Cell: The Building Block of Life

Class 09 Science

The cell is the basic structural and functional unit of all living organisms. All living organisms are made up of cells. The cell represents the basic level at which life exists.

Some organisms, such as bacteria or yeast consist of only one cell (unicellular), while others like plants, fish, birds or humans are made up of millions of cells (multicellular) that work together.

A group of similar cells performing similar functions forms tissues. Different tissues are organised to form an organ and several organs work together to form organ systems.

How to Study Cells?

A cell is usually too small to be seen by the unaided eye. Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope.

A microscope allows us to see very small structures clearly and is an essential tool for studying the cell structure. Over the years, scientists have improved the microscope by improving its three main features - resolution (measure of clarity), contrast (the difference in brightness between various parts of an object), and magnification.

Structure of a Cell

Cells are organised into specialised tissues and organs, and collectively perform specific function. For these cells to function as units, they must be able to interact with one another and with their surroundings. These interactions occur at the cell boundary, where substances move between the cells and their external environment. Even single-celled organisms exchange materials and respond to their environment through cell membrane.

1. Cell membrane - The universal feature of a cell

The cell membrane is a thin boundary that surrounds a cell and protects its contents. It defines the individuality of a cell and is also called the plasma membrane. The cell membrane is selectively permeable, which means it allows some substances to pass through it while blocking others.

The movement of water through a selectively permeable membrane is called osmosis. Diffusion is the net movement of particles from a higher to a lower concentration (which occurs even without a membrane).

Structurally, the cell membrane is extremely thin, about 7 to 10 nanometres (nm) thick. It is made up of lipids (fats) and proteins.

All living cells communicate with their surroundings and their neighbouring cells through the cell membrane. However, cells of plant, fungi, and bacteria have an additional layer around the cell membrane, called the cell wall.

2. Cell wall - The outer covering of cells

Plants cannot move from place to place, so they need a rigid structure to withstand environmental stresses like wind and rain. Therefore, plant cells have an additional covering outside the cell membrane called a cell wall. The cell wall also helps leaves and flowers remain firm, and maintain their shapes and help plants stay upright.

Although rigid the cell wall is permeable, which means water and some dissolved minerals can pass through it. Along with the selective permeability of the cell membrane, the permeability of the cell wall helps plant roots absorb water and nutrients from the soil.

The plant cell wall is primarily made of cellulose, a type of carbohydrate formed by many glucose units linked together. Cellulose in our diet acts as roughage, helping in digestion. Some microorganisms, like fungi and bacteria also have a cell wall to provide protection and structural support to their cells.

Cell Interior

Most cells have three basic parts:

1. A selectively permeable membrane called the plasma membrane
2. A semi-fluid, jelly-like substance called the cytoplasm
3. A prominent nucleus

In addition to the nucleus, the cytoplasm contains several sub-cellular components called organelles, along with other substances present in it.

A bacterial cell lacks a well-defined nucleus and membrane-bound organelles. Such cells are called prokaryotic cells. In prokaryotic cells, most cellular activities take place directly in the cytoplasm. In contrast, plant and animal cells have a well-defined nucleus and several membrane-bound organelles. Such cells are called eukaryotic cells.

Why do eukaryotic cells need these organelles?

Eukaryotic cells carry out various life processes in different cell organelles independently at the same time.

Cell organelles help in building new materials, removing waste, and providing energy to the cell. They work together to perform all functions of a cell. In other words, a cell is like a tiny living factory, with each of its part doing a specific job.

3. Nucleus - House of coded instructions

The nucleus has a double-layered covering called the nuclear membrane, which has pores that allow the transfer of material between the nucleus and the cytoplasm.

The nucleolus is the dense round body in the nucleus, where synthesis of ribosomal subunits take place. These subunits then exit the nucleus to cytoplasm where one large and one small subunits assemble to form ribosome.

The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of characters from parents to the next generation in the form of DNA (Deoxyribonucleic acid) molecules. Chromosomes are composed of DNA and specific proteins. DNA molecules contain the genetic information. The functional segments of DNA are called genes. In a non-dividing cell, this DNA is present as part of chromatin material. Chromatin material is visible as an entangled mass of thread-like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.

Prokaryotic cells do not have a well-defined nucleus. Their DNA is present as a single circular molecule associated with specific proteins. The region containing this genetic material is called the nucleoid.

4. Ribosomes - The protein factories

These are tiny structures may be present either freely in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are the sites of protein synthesis.

5. Endoplasmic Reticulum (ER) - Manufacturing factory

The Endoplasmic Reticulum (ER) is a large organelle that spreads like a network within the cytoplasm of the cell. The ER is continuous with the outer membrane of nuclear envelop. The ER plays a key role in the synthesis and transport of proteins, fats (lipids), and some hormones in some of the specialised cell. The structure of the ER in a cell varies depending on its function.

There are two types of ER:

1. Rough Endoplasmic Reticulum (RER): It looks rough under an electron microscope because it has ribosomes attached to its surface, and is mainly involved in protein synthesis and protein secretion (for example, in gland cells, such as pancreatic cells).
2. Smooth Endoplasmic Reticulum (SER): It does not have ribosomes on its surface, and therefore, looks smooth. It is involved in the synthesis, and storage of fats and hormones.

