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UNIT 2

I.TEXT
THE CELL

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit
of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are
often called the "building blocks of life". The study of cells is called cell biology.

Plan/Plane/Plant

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such
as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell;
including bacteria) or multicellular (including plants and animals). While the number of cells in plants and
animals varies from species to species, humans contain more than 10 trillion (1013) cells. Most plant and animal
cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.

Cells are of two types, eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes
are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Prokaryotic cells were the first form of life on Earth, characterized by having vital biological
processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic cells,
and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the three domains of
life, bacteria and archaea. The DNA of a prokaryotic cell consists of a single chromosome that is in direct
contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the
smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.

Linear

A prokaryotic cell has three architectural regions:

 Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell
wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most
prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria)
and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to
the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell
wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also

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 prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to
a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
 Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of
inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extra chromosomal
DNA elements called plasmids, which are usually circular. Though not forming a nucleus, the DNA is
condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
 On the outside, flagella and pili project from the cell's surface. These are structures (not present in all
prokaryotes) made of proteins that facilitate movement and communication between cells.

Eukaryotic cells

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen
times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main
distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of
membrane-bound organelles (compartments) in which specific metabolic activities take place. Most important
among these is a cell nucleus, an organelle that houses the cell's DNA.

Part of cell

Membrane

The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In
animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually
covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment
and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and
partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane.
Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move
different molecules into and out of the cell. The membrane is semi-permeable, and selectively permeable, in
that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not
pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external
signaling molecules such as hormones.

Cytoskeleton

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps
during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells
after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton
is composed of microfilaments, intermediate filaments and microtubules. There are a great number of proteins

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associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The
prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and
cytokinesis.

Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in
a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication
and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double
membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various
molecules that could accidentally damage its structure or interfere with its processing. During
processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is
then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a
specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing
takes place in the cytoplasm.

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells
use DNA for their long-term information storage. The biological information contained in an organism
is encoded in its DNA sequence. RNA is used for information transport (e.g., mRNA) and enzymatic functions
(e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in
the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules
called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles
like mitochondria and chloroplasts.

Mitochondria and Chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that
occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Respiration occurs in the
cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release
energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Chloroplasts can only be
found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.

Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for
certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The

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ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the
smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release.

Golgi apparatus: The primary function of the Golgi apparatus is to process and package
the macromolecules such as proteins and lipids that are synthesized by the cell.

Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or
worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the
cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a
membrane-bound system.

Centrosome: the cytoskeleton organizer: The centrosome produces the microtubules of a cell – a key
component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are
composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle.
A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.

Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid
filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles,
which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are
usually larger than those of animal cells.

II. PRACTICE
1. Match the cell parts and their functions
Nucleus Nucleolus: produces ribosomes.
Nucleolus Nucleus directs all activities of the cell including
reproduction.
Vacuole Mitochondria: Most of the cell's energy is produced within
these rod-shaped organelles.
Golgi Complex Lysosome These small organelles contain chemicals that
break down food particles and worn out cell parts.
Cytoskeleton Vacuole : The sac within the cytoplasm stores water, food,
waste products, and other materials./ Some animal cells have
them vacuoles that store food, water, waste, and other
materials.
Chloroplast Chloroplast : These organelles capture energy from the
sunlight and use it to produce food for the cell.
Cytoplasm Ribosomes: These small structures function as factories to

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 produce proteins. They may be attached to the endoplasmic
reticulum, or they may be floating the cytoplasm.
Cell Membrane Cytoskeleton : helps to maintain the cell's shape.
Ribosomes Cytoplasm : includes a gel-like fluid in which many different
organelles are found.
Lysosome Golgi Complex : recieve materials from the endoplasmic
reticulum and send them to other parts of the cell. They also
release materials outside the cell.
Mitochondria Rough Endoplasmic Reticulum/Smooth Endoplasmic
Reticulum : This network of passageways carries materials
form one part of the cell to another.
Cell Wall Cell Membrane : forms a barrier between the cytoplasm and
the environment outside the cell.
Rough Endoplasmic Cell Wall : In a plant cell, a stiff wall surrounds the
Reticulum/Smooth membrane, giving the cell a rigid, boxlike shape.
Endoplasmic Reticulum

The prokaryotic cell/ prokaryote

Pilli/ Cilia:
Flagella: Roi
Capsule:
Cell wall:
Plasma membrane:
Cytoplasm:
Cytosol:
Nucleoid:
Chromosome:
Ribosome: small/large sub-unit
The Eukaryotic cell/ Eukaryote
Cell wall: xenlulose/hemixenlulose/pectin…
Flagella: roi
Plasma membrane:
Phospholipid bi-layers
Hydrophobic:
Hydrophilic:
Cytoplasm
Cytosol:
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 Organelle:
Nucleus: nuclear envelope/ nuclear membrane
Nucleolus
Chromosme
Nuclear matrix
Endoplasmic Reticulume (ER): Smooth ER, Rough ER
Golgi/ Golgi complex/Golgi apparatus/ Golgi body
Lyzosome/Peroxisome/Glyxisome
Ribosome: small subunit/ large subunit
Cytoskeleton: Micro filament/ micro tubule/ intermediate filament
Mitochondria: outer membrane/ inner membrane
Chloroplast: outer membrane/ inner membrane
Centrosome: centriole
Vacuole:
Respiration
Photosynthesis:
Cell division:
Mitotic
Mitosic
Meiosis:
Cell cycle:
DNA-DNA: Replication/ Relicate
DNA-RNA: Transcription
RNA-Protein: Translation