THE FUNDAMENTAL UNIT OF LIFE: CELL

The Discovery of the Cell:

Until the invention of microscope, nobody knew that living things were made up of cells. The study of many scientists over the last three centuries has developed our modern understanding of the cells. An historical approach may be appropriate: 1665 - Robert Hooke (examined a slice of cork with the help of a homemade microscope)
1650 - 1700 - Anton Van Leewenhoek (developed some earliest microscope and left written records of the structures he studied)
1838 - A. Schleiden (botanist; was the first to point out that plants are composed of groups of cells)
1839 - T. Schwann (zoologist; was the first to point out that animals are composed of groups of cells)
1840 - Purkinje (used the term protoplasm to refer to the jelly like material that fills the cell)
1855 - Virchow (stated that new cells are formed from only by the division of previously existing cells)

The invention of light microscope and then electron microscope provided biologists to get information about the structure and function of cells. With the help of all these information, Modern Cell Theory is stated. Modern Cell Theory says that:

1. All organisms are made of one or more cells
2. A cell is the structural and functional unit of all living organisms. Cells are similar in structure and composition.
3. All cells carry on their own life activities.
4. New cells can arise only from a previously existing cell by cell division.

TYPES OF CELLS

Cells are the basic structural and functional units of life. They may differ in size, shape and organization. Each cell has special structures called organelles. Each of them carries out a special function within the cell. Most of the organelles have membranes similar to cell membrane around them. Today cells are divided into two groups depending on the presence of membrane-bound organelles such as nucleus, mitochondria, golgi body, E.R., lysosomes, chloroplast, etc.

  1. Prokaryotic Cells:

  • Have no membrane-bound organelles but have lots of ribosomes around DNA
  • Genetic material is in cytoplasm
  • Have a single-circular DNA
  • Ribosomes are 70S
  • Size 0.5-5 micrometer
  • Have mesosomes in aerobic species (similar function to the mitochondria)
  • In photosynthetic prokaryotes, chlorophyll is found in cytoplasm
  • Examples are bacteria and blue-greenalgae

II. Eukaryotic Cells:

  • Have membrane-bound organelles
  • Their genetic material is packaged in nucleus
  • They have linear chromosomes made of DNA and proteins (histones).
  • Ribosomes are 80S
  • Size 20 micrometer
  • Protists (euglena,amoeba,paramecium), fungi, plant and animal cells are eukaryotic.

All cells are surrounded by a "Cell Membrane" which separates cell from its environment.
All cells have both nucleic acids DNA & RNA which carry the genetic information for controlling cellular activities.
General structure of an eukaryotic cell:

Although even the largest cell is too small to see with naked eye, it is important to have an understanding of the relative sizes of cells and organelles.

Some eukaryotic cells are larger than as indicated above. Animal cells are often smaller than plant cells; the yolk of an egg is one cell, each fluid filled vesicle of an orange is one cell.

ORGANIZATION OF CELLS IN MULTICELLULAR ORGANISM

Unicellular organisms like bacteria, protists and some fungi consist of a single cell.
In multicellular organisms cells group together to carry out a special function. For example,muscle cells are specialized to contract, nerve cells are specialized to carry nerve impulses, and red blood cells are specialized to carry oxygen.

  • Groups of cells similar in structure and function form tissues.
  • A group of tissues working together to perform a special function form an organ.
  • A group of organs working together to perform a special function form a system.
  • Several systems together form an organism.

CELL STRUCTURE

A PLANT CELL

AN ANIMAL CELL

THE CELL MEMBRANE

All cells are surrounded by a very thin flexible structural and functional boundary. Cell biologists now accept a working model of cell membrane, Fluid-Mosaic Model of the cell membrane. Singer-Nicolson proposed fluid-mosaic structure in 1972. According to the model, cell membrane is composed of two layers of phospholipids and many variable proteins disposed through it. Phospholipids play important structural and functional role in cell membrane.Recall that a phospholipid consists of a glycerol molecule attached to two fatty acids, and to a phosphate group. The two ends of the phospholipids molecule differ physically as well as chemically. The fatty acid portion of the molecule is hydrophobic (water-hating) and is not soluble in water. However, the portion composed of glycerol and phosphate group is ionized and water-soluble. This end of the molecule is hydrophilic (water-loving).

The formation of a lipid bilayer from phospholipids is a rapid, spontaneous process. The hydrophobic fatty acid chains (the tails) of the phospholipids meet and overlap in the middle, while the hydrophilic heads are directed toward the outside of the membrane. The tails of the phospholipids molecules pack loosely and the fatty acid chains are in constant motion. This results in a fluid state of the membrane that is essential for its function. (Fluid-Mosaic Model of the Membrane)

The fluid-like qualities of lipid bilayer also allow moleculesembedded in them (for example proteins)to move along the plane of the membrane (as long as they are not anchored in some way.

If a membrane is to function properly, its lipids must be in a state of optimal fluidity. The structure of a membrane is weakened if its lipids are too fluid. On the other hand, transport of certain substances is inhibited if the lipid bilayer is too rigid.

