Cell Structure

A cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm. Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of miniscule but distinct structures called organelles. Here we cover them all.

Animal and Plant Cell

STRUCTURAL ORGANISATION OF A CELL (3 parts)

Plasma membrane, nucleus and cytoplasm are three basic features are common to all cells.

1.  Plasma Membrane or Cell Membrane
This is the outermost living, thin and delicate covering of cell. It separates the contents of the cell from its external environment.

(a) Functions of Plasma Membrane

(i) It allows the entry and exit of some selective materials in and out of the cell. The cell membrane, therefore, acts as a semipermeable, selectively permeable, partially permeable and differentially permeable membrane.  

(ii) It helps to maintain the shape of the cell.
(iii) It acts as a mechanical barrier and protects the internal contents of the cell from leaking out.
(iv) It provides protection against microbes and foreign substances.
(v) It gets modified to perform different functions, e.g. microvilli in the intestine of human beings for absorption.
(vi) Its semi-permeability enables the cell to maintain cellular homeostasis.

(b) Transport Across the Membrane
Transportation of substances across the membrane may take place by Diffusion or Osmosis.

(i) Transport Across the Membrane by Diffusion
The spontaneous movement of a substance (solid, liquid or gas) from a region of its higher concentration to a region of its lower concentration is called diffusion.  For example, CO2 (cellular waste, which needs to be excreted out) accumulates in higher concentration inside the cell.  In the cell’s external environment, the concentration of CO2  is lowered compared to the inside of the cell.  Due to this difference in the concentration, CO2  moves out of the cell by the process of diffusion.  Similarly, O2 enters the cell by the process of diffusion, when the level or concentration of O2 inside the cell decreases.

Diffusion is faster in the gases than in liquids and solids.  It plays an important role in gaseous exchange between the cells and also between the cell and its external environment.  In addition to gaseous exchange, diffusion also helps an organism in obtaining nutrition from the environment.

(ii) Transport Across the Membrane by Osmosis
Osmosis is defined as the movement of water molecules from a region of its higher concentration to a region of lower concentration through a semipermeable membrane. Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. The process of osmosis can be seen in a cell placed in a solution of different concentrations (such as hypotonic, isotonic and hypertonic).

(a) Hypotonic Solution: The word “HYPO” means less, in this case, there are less solute (salt) molecules outside the cell, since salt is less water will move into the cell. The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting.

(b) Isotonic Solution: If the concentration of solute (salt) is equal on both sides, the water will move back and forth but it won’t have any result on the overall amount of water on either side.

(c) Hypertonic Solution: The word “HYPER” means more; in this case, there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction. In plant cells, the central vacuole loses water and the cells shrink, causing wilting. The phenomenon is called plasmolysis[It is the phenomenon, in which a living plant cell losses water through osmosis when kept in hypertonic solution].

In animal cells, the cells also shrink. In both cases, the cell may die.

  2. Cell Wall
It is a tough, non-living covering outside the plasma membrane.  It is found in plant and fungal cells. It is freely permeable. It is mainly made up of cellulose, a complex substance that provides structural strength to plants. The cell wall of the fungal cell is made up of Chitin.

Functions of Cell Wall

(i) Cell wall permits the cells of plants, fungi and bacteria to withstand hypotonic  conditions without bursting.

(ii) It has narrow pores, called pits. Through them, fine strands of cytoplasm (or cytoplasmic bridges) called plasmodesmata are able to cross the cell walls. Plant cells interact with each other through these cytoplasmic channels.


3.   Nucleus
Nucleus is the brain of the cell. It is composed of a double-layered covering called nuclear membrane.   It has numerous pores called nuclear pores. They transfer the materials from inside the nucleus to the cytoplasm.  The nucleus contains chromosomes. They which are visible as rod-shaped structures only when the cell is about to divide. It encloses a liquid ground substance called nucleoplasm.   It contains nucleolus and chromatin material.


(i) Nucleolus is a more or less round structure found inside the nucleus. It does not have covering of membranes. It is known as the factory of ribosomes.

