Muscle contraction and nervous system

muscle fibre consists of thin and thick filament.Thin filament is made up of actin filament with troponin attached to it.The thick filament consists of the myosin filament .Myosin filament has a head and a tail.The myosin head contain ATPase enzyme.

For a muscular contraction to be carried out,the nervous system transmits nerve impulse to the neuromuscular junction.This causes the presynaptic membrane to release acetycholine to the synaptic cleft.This neurotransmitters enter the postsynaptic membrane and causes depolarisation of post synaptic membrane.Action potential is generated and nerve impulse is transmited to the sarcolemma and to the sarcoplasmic reticulum.This cause sarcoplasmic reticulum to release calcium ions to the sarcoplasm.The calcium ion binds to the troponin and causes the binding site of the actin filament to be exposed.calcium ion also binds to the myosin head and causes the ATP on the head to undergo hydrolysis to form ADP and P which are still attached to the myosin head.The myosin head is energised and it bind to the binding site of the actin filament to form actomyosin cross-bridge.The myosin head pulls the actin filament slide over the thick filament .Another ATP attaches to the myosin head and causes comformational change of the myosin head.The head detaches from the actin filament and is hydrolysed again.The attaching and detaching processes cause muscle to contract.After muscle contraction, the calcium ions detach from the myosin head and troponin.They are collected back into the sarcoplasmic reticulum.Acetylcholinesterase hydrolyses the acetylcholine.The neuromuscular junction returns to its resting state.the muscle is relaxed.

Phloem

There are 4 types of phloem tissues as well!!

~Sieve tube element

~Companion cells

~Phloem fibres

~Phloem parenchyma'

The sieve tubes and the companion cells are directly involved in the translocation of the organic substances. sieve tubes ,phloem parenchymam and companion cells are living tissues.Phloem fibre is dead tissue.

Sieve tube element:they are joined end to end to form a long tube.Their end walls are perforated with sieve pores to allow cytoplasmic connection between sieve elements and for flow of liquid.They are living cells with thin cellulose cell wall (not lignified) and protoplasm.It has few mitochondria to allow smooth flow of food.However,it has no nucleus ,ribosomes and golgi apparatus that have degenerate.Its metabolic need is supplied by the companion cells.

Companion cell: they can only be found in angiosperms.They are closely related to the sieve elements.Theyb have a nucleus , small vacuoles, cytoplasm, ribosomes and many mitochondrias.

They move sugars and amino acids through plasmodesmata to the sieve elements.

Phloem fibre : They are meant only for providing mechanical support.They are the only dead phloem tissues.

Phloem Parenchyma : They are packaging tissues.They are meant only for storage of organic foods.

Xylem

There are 4 types of xylem tissues:

~tracheid

~Xylem vessel

~xylem fibre

~xylem parenchyma



The main function of xylem tissues is for the transport of water and mineral and for support.



TRANSPORT & SUPPORT:

Tracheid: The tracheids are elongated, dead cells, with tapering ends.It is strengthened with the lignified secondary cell wall. The tapering ends overlap and interlock with each other ,increasing its strength.They have pits to connect to lumens of other cells.This allows water to move from one tracheid to another and to the surrounding cells.tracheids are the main water conducting ducts in ferns and conifer.Tracheids:

Xylem vessel :They are only found in angiosperms.They are cylindrical dead cells in which most water travels.Thus,it has no cytoplasmic content and no protoplasm.They have a bigger diameter than tracheids.The end walls of then vessels are perforated with large pores or completely disintegrated to allow continuous flow of water.The deposition of lignin in the secondary wall is not always uniform. As a result, the xylem vessels exhibit different types of secondary thickenings. On this basis, xylem vessels can be distinguished into five types.
~Annular
~spiral
~scalariform
~reticulate
~pitted

SUPPORT ONLY:
Xylem fibres : they are dead cells with lignified cell walls.they support the plant tissues.

PACKING TISSUES :
Xylem parenchyma : They are the only living xylem tissue. they are used to store food.

The Leaf Structure

Leaf adaptation to photosynthesis:

Photosynthesis is a process whereby green plants uses light energy to convert CO2 and H2O into

various organic compounds such as glucose,fructose and protein.

The leaf functions as the photosynthetic organ.This is because photosynthesis usually occurs in mesophyll cells of leaf.Mesophyll cells can be categorised into spongy mesophyll and palisade mesophyll cells.Just beneath the upper epidermis is the layer of closely packed elongated palisade mesophyll cells.The epidermis layer is a protective layer that is transparent to allow sunlight to penetrate through it.It also secretes a waxy material called cuticle to the leaf surface to prevent excessive loss of water.Water is one of the raw materials of photosynthesis.It is supplied to the leaf by the xylem vessels which transport water as well as mineral salt from the root to other parts of the plant like the leaf.The palisade mesophyll cells have a lots of chloroplasts which are double-membrane organelle which contains green pigment,chlorophyll.Chlorophyll molecules trap and absorb sunlight for phtochemical reactions.There are also accessory pigments like carotenoids in the chloroplast to absorb light of different wavelengths. Ironically,irregularly-shaped spongy mesophyll cells are loosely packed with air spaces.This allows gaseous exchange to be carried out efficiently in the mesophyll. This is important so that by-product of photosynthesis, O2 can passed out through the stomata.Stoma is a pore surrounded by two guard cells to enable transpiration and gaseous exchange.Guard cells regulate the opening of and closure of stomata. CO2 enters the leaf through stomata.The large surface area of the leaf allows maximum exposure to the sunlight and for gas exchange.The synthesised food is transported to other parts of the plant for utilised or storage by phloem sieve tubes.this process is known as translocation.

a good website

http://photoscience.la.asu.edu/photosyn/education/learn.html
http://www.tutorvista.com/content/biology/biology-iii/plant-histology/phloem.php