6. Golgi apparatus - The packaging and shipping centres

Stacks of flattened sac-like structures form the Golgi apparatus. It is functionally linked to the ER, the cell membrane and the other cell organelles. The Golgi apparatus acts like the cell's post office. It modifies, sorts, and packages proteins and lipids into vesicles for transport, secretion, or lysosome formation.

7. Lysosomes - The clean-up system

Cells produce waste materials and damaged, worn-out organelles during their activities. Lysosomes are single membrane-bound sacs filled with enzymes, which can break down unwanted proteins, carbohydrates, fats, and even damaged parts of the cell, keeping it clean and healthy. The products formed by the breakdown are released into the cytoplasm, where they may be reused in other cellular processes.

8. Mitochondria - The powerhouse of the cell

Mitochondria are often called the ‘powerhouses of the cell’ because they supply the energy needed for most cellular activities. Each mitochondrion is surrounded by two membranes:

1. The outer membrane is smooth and porous.
2. The inner membrane is folded into finger-like projections called cristae, which increase the surface area for chemical reactions and facilitate energy production.

In mitochondria, the glucose and other molecules are broken down to release energy during a process called cellular respiration. The energy released is stored in the form of a molecule called Adenosine Triphosphate (ATP), which acts as the energy currency and is used for most of the cellular activities.

9. Plastids - Centre for food synthesis in the plant cells

Plants use special organelles called plastids for food synthesis and storage. Plants prepare their food by the process of photosynthesis in the presence of sunlight.

A green pigment called chlorophyll, which is present in the chloroplast (a type of plastid) absorbs sunlight. Chloroplasts are double-membrane-bound organelles, like mitochondria. Inside the chloroplast, there is a semi-fluid substance called the stroma. Within the stroma are disc-shaped membrane structures that contain chlorophyll. Light energy is absorbed by them during photosynthesis. The sugars synthesised in this process are stored in stroma, along with the starch granules.

How do flowers, fruits, and vegetables acquire varied colours?

In flower petals and fruits, plastids contain pigments other than chlorophyll. These plastids are called chromoplasts. Their pigments (may be yellow, orange or red) are the source of bright colours in such flowers and fruits. Such bright colours help in attracting pollinators for pollination and fruit-eating animals that help in seed dispersal.

Some plastids lack pigments and are thus, colourless. These are called leucoplasts. Leucoplasts store food material, such as starch, oils or proteins and are classified based on the type of food they store. For example, some leucoplasts in potato and taro (Colocasia) cells store starch.

10. Vacuoles - The organelles for storage and support

Plastids, such as chloroplasts help plants produce food and temporarily store it. Other plastids, such as leucoplasts help in storing food.

In a mature plant cell, there is usually one large central vacuole surrounded by a single selectively permeable membrane. The vacuole is filled with a watery fluid called cell sap. The vacuole stores water, minerals, sugars and waste material. By storing large amounts of water, the vacuole helps maintain pressure inside the cell, which keeps a plant cell firm. When a plant does not get enough water, the vacuole loses water, the cells become less firm, and the plant gets wilted.

Cell Division

Cell division is the process by which new cells are formed from pre‐existing cells. It allows living organisms to grow, repair damaged tissues and reproduce.

There are two major types of cell division:

1. Mitosis is important for normal growth, repair, maintenance and asexual reproduction.
2. Meiosis is important for sexual reproduction for creation of genetic diversity.

Mitosis

Every human begins life as a single fertilised egg. This one cell divides repeatedly to form trillions of cells in the body. Cells increase in number through mitosis, which is the most common type of cell division.

Mitosis produces two genetically identical daughter cells from one parent cell. Each new cell gets the same DNA and the same number of chromosomes as the parent cell. This ensures that genetic information is largely maintained across body cells.

Meiosis

Meiosis is a type of cell division that produces gametes and occurs only in the cells of reproductive organs. Gametes produced for sexual reproduction create variations and diversity among living organisms. Therefore, children resemble their parents but are not exactly the same.

In animals including humans, meiosis occurs only in the cells of testes of males to produce sperm and ovaries of females to produce eggs. In plants, meiosis occurs in the anthers (male parts) to form pollen grains (that later produce sperm cells) and the ovaries (female parts) to produces egg cells.

In meiosis, the parent cell divides twice, one after the other, to form four daughter cells. During the first division, the cells divide into two daughter cells and the number of chromosomes in each daughter cell is reduced to half. The second division is similar to mitosis where each daughter cell divides into two thus, forming four daughter cells with half the number of chromosomes. As a result, each gamete has half the number of DNA compared to the parent cell. During fertilisation, when gametes from two individuals combine, the original chromosome number is restored.

Cell Theory

An important observation about living organisms is that all organisms are made up of cells. In 1838, a German botanist named Matthias Schleiden reported that all plants are made up of cells. In 1839, German zoologist Theodor Schwann, found that all animals are also made up of cells. Later, in 1855, a German scientist named Rudolf Virchow, further expanded the Cell Theory by stating that new cells are formed only from pre-existing cells. Together, their work led to the formulation of the Cell Theory.

According to the classical Cell Theory:

• All living organisms are made up of one or more cells.
• The cell is the basic unit of structure and function in living beings.
• All cells arise from pre-existing cells.