Overview of plant structure

The vegetative body of a plant consists of three organs: leaf, stem and root. The stem and leaves together is called the shoot. Flowers are the reproductive organs.

There are two groups of angiosperms (flowering plants): dicotyledons (dicots) and monocotyledons (monocots) based on the number of cotyledons (seed leaves) in the embryo.

Meristems:

Primary growth - occurs from apical meristems - cell division in the meristems is followed by cell enlargement and differentiation.
Secondary growth - involves cell division in two lateral meristems: the vascular cambium and the cork cambium.
At the nodes, axillary buds contain apical meristems that give rise to branches.
Lateral roots come from cell divisions in the pericycle, which is an internal meristematic tissue.

Plant tissues:
Dermal tissue:
Epidermal cells cover all plant surfaces. In the shoot, the epidermal cells are coated with a waxy cuticle. Guard cells are a specialized epidermal cells that surround pores in the surface. Guard cells and the pore are called stomata - control movement of gas and water vapor. Root hair cells are specialized cells that produce an elongated hair-like structure that increases the cells surface area to facilitate uptake of water and nutrients.
Ground tissue make up the bulk of the plant:
Parenchyma tissue consists of thin walled cells with a variety of functions including photosynthesis and storage.
Collenchyma tissue consists of narrow elongated cells with thick primary walls
Scleranchyma tissue consists of sclereids and fibers, cells that have thick secondary walls and are dead at maturity.
In the stem, the ground tissue consists of pith and cortex. The pith is inside the vascular tissue and the cortex is located between the epidermis and vascular tissue.
Roots contain a specialized layer of cells, called the endodermis, between the ground tissue and vascular tissue.
The ground tissue in leaves is called the mesophyll which consists of palisade parenchyma and spongy mesophyll cells.

Vascular tissue:
Xylem tissue conducts water and minerals from the root. The xylem consists of tracheids and vessel elements. These have elaborate secondary cell walls and are dead at maturity.
Phloem tissue distributes photosynthate and other solutes throughout the plant and consists of sieve elements (angiosperms) and sieve cells (gymnosperms).
 
Plant Cells:
Plant cells are surrounded by a cell wall. Walls are connected by the middle lamella. Primary cell walls are usually thin and are found in young growing cells. Secondary cell walls are thicker and stronger and are typically formed when cells stop enlarging. Secondary cells walls contain a material called lignin, which provides the strength.

Membrane-bound systems of the cell.

General properties of membranes.
  • Very permeable to H20.
  • Permeable to non-electrolytes in proportion to lipid solubility.
  • Relatively impermeable to electrolytes (ions) i.e., with organic acids, pH dependent.
  • High electrical resistance.
  • Low surface tension.
  • Fundamental structure is a lipid bilayer of phospholipids and glycolipids with adhering and inserted proteins.
  • Form spontaneously in water due to repulsion of hydrophobic fatty acid chains, leaving more hydrophilic polar groups at the aqueous surface.

    • Membranes function as boundary structures which regulate the passage of metabolites. Membranes isolate the "living" cell (protoplast) from the environment, and isolate the organelles from the bulk cytoplasm, permit the separation of specific biochemical and physiological reaction, and the development of biologically useful energy.

    Specific membranes and membranous organelles

    Endoplasmic reticulum (ER): A network of membrane tubules and flattened sacs (cisternae) pervading the cytoplasm and surrounding the nucleus (nuclear envelope). Enormous surface provided for biochemical reactions - 1 to 10 m2/cm3 of cytoplasm. Passes through the plasmodesmata, connecting the cells of a tissue into a "symplast" and, thus, provides a cell-to-cell transport system. The lumen of the ER, and the vesicles it produces, is topologically outside of the cytoplasm. ER, or vesicles from ER, are the sites of synthesis for secreted or stored proteins. "Rough" ER is coated with protein synthesizing polysomes (= ribosomes + mRNA), which secrete protein into the lumen. Vesicles containing these proteins can fuse with the plasmalemma, thus giving secretion.

