CELL
Definition of cell: Cell is the fundamental, structural and functional unit of life.
CELL HISTORY
Some scientists contributed towards the history of the cell. Some of them include
(1) Robert Hooke an English scientist, is referred to as the father of the cell as he was the first person
to discover cell. He was examining the cork of the bark of an oak tree with his microscope in 1665 and
observed that the cork was consist of numerous honeycomb/box-like structures known to be dead cell
walls surrounding empty holes which he called cells.
(2) Felix Dujardin, a French biologist in 1835 discovered that cells are made up living substance which he
called protoplasm.
(3) Mathias Jakob Schleiden, a German botanist in 1838 revealed that the bodies of plants are made up
of cells referred to as unit of life.
(4) Theodor Schwann, a German zoologist (Physiologist) in 1839 discovered that the bodies of all
animals are made up of cells. He said that every living thing is made up of one or more cells. The
discoveries of Schleiden and Schwann led to the postulation of cell theory.
(5) Rudolf Von Virchow, a German biologist (a doctor) in 1849 concluded in his research that all cells
come from pre-existing ones via cell division.
CELL THEORY
Cell is the fundamental, structural and functional unit of life.
All living things are made up of one or more cells (Unicellular or Multicellular).
All cells come from previously existing ones through cell division.
Cells contain genetic/hereditary material in form of DNA which is passed from mother cell to
daughter cell during cell division.
All cells consist of mostly the same chemicals or all cells are the same in regard to chemical
composition.
CELL STRUCTURE
There are two types of cell namely prokaryotic and eukaryotic cells. Prokaryotic cell is the cell that
lacks membrane bounded organelles within the cytoplasm. It has no nuclear membrane while in
eukaryotic cell, the nucleus and most of the organelles in the cytoplasm are membrane-bounded.
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Examples of organisms with prokaryotic cells are cyanobacteria/blue-green bacteria and other species
of bacteria. Examples of organisms with eukaryotic cells include Vertebrates and invertebrate animals
such as protozoa, nematodes and mammals. Plants as well are examples of eukaryotes with the
exception of bacteria.
Differences between prokaryotic and eukaryotic cells are as follows
Prokaryotic cell Eukaryotic cell
Absence of membrane-bounded organelles Presence of membrane-bounded organelles.
No distinct nucleus, only diffuse area of A distinct membrane-bounded nucleus present
nucleoplasm with no nuclear membrane
No chromosome, only circular strands of DNA Chromosome present where DNA is located
Ribosomes are smaller in size Ribosomes are larger in size
Flagella, if present lack 9+2 fibril arrangement Flagella have 9+2 arrangement
No mitosis or meiosis Mitosis or meiosis occurs
Mesosome used for production of energy (cellular Mitochondria used for production and release of
respiration) energy in the cell (cellular respiration)
Chloroplast and mitochondria absent Chloroplast or mitochondria may be present
ADVANTAGES OF MEMBRANE-BOUNDED ORGANELLES IN A CELL
Enzymes which catalyze metabolic reactions/processes are embedded in membranes.
Increase in size of a cell reduces the proportion of membrane area to cell volume hence the
proportion is increased by the presence of organelles’ membranes.
Membranes contain enzymes for a particular metabolic pathway within the organelles hence
the products of one reaction will always be in close proximity to the next enzyme in the
sequence thereby increasing the rate of reactions.
The rate of any metabolic pathway inside an organelle can be controlled by regulating the
rate at which the membrane surrounding the organelle allows the first reactant to enter.
Potential harmful reactants and enzymes can be isolated inside an organelle so that they
won’t damage the rest of the cell.
PARTS OF THE CELL AND THEIR FUNCTIONS
PLANT CELL: Plant cell is made up of the following
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Cellulose cell wall, cell membrane, cytoplasm, chloroplasts/green plastid, leucoplasts, mitochondria,
sap vacuole, nucleus with nucleolus, rough and smooth endoplasmic reticulum, Golgi apparatus,
ribosomes, middle lamella, plasmodesmata, microtubules, starch grains, and lipid globule.
