Plant Cell

Plant cell Further information: Plant cell Plant cells are quite different from the cells of the other eukaryotic organisms. Their distinctive features are: • •

•

• •

A large central vacuole (enclosed by a membrane, the tonoplast), which maintains the cell's turgor and controls movement of molecules between the cytosol and sap A primary cell wall containing cellulose, hemicellulose and pectin, deposited by the protoplast on the outside of the cell membrane; this contrasts with the cell walls of fungi, which contain chitin, and the cell envelopes of prokaryotes, in which peptidoglycans are the main structural molecules The plasmodesmata, linking pores in the cell wall that allow each plant cell to communicate with other adjacent cells; this is different from the functionally analogous system of gap junctions between animal cells. Plastids, especially chloroplasts that contain chlorophyll, the pigment that gives plants their green color and allows them to perform photosynthesis Higher plants, including conifers and flowering plants (Angiospermae) lack the flagellae and centrioles that are present in animal cells.

Eukaryotes Basic structure The basic eukaryotic cell contains the following: 1. 2. 3. 4.

plasma membrane glycocalyx (components external to the plasma membrane) cytoplasm (semifluid) cytoskeleton - microfilaments and microtubules that suspend organelles, give shape, and allow motion

5. presence of characteristic membrane enclosed subcellular organelles Characteristic biomembranes and organelles

Plasma Membrane A lipid/protein/carbohydrate complex, providing a barrier and containing transport and signaling systems.

Nucleus Double membrane surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm. The nucleolus is a site for synthesis of RNA making up the ribosome.

Mitochondria Surrounded by a double membrane with a series of folds called cristae. Functions in energy production through metabolism. Contains its own DNA, and is believed to have originated as a captured bacterium. Chloroplasts (plastids) Surrounded by a double membrane, containing stacked thylakoid membranes. Responsible for photosynthesis, the trapping of light energy for the synthesis of sugars. Contains DNA, and like mitochondria is believed to have originated as a captured bacterium.

Rough endoplasmic reticulum (RER) A network of interconnected membranes forming channels within the cell. Covered with ribosomes (causing the "rough" appearance) which are in the process of synthesizing proteins for secretion or localization in membranes. Ribosomes Protein and RNA complex responsible for protein synthesis. Smooth endoplasmic reticulum (SER) A network of interconnected membranes forming channels within the cell. A site for synthesis and metabolism of lipids. Also contains enzymes for detoxifying chemicals including drugs and pesticides.

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Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. RNA and DNA differ in three main ways. First, unlike DNA which is double-stranded, RNA is a single-stranded molecule in most of its biological roles and has a much shorter chain of nucleotides. Second, while DNA contains deoxyribose, RNA contains ribose, (there is no hydroxyl group attached to the pentose ring in the 2' position in DNA). These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. Third, the complementary base to adenine is not thymine, as it is in DNA, but rather uracil, which is an unmethylated form of thymine.[14]

The 50S ribosomal subunit. RNA is in orange, protein in blue. The active site is in the middle (red). Like DNA, most biologically active RNAs including tRNA, rRNA, snRNAs and other, non-coding, RNAs are extensively base paired to form double stranded helices. Structural analysis of these RNAs have revealed that they are highly structured. Unlike DNA, this

structure is not long double-stranded helices but rather collections of short helices packed together into structures akin to proteins. In this fashion, RNAs can achieve chemical catalysis, like enzymes.[15] For instance, determination of the structure of the ribosome— an enzyme that catalyzes peptide bond formation—revealed that its active site is composed entirely of RNA.[16]

Monosakarida ialah gula ringkas dan merupakan unit yang paling kecil bagi sesuatu karbohidrat. Monosakarida terbahagi kepada kumpulan aldosa dan ketosa.

[sunting] Pengelasan mengikut bilangan atom karbon Monosakarida boleh dikelaskan kepada triosa, tetrosa, pentosa, heksosa, dan heptosa. Pengelasan yang sama berlaku kepada aldosa (aldo~) dan ketosa (keto~). Diambil daripada "http://ms.wikipedia.org/wiki/Monosakarida" Polisakarida (karbohidrat) termasuk dalam kelas makromolekul. Tidak seperti protein dan asid nukleik, polisakarida tidak memainkan peranan secara langsung di dalam sel. Kebiasaanya polisakarida mengandungi satu jenis unit asas yang diulang beberapa kali untuk membentuk rantaian yang panjang. Rantaian ini akan membentuk polisakarida yang berbeza berdasarkan jenis unit asasnya dan jenis ikatan yang terbentuk. Unit asas di dalam polisakarida ialah monosakarida dimana monosakarida yang terbanyak didapati di dunia ini ialah di dalam bentuk aldoheksa-D-glukosa ataupun secara ringkasnya dikenali sebagai glukosa. Disakarida ialah sejenis karbohidrat yang mengandungi dua unit monosakarida dan dihubungkan oleh ikatan O-glikosida. Disakarida boleh dikelaskan kepada monodisakarida dan heterodisakarida. Disakarida yang lazim dalam diet harian termasuklah maltosa, laktosa dan sukrosa. Contoh lain disakarida ialah trehalosa dan palatinosa. Disakarida dihidrolisiskan oleh enzim disakaridase. Monosakarida dikenali sebagai gula ringkas dengan formula kimia

