Download The Cell

This module is all about the cell as the unit of structure and function of living
things. It discusses the relevance of size and shape to the function the cell performs; the
difference between prokaryotic and eukaryotic cells and plant and animal cells. It
describes the organelles in relation to the function they perform in the cell.
All living things are made up of cells. The cell is the unit of structure and function of life. It
exhibits all the characteristics of living things. It is equipped with structures designed to perform the
various life functions such nutrition, synthesis and respiration, steady state regulation, reproduction,
growth, development and adaptation.
Englishman Robert Hooke is acknowledged as the first to report the cell. He was then
studying under the microscope a thin section of a cork and noticed cube-like structures that reminded
him of small cell rooms in the monastery – thus he called these structures cells. What Hooke actually
saw were plant cell walls, the skeleton of dead plant cells.
Henceforth, interest on the cell prospered and more knowledge about it was known. However,
the discovery and use of the electron microscope hastened the growth of information in cell biology.
Likewise, new discoveries in biochemistry contributed to the in depth understanding of the
functioning cell.
The Cell Theory
The Cell Theory is one of the unifying principles in Biology. Although a lot of scientists
worked on the cell, Matthias Schleiden, a botanist and Theodore Schwann, a zoologist, working
independently of the other were acknowledged for the Cell Theory
The Cell Theory states all living things are made up of cells and cells arise from preexisting
cells. This theory is a product of inductive reasoning - reaching a generalization based on many
concurring observations. The ability of cells to come to divide to form new cells is the basis for all
reproduction and for growth and repair of multicellular organisms.
Cell Size and Shape
In general, cells are small and microscopic. Most plant and animal cells are between 10 to 20
microns (1 micron = 10 –3mm). The bird’s egg is an exception because it is big enough to be seen by
the naked eye.
Cell size is restricted by the relationship between cell volume and surface area. In smaller
cells, the proportion of the cell surface area to volume is greater compared to larger cells. Materials
can move faster in and out of smaller cells. Likewise, cell size is limited by its nuclear regulation. If
cells are large, they have more than one nucleus to be able to regulate their activities. In the
exceptionally big cell, like the bird’s egg cell, a lot of nutrients stored for the developing embryo.
When the egg cell is fertilized and begins it activities of metabolism, it divides first many times
before there is additional increase in volume.
Cell shape is highly diverse. Its shape is adapted to the function of it performs. The other
factors that affect cell shapes are surface tension, pressure exerted by adjoining cells, rigidity of
the cell membrane and viscosity of the protoplasm. Stellar nerve cell and spider-like bone cell are
specialized to communicate with other cells. The filamentous skeletal muscle and spindle shaped
smooth muscle are adapted to shorten and lengthen to bring about movement or contraction. The
squamous, cuboidal and columnar epithelial cells with large surface area are adapted for covering
more surfaces for protection. The oval and round blood cells are suitable for moving from one place
to another.
Prokaryotes vs. Eukaryotes
Cell may vary in size, shape and organization but they all share common essential features.
They all have an outer membrane, which separates the cell from its external environments. They all
have the genetic material containing the hereditary information that directs cell activities and enables
them to reproduce and pass on traits to the offspring. An in their protoplasm, the various important
biological molecules responsible for energy production are found.
Cells are broadly categorized as prokaryotes and eukaryotes. Both cells have protoplasm,
which are colloid of organic and inorganic materials. However, they vary in the organization of the
nuclear material. In eukaryotic cells, the DNA is associated with proteins in structures called
chromosomes. These chromosomes are found in the nucleus surrounded by a nuclear membrane. In
contrast to prokaryotes, only a single chromosome is found this is not contained in a membrane
bound nucleus.
The Prokaryotes
Modern prokaryotes include the bacteria and blue-green
algae. The cell membrane is coated by a cell wall, which is
secreted by the cell itself. The cytoplasm of the prokaryote
contains a lot of very small organelles, the ribosomes. No
membrane bound organelles are found in the cytoplasm of the
prokaryotes.