    Single membraned bodies

  • Dictyosomes. (Golgi bodies) Stacks of fenestrated, flattened vesicles (cisternae) derived from smooth ER. At the forming face the membrane is ER-like; at the maturing face, where the polysaccharide vesicles are budded off, the membranes are plasmalemma-like and can fuse to discharge contents and form additional plasmalemma.
  • Fat bodies (oleosomes). Food reserve - common in seeds. Contain triglycerides, and are high in unsaturated fatty acids. Membrane is thought to be outer half of bilayer only.
  • Protein bodies. Food reserves. Stored proteins are high in dicarboxylic acids and their amides (up to 40% glutamate, glutamine, aspartate and asparagine). Also contain phosphate, magnesium, and calcium reserves. Can be derived from vacuoles in some seeds, and become vacuoles when protein is utilized.
  • Glyoxysomes - have enzymes for fatty acid metabolism Peroxisomes - enzymes for glycolate metabolism
  • Vacuoles. Formed as special Golgi vesicles which coalesce during growth and finally comprise >90% of mature protoplast. Solution of inorganic and organic compounds (nutrient ions, enzymes, organic acids, sugars, flavonoids, tannins, etc.). In some ways acts as a storehouse for solutes, in other ways like a cesspool. Analogous to the digestive lysosomes of animal cells; contains lytic enzymes -- nucleases, phosphatases, proteases, etc. (The lysosomes of animals are budded from the golgi apparatus, and contain hydrolytic enzymes for digestion of unwanted macromolecules, including those taken into the cell by endocytosis. It is debatable if plant cells have the exact counterpart). Contents of vacuole under pressure - in fact, entire cytoplasm normally functions under 3-10 bars pressure, with the plasma membrane forced against the cell wall.

    • Double membrane organelles (nucleus, mitochondria, plastids)

    All of the double membrane organelles contain DNA and RNA and carry on transcription and translation.

  • Nucleus: Largest organelle. Has double membrane with pores, about 20% DNA, 20% RNA, 60% protein. Pores have protein complexes around the periphery and control traffic of molecules (e.g., proteins and mRNA). Nuclear membrane and nucleolus break down at cell division, reform in telophase.
  • Mitochondria: Respiratory organelles responsible for synthesis of most of the ATP utilized in the cytoplasm. They also participate in intermediary metabolism via the Krebs cycle. Outer membrane - Permeable to molecules up to 10,000 daltons. Inner membrane - Impermeable to most ions and solutes except through specific carriers. Has enzymes of respiratory chain and ATP synthesis. Matrix - has soluble enzymes of citric acid cycle (Krebs cycle) and enzymes participating in intermediary metabolism. Contains circular DNA (like bacteria). Also contains ribosomes (bacteria-like), messenger and transfer-RNA. However, nuclear genes and cytoplasmic ribosomes are responsible for most of the proteins; certain inner membrane proteins are coded by mitochondria genome (subunits of cytochrome oxidase and the ATPase). There are from 100-200 mitochnondria in average plant cell. Mitochondria arise from preexisting mitochondria by fission.
  • Plastids: Double-membraned organelle with DNA and RNA, similar in basic organization to the mitochondrion, but larger and with an additional inner membrane (thylakoid membrane). In chloroplasts, the thylakoids contain chlorophyll. Starch forms in the plastid matrix. In some instances chloroplasts lose chlorophyll and gain additional carotinoids, becoming colored chromoplasts, as when a green tomato turns red. All plastids have their origin in the proplastids of meristematic cells. Amyloplasts store the starch in potato tubers, seeds, roots, etc. Amyloplasts appear to be the intracellular "weights" by which the plant senses gravity.

    • Soluble cytoplasm (cytosol): Makes up the basic matrix in which the organelles occur. Contains most of the metabolites and inorganic ions, plus the soluble enzymes for intermediary metabolism and biosynthetic reactions (mostly in the matrix of organelles). The cytosol has sub-structure which associated with microfilaments. These are fibrous strands of protein (actin) that give gel-like features to portions of the cytosol, and drive active streaming in which subcellular particles, such as mitochondria, microbodies, etc., can be seen to move. Hence, plant cytoplasm is fluid and constantly mixed. Microtubules are dynamic structures having roles in cell division (spindle fibers) as well as orientation of wall growth.

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