THE FUNDAMENTAL UNIT OF LIFE – CELL
The fundamental unit of life is the cell. Living organisms are made up of cells, which are the basic building blocks of life. Cells come in different shapes and sizes and are highly organized structures. The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes. In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape. Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. How does a living cell perform these basic functions? We know that there is a division of labour in multicellular organisms such as human beings. This means that different parts of the human body perform different functions. The human body has a heart to pump blood, a stomach to digest food and so on. Similarly, division of labour is also seen within a single cell. In fact, each such cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, such as making new material in the cell, clearing up the waste material from the cell and so on.
Cells were first observed by Robert Hooke in 1665 when he looked at cork slices under a microscope. Leeuwenhoek later discovered free-living cells in pond water. In 1831, Robert Brown discovered the cell nucleus. The cell theory, proposed by Schleiden and Schwann in the 1830s, states that all plants and animals are made up of cells and that cells are the basic units of life. Virchow expanded the theory, suggesting that cells arise from pre-existing cells. The invention of the electron microscope in 1940 allowed scientists to study the complex structure of cells and their organelles.
A cell is made up of several components, including:
Plasma Membrane or Cell Membrane:
- The plasma membrane, also known as the cell membrane, is the outer covering of the cell.
- It is selectively permeable, meaning it allows some substances to enter and exit the cell while blocking others.
- Diffusion is a process where substances like carbon dioxide and oxygen move from an area of high concentration to an area of low concentration across the cell membrane.
- Oxygen enters the cell when its concentration inside decreases, while carbon dioxide moves out when its concentration inside is high.
- Osmosis is the movement of water through the cell membrane.
- Unicellular organisms and plant cells gain water through osmosis, and plant roots absorb water this way too.
- Diffusion and osmosis are important for the exchange of gases, water, and nutrition in cells.
- The plasma membrane is made of lipids and proteins and is flexible.
- It can only be observed through an electron microscope.
- The flexibility of the membrane allows cells to engulf food and other materials through processes like endocytosis, as seen in Amoeba.
- Plant cells have a tough outer covering called the cell wall.
- The cell wall is mainly made of cellulose and gives plants structural strength.
- Plasmolysis is the shrinking of a plant cell when it loses water.
- The cell wall helps plant cells withstand changes in the environment.
- It prevents plant cells from bursting when exposed to dilute solutions.
- Cell walls are found in plant, fungi, and bacterial cells.
- They exert pressure against swollen cells to maintain their shape.
- The nucleus is the control center of a cell and is found in eukaryotic cells.
- It has a double-layered membrane called the nuclear membrane with pores for material transfer.
- Inside the nucleus, there are chromosomes made of DNA and proteins that carry genetic information.
- The nucleus controls cellular reproduction and development.
- Prokaryotes, like bacteria, do not have a nuclear membrane.
- Instead, they have a region called the nucleoid.
- Prokaryotes lack other organelles found in eukaryotic cells.
- Photosynthetic prokaryotic bacteria have chlorophyll associated with membranous vesicles, not plastids like eukaryotic cells.
- Cytoplasm is a large region inside the cell membrane of onion peel and human cheek cells.
- It is a fluid-filled area that contains specialized cell organelles.
- Prokaryotic cells, like bacteria, lack membrane-bound organelles.
- Eukaryotic cells, like plant and animal cells, have a nuclear membrane and membrane-bound organelles.
- Membranes are important for proper cell function.
- Viruses lack membranes and cannot show signs of life until they enter a living body and use its cell machinery to reproduce.
Every cell has a membrane around it to separate its contents from the external environment. Large and complex cells use membrane-bound structures called organelles to carry out different activities within themselves. These organelles are found in eukaryotic cells and are not present in prokaryotic cells. Some examples of organelles are the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids. These organelles perform important functions in cells.
These are specialized structures within the cell that perform specific functions. Some essential organelles include:
a. Endoplasmic Reticulum (ER):
- The endoplasmic reticulum (ER) is a network of membranes in the cell.
- It has two types: rough ER (RER) and smooth ER (SER).
- RER has ribosomes and helps make proteins.
- SER makes fats and helps build the cell membrane.
- The ER transports materials within the cell.
- It provides a surface for biochemical activities.
- In liver cells, SER is important for detoxification.
b. Golgi Apparatus:
- The Golgi apparatus is a structure made of flat sacs called cisterns.
- It helps package and modify molecules made near the endoplasmic reticulum (ER).
- The Golgi apparatus stores, modifies, and packages molecules in small sacs called vesicles.
- It can make complex sugars from simple sugars.
- It plays a part in forming lysosomes, which are important for cell digestion.
(Camillo Golgi was a scientist born in 1843. Golgi received recognition for his work and shared the Nobel Prize in 1906 with Santiago Ramón y Cajal for their research on the nervous system’s structure.)
- Lysosomes are tiny sacs inside cells.
- They are filled with special enzymes that help break down waste material.
- Lysosomes act like a cell’s garbage disposal system.
- They can break down foreign substances and old parts of the cell.
- These enzymes are powerful and can break down organic material.
- If there is damage or disturbance in the cell, lysosomes might burst.
- When lysosomes burst, they can digest the cell itself.
- Because of this, lysosomes are sometimes called “suicide bags” of the cell.
- Mitochondria are the powerhouses of the cell.
- They have two membranes, with the inner one being folded to increase surface area.
- They produce energy in the form of ATP molecules, which are like the cell’s energy currency.
- Mitochondria have their own DNA and ribosomes.
- They can make some of their own proteins.
Plastids are present in plant cells and come in two main types: chromoplasts and leucoplasts.