Some membrane lipids have the ability to help stabilize membrane fluidity within certain limits. One such molecule is cholesterol, a steroid found in animal cell membranes. At low temperatures the cholesterol prevents crystallization of cell membranes. It also helps prevent the membrane from becoming weakened or unstable at higher temperatures. Plant cells have steroids other than cholesterol that carry out similar functions.

The lipid bilayer is very impermeable to ions and polar molecules. Water is an exception and is able to pass in and out of the cell through lipid bilayer with ease. Additionally, even relatively large non-polar molecules, such as other lipids (e.g. hormones) pass with relative ease through cellular membranes.

In the structure of natural cell membrane a variety of proteins are distributed in a mosaic pattern. We can classify membrane proteins into two groups: integral proteins and peripheral proteins.

Integral proteins have regions that are inserted into the hydrophobic regions of the lipid bilayer and are firmly bound to membrane. Some integral proteins pass all of the way through the membrane, whereas others are located mainly on one side of the membrane.

Peripheral proteins usually bind to exposed regions of integral proteins. They can be removed from the membrane without disrupting the structure of the membrane.

The membrane proteins carry out specific functions. The diversity of membrane proteins is a reflection of the number of activities that take place in or on the membrane.

Generally, plasma membrane proteins fall into several functional groups:

a) Cell Adhesion Proteins attach membranes of adjacent cells and may serve as anchoring points for cytoskeletal elements. (Cytoskeletal elements will be discussed later in organelles)

b) Protein Channels allow transfer of small molecules

c) Transport Proteins allow selective passage of essential molecules, either by diffusion or active transport. (Transport mechanisms will be discussed later in this chapter)

d) Receptor Proteins bind external signal molecules (like hormones). Binding of signal molecules cause a change in other membrane proteins so a message is transferred into the cell.

e) Some integral membrane proteins have multiple functions:

  • Transporting specific molecules
  • Serving as attachment sites for cytoskeletal elements
  • Serving as attachment sites for enzymes

f) ATP-driven Pumps actively transport ions from one side to another.

g) Membrane-bound enzymes may have active sites located on either side or in the interior of the membrane.

The proteins to which carbohydrates are attached are called glycoproteins. Glycoprotein serves as the cell's communication system. Some of them enable cells to recognize other cells, leading to cell-to-cell adhesion and tissue formation. Certain types recognize and bind with messenger molecules such as hormones. Others are very significant in the operation of immune system.

The plasma membrane is far more than a barrier. The functions of the plasma membrane as follows:

1.It protects the cell and gives shape to the cell.

2.It separates the cell from surrounding environment.

3.It regulates the passage of materials into and out of the cell.
Plasma membrane is "selectively permeable";

it can prevent the passage of certain substances while permitting, even facilitating, the passage of others, such as the entrance of nutrients or elimination of wastes

4.It receives information that permits the cell to sense changes in its environment and respond them.
Receptor proteins in the plasma membrane receive chemical messages from other cells and environment. Hormones, growth factors and neurotransmitters are among the substances that combine with such receptors.

5.It communicates with neighboring cells and with the organism as a whole.
The plasma membrane is important in internal defense. Cells identify one another by protein receptors on plasma membranes. The body recognizes its own receptors as "self" and identifies cells with other molecules as "non-self", they are able to attack and destroy them.

6.In certain cells, cell transmits impulses, secretes substances, or is involved in cell movement.
These functions may include impulse transmission in nerve cells, secretion by exocytosis, ameboid movements or movement of cell membrane in exocytosis and endocytosis (Exocytosis and endocytosis will be discussed later in this chapter.).

PERMEABILITY OF THE CELL MEMBRANE

The cell membrane doesn't permit the passage of every kind of molecule. Therefore it's called selectively (=semi) permeable. So that it regulates the chemical composition of the cell, by this way, it maintains homeostasis.

  • Small molecules like H2O, CO2, glucose, amino acid, lipid-soluble molecules (=alcohol, ether, chloroform, etc.) can pass more easily than the large molecules (=starch, glycogen, protein, etc.) can not pass through the cell membranes.
  • Neutral molecules can pass more easily than charged ions (Na+, K+, Cl-, etc.) Among the ions, negatively charged ions (-) pass more easily than positively charged (+) ions.
  • The molecules that can dissolve lipids pass easily through lipid bilayer (like alcohol, ether, chloroform).
  • Lipid soluble molecules (like vitamin A, D, E, K) can pass more easily than water-soluble molecules (like vitamins B,C)

TRANSPORT MECHANISMS OF THE MATERIALS ACROSS THE CELL MEMBRANE

Because of the selective permeability of the cell membrane only certain types of molecules may pass across the membrane. There are certain types of transport mechanisms for the transport of the materials through the cell membrane.

Although some of these mechanisms need energy, some others don't need energy (ATP). Mechanisms of transport vary according to the size of the molecules too.

