Mitochondria

Mitochondria are commonly known as power house of the cell ( because it take part in release of energy in the form of ATP by oxidative phosphorylation ). They were first observed by Kolliker in 1850. Benda (1897) gave the present name of mitochondria.
                                                                            Mitochondria are absent in prokaryotes or anaerobic eukaryotes. Their number varies from one in some algae ( Chlorela ), 25 in sperm cell, 300-400 in kidney cell, 1000 in liver cell, 50,000 in giant amoeba and 500,000 in flight muscle cell. The number depends upon cellular activity. They are different in shape - sherical, cylindrical, tubular or filamentous.

Ultrastructure:- The mitochondria contains two membranes and two chambers, outer and in inner. The two membranes form the envelop of the mitochondria.

Outer Membrane:- The membrane is smooth. It is permeable to number of metabolites. It is due to presence of protein channels called porins. A few enzymes connected with lipid synthesis are located in the membrane.
Inner Membrane:- It is permeable to some of metabolites. It is rich in double phospholipids. Protein content is also high. The inner membrane is infolded variously to form involutions called cristae. They are meant for increasing the physiological active area of inner membrane. A cristae encloses a space that is continuation of the outer chamber. The inner membrane as well as cristae posses small tennis-racket like particles called elementary particles, F0 - F1 particles or oxysomes. Each elementary particle or oxysome has a head, a stalk and a base. Elemetary particles or oxysomes contain ATPase enzyme and are connected with ATP formation. They are, therfore, the centres of ATP synthesis.

               

                   
Outer Chamber:- The chamber is the space that lies between the outer and inner membrane of the mitochondrial envelop. It extends to into the spaces of cristae. The chamber contain a fluid havin a few enzymes.
Inner Chamber:- It forms the core of the mitochondria. The inner membrane contains a semi-fluid matrix. The matrix has proteins, ribosomes, RNA, DNA, enzymes of Kreb's cycle, amino acid synthesis and fatty acid metabolism. Mitochondrial ribosomes are 55s to 70s in nature. They thus resemble to the ribosomes of prokaryotes. Because of presence of its own DNA this organelle is semi-autonomous.

Active and inactive state:- Mitochondria occur in two states, active and inactive. In active state, mitochondria are actively engaged in performing Kreb's cycle, electron transport chain and oxidative phosphorylation. In this state core is reduced, cristae randomly distributed and outer chamber space quite large. Active state is also called condensed state. In inactive state, Atp synthesis is reduced. Matrix or core is enlarged while outer chamber is narrow. It is also called orthodox state.

Functions of Mitochondria:-

i) Mitochondria are miniature factories where respiratory substrate is completely oxidised to carbon dioxide and water.The energy is liberated in the form of ATP. ATP comes put of mitochondria and helps perform verious energy requiring processes of the cell.Because of formation of ATP the mitochondria is called power houses of the cell. 

ii) Mitochondria provide important intermediates for the synthesis of biochemicals like chlorophyll, cytochromes, steroids etc.

iii) The inner chamber of the mitochondria has enzymes for the synthesis of fatty acids.

iv) Synthesis of many amino acids occur in the mitochondria.

v) Mitochondria may store and releases calcium when required.

Cell Wall

The cell wall is secreted by the protoplasm. It was first observed by Robert Hooke in 1665 in a thin slice of cork.
              The cell wall is chiefly composed of insoluble polysaccaride, cellulose, certain other compound such as hemicellulose, pectin, lignin, suberin, xylan and silicates of calcium and magnesium also occur in the cell wall.

Structure:- the cell wall is composed of four layers. a) middle lamella b) primary wall c) secondary wall and d) tertiary wall. 
a) Middle lamella:- It is the first formed wall layer, formed at the time of cytokinesis ( division of cytoplasm is called cytokinesis ). It is made up of Ca and Mg pectate. It is common to all the cells, popularly known as the cementing layer.The softening of ripe fruits is caused by partial solubilisation of pectic compounds to produce jelly like consistency.
b) Primary wall:- It is laid down along the middle lamella. It is made up of cellulose and pectin and is about 1-3 micro meter in thickness. Meristematic ( those cells who can divide are called meristematic cells in plants ) and parenchyma cells have primary cell wall only.
c) Secondary wall:- It is thick and rigid and laid down next to the primary wall. It again consists of three sub-layers called S1, S2 and S3. It is made up of cellulose, lignin or suberin. It develops in those cells which cease to grow e.g. in tracheary elements and sclerenchyma.
d) Tertiary wall:- internal to secondary wall is another wall material. It is chiefly composed of xylans. this is found in the tracheids of gymnosperms.

