2. STRUCTURE OF CELL

CELL: STRUCTURE AND FUNCTION

Cell is the basic structural and functional unit of all living organisms. All the living organisms can be categorised into unicellular (e.g., protozoa, yeast, and bacteria) or multicellular organisms (e.g., plants and humans). A human body consists of about 10″ or 100 trillion cells with a size and mass of 10um and 1 nanogram respectively.

Types of Cells

1. Prokaryotic Cells: These cells do not have a well-developed nucleus, though they have some other organelles present as in the eukaryotic cells. For example, archaebacteria, etc.
2. Eukaryotic Cells: These cells have a well-developed nucleus and other organelles. For example, plants, animals, etc.

Functions of a Cell

1. Cell Growth and Metabolism: Cells grow either by increasing its number or its size. During the metabolism, various anabolic and catabolic processes occur. In catabolic processes, break down of complex molecules occurs within the cells to produce energy; while during the anabolic processes, cells utilise energy to synthesise complex biomolecules and perform other biological functions.
2. Cell Division: It involves the splitting of a single cell, i.e., mother cell splits into two daughter cells. This results in the growth of multicellular organisms and breeding (vegetative reproduction) of the unicellular organisms. During the cell division, DNA is also synthesised and the process is known as replication.
3. Protein Synthesis: It occurs within the cells, by two major steps, i.e., transcription and translation.
4. Molecule Transport: The cell must be able to acquire nutrients and other molecules to survive.

COMPONENTS OF CELL

Cell Membrane / Plasma Membrane

The cell membrane is a selectively permeable biological membrane, allowing only certain substances to pass through it. It acts as a barrier between the inner and outer surface of the cell. It is also known as the plasma membrane or plasmalemma. It comprises mainly of proteins and lipids, along with other living molecules. These molecules are important for the normal functioning of the cells, i.e., in an ion channel conductance, cell adhesion, and cell signalling.

Structure (Fluid Mosaic Model) of Cell Membrane

In 1970s, S.J. Singer and G.L. Nicholson proposed a model for the structure of cell membrane named as fluid mosaic model. This theory proposes that proteins exist as globules that are randomly distributed in a mosaic pattern rather than as a continuous layer that stretches from one side of the membrane to the other. The protein’s globules can pierce the membrane to deeper depths; some of them hang onto the membrane’s surface, while others extend through it and stick to the other side. It has fluid-like structure because of its pattern and components present in it such as phospholipids, cholesterol, proteins, and carbohydrates.

Functions of Cell Membrane

1. Protection of Cells: The receptors present on the cell membrane allow chemical messages to pass between the cells and systems, and thus provides protection to cells.
2. Providing Cytoskeleton and Mechanical Support: It provides a specific shape to the cell. It provides mechanical support and maintains the membrane potential.
3. Permeability: It is a selectively permeable membrane as it allows the passage of solvent and several selected solutes while holding back the others.
4. Tissue Formation: Groups of similar cells together form tissues.
5. Particle Transportation: This occurs by various mechanisms like ion channel, carrier proteins, and ion pumps. Chemical exchange of solute between extracellular medium and cytosol is mediated by transport of channel and carrier proteins.
6. Contribution to Enzyme Activity: It controls the activity of enzymes which further play significant role in metabolism and immune system.

Cytoplasm

The cell consists of cytoplasm enclosed within the plasma membrane. The cytoplasm includes various organelles present inside a cell and therefore these organelles are also referred to as cytoplasmic organelles. The cell performs most of the metabolic activities (e.g., glycolysis, and processes like cell division) in the cytoplasm.

Structure of Cytoplasm

1. Cytosol: It is the fluid present inside the cells. This fluid bathes the cytoplasmic organelles. This fluid is responsible for cell metabolism, and has role in signal transduction and glycolytic pathways and act as ribosomes and intracellular receptors.
2. Cytoplasmic Organelles: Cell consists of specialised organelles (viz., Golgi bodies, mitochondria, endoplasmic reticulum, lysosomes, etc.) having some specific characteristics that help in the maintenance of life.
3. Cytoplasmic Inclusions: These are certain bodies which are present within a cell but are unable to perform their own metabolic processes. These structures lack membranes binding them. And include secretory products, pigment granules, water-containing vacuoles, different types of crystals, and nutrients like glycogens (in liver and muscle cells), lipids (fat cells), and carbohydrates.

Functions of Cytoplasm

1. Various vital biochemical reactions, important for the maintenance of life, occur in the cytoplasm.
2. It provides the site for cell growth and expansion.
3. It provides space for organelles to remain suspended in the medium.
4. It also helps in the movement of several elements within the cell.
5. It has enzymes which help in breaking the large macromolecules into smaller ones so that other organelles can use them easily.
6. Other important functions performed by the cytoplasm involve protein synthesis, cytokinesis, anaerobic glycolysis, cell reproduction, etc.

Mitochondria

Most of the eukaryotic cells contain a membrane-bound cell organelle called mitochondria (singular mitochondrion) having the diameter ranging from 0.5-10pum. Mitochondria are also known as the powerhouse of the cell because they generate Adenosine Triphosphate (ATP) that is used as a source of chemical energy. Hence, mitochondria are the chief source of energy in the cell.