FUNCTIONS OF THE PARTS LISTED ABOVE
Cellulose cell wall
It is the outermost part of the plant cell hence protects the internal structures of the cell.
Since it is semi-permeable, it controls the movement of materials in and out of the cell.
It provides mechanical support/strength to the cell thereby helping it to maintain its shape
It serves as a boundary between the cell and its environment.
Cell membrane
It is the second structure after the cellulose cell wall and as well protects the internal
structures of the cell with the exception of cellulose cell wall.
It is selectively/partially permeable hence controlling what enters or exits the cell.
It facilitates the transport of materials /nutrients needed for the survival of the cell.
Cytoplasm/cytosol/cytoplasmic matrix/Hyaloplasm
It is an aqueous material/solution/colloidal suspension of many vital cellular chemicals such as ions of
sodium, phosphate, organic molecules like amino acids, ATP and nucleotides.
Functions
All the cell organelles are embedded in the cytoplasm.
Many biochemical processes like glycolysis occur in the cytoplasm.
Its streaming nature especially in unicellular organisms qualifies it to serve as transport
medium.
Chloroplast/green plastid: It is a double membrane-bounded organelle with Small amount of DNA in
its stroma.
Functions
It is the site for photosynthesis. It has the ability to absorb solar energy which is used for the
photolysis of water during photosynthesis.
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It stores excess glucose in form of starch grains.
Leucoplast/Amyloplast/colourless plastid
It also stores starch grains within the cytoplasm.
Mitochondrion: It is a double membrane-bounded organelle. The matrix contains proteins, lipids and
traces of DNA. The outer membrane controls the entry and exit of chemicals while the inner
membrane is folded forming cristtae within the matrix.
Functions
The main function is to generate most of the energy needed by the cell in form of
ATP/chemical energy (Adenosine triphossphate). They are referred to as the power house of
the cell.
They are abundance in highly active cells like the liver cells where much energy is needed, to
enable the liver to perform its numerous functions.
They are also abundance in striated muscle cells where energy is needed for the contraction
of the muscles especially the flight muscles of insects where rapid contraction is needed.
They are much in sperm tails where energy is required for the propelling of the sperm.
They are plenty in nerve cells especially around the synapses where energy is required for the
production and release of transmitter substances across the synapse.
Many mitochondria occur underneath the microvilli of the epithelial cells of the intestine to
provide the needed energy for the absorption of digested food by active transport.
Apart from supply of energy, mitochondria are involved in other cellular activities such as cell
signaling/cell to cell communication, ion homeostasis in many metabolic pathways, cellular
differentiation, steroid synthesis, apoptosis and programmed cell death as well as cell
senescence.
Sap vacuole: Sap vacuole is surrounded by a membrane called tonoplast which controls the movement
of materials in and out of the vacuole. The vacuole is large and centrally placed. When the vacuole is
turgid, it pushes the cell membrane against the cell wall.
Functions
It stores water and maintains the internal hydrostatic pressure /turgidity of the cell thereby
helping to maintain the shape and rigidity of the cell.
The vacuole conations a solution of mineral salts, sugars, amino acids, waste products like
tannins, pigments like anthocyanins.
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The sugars and amino acids serve as temporarily food store while the anthocyanins have
various colours and may give colour to petals of flowers for attraction of insects for
pollination or give colour to fruits to attract animals for dispersal.
It serves as temporary store for organic wastes such as tannins which may accumulate in the
vacuole of the cell in leaf and being removed when the leaf falls.
The vacuole sometimes contains hydrolytic enzymes which breakdown worn-out tissues.
Nucleus (Double membrane-bounded organelle)
The nucleus contains nucleolus which produces ribosomes.
It is the life wire of a cell since it controls all the activities of the cell. Any cell devoid of
nucleus is dead.
It houses the genetic or hereditary materials, the chromosomes that contain the DNA.
It carries instruction for the synthesis of proteins in the nuclear DNA.
It initiates cell divisions in any cell.
Nuclear membrane controls the movement of materials in and out of the nucleus.