Lacking the membrane-bound organelles, the
prokaryote is much simpler in structure. The DNA is in the
nucleoid region, and no membrane separates the DNA from the
rest of the cell. A prokaryote has large number of ribosomes,
where proteins are synthesized. The border of the cell is the
plasma membrane, which in some prokaryotes folds in to form
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structures called mesosomes. Outside the plasma membrane are fairly rigid cell wall and often an
outer capsule, usually jelly-like. Some bacteria have flagella, locomotion organelles or pili,
attachment structures or both.
The Eukaryotic Cell
Eukaryotes are larger cells and with more complex and membrane-bound organelles. Some
cells have cell walls like plant and fungal cells which are not found in animal cells. All multicellular
organisms are made up of eukaryotic cells.
Animal cells have a lot of sub-cellular organizations called organelles in the cytoplasm. These
organelles are capable of performing some of life functions but not self-sufficient to neither live nor
reproduce once removed from the cell. The organelles perform their specific functions to maintain the
life and well being of the cell.
Inside the cell membrane is the protoplasm, a complex colloid where membranous structures
like the RER, SER and Golgi bodies; formed organelles like the nucleus, mitochondrion, lysosome,
centrioles, microtubules and microfilaments are found. The figure below shows the animal cell.
CELL MEMBRANE. The cell is limited and bounded by the cell membrane or otherwise
known as the plasmalemma. It regulates the passage of the materials into and out of the cells
maintaining the structural and functional integrity
of the cell.
The “fluid mosaic model” is the proposed
structure of the cell membrane, wherein two layers
of phospholipids are formed with integral and
peripheral proteins. This membrane is not static
but rather in quasi fluid and very dynamic.
The cell membrane is semi-permeable
membrane. It has pores that would only allow molecules with smaller diameter than the pore to go in
and out of the cells. The bigger molecules are kept outside these pores. Water is a small molecule and
therefore could easily go in and out of the cell via these pores, however, bigger molecules like
carbohydrates cannot pass through the poles. Moreover, the phospholipid layer serves as a barrier for
entry or exit of molecules, which are not lipids soluble.
Materials are transported through the membrane from area of greater concentration to lesser
concentration through osmosis (diffusion through a semi-permeable membrane). Some molecules,
like glucose, which cannot pass through the pores, may require carrier molecules to pass through the
membrane, as in facilitated transport. In some instances when molecules have to move against the
concentration gradient, from area of lower concentration to area of greater concentration, the carrier
molecules in the membrane with expenditure of energy actively transport them.
Other molecules may enter the cell through endocytosis. Pinocytosis (drinking of the cell)
phagocytosis engulfing of materials
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ENDOPLASMIC RETICULUM. Filling the space between the cell membrane and the
internal nucleus is the cytoplasm is a membranous system similar to the cell membrane that forms
intricate channels and is continuous with the outer layer of the nuclear envelope. This is the
endoplasmic reticulum or otherwise called the ER.
Two types of ER, are found in the cytoplasm, the
smooth ER (SER) and the rough ER (RER).
The RER is characterized by the presence of
attached granules, the ribosomes and is involved in
protein synthesis.
On the other hand, SER is agranulated and
not associates with ribosomes. It is involved in
synthesis
of
glycolipids,
metabolism
of
carbohydrates and detoxification of drugs and other poisons. Enzymes of SER are important to the
synthesis of fatty acids, phospholipids, steroids and other lipids.
RIBOSOME. Ribosome is a granule found attached to the RER or free in the cytoplasm. It is
chemically made up of conjugated RNA and protein. It has a complex structure of two sub-units with
one smaller than the other. Ribosome is the site of protein synthesis
GOLGI APPARATUS. The Golgi body or
Golgi apparatus is a stack of membranes originating
from the endoplasmic reticulum. The membranes at
the bottom are flattened and empty while the ones at
the top are swollen. This structure is involved in the
production and packaging of cellular secretions. It is
the site of lipoprotein and glycoprotein production,
and sometimes referred to as the chemical factory of
the cell. The secretions produced in the Golgi bodies are in vesicles. These may remain inside the
cell to be used or secreted outside through exocytosis.
