BIOLOGY FOR SENIOR HIGH SCHOOL GRADE XI
CHAPTER: 1
STRUCTURE AND FUNCTION OF THE CELL AS THE SMALLEST UNIT OF LIVE
By: Tri Susila Hidayati
Basic Competence: Describing Chemical Compounds, Structure and Function of Cell as
the Smallest Unit of Living.
The word cell comes from the Latin cellula, meaning, a small room. Most cells are too small to be seen
individually with an unaided human eye and typically range in diameter from about 10 to 30 micrometer
(µm) or 0, 01 to 0, 03 mm. The cell is the functional basic unit of life. It was discovered by Robert
Hooke in 1665 and is the functional unit of all known living organisms. Living things are
composed of one or more building blocks known as cells that are the basic unit of structure. Some
organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as
humans, are multicellular Cells carry out the various processes that are characteristic of `being alive`.
In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules"
while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by
Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or
more cells, that all cells come from preexisting cells, that vital functions of an organism occur within
cells, and that all cells contain the hereditary information necessary for regulating cell functions and for
transmitting information to the next generation of cells.
A. History
1. 1632–1723: Antonie van Leeuwenhoek teaches himself to grind lenses, builds a microscope and
draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
2. 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.[6]
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3. 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and
animals are made of cells, concluding that cells are a common unit of structure and development,
and thus founding the cell theory.
4. The belief that life forms can occur spontaneously (generatio spontanea) is contradicted by
Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that
suggested the same conclusion).
5. 1855: Rudolf Virchow states that cells always emerge from cell divisions (Omnis cellula ex
cellula).
6. 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of
Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing
previously unresolvable organelles.
7. 1953: Watson and Crick made their first announcement on the double-helix structure for DNA
on February 28.
8. 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.
B. Chemical Compounds in Living Cells Indicator: To explain the chemical compound in living cellsTime : 2 X 45’
All living organisms, from microbes to mammals, are composed of chemical substances from both
the inorganic and organic world, that appear in roughly the same proportions, and perform the
same general tasks. Of the elements in the living material of cell, hydrogen, oxygen, nitrogen,
carbon, is present in the greatest amount. Phosphorus, sulfur, magnesium, iodine, iron, calcium,
sodium, chlorine and potassium are found in smaller quantities. When combined in various ways,
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M.J Schleiden & Theodor SchwannRudolf Virchow
form virtually all known inorganic and organic biomolecules. There are four general classes of
macromolecules within living cells: nucleic acids, proteins, polysaccharides, and lipids.
They all generally contain a greater variety of proteins than any other type of macromolecule, with
about 50% of the solid matter of the cell being protein (15% on a wet weight basis). Cells generally
contain many more protein molecules than DNA molecules, yet DNA is typically the largest
biomolecule in the cell. About 99% of cellular molecules are water molecules, with water normally
accounting for approximately 70% of the total wet-weight of the cell. Although water is obviously
important to the vitality of all living cells, the bulk of our attention is usually focused on the other
1% of biomolecules.
1. Nucleic Acids
Nucleic acids are a group of organic compounds that are essential to life. These are the
compounds that pass hereditary information from one generation to another, making possible
a remarkable continuity of life within the various species of living things. Genetic information
contained in nucleic acids is stored and replicated in chromosomes, which contain genes. A
chromosome is a deoxyribonucleic acid (DNA) molecule, and genes are segments of intact DNA.
The total number of genes in any given mammalian cell may total several thousand. Nucleic
acids are biological molecules essential for life, and include DNA (deoxyribonucleic acid) and
RNA (ribonucleic acid). Together with protein, nucleic acids make up the most important
macromolecules; each is found in abundance in all living things, where they function in
encoding, transmitting and expressing genetic information. There are five easy parts of nucleic
acids. All nucleic acids are made up of the same building blocks (monomers). Chemists call the
monomers nucleotides. The five pieces are Uracil, Cytosine, Thymine, Adenine, and Guanine.
Just as there are twenty (20) amino acids needed by humans to survive, there are five (5)
nucleotides.
These nucleotides are made of three parts.
1. A five carbon sugar
2. A base that has a nitrogen (N) atom
3. An ion of phosphoric
acid
3
Source:
http://www.chem4kids.com/files/bio_nucleicacids.html
2. Proteins
Many foods contain protein (say: pro-teen), but the best sources are beef, poultry, fish, eggs,
dairy products, nuts, seeds, and legumes like black beans and lentils. Protein builds up,
maintains, and replaces the tissues in your body. Proteins are organic compounds composed of
the elements carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain sulfur. All
proteins are built from small molecular units known as amino acids. A typical protein contains
200–300 amino acids but some are much smaller (the smallest are often called peptides) and
some much larger (the largest to date is titin a protein found in skeletal and cardiac muscle; one
version contains 34,350 amino acids in a single chain!). Proteins with covalently linked
carbohydrate are called glycoproteins.
3. Polysaccharides
Polysaccharides are the complex carbohydrates. Carbohydrates are composed of the elements
carbon, hydrogen, and oxygen. Hydrogen and oxygen atoms are usually present in
carbohydrates in the ratio of 2:1. Glucose (C6H12O6) represents the basic unit of carbohydrate
structure. They are made up of chains of monosaccharides (the sugars) which are linked
together by glycosidic bonds, which are formed by the condensation reaction. The linkage of
monosaccharides into chains creates chains of greatly varying length, ranging from chains of
just two monosaccharides, which makes a disaccharide to the polysaccharides, which consists
of many thousands of the sugars. When all the monosaccharides in a polysaccharide are the
same type the polysaccharide is called a homopolysaccharide or homoglycan, but when more
than one type of monosaccharide is present they are called heteropolysaccharides or
heteroglycans.