Endoplasmic reticulum (Double membrane-bounded organelle
The structure varies from cell to cell. It is the extension of nuclear membrane which forms a series of
sheet that enclose flattened sacs called cisternae. There two type of Endoplasmic reticulum namely
rough endoplasmic reticulum with ribosomes on its surfaces and smooth endoplasmic reticulum
without ribosomes on its surfaces.
Functions
It acts as a port for the entry of proteins into the membranes.
It plays a role in biosynthesis and storage of lipids.
It is connected between cells through the plsmodesmata which aids in cell to communication.
It provides large surface area for chemical reactions.
It provides a pathway for the transport of materials via the cell.
The rough endoplasmic reticulum is concerned with the synthesis of proteins and it is
abundance in cells that are rapidly growing or secreting enzymes.
It collects and stores synthesized materials like proteins and enzymes.
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The smooth endoplasmic reticulum synthesizes lipids and it is abundance in cells producing
lipid-related secretions like sebaceous glands of the mammalian skin and cells secreting
steroids.
It provides a structural skeleton to maintain the shape of some cells e.g. smooth endoplasmic
reticulum helps to maintain the shape of rod cells of the retina of the eyes.
Golgi apparatus/Golgi bodies/Dictyosome
This structure is named after the man Golgi Camillo who discovered it. It is made up of stacks of
flattened sacs filled with fluid. Its position and size varies from cell to cell. It pinches off smaller
membranous sacs called vesicles at its end. It is well developed in secretory cells and nerve cell but
smaller in muscle cells.
Functions
It is responsible for identifying, processing, and packaging macromolecule and transfers them
to appropriate destination.
All the proteins produced by the endoplasmic reticulum are passed via the Golgi apparatus in
a strict sequence e.g. The proteins first of all pass via the cis Golgi network which returns to
the endoplasmic reticulum any proteins wrongly exported by it. From the cis Golgi network
the proteins and lipids pass via the stack of cisternae where the proteins and lipids are
modified and labels are added to allow them to be identified and sorted at the next stage of
network which is the trans Golgi network.
The Trans Golgi network sorts the proteins and lipids and sends them to their final
destination.
The Golgi apparatus produces glycoprotein like mucin required for secretions by adding the
carbohydrate part of the protein.
It produces secretory enzymes like digestive enzymes of the pancreas.
It produces carbohydrates that are involved in production of new cell walls.
It transports and stores lipid.
Ribosomes
If the ribosomes occur in groups, they are referred to as polysomes. They are associated with rough
endoplasmic reticulum.
Function: The main function is the production of proteins.
Middle lamella
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Middle lamella is found in between cells hence joining the adjacent cells.
Plasmodesmata: They are narrow pores carrying fine strands of cytoplasm to join cells together.
They facilitate the movement of materials between cells.
Microtubules
They form a framework along which the cellulose cell wall of plants is laid down.
They provide internal skeleton/cytoskeleton for cells that helps to determine their shape.
They transport materials within the cells by providing routes along which materials move
They are major components of cilia and flagella especially in lower organisms; Contributing to
their movement.
Starch grains: They are products of photosynthesis. After photosynthesis, excess glucose is stored
in plants in form of starch grains. They are temporary food store.
Lipid globules/lipid granules: They are also products of photosynthesis in plants. The oil produced is
stored in the cytoplasm of the cell in form of lipid globules /granules.
ANIMAL CELL
Animal cell is made up of the following parts: cell membrane, cytoplasm, mitochondria, vacuoles,
nucleus with nucleolus, rough and smooth endoplasmic reticulum, Golgi apparatus, ribosomes,
microtubules, lysosomes, flagellum/cilia, centrioles, microvilli, micro bodies/peroxisomes, glycogen and
lipid globule.
FUNCTIONS OF THE PARTS LISTED ABOVE
Cell membrane
It is the outermost structure of the cell which protects the internal structures of the cell.
It is selectively/partially permeable hence controlls what enters or exits the cell.
It facilitates the transport of materials /nutrients needed for the survival of the cell.
