The Fundamental Unit of Life
Chapter: The Fundamental Unit of Life
While observing a thin slice of cork obtained from bark of a tree, Robert Hooke saw that the cork resembles a structure of honeycomb consisting of many small compartments – However, it was in the year 1665 when Hooke made his observation through self-designed microscope. Robert Hooke called these boxes as cell. Cell is a Latin word for little room. That was the very first time someone had observed that living things appear to consist of separate units. The word cell is used to describe these units until today in biology. Let us find out more about cells.
What are living organisms made up of?
Let us just perform a small experiment before we talk about what organisms are made up of.
- Take a small piece of an onion with the help of forceps and peel of the inner skin of the onion.
- This skin is immediately placed in watch glass containing water in order to prevent the peel from getting dry.
- Take a glass slide, put a drop of water on it and transfer a small piece of the peel from watch glass on the slide, make sure that the peel is perfectly flat on the slide
- Now put a drop of safranin solution on this piece followed by placing a coverslip. Take care to avoid air bubbles while putting cover slip with the help of a mounting needle.
- We have prepared a temporary mount of onion peel. We can now observe the slide under low power followed by power of a compound microscope.
What do we observe when we look through the lens? Can you draw an image of what u observe?
What are these structures?
These small structures that we see are the basic building blocks of onion bulb. These structures are called cells. Not only onions but all the living organisms are made up of cells. However, there are single celled organisms that are capable of independent existence. Chlamydomonas, paramaecium and bacteria, these organisms are unicellular (uni = single). On the other hand, many other cells group together in a single body and assume different functions to form various body parts in multicellular organisms.
Example: fungi, plants and animals.
Some organisms have cells of different kinds. Below are different types of human cells.
Scientists who worked on cell:
Antony Von Leeuwenhoek was the first to see and describe a live cell. Robert Brown later discovered the nucleus in the cell. The invention of microscope revealed all the structural details of the cell.
In 1838 Matthias Schleiden, a German botanist, examined a large number of plant cells and observed that all plants are composed of different kinds of cells, which form the tissues of the plant.
Theodor Schwann (1839), a British Zoologist observed different type of animal cells and reported that cells have a thin outer layer that is today known as 'plasma membrane'. He also concluded based on his studies on plant cells that cell wall is a unique character of plants cells.
Rudolf Virchow (1855) first explained that cells divided and the new cells are formed from the pre-existing cells.
Overview of cells:
For example: Mycoplasmas are the smallest cells followed by bacteria.
- A classic plant cell has cell wall, which covers the cell membrane whereas in animal cells, cell membrane is the delimiting structure.
- Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains genetic material (DNA).
- Cells that have a membrane bound nucleus are called eukaryotic cells and the cells that lack membrane bound nucleus are called prokaryotic cells.
- In both prokaryotic and eukaryotic cells, cytoplasm which is a semi-fluid like matrix occupies the volume of the cell. The cytoplasm is the main area where all the cellular activities take place.
- Besides nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), The Golgi complex, lysosomes, mitochondria,
- Micro bodies and vacuoles.
- Ribosomes are found in both prokaryotes and eukaryotes and they are said to be non-membrane.
- Cells differ greatly in size, shape and activities.
- Nerve cells are the longest cells.
- Cells vary greatly in their shapes. They may be disc- like, polygonal, columnar, cuboidal, thread like or even irregular.
- The shape of the cell vary with the function they perform.
Difference between Prokaryotic and Eukaryotic cells:
Structural organization of cells:
If we study cells under a microscope, we would come across three features in almost all the cells, plasma membrane, nucleus and cytoplasm. All the activities inside the cell and the interaction of cells with external environment are possible due to these features.
The cell membrane (also known as the plasma membrane or cytoplasmic membrane) is a biological membrane that separates the interior components of the cell from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of the cells. The basic function of a cell membrane is to protect the cell from its surroundings.
The cell membrane (or plasma membrane or plasma lemma) surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment.
The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it. Since the membrane also maintains the cell potential, it works as a selective filter that allows only certain things to come inside or go outside the cell. There are a number of transport mechanisms occurring in the cell and these involve the biological membranes:
1. Passive osmosis and diffusion: Some substances (small molecules, ions) such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is called as a passive transport process. The molecules and ions occur in different concentration gradients on the two sides of the membrane as the membrane acts as a barrier for certain molecules and ions. Such a concentrated gradient across a semi-permeable membrane creates an osmotic flow of the water.
2. The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. The flexibility of the cell membrane enables the cell to engulf in food and other materials from its external environment. Such processes are known as endocytosis. Amoeba obtains its food by endocytosis.
- It is a non-living rigid structure forming an outer covering for the plasma membrane of fungi and plants.
- Cell wall gives shape to the cell and protects the cell from mechanical damage and infection. It also helps in cell-to-cell interaction and provides barrier to undesirable macro molecules.
- The cell wall of a young plant, the primary wall is capable of growth, which gradually diminishes as the cell matures and a secondary wall is formed on the inner side of the cell.
- The middle lamella layer is mainly made of calcium pectate, which holds neighbouring cells together.
- The cell wall and middle lamella may be traversed by plasma desmata, which connects the cytoplasm of the neighbouring cells.