4. Lipids
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Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Proteins.html
Source:
http://en.wikipedia.org/wiki/Polysaccharide
Assignment 1.1
a. Find out the references from other media (internet or books). That references connective to material about Chemical Compounds in Living Cells
b. Don’t forget you write the web address or the title of book. c. Where can you find out that chemical substance in the living cell body?d. Explain the chemical compounds in living cells.
The lipids are group of organic compounds that include the fats and fat-like substances. A lipid
molecule contains the elements carbon,
hydrogen and oxygen. In lipid molecules
the ratio of hydrogen to oxygen is much
greater than 2:1. A lipid molecule is
made up of two basic units: an alcohol
usually glycerol and a class of
compounds called fatty acids. Fatty Acids are the lipid building blocks: The common building
block for most of the different types of lipids is the fatty acid. Fatty acids are composed of a
chain of methylene groups with a Carboxyl functional group at one end. The methyl chain is the
fatty part, the Carboxyl, the acid. The fatty acid chains are usually between 10 and 20 Carbon
atoms long. The fatty "tail" is non-polar (Hydrophobic) while the Carboxyl "head" is a little polar
(Hydrophillic).
Fatty acids can be saturated (meaning
they have as many hydrogens bonded to
their carbons as possible) or
unsaturated (with one or more double
bonds connecting their carbons, hence
fewer hydrogens). A fat is a solid at
room temperature, while oil is a liquid under the same conditions. The fatty acids in oils are
mostly unsaturated, while those in fats are mostly saturated. Triglycerides: Energy Storage,
Three fatty acids bonded to Glycerol. Triglycerides are Energy-storage molecules. They are
formed by connecting three fatty acids (shown
in black) to the red part of the molecule on
the left, Glycerol. As you can imagine, the
three fatty acids together, contain a lot of
Energy (aka Calories). Fat has a lot of calories.
5
Source:
http://bioweb.wku.edu/courses/biol115/wyatt/biochem/lipid/lipid1.htm
C. Structure and Functions of Living Cells Indicator: 1. To use microscope for observe cell structure of fresh cell or preserves cell2. To picture cell structure 3. To show a part of cell 4. To explain structure and function of cells
Time : 4 x 45’
The structures of prokaryotic cell and eukaryotic cell have many different. Prokaryotic cells
include bacteria and blue green algae. Prokaryotic cell do not have nucleus membrane, so its
genetic material is mixed with cytoplasm. But eukaryotic cell has nucleus membrane so there is
a separation between the nucleus and cytoplasm. Eukaryotic cells can be found in animal and
plant cells. Part compiler of animal and plant cell has some similarities, namely the cell
membrane, nucleus, cytoplasm, cytoskeleton, ribosomes, endoplasmic reticulum, Golgi
apparatus, lysosomes, peroxisomes, and mitochondria.
1. Prokaryotic Cell
The simplest of cells and the first types of cells to evolve, were prokaryotic cells— organisms
that lack a nuclear membrane, the membrane that surrounds the nucleus of a cell. Bacteria
are the best known and most studied form of prokaryotic organisms, although the recent
discovery of a second group of prokaryotes, called archaea, has provided evidence of a third
cellular domain of life and new insights into the origin of life itself.
Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular
forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is
identical and capable of independent existence. The cells may be adjacent to one another
because they did not separate after cell division or because they remained enclosed in a
common sheath or slime secreted by the cells. Typically though, there is no continuity or
communication between the cells. Prokaryotes are capable of inhabiting almost every place
on the earth, from the deep ocean, to the edges of hot springs, to just about every surface
of our bodies.
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Assignment 1.2
For more understand of prokaryotic cell, find out sample of the prokaryotic cells include living things. Take pictures of the living things named. Give explanation of
pictures.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization,
specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular
organelles and structures that are characteristic of eukaryotic cells. Most of the functions of
organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by
the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions:
appendages called flagella and pili—proteins attached to the cell surface; a cell envelope
consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that
contains the cell genome (DNA) and ribosomes and various sorts of inclusions.
2. Eukaryotic Cell
Eukaryotes include fungi, animals, and plants as well as some unicellular organisms.
Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000
times greater in volume. The major and extremely significant difference between
prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound
compartments in which specific metabolic activities take place. Most important among
these is the presence of a nucleus, a membrane-delineated compartment that houses the
eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its
name.
Eukaryotic organisms also have other specialized structures, called organelles, which are
small structures within cells that perform dedicated functions. As the name implies, you can
think of organelles as small organs. There are a dozen different types of organelles
commonly found in eukaryotic cells. In this primer, we will focus our attention on only a
handful of organelles and will examine these organelles with an eye to their role at a
molecular level in the cell.
The origin of the eukaryotic
cell was a
milestone
in the
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evolution of life. Although eukaryotes use the same genetic code and metabolic processes
as prokaryotes, their higher level of organizational complexity has permitted the
development of truly multicellular organisms. Without eukaryotes, the world would lack
mammals, birds, fish, invertebrates, mushrooms, plants, and complex single-celled
organisms.
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This figure illustrates a typical human cell (eukaryote) and a typical bacterium (prokaryote).
