Fundamental Unit Of Life

CELL

Robert Hooke Examined a thin slice of cork and saw that it resembled the structure of a honeycomb consisting of many little compartments through a self-designed microscope  in year 1665. Cork is a substance which comes from the bark of a tree.  He called these boxes cells.

Cell is a Latin word for ‘a little room’.

The fundamental organisational unit of life and the smallest functional unit  is the cell.

A cell can independently perform all necessary activities to sustain life. Hence cell is the basic unit of life.

CELL THEORY:

The theory that all plants and animals are composed of cells is given by schleiden and schwann.

• Organisms can be unicellular and multicellular
• Cells can also be categorized as plant cell and animal cell.

Unicellular:  a single cell may constitute a whole organism as in Amoeba,Chlamydomonas, Paramoecium and bacteria. These organisms are called unicellular organisms (uni = single).

Multicellular :many cells group together in a single body and assume different functions in it to form various body parts in multicellular organisms (multi = many) such as some fungi, plants and animals.

• Every multi-cellular organism has come from a single cell,Cells divide to
  produce cells of their own kind. All cells thus come from pre-existing cells.

Each living cell has the capacity to perform certain basic functions that are characteristic of all living form,each such cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, such as making new material in the cell, clearing up the waste material from the cell and so on, These organelles together constitute the basic unit called the cell.

Lets discuss each in detail;

Three  features in almost every cell; plasma membrane, nucleus and  cytoplasm are common.All activities inside the cell and interactions of the cell with its environment are possible due to these features.

PLASMA MEMBRANE OR CELL  MEMBRANE

• is the outermost covering of the cell that separates the contents of the cell from its external environment
• allows or permits the entry and exit of some materials in and out of the cell so the cell membrane is called a selectively permeable membrane.
•  The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
• we can observe the structure of the plasma membrane only through an electron microscope.

Some substances like CO₂ or O₂ gases can move across the cell membrane by a process called diffusion.

The movement of water molecules (liquid) through such a selectively permeable membrane is called osmosis. Osmosis is the passage ot water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

• If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain water by osmosis. Such a solution is known as hypotonic solution.
•  If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as isotonic solution.
• If the medium has a lower water concentration then the cell will lose water by osmosis. Such a solution is known as hypertonic solution.

Properties

• The  cell membrane is flexible as it is made up of lipids and proteins.
• The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis.

Function

• It permits the entry and exit of some materials in and out of the cell.
• Prevents the  movement of other materials not required.

CELL WALL

• It is  another rigid outer covering of cell in addition to plasma membrane.
• The cell wall lies outside the plasma membrane.
• Only plant cells have cell wall and animal cells don’t.
• The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural
  strength to plants.
• When a living plant cell loses water
  through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis

Function of cell wall

• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting.
• Cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.
• Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.

NUCLEUS

• Also called Brain of cell.

Composition

• The nucleus has a double-layered covering called nuclear membrane.
• The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, i.e., to the cytoplasm.
• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.

Chromosomes

• Chromosomes contain information for inheritance of features from parents to next generation in form of DNA [Deoxyribo Nucleic Acid) molecules.
• Chromosomes are composed of DNA and protein.
• DNA molecules contain the information necessary for constructing and organising cells.
• Functional segments of DNA are called genes.
• In a cell which is not dividing, this DNA is present as part of chromatin material.
• Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to
  divide, the chromatin material gets organised into chromosomes.

Function

• The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.
• It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

Nucleoid

In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear
membrane containing only nucleic acids is called a nucleoid.

Organisms, whose cells lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary; karyote ≈ karyon= nucleus).

Organisms with cells having a nuclear membrane are called eukaryotes.

Prokaryotes	Eukaryotes
Cell lack Nuclear Membrane	Cells have nuclear membrane
Nucleolus is absent	Nucleolus is Present
Single chromosomes	Single or Multichromosomes.
Asexual Reproduction	Sexual or Asexual Reproduction
Always unicellular	Often multicellular
Membrane Bound Cell Organelles are Absent	Cell organelles are present
Centriole is absent	Centriole is present only in animal cells
Cell division is through binary fission	Cell division is through mitosis or meiosis
eg: nacteria, blue green algae etc	eg; plant cell, Animal cell. fungi etc

CYTOPLASM

• The cytoplasm is the fluid content inside the plasma membrane.
• It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.

