What is NEET ?
The National Eligibility cum Entrance Test (NEET) is a competitive examination in India for students aspiring to pursue undergraduate medical and dental courses (MBBS/BDS) in government or private medical colleges across the country. Here's a concise definition in 10 lines:
Fundamental Unit of life
Introduction to Cells:
Definition:
A cell is the smallest structural and functional unit of an
organism, often referred to as the "building block of life." Cells
are the basic units of life, and all living organisms are composed of one or
more cells. The cell is the smallest entity that can carry out all the
processes necessary for life.
Historical
development of the cell theory.
Cell Theory:
The cell theory is a fundamental principle in biology that
describes the basic properties of cells. The cell theory includes three main
postulates:
·
All living organisms are composed of one or more
cells.
·
The cell is the basic unit of structure and
organization in living organisms.
·
All cells come from pre-existing cells through
the process of cell division.
Types of Cells:
·
Prokaryotic
cells:
Prokaryotic cells are simple cells lacking a true nucleus
and membrane-bound organelles. The genetic material in prokaryotic cells is
typically found in the nucleoid region, a central area in the cell that is not
enclosed by a membrane. Bacteria and archaea are the two main groups of
organisms composed of prokaryotic cells. Here's an example of a prokaryotic
cell:
Example:
Escherichia coli (E. coli)
Type: Bacterium
Domain: Bacteria
Escherichia coli is a commonly studied bacterium and is
frequently used in laboratory research. While it is generally harmless and a
part of the normal gut microbiota in humans and other animals, certain strains
of E. coli can cause foodborne illnesses. Studying prokaryotic cells like those
of E. coli provides valuable insights into basic cellular processes and the
biology of microorganisms
·
Eukaryotic
cells
Eukaryotic cells are complex cells that have a true nucleus
and membrane-bound organelles. Eukaryotes include a wide variety of organisms,
such as plants, animals, fungi, and protists. Here's an example of a eukaryotic
cell:
Example: Human
Skin Cell
Type: Animal cell
(specifically, a eukaryotic human somatic cell)
Human skin cells, like many other animal cells, are
eukaryotic and possess these structures, contributing to the overall complexity
and specialization of the cell. Eukaryotic cells exhibit a higher level of
organization and compartmentalization compared to prokaryotic cells, allowing
them to perform a wide range of functions essential for the survival and
functioning of multicellular organisms.
Cell Structure and Components:
Cells have a defined structure
with various components, each playing a specific role in the overall
functioning of the cell. Here is an overview of the main cell structures and
components:
1. Cell Membrane (Plasma Membrane):
Structure: A semi-permeable lipid bilayer made up of phospholipids,
proteins, and carbohydrates.
Function: Regulates the passage of substances into and out of the
cell, maintaining internal balance.
2. Cytoplasm:
Structure: A gel-like substance filling the cell's interior.
Function: Supports cellular structures, provides a medium for
cellular activities, and facilitates the movement of organelles.
3. Nucleus:
Structure: Surrounded by a nuclear membrane and contains genetic
material (DNA).
Function:
Acts as the control center, governing cellular activities and containing the
instructions for protein synthesis and cell reproduction.
4. Endoplasmic Reticulum (ER):
Structure: A network of membranes.
Function:
Rough ER: Studded with ribosomes; involved in protein synthesis and
modification.
Smooth ER: Involved in lipid synthesis, detoxification, and calcium
storage.
5. Ribosomes:
Structure: Small, dense particles composed of RNA and proteins.
Function: Site of protein synthesis, where amino acids are
assembled into polypeptides.
6. Mitochondria:
Structure: Double-membraned organelles with inner folds (cristae).
Function: Powerhouse of the cell; site of cellular respiration, where
energy (ATP) is generated.
7. Lysosomes:
Structure: Membrane-bound vesicles containing digestive enzymes.
Function: Breaks down cellular waste, damaged organelles, and
foreign materials.
8. Cytoskeleton:
Components: Microfilaments, intermediate filaments, and
microtubules.
Function: Provides structural support, maintains cell shape, and
facilitates cell movement.
9. Vacuoles (Plant Cells have a large central vacuole):
Structure: Membrane-bound sacs.
Function: Storage of nutrients, waste products, and pigments;
provides turgor pressure in plant cells.
These components work together to
maintain the integrity and function of the cell, allowing it to carry out
essential processes for the survival and growth of the organism. The specific
structures and functions may vary between prokaryotic and eukaryotic cells and
among different cell types within multicellular organisms.
Comparison of
Plant and Animal Cells:
Plant and animal cells share many similarities, but there
are key structural differences that reflect their distinct functions and
adaptations to their environments. Here are the main structural differences
between plant and animal cells:
1. Cell Wall:
Plant Cells: Have
a rigid cell wall composed of cellulose outside the cell membrane. The cell
wall provides structural support and protection.
Animal Cells:
Lack a cell wall; the outer boundary is formed only by the cell membrane.
2. Shape:
Plant Cells:
Often have a fixed, regular shape due to the rigid cell wall.
Animal Cells: Can
be irregular in shape, and their structure is maintained by the cytoskeleton.
4. Vacuole:
Plant Cells:
Typically have a large central vacuole that stores water, nutrients, and waste
products. It also contributes to turgor pressure and structural support.
Animal Cells:
Have smaller vacuoles, which are more temporary structures and are involved in
storing and transporting substances.
5. Chloroplasts:
Plant Cells:
Contain chloroplasts, the site of photosynthesis, which contain the green
pigment chlorophyll.
