CHAPTER 2: CELLS
CELL STRUCTURES
 All organisms are made of cells. Cells are the fundamental
units of life.
 CELLS are the basic building blocks of all living things.
The human body is composed of trillions of cells. They
provide structure for the body, take in nutrients from food,
convert those nutrients into energy, and carry out
specialized functions.
 Cells contain the body’s hereditary material and can make
copies of themselves.
 Cells provide six main functions. They provide structure
and support, facilitate growth through mitosis, allow
passive and active transport, produce energy, create
metabolic reactions and aid in reproduction.
i.
Provide Structure and Support
Like a classroom is made of bricks, every organism is
made of cells. While some cells such as the
collenchyma and sclerenchyma are specifically
meant for structural support, all cells generally
provide the structural basis of all organisms. For
instance, skin is made up of a number of skin cells.
Vascular plants have evolved a special tissue called
xylem, which is made of cells that provide structural
support.
ii.
Facilitate Growth Through Mitosis
In complex organisms, tissues grow by simple
multiplication of cells. This takes place through the
process of mitosis in which the parent cell breaks
down to form two daughter cells identical to
it. Mitosis is also the process through which simpler
organisms reproduce and give rise to new organisms.
iii.
Allow Passive and Active Transport
Cells import nutrients to use in the various chemical
processes that go on inside them. These processes
produce waste which a cell needs to get rid of. Small
molecules such as oxygen, carbon dioxide and
ethanol get across the cell membrane through
the process of simple diffusion. This is regulated with
a concentration gradient across the cell membrane.
This is known as passive transport. However, larger
molecules, such as proteins and polysaccharides, go
in and out of a cell through the process of active
transport in which the cell uses vesicles to excrete or
absorb larger molecules.
 Some of the smallest organisms, such as yeast and
bacteria, are single-celled organisms (Prokarytic), but
most plants and animals are multicellular (Eukaryotic).
 The two major types of cells are: Prokaryotic cells and
Eukaryotic cells. The most important distinction
between these 2 cells is considered to be the most
important distinction among groups of organisms.
o
o
Prokaryotes are single-celled (unicellular) organisms
(e.g. Bacteria and Archaebacteria) that lack a
membrane-bound nucleus, mitochondria, or any
other membrane-bound organelle.
Eukaryotes are organisms (Animals, Plants, Fungi and
Protists) whose cells (multicellular) have a nucleus
enclosed within membranes, unlike prokaryotes,
which have no membrane-bound organelles.
 Prokaryotes were the only form of life on Earth for
millions of years until more complicated eukaryotic cells
came into being through the process of evolution.
iv.
Produce Energy
 An organism's survival depends upon the thousands of
chemical reactions that cells carry out relentlessly. For
these reactions, cells require energy. Most plants get this
energy through the process of photosynthesis, whereas
animals get their energy through a mechanism called
respiration.
 Cells have many parts, each with a different function. Some
of these parts, called organelles, are specialized structures
that perform certain tasks within the cell. Human cells
contain the following major parts:
o
NUCLEUS

It is a circular structure that contains DNA (deoxyribonucleic
acid) in the form of chromosomes, the cell’s hereditary
(genetic information) material. The nucleus is found in the
cytoplasm.
o
o

This helps the cell make the right sorts of proteins. The
information is kept on the chromosomes, which are
inherited from the organisms’ parents. The
chromosomes are made of DNA.
o

Mitochondria are the ‘POWERHOUSE OF THE CELL’, found
in almost all cells, except those of Prokaryotes
(Prokaryotes are organisms made up of cells that lack a cell
nucleus or any membrane-encased organelles, e.g. Bacteria –
Tuberculosis (TB) and Cholera).
Chromosomes are very long, but extremely thin that
they cannot be seen even by using an electron
microscope. However, when the cell is dividing, they
become short and thick, and can be seen with a good
light microscope.
o
Inside them, oxygen is used to release energy from
glucose, in the process called aerobic respiration.
o
Cells that use a lot of energy have a lot of
mitochondria. Examples:
 Muscle cells are tightly packed with mitochondria.
 Sperm cells, which need energy to swim to the egg
 Nerve cells (Neurones), which need energy to
transmit impulses
Serves as the cell’s command/control center, which controls
cell division, cell development and cell activities.
CYTOPLASM

The black spots in the electron microscope image
(Figure 2.8) are granules of a carbohydrate called
Glycogen. This is similar to starch, as starch is NEVER
found in animal cells – they store Glycogen instead.
Glycogen is a reserve of fuel. When required, it can be
broken down to Glucose, to be used as a fuel by the
mitochondria in the liver cell, or transported in the
blood to other cells that need it.
Within cells, the cytoplasm is made up of a jelly-like fluid
(called the cytosol) which contains particles (especially
proteins) and other structures (organelles) that surround the
nucleus.



