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CELLS: The Building
Blocks of Life
History of the Cell
• Robert Hooke is a scientist from England.
• He experimented with clocks, telescopes
and microscopes.
• He became famous for the observation of
thin slices of bark through the
microscope in 1667.
• It was this observation that led him
to….THE DISCOVERY OF THE CELL !
How did he discover the Cell?
1. He sharpened his knife as keen as a razor
blade.
2. He cut off a very thin piece of cork with his
knife.
3. He placed the thin piece of cork on a black
object plate.
4. Through the microscope, he observed that
the cork cells were perforated and porous,
much like a honeycomb.
CORK CELLS
Microscopes
•Light microscope can
magnify objects up to
1000 x
•Thus, many organelles
cannot be seen clearly
• Electron microscope
can magnify objects up
to > 200 000x
•Thus, many organelles
can be seen more
clearly
Micrographs
What is it?
A camera can be fitted to a microscope to
take pictures. This picture is called the
micrograph.
Micrographs
Photomicrograph
•Taken using a light
microscope
•Pictures are in colour
Electronmicrograph
• Taken using an
electron microscope
•Pictures are in blackand-white but images
can be artificially
colourised.
What does a cell consist of?
What does a cell consist of?
cell wall
cytoplasm
nucleus
What does a cell consist of ?
Protoplasm
- Nucleus + Cell surface membrane +
Cytoplasm
Organelles
- Specialised structures within the cell
Parts of a Cell
The protoplasm of a cell is made up of three parts:
Cell surface membrane
Cytoplasm
• Surrounds the
cytoplasm
• Protoplasm between the cell
surface membrane and nucleus
• Partially permeable
• Where most cell activities occur
• Hence controls
substances entering or
leaving the cell
• Contains enzymes and
specialised structures (organelles)
nucleoplasm nuclear nucleolus
envelope
chromatin
Nucleus
• Consists of nucleoplasm, nuclear envelope, one or more nucleoli and chromatin
• Functions: controls cell activities (e.g. cell growth and repair of worn out parts) ;
;essential for cell division. Cells without nucleus cannot divide and have short
lifespans.
Cell surface membrane (plasma membrane)
Cell surface membrane
Animal cell
Plant cell
Cell surface membrane (plasma membrane)
Photomicrograph
Cheek cell
Cell surface membrane
Onion epithelial cell
Cell wall
• Non-living cell wall made of cellulose that
encloses the entire plant cell, surrounding
the cell membrane.
• The cell wall is fully permeable and is
absent in animal cells.
Function:
• It protects the cell from injury and gives
the plant cell a fixed shape.
What does a cell consist of?
Cell wall

Only exist in plant cells

Absent in animal cells
What does a cell consist of?
Photomicrograph
Cell wall
Parts of a Cell
The protoplasm of a cell is made up of three parts:
Cell surface membrane
Cytoplasm
• Surrounds the
cytoplasm
• Protoplasm between the cell
surface membrane and nucleus
• Partially permeable
• Where most cell activities occur
• Hence controls
substances entering or
leaving the cell
• Contains enzymes and
specialised structures (organelles)
nucleoplasm nuclear nucleolus
envelope
chromatin
Nucleus
• Consists of nucleoplasm, nuclear envelope, one or more nucleoli and chromatin
• Functions: controls cell activities (e.g. cell growth and repair of worn out parts) ;
;essential for cell division. Cells without nucleus cannot divide and have short
lifespans.
Cytoplasm
Cytoplasm
Cytoplasm
Cytoplasm
Drawing of Onion epithelial cell
Cheek cell
Parts of a Cell
The protoplasm of a cell is made up of three parts:
Cell surface membrane
Cytoplasm
• Surrounds the
cytoplasm
• Protoplasm between the cell
surface membrane and nucleus
• Partially permeable
• Where most cell activities occur
• Hence controls
substances entering or
leaving the cell
• Contains enzymes and
specialised structures (organelles)
nucleoplasm nuclear nucleolus
envelope
chromatin
Nucleus
• Consists of nucleoplasm, nuclear envelope, one or more nucleoli and chromatin
• Functions: controls cell activities (e.g. cell growth and repair of worn out parts) ;
;essential for cell division. Cells without nucleus cannot divide and have short
lifespans.
Nucleus
Nucleus- what is inside?
Chromatin






-A network of thread-like structures in the
nucleus
-It controls the activities of the cell, such as
cell division
-Each chromatin is made up of proteins and
deoxyribonucleic acid (DNA)
DNA contains:
-Hereditary information
-Instructions that a cell needs for carrying out
all chemical reactions within itself.
Nucleus- what is inside?

