Biology Form One Notes – Cell Structure and Organization
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TOPIC THREE
CELL STRUCTURE AND ORGANIZATION
‘The Concept of Cell
‘The Meaning ofthe Cell
Explain the meaning ofthe cell
Plants and animals are made up of units called cells. The cells are microscopic in such a way that
they cannot be seen by our naked eyes. Some organisms like protozoa, diatoms and bacteria
consist of one cell and are called single-celled or unicellular organisms. Some are made up of
‘many cells and are called multicellular organisms.
“The cell structure of living things was first seen by Robert Hooke in 1667 when he examined
fine slices of cork. Robert Hooke believed thatthe cells were empty and thatthe cell wall and
cell membrane were the most important parts of the cell but now cell contents are seen to be the
‘most important ones. Therefore a cell can be defined as the smallest unit of living things or cell
is abasic unit of ie
The Characteristics of the Cell
Characteristics ofthe cell include the following:
1 Cells are small microscopic structures which cannot be seen by our naked eyes.
2. Calls are capable of dividing by mitotic process or meiotic process,
3. Cells contain structures called organelles.
‘The cell theory
4. Acell isa basic unit of structure and function in living organisms.
b. New cells only come from pre-existing ones.
€. Cells contain structures called organelles.
‘The cell theories were proposed jointly by two scientists namely Schleiden, a Belgian botanist, in
1838 and Schwann, German Zoologist, in 1839
Difference Between Various Types of Cells
Differentiate various ypes of cells
PROKARYOTIC AND EUKARYOTIC CELLS.
Prokaryotic cells are cells with no membrane-bound nucleus. The DNA lies free in the
cytoplasm in a region known as nucleoid, They have no true nuclei, Examples of prokaryotic
‘organisms are bacteria,
Eukaryotic cells are cells whose nuclei are bounded by nuclear membrane. They are surrounded
by two nuclear membranes called nuclear envelope. Examples of eukaryotic organisms are
protoctsts, fungi, plants and animal cells.
Differences between cell wall and cell membrane
Cell wall Cell membrane
Iisa non-tiving structure Itis living structure
is made up of cellulose Its made up of lipoprotein
kis freely permeable Itis selectively permeable
Different between Prokaryotic and Eukaryotic cells
Eukaryotic cells Prokaryotic cell
‘They have nuclear membrane ‘Lack nuclear membrane
Organelles are surrounded by envelopes Organelles are not surrounded by envelopes
Have tre nucleus Have no true nucleus
‘The Functions of Different Parts of Plant and Animal Cells
Basically a cell has three main parts
1, Cell membrane
2. Cytoplasm
3. Nucleus
Cell membrane (plasmalemma)
‘This is a thin flexible membrane made of protein and oil thas the following Functions
1 The cell membrane encloses the contents ofthe cell.
2. Itis freely permeable to water and gases only and selectively permeable to other molecules eit
allows food in but keeps unwanted molecules out. Thus cell the membrane controls the
substances entering and leaving the cell,
Gytoplasm
CCytoplasm is a transparent jellylike fluid and may contain particles such chloroplasts or starch
trains or oil droplets. It contains up to 80% water and the remainder is mainly protein, It is @
place where chemical reactions take place
Nucleus
A nucleus is a ball-shaped or oval body located inside the eytoplasm. It cannot usually be seen
unless the cell has been stained with certain dyes. It consists of nucleoplasm bounded by nuclear
‘membrane, The nucleus isa cell contro centre
The following are the functions of the nucleus:
1 Itcontrols the formation and development ofa cel
2. ‘The nucleus also controls chemicals which the cell manufactures,
3. ‘The nucleus contain chromosomes which carry genetic material Le. DNA which is responsible
for controlling genetic information
Celt walt
‘The cell wall is only found in plant cells. It is made up of cellulose. When the cell is growing the
cel wall is fairy plastic and extensible, It becomes tough and resists stretching when the cell has
reached fll size. The cell wal s non-living. I has the following functions:
1 Itgives the cell its shape.
2. Iti frely permeable to all kinds of molecules.
3. It supports and protects the cell
4. Itsupports non-woody plant organs, such as leaves, by turgor pressure,
5. Itosmoregulates by resisting entry of excess water into cell,
Vacuole
In animal cells, vacuoles are small droplets of fuid in the eytoplasm variable in size and
Position. In plant cells, the vacuole is a large, permanent Muid-filled cavity which occupies a
areater part ofthe cell In plants, the fuid is called cell sap. The cell sap may contain salts, sugar
and pigments dissolved in water.
“The vacuole performs the following Functions
1 lis responsible for food storage and osmoregulation,
2. ‘The outward pressure of the vacuole on the cell wall makes the plant cells firm, giving strength
and resilience tothe tissues,
Mitochondria
Mitochondria are found in all aerobic eukaryotic cells. A mitochondrion is surrounded by an
envelope of two membranes, the inner being folded to form cristae (singular: erst) It contains a
‘matrix witha few ribosomes, a circular DNA molecule and phosphate granules
Mitochondrion
‘A mitochondrion is a power house of a cell. It contains respiratory enzymes involved in
respiration. It absorbs oxygen and glucose. The glucose is broken down to CO» and H:0. Energy
is released from glucose bonds to form ATPs (for use in other vital functions eg. growth,
‘movement, et
Chloroplast
Chloroplasts are dise-shaped organelles. They are found in plant cells and algae cells. A
chloroplast contain a green substance called chlorophyll. I is surrounded by an envelope of two
‘membranes and contains gel-like stroma through which runs a system of membranes that are
stacked in places to form grana
aca
Circular DNA stroma
Envelope (two
\ oN membrane)
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Z GY Lipid droplet
yy
starch pains
Coop i anole n which potoryetei aes place, pedcing sas om carton
iia water eng ih eer aped by horny
Golgi body
Gol boies resco ated membrane-bound ss
Golgi vesicles
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nt ct
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rere ANIMALCELL
() Have definite shape Have no definite shape
(ii) Have chloroplasts Have no chloroplasts
ii)Have large permanent vacuoles Have small temporary vacuoles
Gv) Centrioles usually absent Centrioles present
(¥) Have cell wall made of cellulose Have no cell wall
(vi) The nucleus is located atthe periphery ‘The nucleus is centrally located
Similarities between animal cell and plant cell
1 Both have cell membranes
2. Both contain eytoplasm
3. Both have nucleus
4. Both have mitochondria
5. Both have golgi bodies
6. Both have ribosomes
Cell Differentiation
‘The Concept of Cell Differentiation
Explain the concept of cell differentiation
‘When most cells have finished dividing they become specialized. This means that:
+ They do one particular job
+ They develop a distinct shape
+ Special kinds of chemical change take place in their cytoplasm
‘The changes in shape and chemical reactions enable the cell to carry ou its special function. The
process by which cells are specialized to perform a particular function is called cell
differentiation or ‘division of labour” within the organism. Similarly, the special functions of
‘mitochondria, ribosomes and other cell organelles may be termed as division of labour within the
call
‘The Importance of Cell Differentiation and Formation of Tissues, Organs and Body
Systems
Humans have many different types of eells with different jobs, such as blood eels that carry
‘oxygen and nerve eells that transmit signals to all parts of the body. Cell differentiation is the
process by which cells become specialized in order to perform different functions.
Even multicellular organisms begin as just a single cell. Geting from one single cell to billions
of specialized cells that perform different functions isa process that happens with the regulation
of DNA and RNA. It can also be influenced by factors inthe environment
Cell differentiation holds a lot of importance for two basic reasons. Firstly, it helps to identify
stem cells, which could be used in the future to deal with conditions that require transplant and
form the basis of embryonic stem cell research. Also, in cytopathology, the level of cellular
differentiation is used as a measure of cancer progression, where the term ‘grade’ is used as a
‘marker to determine how differentiated a cel in a tumor is. Thus, the importance ofthis process
cannot be underestimated as it could hold the key to future treatments for fatal diseases.
