Cell biology is the branch of biology that studies the basic unit of life, the cell. Every living organism begins with a cell, whether it is a unicellular or multicellular organism. The cell is not only the structural unit but also the center of all biological functions.
The cell membrane on the outer surface of the cell is very important because it protects the cell and controls the exchange of certain substances. There are several organelles inside the cell, such as the nucleus, which is called the brain of the cell, the mitochondria, which produce energy, and the ribosomes, which produce proteins. All these components work together to keep the cell active and alive.
The cell does not just exist; it undergoes a regular process called the cell cycle. In this cycle, the cell grows, copies its material content, and divides itself into two new cells. This division is called cell division, which is of two types: mitosis and meiosis. Cells communicate with each other using a cellular messaging system, which keeps cells informed of environmental changes.
Plant Cell
A plant cell is a spherical (rectangular) cell that has specialized organelles such as a cell wall, chloroplasts, and vacuoles. This cell produces energy through photosynthesis and provides strength to the cell structure.
Cell Wall – Rigid outer layer that provides support and protection.
Cytoplasm – Jelly-like fluid where cell activities occur.
Vacuole – Stores water, nutrients, and waste; maintains cell shape.
Nucleus – Controls cell activities and contains DNA.
Ribosomes – Make proteins.
Chloroplast – Site of photosynthesis; contains chlorophyll.
Endoplasmic Reticulum – Transports materials within the cell.
Golgi Apparatus – Packages and sends proteins and lipids.
Mitochondria – Produce energy (ATP) for the cell.
Animal Cell
An animal cell is a round or irregularly shaped cell that does not have a cell wall or chloroplasts. This cell has a central nucleus and uses mitochondria to obtain energy. It usually contains small vacuoles.
Structure of the Cell
Each cell is made up of three main components: the outermost cell membrane, which protects the cell from the external environment and controls the movement of substances in and out; the cytoplasm, which is a jelly-like substance in which most of the cell floats. This cytoplasm also contains organelles such as the nucleus, where genetic information is stored and the cell’s activities are controlled.
Furthermore, the cytoskeleton is a network of strong but flexible fibers that helps maintain the shape of the cell, holds internal components in place, and aids in cellular movement. This framework plays a central role in the transport of materials throughout the cytoplasm, during division, and in the structural stability of the cell.
Function of a Cell
Energy production (metabolism): The cell breaks down nutrients to produce energy to provide the energy needed to live and function.
Transport of substances: The cell moves nutrients, gases, and wastes in and out of the cell to maintain proper balance.
Cell division (reproduction): The cell divides to make new cells so that the body can grow, repair, and reproduce.
Communication (cellular messaging): Cells communicate with each other through chemical signals to keep the body’s systems in sync.
Responding to the environment: The cell senses external changes and responds accordingly, such as changes in temperature or light.
Maintaining homeostasis (balance of comfort): The cell maintains a stable internal environment so that cellular functions can continue properly.
Cell cycle
The cell cycle is the process by which a cell is born, grows, replicates its DNA, and finally divides to form two new cells. This process naturally completes the stages of a cell’s life.
For example, Human skin cells replace old cells every day with new ones. These new cells go through a process called the cell cycle, which keeps the skin healthy and intact.
Phases of the Cell Cycle
Interphase
Interphase is the phase of the cell cycle that occurs before cell division. During this phase, the cell prepares for growth, DNA replication, and the production of essential proteins. This phase is the longest, and the cell spends most of its life in it.
For example, when there is a wound in the human body, before new cells are formed, old cells go through interphase so that they can duplicate their DNA and get ready to make new cells.
G₁ Phase
G₁ Phase is the first phase of Interphase, in which the cell begins to grow again after division.
In this phase:
- The cell increases in size
- New proteins and RNA are produced
- New organelles begin to form
- The cell prepares itself for the next phase (S Phase)
S Phase
S Phase (Synthesis Phase) is the phase that occurs during Interphase, and in it the cell replicates its entire DNA so that both new cells receive equal hereditary material at the time of division.
