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2021-08-17 09:38:24 (ii) Exchange of gases:

(a) Exchange of gases in lungs: It is also called external respiration. In this gaseous exchange oxygen passes from alveoli to pulmonary capillary blood and CO2. Comes to alveoli from pulmonary capillary.

(2) Release of CO2 by the blood: The PCO2 (partial pressure of carbon dioxide) of blood reaching the alveolar capillaries is higher than the PCO2 of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air.

(b) Exchange of gases in tissues: In the tissues, exchange of gases occurs between the blood and the tissue cells. This exchange occurs via tissue fluid that bathes the tissue cells. The blood reaching the tissue capillaries has PO2 higher than that in the tissue cells and PCO2 lower than that in the tissue cells.

(iii) Transport of gases: Blood carries O2 from respiratory organs to the tissue cells for oxidation and CO2 from tissue cells to respiratory organs for elimination. Blood should be slightly alkaline to help the transport of O2 and CO2 properly. Open / Comment

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2021-08-17 09:13:04 Notes on Anatomy of Flowering Plants

A tissue may be defined as, “a group of similar or dissimilar cells having common origin and performing a specific functions.”

Tissues are mainly divided into three categories:

(A) Meristematic tissues or Meristems

(B) Permanent tissue

(C) Secretory tissue

Meristematic Tissues or Meristems
(1) They contain immature and young cells and are capable of repeated divisions.

(2) Intercellular spaces are not present in meristematic tissue.

(3) They contain a homogeneous thin wall.

(4) They contain large nuclei associated with abundant cytoplasm.

(5) They are metabolically very active but they do not store food material.

(6) Only proto-plastids are present instead of plastids, chloroplast absent.

(7) Dense cytoplasm is present which contains several premature mitochondria.

(8) Vacuoles are absent.

(9) Meristematic cells are isodiametric in shape.

Types of meristems
The meristems may be classified on the basis of their mode of origin, position or function:

(i) According to origin and development: On the basis of origin, meristematic tissues are of three types :

(a) Promeristem or Primordial meristem: The promeristem originates from embryo and, therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated.

(b) Primary meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia.

(c) Secondary Meristem: They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature.

(ii) According to position: On the basis of their position in the plant body meristems are classified into three categories:

(a) Apical meristem: This meristem is located at the growing apices of main and lateral shoots and roots. These cells are responsible for linear growth of an organ.

(b) Intercalary meristem: These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis-Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyts like Equisetum) or at the base of the leaf (e.g., Pinus).

(c) Lateral meristem: These meristems occur laterally in the axis, parallel to the sides of stems and roots. This meristem consists of initials which divide mainly in one plane (periclinal) and result increase in the diameter of an organ.

(iii) According to function: Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :

(a) Protoderm: It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.

(b) Procambium: It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue constitute the procambium.

(c) Ground meristem: It constitutes the major part of the apical meristem develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.

(iv) According to plane of cell division: On the basis of their plane of cell division meristem are classified into three categories :

(a) Mass meristem: The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.

(b) Plate meristem: The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.

(c) Rib or File meristem: The cells divide anticlinally in one plane, so row or column of cells is formed. e.g,, formation of lateral root. Open / Comment

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2021-08-16 18:06:24 Menstrual Cycle

(1) Menstruation occurs in human, apes and old world monkeys.
(2) Menstruation is bleeding from the uterus of adult females at intervals of one lunar month.
(3) Beginning of menstruation or first menstruation is called menarche.
(4) The beginning of menstruation varies. It usually occurs between 12 and 15 years.
(5) The cycle of events starting from one menstruation till the next one is called Menstrual Cycle.
(6) In human females, menstruation is repeated at an average interval of about 28/29 days.
(7) One ovum is released (ovulation) during the middle of each menstrual cycle.
(8) It is regulated by certain hormones, some of which are secreted by the pituitary gland.
(9) The pituitary gland is stimulated by releasing factors produced in the hypothalamus.
(10) The hormones produced by the pituitary gland influence the ovaries. The hormones secreted by the ovaries affect the walls of the uterus.

Phases of Menstrual Cycle
The menstrual cycle consists of following four phases:

(1) Menstrual Phase:
(i) In a 28 days menstrual cycle,the menses takes place on cycle days 3-5.
(ii) The production of LH from the anterior lobe of the pituitary gland is reduced.
(iii) The withdrawal of this hormone causes degeneration of the corpus luteum and, therefore progestrone production is reduced.
(iv) Production of oestrogen is also reduced in this phase.
(v) The endometrium of uterus breaks down & menstruation begins.
(vi) The cells of endometrium secretions, blood & unfertilised ovum constitutes the menstrual flow.

(2) Follicular Phase:
(i) This phase usually includes cycle days 6-13 or 14 in a 28 days cycle.
(ii) The follicle stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete oestrogens.
(iii) Oestrogen stimulates the proliferation of the endometrium of the uterine wall.
(iv) The endometrium becomes thicker by rapid cell multiplication and this is accompanied by an increase in uterine glands & blood vessels.

Various Events during a Menstrual Cycle

(3) Ovulatory Phase:
(i) Both LH & FSH attain a peak level in the middle of cycle (about 14th day).
(ii) Oestrogen concentration in blood increases.
(iii) Rapid secretion of LH induces rupturing of graffian follicle and thereby the release of ovum.
(iv) In fact LH causes ovulation.

(4) Luteal Phase:
(i) Includes cycle days 15 to 28.
(ii) Corpus luteum secretes progestrone.
(iii) Endometrium thickens.
(iv) Uterine glands become secretory.

Hormonal Control of MC
(i) FSH stimulates the ovarian follicles to produce oestrogens.
(ii) LH stimulates corpus luteum to secrete progestrone.
(iii) Menstrual phase is caused by the increased production of oestrogens.
(iv) LH causes ovulation
(v) Proliferative phase is caused by the increased production of oestrogens.
(vi) Secretory phase is caused by increased production of progestrone. Open / Comment

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