LYSOSOME. Lysosome is ovoid or sometimes irregularly shaped body abundant in animal
cell. It originated from Golgi bodies and contains a lot of hydrolytic enzymes capable of breaking
down proteins, carbohydrates, lipids and nucleic acids. It causes death or destruction of old and
damaged cells through the process of autolysis. It is commonly referred to as the suicide bag of the
cell.
Mitochondrion. Mitochondrion is a minute rodlike organelle surrounded by two membranes. The outer
membrane limits the organelle while the inner one is
highly folded forming the cristae. Cristae increase the
surface area of the mitochondrion for enzyme attachment.
The mitochondrion is involved in energy
production as it is the site of cellular respiration. It
contains the necessary respiratory enzymes for the
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chemical reactions of cellular respiration. Likewise, DNA is found in mitochondrion required for
production of these enzymes. The energy produced in cellular respiration is stored in the form of
ATP (adenosine triphosphate). ATP is the immediate source of energy of the cell for all its
activities thus, the mitochondrion is called the powerhouse of the cell.
MICROFILAMENT
AND
MICROTUBLE . The cytoskeleton is
web-like structure in the cytoplasm of
all eukaryotic cells and in which the
organelles are suspended. This
intracellular scaffolding acts as both
muscle and skeleton for the cell; allows
the cell its complex surface, and its
organelles to move, and gives the cell
its normal shape and holds its parts in
proper spatial relationship to each
other.
Microfilament
is
thin,
elongated rod found frequently in
dense bodies under the cell membrane. Microtubule is a hollow, fluid elongated cylinder.
Microfilament through contraction causes movement in the cell while the microtubules provide
rigidity and structural reinforcements. Cilia and flagella are made up of microtubules.
CENTRIOLE. This is an animal cell structure found near the nucleus and consists of two
bodies, it is made up of microtubules responsible for the formation of spindle fibers and astral rays
during cellular reproduction.
NUCLEUS. The nucleus is the largest
organelle in the cell. It is bounded by a nuclear
envelope or nuclear membrane. This membrane is
semi-permeable controlling the passage of materials in
and out of the nucleus.
Inside the nucleus is a dense unbounded
nucleolus. This is a concentration of RNA and is
known as the pacemaker of the cell. The chromatin materials of the nucleus are found in the form of
chromosomes. These are longs chains of DNA and nucleoproteins. These chromosomes are the gene
carriers and control the cellular activities.
Extracellular Matrix of Animal Cells. Although animals lack walls, they have an elaborate
extracellular matrix (ECM) or otherwise referred to as glycocallyx. The main components of ECM
are glycoproteins like collagen, which forms strong fibers outside the cells. The ECM functions for
support, adhesion, movement and development in animal cells.
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Intercellular Junctions. Many plant and animal cells are integrated into one functional
organism. Neighboring cells adhere, interact and communicate through the special patches of direct
physical contact.
In animals, there are three
main types of intercellular
junctions, the tight junction, the
desmosome and gap junction.
The
tight
junction
forms
continuous belts around the cell,
the membranes of neighboring
cells are actually fused at a tight
junction forming a seal that
prevents leakage of extracellular
fluids across a layer of cells.
Desmosomes
(anchoring
junctions) function by fastening
cells together into sheets. Gap
junctions (communicating junctions) provide connection cytoplasmic channels between adjacent
cells. Gap junctions are especially common in animal embryos where chemical communication
between cells is essential for development
Comparison of Plant and Animal Cells
The basic patterns of the plant and animal cells differ. Plant cells produce their own food
because they have the green machinery in their cells– the chloroplasts and related storage organelles,
the choromoplast and leucoplast and vacuoles. Furthermore, they are more rigid cells as they are
bounded by cell walls. On the other hand, animal cells have lysosomes, not commonly found in plant
cells. Instead other vesicular organelles like glyoxysomes and peroxisomes maybe found. Animal
cells have centrioles that give rise to astral rays and spindle fiber during cell division. They vary more
in shape as they do not have cell walls.