It also serves as a boundary between the cell and the environment.
Cytoplasm/cytosol/cytoplasmic matrix/Hyaloplasm
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It is an aqueous material/solution/colloidal suspension of many vital cellular chemicals such as ions of
sodium, phosphate, organic molecules like amino acids, ATP and nucleotides.
Functions
All the cell organelles are embedded in the cytoplasm.
Many biochemical processes like glycolysis occur in the cytoplasm.
Its streaming nature especially in unicellular organisms qualifies it to serve as transport
medium.
Mitochondrion: It is a double membrane- bounded organelle. The matrix contains proteins, lipids and
traces of DNA. The outer membrane controls the entry and exit of chemicals while the inner
membrane is folded forming cristtae within the matrix.
Functions
The main function is to generate most of the energy needed by the cell in form of
ATP/chemical energy (Adenosine triphossphate). They are referred to as the power house of
the cell.
They are abundance in highly active cells like the liver cells where much energy is needed for
the liver to perform its numerous functions.
They are also abundance in striated muscle cells where energy is needed for the contraction
of the muscles especially the flight muscles of insects where rapid contraction is needed.
They are much in sperm tails where energy is required for the propelling of the sperm.
They are plenty in nerve cells especially around the synapses where energy is required for the
production and release of transmitter substances across the synapse.
Many mitochondria occur underneath the microvilli of the epithelial cells of the intestine to
provide the needed energy for the absorption of digested food by active transport.
Apart from supply of energy, mitochondria are involved in other cellular activities such as cell
signaling/cell to cell communication, ion homeostasis in many metabolic pathways, cellular
differentiation, steroid synthesis, apoptosis and programmed cell death as well as cell
senescence.
Nucleus (Double membrane-bounded organelle)
The nucleus contains nucleolus which produces ribosomes.
It is the life wire of a cell since it controls all the activities of the cell. Any cell devoid of
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nucleus is dead.
It houses the genetic or hereditary materials, the chromosomes that contain the DNA.
It carries instruction for the synthesis of proteins in the nuclear DNA.
It initiates cell divisions in any cell.
Nuclear membrane controls the movement of materials in and out of the nucleus.
Endoplasmic reticulum (Double membrane-bounded organelle
The structure varies from cell to cell. It is the extension of nuclear membrane which forms a series of
sheet that enclose flattened sacs called cisternae. There are two types of Endoplasmic reticulum
namely rough endoplasmic reticulum with ribosomes on its surfaces and smooth endoplasmic
reticulum without ribosomes on its surfaces.
Functions
It acts as a port for the entry of proteins into the membranes.
It plays a role in biosynthesis and storage of lipids.
It provides large surface area for chemical reactions.
It provides a pathway for the transport of materials via the cell.
The rough endoplasmic reticulum is concerned with the synthesis of proteins and it is
abundance in cells that are rapidly growing or secreting enzymes.
It collects and stores synthesized materials like proteins and enzymes.
The smooth endoplasmic reticulum synthesizes lipids and it is abundance in cells producing
lipid-related secretions like sebaceous glands of the mammalian skin and cells secreting
steroids.
It provides a structural skeleton to maintain the shape of some cells e.g. smooth endoplasmic
reticulum helps to maintain the shape of rod cells of the retina of the eyes.
Golgi apparatus/Golgi bodies/Dictyosome
This structure is named after the man Golgi Camillo who discovered it. It is made up of stacks of
flattened sacs filled with fluid. Its position and size varies from cell to cell. It pinches off smaller
membranous sacs called vesicles at its end. It is well developed in secretory cells and nerve cells but
smaller in muscle cells.
Functions
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It is responsible for identifying, processing, and packaging macromolecule and transfers them
to appropriate destination.
All the proteins produced by the endoplasmic reticulum are passed via the Golgi apparatus in
a strict sequence e.g. The proteins first of all pass via the cis Golgi network which returns to
the endoplasmic reticulum any proteins wrongly exported by it. From the cis Golgi network
the proteins and lipids pass via the stack of cisternae where the proteins and lipids are
modified and labels are added to allow them to be identified and sorted at the next stage of
network which is the trans Golgi network.
