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Agriculture LibreTexts

Section 3

Cytoplasm(Strasburger, 1882) is a homogeneous, mostly clear jelly-like liquid lies between the nucleus and cell membrane, consisting of cytosol and the cellular organelles.

Cytosol is composed of water, salts, organic molecules, and the many enzymes necessary for the cell to catalyze reactions. 80% of the cytoplasm is aqueous and composed of ions and soluble, primarily organic, macromolecules.The 20% of the cytoplasm that is not aqueous is made up of organelles, mitochondria, chloroplasts, vacuoles, the cytoskeleton, and endoplasmic reticulum. The aqueous part is also called hyaloplasm. It behaves like a gel sometimes, depending on the activity phase of the cell; in this state, it is called cytogel. Cytogel lines the plasma lemma and known as ectoplasm. When instead it behaves like a liquid, it's called cytosol. It is known as endoplasm..

Vacuole

A vacuole is a large fluid-filled sac in the cytoplasm, bounded by a single membrane called the tonoplast, it plays the major role in all these functional activities of the vacuole Vacuoles are filled with watery fluid termed as cell sap.It is acidic. The chemical composition of cell sap differs markedly from that of the surrounding cytoplasm. In most cases mature plant cells have single large central vacuole. So that the plant cytoplasm lies as a thin layer positioned between the plasma membrane and the tonoplast. The vacuole contains nutrients, metabolites, pigments and waste products. Pigment anthocyans are present ,which is mainly responsible for the colours in flowers.

Function of vacuole

In mature plant cells, vacuoles tend to be very large and are extremely important in providing structural support, as well as serving functions such as:-1 ) storage; 2) water storage; 3) hydrolytic enzymes; 4) osmoregulation and 5) turgor Pressure.Osmoregulation:- Most of the water in mature plant cells occursin the vacuoles Water enters the cell sap by osmosis(osmosis is diffusion of water across a membrane) through the differentially permeable tonoplast.water enters the cell (cell vacuole) and creates turgor pressure . Turgor pressure drives cell growth . Opening and closing of stomata is due to changes in turgor pressure of guard cells The pressure applied by the vacuole, called turgor, is necessary to maintain the size of the cell. Plant cells don't increase in size by expanding the cytosplasm, rather they increase the size of their vacuoles.

Lysosomes

(DE Duve, 1955)Lysosomes are a single membrane bound small vesicular organelles. Shape and size of lysosome is variable. A granulated stroma and a vacuole are located inside the membrane. Lysosomes are found in fungi and root tips of cells.Lysosomes are little sacs of hydrolyzing enzymes, acid hydrolases that are used by the cell to break down food as well as debris and the byproducts are used in biosynthetic activity of cells.. These sacs are separate from the cell because the enzymes could destroy the cell if they were mixed with the cytoplasm. Only the waste products or nutrients leave the lysosome. Lysosomes are involved amongst other things in the intracellular digestions of particles scavenged by some types of cell during endocytosis. The enzymes contained in the lysosome are synthesized on rough endoplasmic reticulum and are transported to golgi body.

Lysosomes are of four types depending upon their funciton:

  • Primary lysosome,
  • Secondary lysosome,
  • Residual bodies,
  • Autophagic vacuoles.

Its functions are defence against bacteria and viruses and in destroying old and worn out organelles. They have been found occasionally to be digesting the whole cell or part of cell(autolysis or self-destruction); therefore lysosomes are called suicide bags.

Endoplasmic reticulum - Some of the floating membranes in the cytosol include a network of tiny sacs, tubules and vesicles that interconnect throughout the inside of the cell. This network is called endoplasmic reticulum (Porter et al.in1945 and Thompson in 1945).According to the metabolic activities of cell endoplasmic reticulum assumes three forms in different cell:

Figure 2.3.1.

 

The endoplasmic reticulum forms a continuous sheet enclosing internal space or a narrow lumen called endoplasmic reticulum lumen or endoplasmic reticulum cisternal space.

Endoplasmic reticulum is of two types:

  1. Rough endoplasmic reticulum (R.E.R)The membrane of endoplasmic reticulum on its external surface carries granular structures known as ribosomes, these regions are known as rough endoplasmic reticulum These ribosomes are made of protein and RNA and are actively involved in the synthesis of proteins.

 

Figure 2.3.2: Rough Endoplasmic Reticulum (RER)

  1. Smooth endoplasmic reticulum (SER), which has a tubular structure and no ribosomes attached to the membrane surface. It consists of smooth membrane segments and found in regions poor in protein synthesis.

Function of endoplasmic reticulum

The endoplasmic reticulum near the ribosomes is responsible for moving much of the protein through its tubules to other parts of the cell. and maintains the integrity of the membranes surrounding the nucleus. This is the conveyor belt of the cell. The large net work of endoplasmic reticulum provide increased surface for enzyme synthetic activity.

