygen and giving out carbon-dioxide. Respiration is a catabolic process of breakdown of energy rich molecules to produce energy needed for the survival of the organism.

Respiratory organs

Different organisms have different respiratory organs depending upon the habitat and level of organization.

Earthworm has moist skin that participates in respiration. This type of respiration is known as Cutaneous Respiration. Insects have tracheal tubes that are respiratory in function. Aquatic animals have gills as respiratory organs. Higher animals have lungs for respiration, including human beings.

Fig. 1. Different respiratory organs

Human Respiratory System

Humans have a pair of nostrils which leads into the nasal passage. Nasal chamber then leads to pharynx which is common passage for food as well as air. The pharynx opens through the larynx region into the trachea. Larynx is a sound box which helps in sound production. During eating larynx is covered by a flap known as Epiglottis to prevent the entry of the food into the larynx. Trachea is straight tube that divides into left and right primary bronchi. Primary bronchi further divide into secondary and tertiary bronchi and bronchioles. Each terminal bronchioles give rise thin, vascularized bag-like structure known as Alveoli.

Fig. 2. Human respiratory system

Humans have pair of lungs that are covered by a membrane known as Pleural Membrane. Between the outer and the inner pleural membrane is pleural fluid that prevents friction on the lung surface.

Respiration involves following steps:

• Pulmonary ventilation which involves taking in atmospheric air and releasing carbon-dioxide rich air.
• Diffusion of oxygen and carbon-dioxide across the alveolar membrane.
• Transport of gases by the blood.
• Diffusion of O2 and CO2 between blood and tissues.
• Utilization of oxygen by the cells for catabolic reactions and release of carbon-dioxide.

Mechanism of Breathing

Breathing involves two processes- Inspiration and Expiration.

Inspiration:

• Process of taking atmospheric air in is known as Inspiration.
• It is an active process.
• Pressure inside the lungs is less than the atmospheric pressure.
• Contraction of diaphragm increases the volume of the thoracic cavity.
• External inter-coastal muscles also contract which raises the ribs to increase the volume of thoracic cavity.

Fig. 3. Mechanism of breathing

Expiration:

• Process of giving out carbon-dioxide is known as Expiration.
• It is a passive process.
• Pressure inside the lungs is more than the atmospheric pressure.
• Relaxation of diaphragm decreases the volume of the thoracic cavity.
• External inter-coastal muscles relax which lowers the ribs to decrease the volume of the thoracic cavity.

Respiratory volumes and capacities

Tidal Volume (TV) is the volume of air that is inspired or expired during normal respiration. It is approximately 500 ml for healthy person.

Inspiratory Reserve Volume (IRV) is the additional air a person can inspire after forceful inspiration. It is about 2500 ml to 3000 ml.

Expiratory Reserve Volume (ERV) is the additional air a person can expire after forceful expiration. It is about 1000 ml to 1100 ml.

Residual Volume (RV) is the volume of air left in the lungs after forceful expiration. It is about 1100 ml to 1200 ml.

Inspiratory Capacity (IC) is the volume of air a person can inspire after normal expiration. Tidal volume and inspiratory reserve volume together forms the inspiratory capacity.

Expiratory Capacity (EC) is the volume of air a person can expire after normal inspiration. Tidal volume and expiratory reserve volume forms the expiratory capacity.

Functional Residual Capacity (FRC) includes expiratory reserve volume and residual volume.

Vital Capacity is the volume of air a person can breathe after forceful expiration.

Total lung capacity includes RV, ERV, TV and IRV.

Exchange of gases

Exchange of gases occurs in alveoli. Exchange of gases occurs by diffusion based on pressure or concentration gradient. Two important parameters that affects the rate of diffusion are – solubility of gases and thickness of membrane.

Fig. 4. Structure of alveoli

Pressure contributed by each gas in a mixture of gas is known as partial pressure. Partial pressure of oxygen and carbon-dioxide is represented by pO2 and pCO2 respectively.

The partial pressure of oxygen in alveoli is 104 mmHg whereas in blood is 40 mmHg. Similarly, the partial pressure of carbon-dioxide is 40 mmHg in alveoli and 45 mmHg in blood. This creates a concentration gradient between the blood and the alveoli. The diffusion membrane is made up of 3 layers – the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them.

Fig. 5. Exchange of gases

Transport of gases/how oxygen and carbon-dioxide transported in blood?

The transport of oxygen and carbon-dioxide occurs via blood. About 97% of transport of oxygen occurs by blood. And remaining 3% is transported by plasma. Similarly, 70% of carbon-dioxide is transported in the form of bicarbonate. Around 25% is transported via red blood cells. Around 7% is transported in dissolved state via plasma.

