Types of Cells in Human Body

Introduction – Cells as Building Blocks

Cells are the basic structure of all living things, including the human body. Each cell operates as a separate entity that can perform the fundamental processes of life. In humans, cells work together to maintain health and proper functioning otherwise, human existence would not be there.

The human body is composed of a wide variety of cell types, performing several functions. Each of these cell types has a special role in helping to form the functioning whole that is the human body. That allows for a very formalised performance, yet specialised, in the performance of very specific tasks necessary for life.

Classification of Human Cells

The cells are classified as:

Epithelial Cells

• An epithelial tissue consists of cells arranged in continuous sheets, in either single or multiple layers.

• Because the cells are closely packed and are held tightly together by many cell junctions, there is little intercellular space between adjacent plasma membranes. Epithelial tissue is arranged in two general patterns in the body: (1) covering and lining various surfaces and (2) forming the secreting portions of glands.

• Functionally, epithelial tissue protects, secretes (mucus, hormones, and enzymes), absorbs (nutrients in the gastrointestinal tract), and excretes (various substances in the urinary tract).

Examples:

• Skin cells

• The lining of the digestive tract

Connective Tissue Cells

• Connective tissue is one of the most abundant and widely distributed tissues in the body.

• It protects and insulates internal organs, compartmentalizes structures, serves as the major transport system within the body (blood), primary location of stored energy reserves (adipose, or fat, tissue) and is the main source of immune responses.

Examples:

• Bone cells (osteocytes)

• Blood cells (red and white blood cells)

• Cartilage cells (chondrocytes)

Muscle Cells

• Muscular tissue consists of cells called muscle fibers or myocytes that can use ATP to generate force.

• It produces body movements, maintains posture and generates heat. It also provides protection.

• Based on location and certain structural and functional features, muscular tissue is classified into three types: skeletal, cardiac, and smooth.

Examples:

• Skeletal Muscle Cells: Usually attached to bones by tendons

• Cardiac Muscle Cells: Heart wall

• Smooth Muscle Cells: Iris of eyes, walls of hollow internal structures such as blood vessels, airways to lungs, stomach, intestines, gallbladder, urinary bladder and uterus

Nerve Cells (Neurons)

• Nervous tissue consists of neurons (nerve cells) and neuroglia, which do not generate or conduct nerve impulses but have other important supporting functions.

• Neurons convert stimuli into electrical signals called nerve action potentials and conduct these action potentials to other neurons, muscles or glands.

Types Of Neurons:

• Sensory Neurons

• Motor Neurons

• Interneurons

Stem Cells

• Stem cells are unspecialized cells that have the ability to divide for indefinite periods and give rise to specialized cells.

• They are two types of stem cells: embryonic stem cells and adult stem cells.

Examples:

• Zygote, which can become any cell

• Hematopoietic stem cell in bone marrow

• Neural stem cells in brain

Detailed Study of Major Cell Types

The major cell types are described below

Red Blood Cells (Erythrocytes)

• The erythrocytes or red blood cells (RBCs), are biconcave, disc-shaped cells that do not have a nucleus.

• This shape gives a larger surface area to the cell for an increased rate of oxygen transport.

• These cells are full of haemoglobin, which can bind and carry oxygen from the lungs to the tissues. In return, they bring back carbon dioxide to the lungs for exhalation.

• Erythrocytes live for approximately 120 days and then get phagocytosed in the spleen and liver. However, macrophages constantly replace them by erythropoiesis in the bone marrow.

White Blood Cells (Leukocytes)

• Leukocytes or white blood cells (WBCs) compose the defence system of the body with different types of specialised defence strategies.

• Neutrophils, being the most numerous, take up the role of first attack against invading microorganisms. They engulf the pathogens and destroy them.

• The lymphocytes consist of B cells and T cells, are involved in adaptive immunity, where the B cells synthesise and present antigens and T cells attack infected cells.

• The monocytes give rise to the macrophages and the dendritic cells, which are phagocytic and antigen-presenting cells.

• Eosinophils kill multi-cellular parasites and also have a role in allergic responses.

• Basophils are responsible for releasing histamine and other chemical mediators of inflammation in the inflammatory response.

Platelets (Thrombocytes)

• Platelets are small, disc-like cell fragments, the products of megakaryocyte cellular meningitis in the bone marrow.

• As they lack a nucleus, they are packed full of granules containing enzymes and factors that help blood to clot.

• Injured vessels expose collagen fibrils to the platelets which then release chemical factors that initiate a series of clotting reactions.

• This represents a method by which excessive loss of blood is prevented and wounds can heal.

Muscle Cells (Skeletal, Cardiac, Smooth)

• Myocytes are highly specialised to contract, with mainly three different types.