The drawing on the left highlights the internal structures of eukaryotic cells, including the
nucleus (light blue), the nucleolus (intermediate blue), mitochondria (orange), and ribosomes
(dark blue). The drawing on the right demonstrates how bacterial DNA is housed in a
structure called the nucleoid (very light blue), as well as other structures normally found in a
prokaryotic cell, including the cell membrane (black), the cell wall (intermediate blue), the
capsule (orange), ribosomes (dark blue), and a flagellum (also black).
Source:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
Activity 1.1 Observation of cell component
Time: 2 X 45’
Purpose : observe at components of cell
Tools and materials:
Pencil knife or Gillette
Spatula or tooth pick
Microscope
Microscope slide and glass cover
Optilab
Computer or Laptop
Stem of Manihot utilisima
Epidermis of Red onion
Membrane epithelium of mucous mouth ( Epithelium mucosa cavum oris ) or membrane epithelium of intestine
Neutral red 1% diluted in distilled water.
Procedures:
Make thin across section of the corky or spongy wood of Manihot utilisima, length section of Red onion epidermis, scrape on the mouth mucous membrane or intestine membrane with a clean spatula or blunt end of tooth pick.
Put down on the microscope slide, then dropped with neutral red or water
Set optilab in the microscope.
Observed under a microscope immediately.
Connective microscope, optilab with computer.
Find out a good picture the epidermis cell of corky or spongy wood of Manihot utilisima, the epidermis cell of red Allium, the epithelium cell of mucous or intestine.
Take that cell pictures. Write down what elements.
Make a table difference of the three cells named.
Question:
What is the name of each structure that you see under the microscope? Describe in your paper.
What is the function of every specimen’s structure that you observe?
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Cell Structures
1. The Plasma Membrane—A Cell's Protective Coat
All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their
contents and serves as a semi-porous barrier to the outside environment. The membrane
acts as a boundary, holding the cell constituents together and keeping other substances
from entering. The plasma membrane is permeable to specific molecules, however, and
allows nutrients and other essential elements to enter the cell and waste materials to leave
the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely
across the membrane, but the passage of larger molecules, such as amino acids and sugars,
is carefully regulated.
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Assignment 1.3.
For more understand of eukaryotic cell, find the pictures of plasma membrane, nucleus and cell organelles. Describe the structure of each picture.
Source:
http://micro.magnet.fsu.edu/cells/plasmamembrane/plasmamembrane.html
According to the accepted current theory, known as the fluid mosaic model, the plasma
membrane is composed of a double layer (bilayer) of lipids, oily substances found in all cells
(see Figure 1). Most of the lipids in the bilayer can be more precisely described as
phospholipids, that is, lipids that feature a phosphate group at one end of each molecule.
Phospholipids are characteristically hydrophilic ("water-loving") at their phosphate ends
and hydrophobic ("water-fearing") along their lipid tail regions. In each layer of a plasma
membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate
groups are aligned so they face outwards, either toward the aqueous cytosol of the cell or
the outside environment. Phospholipids tend to spontaneously aggregate by this
mechanism whenever they are exposed to water.
Within the phospholipid bilayer of the plasma membrane, many diverse proteins are
embedded, while other proteins simply adhere to the surfaces of the bilayer. Some of these
proteins, primarily those that are at least partially exposed on the external side of the
membrane, have carbohydrates attached to their outer surfaces and are, therefore,
referred to as glycoproteins. The positioning of proteins along the plasma membrane is
related in part to the organization of the filaments that comprise the cytoskeleton, which
help anchor them in place. The arrangement of proteins also involves the hydrophobic and
hydrophilic regions found on the surfaces of the proteins: hydrophobic regions associate
with the hydrophobic interior of the plasma membrane and hydrophilic regions extend past
the surface of the membrane into either the inside of the cell or the outer environment.
Plasma membrane proteins function in several different ways. Many of the proteins play a
role in the selective transport of certain substances across the phospholipid bilayer, either
acting as channels or active transport molecules. Others function as receptors, which bind
information-providing molecules, such as hormones, and transmit corresponding signals
based on the obtained information to the interior of the cell. Membrane proteins may also
exhibit enzymatic activity, catalyzing various
reactions related to the plasma membrane.
2. The Cytoskeleton—A Cell's Scaffold
The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize
and maintains the cell's shape; anchors organelles in place; helps during endocytosis, the
uptake of external materials by a cell; and moves parts of the cell in processes of growth
and motility. There are a great number of proteins associated with the cytoskeleton, each
controlling a cell’s structure by directing, bundling, and aligning filaments. Three types of
filaments make up the cytoskeleton.
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1. Microfilaments are the thinnest and most abundant of the cytoskeleton proteins. They are
composed of actin, a contractile protein, and can be assembled and disassembled quickly
according to the needs of the cell or organelle structure.
2. Intermediate filaments are slightly larger in diameter and are found most extensively in
regions of cells that are going to be subjected to stress. Once these filaments are assembled
they are not capable of rapid disassembly.
3. Microtubules are hollow tubes composed of a protein called tubulin. They are the thickest
and most rigid of the filaments. Microtubules are present in the axons and long dendrite
projections of nerve cells. They are capable of rapid assembly and disassembly according to
need. Microtubules are structured around a cell region called the centrosome, which
surrounds two centrioles composed of 9 sets of fused microtubules. These are important in
cell division when the centrosome generates the microtubluar spindle fibers necessary for
chromosome separation.