Functions of cell wall:- i) It maintains the shape of the cell.
ii) It protects the cell from mechanical injury.
iii) It allows the material to pass in and out of the cell.
iv) Many enzymatic activities are known to occur in the cell wall.
v) Cutin and suberin check loss of water by transpiration.
vi) It provides mechanical support aganist gravity. It is due to the rigid cell wall that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.
vii) It counteracts osmotic pressure.

Diffusion

The movement of molecules, atoms, ions of gases, liquid or solids from region of higher concentration to region of lower concentration until the molecules get uniformly distributed irrespective of the law of gravitation.

Diffusion Pressure:- It can be defined as the potential ability of a molecule or ions ( solid, liquid or gas ) to diffuse from the area of greater concentration to an area of lesser concentration. The diffusion pressure is directly proportional to the concentration or number of diffusing particles.

Factors affecting diffusion:- i) Rate of diffusion is directly proportional to the temperature.
ii) The rate of diffusion is inversely proportional to sequare root of the density of the diffusing substance.  
iii) The rate of diffusion is directly proportional to the surface area from which diffusion occurs.
iv) Rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends.

Importance:- i) The exchange of gases e.g CO2 intake and O2 output in photosynthesis and CO2 output or O2 intake in respiration takes place by the principle of independent diffusion.

ii) The process of diffusion involved in the transpiration of water vapours.

iii) The ions are absorbed by the simple diffusion during passive salt intake.

iv) Diffusion is an effective means of transport of substances over a very short distances.

Imbibition

Imbibition is the phenomena of adsorption of water or any other liquid by the solid particles of a substance without forming a solution. The solid particles which adsorb water are called imbibants. The imbibants are rich in hydrophilic colloids and have strong affinity with water.
                                                                                     Imbibition results in as increase in volume, liberation of heat and development of pressure called imbibition pressure.

Importance:- i) Absorption of water by young cells is mostly through imbibition.
ii) Imbibition plays important role in germination of seeds.
iii) Imbibition causes the swelling of seeds results in the breaking of seed coat.
iv) Seedling is able to come out of soil due to development of imbibition pressure.
v) Jamming of wooden frames during rains is caused by swelling of wood due to imbibition.
vi) In older times, it was used in breaking the rocks.

Factors affecting imbibition:-
i)  The rate of imbibition increases with rise in temperature.
ii) The rate of imbibition decreases with increasing the concentration of solutes in medium.
iii) Imbibition also influenced by texture like more looseness more imbibition and compactness less imbibition.

Imbibition pressure:- The imbibant, after imbibition of water or liquid, exerts the pressure which is called imbibition pressure. The germinating seeds have imbibition pressure of about 1000 aiomspheres.

Plant-Water Relations

Water is one of the most important constituent of the protoplasm and body fluids. It forms 50 - 90% of the total body weight of living beings and is the universal solvent.
                                                                            Gases, minerals and other solutes enter the plants and move from cell to cell dissolved in water. It provides turgidity to growing cells which is essential for plants to maintain their form and structure. It helps in regulating the body temperature.

Osmoregulation in plants:- Osmoregulation is the homeostatic process in which organisms maintain their internal conditions constant or in which organisms maintain the concentration of their body fluids and temperature at a steady state. In plants this is maintained by absorption, transport and retention of water in their bodies. Transpiration ( loss of water in the form of water vapour from the exposed surface of the plant ) results in the loss of water by plants but this loss is made up by absorption of water.
                The forces like imbibition, diffusion and osmosis are responsible for exchange of material between cells and environment and between cells.