Difference between Cells, Tissues Organs and Body Systems
Differentiate cells, tissues organs and body systems
Animal cells
Nerve cells
‘These are specialized for conducting impulses of an electrical nature along the fibre. The fibre
may be very long eg. from the foot tothe spinal column. They ae the longest cells known,
Cell body
Axon
Nucleus
y b – ‘Axon
Dendrites eau ————- Jecrainal
Myelin sheath
Nerve cell
Nerve cell
Blood ells
«White Blood cels‘Tese cll oceut in blood steam and are specialized for engulfing harmful
bacteria They are able to change their shapes and move about, even trough the walls of blood
«Red bled cells:These cells are responsible for transportation of oxygen from the lungs tall
parts ofthe by
+ Platelet cells: Platelet cells are found in the blood, They are also called blood platelets. Their
function is to help clot the blood at wounds and so stop bleeding
Reproductive cells,
+ Sperm cell: ts a male gamete produced inthe testes.
+ Egg ce:tis a female gamete produced inthe ovary
Muscle cells
—”
Og he =
Phloem cells
‘These are food conducting cells ina plant, joined end to end, and where they meet, perforations
‘occur inthe walls. Through these holes, the cytoplasm of one cell communicates with the next.
Xylem cells
“The cells conduct water and mineral salts form the soil to all pats ofthe plant. They are also
responsible for mechanical support
+ Parenchyma cells,
+ Collenchyma cells
+ Sclerenchyma cells
+) Epidermal cells,
‘TISSUES
A tissue is a group oF collection of similar cells performing a specific Function. Tissues vary in
size, shape and function,
Examples of tissues
Animal tissues
‘Animal tissues include epithelial tissue, muscle tissue, nervous tissue, blood tissue and bony
tissue
Epithelial tissue: Epithelial tissues cover the body lining cavities, hollow organs and tubes. They
are responsible for () protection ofthe underlying structure from dehydration, and chemical and
‘mechanical damages; (i) secretion; and (ii) absorption.
epithelial cells 4
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test =
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realtor sytem
This sytem consists of the heat, arteries and vein.The role ofthe creultry system is to
transport gases, fod, hormones and distribute heat
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the lymphatic system isto transport materials and protect against.
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The winary system
The min erp of the rnary sym ae kidney, enter, bladder and wrt. The sary
system plays a role in removing metabolic waste products from the body and also itis
responsible fr asmoregulation
Kidney
Ureter
Bladder
Urethra
y
fe } Skeletal muscles
Tendon Ke N \S
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The reproductive system
consists of the ovaries, testes and uterus Is role is to produce offipring,
peter Lr
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Pituitary: at base of forebrain
(any hormones)
‘Thyroid: in neck in
front of trachea
(hyroxine)
Pancreat: blow
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6 Adrenals: on topof
v~ ‘dae (adrenaline)
vases: near
oviducts estrogen
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“Tested i sre se
(testosterone)
Steel system
I consists ofa system of skeletons. Ils faction is to support the body, protect intemal organs
and produce blod cll
(ojo cranium
we
— Mandible
Sesouta FS 2, strum
wumerue—t, [6 Gite
h¥ = Vaan
pS AA sacrum
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ina AAS) Carvate
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Fibula” | | Tarsals
Metatarsals
AC tnatanges
Slt sem
Examples of organ stem in plats
oo system -consist of ro
Hose plant inline oi
«Abortion wat and mina ss om the soi
+ Somaimes thy at storage organs in some plans
Shoot stem consis of horn Nowe, is, tm and eaves
Function
+ Site of reproduction
+ Transport of substances
+ Photosynthesis occurs in the shoot
+ Transpiration proceses