3. The Cytoplasm—A Cell's Inner Space
Inside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the
cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the
cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of
the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste
products, and moves material around the cell through a process called cytoplasmic
streaming. The nucleus often flows with the cytoplasm changing its shape as it moves. The
cytoplasm also contains many salts and is an excellent conductor of electricity, creating the
perfect environment for the mechanics of the cell. The function of the cytoplasm, and the
organelles which reside in it, are critical for a cell's survival.
Cell Organelles
The human body contains many different organs, such as the heart, lung, and kidney, with each
organ performing a different function. Cells also have a set of "little organs", called organelles,
12
Source:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
which are adapted and/or specialized for carrying out one or more vital functions. Organelles
are found only in eukaryotes and are always surrounded by a protective membrane. It is
important to know some basic facts about the following organelles.
1. The Nucleus—A Cell's Center
The nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's
chromosomes and is the place where almost all DNA replication and RNA synthesis occur.
The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called
the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various
molecules that could accidentally damage its structure or interfere with its processing.
During processing, DNA is transcribed, or synthesized, into a special RNA, called mRNA. This
mRNA is then transported out of the nucleus, where it is translated into a specific protein
molecule. In prokaryotes, DNA processing takes place in the cytoplasm. Inside the Nucleus,
there are:
Chromosomes
- Usually in the form of chromatin
- Contains genetic information
- Composed of DNA
- Thicken for cellular division
- Set number per species (i.e. 23 pairs for human)
Nuclear membrane
- Surrounds nucleus
- Composed of two layers
- Numerous openings for nuclear traffic
Nucleolus
- Spherical shape
- Visible when cell is not dividing
- Contains RNA for protein manufacture
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Source:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
Pictures:
http://library.thinkquest.org/12413/structures.html
Sources:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
http://www.biology4kids.com/files/cell_ribos.html
Pictures:
http://library.thinkquest.org/12413/structures.html
2. The Ribosome—the Protein Production Machine
Ribosomes are found in both prokaryotes and eukaryotes.
The ribosome is a large complex composed of many
molecules, including RNAs and proteins, and is responsible
for processing the genetic instructions carried by an mRNA.
The process of converting an mRNA's genetic code into the
exact sequence of amino acids that make up a protein is called translation. Protein
synthesis is extremely important to all cells, and therefore a large number of ribosomes—
sometimes hundreds or even thousands—can be found throughout a cell.
Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the
endoplasmic reticulum. A ribosome is not just one piece. There are two pieces or subunits.
Scientists named them 60-S (large) and 40-S (small). When the cell needs to make protein,
mRNA is created in the nucleus. The mRNA is then sent into the cell and the ribosomes.
When it is time to make the protein, the two subunits come together and combine with the
mRNA. The subunits lock onto the mRNA and start the protein synthesis.
The 60-S/ 40-S model works fine for eukaryotic
cells. Prokaryotic cells have ribosomes made of
50-S and 30-S subunits. It's a small difference,
but one of many you will find in the two
different types of cells. Scientists have used this
difference in ribosome size to develop drugs that
can kill prokaryotic microorganisms that cause
disease.
3. Mitochondria and Chloroplasts—The Power Generators
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Mitochondria are self-
replicating organelles that occur
in various numbers, shapes, and
sizes in the cytoplasm of all
eukaryotic cells. As mentioned
earlier, mitochondria contain
their own genome that is
separate and distinct from the
nuclear genome of a cell.
Mitochondria have two
functionally distinct membrane
systems separated by a space: the outer membrane, which surrounds the whole organelle;
and the inner membrane, which is thrown into folds or shelves that project inward. These
inward folds are called cristae. The number and shape of cristae in mitochondria differ,
depending on the tissue and organism in which they are found, and serve to increase the
surface area of the membrane.
Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process
involves a number of complex pathways. Let's break down each of these steps so that you
can better understand how food and nutrients are turned into energy packets and water.
Some of the best energy-supplying foods that we eat contain complex sugars. These
complex sugars can be broken down into a less chemically complex sugar molecule called
glucose. Glucose can then enter the cell through special molecules found in the membrane,
called glucose transporters. Once inside the cell, glucose is broken down to make adenosine
triphosphate (ATP), a form of energy, via two different pathways.
The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic
metabolism. Glycolysis occurs in the cytoplasm outside the mitochondria. During glycolysis,
glucose is broken down into a molecule called pyruvate. Each reaction is designed to
produce some hydrogen ions that can then be used to make energy packets (ATP).
However, only four ATP molecules can be made from one molecule of glucose in this
pathway. In prokaryotes, glycolysis is the only method used for converting energy.
The second pathway, called the Kreb's cycle, or the citric acid cycle, occurs inside the
mitochondria and is capable of generating enough ATP to run all the cell functions. Once
again, the cycle begins with a glucose molecule, which during the process of glycolysis is
stripped of some of its hydrogen atoms, transforming the glucose into two molecules of
15
Source:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
Pictures:
http://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html
pyruvic acid. Next, pyruvic acid is altered by the removal of a carbon and two oxygens,
which go on to form carbon dioxide. When the carbon dioxide is removed, energy is given
off, and a molecule called NAD+ is converted into the higher energy form, NADH. Another
molecule, coenzyme A (CoA), then attaches to the remaining acetyl unit, forming acetyl
CoA.
Acetyl CoA enters the Kreb's cycle by joining to a four-carbon molecule called oxaloacetate.