Many different metabolic reactions (the chemical reactions
of life) take place in the cytoplasm.
Cytoplasm is nearly all water (70% is water in many cells)
The cell membrane (cell surface membrane) is a thin
semi-permeable membrane (outer lining of the cell –
made of protein and fat) that surrounds the cytoplasm.

Cytoplasm is contained by the cell membrane

All cells have a cell membrane (also called cell surface
membrane) around the outside.
o Very difficult to see cell membrane in a plant cell
because it is right against the cell wall.

Its function is to separate the cell from its environment
and allows materials to enter and leave the cell.
o Cell membrane is important because it controls what
enters and leaves the cell.
o It protects the integrity of the interior of the cell by
allowing certain substances into the cell while keeping
other substances out.

CELL MEMBRANE (CELL SURFACE MEMBRANE)
MITOCHONDRIA



Mitochondria (singular: Mitochondrion) are complex
organelles that convert energy from food into a form that
the cell can use.
Mitochondria are tiny structures that are almost impossible
to see with a light microscope, thus electronic micrographs
are used.
They have their own genetic material, separate from the
DNA in the nucleus, and can make copies of themselves.

The main difference between cell wall and cell
membrane is, cell membrane is present in all cells but cell
wall is only present in plants, bacteria, fungi and algae. It is
not present in the Protozoa and animal cell.. Cell wall (fully
permeable) covers the cell membrane of plants
plants, which is a
partially permeable (semi-permeable) membrane
membrane.
RIBOSOMES


Ribosomes are a cell structure that makes protein
(Proteinsythesis - Proteinss are made by joining small
molecules called amino acids together. This process is
called protein synthesis and happens in small structures in
the cytoplasm of all cells called ribosomes.
o Ribosomes are found in all types of cells – bacteria,
protoctists, fungi, plants and animals all have
ribosomes in their cells.
o Protein is needed for many cell functions such as
repairing damage or directing chemical processes
processes.
o Ribosomes can be found floating within the cytoplasm
or attached to the endoplasmic reticulum
reticulum.
Ribosomes are organelles that process the cell’s genetic
instructions to create proteins.
o Ribosomes can float freely in the cytoplasm or be
connected to the endoplasmic reticulum
reticulum.
 Endoplasmic reticulum (ER) is a type of organelle
found in eukaryotic cells (An organism
whose cells contain a nucleus surrounded by a membrane
and whose DNA is bound together by proteins (histones)
into chromosomes) that forms an interconnected
network of flattened, membrane-enclosed
enclosed sacs or
tube-like
like structures known as cisternae
cisternae.
 The membranes of the ER are continuous with the
outer nuclear membrane, flattened disks organelles
that help process molecules created by the cell.
 The endoplasmic reticulum also transports these
molecules to their specific destinations either inside
or outside the cell.
 Rough Endoplasmic Reticulum is the laye
layer that
surrounds the nucleus and has little black dots
(Ribosomes) attached on to it.
 Smooth Endoplasmic Reticulum is the layer that is
found away from the nucleus, closer to the cell
membrane and does not have black dots
(Ribosomes) attached to them.

Ribosomes are even tinier structures/organelles than
mitochondria and can only be seen with an electron
microscope (Figure 2.10).
o Ribosomes look like tiny dots attached to a network of
membranes that run throughout the cytoplasm
cytoplasm. This
network is called the rough
ugh endoplasmic reticulum.
Plant cells contain all the Animal cells
mentioned and defined above, but also have 3
additional parts:
1. Cell wall
2. Chloroplast, and
3. Vacuole
CELL WALL

A cell wall is a tough, fully permeable,
permeable outermost layer
of cells in plants, bacteria, fungi, and many algae that gives
shape to the cell and protects it from infection and
bursting when the cell takes in too much water.
water

Cell wall is made out of cellulose, determines tissue
texture, and often is crucial to cell function.
o Cellulose belongs to a group of substances called
Polysaccharides.
o Cellulose forms fibres which criss-cross
criss
over one
another to form a very strong covering to the cell.
The above is a picture of cellulose fibres of a plant cell wall
CHLOROPLAST


Chloroplasts are tiny plant powerhouses that capture light
energy to produce the starches and sugars that fuel plant
growth.
They are found inside plant cells (within the cytoplasm) in
plant leaves and in green and red algae as well as in
cyanobacteria.
o Chloroplasts are only found in the parts of the plant
that are capable of Photosynthesis.
o The majority of chloroplasts are found in the lea
leaves of
the plant because these structures have the greatest
surface area for absorption.. The outer part of a plant
stem may also contain chloroplasts.

