Cell – Structure and Functions

(1) All living beings are made up of organs. These organs are made up of small units which are called cells.

Cells are the basic complex structural units of the living organisms.

Robert Hooke invented the cell when he was observing slices of cork under the microscope in 1665. He noticed compartments or boxes which looks like honey comb. He called each box cell. But, it took 150 years to know more about the cell after Robert Cook.

Different organisms show different number of cells, different shape and size of cells. Scientists observed it by using stains (Dye) to colour the parts of the cell for study.

(i) Cell number

(a) The number of cell in an organism is one, is called unicellular cell or single cell.

Eg:- Amoeba, paramecium

schoolhelpbygunjan.wordpress.com | Amoeba | Diagram | NCERT | Class 8

(b) The number of cell in an organism is greater than one is called multicellular cell. The number of cells runs into billions to trillions in many organisms.

Eg:- Human body has trillions of cells which varies in shape and size.

These singled cell organisms performs all functions which multicellular organism can perform like capturing food, digesting food, excretes, grow and reproduces. The only difference between both is functions in the multicellular organisms are performed by the different group of the cells or specialised cells forming different tissues and all functions are performed by one cell in single – celled organisms.

(ii) Shape of cells –

(a) Unicellular has no definite shape. They keep on changing their shape. The different lengths projections are protruding out of their body are called pseudopodia which are very helpful in moving and capturing their food. A white blood cell is single cell in human blood. It can change its shape. But a white blood cell is a cell and amoeba is an organism.

(b) Multicellular organism has definite shape but their cells have different shapes. Generally cells are round, spherical or elongated.

Eg:- Spherical red blood cells

schoolhelpbygunjan.wordpress.com | Spherical red blood cells of humans | NCERT | Class 8

Some cells are long and pointed at both ends.

Eg:- Spindle shaped muscle cells

schoolhelpbygunjan.wordpress.com | Spindle shaped muscle cells

Some cells are like branch of a tree. These cells receive and transfers messages. Thus, control and coordinate the working of different parts of the body.

Eg: – Nerve cells or a neuron

schoolhelpbygunjan.wordpress.com | Long branched nerve cell | Diagram | Cell - structure and functions | NCERT | Class 8

Cells are enclosed in a membrane and it is surrounded by a cell wall. Cell wall gives shapes and rigidity to these cells.

(iii) Size of the cell –

Most of the cells are very small in size and can be visible with the help of microscope. The smallest cell is 0.1 to 0.5 micrometre in bacteria and the largest cell measuring 170mm  130 mm, is the egg of the ostrich.

The size of the cell does not depend on the size of the body of organism. The size of the cell is related to its function. The nerve cell will be long and branched in small and big organisms and will perform same function.

(2) Cell structure and functions

A group of similar cells performing a specific function in each organ is called tissues.

Organ is made up of tissues  and tissues are made up of cell.

(3) Parts of the cell

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(i) The boundary of a cell is known as cell membrane which separates one cell from another and also separates from the surrounding medium. It has nucleus and cytoplasm in it. The cell membrane is also called plasma membrane.

The plasma membrane is flexible, porous, made up of organic molecules (lipids and proteins) and allows the movement of substance or material both inward and outward of the cell. Therefore it is called semipermeable membrane. But, it also prevents the movement of some other materials. Therefore, cell membrane is also called selectively permeable membrane.

The plasma membrane can be seen only by electronic microscope.

Process by which substances move inward and outward from the cell –

(a) Some substances can move across the cell membrane by diffusion.

Eg:-

(A) CO2 is a cellular waste and requires to be excreted out by the cell. The CO2 is accumulated in high concentration in side the cell. The concentration of the CO2 is very low outside the cell. Therefore, Co2 moves out from the cell by process of diffusion.

(B) Oxygen enters in to the cell by the diffusion because the concentration of the oxygen is high outside the cell.

(b) Osmosis is another process by which substances can move across the cell membrane. Osmosis is special case of diffusion through a selectively permeable membrane.

Eg:- Water movement across the plasma membrane is also affected by the amount of substances dissolved in water. Water moved towards high concentration of substance from the low concentration of the substances till equilibrium reached.

Different situations if an animal cell or plant cell into a solution of salt or sugar are tabulated below –

SN Condition Solution Result
1. Outside solution is less concentrated (or solute is high concentrated) than solution of inside cell Hypotonic solution More water will come inside the cell than will leave. Therefore, cell will swell up.
2. Outside solution is equally concentrated (or solute is equally concentrated) as solution of inside cell Isotonic Solution No overall movement of water. The cell will be same in size.
3. Outside solution is higher concentrated (or solute is less concentrated) than solution of inside cell Hypertonic solution Less water will come inside the cell than will leave. Therefore, cell squeezes or shrinks.