Components of Prokaryotic, Plant and Animal Cells
Component
Cell Wall
Glycocallyx
Plasma membrane
Cytoskeleton
Nucleus
chromosomes
Mitochondria
Plastids
Ribosomes
ER
Golgi Complex
Vacuoles
Lysosomes
Cilia (9+2)
Flagellum
Centrioles
PROKARYOTE
Present
Absent
Present
Absent
Absent
Single
Absent
Absent
Present
Absent
Absent
Absent
Absent
Absent
Often Present,
Absent
PLANT CELL
Present
Absent
Present
Present
Present
Multiple
Present
Often Present
Present
Present
Present
Present
Often Absent
Absent in most
Absent
Absent in most
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ANIMAL CELL
Absent
Present
Present
Present
Present
Multiple
Present
Absent
Present
Present
Present
Present
Present
Present in some
Present in some
Absent
6
ANIMAL CELL
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Name _______________________________
Date __________________
Task 3.1
Size and Shapes of Cells
1.
Observe the different animal cells above and take note of
their shapes. List down the shape and function of each cell
seen in the figures. Relate the functions to the shapes of
cells.
Shape
2.
Function
Are all cells microscopic? If your answer is no, what are some exceptions to this? Why
should cells be small?
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Name _________________________
Date _______________________
Task No. 3.2
Cellular Organelles
1. What cellular structures are common among all cells? Why are these organelles present in all
cells?
2. Differentiate prokaryotic from eukaryotic cell? Animal cell from plant cell?
a. Prokaryotic cell vs. eukaryotic cell
b. Plant cell vs. animal cell
3. In a tabular form, summarize the different cellular structures and their functions.
Cell Structure
Function
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4. Why is the cell considered as the unit of life?
5. Presence and number of organelles vary in different cells because they have different
specialized functions. Which organelle(s) do you think are predominantly present in great
number in the following cells. Cite the reasons for your answers.
a. heart muscle
b. liver cell
c. sperm cell
d. red blood cell
6. Research further on the cell membrane fluid mosaic model. Explain how this can explain why
the cell membrane is semi-permeable. Relate the functions performed by cell membrane with
the different structures present in them.
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Some Review Questions
True or False.
1. Cells are considered living.
2. Water is a source of energy for living things.
3. Deoxyribose nucleic acids are found in the nucleus, mitochondrion and chloroplast.
4. Glucose molecules pass through the cell membrane by facilitated diffusion.
5. The cell theory states that all living things are made up of cells and came from pre-existing cells.
6. All substances can get in and out of the cell.
7. Bigger cells generally have more nuclei.
8. The ribosomes, nucleus and cell membrane are found in all cells
9. Energy can be changed from one form to another but it cannot be created nor destroyed in the
cell.
10. ATP is the principal energy carrier in living systems.
Matching type: For Nos. 1-20. To which group do the following descriptions and examples belong?
Choose form the answer pool below and write the letter of your choice.
A. plant cell
B. animal cell
C. prokaryotic cell
D. eukaryotic cell
E. all cells
___ with cell membrane
___with nucleus
___with ribosomes
___ with lysosome
___with chloroplast
___with cellulose wall
___ with chromosomes
___ with golgi bodies ___with mitochondria
___eukaryotic and undergoes photosynthesis ___threadlike sperm cell
___undrgoes cellular respiration
___has single chromosome
___no nucleus
___capable of reproduction
___no cell wall
___with large vacuole
Multiple Choice. Write the letter corresponding to your answer. Then give the reason for your
choice
1. Which shape of the cell is adapted for moving and carrying a lot of materials (a. round b. stellar
c. filamentous d. threadlike).
2. Which structures are present in all cells? (a. cell membrane, chromosome , ribosome b. cell
membrane, nucleus, ribosome c. ribosome, chromosome, cell wall d. cell wall, nucleus ,
ribosome)
3. Which cell do you think would have more mitochondria? (a. liver cell b. skin cell d. sperm cell
d. cardiac muscle cell)?
IV. Complete the following statements
1.
The cell membrane is semi-permeable because _____________________.
2.
The lysosome is the suicide bag of the cell because _________________.
3.
The powerhouse of the cell is the mitochondrion because_____________.
4.
The nucleus is called the director of the cell because ________________.
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