Chromosome
- When the cell is dividing, chromatin condenses
and becomes highly coiled structures called
chromosomes (which appear as thick, rod-shaped
structures under light microscope)
chromosome
The Nucleus
Nucleus
The nucleus
Nucleus
Cheek cell
Onion epithelial cell
Vacuole

-Fluid-filled space enclosed by a membrane

-Contained in the cytoplasm
Function:
-Storage of substances within the cell

Vacuole
Animal
Plant
Size and
number
Small and
many
Large and only
one
Contents
Water; food
substances
Cell sap (contains
How long do
they exist?
Temporarily
dissolved substances
such as sugars, mineral
salts and amino acids,
enclosed by a membrane
called tonoplast)
More
permanent
Vacuole
Vacuole
Vacuole
Chloroplast
-Oval structures found in plant cells
 -Found in the cytoplasm



Function:
-Contains the green pigment chlorophyll,
which is essential for plants to make food
(the process of photosynthesis)
The chloroplast
Chloroplast

Only exist in plant cells

Absent in animal cells
Chloroplast
Photomicrograph
Chloroplasts
Chloroplast
Electronmicrograph
Mitochondria (sin: Mitochondrion)
-Small, sausage-shaped
 -Can be seen clearly under electron
microscope

Function:
- Aerobic respiration occurs in the
mitochondria
 -Aerobic respiration is the process
whereby food substances are oxidised to
give energy the cell needs to carry out
activities like growth and reproduction
Mitochondria
Electronmicrograph
of mitochondrion
Mitochondria
Electronmicrograph:
Mitochondria
Electronmicrograph:
(b) A plant cell
Mitochondria
Electronmicrograph:
Ribosome
Small, round structures
 Either attached to certain membranes
in the cells or lie freely in the cytoplasm.