(ii) Chromatin is an entangled network of long, thread-like structures.  It condenses to form chromosomes during cell division. Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA (Deoxyribonucleic Acid). Chromosomes are composed of two components, i.e.  DNA and protein. The DNA molecules contain information necessary for constructing and organizing cells. The functional segments of DNA are called genes.

Nucleus also contains RNA that directs protein synthesis.  

Functions of Nucleus

(i) Nucleus plays an important role in cellular reproduction. In this process, a cell divides to form two new cells.
(ii) It determines  the cell development and maturity by directing the chemical activities of the cell.
(iii) It helps in the transmission of hereditary traits from parents to offspring.
(iv) It controls all metabolic activities of cell. If it is removed, the protoplasm dries up.

In some organisms like bacteria, the nuclear region of the cell is poorly defined because of the absence of nuclear membrane. The nuclear region in these organisms contains only nucleic acid. Such an undefined nuclear region is called nucleoid.  Organism who lack a nuclear membrane are called prokaryotes. The organisms with cells having a well-defined nucleus enclosed in nuclear membrane are called eukaryotes. Eukaryotic cells are further categorized into plant and animal cells.

Prokaryotic Cell & Eukaryotic Cell

Prokaryotic and Eukaryotic Cell

GENE – A piece of DNA which carries information for a specific character in organisms.

4.  Cytoplasm
The large region of each cell enclosed by a cell membrane is called cytoplasm.  It is the fluid content present inside the plasma membrane. It contains many specialised cell organelles, each of which performs a specific function for the cell.

Functions of Cytoplasm

(i) It helps in the exchange of material between cells organelles.
(ii) It acts as a storehouse of vital molecules such as amino acid, glucose, vitamin, iron etc.
(iii) It acts as the site for certain metabolic pathways such as glycolysis etc.

Protoplasm
It is the living content of a cell. It consists of the cytoplasm and nucleoplasm.

CELL ORGANELLES

 A membrane-bound compartment or structure in a cell that performs a special function.  A eukaryotic cell contains many organelles, for example, the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid).

1. Endoplasmic Reticulum (ER)
It is a large network of membrane-bound tubes and sheets. It extends from outer nuclear membrane into the cytoplasm. It looks like long tubules round and oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. It occurs in three forms, i.e. cisternae, vesicles and tubules.  Depending upon nature of its membrane, ER is of two types:

(i) Rough   Endoplasmic Reticulum (RER): It contains ribosomal particles, due to which its surface is rough. The ribosomes are the site of protein synthesis. RER is mainly formed of cisternae.

(ii) Smooth Endoplasmic Reticulum (SER): It helps in manufacture of fat molecules or lipids. It is formed of vesicles and tubules.  Its surface is smooth due to the absence of ribosomes. ER appears in varying forms in different cells. It always forms a network system of vesicles and tubules.

Functions of Endoplasmic Reticulum

(i) Ribosomes present in all active cells act as sites for protein synthesis.  Proteins manufactured here are transported throughout the cell by endoplasmic reticulum.
(ii) Fat and lipid molecules manufactured by SER helps in building cell membrane. This process is called membrane biogenesis.   
(iii) Some other proteins and lipids synthesized by ER function as enzymes and hormones.
(iv) SER plays a crucial role in detoxification of poisons and drugs in liver cells of vertebrates (group, of animals).
(v) It forms a network system, providing channels for the transport of materials especially proteins. It transports between various regions of the cytoplasm or between the cytoplasm and the nucleus.
(vi) It functions as cytoplasmic framework. It provides a surface for some of the biochemical activities of the cell.
(vii) It gives mechanical support to the cells.

Endoplasmic reticulum

2. Golgi Apparatus
It consists of a system of membrane-bound, fluid-filled vesicles, large spherical vacuoles and smooth, flattened cisternae.  These are stacked parallel to each other. Each of these stacks is called cistern. The golgi apparatus (or dictyosomes) arises from the membrane of smooth ER.  Therefore, it constitutes another portion of a complex cellular membrane system. The material that is synthesised near Endoplasmic Reticulum (ER) is packaged and dispatched to various parts of the cell through Golgi apparatus. They were discovered by Camillo Golgi.