Function

• It helps in exchange of materials between cell Organelles.
• It acts as a source of vital chemicals  such as amino acid, glucose, vitamin , iron etc.
• It acts as site of certain metabolic pathways like glycolysis.

Cell Organelles

Every cell has a membrane around it to keep its content separate from the external environment.

The different components of cell perform different function and these components are called cell organelles.

(i) ENDOPLASMIC RETICULUM (ER)

• ER is a large network of membrane-bound tubes and sheets.
• It looks like long tubules or round or oblong bags.
• ER membrane is similar to structure of plasma membrane.

There are two types of ER

• Rough endoplasmic reticulum [RER]
• Smooth endoplasmic reticulum [SER].

functions

RER-It has particles called ribosomes attached to its surface. The ribosomes are the sites of protein manufacture.

SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.

In the liver cells of the group of animals called vertebrates , SER plays a crucial role in detoxifying many poisons and drugs.

The one function of ER is to serve as channels for the transport of materials between various regions of the cytoplasm or between the cytoplasm and the nucleus.

The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

(ii) GOLGI APPARATUS

• 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.

Function

•  The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
• Its functions include the storage, modification and packaging of products in vesicles.
• In some cases, complex sugars may be made from simple sugars in the Golgi apparatus.
• The Golgi apparatus is also involved in the formation of lysosomes.

(iii) LYSOSOMES

• Lysosomes are a kind of waste dispatch and disposal system of the cell.
• Lysosome help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
• Lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.

Function

• Foreign materials entering the cells such as bacteria or food, as well as old organelles, end up in the lysosome, which break them up into small pieces.
• They are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.
• Under abnormal condition, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore they are also known as “suicide bags”.

(iv) MITOCHONDRIA

• Mitochondria are known as powerhouses of the cell.

Structure

• Mitochondria have two membrane coverings .
• The outer membrane is very porous while the inner membrane is deeply folded.
• These folds create a large surface area for ATP-generating chemical reactions.
• Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

Function

• The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP[Adenosine Triphosphate] molecules .
• ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical
  work.

(V) PLASTIDS

• Plastids are present only in plant cells.

There are two types of plastids   chromoplasts  and leucoplasts

• Chromoplasts are the coloured plastids present in leaves, flowers and fruits.
•  Plastids containing the pigment chlorophyll are known as chloroplasts. They are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.
•  Leucoplasts are found primarily in organelles in which materials such as starch, oils and protein granules are stored.
• The internal organisation of the plastids consists of numerous membrane layers embedded in a material called stroma.
• Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

(vi) VACUOLES

•  Vacuoles are storage sacs for solid or liquid contents.
•  They are small-sized in animal cells while plant cells have very large vacuoles [50% to 90% cell volume].
•  In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
• Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.
• In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has
  consumed.
• In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

1)Mitochondria are sites of

(a) Aerobic respiration

(b) Krebs cycle of aerobic respiration

(c) Glycolysis of aerobic respiration

(d) Anaerobic respiration

Ans- (a)

2)  The history of the cell began in 1665 with the publication of Micrographia in London by

(a) Robert Hooke

(b) Robert Brown

(c) Straburger

(d) Dujardin

Ans- (a)

. 3)Which of the following organelles is smallest in size?
a. Ribosome
b. Mitochondrial
c. Chloroplast
d. Lysosome

Ans- (a)

4)3. Which of the following organelle is not present in an animal cell?
a. Ribosome
b. Plastid
c. Mitochondrial
d. Nucleus

Ans- (b)

5). Plastid that is colourless is
a. Chromoplast
b. Leucoplast
c. Chloroplast
d. Lysosome

Ans- (b)

6)Plant cell wall is mainly composed of
a. Cellulose
b. Lipid
c. Protein
d. Sugar

Ans- (a)

7)Cellular respiration is related to ______ as __________ is related to chloroplasts.
a. Mitochondria, Light
b. Mitochondria, Photosynthesis
c. Chloroplast, Light
d. Chloroplast, Photosynthesis

Ans- (b)

8) A cell “X” contains a cell wall, large central vacuole and a nucleus at the periphery. The cell “X” is _______.
a. Plant cell
b. Animal cell
c. Bacterial cell
d. Prokaryotic cell

Ans- (a)