CHAPTER VIII- The Cell- Review - Agri-team - Homeagriteam.weebly.com/.../14656198/chapter_viii-_the_cell-_review_1.pdf · complexes of subcellular compartments with specialized metabolic

4/6/2008 DMA: Chapter 8 Hartmann's Plant Science, 4th edition

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THE CELL

• The plant cell is the basic structural and physiological unit of plants, in which most reactions characteristic of life occur.

• Cytology is the branch of biology involved in the study of the components of cells and their functions.

• Cells vary greatly in size. The smallest must be measured in micrometers (1/1,000 of a millimeter), but some wood fiber cells are several centimeters long.

• Early cytological studies were conducted with light microscopes, which can demonstrate general cellular features but which cannot resolve all the fine details within cells.


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• Electron microscopes and enhanced light microscopes have revealed that living cells are not empty chambers but highly organized complexes of subcellular compartments with specialized metabolic functions.

• In the living cell, these complexes are distributed through a dynamic and orderly flow of materials within the cytoplasm.


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CELL STRUCTURE

There are two types of cells: • Prokaryotic cells have no separate subcellular units; for

example, nuclear material is not enclosed in a membrane.

• (These cells, considered primitive, are found in bacteria). • Eukaryotic cells are made up of compartments

bounded by membranes, with specialized structures and functions.

• These units, called organelles, include the nucleus, mitochondria, plastids, microbodies, vacuoles, dictyosomes, and endoplasmic reticulum (Fig. 8-5).

• Plant cells are eukaryotic cells.


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The Protoplast

• The organelles of the plant cell are contained within a membrane-bounded protoplast, which in turn is encased within a cell wall.

• The plasma membrane, also called the plasmalemma, is a lipid bilayer surrounding the cytoplasm.


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Plasma membrane is important in:1. Maintaining a surface area for selective

absorption and secretion by the cell.2. Plays a role in generating energy as well.

• Proteins embedded within the bilayer can function as:

1. Enzymes. 2. Surface receptors, both on the inside surface

and in communication with the cell environment. 3. Channels for the uptake and efflux of ions.


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Cytoplasm• The cytoplasm is a viscous fluid composed of

matrix proteins, bounded by the semi-permeable plasma membrane.

• The flow of organelles within the cytoplasmicmatrix, called cytoplasmic streaming.

• Also within the cytoplasm is a very important network of membranes, the endoplasmic reticulum (ER).


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• Proteins are synthesized on the surfaces of the ER throughout the cell, on small discrete structures called ribosomes.

• Plastids of several types are located within the cytoplasm.

• Leucoplasts (colorless) serve as storage bodies for oil, starch, and proteins.

• Chromoplasts contain the various plant pigments, including chlorophyll.

• Chromoplasts with chlorophyll are called chloroplastsand are responsible for photosynthesis in leaves and in some stems.


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• Mitochondria are cytoplasmic bodies that are smaller than plastids.

• The mitochondria are sites of respirationand are also involved in protein synthesis. They produce energy-rich compounds such as adenosine triphosphate (ATP).


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The Nucleus• The nucleus is a prominent organelle within the cell,

enclosed by a double membrane and containing one or more bodies called nucleoli.

• Within the nucleus are the chromosomes, long lengths of deoxyribonucleic acid (DNA) and associated proteins that contain the genetic information.

• DNA is also found outside the nucleus in the mitochondria and in the chloroplasts, thereby giving these bodies a role in heredity independent of the nucleus.

• Unlike nuclear DNA, mitochondrial DNA and chloroplastic DNA are inherited only from the female parent. (There is no sexual segregation of genetic traits).


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Vacuoles• Vacuoles may occupy a major portion of the interior of

plant cells: 1. In actively dividing cells, vacuoles are very small, 2. In but mature cells, vacuoles account for up to 90% of

the cell volume).

• The membrane surrounding the vacuole is called the tonoplast, and it serves an important role in:

1. Regulating ion flow within the cell, 2. Maintaining cell turgor, and 3. Other functions.

• The vacuole serves as a storage reserve for water and salts, as well as for toxic products.


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The Cell Wall• The cell wall functions:1. Protects the protoplast, 2. Provides an external structure, 3. In some tissues (e.g., bark, wood) may act as a strong

support for the plant.

• The cell wall is nonliving, made up of 1. Cellulose, 2. Pectic substances, and 3. Lignins.

• Between cells lies an intercellular layer called the middle lamella, which contains many of the mucilaginous pecticcompounds that hold adjacent cell walls together.


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• Adjacent to the middle lamella is the primary wall, which is composed mostly of cellulose.

• The secondary wall layer, which lies within the primary wall and is laid down only after the primary wall is complete, is usually thicker than the primary wall when fully developed.

• Individual cells in a tissue are connected to one another via strands of cytoplasmic material, called plasmodesmata, which extend through the plasma membrane.

• Water and dissolved materials can move from cell to cell through these connections.

Primary and Secondary wall

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CHAPTER VIII- The Cell- Review - Agri-team - Homeagriteam.weebly.com/.../14656198/chapter_viii-_the_cell-_review_1.pdf · complexes of subcellular compartments with specialized metabolic