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THE FUNDAMENTAL UNIT OF LIFE

 THE FUNDAMENTAL UNIT OF LIFE

 CELL: A cell is the structural and functional unit of all living organisms.

Discoveries about Cells – The Fundamental Unit of Life

Robert Hooke

1665

noticed the presence of dead cells in a cork slice

Leeuwenhoek

1674

found the presence of living cells in the pond water

 Unicellular Organisms – The organisms that consist of a single cell such as Amoeba.

  • Multicellular Organisms – The organisms which contain various cells that perform different functions in the organism such as plants fungi and animals

 The Shape of the Cell

  • The shape of the cell may vary depending upon the type of function they perform in an organism.
  • Cells are capable of changing their shape. For example, the white blood cells and amoeba can change shapes on their own.

 STRUCTURE OF CELL

1. The Nucleus: Nucleus is a prominent organelle present in the cell which is the controlling centre of all activities of the cell.

  • A nucleus has a nuclear membrane that covers it all around.
  • There are pores present on the nuclear membrane that allow the movement of substances in and out of the nucleus.
  • There are chromosomes, rod-shaped structures present in the nucleus which contain genetic information.

The chromosomes contain two types of things -

1. DNA - This is responsible for organising and constructing new cells

2. Proteins - These help in the packaging and condensation of DNA.

Nucleolus: It is called the Brain of the Nucleus. It comprises 25% of the volume of the nucleus. It consists of proteins and ribonucleic acids (RNA). It helps in the formation of ribosomes which help in the formation of proteins inside the cell.

Nucleoid: Sometimes cells do not have a well-defined nucleus because they lack a nuclear membrane. Such a nucleus with no definite nuclear boundaries is called a Nucleoid.

Prokaryotes: Organisms whose cells do not have a definite cell membrane are called Prokaryotes.

Eukaryotes: Organisms whose cells contain a well-defined nuclear membrane are called Eukaryotes.

 

Differences between prokaryotes and eukaryotes

Prokaryotes

Eukaryotes

There is no presence of nucleus

The nucleus exists in the cells

A single chromosome is present  

There are multiple chromosomes

They undergo asexual reproduction

They undergo sexual as well as a sexual reproduction

They are generally unicellular organisms

They are generally multicellular organisms

There are no membrane-bound cell organelles

There are membrane-bound cell organelles present inside the cells

Example – Bacteria, Blue-green algae (Cyanobacteria)

Example – Fungi, Plants and Animals

 2. Cytoplasm

  • The plasma membrane has a fluid-like substance in it which is called the cytoplasm.
  • The cytoplasm contains several organelles that can perform distinct functions of the cell

 Functions of Cytoplasm

  • It supports and suspends the cell organelles and molecules.
  • The cellular processes occur in the cytoplasm such as the formation of proteins.
  • It allows the movement of substances in the cell such as hormones.
  • It dissolves cellular wastes.

 3. Plasma Membrane

  • It is just like an envelope that covers the whole cell. Therefore, a cell gets separated from the external environment because it has a plasma membrane.
  • The plasma membrane can decide which materials should enter or leave the cell and which should not. That is why it is also called a ‘Selectively Permeable Membrane’.

QUE. How can substances move in and out of a cell?

Gaseous Exchange between the Cell and its External Environment –

  • Movement of Oxygen and Carbon dioxide to and from the cell is carried out through diffusion.
  • Gaseous substances tend to move to areas where their concentration is less from the areas where there is higher. This movement is defined as the process of diffusion. Diffusion can take the place of solids, liquids, and gases.

CELL ORGANELLS

The cells perform several functions. The organelles are useful because they allow the separation of different functions that are being performed by the cell.

Organelles which carry out important activities in a Cell – 

1. Endoplasmic Reticulum

2. Golgi Apparatus

3. Lysosomes

4. Mitochondria

5. Plastids

6. Vacuoles

1. Endoplasmic Reticulum (ER): The structure of the ER is quite similar to that of the plasma membrane. It is a network-like structure that consists of membrane-bound tubes and sheets.

Functions of ER

       I.          Transportation of material between different parts of the cytoplasm and also between the nucleus and cytoplasm

     II.          Folding of proteins which are synthesised by ribosomes on RER.

   III.          Detoxifying poisons and drugs out of the cell is the function of SER.

Two types of ER –

·        Rough ER: Rough ER contains ribosomes that are responsible for the manufacturing of proteins in the cells. They give a rough texture to the cell.

·        Smooth ER: The smooth ER manufactures fats or lipids in the cell which allow the functioning of the cell.

QUE. What are the functions of lipids and proteins?