Animal Cells:
Lack chloroplasts, as they do not carry out photosynthesis.
6. Plastids:
Plant Cells:
Besides chloroplasts, may contain other types of plastids like chromoplasts
(for pigment storage) and amyloplasts (for starch storage).
Animal Cells:
Lack plastids other than occasional small vesicles.
7. Lysosomes:
Plant Cells:
Lysosomes are rare and less prominent.
Animal Cells:
Typically have many lysosomes, which are involved in intracellular digestion.
8. Centrioles:
Plant Cells: Lack
centrioles in the centrosome (though exceptions exist in certain plant cells).
Animal Cells:
Contain centrioles in the centrosome, which are involved in organizing
microtubules during cell division
9. Motility:
Plant Cells:
Generally, plant cells are stationary and do not exhibit significant movement.
Animal Cells:
Many animal cells are mobile and can exhibit various forms of movement.
10. Size and Number:
Plant Cells: Tend
to be larger than animal cells.
Animal Cells:
Smaller and often more numerous within multicellular organisms.
11. Organization:
Plant Cells: Tend
to be organized into tissues, and multiple cells make up plant organs.
Animal Cells:
Also organized into tissues, but the overall structure of animals is often more
complex, with various organ systems.
Understanding these structural differences is essential for
comprehending the specialized functions of plant and animal cells and their contributions
to the overall biology of the respective organisms.
Cell Division:
Cell division is a fundamental biological process that
allows living organisms to grow, develop, repair damaged tissues, and
reproduce. It is a precisely regulated mechanism that ensures the accurate
distribution of genetic material (DNA) to daughter cells. Cell division occurs
in two main types: mitosis and meiosis.
1. Mitosis:
·
Definition:
Mitosis is the process of cell division in which a single cell divides into two
genetically identical daughter cells.
·
Purpose:
Mitosis is primarily involved in growth, development, and tissue repair. It
ensures that each daughter cell receives an identical set of chromosomes as the
parent cell.
·
Stages:
Mitosis consists of several stages, including prophase, metaphase, anaphase,
and telophase. During these stages, the cell undergoes changes in its nuclear
and cellular components, ultimately resulting in the formation of two identical
daughter cells.
2. Meiosis:
Definition:
Meiosis is a type of cell division that occurs in specialized cells called germ
cells (sperm and egg cells) and reduces the chromosome number by half,
resulting in the formation of four non-identical daughter cells.
·
Purpose:
Meiosis is crucial for sexual reproduction, introducing genetic diversity among
offspring by shuffling genetic material between homologous chromosomes.
·
Stages:
Meiosis involves two sequential divisions (meiosis I and meiosis II), each
consisting of prophase, metaphase, anaphase, and telophase. The process
generates four haploid cells (gametes) with unique genetic combinations.
Importance of
Cells:
1. Cellular
Organization:
Significance:
Cells organize into tissues, tissues into organs, and organs into organ
systems. This hierarchical organization enables specialized functions and
complexity.
Implication:
Cellular organization allows for the division of labor and the efficient
performance of specific tasks within an organism.
2. Metabolism:
Significance:
Cells carry out metabolic processes, including the conversion of nutrients into
energy, synthesis of molecules, and the breakdown of waste products.
Implication:
Metabolism is essential for sustaining life, providing energy for cellular
activities, and maintaining homeostasis.
3. Growth and Development:
Significance:
Cells divide and differentiate to promote growth and development in organisms.
Implication: The
increase in cell number and specialization of cell types contribute to the
growth of an organism and the formation of distinct tissues and structures.
4. Reproduction:
Significance:
Cells undergo division for reproduction, either through mitosis (asexual
reproduction) or meiosis (sexual reproduction).
Implication:
Reproduction ensures the continuity of life, passing genetic information from
one generation to the next.
5. Response to
Stimuli:
Significance:
Cells can respond to external and internal stimuli, adapting to changes in
their environment.
Implication:
Responsiveness allows organisms to interact with their surroundings, seek resources,
and respond to challenges for survival.
6. Homeostasis:
Significance:
Cells contribute to the maintenance of internal stability through homeostasis,
regulating internal conditions such as temperature, pH, and nutrient levels.
Implication:
Homeostasis is critical for the proper functioning of cellular processes and
overall organismal health.
7. Communication:
Significance:
Cells communicate with each other through chemical signals, allowing for
coordinated responses and the integration of cellular activities.
Implication:
Cellular communication is essential for the proper functioning of tissues,
organs, and organ systems.
8. Adaptation and
Evolution:
Significance:
Cells can undergo genetic changes, contributing to adaptation and evolution
over time.
Implication:
Cellular mechanisms such as mutation and natural selection drive the diversity
and adaptability of living organisms.
9. Genetic
Information:
Significance:
Cells contain genetic material (DNA) that carries instructions for the
synthesis of proteins and the regulation of cellular activities.
Implication:
Genetic information is passed on during reproduction, influencing the traits
and characteristics of offspring.
Understanding the significance of cells is foundational to
comprehending the biological principles that govern life. The study of cells
provides insights into the complexity, diversity, and interconnectedness of
living organisms across various levels of biological organization.
The fundamental unit of life chapter serves as a
foundational unit for further studies in biology. It lays the groundwork for
understanding the complexity of living organisms and their various functions at
the cellular level. Students often perform practical experiments to observe
cells under a microscope and gain hands-on experience with the concepts covered
in the chapter.