Chloroplasts allow plants to produce the complex
chemicals necessary for life from simple, inorganic
substances
tances such as carbon dioxide, water and minerals.

Chloroplasts contain the molecule chlorophyll
chlorophyll, which are
responsible for absorbing sunlight for photosynthesis
photosynthesis.

Chloroplasts often contain starch grains,, which have been
made by photosynthesis.

Animal cells never contain starch grains.. Some animal cells,
however do have granules (tiny grains) of another
substance similar to starch grains, called glycogen (found in
the cytoplasm).
Table shows some feature comparison of Plant Cells and
Animal Cells:
CELL TYPE
A vacuole is membrane-bound
bound organelle in the cytoplasm of
most cells, especially plant cells, containing water and
dissolved substances such as salts, sugars, enzymes, and
amino acids.
o
Plant cells have very large vacuoles,, which contain a
solution of sugars and other substances,, called CELL SAP.
o
Animal cells have much smaller
maller membrane
membrane-bound
spaces, called VESICLES, which contain food or water
water.
A full vacuole presses outwards on the rest of the cell, and helps
to keep the cell in shape.
(have STARCH
GRAINS)
GRAINS
(never have starch
grain, but
sometimes
GLYCOGEN
GRANULES)
(have larger
vacuoles
containing CELL
SAP)
SAP
(have smaler
bound spaces
containing food
and water, called
VESICLES)
REGULAR SHAPE
IRREGULAR SHAPE
CYTOPLASM
RIBOSIMES
MITOCHONDRIA
ROUGH
ENDOPLASMIC
RETICULUM (ER)
SMOOTH
ENDOPLASMIC
RETICULUM (ER)
CELL MEMBRANE
CELLULOSE CELL
WALL
CHLOROPLAST
(with
CHLOROPHYLL)
CELL SHAPE

ANIMALS
NUCLEUS
VACUOLE
VACUOLE
PLANTS
MICROSCOPES
CELLS AND ORGANISMS


One of the complexities of life as a multicellular
organism is that the trillions of bits and pieces that make
up your body must somehow all work together to
accomplish the basic functions that keep you alive.
i.
Biologists refer to the relationship between cells,
tissues and organs as the levels of organization of
the human body.


In the human body, cells are the basic units of life. Groups of
cells working together for a specific function form tissues.
Organs are two or more tissues operating together. Even
separate organs work together, forming body systems.
CELLS FORM TISSUES
Groups of cells organized together for a specific function
form tissues.


There are four basic types of tissue in the human body:
i.
Epithelial - Epithelial tissue covers the exterior of
the body as well as the linings of the organs and
cavities of the body,
ii.
Muscle - Muscle tissue contains cells that are
sometimes called “excitable” because they are
able to contract and enable movement
iii.
Nerve - Nerve tissue conducts electrical impulses
and send signals through the body
iv.
Connective - Connective tissue holds the body
together and includes both bones and blood.
EXAMPLE: Stomach Lining is a tissue, a layer of cells
lining your stomach. These cells make enzymes to help
digest your food.


The stomach also has other tissues. For example, the layer
of muscle in the stomach wall, made of cells which can
move.
o This muscle tissue makes the walls of the stomach
move in and out, churning the food and mixing it up
with the enzymes.
The stomach also has other tissues. For example, the layer
of muscle in the stomach wall, made of cells which can
move.
o This muscle tissue makes the walls of the stomach
move in and out, churning the food and mixing it up
with the enzymes.
In plant anatomy, tissues are categorized broadly into
three tissue systems: the epidermis, the ground tissue, and
the vascular tissue.
Epidermis tissue - Cells forming the outer surface of the
leaves and of the young plant body. E.g. epidermis
tissue from an onion bulb, Palisade tissue – a layer of
cells in a leaf in which the cells are specialized to carry
out photosynthesis
Vascular tissue - a term referring to the parts of plants
that convey water and nutrients from one part of the
organism to another (Xylem and Phloem). Xylem
tissue consists of a variety of specialized, waterconducting cells known as tracheary elements.
TISSUES FORM ORGANS
An organ is two or more tissues that come together to
form a single unit with a unique structure and function.