• Skeletal muscle cells are long, multinucleated fibres that have a striated appearance due to the arrangement of contractile units called sarcomeres. They are in charge of voluntary movements, which include walking and running, and they are attached to the bones using tendons.

• The cardiac muscle cells are located in the wall of the heart. They are striated, have a single nucleus, and their fibres are branched.

• Non-striated, fusiform smooth muscle cells are embedded in the walls of hollow organs, the intestines and blood vessels bringing about involuntary movements like peristalsis and vasoconstriction.

Skin Cells (Keratinocytes, Melanocytes, Langerhans cells, Merkel cells)

Skin is the largest organ of the body, made up of different cell types.

Bone Cells (Osteoblasts, Osteoclasts, Osteocytes)

• Cells that give bones their tissue form are known as bone cells.

• There are three different categories of cells, that is, osteoclasts, osteoblasts, and osteocytes, which are categorised according to function.

• Osteoclasts resorb and break down bone tissue and produce new bone tissue.

• Osteocytes, in fully developed bones, respond to mechanical stress, either signal for bone resorption or formation, and control mineral homeostasis both at the local and systemic levels.

Fat Cells (White & Brown adipocytes)

• Fat cells, otherwise adipocytes, also have a function as far as weight, size, and number are concerned.

• There are two types of these cells: white adipocyte cells and brown adipocyte cells.

• White adipocytes store energy in the form of triglycerides, while brown adipocytes burn it to release heat through thermogenesis.

• Their production changes with age, shifting to fewer white fat cells and a larger number of brown fat cells, thus affecting the metabolic process of energy and the regulation of body weight.

Functions of Specialised Cells

Specialised connective tissues have a variety of functions in the human body.

• Provides structural support and strength to the body.

• Helps in transport of substances like gases, nutrients, wastes, and fluids.

• Plays a role in protection and defense, including immune responses.

• Serves as a site for storage of energy and minerals.

• Maintains homeostasis and cushioning.

NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Classification of Cells in human body

• Blood cells (RBCs, WBCs, Platelets)

• Muscle Cells (Skeletal, Cardiac, Smooth)

Practice Questions for NEET

Q1. An example of a response to stimuli by the protoplasm is

1. Eating when hungry

2. Retinal cells respond to light.

3. Running when the dog is chasing

4. Fever when having a viral infection

Correct answer: 2) Retinal cells respond to light.

Explanation:

Protoplasm is irritative, thus the property of reacting towards stimuli. This characteristic of living organisms is very important because living organisms can be able to respond to the environment as well. For example, cells in the retina of the eyes react to light, whereby vision is enabled and vital procedures such as adaptation and survival, especially in conditions of light take place.

Hence, the correct answer is option 2) Retinal cells responding to light.

Q2. Energetically unfavorable reactions occur in human cells through :

1. Heat energy supplied by the body

2. Heat energy released through exercise

3. Coupling of energetically favourable reactions with unfavourable ones

4. Photosynthesis

Correct answer: 3) Coupling of energetically favourable reactions with unfavourable ones

Explanation:

In human cells, many energetically unfavorable reactions are made to occur by coupling them with energetically favorable reactions, most commonly the hydrolysis of ATP. The breakdown of ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and inorganic phosphate (Pi) is a highly exergonic reaction, meaning it releases a significant amount of energy. This energy is then used to drive endergonic (unfavorable) processes, such as the synthesis of molecules, active transport across membranes, and muscle contractions. This process is called energy coupling, and ATP acts as the "energy currency" of the cell, linking the energy released from its hydrolysis to power a variety of essential biological functions.

Hence, the correct answer is option 3) coupling energetically favourable reactions with unfavourable ones.

Q3. Prokaryotic genetic material is

1. Linear DNA + histones

2. Circular DNA + Histones

3. Linear DNA without histones

4. Circular DNA without histones

Correct answer: 4) Circular DNA without histones

Explanation:

Cells are broadly classified into two categories based on the organization of DNA (nucleus), biomembranes, and the variety of cytoplasmic organelles: prokaryotic and eukaryotic. Prokaryotic cells are simpler in structure, lack a defined nucleus, and have fewer organelles, with DNA generally floating freely in the cytoplasm. Eukaryotic cells, on the other hand, have a well-defined nucleus that contains the DNA and a variety of membrane-bound organelles, allowing for greater complexity and specialization. Cells can also vary significantly in size, shape, and other characteristics, depending on their function and the organism they belong to.

Hence, the correct answer is option 4) Circular DNA without histones.

Also Read:

• MCQs on Different types of Cells

• Plasma in Blood

• Semiautonomous Organelles