Cell Biology: Organelles and Plant Cell Structure

1. Rough endoplasmic reticulum is specialised in synthesising proteins.
2. Some of the enzymes have been located in mitochondria which help in the synthesis of lipids like lecithin from fatty acids and glycerol.
3. Chloroplast stroma contains enzymes for conversion of carbon dioxide into carbohydrates. Lysosomes help in intracellular digestion of food particles.

The functions of Endoplasmic reticulum are:
Detoxification:- E.R helps in detoxification of many endogenous and exogenous compounds.
Glycogenolysis:- Synthesis of glycogen occurs in the cytoplasmic matrix but the SER is involved in glycogenolysis.
Lipid synthesis:- The SER is concerned with synthesis of lipid e,g, triglyceride. The membranes of SER also appear to be involved in the formation of lipo – protein complexes. The SER is also involved in the initial stages of the breaking down of fatty acids.
The nucleoli present in the nucleus helps in the formation of ribosomes.

A. Cells are enclosed by a plasma membrane composed of lipids and proteins.
B. In plant cells, the cell wall composed mainly of cellulose is located outside the cell membrane. In animal cells, the cell wall is mainly composed of lipids and phospholipids.
C. The nucleus in eukaryotes is separated from the cytoplasm by a double-layered membrane and it directs the life processes of the cell.
D. Most plant cells have large membranous organelles called plastids, which are of two types - chromoplasts and leucoplasts. Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis.

Plastids are small bodies found free in the cytoplasm of most plant cells but they are absent in animal cells as well as in  bacteria, fungi and cynobacteria.

Nucleus is a dense protoplasmic body that contains hereditary information for controlling cell activities as well as for transfer to next generation. The nucleus contains genetic material DNA, so it is needed for the inheritance of characters from one generation to another.
Smooth endoplasmic reticulum is specialized to synthesize lipids and steroids. Ribosomes help in protein synthesis inside the cell.
Plasma membrane is the outer covering of most of the animal cells, living, ultra thin, elastic, semi – permeable in nature. It provides the mechanical support and external frame to the protoplasm (cytoplasm + nucleus).
Hence, the correct sequence of the codes is i - 4, ii - 3, iii - 1, iv - 2.

A. The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets.
B. The function of the endoplasmic reticulum is to serve as a channel for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
C. In the liver cells of the group of animals called vertebrates, smooth endoplasmic reticulum plays a crucial role in detoxifying many poisons and drugs.

Lysosomes are cellular organelles that contain acid hydrolase enzymes that break down waste materials and cellular debris. They can be described as the stomach of the cell. Lysosomes digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria.

Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored. Lysosomes help in removing dead and worn out cellular organelles (cell debris) by digesting them.

A. Suberin is a waxy substance found in higher plants. Suberin is the main constituent of cork, and is named after the Cork Oak, Quercus suber.
B. Abnormal secondary growth does not follow the pattern of a single vascular cambium, a thick hair clit producing xylem to the inside and phloem to the outside. Some dicots have anomalous secondary growth, e.g. in Bougainvillea, Dracaena. Withania shows normal secondary growth.
C. In angiosperms, intercalary meristems occur only in monocots (in particular, grass) stems at the base of nodes and leaf blades and respond to primary growth of stem and root.
Hence, B and C are false.

In monocotyledon roots vascular bundles are present as hexarch or polyarch system. Pith is large and well developed. Cambium is absent.
Hence, A and D both characters are absent in monocotyledon roots.

Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified walls. These have a shape of a star. They are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples and the gritty texture of pears. Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked or elaborately branched. They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells within parenchyma tissues. But compared with most fibres, sclereids are relatively short. Characteristic examples are the stone cells (called stone cells because of their hardness) of pears (Pyrus communis) and quinces (Cydonia oblonga) and those of the shoot of the wax plant (Hoya carnosa).
Hence, A, C and D plant species have sclerenchyma cell, referred to as stone cells.

The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.
Ribosomes are sub - microscopical dense granules about 150 to 2500 Angstrom in diameter and were seen by Plade in 1955 under electron microscope.
The term Nucleolus was coined by Bowman in 1840.
Endoplasmic reticulum was discovered and named so by Porter. Endoplasmic reticulum is an interconnected system of membrane-lined channels that runs through the cytoplasm.
Hence, the correct answer using the codes is P - 3, Q - 1, R - 2, S - 4.

Haplostele - consisting of a cylindrical core of xylem surrounded by a ring of phloem. An endodermis generally surrounds the stele. A centrarch (protoxylem in the center of a metaxylem cylinder) haplostele is prevalent in members of the rhyniophyte grade, such as Rhynia.
Actinostele - a variation of the protostele in which the core is lobed or fluted. This stele is found in many species of club moss (Lycopodium and related genera). Actinosteles are typically exarch (protoxylem external to the metaxylem) and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem. Exarch protosteles are a defining characteristic of the lycophyte lineage.
Plectostele - a protostele in which plate-like regions of xylem appear in transverse section surrounded by phloem tissue. In fact, these discrete plates are interconnected in longitudinal section. Some modern club mosses have plectosteles in their stems. The plectostele may be derived from the actinostele.
Solenostele - if the cylinder of vascular tissue contains no more than one leaf gap in any transverse section (i.e. has non-overlapping leaf gaps). This type of stele is primarily found in fernstems today.
Eustele - in this arrangement, the primary vascular tissue consists of vascular bundles, usually in one or two rings around the pith. In addition to being found in stems, the eustele appears in the roots of monocot flowering plants. The vascular bundles in a eustele can be collateral (with the phloem on only one side of the xylem) or bicollateral (with phloem on both sides of the xylem, as in some Solanaceae).
Hence, A is the only unmatched pair.

Xylem is one of the two types of transport tissue in vascular plants. Almost all plant cells produce primary cell walls. The major component of most primary walls in xylem is a disorganized network of cellulose fibrils, which allows the wall to stretch and expand as the cell grows. The secondary wall is deposited on the inner side of the primary wall during and after the cell has elongated or enlarged. The cellulose fibrils in the secondary wall are arranged in a regular fashion with alternating layers at fixed angles to the main axis of the cell. This reinforces the plant cell, while preserving the elastic nature of the primary wall. Most of the cell types in xylem can be distinguished based on the shape and features of the secondary cell wall.
The walls of tracheids are made up of lignin. Xylem parenchyma cells store water, mineral nutrients and carbohydrates, and respond to wounding. Sclerenchyma cells of xylem provide mechanical support, defence and water transport.

Leeuwenhoek first saw and described a living cell.
Mitochondria was first observed by Kolliker in 1850 as granular structure in striated muscles.
Endoplasmic reticulum was discovered by Porter and Thompson in 1945 and was named in 1953 by Porter.
Robinson and Brown discovered ribosomes in 1953 in plant cell and by Palade in animal cell (1955).
Hence, the sequence P - 1, Q - 4, R - 2, S - 3 is correctly matched.