Once the two molecules are joined, they make a six-carbon molecule called citric acid. Citric
acid is then broken down and modified in a stepwise fashion. As this happens, hydrogen
ions and carbon molecules are released. The carbon molecules are used to make more
carbon dioxide. The hydrogen ions are picked up by NAD and another molecule called
flavin-adenine dinucleotide (FAD). Eventually, the process produces the four-carbon
oxaloacetate again, ending up where it started
off. All in all, the Kreb's cycle is capable of
generating from 24 to 28 ATP molecules from
one molecule of glucose converted to pyruvate.
Therefore, it is easy to see how much more
energy we can get from a molecule of glucose if
our mitochondria are working properly and if
we have oxygen.
One of the most widely
recognized and important
characteristics of plants are
their ability to conduct
photosynthesis, in effect, to
make their own food by
converting light energy into
chemical energy. This
process occurs in almost all
plant species and is carried
out in specialized organelles
known as chloroplasts. All of the green structures in plants, including stems and unripened
fruit, contain chloroplasts, but the majority of photosynthesis activity in most plants occurs
in the leaves. On the average, the chloroplast density on the surface of a leaf is about one-
half million per square millimeter.
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Chloroplasts are one of several different types of plastids, plant cell organelles that are
involved in energy storage and the synthesis of metabolic materials. The colorless
leucoplasts, for instance, are involved in the synthesis of starch, oils, and proteins. Yellow-
to-red colored chromoplasts manufacture carotenoids, and the green colored chloroplasts
contain the pigments chlorophyll a and chlorophyll b, which are able to absorb the light
energy needed for photosynthesis to occur. All plastids develop from tiny organelles found
in the immature cells of plant meristems (undifferentiated plant tissue) termed proplastids,
and those of a particular plant species all contain copies of the same circular genome. The
disparities between the various types of plastids are based upon the needs of the
differentiated cells they are contained in, which may be influenced by environmental
conditions, such as whether light or darkness surrounds a leaf as it grows.
The ellipsoid-shaped chloroplast is enclosed in a double membrane and the area between
the two layers that make up the membrane is called the intermembrane space. The outer
layer of the double membrane is much more permeable than the inner layer, which
features a number of embedded membrane transport proteins. Enclosed by the chloroplast
membrane is the stroma, a semi-fluid material that contains dissolved enzymes and
comprises most of the chloroplast's volume. Since, like mitochondria, chloroplasts possess
their own genomes (DNA), the stroma contains chloroplast DNA and special ribosomes and
RNAs as well. In higher plants, lamellae, internal membranes with stacks (each termed a
granum) of closed hollow disks called thylakoids, are also usually dispersed throughout the
stroma. The numerous thylakoids in each stack are thought to be connected via their
lumens (internal spaces).
Light travels as packets of energy called photons and are absorbed in this form by light-
absorbing chlorophyll molecules embedded in the thylakoid disks. When these chlorophyll
molecules absorb the photons, they emit electrons, which they obtain from water (a
process that results in the release of oxygen as a byproduct). The movement of the
electrons causes hydrogen ions to be propelled across the membrane surrounding the
thylakoid stack, which consequently initiates the formation of an electrochemical gradient
that drives the stroma's production of adenosine triphosphate (ATP). ATP is the chemical
energy "currency" of the cell that powers the cell's metabolic activities. In the stroma, the
light-independent reactions of photosynthesis, which involve carbon fixation, occur, and
low-energy carbon dioxide is transformed into a high-energy compound like glucose.
Plant cells are remarkable in that they have two organelles specialized for energy
production: chloroplasts, which create energy via photosynthesis, and mitochondria, which
17
Source:
http://micro.magnet.fsu.edu/cells/chloroplasts/chloroplasts.html
generate energy through respiration, a particularly important process when light is
unavailable. Like the mitochondrion, the chloroplast is different from most other organelles
because it has its own DNA and reproduces independently of the cell in which it is found; an
apparent case of endosymbiosis. Scientists hypothesize that millions of years ago small,
free-living prokaryotes were engulfed, but not consumed, by larger prokaryotes, perhaps
because they were able to resist the digestive enzymes of the engulfing organism. According
to DNA evidence, the eukaryotic organisms that later became plants likely added the
photosynthetic pathway in this way, by acquiring a photosynthetic bacterium as an
endosymbiont.
4. The Endoplasmic Reticulum and the Golgi apparatus—Macromolecule Managers
The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain
modifications and specific destinations, as compared to molecules that will float freely in
the cytoplasm. The ER has two forms: the rough ER and the smooth ER. The rough ER is
labeled as such because it has ribosomes adhering to its outer surface, whereas the smooth
ER does not. Translation of the mRNA for those proteins that will either stay in the ER or be
exported (moved out of the
cell) occurs at the ribosomes
attached to the rough ER. The
smooth ER serves as the
recipient for those proteins
synthesized in the rough ER.
Proteins to be exported are
passed to the
Golgi
apparatus,
sometimes
called a Golgi
body or Golgi
complex, for
further
processing,
packaging,
18
Sources:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
http://www.cytochemistry.net/Cell-biology/golgi.htm
and transport to a variety of other cellular locations. The Golgi complex controls trafficking
of different types of proteins. Some are destined for secretion. Others are destined for the
extracellular matrix. Finally, other proteins, such as lysosomal enzymes, may need to be
sorted and sequestered from the remaining constituents because of their potential
destructive effects. This figure shows the two types of secretory pathways. The regulated
secretory pathway, as its name implies, is a pathway for proteins that requires a stimulus or
trigger to elicit secretion. Some stimuli regulate synthesis of the protein as well as its
release. The constitutive pathway allows for secretion of proteins that are needed outside
the cell, like in the extracellular matrix. It does not require stimuli, although growth factors
may enhance the process.
