Biology Cell structure function types

 ​Cell Notes 1:

CONTENTS

  The Cell

  History

  Structure of Cell

   Cell Organelles



 THE CELL

Cell is the functional & structural unit of life as all the metabolic activity takes place in a cell. The study of structure & composition of cell is known as cytology.

 HISTORY

Robert Hooke used a primitive compound microscope to examine thin slices of cork from the bark of an oak tree.

He observed the hollow box like structure similar to honey comb for which he coined the term cell'. 

 Unicellular Organisms :

  In these organisms, all the function like nutrition, respiration, excretion & reproduction are carried out by the single cell.

Example  Amoeba.

 Multicellular Organisms :

  Organisms which are made up of a few to billions of cells.

Example Plants & Animals.

 Shape of Cell :

  The Cells of animals & plants may be oval, spherical, cuboidal, columnar, polygonal or spindlelike. Neuron is the longest cell in animal body.

Figure : VARIOUS TYPES OF CELLS SHOWING DIFFERENT SHAPES

​ Size of Cell :

 The cell of animal & plants show variation in their size.

For example The largest living cell is egg of an ostrich which is about 170 mm in diameter. The smallest cell is PPLO which is 1micrometer small.

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 STRUCTURE OF CELL 

 Cell Membrane :

 Cell membrane is also called the plasma membrane. In an animal cell, it is the outermost layer, where as in a plant cell, it is protected by the cell wall. It is a living structure and controls the entry and exist of some substances in and out of the cell. It also protects the internal components of the cell. It gives shape to the cell.

 Cell Wall :

 In addition to the cell membrane, plant cells have an outer thick layer called cell wall. It is nonliving and it mainly composed of cellulose. It is protective in function and mainly determines the shape of the cell. It is absent in animal cells. 

 Cytoplasm :

 It is a transparent , jelly-like living substance which fills the interior of an animal and plant cell. It is present between the cell membrane and the nucleus. It contains a number of minute living structures known as the cell organelles and many non-living substances known as cell inclusions.

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 Nucleus :

 Nucleus is the most important part of a cell. It is usually spherical or oval in shape. It controls all the vital functions of the cell. It is made up of the nuclear membrane, nuceloplasm, nucleolus and chromosomes. The nuclear membrane surrounds the nucleus and separates it from the cytoplasm. It is permeable and controls the passage of materials through and from the nucleus.

 The nucleoplasm or nuclear sap makes up the body of the nucleus. It is deser than the cytoplasm.

 The nucleolus is a spherical body in the nucleus. It is composed of the nucleoprotein RNA (ribonucleic acid). It is responsible for protein synthesis.

 Nucleus also contains thread-like structures called chromosomes, which are composed of nucleoprotein DNA (deoxyribonucleic acid). The hereditary units of chromosomes are the genes. They are responsible for the transmission of characters from the prarents to the offspring. 

 Those organisms which do not have well defined nucleus and nuclear membrane are called prokaryotes. For example, bacteria, and some blue green algae.

 Those organisms which have well organized nucleus with the nuclear membrane are called eukaryotes. For example man, elephant, onion and cheek cells.

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 CELL ORGANELLES

These are active, living, permanent small structure present in cytoplasm & concerned with cell function.

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Vacuoles :

 The central part of most plant cells is occupied by a large vacuole. It is a sac like structure filled with fluid. Food, wastes pigments and other substances are dissolved in the fluid. Some plant cells have a number of large vacuoles.

Vaculoes are not so common in animal cells.

 When they occur, they are much smaller in size. 

 Plastids :

 These organelles are not present in animal cells. Chloroplasts (a type of plastid) contain the green pigment chlorophyll and are responsible for photosynthesis. Only green parts of plants have chloroplasts.

 There are two other types of plastids called chromoplasts and leucoplasts. Chromoplasts contain pigments which give fruits and flowers their colours. Leucoplasts store food and are found in the storage organs of plants.

​Figure : CHLOROPLAST

 Endoplasmic reticulum (ER) :

 The endoplasmic reticulum is a network of tubelike structures running through the cytoplasm. If ribosomes are attached to it, the reticulm is rough, otherwise it is smooth.

 Function It gives internal support to the colloidal matrix (cytoplasm).

 Rough endoplamic reticulum (RER) is associated with the synthesis of proteins.

 Ribosomes :

 Ribosomes are extremely Small, round bodies found either in the state in the cytoplasm or attached to the surface of the ER. They are composed of ribonucleoprotein (ribonucleic acid and protein).

 Functions The main function of ribosomes is to act as a platform or work place for the synthesis of proteins.

Figure : RIBOSOMES

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Mitochondria :

 Mitochondria are small, rod-shaped organelles found in large numbers. Each mitochondria is
bounded by two membranes-outer and inner. The outer membrane is smooth and the inner membrane is pushed inwards at intervals forming crests called cristae. The cristae lie in a ground substane called matrix. Mitochondria process enzymes necessary for the oxidation of carbohydrates. This process releases energy in the form of ATP. This is why mitochondria are known as the powerhouses of the cell. Mitochondria have their own DNA and ribosomes. They can synthesize their own proteins.

