Structure and Function of Human Cell

Cytology is a branch of science that deals with the study of cells. The cell is the structural and functional unit of living organisms. Human body is made up of 75 trillion cells. Robert hook who first coined the term cell in 1665.

FUNCTIONS OF CELL

1. Irritability: The cell has the ability to detect and respond to change in its environment.

2.Nutrition: The cell is capable of absorbing fluids and dissolved substances directly through Cell membrane and these can be used by the cell growth and repair, to provide energy.

3. Respiration: The cell has the ability to use oxygen combined with food substances to form CO2 and water, while releasing energy for intracellular activity.

4. Excretion: The cell is able to discharge unused and waste material through cell membrane.

5. Growth and reproduction: The cell has the ability to increase in size and when it reaches the limit of its growth it reproduce by dividing in to 2 smaller cells.

6. Movement: Some cells have power of movement.

Composition of Cell:  

A. Plasma membrane

B. Subcellular organelles.

  1. Nucleus.
  2. Endoplasmic reticulum
  3. Mitochondria
  4. Ribosomes
  5. Golgi apparatus
  6. Lysosomes
  7. Peroxisomes
  8. centrosomes

A. CELL MEMBRANE

  • The cell membrane is a protective sheath of the cell body.
  • It measures approximately 70-100 Angstron unites in thickness.
  • The cell membrane is the semipermeable membrane. So, there is free exchange of certain substances between the extracellular and intracellular fluids.
  • The structure of cell wall is Fluid mosac model.
  • The cell membrane is made up of proteins, lipids and a little amount of carbohydrates.

COMPOSITION:

1. PROTEINS: 55%

2. PHOSPHOLIPIDS: 25%

3. CHOLESTEROL: 13%

4. OTHER LIPIDS: 4%

5. CARBOHYDRATES: 3%

FUNCTIONS:

1. Protective: It forms outermost boundary of the cell organelles.

2. Digestives: Takes in food materials

3. Excretion: It excretes waste products

4. Property of Selective Permeability

Subcellular organelles:

  1. Cytoplasm:
  • It is a gel like substance which is enclosed by the plasma membrane and external to the nucleus of the cell.
  • The semifluid portion of the cytoplasm in which the cell organelles are suspended is called “Cytosol or Intracellular fluid.”
  • The cell organelles are embedded in the cytosol.
  • Cytosol is a clear viscous fluid containing almost 75-90% of water with substances like proteins, fats, lipids, inorganic substances either dissolved or suspended.

2. NUCLEUS:

  • The control centre for all activity. Large Oval body near the centre of the cell.
  • Surrounded by a nuclear membrane, but it has tiny pores through which some substances can pass between it and the cytoplasm.
  • Every cell in the body has a nucleus, with the exception of mature erythrocytes (red blood cells).

NUCLEOPLASM: Is the protoplasm in the nucleus. It contains genetic material,   CHROMOSOMES (DNA).

NUCLEOLUS: It is found in the nucleus. It contains more genetic information (RNA)

Function of nucleus:

Controls every organelle in the cytoplasm and the cell reproduction process because it contains the body’s genetic material i.e. 46 chromosomes which contain DNA, the genetic code, which directs the activities of the cell.

3. Mitochondria:

  • This special cell organelle is involved in cellular respiration and generates ATP hence called “Powerhouse of the Cell”.
  • The number of mitochondria in the cell varies as per nature of the cell. 
  • Shape varies, but usually ellipsoidal.
  • Mitochondria contains two membranes made up of lipoproteins.
  • Outer membrane is intact and covers the entire structure of the mitochondria.
  • Inner membrane is folded and called “Cristae” while a single fold is called “Crista”.
  • The space between two membranes is called intermembrane space.
  • The centre cavity of the mitochondria is filled with fluid called “Mitochondrial Matrix”.
  • Mitochondrial matrix contains various enzymes needed for cellular respiration.
  • Mitochondria also contains DNA, which is received from the mother of the individual i.e. Mitochondrial DNA is maternally inherited.
  • Functions:
    • ATP synthesis.
    • Mitochondria are involved in apoptosis, cancer, aging, and pathologies like Parkinson disease and diabetes.
    • Mitochondria also plays a role in cell signalling.
  1. Golgi complex:
  • Also called “Golgi Apparatus”.
  • It is present near the nucleus.
  • It consists of 4 to 6 flat sacs called “Cisterns” stacked upon each other in bent semicircular shape.
  • Each cistern is divided into three prominent regions called,
    • Cis
    • Intermediate.
    • Trans.
  • Cis region is receiving point from where substances enter the golgi apparatus.
  • Intermediate region deals with modification and storage of substances.
  • Trans region is a region from where substances leave the golgi apparatus.
  • Golgi apparatus specifically deals with modification and storage of chemicals synthesized in the cells.
  • The substances formed or modified in the apparatus are enclosed in vesicles which are then used or released from the cells.
  • The cells having secretary functions show well developed golgi apparatus.
  1. Ribosomes:
  • These are the sites of protein synthesis in the cell.
  • They are made up of two subunits
    • Smaller subunit (40S): 
    • Lager Subunit (60S):
  • Ribosomes receive a message of protein synthesis in the form of mRNA from the nucleus and accordingly they join amino acids to form the proteins.
  • They are of two types,
    • Membrane bound: Bound to endoplasmic reticulum.
    • Free: Found free in cytoplasm.
  1. Endoplasmic reticulum:
  • It is the transport system of the cell.
  • It is made up of membrane bound channels called “Cisterns” of different shapes.
  • It is an interconnected network of internal membranes.
  • It is of two types,
    • Rough Endoplasmic Reticulum. (RER)
    • Smooth Endoplasmic Reticulum. (SER)
  • Rough Endoplasmic Reticulum:
    • Ribosomes are attached to the external surface and hence they appear granular.
    • These are responsible for synthesis of secretory proteins and membranes.
  • Smooth Endoplasmic Reticulum:
    • Ribosomes are not attached to the external surface and hence appear smooth.
    • They are involved in fatty acids, phospholipids and sterol synthesis.
    • These are also involved in inactivation of certain chemicals like alcohol, carcinogens, pesticides etc.
  1. Lysosomes:
  • They are vesicles containing many hydrolytic enzymes.
  • They are formed by the Golgi Complex.
  • They contain about 60 different hydrolytic enzymes.
  • As the hydrolytic enzymes are capable of digesting all cellular components when released the Lysosomes are also called Suicidal Bags.

8. Centrioles:

  • A centriole is a cylindrical organelle composed mainly of a protein called tubulin.
  • Centrioles are found in most eukaryotic cells.
  • A bound pair of centrioles, surrounded by a highly ordered mass of dense material, called the pericentriolar material makes up a structure called a centrosome.
  • They play a major role in cell division.
  1. Peroxisomes:
  • These are similar to lysosomes but are much smaller in size.
  • These are smaller vesicles containing many oxidase enzymes capable of causing oxidation of many substances like amino acids, fatty acids, uric acid etc.
  • The oxidation of fatty acids is a major source for metabolic energy, it produces Hydrogen Peroxide as a byproduct which is harmful to the cell.
  • Peroxisomes contain one enzyme called “Catalase” that causes deactivation of the formed hydrogen peroxide hence protecting the cell.

Functions of Cell:

  1. Movement of substances across the cell membrane, 
  2. cell division to make new cells, and 
  3. protein synthesis.
  4. Phagocytosis for protection.
  5. Secretion of enzymes / hormones.
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