Eg:- Put raisins in plain water. The low concentrated water will move inside the raisins and raisins swells up.

Now put raisins in sugar solution. The low concentrated water from cell will move outside towards high concentrated sugar solution. As a result raisin will shrink.

(c) The process in which flexibility of the cell membrane enables the cell to engulf in food and other material from external environment is called endocytosis.

Eg:- Amoeba acquires its food through endocytosis.

Uses of osmosis in animal or plant cell-

(a) Unicellular freshwater organisms gain water through osmosis.

(b) Plant cells received water from roots through osmosis.

Uses of diffusion in animal or plant cell-

(a) The cell can exchange water and gases through diffusion.

(b) Different molecules of nutrition transport nutrition by moving in and out of the cell through diffusion.

Cell membrane is covered with a thick covering which is called cell wall. It is very helpful in plants. It protects the plant cell from the variation in temperature, high wind speed, atmospheric moisture etc. The plants exposed to the atmospheric changes because they cannot move. The plant cell wall is mainly made up of complex substance cellulose which provides plant strength.

The process in which cell shrinkages because of a living plant loses water through osmosis, is called plasmolysis.

(ii) A jelly like structure found in between nucleus and cell membrane which has many organelles of cells like mitochondria, golgi bodies, ribosomes etc. is called cytoplasm.

 (iii) The round body in the centre of the cell which acts as control center of the activities of the cell is called nucleus of the cell.

A membrane which is porous and separates cytoplasm and nucleus is called nuclear membrane. It allows the movement of the materials between the cytoplasm and the inside of the nucleus. The cells having well organised nucleus with a nuclear membrane are called as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes.

The cells having nuclear material without nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes.

Eg:- Bacteria and blue green algae.

An irregularly shaped and undefined nuclear region within the cell of prokaryotes is called nucleoid.

schoolhelpbygunjan.wordpress.com | The fundamental unit of life | Cell | Prokaryotic cell | NCERT | Class 9

Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.

Parts of the nucleus

(a) A smaller spherical body inside the nucleus is called nucleolus.

(b) The thread like structures of nucleus which carry genes (Functional segments of DNA) and can be seen only when cell divides are called chromosomes. Chromosomes are composed of DNA and protein.

Functions of nucleus-

(a) Gene (Functional segments of DNA) is a unit of inheritance in living organisms. It was carry by chromosomes of living organism. Genes carry the characteristics of their parents which they transfer to their offspring.

Eg:- Colour of eye, hair, habits etc.

(b) It acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. Protoplasm is called the living substance of the cell.

(c) The nucleus plays an important role in cellular reproduction. A single cell divides and forms two new cells.

(iv) Organelle is a small structure lies in the cell and performs functions of the cell. Organ is a collection of tissues which makes large part of body and performs functions for that part.

Cell organelles are enclosed by the membrane.

(a) In prokaryotes cell organelles are also absent.

(b) In eukaryotic cell organelles are also present.

Organelle cells are necessary to keep chemical activities to support complicated structure of the multicellular organisms (eg:- human)

Eg:- Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.

Eg:- endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.

(a) Endoplasmic Reticulum (ER) –

(A) It is a large network of membrane bound tubes and sheets.

(B) It looks like long tubules or round or oblong bags (vesicles).

(C) The ER membrane is similar in structure to the plasma membrane.

(D) The ER varies greatly in appearance in different cells, it always forms a network system.

Types of Endoplasmic Reticulum (ER)

Rough Endoplasmic Reticulum (RER)-

Ribosomes are attached to the surface of the endoplasmic reticulum. Therefore, it looks rough when we see it under microscope. Ribosomes are protein builders of the cell. Then this manufactured protein is sent to various places sing ER.

Smooth Endoplasmic Reticulum (SER)–

(A) SER looks smooth under microscope.

(B) They help in manufacturing of fat molecules or lipids. These are important for cell function.

(C) Proteins and lipids help in building the cell membrane. The process is known as biogenesis.

(D)Some other proteins and lipids function as enzymes and hormones.

Functions of the ER-

(a) It serves as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and nucleus.

(b) It also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

(c) It also works as detoxifying many poisons and drugs.

(b) Golgi Apparatus –

It is described by Camilio Golgi.

It consists of membrane bound vesicles (Flattened sacs) arranged approximately parallel to each other in stacks called cisterns.

These membranes constitute another portion of a complex cellular membrane system because they are connected to ER Membrane.