The heart, for example, is an organ that contains all four
types of tissue (Epithelial, Muscle, Nerve and Connective
tissues) to accomplish its very important task.

There are 78 organs in the human body, including five
organs considered vital for life.

There are 5 vital organs considered vital for life: brain,
heart, lungs, kidney, and liver.

The largest human organ is the skin, which can weigh
around 20 pounds.
Of course, the levels of organization of the human body don’t
stop with the organs. Individual organs work together in nine
major organ systems. And, at the very top of the ladder, all
those systems, organs, tissues and cells come together to
form an organism: you!

There are 4 organs in the Plant: the leaf, stem, root, and
reproductive structures.
ORGANS FORM ORGAN SYSTEMS
An organ system is a group of organs that work
together as a biological system to perform one or more
functions. Each organ does a particular job in the body,
and is made up of distinct tissues.

There are 11 major organ systems in the human body.
The most important systems are:
o
Nervous System is the main command system that
directs function for all other systems. The nervous
system sends signals throughout the body to control
function and movement. It is composed of the brain,
spinal cord and peripheral nervous system. It directs
quick responses to stimuli, such as automatic
reflexes. The nervous system works in conjunction
with the endocrine system to control metabolism
and other body functions.

o
Endocrine System uses chemical messengers. It
secretes hormones into blood and other body fluids.
Water balance, body growth and responses to stress
are some of the activities regulated by the endocrine
system. Glands that secrete hormones include the
pituitary, thyroid, adrenal, pancreas and
hypothalamus.
o
Cardiovascular system (Circulatory system) is
occasionally referred to as the circulatory system. It
contains the heart, blood vessels and blood. The
blood transports nutrients, hormones, gases and
waste products by using the blood vessels. The heart
pumps the blood throughout the body and maintains
blood pressure. Arteries pump blood away from the
heart, and veins return blood toward the heart.
o
Respiratory system contains the nasal cavities,
throat areas and lungs. The pharynx is shared with
the digestive
stive tract. Air moves from the pharynx to
the larynx, which protects the opening to the
trachea. The trachea is the main passageway to the
lungs. It acts as an air filter. Inside the lungs, oxygen
is extracted from the air, and carbon dioxide is
exhaled as a waste product.
o
Digestive system food is absorbed and processed by
the body. After being swallowed through the mouth,
food moves through the esophagus and into the
stomach.. The stomach breaks down the food
mechanically and chemically so it can be digest
digested by
the small intestine and used for nutrition. Any
undigested material is then moved through the large
intestine and excreted through the anus. The liver is
also considered part of the digestive system. It
releases bile to aid in digestion.
A plant has two organ systems: 1) the shoot system, and 2)
the root system:
o
Shoot system is above ground and includes the
organs such as leaves, buds, stems, flowers (if the
plant has any), and fruits (if the plantt has any)
any).
o
Root system includes those parts of the plant below
ground, such as the roots, tubers, and rhizomes.
ORGANS SYSTEMS FORM ORGANISMS
The way in which organisms are built up can be
summarized in the diagram below:
Figure above: From an atom to the entire Earth, biology
examines all aspects of life.
RELATING SPECIALISED CELL STRUCTURE TO
THEIR FUNCTIONS (ADAPTATION)
ROOT HAIR CELLS

Plants absorb water from the soil by osmosis. They
absorb mineral ions by active transport, against the
concentration gradient. Root hair cells are adapted for
taking up water and mineral ions by having a large
surface area to increase/maximise
/maximise the rate of absorption.

Root hair cells are found on the root of a plant are
a on the
edge of the root and a little way up from the root tip.

