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Plant Cell: Structure and Its Types

What happens when we are on the street, without an umbrella, and it starts raining suddenly? Most of us would run to the nearest shade to protect ourselves from the rain, right? But, have you ever seen a plant doing so? I am sure you haven’t. How do you think the plants protect themselves from the environmental stresses in spite of being sedentary organisms?? The secret lies in their cell structure which is highly different from that of the animals.

In fact, we are all aware that the plants have this amazing ability to prepare their own food using sunlight, water and carbon dioxide, whereas we do not. What makes them so special? Again, the unique structure of the plant cell plays an important role in this. Sounds intriguing right? Come let us learn more about the structure of the plant cell.

Introduction

The plant cell is the structural and functional unit of the plant. Plant cells are eukaryotic and have a well-defined nucleus surrounded by a nuclear membrane, just like the animal cell.

Plant cell contains a nucleus (having chromosomes inside) and organelles like mitochondria, cell walls, endoplasmic reticulum, ribosomes, Golgi bodies, etc. Let us have a detailed look into the structure of the plant cell.

Structure of Plant Cell

The most unique thing about the plant cell is the presence of an additional cell wall, outside the cell membrane, which is absent in animal cells. Just like different organs within the body, plant cell structure includes various components known as cell organelles that perform different functions for the sustenance of the cell.  

Structure of a typical plant cell

Cell Wall

It is a rigid layer which is composed of polysaccharides cellulose, pectin and hemicellulose. It is located outside the cell membrane. It also comprises glycoproteins and polymers such as lignin, cutin, or suberin.

The primary function of the cell wall is to protect and provide structural support to the cell. The plant cell wall is also involved in protecting the cell against mechanical stress and providing form and structure to the cell. It is freely permeable and allows the entry and exit of substances to and from the cell. The presence of the cell wall helps the plant cells to withstand environmental stress.

The formation of the cell wall is guided by microtubules. It consists of three layers, namely, primary, secondary and the middle lamella. The primary cell wall is formed by cellulose laid down by enzymes. Removal of the cell wall results in the formation of a protoplast which consists of the cell membrane and cellular constituents.

Plant cell wall structure

Cell membrane

It is the semi-permeable membrane that is present internal to the cell wall. It is composed of a thin layer of protein and phospholipids.

The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell. For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across.

Structure of Cell membrane

Nucleus

The nucleus is a membrane-bound structure that is present only in eukaryotic cells. The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism and growth.

The nucleus has a dense spherical structure suspended in the nucleoplasm (semisolid matrix enclosed by the nuclear membrane) known as the nucleolus. It is not bound by a membrane. Thin threads-like networks of nucleo-protein fibres are present. This includes:

  • DNA
  • RNA
  • Histone proteins (which contain basic amino acids)
  • Non-histone proteins 

It is the site of ribosome assembly (ribosomes are non-membrane bound organelles involved in protein synthesis). Cells may have one or more nucleoli.

Small gaps in the nuclear membrane, known as nuclear pores, allow the movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.

Typical nucleus

Plastids

They are membrane-bound organelles that have their own DNA. They are necessary to store starch and to carry out the photosynthesis process. Some of the vital types of plastids and their functions are stated below:

Leucoplasts

Leucoplasts are the structures that contain no pigments, thus they are colourless and named leucoplasts. Due to the absence of pigments they can not take part in photosynthesis hence found in the non-photosynthetic tissue of plants. They are used for the storage of protein, lipid and starch. Leucoplasts are of three types -

  • Amyloplasts - store starch
  • Aleuroplasts - store proteins
  • Elaioplasts - store fat

Leucoplasts

Chloroplasts

It is an elongated organelle enclosed by a double layer of phospholipid membrane. The chloroplast is shaped like a disc which contains following parts:

  1. Stroma: It is a transparent fluid present inside the chloroplast, containing genetic material DNA,ribosomes ,starch granules and some enzymes.
  2. Thylakoid: Stroma contains some membrane-bound, flattened sac-like structures known as thylakoids.The thylakoid membrane encloses the thylakoid lumen. The thylakoid membrane has light capturing bodies called quantasomes. Each quantasome contains chlorophyll molecules.
  3. Grana: Thylakoids are disc-like structures stacked upon each other. These stacks of thylakoids are known as grana (sing. granum).
  4. Stroma lamellae: The thylakoids of different grana are connected to each other with some flat channels known as stroma lamellae.

 Each chloroplast contains a green coloured pigment called chlorophyll required for the process of photosynthesis. The chlorophyll absorbs light energy from the sun and uses it to transform carbon dioxide and water into glucose.

Structure of Chloroplast

Chromoplasts

They are heterogeneous, coloured plastids which are responsible for pigment synthesis and for storage in photosynthetic eukaryotic organisms. Chromoplasts contain fat-soluble carotenoid pigments like carotene and xanthophylls which impart the colours - yellow, orange, and red.

Central Vacuole

It occupies around 30% of the cell’s volume in a mature plant cell. Tonoplast is a membrane that surrounds the central vacuole. The vital function of the central vacuole apart from storage is to sustain turgor pressure against the cell wall. The central vacuole consists of cell sap. It is a mixture of salts, enzymes and other substances.

Central Vacuole

Endoplasmic Reticulum

This is a double-membrane system that helps in the transport of proteins within and outside the cell. The two types of reticulum found in a plant cell are rough endoplasmic reticulum and smooth endoplasmic reticulum. The rough endoplasmic reticulum has ribosomes attached to it which help in protein synthesis.