G₂ Phase
G₂ Phase is the third and final phase of Interphase. In this phase, the cell completes its final preparations so that it can enter the division phase (M phase).
In this phase:
- The cell makes more proteins and enzymes
- Error checking of DNA
- Internal components of the cell (organelles) are replicated
- The cell increases in size
- Checkpoints are used to determine whether the cell is capable of dividing or not.
Mitotic Phase
Mitotic phase is the stage in which a cell divides to form two identical cells. It involves the division of the nucleus (central part) and then the division of the cytoplasm. This process occurs towards the end of the cell’s life and is the final stage of the cell cycle. It has two major parts:
- Mitosis – where DNA divides
- Cytokinesis – where the cytoplasm divides
Mitosis
Mitosis is a process of cell division in which a parent cell divides into two identical daughter cells. This process occurs in somatic cells and ensures equal distribution of genetic material so that each daughter cell contains the same DNA as the parent cell.
During mitosis, the number of chromosomes does not change, but they are copied equally. This process is of great importance in development, cell repair, and tissue renewal.
The process of mitosis consists of different stages, including prophase, metaphase, anaphase, and telophase, after which the division of the cytoplasm (cytokinesis) is completed. During these stages, the chromosomes move successively, pull toward the two poles, and finally divide the cell. The correct performance of mitosis is essential for the biological health of every organism and the preservation of hereditary information.
Cytokinesis
Cytokinesis is a cellular process in which the cytoplasm divides to form two separate cells. This process occurs after mitosis or meiosis so that each daughter cell receives not only a complete set of genes but also its cytoplasm, organelles, and cell membrane. Cytokinesis allows for the complete separation of the cell.
Mitosis Stages
Prophase
Prophase is the first stage of mitosis. In it, the cell’s DNA becomes visible in the form of chromosomes, and spindle fibers begin to form. The nuclear membrane gradually begins to break down. For example, when plant or animal cells are ready to divide, their chromosomes begin to become visible in prophase, like a coil of wire.
Metaphase
In metaphase, all the chromosomes line up in a row at the center of the cell (the equator), and the spindle fibers grab them at both ends. For example, just as students sit in a straight line in class, the chromosomes line up in the middle of the cell.
Anaphase
In anaphase, the two chromatids of each chromosome separate and are pulled to opposite ends of the cell. That is, the genetic material begins to divide into equal parts.for example Like a pair of ropes pulled by two people, and they break in the middle, the two halves move in opposite directions.
Telophase
In telophase, new nuclear membranes form around the separated chromatids, and the chromosomes reassemble into thin strands. The cell is now roughly divided into two halves.
For example, it’s like dividing a batch of dough into two equal parts and placing them in separate bowls. Each part has its membrane.
Cellular Respiration
Cellular respiration is a biological process during which cells break down glucose (sugar) in the presence of oxygen to produce energy (ATP). This energy is essential for all body functions.
For example, when we eat rice, bread, or any other food, our digestive system breaks it down and creates glucose. This glucose reaches the cells, and when we breathe, oxygen reaches the cells through the blood. The cells use both of these to produce energy so that the body can move, think, and live.
Stages of Cellular Respiration
Glycolysis
Glycolysis is a primary cellular respiration process that occurs in all living organisms. During this process, simple carbohydrates such as glucose are broken down into pyruvic acid through various chemical reactions.
This process is completed with the help of enzymes and does not require oxygen or specific aerobic or anaerobic conditions for its completion. This stage of glycolysis provides the cell with immediate energy to continue its basic functions.
During this process, two molecules of ATP and two molecules of NDH are produced, which are used as energy sources. Glycolysis is not only the first stage of aerobic cellular respiration but also provides the basis for anaerobic respiration.
This process takes place in the cytoplasm of the cell and is considered an effective means of obtaining energy in living organisms. Since glycolysis can be completed without oxygen, it helps living organisms survive even in harsh environmental conditions.