Finally, this cartoon also shows the packaging of lysosomes which will be discussed in more
detail later
5. Lysosomes and Peroxisomes—the Cellular Digestive System
Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell.
Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive
enzymes, naturally occurring proteins that speed up biochemical processes. For example,
lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic
acids, and certain sugars called polysaccharides. All of these enzymes work best at a low pH,
reducing the risk that these enzymes will digest their own cell should they somehow escape
from the lysosome. Here we can see the importance behind compartmentalization of the
eukaryotic cell. The cell could not house such destructive enzymes if they were not
contained in a membrane-bound system. Lysosomes also degrade worn out organelles such
as mitochondria. In this cartoon, a section of rough endoplasmic reticulum wraps itself
around a mitochondrion and forms a vacuole. Then, vesicles carrying lysosomal enzymes
fuse with the vesicle and the vacuole becomes an active secondary lysosome.
A third function
for lysosomes is
to handle the
products of
receptor-
mediated
endocytosis such
as the receptor,
ligand and
19
associated membrane. In this case, the early coalescence of vesicles bringing in the receptor
and ligand produces an endosome. Then, the introduction of lysosomal enzymes and the
lower pH causes release, and degradation of the contents. This can be used for recycling of
the receptor and other membrane components.
All eukaryotes are comprised of one or more cells that contain peroxisomes. The organelles
were first discovered by the Belgian scientist Christian de Duve, who also discovered
lysosomes.
Peroxisomes contain a variety of enzymes, which primarily function together to rid the cell of toxic
substances, and in particular, hydrogen peroxide (a common byproduct of cellular metabolism).
These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the
potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as
those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from
the poisons to molecules of oxygen (a process termed oxidation). Others are more important for
their ability to initiate the production of phospholipids, which are typically used in the formation of
membranes.
In order to carry out their activities, peroxisomes use significant amounts of oxygen. This
characteristic of the organelles would have been extremely important millions of years ago, before
cells contained mitochondria, when the Earth's atmosphere first began to amass large amounts of
oxygen due to the actions of photosynthetic bacteria. Peroxisomes would have been primarily
responsible at that time for detoxifying cells by decreasing their levels of oxygen, which was then
poisonous to most forms of life. The organelles would have provided the cellular benefit of carrying
out a number of advantageous reactions as well. Later, when mitochondria eventually evolved,
peroxisomes became less important (in some ways) to the cell since mitochondria also utilize oxygen
to carry out many of the same reactions, but with the additional benefit of generating energy in the
form of adenosine triphosphate (ATP) at the same time.
Peroxisomes are similar in
appearance to lysosomes,
another type of
microbody, but the two
have very different
origins. Lysosomes are
generally formed in the
Golgi complex, whereas
20
Sources:
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
http://www.cytochemistry.net/cell-biology/lysosome.htm
http://micro.magnet.fsu.edu/cells/peroxisomes/peroxisomes.html
peroxisomes self-replicate. Unlike self-replicating mitochondria, however, peroxisomes do not have
their own internal DNA molecules. Consequently, the organelles must import the proteins they need
to make copies of themselves from the surrounding cytosol. The importation process of
peroxisomes is not yet well understood, but it appears to be heavily dependent upon peroxisomal
targeting signals composed of specific amino acid sequences. These signals are thought to interact
with receptor proteins present in the cytosol and docking proteins present in the peroxisomal
membrane. As more and more proteins are imported into lumen of a peroxisome or are inserted
into its membrane, the organelle gets larger and eventually reaches a point where fission takes
place, resulting in two daughter peroxisomes. Illustrated in Figure 2 is a fluorescence digital image of
an African water mongoose skin fibroblast cell stained with fluorescent probes targeting the nucleus
(red), actin cytoskeletal network (blue), and peroxisomes (green).
The Organelles of Plant and Animal Cells
Plant Cell
Plant Cell’s special organelles are Cell Wall, Vacuoles, and Plastids.
a. Cell Wall
Cell walls made of cellulose are only found around plant cells.
Cell walls are made of specialized sugars called cellulose.
Cellulose provides a protected framework for a plant cell to
survive. It's like taking a water balloon and putting it in a
cardboard box. The balloon is protected from the outside
world. Cellulose is called a structural carbohydrate (complex
sugar) because it is used in
protection and support.
Cell walls also help a plant keep its shape. While they do protect the
cells, cell walls and cellulose also allow plants to grow to great heights.
While you have a skeleton to hold you up, a 100-foot tall redwood tree
does not. It uses the strong cell walls to maintain its shape. For smaller
plants, cell walls are slightly elastic. Wind can push them over and then
they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).
A cell wall is not a fortress around the delicate plant cell. There are small holes in the wall that let
nutrients, waste, and ions pass through. Those holes are called plasmodesmata. These holes have a
problem: water can also be lost. But even when the plant cell loses water, the basic shape is
maintained by the cell walls. So if a plant is drooping because it needs water, it can recover when
water is added. It will look just the same as when it started.
b. Vacuoles
21
Vacuoles are membrane-bound sacs within the cytoplasm of a cell that function in several
different ways. In mature plant cells, vacuoles tend to be very large and are extremely
important in providing structural support, as well as serving functions such as storage,
waste disposal, protection, and
growth. Many plant cells have a
large, single central vacuole that
typically takes up most of the room
in the cell (80 percent or more).
