in this discussion class we are looking at cells, the basic units of life

DISCUSSION CLASS 1: Introduction to the Cell. Bio Mechanics Discussion Group, ME DEPARTMENT IISc 31/8/2006

Ashwini.B.M, Project Assistant Force Microscopy Lab ME Department, IISc

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Introduction to the Cell In this discussion class we are looking at cells, the basic units of life. Even though cells are the basic units, they are still organized and made of smaller structures. The basic properties of a cell are,

• Cells are highly complex and organized. The more complex a structure, the greater the number of parts that must be in their proper place, the less tolerance of errors in the nature and interactions of the parts and more control and regulation must be exerted to maintain the system.

• Cells possess a genetic program and the means to use it. Organisms are built according to the genetic information encoded in a collection of genes. This vast amount of information is packaged into a set of chromosomes that occupy the space of a cell nucleus. The molecular structure of genes allows for changes in genetic information that leads to variation among individuals which forms the basis of biological evolution.

• Cells are capable of producing more of themselves Just like the individual organisms are reproduced by organisms, even cells are reproduced to individual cells. Cells reproduce by division, a process in which the contents of a mother cell are distributed into two daughter cells. Prior to division the genetic material is faithfully duplicated and each daughter cell receives a complete and equal share of genetic information.

• Cells acquire and utilize energy. To develop and maintain the complexity of the cell, it needs an input of energy. All of the energy required on the earth surface arrives in the form of electromagnetic radiation from the sun. Light energy is converted by photosynthesis (due to chloroplasts in the plant cell) into chemical energy that is stored in energy rich carbohydrates, such as sucrose or starch. Once in a cell, the glucose is disassembled in such a way that its energy content can be stored in a readily available form (usually as ATP) that is later put to use in running all of the cell’s myriad energy-requiring activities.


Figure 1: The figure shows the helical structure of chloroplast in Spirogyra

• Cells carry out a variety of chemical reactions. Cells function like miniaturized chemical plants. Even the simplest bacterial cell is capable of hundreds of different chemical transformations, none of which occurs at any significant rate in the inanimate world. Virtually all chemical changes that take place in cells require enzymes-molecules that greatly increase the rate at which a chemical reaction occurs. The sum total of the chemical reactions in a cell represents that cell’s metabolism.

• Cells engage in numerous mechanical activities. Cells are sites of bustling activity. Materials are transported from place to place, structures are assembled then rapidly disassembled, and, in many cases the entire cell moves itself from one site to another. These types of activities are based on dynamic, mechanical changes within the cells, most of which are initiated by changes in the shape of “motor” proteins. Motor proteins are just one of the molecular machines employed by cells to carry out mechanical activities.

• Cells are able to respond to stimuli. Some cells respond to stimuli in obvious ways: a single celled protest, for example, moves away from an object in its path or moves toward a source of nutrients. Cells within a multicellular plant or animal respond to stimuli less obviously. Most cells are covered with receptors that interact with substances in the environment in highly specific ways. Cells possess receptors to hormones, growth factors, extracellular materials, as well as to substances on the surfaces of other cells. A cell’s receptors


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provide pathways though which external agents can evoke specific responses in target cells. Cells may respond to specific stimuli by altering their metabolic activities, preparing for cell division, moving from one place to another, or even suicide.

• Cells are capable of self-regulation.

The importance of a cell’s regulatory mechanisms becomes most evident when they break down. For example, failure of a cell to correct a mistake when it duplicates its DNA may result in a debilitating mutation, or a breakdown in a cell’s growth control can transform the cell into a cancer cell with capability of destroying the whole organism.

• Cells evolve.

If you were to observe the bacterial cell living in the human respiratory tract and the cell lining the intestinal tract, you would be struck by the differences between the two cells. Yet, both have evolved from a common ancestral cell that lived more than three billon years ago. The structures, which are shared by these two distantly related cells such as the plasma membrane and the ribosome, must have been present in the ancestral cell. Evolution is not simply an event of the past, but an ongoing process that continues to modify the properties of cells that will be present in organisms that have yet to appear.

CELLS ARE FUNDAMENTALLY DIVIDED INTO 2 CLASSES. Once the electron microscope became widely available, biologists ere able to examine the internal structure of a wide variety of cells. It became apparent from these studies that there were two basic classes of cells- prokaryotic and eukaryotic- distinguished by their size and the internal structures, or organelles, they contain. The existence of two distinct classes of cells, without any known intermediates, represents one of the most fundamental evolutionary separations in the biological world. The structurally simpler, prokaryotic cells include bacteria, whereas the structurally more complex eukaryotic cells include protists, fungi, plants and animals. STRUCTURAL ORGANISATION OF THE CELL Just as the body is made of organs, each having different shapes and functions, so the cells are made of organelles, which also have their own shape and function. We will be studying the following organelles:

1. Centrioles 2. Cilia and Flagella



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3. Endoplasmic reticulum 4. Endosomes 5. Golgi apparatus 6. Intermediate filaments 7. Lysosomes 8. Microfilaments 9. Microtubules 10. Mitochondria 11. Nucleus 12. Peroxisomes 13. Plasma membrane.


