What is Cell?
Cell is the basic Structural and functional unit of living organisms.
In other words, cells make up living things and carry out activities that
keep a living thing alive.
Cell Theory
Cell theory is a collection of ideas. And conclusions from many different
scientists over time that describes cells and how cells operate.
1. All known living things are made up of one or more cells.
2. All living cells arise from pre-existing cells by division.
3. The cell is the basic unit of structure and function in all living
organisms.
Shape of Cells
1. Cells vary in shape.
2. Variation depends mainly upon the function of cells.
3. Some cells like Euglena and Amoeba can change their shape, but
most cells have a fixed shape.
Human RBCs are circular biconcave for easy passage through
human capillaries.
Nerve cells are branched to conduct impulses from one point to
another.
Human WBCs can change their shape to engulf the microorganisms
that enter the body.
Structure Of Cell
The detailed structure of a cell has been studied under compound
microscope and electron microscope.
Certain structures can be seen only under an electron microscope.
The structure of a cell as seen under an electron microscope is
called ultrastructure.
If we study a cell under a microscope, we would come across three
features in almost every cell: plasma membrane, nucleus and
cytoplasm,
All activities inside the cell and interactions of the cell with its
environment are possible due to these features.
Plasma Membrane
Extremely delicate, thin, elastic, living and semi-permeable
membrane
Made up of two layers of lipid molecules
In which protein molecules are floating Thickness varies from 75-
110 A Can be observed under an electron microscope only
Functions:
Maintains shape & size of the cell
Protects internal contents of the cell
Regulates entry and exit of substances in and out of the cell
Maintains homeostasis
Nucleus
Dense spherical body located near the centre of the cell
Diameter varies from 10-25 µm
Present in all the cells except red blood cells and sieve tube cells
Well developed in plant and animal cells
Undeveloped in bacteria and blue-green algae (cyanobacteria)
Most of the cells are uninucleated (having only one nucleus)
Few types of cells have more than one nucleus (skeletal muscle
cells)
Nucleus has a double layered covering called nuclear membrane
Nuclear membrane has pores of diameter about 80-100 mm
Colourless dense sap present inside the nucleus known as
nucleoplasm
Nucleoplasm contains round shaped nucleolus and network of
chromatin fibres
Fibres are composed of deoxyribonucleic acid (DNA) and protein
histone
These fibres condense to form chromosomes during cell division
Chromosomes contain stretches of DNA called genes
Genes transfer the hereditary information from one generation to
the next
Functions:
Control all the cell activities like metabolism, protein synthesis,
growth and cell division
Nucleolus synthesizes ribonucleic acid (RNA) to constitute ribosomes
Store hereditary information in genes
Cytoplasm
Jelly-like material formed by 80% of water
Present between the plasma membrane and the nucleus
Contains a clear liquid portion called cytosol and various particles
Particles are proteins, carbohydrates, nucleic acids, lipids and
inorganic ions
Also contains many organelles with distinct structure and function
Some of these organelles are visible only under an electron
microscope
Granular and dense in animal cells and thin in plant cells
Endoplasmic Reticulum
Network of tubular and vesicular structures which are
interconnected with one another
Some parts are connected to the nuclear membrane, while others
are connected to the cell membrane
Two types: smooth(lacks ribosomes) and rough(studded with
ribosomes)
Functions:
Gives internal support to the cytoplasm
RER synthesize secretory proteins and membrane proteins
SER synthesize lipids for cell membrane
In liver cells SER detoxify drugs & poisons
In muscle cells SER store calcium ions
Golgi Body
Discovered by Camillo Golgi
Formed by stacks of 5-8 membranous sacs
Sacs are usually flattened and are called the cisternae
Has two ends: cis face situated near the endoplasmic reticulum and
trans face situated near the cell membrane
Functions:
Modifies, sorts and packs materials synthesized in the cell
Delivers synthesized materials to various targets inside the cell and
outside the cell
Produces vacuoles and secretory vesicles
Forms plasma membrane and lysosomes
Lysosomes
Small, spherical, single membrane sac
Found throughout the cytoplasm
Filled with hydrolytic enzymes
Occur in most animal cells and in few type of plant cells
Functions:
Help in digesting of large molecules
Protect cell by destroying foreign invaders like bacteria and viruses
Degradation of worn out organelles
In dead cells perform autolysis
Vacuoles
Single membrane sac filled with liquid or sap (water, sugar and lons)
In animal cells, vacuoles are temporary, small in size and few in
number
In plant cells, vacuoles are large and more.