Vacuoles in animal cells, however,
tend to be much smaller, and are
more commonly used to
temporarily store materials or to
transport substances.
The central vacuole in plant cells (see Figure 1) is enclosed by a membrane termed the
tonoplast, an important and highly integrated component of the plant internal membrane
network (endomembrane) system. This large vacuole slowly develops as the cell matures by
fusion of smaller vacuoles derived from the endoplasmic reticulum and Golgi apparatus.
Because the central vacuole is highly selective in transporting materials through its
membrane, the chemical palette of the vacuole solution (termed the cell sap) differs
markedly from that of the surrounding cytoplasm. For instance, some vacuoles contain
pigments that give certain flowers their characteristic colors. The central vacuole also
contains plant wastes that taste bitter to insects and animals, while developing seed cells
use the central vacuole as a repository for protein storage.
Among its roles in plant cell function, the central vacuole stores salts, minerals, nutrients,
proteins, pigments, helps in plant growth, and plays an important structural role for the
plant. Under optimal conditions, the vacuoles are filled with water to the point that they
exert a significant pressure against the cell wall. This helps maintain the structural integrity
of the plant, along with the support from the cell wall, and enables the plant cell to grow
much larger
without
having to
synthesize
new
cytoplasm.
In most cases, the plant cytoplasm is confined to a thin layer positioned between the
22
plasma membrane and the tonoplast, yielding a large ratio of membrane surface to
cytoplasm.
The structural importance of the plant vacuole is related to its ability to control turgor
pressure. Turgor pressure dictates the rigidity of the cell and is associated with the
difference between the osmotic pressure inside and outside of the cell. Osmotic pressure is
the pressure required to prevent fluid diffusing through a semi permeable membrane
separating two solutions containing different concentrations of solute molecules. The
response of plant cells to water is a prime example of the significance of turgor pressure.
When a plant receives adequate amounts of water, the central vacuoles of its cells swell as
the liquid collects within them, creating a high level of turgor pressure, which helps
maintain the structural integrity of the plant, along with the support from the cell wall. In
the absence of enough water, however, central vacuoles shrink and turgor pressure is
reduced, compromising the plant's rigidity so that wilting takes place.
Plant vacuoles are also important for their role in molecular degradation and storage.
Sometimes these functions are carried out by different vacuoles in the same cell, one
serving as a compartment for breaking down materials (similar to the lysosomes found in
animal cells), and another storing nutrients, waste products, or other substances. Several of
the materials commonly stored in plant vacuoles have been found to be useful for humans,
such as opium, rubber, and garlic flavoring, and are frequently harvested. Vacuoles also
often store the pigments that give certain flowers their colors, which aid them in the
attraction of bees and other pollinators, but also can release molecules that are poisonous,
odoriferous, or unpalatable to various insects and animals, thus discouraging them from
consuming the plant.
c. Plastids
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of
manufacture and storage of important chemical compounds used by the cell. Plastids often
contain pigments used in photosynthesis, and the types of pigments present can change or
determine the cell's color.
Plastids are responsible for photosynthesis, storage of products like starch and for the
synthesis have the ability to differentiate, or
redifferentiate, between these and other forms.
All plastids are derived from proplastids
(formerly "eoplasts", eo-: dawn, early), which are
23
Source:
http://micro.magnet.fsu.edu/cells/vacuole/vacuole.html
present in the meristematic regions of the plant. Proplastids and young chloroplasts
commonly divide, but more mature chloroplasts also have this capacity.
In plants, plastids may differentiate into several forms, depending upon which function they
need to play in the cell. Undifferentiated plastids (proplastids) may develop into any of the
following plastids:
-Chloroplasts: for photosynthesis; see also etioplasts, the predecessors of chloroplasts (See
Chloroplasts above)
-Chromoplasts: Chromoplasts are plastids responsible for pigment synthesis and storage.
They, like all other plastids (including chloroplasts and leucoplasts), are organelles found in
specific photosynthetic eukaryotic species.
Chromoplasts in the traditional sense are found in colored organs of plants such as fruit and
floral petals, to which they give their distinctive colors. This is always associated with a
massive increase in the accumulation of carotenoid pigments. The conversion of
chloroplasts to chromoplasts in ripening is a classic example.
Chromoplasts synthesize and store pigments such as orange carotene, yellow xanthophylls,
and various other red pigments; as such, their color varies depending on what pigment they
contain. The probable main evolutionary role of chromoplasts is to act as an attractant for
pollinating animals (e.g., insects) or for seed dispersal via the eating of colored fruits. They
allow the accumulation of large quantities of water-insoluble compounds in otherwise
watery parts of plants. In chloroplasts, some carotenoids are also used as accessory
pigments in photosynthesis, where they act to increase the efficiency of chlorophyll in
harvesting light energy. When leaves change color in autumn, it is due to the loss of green
chlorophyll unmasking these carotenoids that are already present in the leaf. In this case,
relatively little new carotenoids are produced. Therefore, the change in plastid pigments
associated with leaf senescence is somewhat different from the active conversion to
chromoplasts observed in fruit and flowers.