The cell can be compared to a factory. Like a factory, it makes products that need to be packaged and delivered to places inside or outside the cell. It needs energy to make its products, and blueprints to work from. Our goal in this discussion will be to understand how these organelles work together to help the cell do its work.


5 Figure 2: The Figure shows the anatomy of animal cell (http://micro.magnet.fsu.edu/cells/animals/animalmodel.html)


Table 1: The table gives description of the components and their function

S. No

Components Description Function Special Remarks

Structure

1 Centrioles Centrioles are self-replicating organelles made up of nine bundles of microtubules

Centrioles aid in cell division or mitosis of animal cells

Present only in animal cells

2 Cilia and

Flagella

Mobile cellular appendages

Cilia and flagella help to move a cell or group of cells or to help transport fluid or materials past them

Can cause disease due to malfunction.

Example: Bronchitis, Kartagener's syndrom


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3 The Endoplasmic Reticulum

The endoplasmic reticulum is a network of sacs

The endoplasmic reticulum produces and transports chemical compounds for use inside and outside of the cell.

Endoplasmic reticulum helps organelles to share information.

4 Endosomes

Endosomes are small sac or cyst covered with membranes


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5 The Golgi Apparatus

Composes a series of five to eight cup-shaped, membrane-covered sacs called cisternae

Distributes the cell's chemical products

Found in both plant and animal cells

6 Intermediate Filaments

Intermediate filaments are a very broad class of fibrous proteins

With size of 8 to 12 nm, intermediate filaments function as tension-bearing elements to help maintain cell shape and rigidity, and serve to anchor in place several organelles.

Mutations in intermediate filament genes can cause uncommon diseases.

7 Lysosomes Lysosomes are spherical organelles covered by a single layer membrane

Lysosomes break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds, which are transferred back into the cytoplasm as new cell-building materials

Present in large numbers in white blood cells


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8 Microfilaments

Microfilaments are solid rods made of globular proteins called actin

Microfilaments are 5 to 9 nm in diameter and can bear large amounts of tension.

Along with myosin, microfilaments help to generate the forces used in cellular contraction and basic cell movements

9 Microtubules

Microtubules are straight, hollow cylinders found throughout the cytoplasm of all eukaryotic cells (Not present in prokaryotes)

Microtubules, about 25 nanometers in diameter, form part of the cytoskeleton that gives structure and shape to a cell.

Eukaryotic cells depend upon the microtubules and other cytoskeletal filaments to maintain their structure.

To survive many plants produce natural toxins to help build the microtubules


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10 Mitochondria Mitochondria are rod-shaped organelles

Mitochondria are power generators of the cell, converting oxygen and nutrients into adenosine triphosphate (ATP)

Mitochondrial DNA is also used in forensic science Causes genetic diseases, such as Alzheimer's disease and diabetes.

11 The Cell

Nucleus

The nucleus is a highly specialized organelle enclosing the nucleoplasm, chromatin and nucleioli

The nucleus coordinates the cell's activities (growth, intermediary metabolism, protein synthesis, and reproduction (cell division).


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12 Peroxisomes

Peroximes are Microbodies(group of organelles) which are found in the cytoplasm of almost all cells. They are roughly spherical, and bound by a single membrane

Peroxisomes contain a variety of enzymes, which function together to get rid of toxic substances, in particular, hydrogen peroxide (a common byproduct of cellular metabolism).

Zellweger syndrome (lack of enough peroxisomes), has no cure and causes death within the first year of life.

13 Plasma

Membrane

A plasma membrane encloses all living cells, prokaryotic and eukaryotic.

The plasma membrane controls transfer of molecules in and out of the cell.


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14 Ribosomes

Ribosomes are tiny organelles(60 % ribosomal RNA (rRNA) and 40% protein)

Protein synthesis


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Figure 3: The figure shows generalized overview of a plant cell and an animal cell

BIBLIOGRAPHY 1. GERALD KARP. Cell and Molecular Biology-Concepts and Experiments. 4th edition. John Wiley and Sons. 2005. 2. The description of the organelles above has been adapted from http://micro.magnet.fsu.edu/cells/animalcell.html.


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in this discussion class we are looking at cells, the basic units of life

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