Cells are the basic unit of life. All living things are made up of one or more cells that carry out basic functions like taking in nutrients, converting energy, eliminating waste, and reproducing by dividing. There are two main types of cells - eukaryotes which have internal membranes and organelles like a nucleus, and prokaryotes which lack internal membranes and organelles. Cells come in different shapes and sizes depending on their function, but all cells share the same basic structures and processes that allow living things to survive.

1. Cell is a basic unit of life
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2. • For example, a cell can take in fuel, convert it
to energy, and eliminate wastes, just like the
organism as a whole can.
• But, the structures inside the cell cannot
perform these functions on their own, so the
cell is considered the lowest level.
• Each cell is capable of converting fuel to
useable energy.

3. • Therefore, cells not only make up living things;
they are living things.
• Cells are found in all plants, animals, and
bacteria.
• Many of the basic structures found inside all
types of cells, as well as the way those
structures work, fundamentally are very
similar, so the cell is said to be the
fundamental unit of life.

4. • The most important characteristic of a cell is
that it can reproduce by dividing.
• If cells did not reproduce, you or any other
living thing would not continue to live.
• Cell division is the process by which cells
duplicate and replace themselves.

5. • Increasingly more complex organisms are
made up of increasingly more groups of cells
(for example, in humans, groups of cells make
up each organ and muscle tissue), and the
organisms survive based on products that the
cells make.

6. • For example, cells in the pancreas make
insulin, which is necessary to ensure that the
blood glucose level doesnot skyrocket.
• Without insulin, the blood glucose can reach a
level that is lethal. So, without that cellular
product, you would die.

7. • Examining eukaryotes and prokaryotes
• Cells fall into two major categories: eukaryotes
and prokaryotes.
• Eukaryotes are organisms that contain
chromosomes, including plants and animals,
as well as fungi (like mushrooms), protozoa,
and most algae. Eukaryotes have the following
characteristics:

8. • They have a nucleus that stores their genetic
information.
• Animal cells have an organelle called
a mitochondria that effectively combines
oxygen and food to convert energy to a
useable form.
• Plant cells have chloroplasts, which use
energy from sunlight to create food for the
plant.

9. • Eukaryotic cells have internal membranes,
which create compartments inside the cells
that have different functions.
• Plants cells have a cell membrane and a cell
wall, which is rigid; animal cells have only a
cell membrane, which is soft.
• The cytoskeleton, which reinforces
the cytoplasm of the cell, controls cellular
movements.

10. • Prokaryotes are cellular organisms that do not
have a “true” nucleus.
• A nucleus is the control center of a cell.
• A nucleus contains the genetic material
packed into chromosomes, and it is associated
with other organelles that function in the
production of amino acids and proteins based
on what the genetic material dictates.

11. • Prokaryotes have some genetic material, but it
is not as well organized as it is in eukaryotes.
• Still, prokaryotes are able to reproduce.
Examples of these organisms include bacteria
and algae

12. Why cells are called basic unit of life?
• Cells are described as the basic unit of life
because all living things are made of cells.
• Cell theory which forms the basis for this
statement basically states three things:
**all living things or organisms are made of cells
and their products,
**new cells are created by old cells through
division, and
**that cells are the basic building blocks of life.

13. Cell size and shape
• There exist cells which have a variable shape,
such as the leukocytes, and some connective
tissue cells and cells with a stable shape, such
as the erythrocytes, epithelial cells, muscle
cells and nerve cells.
• These stable cells always have á typical more
or less fixed shape which is a specific
characteristic of each cell type.

14. • The shape of the cell depends partly on the
surface tension and viscosity of the cytoplasm,
the mechanical action which the adjoining
cells exert, the rigidity of the membrane and
the functional adaptation.
• Many cells when isolated in a liquid medium
tend to take a spherical form, obeying the
laws of surface tension.

15. • This is the case with the leukocyte which in
the circulating blood are spherical, but by the
influence of adequate stimuli can emit
pseudopodia (amoeboid movement) and
become completely irregular in shape.
• The cells of many plant and animal tissues
have a polyhedral shape, statistically more or
less constant, determined principally by
pressure from adjacent cells.

16. • In these cases, the original spherical form is
modified by contact with the other cells.
• Individual cells in a large mass appear to
behave like polyhedral solids of minimal
surface packed without gaps.

17. • All cells are similar to each other because they
all have the same basic structure inside.
• They all have a membrane that encloses the
jelly-like cytoplasm and a nucleus that controls
the cell.
• But apart from these similarities, cells can be
very different from each other.

18. • There are more than 200 different shapes and
sizes of cell in your body doing many different
jobs.
• Here are a few examples:
•

19. Nerve cells can be as long as 100 cm, but they are very
thin so they can speed electrical messages up and down
the length of the body, to and from the brain

20. Nerve cells in the brain have lots of branches spreading
out to make connections with as many other nerve
cells as they can

21. Gland cells that make a hormone for your body or oil
for your skin have ruffled surfaces so that they can
pump out their product as quickly as possible

22. Sperm cells need to swim long distances so they have a wiggly
tail to propel themselves.

23. Muscle cells are stretchy so they can squeeze and
release and change their size and shape.