CILIATED CELLS

Ciliated epithelial cells are covered with cilia, which
are produced by the cytoplasm, and are tiny hairs that
push dust, mucus, bacteria, or hair up the trachea to the
back of the throat where they can be swallowed
swallowed.
o

o
They are long epidermal cells
o
They don’t live for very long and are replaced by new
ones as the root grows.
Water moves into the root hair via osmosis. The cytoplasm
and cell sap inside the root hair is quite concentrated with
mineral ions which enters the root hair by active transport.
o
The water is absorbed into the root by the root hairs
and taken to the Xylem vessels to be transported to
other parts f the plant.
Ciliated cells are designed to stop damage to the
lungs and other organs.
Ciliated cells are found in epithelial surfaces
(Epithelial tissues line the outer surfaces of organs and blood
vessels throughout
hout the body, as well as the inner surfaces of
cavities in many internal organs) of the Respiratory system in
such structures as the trachea, bronchial tubes, and nasal
cavities.
o
It often occurs in the vicinity of mucus-secreting
secreting
goblet cells – mucus trapss the dust and bacteria and
moved away by the mucus.

Goblet cell is a column-shaped
shaped cell found in the
respiratory and intestinal tracts, which secretes the
main component of mucus.

Cilia move rhythmically; they beat constantly,
moving waste materials such as dust, mucus, and
bacteria upward through the airways, away from
the lungs and toward the mouth.
XYLEM VESSELS
The xylem transports water and minerals from the roots up
the plant stem and into the leaves.
leaves In a mature flowering
plant or tree, most of the cells that make up
the xylem are specialised cells called vessels.

Xylem vessel have no cytoplasm or nuclei.

Their walls are made of cellulose and lignin.
o
Lignin is very strong substance, so the xylem vessels
help keep plants upright.

Wood is made almost entirely of lignified xylem
vessels.
PALISADE MESOPHYLL CELLS
The palisade mesophyll cells are plant cells located on the
leaves, the upper part of the leaf, right below the epidermis
and cuticle (in the palisade mesophyll layer) and is where
most of the photosynthesis takes place.


Palisade cells contain a lot of chloroplasts, which contains
chlorophyll, to help them perform photosynthesis.
o
The many chloroplasts circulate round the palisade cell,
which maximizes the absorption of light.
o
Moving chloroplasts helps to avoid shadowing effect.
Palisade cells are closely packed together to maximize light
absorption.
o

Palisade cells have large vacuole which pushes the
chloroplasts and cytoplasm to the edge of the cell.
o

Xylem is located in roots, stems and leaves of the plant
and it transports water and minerals from plant roots all
the way to the leaves.
o
With phloem it forms vascular bundles. Dead cells
in Xylem contribute to wooden parts of the plant.

Xylem vessel is like a long drain pine (see above Figure 8.3)
made up of many hollow, dead cells joined end to end (see
Figure 8.2 below).

They don’t live for very long and disappear over time (as
the plant grows and replaces new ones) to become a
hollow structure/vessels.
o
As the content of the cells die, it creates the hollow
vessel that allows transport of water and nutrients to
the rest of the plant all the way to the leaves.
Palisade cells are long and thin, arranged side by side
near the top surface of the leaf
This exposes the chloroplasts more to enable them to
absorb more light energy
NERVE CELLS (NEURONE)
RED BLOOD CELLS
The human Nervous system is made of special cells
called neurones or nerve cells (Figure 13.2 below is a
particular of neurone called a motor neurone).
Blood is a complex liquid tissue. It transports oxygen,
dissolved substances and heat around the body. It is also
involved in the body’s immune response.
Nerve cells (Neurones) carry (conduct) impulses
(messages) from one part of the body to another. Neurones
need to be able to rapidly transmit electrical impulses
along their length. They also need to exchange signals with
neighbouring neurones across the gaps, known as
synapses, that separate them.
Red blood cells transport oxygen for aerobic respiration
(respiration that requires oxygen). They must be able to
absorb oxygen in the lungs, pass through narrow blood
vessels, and release oxygen to respiring cells.

Nerve cells/Neurones contain the same basic parts as any
animal cell – Nucleus, Cytoplasm, and a Cell Membrane.
However, their structure is specifically adapted to be able
to carry messages very quickly.

To achieve these functions, nerve cells have several
adaptations.
o
The axon, the long, thin part of the neuron along
which the nerve impulse passes, is covered in a fatty
myelin sheath, which acts as an electrical insulator.