Functions of Golgi Apparatus

(i) Golgi apparatus stores, modifies and packs products in vesicles.
(ii) It is involved in the formation of lysosomes.
(iii) It forms complex sugars from simple sugars in some cases.
(iv) It is involved in the synthesis of cell wall and plasma membrane.

Golgi Apparatus

3. Lysosomes
They are formed by the RER. They are waste disposal system of cell, they store digestive enzymes in them which are capable of digesting all materials. They clean up the cell by digesting any worn-out part of cell or any material entering the cell. They are known as ‘suicidal bags’ of cell as they burst and digest their own cell in case there is any disturbance in the cell.

Functions of Lysosomes

(i) They help to keep the cell clean by digesting any foreign material that enters the cell as well as worn out cellular organelles. Hence, called scavengers and cellular housekeepers.
(ii) They remove foreign material by breaking it into small pieces through its powerful digestive enzymes. These enzymes can break-down all organic materials.
(iii) During starvation, the lysosomes digest stored food contents by autophagy and supply energy to the cell.

Lysosomes

4. Ribosomes
These are tiny granular structures. These are not bounded by any membrane formed in the nucleolus and are freely dispersed in cytoplasm or attached with the endoplasmic reticulum.

Functions of Ribosomes
Ribosome is involved in protein synthesis. It is the only organelle which is also found in Prokaryotic cell.

5. Mitochondria
Mitochondria were first observed by Kolliker in 1880. It is a double membrane-bounded cell organelle. The outer membrane is very porous.  The inner membrane is deeply folded into finger-like projections called cristae. It creates large surface area for ATP generating chemical reactions.      There are about one million elementary particles in one mitochondrion. They are strange in having their own DNA and ribosomes for making some of their own proteins themselves.  Due to this they are also known as semi-autonomous bodies.

Functions of Mitochondria

(i) It generates energy for various activities of cell. It is known ·as the power-house of the cell. Mitochondria are sites of cellular respiration.  They release energy required by the cell in the form of ATP (Adenosine Triphosphate). This ATP is known as energy currency of the cell.

(ii) Whenever, the cell requires energy, ATP molecule breaks down.  It generate energy to be used for metabolic activities of the body.
(iii) Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Hence, they are able to make some of their own proteins.
(iv) They provide intermediates for the synthesis of various chemicals like fatty acids, steroids, amino acids etc.

Mitochondria

6. Plastids
These are pigment-containing organelles. These are found in plant cells. Many plastids have one or more than one pigments.

Plastids are of three types which are as follows:

(i) Chloroplasts: These are most important plastids. These are green in colour and found in green parts of plant. These contain chlorophyll which helps in photosynthesis. The study of ultra-structure reveals that it is bounded by a double membrane. They also have their own DNA and ribosomes and are known as semi-autonomous bodies. Inside the chloroplast, there is present a semi-fluid matrix called stroma, which is made up of proteins and other chemicals. The inner membrane forms stacked membrane system which becomes suspended in the stroma. Each membrane stack is called granum (plural grana). The membranes of grana are the sites where photosynthesis occurs in the presence of sunlight.

Functions: In chloroplasts, photosynthesis takes place and food is prepared for plant.

(ii) Chromoplasts: These are second type of plastids. These are of various colours other than green. In plants, colours other than green are due to chromoplasts. These are present in the petals of the flowers and in the ripened fruit.

Functions: These help the plants in pollination. These impart various colours to petals and fruits.

(iii) Leucoplasts: These are third type of plastids. These are colourless plastids. These are Triangular, tubular or of any other shape. Functions: These are found in food storage parts of the plant especially the roots and tubers

Plastids

7. Vacuoles
These are the storage sacs for solid or liquid contents. In animal cells, vacuoles are small-sized, but in plants, the vacuoles are large-sized.   Some may occupy 50-90 per cent of the total cell volume.   The vacuole is bounded by a membrane called tonoplast.

Functions of Vacuoles

(i) Vacuoles are full of cell sap and provide turgidity and rigidity to cells in plants.
(ii) Many substances like amino acids, sugars, organic acids and proteins are stored in vacuoles.
(iii) In some unicellular organisms, vacuoles also play an important role in expelling excess water and some wastes from the cell.

vacuole

Difference between Plant Cell and Animal Cell

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