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

29

Endocytosis

Exocytosis

Active 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