The Trans Golgi network sorts the proteins and lipids and sends them to their final
destination.
The Golgi apparatus produces glycoprotein like mucin required for secretions by adding the
carbohydrate part of the protein.
It produces secretory enzymes like digestive enzymes of the pancreas.
It produces carbohydrates that are involved in production of new cell walls.
It transports and stores lipids
It produces lysosomes.
Ribosomes
If the ribosomes occur in groups, they are referred to as polysomes. They are associated with rough
endoplasmic reticulum.
Function
The main function is the production of proteins through the interpretation of cellular
information from the nucleus while they move along messenger RNA in a succession.
Microtubules
They are major components of cilia and flagella especially in lower organisms; Contributing to
their movement.
They provide internal skeleton (cytoskeleton) for the cells thereby helping in determining their
shape.
They transport materials within the cells by providing routes along which materials move
In animal, during cell divisions within the centrioles where the spindle is formed, they help to
draw the sister chromatids to the opposite poles of the spindle.
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Lipid globules/lipid granules: The oil produced in the body is stored in the cytoplasm of the cell in
form of lipid globules /granules.
Lysosomes: They are spherical bodies which contain about 50 enzymes mainly hydrolases, in acid
solutions. They are abundance in secretory cells and phagocytic cells.
Functions
They digest materials/food the cell consumes from the environment. For instance, bacteria
engulfed by white blood cells or food consumed by protozoa via phagocytosis.
They digest/breakdown worn-out organelles or dead cell as a whole.
They release their enzymes outside the cell (exocytosis) in order to breakdown other cells. E.g.
re-absorption of tadpole tail during metamorphosis.
Micro-bodies/Peroxisomes
They are small membrane-bounded bodies that contain some important enzymes especially catalase,
which catalyses the breakdown of hydrogen peroxide which is a toxic by-product of many biochemical
reactions within an organism. They are numerous in actively metabolizing cells e.g. liver cells.
Centrioles: They are hollow cylinders that arise in a distinct region of the cytoplasm known as
centrosome. The centrosome contains two centrioles whose function during cell division is to move to
the opposite poles of the cell to synthesize microtubules of the spindle to separate sister chromatids
during the anaphase.
Cilia and flagella: The two are almost identical only that cilia are shorter and more numerous than
flagella.
The flagella are mainly used for movement in some organisms especially lower organisms like
Trypanosomma species, Euglena, Chlammydomonas etc. while cilia are used for locomotion and
feeding in some organisms such as Paramecium caudatum, Balantidium coli etc. Cilia also are used in
respiratory tract to move mucus towards the throat.
Microfilament: They are very thin strands that are made of protein actin and a smaller proportion of
myosin. The two proteins are involved in the contraction of muscles .
Microvilli/Brush border: They are tiny finger-like projections on the membrane of certain cells like
those of intestinal epithelium and kidney tubules.
The actin filaments within the microvilli allow them to contract and with their large surface area
facilitates absorption of digested food/materials.
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Differences between plants and animal cells
Plant cell Animal cell
Cellulose cell wall present cellulose cell wall absent
Pit and plasmodesmata present Pits and plasmodesmata absent
Middle lamellae present Middle lamellae absent
Plastids/chloroplast and leucoplast present Plastid/chloroplast and leucoplast absent
Presence of one large centrally placed vacuole Presence of many small scattered vacuoles
Tonoplast present Tonoplast absent
Nucleus placed at the edge of the cell Nucleus is mostly centrally placed in a cell
Lysosomes absent Lysosomes present
Cilia and flagella absent in higher plants Cilia and flagella often present
Centrioles absent Centrioles present
Excess carbohydrates stored in form of starch Excess carbohydrates stored in form of glycogen
Few secretions are produced A wide variety of secretions are produced
Only some cells are capable of division Almost all the cells are capable of division
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Diagram of a Bacterium (Example of prokaryotic cell)
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Diagram of animal cell (Example of eukaryotic cell)
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