Structure and Function of Plant Cells-I, Biology tutorial

Introduction:

Every plant comprises of cells that are simplest units of plant which can live separately. Smallest organisms are single cells, but plants are composed of billions of cells. Plants cells have several shapes and sizes. Smallest of which are dividing cells at tips of roots and stems. Every cell in plant comprises of cell with which surrounds plasma membrane that surrounds several smaller parts known as organelles. As each organelle has its own Sets of functions, job of every cell in plant is find out by how many and which organelles it comprises, and what Organelles do.

Components of Plant Cell:

Plant cell is made up of two basic parts: Cell wall and protoplast. Cell wall contain given parts.

  • Middle lamella
  • Primary cell wall
  • Secondary cell wall
  • Plasmodesmata

Protoplast contains two major divisions: Protoplasm and Ergastic substances

1) Ergastic substances comprise:

  • Starch grains
  • Vacuole sap with waste substances such as tannins,
  • Crystals etc.

2) Protoplasm contains two major divisions Nucleus composed of:

  • Nucleomembrane
  • Nucleoplasm
  • Chromalin
  • Nucleolus.

Cytoplasm composed of:

  • Plasmalemma
  • Hyaloplasm
  • Tonoplasm
  • Microbodies
  • Endoplasmic Reticulum (Er)
  • Dictyosomes
  • Spherosomes
  • Mitochondria
  • Plastids.

Few components of cytoplasm have other smaller components, which are given below:

Hyaloplasmà ground plasma, Ribosomes, Oil droplets, Cytoskeleton, Microtubules, Peroxysomes, Lysosomes, Glyoxysomes, Peroxysomes, Lysosomes, Glyoxysomes and Lipid bodies

Microbodiesà Peroxysomes and Lysosomes

SpherosomeàLipid bodies, Wax bodies, Fatty bodies, Proplastids, Leukoplast, Amyloplasts, Chloroplasts, Chromosplasts

Plastidsà Proplastids

Why Cell are so small:

For convince, majority of contents of cell are referred to as its cytoplasm. Only component of cell which is not part of cytoplasm is nucleus. Plasma membrane that surrounds cytoplasm is barrier which protects cell from dangerous substances. It has consistency of salad oil and should permit passage of gases and nutrients in and out of cell. Though, surface area of membrane can service only so much cellular volume which is surface to-volume ratio should have lower limit. Multicellular organisms avoid the limit by creating more but smaller cells for given volume. Cells are thus small so as to create room for more plasma membranes.

Membranes and Cell Compartment:

Several metabolic functions in cell happen in or on membranes. Plasma membrane alone is insufficient for all of the processes regardless of cell size. Extra internal membranes attached to plasma membrane or included in organelles compensate for deficiency of plasma membrane. Other membranes also create compartment which preserve different environments within cell. For instance, contrary reactions, like hydrolysis and dehydration of carbohydrates are isolated from each other in different compartments. Such compartments permit several different types of reactions to happen in cell simultaneously. Biological membranes are generally very small and comprise of generally phospholipids and proteins.

The Cytoskeleton:

The cytoskeleton is the network of filaments which forms mechanical support system in cell. The filaments also assist keep direct cell expansion, organelle position and control movement of chromosomes in nuclear division. Groups of filaments may form channels for transporting large molecules in cell. There are at least 3 types of filaments which comprise cytoskeletons of cells in plants and animals. Largest filaments are Microtubules that are hollow tubules. Microtubules are composed of two kinds of globular proteins, alpha tubulin and beta tubulin. Such proteins are bound to each other as dimmers, that aggregate in microtubules. Smallest filaments in cytoskeleton are actin filaments that comprise of 2 intertwined strands of globular protein actin.

How cytoskeleton works?

Cells frequently change shape, and organelles and other cellular components are in steady motion. This means that cytoskeleton can't be a firm structure. This statement is established by observations that expansion of and internal movements in cells happen in conjunction with growth and breakdown of microtubules and actin filaments. Observations are indirect proof that cytoskeleton has several dynamic functions in cell.

The least known parts of cytoskeleton are intermediate filaments. As their proteins are fibrous, intermediate filaments don't have globular subunits which are simply assembled and disassembled like microtubules and action filaments. Therefore, intermediate filaments may be comparatively static, tension bearing structures which are not as dynamic as larger and smaller counterparts in cytoskeleton.

The Cell Wall:

Most easily observed part of plant cell is cell wall. In few cells, like cork cell in bark, cell wall is only remnant of formerly living cell. Cell walls are dynamic parts of cells which can grow and modify their shape and composition. Their composition differs in different cell kinds and from one species to another. Up to 60% of cell wall may be cellulose; other components comprise hemicelluloses, pectins, Lignins and proteins. Approximately all plant cells contain cellulose - comprising cell walls.

How Cell Walls Grow:

All materials essential for making cell wall come from inside cell. Dictyosomes play the role in process by transporting pectins, hemicelluloses, and glycoproteins to plasma membrane. The substance passes by membrane in region of cell-wall synthesis, where they are gathered inside existing wall. This is how cell walls thicken. Cellular expansion needs cell wall to stretch or deform as cell absorbs more water. As wall expands, existing cellulose micro fibrils should separate as they can't stretch. In expansion, new cellulose micro fibrils are deposited inside loosened cell wall in different patterns, relying on cell type. In stem cells micro fibrils are oriented generally perpendicular to direction of cell expansion comparison, micro fibrils are settled, in random arrays in cells of storage tissues and tissues and tissue cultures. This pattern allows development in such cells to be more or less uniform in every direction.

Link between Cells:

Primary cell walls contain thin areas where several tiny connections, known as plasmodesmata happens between adjacent cells. Plasmodesmata are lined by plasma membrane, thus forming uninterrupted channel for movement of materials, from one cell to another. This signifies that all cells in plant are consistent and have potential to exchange substances through plasmodesmata. Plasmodesmata frequently happen in clusters where primary cell walls are mainly thin. These regions are known as primary pit-fields. Primary pit-fields and plasmodesmata are plentiful in conducting cells and secretory cells, like those in nectar glands or oil glands. Structure of plasmodesmata and frequency of these incidences in conducting and glandular cells propose that the connections function in transport between cells. Direct proof for the function comes from experiments having dyes and electric currents. When the dye which doesn't simply cross plasma membrane is injected in one cell, it rapidly passes in neighboring cells.

The Nucleus:

The nucleus is generally most conspicuous organelle in cell, when stained, it can be simply seen with light microscope. The nucleus has most of the cell's DNA that occurs with proteins in threadlike chromosomes. The nucleus is enclosed by two membranes. Outer membrane is continuous with membrane of endoplasmic reticulum. Inner and outer nuclear membranes are divided by very small space, but in few cases they fuse to form pores in envelope. Such nuclear pores are tiny circular openings bordered by proteins which probably influence passage of molecules between nucleus and rest of cell.

Ribosomes:

Ribsomes are where proteins are composed in cell. They are generally small and comprise of roughly equal amounts of protein and ribosomal RNA (RNA). Every ribosome is gathered from two subunits which are produced in nucleus and exported to cytosol. Two sub-units are connected when they join to the molecule of messenger RNA (mRNA). Ribosomes generally happen in clusters known as polysomes on single molecules of mRNA. Unlike nucleus and other organelles, ribosomes are not enclosed by membranes. Ribosomes are either joined to membranes or move freely in Cytosol. Proteins composed by cytosolic ribosomes are generally also Cytosolic; that is, they are not related with membranes. Such proteins comprise enzymes which degrade sucrose and glucose in cytosol.

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