Monday, January 29, 2007 @ 8:22 PM
If the animal cell is to have a cell wall, they will provide rigidity to the cell for structural and mechanical support, maintaining cell shape, the direction of cell growth, and ultimately the architecture of the plant. The cell wall also prevents expansion when water enters the cell. As the young cell forms, the cell wall is initially rather thin and consists of a middle lamella (a sort of pectin glue that holds adjacent cells together) and a primary cell wall. The primary cell wall is flexible enough to allow the cell to grow and mature. Once mature the cell stops growing and the cell wall is strengthened, either by the deposition of secondary substances such as lignin or by the development of a secondary cell wall. This secondary wall often consists of a number of layers and affords great mechanical strength. It is this secondary wall that gives wood its useful properties. So, everyone will be as hard as wood and movement will not really be easy.
The term turgor is used to describe this pressure that is induced by excess water inside the plant cell. Cell walls protect against pathogens in the environment and can store carbohydrates for the cell. Hence, maybe the animal cell can also make by itself in the future The cell wall is constructed primarily from a carbohydrate polymer called cellulose.
Animal cells that have a cell wall will not break open oftenly in solutions whose salt concentrations are either too low (hypotonic) or too high (hypertonic). If the ionic strength of the solution is much smaller than the cell, osmotic pressure forces water into the cell to bring the system into balance, which causes the cell to burst. If the ionic strength of the solution is too high, osmotic pressure forces water out of the cell, and the cell breaks open as it shrinks. The cell wall provides the mechanical strength that can also help protect animal cells that live in fresh-water ponds (too little salt) or seawater (too much salt) from osmotic shock. The cell wall also provides the mechanical strength that allows cells to support the weight of other cells.
@ 8:17 PM
Plasma membrane
Smooth Endoplasmic Reticulum
Vacuole
Golgi apparatus
Mitochondrion
Ribosomes
Centrioles
Rough Endoplasmic Reticulum
--Parts of Nucleus--
Nuclear envelope
Nucleolus
Chromatin
Nuclear pore
- The plasma membrane, which serves as a diffusion barrier between the cell and its environment. A "diffusion barrier" prevents the loss of cellular materials by interfering with the physical tendency of molecules to spread out. It prevents substances from diffusing into the cell and also controls the movements of various substances and objects into and out of the cell.
- Endoplasmic Reticulum (ER) interconnects with the plasma membrane and the nuclear envelope. "Endo" means "inside," "plasmic" refers to the cytoplasm, and "reticulum" means, "network." It comes in two types--smooth and rough.
- Rough ER helps to compartmentalize the cell, and it serves as routes for the transport of materials from one part of the cell to another. It's associated with lipid synthesis and protein synthesis. It has ribosomes attached to its outer surface.
- Each cell contains Golgi apparatus. Their function is to process materials manufactured by the cell, then package those products into small structures called "Golgi vesicles." They contain materials, usually enzymes, which the cell needs, but which must remain packaged away from the cell's other contents.
- Lysosomes contain digestive enzymes, which, if released into the cell, would digest the vital components of the cell, and kill it. "Lysis" means "breakage," and "some" means "body."
- The other kind of Golgi vesicle contains materials to be exported from the cell. These materials are not waste products--they are chemicals intentionally manufactured by the cell for export.
- Mitochondria are very complex, double-membrane-bound organelles. Their function is to perform the aerobic portions of aerobic cellular respiration, the essential energy-producing process of the cell. Mitochondria contain their own naked, circular DNA and their own ribosomes.
- Ribosomes are to make protein, following instructions sent from the DNA's genes.
- The nucleus of the cell is a chamber specialized in DNA functions. A double layer of membrane called the nuclear envelope encloses it. The function of the nuclear envelope is to confine the materials necessary for DNA synthesis inside the nucleus, and controlling movement into and out of the nucleus.
- Assembly of ribosomes takes place in nucleolus, found in the nucleus.
- The nucleus contains a number of chromosomes, which are composed of DNA and histone proteins. Chromosomes carry the information archive of the cell. Each gene is a set of instructions for the construction of a specific protein.
- Nuclear pores perforate the double nuclear envelope. These pores do have structures in them that open and close to control movement through the pores. The nuclear pores are important for ribosomes to leave the nucleus. Though ribosomes are constructed in the nucleus, they must move to the cytoplasm in order to function.
- The nucleoplasm is substance that fills the interior of the nucleus. It is mostly composed of water, containing a complex assortment of materials. Nucleoplasm would be distinct from cytoplasm due to the high concentration of materials like nucleotides, which are used to make DNA, and the suite of enzymes, which control the DNA construction reactions.
Labels: Qn 1
Tuesday, January 16, 2007 @ 4:45 PM
Leeches are segmented worms; their bodies are divided into 34 segments, with a powerful clinging sucker at each end. The mouth is in the anterior sucker and the anus is on the dorsal surface just in front of the rear sucker.
Respiration takes place through the body wall. Sensory organs on the head and body surface enable a leech to detect changes in light intensity, temperature, and vibration. Chemical receptors on the head provide a sense of smell.
As hermaphrodites, leeches have both male and female sex organs. Like the earthworms they also have a clitellum, a region of thickened skin, which is only obvious during the reproductive period. Mating involves the intertwining of bodies where each deposits sperm in the others' clitellar area. Rhyncobdellids have no penis but produce sharp packages of sperm that are forced through the body wall.
The sperms then make their way to the ovaries where fertilisation takes place. The clitellum secretes a tough gelatinous cocoon that contains nutrients, and it is in this that the eggs are deposited.
The leech shrugs itself free of the cocoon, sealing it as it passes over the head.
The cocoon is either buried or attached to a rock, log or leaf and dries to a foamy crust. After several weeks or months, the young emerge as miniature adults. Studies show that the cocoons are capable of surviving the digestive system of a duck.
Using its many toothed jaws like minute saws, it makes an incision in the skin and excretes a mucous from the nephropores (external openings from the kidney-like organs). This helps the sucker to adhere. A salivary secretion containing the anticoagulant and histamine floods the wound and the leech relaxes its body to allow the blood to be ingested. This mixture allows the blood to flow and also prevents clotting once inside the leech. A bacterium in the gut of the leech assists the digestion of the blood and also prevents growth of other bacteria that may cause the ingested blood to putrefy.
Thursday, January 11, 2007 @ 6:41 AM
Haha!! I changed my chinese blog into a bio blog, cos the computer is a bit spoiled. Yup and I am too lazy to make another one. Hope the teacher wun come running after me....