Function:
 Synthesize proteins in the cell
Ribosomes
Electronmicrograph:
In summary- Most cells share certain
features:
Protoplasm
Nucleus
Cell surface membrane
Nucleoli- Building proteins
Nucleoplasm- small round
mass of protoplasm called
the nucleoplasm
Nuclear envelopeSeparates nuclear content
from the surrounding
Cytoplasm
Chromatin threads- Found in
nucleoplasm, contain
hereditary materials which
condense to form chromosomes
•Partially permeable
membrane covering
of cell
•Controls substances
entering and leaving cell
Cytoplasm
•Gel-like substance
forming major part of cell
•Site where most
organelles are found:
Mitochondria (sausage
shaped)
-for cell respiration
Centrioles involved in cell division
Chloroplasts Contains chlorophyll,
green pigment needed for
photosynthesis
Vacuoles fluid-filled storage sacs
RibosomesSynthesise proteins
Cell seen under
electron microscope
Rough endoplasmic reticulum
• Rough endoplasmic reticulum (RER) has a surface that
appears rough because of small particles called
ribosomes that are attached to its outer surface. The
outer surface of the RER is continuous with the nuclear
envelope.
• Ribosomes are small round structures that are either
attached to the membrane of the RER or lie freely in the
cytoplasm. They are needed to synthesise proteins.
• The ribosomes attached to the RER make proteins that
are usually transported out of the cell
• The ribosomes lying freely in the cytoplasm make
proteins that are used within the cytoplasm of that cell.
• The RER transports proteins made to the Golgi
apparatus for secretion out of the cell.
Smooth endoplasmic reticulum
• The smooth ER (SER) does not have
ribosomes attached to its membrane. SER
is more tubular that than the RER. It is
connected to the RER.
• The SER synthesises fats and steroids
and converts harmful substances into
harmless materials.
Golgi apparatus
• Golgi apparatus or Golgi body is shaped like a
disc.
• It consists of a stack of flattened spaces
surrounded by membranes. Vesicles or tiny
spherical spaces enclosed by a membrane can
be seen fusing with one side of the Golgi
apparatus and pinching off from the opposite
side.
• The Golgi apparatus stores and modifies
substances made by the ER and packages
these substances in vesicles for secretion out of
the cell.
Plant and animal cells: Similarities
•
•
•
•
Both have nuclei
Both have cell membranes
Both have cytoplasm
Both have mitochondria, ribosomes, ER,
Golgi apparatus, vesicles
Plant and animal cells:
Differences
Animal cell
Plant cell
Many small vacuoles
A large central vacuole
No chloroplasts
Chloroplasts present
No cell wall
Cellulose cell wall
centrioles present
No centrioles
No regular shape
Regular shape
Differentiation
• Growth in an organism involves cell division.
• New cells produced may change and develop into new
tissues.
• This process by which a cell becomes specialised for a
specific function is called differentiation. Cells may
develop special structures or lose certain structures for
them to carry out specific functions.
e.g. Human cells become differentiated to form various
types of cells:
liver cells, muscle cells, sperm cells, epithelial cells, blood
cells, etc
e.g. Plant cells differentiate to form xylem cells, phloem
cells, etc
Root Hair Cell
Functions
Absorption of water &
mineral salts from soil
Adaptations
Long and narrow to increase the
surface area: volume ratio,
thus water and mineral salts
can be efficiently absorbed from
the soil.
Surface Area to Volume Ratio
Cube
Surface
area/cm2
Volume/cm3 Surface area:
volume
Side 1 cm
6 cm2
1 cm3
6:1
Side 2 cm
24 cm2
8 cm3
3:1
27 cm3
2:1
Side 3 cm
54 cm2
Xylem Vessels
Functions
To conduct water & salts
from roots to leaves
To provide support for
stem & leaves
Adaptations
Long & thin tubes,
no cross walls or
protoplasm which enables
water to move easily through
the lumen (central space)
Lignin deposited on wall
Red Blood Cells
Functions
To carry oxygen
around the body
Adaptations
-Circular biconcave, increases
the SA to Vol. ratio.
-no nucleus, enabling it to
contain more haemoglobin
Tissues
muscle cell
muscle tissue made
up of muscle cells
epithelial
cell
epithelial cell
gland cell
neurones
epithelium in the trachea
(tissue) made up of epithelial
cells and gland cells
epithelium in the lungs
(tissue) made up of
epithelial cells
nervous tissue made up of
neurones (nerve cells)
intestinal
epithelial cell
intestinal epithelium (tissue) made
up of intestinal epithelial cells
connective
tissue cell
connective
tissue cell
cartilage in the trachea
(tissue) made up of
connective tissue cells
connective tissue in the
lungs made up of
connective tissue cells
A group of cells work together to form a tissue.
Tissue
A group of similar cells which work together
to perform a specific function
-Cells of the same type form simple tissue
E.g. epidermis tissue (plants) and epithelial tissues (animals)
-Cells of different types form complex tissue
E.g. connective tissue (animals), blood, vascular tissue (plants)
Organs
muscle tissue
epithelium (tissue)
stomach
trachea
(organ)
(organ)
nervous tissue
cartilage (tissue)
intestinal epithelium
epithelium (tissue)
(tissue)
lungs
(organs)
intestine
muscle tissue
(organ)
connective tissue
Different tissues combine to form an organ.
Systems
stomach
trachea
(organ)
(organ)
respiratory system
(organ system)
intestine
digestive system
lungs
(organ system)
(organs)
(organ)
Several organs work together to make up an organ system.
Specialised Cells, Tissues, Organs & Systems
digestive system
respiratory system
(organ system)
(organ system)
human
(organism)
Various organ systems work together to make up an organism.
CELL
Tissues
Organs
Organ systems
made up of
Protoplasm
consists of
Cellulose cell wall
(found only in plants)
Nucleus
Cytoplasm
which
which
controls cell activities
and contains
Chromatin (made
up of DNA)
surrounds the nucleus and is where
most cell activities occur
contains organelles
Organism
Cell surface membrane
which
is the partially permeable,
outer covering enclosing the
cytoplasm
Mitochondria (sites of
energy release)
Ribosomes (involved in
protein synthesis)
Chloroplasts (in plant cells)
Vacuoles (contain water and
food substances)
You should be able to:
• Know the functions and identify from diagram and
photomicrographs, the nucleus, chloroplast, ribosome,
mitochondrion, endoplasmic reticulum.
• Examine under the microscope an animal cell
(from fresh liver& human check cell) and
a plant cell (from Hydrilla & onion )
• Compare and contrast the structures of plant and animal
cell
• Relate the adaptations of a cell structure to its functions
for the following: Absorption-root hair cells/ Conduction
and support- xylem vessels /Transport of oxygen-red
blood cells
• Differentiate between the terms cell, tissues, organ and
organ system.