This increases the speed of transmission by
forcing the impulse to jump between gaps in the
sheath, known as Nodes of Ranvier, rather than
passing along the full length of the axon.
o
Nerve cells also have a lot of mitochondria, which
provide the energy to synthesise neurotransmitters
such as acetylcholine that pass messages across
synpases.
o
In addition, numerous fine/shorter fibre extensions to
the cell body at one end of the cell, known as
dendrites or dendrons, provide a high surface
area allowing the nerve cell to form synapses with
many others. Similar branching occurs at the other
end of the cell, and synaptic knobs at the end of
branches store neurotransmitter, ready to diffuse
across the synapse as soon as an impluse arrives.
Red blood cells have adaptations that make them suitable for
this:
Adaptations for efficient diffusion of oxygen

Red blood cells have very thin cell membranes – this lets
oxygen diffuse through quickly.
o

The cells themselves are thin, so there is only a short
distance for the oxygen to diffuse to reach the centre
of the cell.
The biconcave shape provides a large surface
area compared to the volume of the red blood cell, allowing
diffusion to happen efficiently.
Adaptations for efficient carriage of oxygen

Red blood cells do not contain a nucleus so they can
contain more haemoglobin.
o
Oxygen combines with haemoglobin to form
oxyhaemoglobin:
o
Oxygen binds reversibly to oxygen.
SPERM CELLS
Humans typically reproduce through sexual reproduction.
Sexual reproduction produces offspring that are unique
individuals. Half of their genes come from each parent.
In sexual reproduction, a male gamete and a female gamete
join together. This is fertilisation.
A gamete is a mature haploid male or female germ cell
which is able to unite with another of the opposite sex in
sexual reproduction to form a zygote.
o
o
Gametes are the sex cells.
Gametes have adaptations to increase the chances of
fertilisation and successful development of an embryo.

The male gametes, called spermatozoa or sperm, are
made in two testes (singular testis). These are outside the
body, in two sacs of skin called the scrotum.

Sperm cells have these adaptations:
o
sperm cells are produced in large numbers to increase
the chance of fertilisation.
o
a tail to move them towards an egg cell
o
many mitochondria to provide energy
o
an acrosome (part of the tip of the head) that releases
enzymes to digest the egg membrane.
o
Sperm cell also has a nucleus, which contains the
genetic material (23 chromosomes) for fertilization.
EGG CELLS
The ovary produces egg cells which are the female gametes.
The main function of egg cells is to join with
male cells (sperm) for the purpose of reproduction.
Adapted to have:
o
The egg cell is large in size because it stores
nutrients.

o
has cytoplasm which has york to stores energy
(Figure 17.6)
The egg cell nucleus consists of half the number of
chromosomes (23) that are found in female
body cells.
o Walls are thin to allow the sperm to penetrate.
VIDEOS & SUPPLEMENT MATERIAL

CELLS STRUCTURE FOR DUMMIES
https://youtu.be/URUJD5NEXC8

DIFFERENCES BETWEEN EUKARYOTIC AND
PROKARYOTIC CELLS
https://youtu.be/4ASXK6RG_f0

CELL STRUCTURE AND ORGANIZATION
Https://youtu.be/dkWhwnLpquY

THE STRUCTURE AND FUNCTION OF A CELL
https://sciencing.com/structure-function-cell5105947.html

ORGANELLES OF CELLS
https://youtu.be/RKmaq7jPnYM

CELL STRUCTURE OF ANIMALS
https://sciencing.com/cell-structure-definitions5043056.html

LEVELS OF ORGANIZATION OF SPECIALIZED CELLS
https://www.youtube.com/watch?v=HBlk_f0v4Q&pbjreload=10

RELATIONSHIP BETWEEN CELL STRUCTURE AND
FUNCTION
https://sciencing.com/relationship-between-cellstructure-function-5154975.html

IGCSE SPECIALIZED CELLS
https://www.youtube.com/watch?v=tLPOHbRreQU

DIFFUSION: WHAT IS IT AND HOW DOES IT HAPPEN?
https://sciencing.com/diffusion-what-is-it-how-does-ithappen-13718021.html

ACTIVE TRANSPORT: AN OVERVIEW
https://sciencing.com/active-transport-an-overview-ofprimary-secondary-13718016.html

METABOLIC REACTIONS: ANABOLIC AND CATABOLIC CELL
METABOLISM
https://sciencing.com/anabolic-vs-catabolic-cellmetabolism-definition-examples13717911.html?q2201904

5 STAGES OF MITOSIS
https://sciencing.com/5-stages-mitosis-13121.html

WHAT IS MEIOSIS
https://sciencing.com/what-is-meiosis-13714447.html

THE NEURONE
http://youtu.be/1bC0FGa0W0