       I.          Proteins and lipids synthesised on ER are used for making cell membranes. The process is known as Membrane Biogenesis.

     II.          Proteins can act as an enzyme

   III.          Both protein and lipids can act as hormones

2. Golgi Apparatus: It contains vesicles that are arranged parallel in stacks. These stacks are called Cisterns. These vesicles have their own membranes. These membranes are sometimes connected to those of the ER. 

Camillo Golgi discovered the Golgi Apparatus.

Functions of Golgi Apparatus

       I.          Golgi apparatus carries materials synthesised by the ER to different parts of the cell. The material is stored and packaged in vesicles.

     II.          Formation of complex sugar

   III.          Formation of lysosomes.

 3. Lysosomes: They are single-membrane vesicles that are responsible for cleaning the cell. They can digest any foreign material such as food or bacteria and even the worn-out cell organelles.

QUE. How can lysosomes digest any foreign material that enters the cell?

Lysosomes are capable of doing so because they have digestive enzymes in them. These enzymes break the materials and digest them. These enzymes are synthesised by RER and packaged into lysosomes by Golgi bodies.

QUE. Why lysosomes are called ‘suicide bags’?

If the cell’s own material gets damaged or dead, there are chances that lysosomes burst out, thus digesting its own cell.

 4. Mitochondria:  It is a double membrane organelle that has its own DNA and that is why often called ‘Semi-Autonomous Organelle’

The cell requires energy in order to carry out several activities. This energy is generated by mitochondria which are often called the ‘Powerhouse’ of the Cell. Mitochondria are the site of cellular respiration. They use oxygen from the air to oxidise the carbohydrates and thereby release energy.

QUE. What are the energy currencies of a cell?

The Mitochondria generates ATP (Adenosine Triphosphate) which are energy giving molecules of the cell that are often called their ‘Energy Currency’.

 5. Plastids: Just like mitochondria, Plastids are also double membraned organelles that have their own DNA and ribosome.

Plastids exist in plant cells only. Depending upon the type of function they play in the cell they can be classified as –

Types of Plastids

Chromoplast

Leucoplast

Coloured in nature, contain a pigment called chlorophyll

Colourless in nature

Cause photosynthesis in plants

Act as storage spaces of the cells

Contain orange and yellow pigments

Contain starch, proteins and oil

Can further be divided into Chloroplasts

Can further be divided into amyloplast, elaioplast and proteinoplast or aleuroplast.

Chloroplasts

  • Chloroplasts are cell organelles that conduct photosynthesis in plants.
  • Chloroplast is derived from two Greek words Chloro and Plasts which means green and plants respectively.
  • Chloroplasts contain photosynthetic pigments called ‘Chlorophyll’ along with lipids, carbohydrates, minerals, DNA, RNA, grana, thylakoids and stroma.
  • The main functions of chloroplasts are:

a.     Conducting photosynthesis in plants.

b.     Protein synthesis

c.     Releases oxygen

d.     Storage of Starch

 6. Vacuoles: Vacuoles are the places where cells can store liquids and solids. They are present in both plants and animals but the plant vacuoles are bigger in size than the animal vacuoles.

Plant Cell Vacuoles

Animal Cell Vacuoles

Plant cell vacuoles store all the material that is required for the plant to stay alive such as water

Animal cell vacuoles contain food items in unicellular organisms

Plant vacuoles maintain the turgidity of the plant cell

Animal vacuoles can also expel water and waste out of the cell

Plant cells generally contain a single large vacuole

Animal cell contains several small vacuoles

Plant vacuoles are present in the centre of the cell

Animal vacuoles are scattered throughout the cell

 Movement of Water between the Cell and its External Environment

It is carried out by the means of osmosis. Osmosis is a process in which water moves from the region of high concentration to one where its concentration is low through a semipermeable membrane. Therefore, we can say that Osmosis is just a special case of the process of diffusion.

  

1.     Hypotonic Solutions

  • If the concentration of water outside the cell is higher than the concentration of water inside the cell gains water by the process of osmosis.
  • Water can move into the cell from the cell membrane. In the case of hypotonic solutions, more water enters the cells which result in swelling of the cells. 

2.     Isotonic Solutions

  • If the cells are put in an environment that has a similar concentration of water as present inside. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side.
  • Therefore, the size of the cell does not vary in an isotonic solution because there is no net movement of water.

3.     Hypertonic Solutions

  • If the cells are kept in an environment that has a lower concentration of water than what is present inside the cells then due to the process of osmosis water moves out of the cells.

This results in a decrease in the size of the cells (they shrink) as more water comes out of the cell. 

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