Endoplasmic reticulum

Golgi Apparatus

They are found in all eukaryotic cells, which are involved in distributing synthesised macromolecules to various parts of the cell. Golgi bodies are referred to as dictyosomes in plant cells.

Golgi body and Endoplasmic reticulum

Ribosomes

They are the smallest non-membrane bound organelles which comprise RNA and protein. They are the sites for protein synthesis, hence, also referred to as the protein factories of the cell. Ribosomes in plant cells are of 80S type and have a 60S and a 40S subunit. Ribosomes are either scattered in the cytoplasm or remain attached to the surface of the rough endoplasmic reticulum.

Ribosomes

Mitochondria

These are the double-membraned organelles found in the cytoplasm of all eukaryotic cells. They provide energy by breaking down carbohydrate and sugar molecules, hence they are also referred to as the “Powerhouse of the cell.”

Mitochondria

Types of Plant Cells

There are five main types of plant cells that are present. Each of them has their specific characteristics and importance. The different types are discussed below:

Meristematic cells

Meristematic cells are also known as meristems. These cells are located at the root and shoot tips and beneath the bark on lateral sides of the plant. These cells can divide indefinitely and help in increase in length and girth of the plant. Due to their rapid division, meristems do not have stored food. These cells have no intercellular space present in them.

Parenchyma cells

These cells have fragile walls and are mainly focused on processes like photosynthesis, storage of food, secretion of substances, and loading of phloem. They are mainly present in the vascular bundles, epidermis, and leaves. These cells are responsible for the opening and closing of the stomata.

Collenchyma cells

The growing shoots and leaves contain collenchyma cells that play an essential role in providing structure to the plant. They have a thicker primary cell wall, and the secondary cell wall is absent.

Sclerenchyma cells

Sclerenchyma cells are characterised by the presence of thick secondary cell walls. These cells are not alive at the time of maturity since they lose their protoplast.

Reproductive cells

Reproductive cells are also called gametes and are of two types namely female and male gametes. The genetic components of both the parent gametes combine to form offspring. This process is known as fertilisation.

Practice problems of Plant Cell

Question1. Which one of the following is not a type of plastid?

A. Leucocytes
B. Chromoplasts
C. Chloroplasts
D. Leucoplasts

Answer: Plastids are membrane-bound organelles that have their own DNA. They are necessary to store starch and to carry out the process of photosynthesis. It is also used in the synthesis of many molecules, which form the building blocks of the cell. Some of the vital types of plastids and their functions are stated below:

  • Leucoplasts
  • Chloroplasts
  • Chromoplasts

Leukocytes are a type of lymphocyte/white blood cell found in animals. Thus, the correct option is (a).

Question2. During the somatic hybridisation, two plant cells are fused together but their cell wall is removed before the fusion. The structure without a cell wall is termed as

A. Tonoplast
B. Protoplast
C. Symplast
D. Apoplast

Answer: Cell wall is a rigid layer which is composed of polysaccharides cellulose, pectin and hemicellulose. It is located outside the cell membrane. It also comprises glycoproteins and polymers such as lignin, cutin, or suberin. If the cell wall is removed from the cell, the remaining structure which is surrounded only by a cell membrane is termed as ‘protoplast’. Hence the correct option is (b).

Question3. There is a cell organelle in the plant cell which functions to store the substances. Apart from storage ,it also sustains turgor pressure against the cell wall. Name the membrane around this organelle.?

A. Tonoplast
B. Elaioplast
C. Cytoplast
D. Amyloplast

Answer: The name of the given organelle is vacuole which occupies around 30% of the cell’s volume in a mature plant cell. Tonoplast is a membrane that surrounds the central vacuole. The vital function of the central vacuole apart from storage is to sustain turgor pressure against the cell wall. The central vacuole consists of cell sap. It is a mixture of salts, enzymes and other substances. Hence option (a) is correct.

Question4. Which of the following pairs is correct?

A. (a) Aleuroplast - stores fats
B. (b) Ribosomes - cell wall synthesis
C. (c) Mitochondria - maintaining turgidity
D. (d) Golgi bodies - distributing synthesised macromolecules to various parts of the cell

Answer: Golgi bodies are membrane bound organelles which appear as stacks of flattened disc-shaped sacs. They are responsible for distributing synthesised macromolecules to various parts of the cell.

Aleuroplasts store proteins. Ribosomes help in protein synthesis and the mitochondria are the sites of aerobic respiration in plant cells.

Thus, the correct option is (d).

FAQs of Plant Cell

Question1. Which parts of a plant cell are not found in animal cells?

Answer: The cell wall, the plastids and the large central vacuoles are found only in plant cells but not in animal cells.

Question2.  What is the reason that plants are so tolerant to environmental changes?

Answer: The reason behind the ability of plant cells to tolerate environmental changes is their rigid structure due to the presence of an additional cell wall, external to the cell membrane.

Question3.  What gives green colour to the leaves?

Answer: The green colour in the leaves is due to the presence of the pigment chlorophyll within the green coloured plastids known as chloroplasts. The chloroplasts are responsible for trapping the light energy for photosynthesis.

Question4. Which plant cells are responsible for the growth of the plant?

Answer: The meristematic cells present at growing tips of the plant such as the root tips and shoot tips are responsible for the growth in length of the plant. The meristematic cells present on the lateral sides of the plant help in the increase in girth of the plant.

Related Topics to Cell Envelopes and Cell Membrane in Biology

 

NCERT Class 11 Biology Chapters

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

 

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