- Chromoplasts, including chloroplasts, contain chlorophyll and play a crucial role in photosynthesis. They also house various pigments.
- Leucoplasts are responsible for storing substances such as starch, oils, and proteins.
- Chloroplasts possess internal membranes and a region called the stroma.
- Similar to mitochondria, plastids have their own DNA and ribosomes.
- Vacuoles are storage sacs found in cells.
- Plant cells have large vacuoles filled with cell sap, which gives them rigidity and stores important substances.
- Animal cells have smaller vacuoles compared to plant cells.
- Vacuoles play roles in storing food, expelling waste, and maintaining cell structure.
- Cells are the fundamental units of living organisms.
- They have a structural organization that allows them to perform functions like respiration, nutrition, waste clearance, and protein synthesis.
Cell division is the process through which new cells are formed in organisms for growth, replacement of old cells, and reproduction. There are two types of cell division: mitosis and meiosis.
Mitosis is the process in which a mother cell divides into two identical daughter cells with the same number of chromosomes. It helps in the growth and repair of tissues in organisms. Meiosis is a different process in which specific cells of reproductive organs divide to form gametes (sperm and egg cells). Meiosis involves two consecutive divisions, resulting in the production of four new cells. These new cells have half the number of chromosomes compared to the mother cells.
The reduction in chromosome number is necessary for sexual reproduction. When the gametes with half the number of chromosomes combine during fertilization, the resulting offspring will have a complete set of chromosomes.
- Have a cell wall made of cellulose, providing structural support and protection.
- Contain chloroplasts for photosynthesis, allowing them to produce their own food.
- Have larger vacuoles for storing water and maintaining turgidity.
- May contain plastids such as chromoplasts and leucoplasts, which are absent in animal cells.
- Cell division occurs through the formation of a cell plate during cytokinesis.
- Lack centrioles, which are essential for animal cell division.
- Lack a cell wall but may have an extracellular matrix for support.
- Lack chloroplasts, so they cannot produce their own food through photosynthesis.
- Have smaller vacuoles or no vacuoles at all.
- Lack plastids.
- Cell division occurs through the formation of a cleavage furrow during cytokinesis.
- Have centrioles that play a role in cell division.
- Simpler and smaller in size.
- Lack a true nucleus and other membrane-bound organelles.
- Genetic material (DNA) is present in the cytoplasm, not enclosed within a nucleus.
- Usually have a single circular chromosome.
- Lack complex internal compartments.
- Found in bacteria and archaea.
- More complex and larger in size.
- Have a true nucleus, which houses the genetic material (DNA) enclosed within a nuclear membrane.
- Contain membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.
- Genetic material is organized into multiple linear chromosomes.
- Can be found in plants, animals, fungi, and protists.
If the plasma membrane breaks down:
- The cell loses its protective barrier.
- Molecules move in and out without control.
- Essential molecules may leak out, and harmful substances may enter.
- Cellular processes are disrupted.
- Homeostasis is compromised.
- The cell may malfunction or die.
- Tissues and organs can be damaged.
- The plasma membrane is crucial for cell survival and normal functioning.
If a cell lacks the Golgi apparatus:
- Protein and lipid processing, modification, and packaging would be disrupted.
- Proper communication and secretion would be compromised.
- Transportation of important molecules would be challenging.
- Maintaining homeostasis would become difficult.
- Overall cellular functioning would be severely affected.
Mitochondria are the powerhouse of the cell.
- They generate energy (ATP) through cellular respiration.
- Mitochondria have their own DNA and specialized membranes.
- They produce energy from nutrients like glucose.
- This energy is vital for cellular processes, growth, and movement.
- Mitochondria fuel the functioning of other organelles in the cell.
- Without mitochondria, cells would lack energy for their functions.
The endoplasmic reticulum (ER) is a network of sacs and tubes in the cell's cytoplasm.
- It is responsible for synthesizing lipids and proteins that make up the cell membrane.
- The rough ER (RER) has ribosomes on its surface, where protein synthesis occurs.
- Proteins are folded and modified within the ER.
- The ER also synthesizes lipids, including phospholipids found in the cell membrane.
- After synthesis, lipids and proteins are sent to the Golgi apparatus for further processing and distribution to different parts of the cell.
- Amoeba obtains food through a process called phagocytosis.
- It extends temporary projections called pseudopodia around its food.
- The food, such as tiny organisms or organic particles, is engulfed and enclosed in a food vacuole.
- Digestive enzymes are secreted into the vacuole to break down the food.
- The cell absorbs the digested nutrients for nourishment and energy.
- Undigested waste is expelled from the cell.
- This process of phagocytosis allows Amoeba to obtain food for survival and growth.
Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?
- Mitosis is cell division for growth and repair of the body.
- It produces two identical daughter cells with the same number of chromosomes as the parent cell.
- Mitosis ensures that the body grows and replaces damaged or worn-out cells.
- Meiosis is cell division for the formation of gametes (reproductive cells).
- It involves two rounds of division, resulting in four non-identical daughter cells.
- Each daughter cell has half the number of chromosomes as the parent cell.
- Meiosis ensures genetic diversity and is essential for sexual reproduction.
- Osmosis is the movement of water molecules across a selectively permeable membrane.
- Water moves from an area of higher water concentration to an area of lower water concentration.
- It helps balance the concentration of solutes on both sides of the membrane.
- Aquaporins are small openings in the membrane through which water molecules pass during osmosis.
- Osmosis is important for maintaining water balance in cells.
- It plays a vital role in nutrient absorption in plants.
- Osmosis is also involved in kidney function in animals.