Ribosomes 

Ribosomes are small ,non membranous, spherical bodies mainly found bound to the endoplasmic reticulum as well as freely scattered throughout the cytoplasm, in all types of cells. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct ribosomes as well.Ribosomes always have two subunits which interlock and behave as a single entity.It is composed of approximately 60 percent ribosomal RNA (rRNA) and 40 percent protein. Ribosomes are sometimes referred to as simply RNA. Like DNA, they are long chains of amino acids, but their base pairs are different and they are usually not as long as DNA.

Function of ribosome

Their main function is to produce a variety of proteins from simple genetic instructions which propagate outwards from the cellular nucleolus in the form of messenger RNA (mRNA). Ribosomes are actively involved in the protein synthesis, the process that generates organic tissue. Genetic instructions for the creation of new proteins come from mRNA. Ribosomes are thus a kind of protein-synthesis "machine." This process of using the information in RNA to make a protein is called translation; it is the complement of transcription. The protein products of the translation include the enzymes ("workers") of the cell. These proteins are shipped throughout the cell for functional uses.

Golgi Apparatus

(Golgi complex, or Golgi body or Dictyosomes) in 1898 by Camillo Golgi:- the Golgi body consists of a series of five to eight cup-shaped, membrane-bounded, flattened sacs. called cisternae. The Golgi body are surrounded by numerous, small, membrane-bounded vesicles. 60 cisternae may combine to make up the Golgi apparatus in some unicellular flagellates and the number of Golgi bodies in a cell varies according to its function. This complex is usually located close to the cell nucleus.

Figure 2.3.3: Golgi Complex

Function of golgi body

The Golgi body and its vesicles function in the sorting, modifying, and packaging of macro-molecules that are secreted by the cell or used within the cell for various functions. The Golgi complex in plant cells produces pectins and other polysaccharides specifically needed by for plant structure and metabolism. The products exported by the Golgi apparatus through the trans face eventually fuse with the plasma membrane of the cell. Among the most important duties of the Golgi apparatus is to sort the wide variety of macromolecules produced by the cell and target them for distribution to their proper location. Specialized molecular identification labels or tags, such as phosphate groups, are added by the Golgi enzymes to aid in this sorting effort. . Each Golgi body has two faces , a cis face- entry face, Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles. The trans face- exit face, the modified and packaged macromolecules are transported to their destinations in the form of smaller detached vesicles. The cis face is found near the endoplasmic reticulum, from where most of the material it receives comes, and the trans face is positioned near the plasma membrane of the cell, to where many of the substances it modifies are shipped.

Mitochondria

(singlular- mitochondrion; Benda, -1897) are rod-shaped, filamentous or granular structures distributed through the cytosol of most eukaryotic cells. Their number within the cell depends upon the metabolic activity of that cell, and may range from a single large mitochondrion to thousands of the organelles. Each mitochondrion is surrounded by two layered membranes forming 5 distinct compartments: outermembrane, intermembrane space, inner membrane, cristae space (formed by invaginations of the inner membrane), and the matrix (space within the inner membrane). Much of the activity of the mitochondria occurs within the inner mitochondrial membrane.

Figure 2.3.4: Structure of Mitochondrion

The number of mitochondria in a cell can:

  • Increase by their fission (e.g. following mitosis);
  • Decrease by their fusing together.

The biochemical activities of the mitochondria are regulated by a set of enzymes.

  1. Outer membrane contains:- Monoamine oxidase, NADH-cytochrome C reductase, Kynurenine hydroxylase, Fatty acid Co. A ligase
  2. Intermembrane space contain Adenylate kinase and Nucleoside diphosphokinase
  3. The inner membrane contains 5 complexes of integral membrane proteins: NADH dehydrogenase (Complex I) succinate dehydrogenase (Complex II) cytochrome c reductase (Complex III; also known as the cytochrome b-c1 complex) cytochrome c oxidase (Complex IV) ATP synthase (Complex V)
  4. The matrix contains:- Malate and isocitrate dehydrogenases, Fumarase and aconitase, Citrate synthetase,a-keto acid dehydrogenase, b-oxidation enzymes.

Function of mitochondria

The most important role of mitochondria is to synthesize ATP with energy supplied by the electron transport chain and a process called oxidative phosphorylation, therefore mitochondria has been called power house of the cell.

CH2O + O2 --------> CO2 + H2O + energy.

This energy is produced through a series of steps. Each one of which is governed by an enzyme (present in cristae and matrix) . Carbohydrates, fat and proteins are broken into smaller molecules which involves (i) glycolysis (ii) oxidative decarboxylation and oxidative phosphorylation including (a) Kreb’s cycle and (b) respiratory chain.

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