In number
May be contractile or non-contractile
Functions:
Store various substances including waste products
Maintain osmotic pressure of the cell
Store food particles in amoeba cells
Provide turgidity and rigidity to plant cells
Mitochondria
Small, rod shaped organelles bounded by two membranes inner and
outer
Outer membrane is smooth and encloses the contents of
mitochondria
Inner membrane is folded in the form of shelf like inward projections
called cristae
Inner cavity is filled with matrix which contains many enzymes
Contain their own DNA which are responsible for many enzymatic
actions
Functions:
Synthesize energy rich compound ATP
ATP molecules provide energy for the vital activities of living cells
Plastids
Plastids are double membrane-bound organelles found inside plants
and some algae.
They are responsible for activities related to making and storing
food.
They often contain different types of pigments that can change the
colour of the cell.
Chromoplasts
Chromoplasts are plastids that produce and store pigments. They are
responsible for different colours found in leaves, fruits, flowers and
vegetables.
Leucoplasts
Leucoplasts are colourless plastids that store foods. They are found in
storage organs such as fruits, tubers and seeds.
Choloroplast
Double membrane-bound organelles found mainly in plant cells
Usually spherical or discoidal in shape
Shows two distinct regions-grana and stroma
Grana are stacks of thylakoids (membrane- bound, flattened discs)
Thylakoids contain chlorophyll molecules which are responsible for
photosynthesis
Stroma is a colourless dense fluid
Functions:
Convert light energy into chemical energy in the form of food
Provide green colour to leaves, stems and vegetables
Centrosome
Centrosome is the membrane bound organelle present near the
nucleus
Consists of two structures called centrioles
Centrioles are hollow, cylindrical structures made of microtubules
Centrioles are arranged at right angles to each other
Functions:
Form spindle fibres which help in the movement of chromosomes
during cell division
Help in the formation of cilia and flagella
Cytoskeleton
Formed by microtubules and microfilaments
Microtubules are hollow tubules made up of protein called tubulin
Microfilaments are rod shaped thin filaments made up of protein
called actin
Functions:
Determine the shape of the cell
Give structural strength to the cell
Responsible for cellular movements
Prokaryotic cell
1. Nucleus is undeveloped
2. Only one chromosome is present
3. Membrane bound organelles are absent
4. Size ranges from 0.5-5 µm
5. Examples: Bacteria and blue green algae
Eukaryotic cell
1. Nucleus is well developed
2. More than one chromosomes are present
3. Membrane bound organelles are present
4. Size ranges from 5-100 µm
5. Examples: All other organisms
CELL THEORY
Cells are the basic units of living organisms.
The cell theory states that:
All living things are made of one or more cells.
Cells are the basic unit of structure and function in living things.
All cells come from other cells.
CELL TYPES
Two categories:
1. Cell that have
membrane-bound organelles
Called Eukaryotic Cells
2. Cells that do not have membrane-bound organelles
called prokaryotic cells
Unicellular organisms such as bacteria
are examples of prokaryotes.
Cell Types
Eukaryotic cells-
Cells that contain organelles which are
held together by membranes
Examples include plant and animal cells.
Eukaryotic Cell Structure
The plasma membrane/cell
membrane
the flexible boundary of a
cell
separates a cell from its
surroundings
Plasma Membrane/Cell Membrane continued:
allows nutrients to enter the cell and
waste to be removed
This is referred to as selective permeability.
*(Selective=Chooses, Permeability=filter
through)*
keeping a healthy balance of
nutrients and water within the cell
is called homeostasis
Overview of Organelles
Nucleus-
Largest organelle in the cell and it is the most inner compartment
of the cell
contains chromatin (DNA); genetic information on strands called
chromosomes
“control center” for cell metabolism and reproduction
Chromatin- Directions on how to make proteins
Nucleolus- Found inside nucleus; ribosomes are made here
Ribosomes- make proteins (made up of RNA and protein);
thought of as “factories”
Cytoplasm- clear gel like fluid inside the cell, which suspends
all organelles
Endoplasmic Reticulum- extensive network of
membranes
Rough ER: with ribosomes
Smooth ER: with no visible ribosomes
Golgi
Apparatus-
Lysosomes- sorts
organelles thatproteins made by
are filled the ribosomes and
with
sends them to needed places in the cell
digestive enzymes to remove waste and invading
bacteria
Mitochondria- often referred to as the
“powerhouse” of the cell
release energy for the cell
It converts the energy stored in glucose into ATP for the cell
Vacuoles- fluid filled organelles enclosed by a
membrane
Eukaryotic plant cell
Plant cells are also
Eukaryotic cells, but
plant cells contain
some organelles that
are not found in
animal cells.
Plant Cell Organelles
Cell wall- rigid wall outside the plasma membrane.
It provides the cell with extra support.
Chloroplasts- captures light and energy; and
converts it into chemical energy.
Chlorophyll- green pigment found inside the
chloroplast.
Plastids- organelles that store things such as food in
the plant cell.