- Algae have cell walls made of cellulose, galactans, manons and minerals like calcium carbonate, while the plant cell wall is made up of cellulose, hemi-cellulose, pectin and protein.
The Nucleus has a doubled layered covering called Nuclear Membrane, having spores that allow the transfer of materials from inside the nucleus to outside. Nucleus, as a cell organelle, was first described by Robert Brown. Let us perform an activity to view the nucleus under microscope:
What do you observe?
A dark colored spherical or oval, dot like structure localized near the center of each cell is observed. This is called nucleus. Nucleus contains chromosomes, which are visible as rod shaped structures only when the cell is about to divide. Chromosomes contain genetic information in the form of DNA. DNA molecules contain the information necessary for constructing and organizing cells. Functional segment of DNA are called Genes.
The nucleus plays a central role in cellular reproduction. Reproduction is defined as the process by which, a single cell divides and forms two new cells. Nucleus directs the chemical activities in the cell and thereby helps in determining the way the cell develop and in what form it will exhibit at maturity. In some organisms like bacteria, the nuclear region of the cell is not well defined due to the absence of a nuclear membrane. Such a region is called nucleoid.
Endoplasmic reticulum is a large network of membrane-bound, fluid filled interconnecting tubules and sheets. ER spreads from the nuclear membrane to the plasma membrane.
Endoplasmic reticulum is of two types
1. Rough endoplasmic reticulum (RER):
It looks rough under an electron microscope as it has particles called ribosomes attached to it. Ribosomes are the sites of protein synthesis. RER helps to transport the proteins synthesized by ribosome to that part of cell where they are needed.
2. Smooth endoplasmic reticulum (SER):
SER is without ribosome and is meant for manufacture of fat molecules or lipids.
Functions of ER:
Golgi apparatus was first described by a German scientist “Camello Golgi”. It comprises of 5 -8 sacks, which resemble deflated balloons placed approximately parallel to each other. These sacks are called Cisternae and are filled with different enzymes. It is considered as the packaging and forwarding department of the chemical factory of a cell.
- It is the secretary organ of the cell.
- It modifies sorts and packs materials synthesized in the cell (enzymes, mucous secretions, proteins, lipids, pigments, etc) and dispatches them to various targets inside the cell (plasma membrane, lysosomes) or outside the cell.
- It produces vacuoles and secretary vesicles.
- It plays the role in formation of the cell wall, plasma membrane and lysosomes.
Lysosomes are simple one membrane bound sacs filled with digestive enzymes. These powerful enzymes are made by RER and are capable of digesting and breaking down all organic matter. It is the cell’s waste disposal system.
- As it is the digestive system of the cell, it protects the cell by destroying any foreign material like bacteria and viruses that invade the cell.
- They remove worn out cellular organelles and remove all the organic debris. This process is call autolysis.
- When a cell gets old or damaged, lysosomes burst and digest their own cells. Therefore lysosomes are also called as “Suicide bags”.
It is called the power house of the cell as it produces the energy for the cell. Mitochondrion is a double membrane bound structure. The outer membrane is porous while the inner membrane is deeply folded. These folding are called Cristae. They create a larger surface area for efficient functioning. The inner cavity is filled with proteinaceous gel like matrix, which contains ribosome, phosphate granules and DNA. Mitochondria oxidize carbohydrates and fats present in the cells with the help of enzymes. During this process, large amount of energy is released, which is used by the mitochondria to produce energy rich compound ATP.
- Mitochondria produce the energy rich compound ATP which is the energy source of the cell.
- The body uses the energy stored in ATP to synthesize chemical compounds (proteins, carbohydrates, lipids) and to do mechanical work (movement of muscles, conduction of nerve impulses, production of heat).
Plastids are double membrane structures present only in plant cells. They are of two types – Chromoplasts (coloured plastids) and Leucoplastids (white or colourless plastids).
Chloroplasts are a type of chromoplasts, which are green in color due to the presence of a green colored pigment called Chlorophyll. Chloroplasts are important for photosynthesis in plants. These are energy factories of plant cells. Each chloroplast has two distinct regions:
1) Stroma: It is the colorless proteinaceous matrix having various photosynthetic enzymes, DNA, Ribosomes and starch granules.
2) Granum (Grana): It is a stack of thylakoids, which are the disks containing chlorophyll and other photosynthetic pigments.
- Chloroplasts help to trap solar energy and convert it into chemical energy in the form of food.
- Chromoplasts give color to flowers and fruits.
- Glucoplasts are involved in the synthesis and storage of various kinds of foods like starch, oil and proteins.
Vacuoles are storage sacs for solids or liquid contents. They do not have any basic shape or size. The structure of the vacuole changes accordingly as per the need of the cell. Plant cells have large number of vacuoles compared to animal cells. Vacuole is bound by a single membrane.
- Vacuoles maintain osmotic pressure of the cell.
- They store metabolic by products and end -products e.g.: Glycogen, proteins and water.
- They store waste products and food in animal cell e.g.: In amoeba, it stores food, so it is called food vacuole.
- In plants, vacuoles are full of cell sap and provide turgidity and rigidity to them.