Transport of oxygen

Red blood cells contain iron containing red colored pigment known as Hemoglobin. Hemoglobin binds oxygen in a reversible manner to form oxy-hemoglobin. Single hemoglobin molecule can bind 4 oxygen molecules. Partial pressure of oxygen determines the binding of oxygen with hemoglobin. When percent saturation of hemoglobin is plotted against partial pressure of oxygen, a sigmoid curve is obtained. This is known as Oxygen Dissociation curve. High partial pressure of oxygen, low partial pressure of carbon-dioxide, low temperature promotes oxy-hemoglobin formation whereas low partial pressure of oxygen, high partial pressure of carbon-dioxide in tissues promotes dissociation of oxygen from hemoglobin.

So, oxygen binds to hemoglobin in lungs and gets dissociated in tissues.

Transport of carbon-dioxide

About 20-25% of carbon-dioxide is transported in the form of carbamino-hemoglobin. This occurs when partial pressure of carbon-dioxide is high is tissues. The dissociation of carbon-dioxide from hemoglobin occurs in alveoli where partial pressure of carbon-dioxide is low. Red blood cells contain a very important enzyme known as Carbonic Anhydrase. The following reactions occurs in presence of carbonic anhydrase:

Fig. 6. Transport of carbon-dioxide

Regulation of Respiration

Neural system regulates the respiration. Respiratory control is located in medulla region of the brain. Another center that controls the respiration is known as Pneumotaxic center located in pons.

Fig. 7. Regulation of respiration

Cell Cycle and Cell Division

 

Revision Notes on Cell Cycle and Cell Division

1. Introduction: It is the process by which a mature cell divides and forms two nearly equal daughter cells which resemble the parental cell in a number of characters.

2. Discovery: Prevost and Dumas (1824) first to study cell division during the cleavage of zygote of frog.

3. Nagelli (1846) was the first to propose that new cells are formed by the division of pre-existing cells.

4. Rudolf virchow (1859) proposed “omnis cellula e cellula” and “cell lineage theory”.

5. A cell divides when it has grown to a certain maximum size which disturb the karyoplasmic index (KI)/Nucleoplasmic ratio (NP)/Kernplasm connection.

6. Two processes take place during cell reproduction.

• Cell growth: (Period of synthesis and duplication of various components of cell).

• Cell division: (Mature cell divides into two cells).

7. Cell cycle: Howard and Pelc (1953) first time described it. The sequence of events which occur during cell growth and cell division are collectively called cell cycle. Cell cycle completes in two steps:

• Interphase

• M-phase/Dividing phase

(i) Interphase : It is the period between the end of one cell division to the beginning of next cell division. It is also called resting phase or not dividing phase. But, it is actually highly metabolic active phase, in which cell prepares itself for next cell division. In case of human beings it will take approx 25 hours. Interphase is completed in to three successive stages.

(a) G1 phase/Post mitotic/Pre-DNA synthetic phase/Gap Ist

(b) S-phase/Synthetic phase

(c) G2-phase/Pre mitotic/Post synthetic phase/gap-IInd

(ii) M-phase/Dividing phase/Mitotic phase

(a) Nuclear division i.e. karyokinesis occurs in 4 phases – prophase, metaphase, anaphase and telophase. It takes 5-10% (shortest phase) time of whole division.

(b) Cytokinesis : Division of cytoplasm into 2 equal parts. In animal cell, it takes place by cell furrow method and in plant cells by cell plate method.

8. Duration of cell cycle: It depends on the type of cell and external factors such as temperature, food and oxygen. Time period for G1, S, G2 and M-phase is species specific under specific environmental conditions. e.g. 20 minutes for bacterial cell, 8-10 hours for intestional epithelial cell, and onion root tip cells may take 20 hours.

9. Regulation of cell cycle: Stage of regulation of cell cycle is G1 phase during which a cell may follow one of the three options.

• It may start a new cycle, enter the S-phase and finally divide.

• It may be arrested at a specific point of G1 phase.

• It may stop division and enter G0 quiscent stage. But when conditions change, cell in G0 phase can resume the growth and reenter the G1 phase.