 Function: Mitochondria provide energy for the vital activities of living cells.

Figure : MITOCHONDRIA

  Golgi Body :

 These are small, hollow, plate-like or cup-shaped bodies found in animal cells. They synthesise, store and secrete enzymes and proteins. The Golgi apparatus in plants is known as dictyosome.

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 Lysosome :

 They are present in animal cells only. They contain enzymes for cellular digestion. If they burst, the cell may get damaged or destroyged. Hence, they are called the suicidal bags of the cell.



 Centrosome :

 It is a star-like structure found mostly in animal cells. It consists of cenrioles. It helps in cell division.


Cell Notes 2

Cell — Structure and Functions

• Cell: The smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.

• All organisms are made of smaller parts called organs.

• Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.

• Cells were first observed in cork by Robert Hooke in 1665.

• Cells exhibit variety of shapes and sizes.

• Number of cells also varies from organism to organism.

• Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.

• Some organisms are single-celled, while others contain large number of cells.

• The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.

• The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.

• Cell membrane: The basic component of a cell. The cytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall.

• Cytoplasm: The jelly-like substance present between the cell membrane and the nucleus. Various organelles present in the cytoplasm are:

(i) Mitochondria

(ii) Plastids (present only in plant cell)

(iii) Endoplasmic reticulum (ER)

(iv) Ribosome

(v) Lysosome

(vi) Vacuole

(vii) Golgi body

(viii) Centrosome (present only in animal cell).

• Nucleus: Nucleus is separated from cytoplasm by a nuclear membrane. It is generally spherical in the centre of the cell.

​• Nuclear membrane: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrance.

• Nucleolus: Nucleus contains a still smaller round body known as nucleoulus.

• Chromosomes: Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of character from the parents to the offsprings

• Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.

• Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall.

• Coloured bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts.

• Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.

• Size of cells: The size may be as small as a millionths of a metre or may be as large as a few centimetres. Size of the cells has no relation with the size of the body of the animal or plant. It is related to its function.

Cell Notes 3

​Class 8 Biology Cell Introduction

Introduction

Cell is the structural and functional unit of life. All living organisms are made of cells. Cells are similarto bricks that are assembled to make buildings. Similarly, Cells are assembledto make the body of every organism. Therefore, cells are the building blocks of an organism.

​Class 8 Biology Cell Discovery of cells

Discovery of cells

Robert Hook discovered cells in 1665. When he observed thin slices of cork (part of bark of a tree) under a self-designed microscope, he noticed that there are many partitioned boxes or compartments like a honeycomb. Each box was separated from the other by a wall. Hook coined the term “cell” for each box.

​Class 8 Biology Cell Cell Theory

Cell Theory

Mathias Schleiden and Theodor Schwann in 1838 postulated Cell Theory. The Cell Theory states that:

All living organisms are formed of one or more cells. New cells arise from pre-existing cells by cell division.

Cell is the fundamental unit of structure and function in living organism.

​Class 8 Biology Cell Number Of Cells The number of cells in an organism may vary

Number Of Cells

The number of cells in an organism may vary from one to millions and trillions. According to the number of cells, organisms are classified into Unicellular and Multicellular organisms.

Unicellular Organism (uni: One; cellular: cell): Organisms composed of one single cell. Single cell performs all the vital activities needed for sustaining life like growth, reproduction, locomotion, excretion etc. Ex: Amoeba, Paramecium, Chalmydomonas etc.

Multicellular Organisms (multi: many; cellular: cell): Organisms composed of more than one cell. The cells of these organisms usually perform specialized functions. Example: Plants, animals, etc.

​Class 8 Biology Cell Shape Of Cells

Shape Of Cells

The cell shows variety of shapes depending on the function they perform. Different shapes of cells are: Oval, round or spherical: Human RBC, Ovum.

Spindle shaped (long and pointed at both ends): Muscle cell.

Branched and elongated: Nerve Cells.

Kidney Shaped: Guard Cells in leaves.

Some cells keep on changing their shape.

Example: WBC in human, Amoeba (single-celled) keeps on changing its shape during locomotion through pseudopodia (false feet) and digestion of food vacuole.

The cell and its components are enclosed in a membrane called cell membrane. These membranes provide shape to the cells of plants and animals. Plant cells have an additional covering over membranes called cell wall. Cell wall gives shape and rigidity to the cells.

(a)Spherical Human RBC (b) Spindle shaped Muscle Cell (c) Long branched Nerve Cell (d) Amoeba

​Class 8 Biology Cell Size Of Cells

Size Of Cells

Cell sizes in living organism are measured in micrometer.

Most cells range from 5-15 micrometer.

Smallest Cell is 0.1 to 0.5 mm in bacteria.

Largest Cell is 170mm * 130mm is the egg of ostrich.

Longest Cells in humans are Nerve Cells (1-2m).

Size of cells is independent on the size of an organism but depends on its function.

Example: Nerve cell in both elephant and rat are long and branched. They both perform the same function of transferring messages.

