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Agriculture LibreTexts

Section 1

Introduction

All plants are made up of one or more units called cells. The cell also brings structural and functional stability to plants. Plants are of two types:

  1. Unicellular plants: Many plants consist of a single cell e.g. Chlamydomonas (figure 2.1.1), Anacystis, Chroococcus, Synechococcus.

 

Figure 2.1.1: single celled Chlamydomonas

In these plants all the vital functions are carried out by the single cell. This type is also known as acellular.

  1. Multicellular plants: Most plants consists of many cells. All the function of life are carried out by one or other group of cells. e.g. Spirogyra (figure 2.1.2), Sphagnum, Lycopodium etc.

Figure 2.1.2: Spirogyra

In multicellular living things, a collection of cells that work together to perform similar functions is called a tissue; various tissues that perform coordinated functions form organs; and organs that work together to perform general processes form body systems. A cell is a basic structural and functional unit of plants.

Discovery of Cell

Marcello Malpighi (1661): found that plants consist of separate structural units "utricles".

Robert Hooke (1665): observed a thin slice of cork under a very primitive microscope. Cork was composed of box-ike compartments, forming a honey comb structure. He name it as cells (Latin: cellula-a hallow space).

Antony van Leenwenhoek (1674): observed under improved microscope living cells-bacteria, protozoa, spermatozoa etc.

Nehemiah Grew (1682): Cells in the forms of cavities with cellulose walls were discovered in different plants. He wrote The Anatomy of Plans.

H.J. Dutrochet (1824): expressed the idea of individuality of cells i.e., cells were not just spaces between a network of fibers, but these were separate and separable units.

Cell Theory

The cell theory or cell doctrine was proposed by Schleiden and Schwann in 1839 describe that all organisms are composed of simiar units of organisation, called cells.

In 1838 Matthias Schleiden found that all plant cells have basically similiar structure.

T. Schwann (1839) observed that animal cells differ from plant cells in lacking cell wall but are otherwise similar amongst themselves. He declared that all animals and plants are made up of cells.

Schleiden (1838) summarized his observations into three conclusions about cells:

  1. The cell is the unit of structure, physiology, and organization in living things.
  2. The cell retians a dual existence as a distinct entity and a building block in the construction of organisms.
  3. Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).

Rudolph Virchow (1858) states that "Omnis cellula e cellula" - that the new cells arise from pre-existing cells only. The main features of cell theory as known nowadays are:

  1. All living organisms are composed of cells. Therefore a cell is the structural unit of living organisms.
  2. All cells arise from the pre-exisitng cells of similar type. Thus cell is a unit of heredity.
  3. All cells are basically alike in chemical composition and metabolism activities
  4. The function of an organism as a whole is the result of the activities and interactions of its cells.

Exceptions to the Cell Theory

  1. Virus - Protoplasm and nucleus absent. DNA and RNA is the genetic material
  2. Bacteria and Blue Green Algae: Nuclear membrane is absent, thus the nuclear content is in direct contact with the cytoplasm. No true cells.
  3. Some mold fungi: Body is made-up of undivided mass of protoplasm in which several nuclei are scattered.

Cell Size

Cell size varies widely in various plants. The size varies from micron to mm (a micron is a one thousandth part of a millimeter). The smallest cells are found in bacteria: the cell of Mycoplasma laidlawii with a diameter of about 0.1-0.15 micron is considered to be the smallest. In plants the longest cells are fibers producing e.g. Jute, Cotton, ramie, which are about 55cm long. A single celled alga Acetabularia measures about 10cm in height.

Figure 2.1.3: Acetabularia

Cell shape

The shape of the cells varies with their position in plants. It can be polygonal, spherical, elliptical, spindle-shaped, cuboidal, plate-like or irregular.

In multicellular plants cell shape also depends on their location and nature of function. The cell shapes are influenced by the surface tension and viscosity of the protoplasm, pressure exerted by the surrounding cells and the rigidity of cell membrane.

Cell types

Two types of living cells were recognized in plants by Dougherty (1957). This division is based on internal complexity

  • Prokaryotic (pro=primitve, karyon=nucleus) cell: a. The nuclear material is not bounded by nuclear membrane, the nuclear materials remain diffused with cytoplasm. b. an organized nucleus is absent in the protoplasm, i.e., the nucleus is without nuclear membrane, nuclear reticulum, nucleoplasm and nucleolus. Only monera i.e. Blue green algae (Cyanobacteria) and Bacteria are prokaryotic cell known as prokaryotes. c. the genetic material contain DNA, histone absent.

Figure 2.1.4: Prokaryotic Cell

  • Eukaryotic cell (eu=true, karyon=nucleus): This type of cells have: a. A true nucleus witha definite nuclear membrane by which nuclear material is bounded. b. the genetic material contains DNA complex with histone proteins to form well-organized chromosomes. c. Nucleoli present. The majority of plants are composed of eukaryotic cells. Distinct organelles, the small structures that each perform a specific set of functions, are present within eukaryotes.

Comparison of prokaryotic and eukaryotic plants

Feature Prokaryotic Cells Bacteria, and Cyanobacteria Eukaryotic Cells Protists Fungi, Plants
Cell size and cell organization 1-10mm: unicellular 5-100mm: multicellular
Organelles Few or none nuclear, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, etc
DNA Circular (or linear) DNA organized in a nucleoid   Linear DNA molecules organized in chromosomes bounded in a nucleus
Synthesis of RNA and proteins RNA and proteins synthesis occur in same place, RNA synthesis occur in nucleus and proteins synthesis in the cytoplasm
Chromosomes Single Multiple
Ribosomes 70s(50s+20s) 80s(60s+40s)
Metabolic Activity Photosynthetic and respiratory enzymes are found on plasma membrane Photosynthetic and respiratory enzymes are found on chloroplast and mitochondria respectively
Cytoskeleton not present Present
Cell Division Amitosis, spindle apparatus absent during cell division; replicated DNA pulled by attachment to plasma membrane Mitosis, meiosis, spindle apparatus present and chromosomes pulled by it
Metabolic Activity Anaerobic (respiration in absence of oxygen) or aerobic Aerobic (in presence of oxygen)