Functions of the Golgi apparatus-

(A) The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through Golgi apparatus.

(B) In some cases complex sugars may be made from the simple sugars in the Golgi apparatus.

(C) It is also involved in the formation of lysosomes.

(c) Lysosomes –

(A) Lysosomes are membrane – bound sacs filled with digestive enzymes that are made by RER.

(B) These are a kind of waste disposal system of the cell.

(C) Lysosomes are called ”suicide bags” because strong enzymes in it digest their own cell during the disturbance in cellular metabolism.

Eg:- If cell damaged and lysosomes burst, then enzymes digest their own cell.

Functions of the lysosomes –

The functions of lysosomes are to keep cell clean by digesting any foreign material (Food or bacteria) as well as worn – out cell organelles. Lysosomes break the complex substances into simpler substances with the help of powerful enzymes that are filled in lysosomes.

(d) Mitochondria –

Mitochondria are also known as “Power house” of the cell.

It has two membrane covering.

(A) Outer membrane is porous.

(B) Inner membrane is deeply folded. These folds increase surface area of for ATP – generating chemical reactions. ATP (Adenosine triphosphate) molecules are the form of energy released by mitochondria needed for various chemical activities necessary for life.

ATP is known as “energy currency” for the cell. The stored ATP energy is used in making new compounds and for mechanical work.

Mitochondria are able to make some of their own proteins because they have their own DNA and ribosomes. Therefore, mitochondria are considered as strange organelles.

(e) Plastids-

The small coloured bodies scattered in the cytoplasm of the cell of leaf is called plastids. They are of different in colours

It presents only in plants.

Plastids have their own DNA and ribosomes like mitochondria.

Types of plastids-

Chromoplasts –

The plastids that have colour are known as chromoplasts.

(A) The chromoplasts containing the pigments chlorophyll are known as chloroplasts.

(B) These chloroplasts are important for photosynthesis in plants.

(C) Chloroplasts also contain various yellow or orange pigments.

(D) The chloroplasts consist of numerous membrane layers embedded in a material called the stroma. Storma have similar external structure like mitochondria.

Leucoplasts-

The plastids that have no colour or white are known as leucoplasts.

These are primarily organelles in which materials such as starch, oils and protein granule are stored.

(f) Vacuoles –

Vacuoles find in plants and animals.

These are storage sacs for solid or liquid contents.

Vacuoles in animals-

(A) These are small in size in animal cells.

(B) In single celled organisms like Amoeba, the food vacuoles contain the food items an Amoeba has consumed.

(C) Vacuoles expellees excess water and some wastes from the cell in some unicellular organisms.

Vacuoles in plants-

Vacuole is a space within the cytoplasm of the cell but surrounded by a membranes and filled with fluid.

(A) These are very large in plants cell. The central vacuoles of some plant cells may occupy 50 – 90% of the cell volume.

(B) Vacuoles are full of cell sap.

(C) Vacuoles provide turgidity and rigidity to the cell.

(D) All necessary substances like sugar, some proteins, various organic acids and amino acids are preset in vacuoles.

schoolhelpbygunjan.wordpress.com | Cell | Plant cell | Diagram | NCERT | Class 9schoolhelpbygunjan.wordpress.com | Cell | Animal cell | Diagram | NCERT | Class 9Comparison of the Plant and animal cell

SN Part Plant Cell Animal Cell
1. Cell membrane Present Present
2. Cell wall Present Absent
3. Nucleus Present Present
4. Nuclear Membrane Present Present
5. Cytoplasm Present Present
6. Plastids Present Absent
7. Vacuole Present Present

(5) The cell is a fundamental structural unit of living organisms.
The organisation of cells membrane and organelles acquire its structure and ability to function. Therefore, cell has basic structural organisation. This helps the cell to perform functions like respiration, obtaining nutrition and clearing of waste material, or forming new proteins.

(6) The process by which new cells are made is called cell division.

Types of cell division-

Mitosis

The process of cell division by which most of the cells divide for growth is called mitosis.

(A) Each cell called mother cell which divides to form two identical daughter cells.

(B) The daughter cells have same number of chromosomes as mother cell. The chromosomes help in growth and repair of tissues in organisms.

schoolhelpbygunjan.wordpress.com | Cell | Mitosis | Diagram | NCERT | Class 9

 

Meiosis

Meiosis is a process of cell division by two consecutive divisions, produces four new daughter cells, which have half the number of chromosomes than that of mother cell.

schoolhelpbygunjan.wordpress.com | Cell | Meiosis | Diagram | NCERT | Class 9

 

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