​Class 8 Biology Cell Level Of Organization

Level Of Organization

In unicellular organism, single cell perform all the vital functions.It functionsindependently. Whereas, multicellular organism have various level of organization within them. Individual cells may perform a specific function or work together as a functional unit for the organism. The cells are dependent on one another.

There are 5 levels of organization in multicellular organisms from simplest to most complex. Cell: The fundamental unit of structure and function in living organism.

Example: Blood cells, Nerve cells, etc.

Tissue: Group of cells that have similar structure and function to perform specific activity forms a Tissue.

Example: Connective, epithelial, muscle, nerve tissue, etc.

Organ: Group of tissue, which are same at their structure and perform similar activity constitute an organ.

Example: Heart, Brain, Skin, etc.

Organ System: Many organs together form an organ system. Organs made up of group of tissues organize themselves to perform a specific activity for an organism.

Example: Circulatory system, Nervous System, etc.

Organism: Aggregation of Organ system constitutes organism. The human has 11 organ systems.

Therefore, the correct order for level of organization in multicellular organism is: Cells -> Tissues -> Organs -> Organ Systems -> Organisms

​Class 8 Biology Cell Parts Of the Cell

Parts Of the Cell

Cell is made up of basically 3 components: Cell Membrane, cytoplasm and nucleus.

Cell Membrane

The outer boundary of cell enclosing nucleus and cytoplasm is called cell membrane or plasma membrane.

The cell membrane separates cell and its components from the outside environment. Also, it separates cells from one another.

The cell membrane is porous to specific ions and organic molecules and controls the movement of substances in and out of the cells. Therefore, the cell membrane is selectively permeable.

It gives shape to the cell.

In plant cells, there is an additional thick layer surrounding cell membrane called cell wall. The cell wall gives rigidity to the cell. It protects the plant cells against different environmental conditions like high wind speed, atmospheric pressure, etc. We can observe cells in the leaf peel of Tradescantia, Elodea, Rhoeo or onion peel.

Cytoplasm

It is a jelly like fluid present between nucleus and cell membrane.

​Organelles like chloroplast, mitochondria, golgi apparatus, ribosomes, etc. are dispersed in cytoplasm.

Nucleus

Generally, it is spherical in shape and located in the center of the cell.

A membrane called nuclear membrane separates nucleus from the cytoplasm.

The nucleus is the control center as it regulates all the activities of the cell.

When nucleus is observed under microscope, a small spherical body in the nucleus was seen and called nucleolus. Also, it contains thread-like structures called chromosome. These chromosomes carry genetic material (genes) and help in inheritance/transfer of characters from parents to offspring.

The nuclear membrane is porous and allows the movement of materials between cytoplasm and nucleoplasm (Nucleolus and Chromosomes).

​Class 8 Biology Cell Prokaryotes and Eukaryotes

On the basis of presence of nuclear membrane, cells are classified into 2 categories as follows:

Prokaryotic Cells (pro: primitive; karyon: nucleus): The cellswithouta well-definednucleus and the nuclear material are in the form of nucleoid dispersed in the cytoplasm are called prokaryotic cells. These cells are present in primitive organisms called prokaryotes. Example: Bacteria and blue green algae.

Eukaryotic Cells (eu: true; karyon: nucleus): The cells withwell-defined nucleus and a distinct nuclear membrane are called eukaryotic cells. Except bacteria and blue green algae, all other organisms are called Eukaryotes.

​Class 8 Biology Cell Protoplasm

Protoplasm

It is the living substance of the cell.

It includes both cytoplasm and nucleus.

​Class 8 Biology Cell Vacuole

Vacuole

Vacuoles are membrane bound transparent organelle present in both animal and plant cell.

They are small and many in animal cells whereas large and one in most of the plant cells.

Vacuoles preserve the turgidity of the cell and also store waste material in them.

Vacuoles are occupied with water containing inorganic and organic molecules.

​Class 8 Biology Cell Plastids

Plastids

Plastids are mainly present only in plants and cyanobacteria.

They are several small colored bodies scattered in the cytoplasm of leaf cells.

There are 3 different types of plastids as follows:

Chloroplasts: Green colored plastids are called chloroplasts. The green color is due to the presence of pigment chlorophyll. Chlorophyll absorbs energy and is compulsory for the process of photosynthesis.

Chromoplasts: Colored Plastids, which provide colors to different parts of the plant (flowers and fruits) are called Chromoplasts. They manufacture and accumulate carotenoid pigments.

Leucoplasts: Colorless plastids occur in the storage part of the plant.

​Class 8 Biology Cell Comparison of plant and animal cell

Comparison of plant and animal cell

The differences and similarities between plant and animal cells are as follows:

Part of the Cell Plant Cell Animal Cell

Shape of the cell Quadrangular or hexagonal Round

Cell wall Distinct Cell Wall and protective in function Absent

Nucleus Present Present

Nuclear membrane Present Present

Cytoplasm Present Present

Plastids Help plants to synthesize and store their food Absent

Vacuole Mostly one and big in size Many and smaller in size

Cell Membrane Present Present

Lysosomes Absent or scanty Many

Cilia and flagella Absent Present

Centrioles Absent Present required for cell division​.