10. Cell division is of three types, Amitosis, Mitosis and Meiosis.

11. Difference between cell Mitosis and Meiosis

S.No	Characters	Mitosis	Meiosis
I. General
(1)	Site of occurrence	Somatic cells and during the multiplicative phase of gametogenesis in germ cells.	Reproductive germ cells of gonads.
(2)	Period of occurrence	Throughout life.	During sexual reproduction.
(3)	Nature of cells	Haploid or diploid.	Always diploid.
(4)	Number of divisions	Parental cell divides once.	Parent cell divides twice.
(5)	Number of daughter cells	Two.	Four.
(6)	Nature of daughter cells	Genetically similar to parental cell. Amount of DNA and chromosome number is same as in parental cell.	Genetically different from parental cell. Amount of DNA and chromosome number is half to that of parent cell.
II. Prophase
(7)	Duration	Shorter (of a few hours) and simple.	Prophase-I is very long (may be in days or months or years) and complex.
(8)	Subphases	Formed of 3 subphases : early-prophase, mid-prophase and late-prophase.	Prophase-I is formed of 5 subphases: leptotene, zygotene, pachytene, diplotene and diakinesis.
(9)	Bouquet stage	Absent.	Present in leptotene stage.
(10)	Synapsis	Absent.	Pairing of homologous chromosomes in zygotene stage.
(11)	Chiasma formation and crossing over.	Absent.	Occurs during pachytene stage of prophase-I.
(12)	Disappearance of nucleolus and nuclear membrane	Comparatively in earlier part.	Comparatively in later part of prophase-I.
(13)	Nature of coiling	Plectonemic.	Paranemic.
III. Metaphase
(14)	Metaphase plates	Only one equatorial plate	Two plates in metaphase-I but one plate in metaphase-II.
(15)	Position of centromeres	Lie at the equator. Arms are generally directed towards the poles.	Lie equidistant from equator and towards poles in metaphase-I while lie at the equator in metaphase-II.
(16)	Number of chromosomal fibres	Two chromosomal fibre join at centromere.	Single in metaphase-I while two in metaphase-II.
IV. Anaphase
(17)	Nature of separating chromosomes	Daughter chromosomes (chromatids with independent centromeres) separate.	Homologous chromosomes separate in anaphase-I while chromatids separate in anaphase in anaphase-II.
(18)	Splitting of centromeres and development of inter-zonal fibres	Occurs in anaphase.	No splitting of centromeres. Inter-zonal fibres are developed in metaphase-I.
V. Telophase
(19)	Occurrence	Always occurs	Telophase-I may be absent but telophase-II is always present.
VI. Cytokinesis
(20)	Occurrence	Always occurs	Cytokinesis-I may be absent but cytokinesis-II is always present.
(21)	Nature of daughter cells	2N amount of DNA than 4N amount of DNA in parental cell.	1 N amount of DNA than 4 N amount of DNA in parental cell.
(22)	Fate of daughter cells	Divide again after interphase.	Do not divide and act as gametes.
VII. Significance
(23)	Functions	Helps in growth, healing, repair and multiplication of somatic cells. Occurs in both asexually and sexually reproducing organisms.	Produces gametes which help in sexual reproduction.
(24)	Variations	Variations are not produced as it keeps quality and quantity of genes same.	Produces variations due to crossing over and chance arrangement of bivalents at metaphase-I.
(25)	In evolution	No role in evolution.	It plays an important role in speciation and evolution.

12. Types of Mitosis

• Anastral mitosis: It is found in plants in which spindle has no aster.

• Amphiastral mitosis: It is found in animals in which spindle has two asters, one at each pole of the spindle. Spindle is barrel-like.

• Intranuclear or Promitosis: In this nuclear membrane is not lost and spindle is formed inside the nuclear membrane e.g. Protozoans (Amoeba) and yeast. It is so as centriole is present within the nucleus.

• Extranuclear or Eumitosis: In this nuclear membrane is lost and spindle is formed outside nuclear membrane e.g. in plants and animals.

• Endomitosis: Chromosomes and their DNA duplicate but fail to separate which lead to polyploidy e.g. in liver of man, both diploid (2N) and polyploid cells (4N) have been reported. It is also called endoduplication and endopolyploidy.

• Dinomitosis: In which nuclear envelope persists and microtubular spindle is not formed. During movement the chromosomes are attached with nuclear membrane.

13. Types of meiosis: On the basis of time and place, meiosis is of three types

• Gametic/Terminal meiosis: In many protozoans, all animals and some lower plants; meiosis takes place before fertilization during the formation of gametes. Such meiosis is described as gametic or terminal.

• Zygotic or Initial Meiosis: In fungi, certain protozoan groups, and some algae fertilization is immediately followed by meiosis in the zygote, and the resulting adult organisms are haploid. Such a meiosis is said to be zygotic or initial. This type of life cycle with haploid adult and zygotic meiosis is termed the haplontic cycle.

• Sporogenetic Meiosis

(a) Diploid sporocytes or spore mother cells of sporophytic plant, undergo meiosis to form the haploid spores in the sporangia.

(b) Haploid spore germinates to form haploid gametophyte which produces the haploid gametes by mitosis.

(c) Haploid gametes fuse to form diploid zygote which develops into diploid sporophyte by mitotic divisions. e.g.  in higher plants like pteridophytes, gymnosperms and angiosperms.