C6H12O6.

Semua monosakarida mempunyai kuasa penurunan. Apabila dipanaskan bersama larutan Benedict atau larutan Fehling, sebatian kuprum (II) sulfat yang berwarna biru akan diturunkan kepada kuprum(I) oksida yang berupa mendakan merah bata. Gula yang mempunyai kuasa penerunan juga dikenali sebagai gula penurun. Contoh monosakarida adalah glukosa, fruktosa, dan galaktosa. Polimer ialah rangkaian atom yang panjang dan berulang-ulang dan dihasilkan daripada sambungan beberapa molekul lain yang dinamakan monomer. Monomer-monomer ini mungkin serupa, atau mungkin juga mempunyai satu atau lebih kumpulan kimia yang diganti. Perbezaan-perbezaan ini boleh mempengaruhi sifat-sifat polimer seperti keterlarutan, kebolehan untuk dilenturkan atau kekuatan. Dalam protein, perbezaanperbezaan ini membolehkan polimer menjadi suatu struktur tertentu, bukannya menjadi lingkaran rawak. Sungguhpun kebanyakan polimer ialah polimer organik, terdapat juga polimer inorganik, yang juga dikenali sebagai polimer sintetik Properties of Polymers: The physical properties of a polymer, such as its strength and flexibility depend on:

1. 2.

Chain length - in general, the longer the chains the stronger the polymer;

3.

Branching - straight, unbranched chains can pack together more closely than highly branched chains, giving polymers that are more crystalline and therefore stronger;

4.

Cross-linking - if polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to melt.

Side groups - polar side groups give stronger attraction between polymer chains, making the polymer stronger;

Phases of the cell cycle

The cell cycle consists of four distinct phases: G1 phase, S phase, G2 phase (collectively known as interphase) and M phase. M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm divides forming distinct

cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.

[edit] M phase The relatively brief M phase consists of nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). In plants and algae, cytokinesis is accompanied by the formation of a new cell wall. The M phase has been broken down into several distinct phases, sequentially known as prophase, Prometaphase, metaphase, anaphase and telophase leading to cytokinesis.

[edit] Interphase After M phase, the daughter cells each begin interphase of a new cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division.

[edit] G1 phase The first phase within interphase, from the end of the previous M phase till the beginning of DNA synthesis is called G1 (G indicating gap or growth). During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same species.[1]

[edit] S phase The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. Rates of RNA transcription and protein synthesis are very low during this phase. An exception to this is histone production, most of which occurs during the S phase.[2][3] The duration of S phase is relatively constant among cells of the same species.[4]

[edit] G2 phase The cell then enters the G2 phase, which lasts until the cell enters mitosis. Again, significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.

[edit] G0 phase The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the selfdestruction of such a damaged cell by apoptosis. http://www.biologycorner.com/bio1/cellcycle.html

All cellular life has the following characteristics in common. •

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• • • • •

All cells have a CELL MEMBRANE that separates the CHAOS outside a cell from the high degree of organization within the cell. A cell without a cell membrane is #NOT A CELL. All cellular life CONTAINS DNA as its genetic material. All cells contain several varieties of RNA molecules and PROTEINS, most of the latter are enzymes. All cells are composed of the same BASIC CHEMICALS: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins. All cells REGULATE the flow of nutrients and wastes that enter and leave the cell. All cells REPRODUCE and are the result of reproduction. All cells require a SUPPLY OF ENERGY. All cells are HIGHLY REGULATED by ELABORATE SENSING SYSTEMS (chemical "noses") that allow them to be aware of every reaction that is occurring within them and many of the environmental conditions around them; this information is continually PROCESSED to make metabolic decisions. o For example: Many flowers close up at night and open during the day. How do they know when the sun is up? o When you were developing in your mother's uterus how did the cell that eventually became your eyes know to turn into an eye and in the right place at the correct moment in fetal development and relative to the other cells around it that will be turning into something else?