Membrane Transport

Passage of substances across biomembranes occur by three methods -
1) Passive transport
2) Active transport
3) Bulk transport

1) Passive transport:- It is the mode of membrane transport in which cell does not spend any energy. it is of two types:-
a) Passive diffusion by tunnel protein molecules:- The tunnel protein molecules have the channels. these channels permit water and water soluble substances to pass through them. The channels only permit the molecules to pas from higher concentration to lower concentration gradient.
b) passive transport by carrier proteins:- The proteins are known as permeases. They transport the material down the concentration gradient. A carrier protein combines with a specific substances to be transported and moves it from one side of the membrane to another through a channel in it.
                                                             Facilitated diffusion moves glucose molecules in liver and it occurs without expenditure of energy.

2) Active transport:- Active transport moves substances against the concentration by utilizing energy which is privided by ATP ( Adenosine triphosphate ). The carries protein has a binding site for ATP in addition to the binding site for the substrate. ATP molecules binds to the carrier protein. The energy set free brings the substrate binding site of the protein to the surface of the membrane. The substrate present in the medium joins the binding site of protein, forming carrier-substrate complex. It undergoes conformational changes and carries the substrate through a channel in it to the cytoplasmic site of the membrane. Here the substrate is released. This type of transport by using energy is supported by various evidences:- i) Absorption is reduced with the decrease in oxygen content of the surrounding environment ( cells are using oxygen in the respiration involved in the liberation of energy-ATP).
ii) Metabolic inhibitors like cyanides inhibit absorption.
iii) Absorption of different substances is selective.
iv) Decrease in temperature decreases absorption. ( again respiration process is regulated by temperature)
v) Active transport is more rapid then diffusion.

3) Bulk transport:- It occurs by two methods:- pinocytosis and phagocytosis. They involve the enclosure of the material under transport in the vesicles of the membrane. The inward transport by means of carrier vesicles is called endocytosis. The outward transport of substance by means of carrier vesicles is known as exocytosis.
Pinocytosis:- It is the bulk trasport of fluid matter and substances dissolved in it. pinocytosis is also called cell drinking.
Phagocytosis:- It is also called cell eating. Phagocytosis is the transport of solid matter like food, foreign particles, pathogens etc. across the membrane by forming vesicles. These vesicles are called phagosomes. phagosomes fuses with lysosomes to produce a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole with indigestible substance is called residual vacuole. The undigested part are usually thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood cells is an important defence mechanism of the animal body.

Function of Cell Membrane

These are the important functions of cell membrane.
1) The cell membrane cause compartmentalisation. As plasma membranes they separates the cell from external environment.
2) Plasma membrane protect the cell from injury.
3) The plasma membrane allow the flow of materials and information between one cell and another, the cell membrane makes metabolism possible.
4) Plasma membranes of the adjacent cells form various types of junctions for keeping the cells together.
5) plasma membranes as well as other membranes of the organelles have selective permeability, they allow only selected substances to pass inwardly. The membranes are impermeable to others.
6) Membranes have carrier proteins for active transport.
It helps certain cells in movement by forming psudopodia as in Amoeba and leucocytes.
7) Substances attached to cell membrane determine antigen specificity.
8) In nerve cells the cell membrane takes part in transmission of impulses.
9) Membrane infolds are used for bulk intake of materials by endocytosis.
10) Cell membranes contain enzymes for performing certain reactions on their surfaces. e.g. ATPase
11) As microvilli the membrane becomes specialized for the absorption of substances.

Different types of membranes:-
a) Permeable membrane :- These membranes allow all types of solute, solvent and ions through them e.g. cellulose wall of cells.
b) Impermeable membrane :- These types of membranes does not allow the diffusion of both solute and solvent molecules. e.g. heavily cutinised cell walls in plants.
c) Semipermeable membranes :- These types of membranes allow diffusion of solvent molecules but do not allow the passage of solute particles. e.g. parchment paper.
d) Differentially permeable membranes :- These are also called selectively permeable membranes. These type of mmbranes allow only selected solute molecules and slovent molecules to pass through them. e.g. plasma membrane.