-Gerontoplasts: control the dismantling of the photosynthetic apparatus during senescence
-Leucoplasts: Leucoplasts are a category of plastid and as such are organelles found in plant
cells. They are non-pigmented, in contrast to other plastids such as the chloroplast.
24
Leucoplasts, specifically, amyloplasts
Lacking pigments, leucoplasts are not green, so they are predictably located in roots and
non-photosynthetic tissues of plants. They may become specialized for bulk storage of
starch, lipid or protein and are then known as amyloplasts, elaioplasts, or proteinoplasts
respectively. However, in many cell types, leucoplasts do not have a major storage function
and are present to provide a wide range of essential biosynthetic functions, including the
synthesis of fatty acids, many amino
acids, and tetrapyrrole compounds such
as haem. In general, leucoplasts are much
smaller than chloroplasts and have a
variable morphology, often described as
amoeboid. Extensive networks of
stromules interconnecting leucoplasts
have been observed in epidermal cells of
roots, hypocotyls, and petals, and in
callus and suspension culture cells of tobacco. In some cell types at certain stages of
development, leucoplasts are clustered around the nucleus with stromules extending to the
cell periphery, as observed for proplastids in the root meristem.
Leucoplasts sometimes differentiate into more specialized plastids:
o Amyloplasts : for starch storage and detecting gravity
o Elaioplasts : for storing fat
o Proteinoplasts : for storing and modifying protein
Animal Cell
Animal Cell’s special organelles are Centrioles and Lysosome
a. Centrioles
Found only in animal cells, these paired organelles are typically located together near the
nucleus in the centrosome, a granular mass that serves as an organizing center for
microtubules. Within the centrosome, the centrioles are positioned so that they are at right
25
Sources:
http://en.wikipedia.org/wiki/Plastids
http://en.wikipedia.org/wiki/Chromoplast
http://en.wikipedia.org/wiki/Leucoplast
Sources:
http://micro.magnet.fsu.edu/cells/centrioles/centrioles.html
angles to each other, as illustrated in Figure 1. Each centriole is made of nine bundles of
microtubules (three per bundle) arranged in a ring.
Centrioles play a notable role
in cell division. During
interphase of an animal cell,
the centrioles and other
components of the
centrosome are duplicated,
though scientists are not yet
sure how this duplication
takes place. At first the two
pairs of centrioles remain in close proximity to each other, but as mitosis initiates, the
original centrosome divides and the pairs are split up so that one set of centrioles is located
in each of the new microtubule-organizing centers. These new centers radiate microtubules
in star-shaped clusters known as asters. As the asters move to opposing poles of the cells,
the microtubules, with the help of the centrioles, become organized into a spindle-shaped
formation that spans the cell. These spindle fibers act as guides for the alignment of the
chromosomes as they separate later during the process of cell division.
b. Lysosome
(See Lysosome above)
Cell Transport
The cell's plasma membrane does not simply form a "sack" in which to keep all the cytoplasm
and other cellular organelles. The plasma membrane is a very important structure which
functions to allow certain substances to enter or leave the cell. It can "pump" other substance
into the cell against the concentration gradient or pump other "wastes" etc. out of the cell.
Some of the transport process happens "passively" without the cell needing to expend any
energy to make them happen. These processes are called "passive transport processes".
Other transport processes require energy from the cell's reserves to "power" them. These
processes are called "active transport processes"
Passive Transport
26
Passive transport is the movement of a substance across a cell membrane without the input of the cell's energy.
Simple Diffusion
Simple Diffusion involves the movement of atoms across the cytolemma from a region of higher concentration to a region of lower concentration. Atoms move across the cell membrane by going between the lipid molecules that make up the cell membrane. Small atoms diffuse the easiest across the membrane. No outside chemical energy is needed for simple diffusion.
Diffusion
Facilitated Diffusion
Diffusion is facilitated by cell membrane proteins that provide a way for atoms or molecules to more easily diffuse across the membrane.
Osmosis
Osmosis is the simple diffusion of water molecules across a semi permeable membrane. It occurs when the concentrations of solutes in the solution on the two sides of a semipermeable membrane are different moves from a solution with a higher water concentration to a solution with lower water concentration.
Active Transport
Chemical energy in the form of ATP is used to begin this process. A membrane carrier is used and the direction can be from high to low concentration or from low to high concentration. Active transport can enable a cell to move items across the membrane against a concentration gradient.
Exocytosis
In exocytosis wastes and cell products are packaged by Golgi apparatus in sacs called Golgi vesicles. Golgi vesicles fuse with the cell membrane and the materials in the vesicles are secreted out of the cell.
27
Endocytosis
The cell membrane surrounds desirable macromolecules outside the cell. The cell pinches off a saclike portion of its outer membrane to form a tiny new vesicle. The vesicle moves into the cell where it releases its contents into the cytoplasm.
Pinocytosis
In Pinocytosis the cell membrane encloses a droplet of fluid and its solutes and brings the droplet into the cell.
Phagocytosis
In Phagocytosis the cell engulfs a food particle. The vesicle containing food then fuses with a lysosome carrying digestive enzymes.
28
Active Transport
Diffusion
Types of Cell Transport
Differences between Animal and Plant Cell
EDITED BY: M. YUSUF ADY H.
CLASS XI SCIENCE 5 / 19
15497
30
Source: http://library.thinkquest.org/trio/TR0110561/transport.htm#
Pictures:
http://kentsimmons.uwinnipeg.ca/cm1504/membranefunction.htm