Krebs Cycle (Citric Acid Cycle)
The Krebs cycle is a major chemical reaction within the cell that plays a fundamental role in energy production. This process occurs within the mitochondria and is completed only in aerobic conditions, i.e., in the presence of oxygen.
This cycle begins with the formation of citric acid, which is formed from pyruvic acid. During this, carbon dioxide is released through various chemical steps, and hydrogen carrier molecules are produced.
In this process, compounds such as HDH and FADH2 are formed, which are later used to produce ATP. The main purpose of the Krebs cycle is to make energy production during cellular respiration more efficient.
The hydrogen molecules produced by this cycle are later used in the electron transport chain, providing a large amount of energy. Thus, this process provides the energy necessary for the continuation of life and the performance of cellular functions.
Electron Transport Chain (ETC)
The electron transport chain is the last and most important step that is completed during cellular respiration. This process takes place on the inner membrane of the mitochondria, where electrons obtained from NADH and FADH2 are transferred by specific protein complexes.
During this, hydrogen ions accumulate on one side of the membrane, which creates a proton gradient. This entire process is possible only under aerobic conditions because the role of oxygen is very important, which finally accepts the electrons and forms water.
The main purpose of this step is to produce a large amount of energy through ATP synthase. When the hydrogen ions come back to the other side of the membrane, they stimulate a specific enzyme that produces ATP molecules. About 34 energy molecules are produced in this step, which are used for all the activities of the cell. Thus, the electron transport chain plays a central role in providing the energy necessary for the survival of life.
Types of Cellular Respiration
Aerobic Respiration
Aerobic respiration is a biological process in which cells break down glucose in the presence of oxygen to produce energy (ATP). Carbon dioxide and water are released as byproducts during this process. This process occurs in most cells of plants, animals, and humans and provides the energy needed for physical movement, growth, and cellular functions.
For example, when humans run or exercise, their cells break down glucose with oxygen through aerobic respiration to obtain energy so that muscles can function efficiently.
Anaerobic Respiration
Anaerobic respiration is the cellular process in which energy is obtained by breaking down glucose without oxygen. This process produces small amounts of energy and produces byproducts such as lactic acid or alcohol.
For example, when we run fast for a long time, the muscles become deprived of oxygen, so the cells produce energy through anaerobic respiration, which produces lactic acid and fatigue.
Fermentation
Fermentation is a biological process in which yeast or other microorganisms break down carbohydrates such as glucose in the absence of oxygen to obtain energy. The process usually produces alcohol, lactic acid, or other organic compounds. For example, the making of yogurt or the rising of bread by yeast are clear examples of fermentation.
Photsynthesis
What is photosynthesis
Photosynthesis is a natural biological process by which green plants, algae, and some bacteria, with the help of sunlight, convert water and carbon dioxide into glucose (food) and oxygen. This process is carried out in the leaves of plants with the help of a green pigment called chlorophyll.
For example, when sunlight falls on the leaves of grass or any other green plant, the plant takes in carbon dioxide from the air and water from the roots, and uses the sunlight to make glucose (food) and oxygen. This process is called photosynthesis.
Equation of Photosynthesis
- Word Equation:
Carbon dioxide + Water + Sunlight → Glucose + Oxygen - Balanced Chemical Equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
What is a Chloroplast
A chloroplast is a cellular organelle (a specialized part of a cell) found only in green plants and algae. It is where the complete process of photosynthesis takes place. The green pigment chlorophyll in it absorbs sunlight to make glucose and oxygen. When sunlight falls on a plant’s leaves, the chloroplasts in the leaf cells absorb this light and produce food (glucose) and oxygen from water and carbon dioxide.
Function of Chloroplast
- Carrying out the process of photosynthesis: This is where plants make food.
- Absorbing sunlight: The green pigment called chlorophyll absorbs light.
- Producing glucose: Converts energy into a safe form, glucose, which meets the plant’s needs.
- Releasing oxygen: Oxygen is produced during photosynthesis, which is essential for living organisms.