I. Passive transport: It doesn’t require energy (ATP)
a) Simple diffusion
b) Facilitated diffusion
c) Osmosis

II. Active transport: It requires energy III. Bulk transport: It requires energy
a) Endocytosis (pinocytosis, phagocytosis)
b) Exocytosis

I. PASSIVE TRANSPORT
A) Simple diffusion: According to the kinetic theory of matter, the molecules of all matter are always moving randomly. The molecules of solid matters cannot move very far, however, gas molecules can move freely. Molecules and ions in solutions can also move freely.
Atoms and molecules tend to diffuse down a concentration gradient, that is, from where they are more concentrated to where they are less concentrated.

The movement of molecules of a substance from an area of higher concentration to an area of lower concentration due to random movement of individual particles is called diffusion.It will continue until particles are evenly distributed in equal concentrations.
Example: The addition of a drop of ink into a beaker of water or opening of a bottle of a perfume in the classroom.

What's the relation between the concentration gradient and diffusion?
Concentration gradient causes diffusion. If concentration gradient is equal to zero, diffusion stops.

B) Facilitated Diffusion (Carrier - Mediated Diffusion) :
Some molecules, although, tend to pass through the cell membrane due to concentration difference, because of their large size, they can't. For the transport of these molecules, within the cell membrane there're channel - like proteins (channel proteins have already discussed in the structure of cell membrane), known as PERMEASES.
Transport of glucose, urea and glycerol is done via channel like proteins. This type of transport is known as "FACILITATED DIFFUSION". Facilitated diffusion does not require ATP and movement is along concentration gradient.

C) Osmosis:
Movement of water molecules from a region of high water concentration to a region of low water concentration through a semi - permeable membrane is called “osmosis”.

Effects of osmosis on the cell:
According to their solute concentration, there are 3-types of solutions:

I. Isotonic solution: If a solution has the same concentration of dissolved materials as the cell has it is said to be isotonic solution. In isotonic solution the water molecule concentration is also equal. Therefore, water moves in and out of the cells at the same rate. The net movement of the water molecules is zero.

II. Hypertonic Solutions: A solution that contains a higher concentration of dissolved substances (e.g. salt, sugar etc.) than the cell has is said to be hypertonic.
The movement of water is inside to outside that means cell loses waterin hypertonic solution. As a result of this cell shrinks. Shrinking of cell in a hypertonic solution is called plasmolysis.

III. Hypotonic Solutions: A solution that contains a lower concentration of dissolved substances than the cell has is said to be hypotonic.
If you place a plasmolyzed cell into a hypotonic solution it regains its original shape (DEPLASMOLYSIS).

  • If an animal cell is kept too long within hypotonic solution it will swell and finally burst. This is called as cytolysis.
  • If a plant cell is kept too long within a hypotonic solution the excess water is collected in central vacuole. It will not burst because of the presence of rigid and flexible cell wall. The pressure that is applied over the cell wall of a plant cell due to the water entering is called turgor pressure. It is the pressure applied over the cell wall due to the internal water molecules.
  • The Osmotic pressure of a solution is the measure of tendency of water to move into that solution by osmosis.
    The Suction force is themeasure of tendency of a cell to draw in water

S.P= O.P - T.P

EFFECTS OF HYPERTONIC AND HYPOTONIC SOLUTIONS ON ANIMAL AND PLANT CELLS.

II. ACTIVE TRANSPORT

If the movement of materials across a cell membrane requires the expenditure of cellular energy (ATP), the process is called active transport. Active transport usually involves the movement of materials against a concentration gradient.That is, materials move from the area of lower concentration to the area of higher concentration. Active transport makes it possible for cells to maintain internal conditions that are chemically different from the surrounding medium. For example, in a nerve cell, the concentration of potassium is higher inside the cell than it is in the medium outside the cell. The concentration of sodium, on the other hand, is lower inside the cell than it is outside. The cell uses active transport to maintain these differences in concentration. In an active transport, a substance is moved across a membrane by transport proteins. Each molecule that is transported must first bind to a transport protein on one side of a membrane and then be released by the same protein on the other side of the membrane.

As in facilitated diffusion, the transport protein is specific for each kind of transported substances. Unlike facilitated transport, the process requires the expenditure of cellular energy in the form of ATP.

III. BULK TRANSPORT

a)ENDOCYTOSIS:

A part of the cell membrane forms a sac or vesicle around the particles. The vacoule is released into the cytoplasm. The process requires energy (ATP).

1. Phagocytosis (cell eating): The process in which large solid particles (ex. bacteria or food) or small organisms are ingested into a cell. For example, white blood cells ingest bacteria.

2. Pinocytosis (cell drinking): The process by which liquids (or very small particles dissolved in liquids) from the surrounding area are taken into a cell by inpocketing of the cell membrane. For example, cell takes in needed solute dissolved in tissue fluid.

b) EXOCYTOSIS: Many substances are exported from cells in vesicles formed by golgi bodies and endoplasmic reticulum. This process is called exocytosis. Cell may also eject waste products or secrete. Substances such as hormones are released by exocytosis

Comparison of Membrane Transport Mechanisms