Structure of Mitochondria

1. Outer Membrane: This membrane comprises of proteins and is permeable to small protein-like molecules.
2. Intermembrane Space: These are the gaps present in the middle of outer and inner membranes.
3. Inner Membrane: This membrane is made of phospholipids which do not permit molecules to pass through it. Specific carriers are required to transport substances.
4. Cristae: These are irregular folds of the inner membrane that increase the surface area and facilitate the chemical reactions.
5. Matrix: This is the fluid which is present in inner membrane that has all essential enzymes for ATP production. Matrix also includes components, like ribosomes, mitochondrial DNA, inorganic and organic molecules, etc.

Functions of Mitochondria

1. It performs aerobic respiration to create energy for the functioning of the cell.
2. It regulates plasma membrane potential and cellular anabolism, and catabolism.
3. It actively participates in apoptosis.
4. It helps in synthesis of steroids.
5. It is involved in haem synthesis and various other reactions.
6. It performs cellular signalling and regulates cellular proliferation.
7. It performs detoxification of ammonia in liver cells.

Golgi Bodies

A Golgi body, often referred to as a Golgi apparatus, is an organelle found in cells that aids in the processing and packaging of proteins and lipid molecules, particularly proteins intended for cell export. The Golgi body, which bears Camillo Golgi’s name, resembles a stack of membranes.

Structure of Golgi Bodies

1. Cis-Golgi Network: It associates with nucleus protein and forms a connection with the endoplasmic reticulum to serve as the entry point into the Golgi apparatus.
2. Cis-Golgi: It is main processing region which is responsible for biochemical alterations.
3. Medial-Golgi: It is main processing region which is responsible for biochemical alterations.
4. Trans-Golgi: It is main processing region which is responsible for biochemical alterations.
5. Trans-Golgi Network: It acts as an exit point for vesicles released from the golgi surface, organises and packs biochemicals into vesicles according to their destination.

Functions of Golgi Bodies

1. Modifying, Sorting, and Packaging: The Golgi complex plays an important role in sorting, modifying, and packaging of substances for use within the cell and for cell secretion.
2. Modification of Proteins: Proteins delivered by the RER are modified by the Golgi body. It also takes part in lysosome formation and in the transport of lipids to the adjoining areas of the cell.
3. Transportation of Substances: Enzymes present in the cisternae bind to the carbohydrate and phosphate groups by the process of glycosylation and phosphorylation respectively, in order to modify substrate by the transport of substances.
4. Phosphorylation of Molecules: Phosphorylation of various biochemical molecules occur during transportation of ATP molecules into the lumen, in the presence of kinases enzyme (like casein kinases).
5. Excretion of Water: These organelles help in excreting excess amount of water.

Lysosomes

In 1949, a Belgian cytologist Christian de Duve had discovered the cell organelle ‘Lysosome’. The word lysosome has been derived from the Greek word lysis meaning to separate, and soma meaning body. They are generally known as suicide-sacs or suicide-bags because of their role in autolysis. Lysosomes are membrane-enclosed organelles that comprise a range of enzymes which are capable of breaking down several varients of biological polymers, such as proteins, nucleic acids, carbohydrates, and lipids.

Functions of Lysosomes

1. It provides defence against invading microorganisms via circulating macrophages.
2. It causes sperm penetration in the ovum.
3. It assists in bone restoration and re-absorption of water and small molecules in kidney and urinary bladder.
4. It decomposes matured RBC and dead cells.
5. It engulfs the damaged (worn-out) cellular organelles.
6. It helps in breakdown of cellular components during metamorphosis and cell differentiation.
7. It provides nutrition via autophagy during starvation.

Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) or microsomes has a large and dynamic structure. It is responsible for different functions in the cell, like calcium storage, protein synthesis and lipid metabolism. The versatile nature of this organelle needs coordination and response to numerous proteins, distinctive physical structures, and variations in the intracellular environment. In microsomes, lipid content is approximately 30-50% in which around 70% is a phospholipid. Approximately, 50-90% of the phospholipid is formed of lecithin and cephalin.

Structure of Endoplasmic Reticulum

1. Rough Endoplasmic Reticulum (RER): Small particles of RNA known as ribosomes are found in close association with the membrane of the endoplasmic reticulum which gives a rough appearance to the membrane.
2. Smooth Endoplasmic Reticulum (SER): The membranes of SER do not contain ribosomes and help in the metabolism of carbohydrates and lipids. They are involved in the synthesis of steroid hormones in adrenals and testes.
3. Sarcoplasmic Reticulum (SR): Most of the smooth and striated muscles consist of a unique type of smooth ER called sarcoplasmic reticulum. The SER and SR differ in their structural composition, i.e., in having a mixture of proteins.

Functions of Endoplasmic Reticulum

1. It acts as a structural framework of the cytoplasm.
2. It exchanges material with the cytosol by active transport, osmosis, and diffusion.
3. Proteins synthesised by its surface-bound ribosomes are collected and stored by RER.
4. It has different metabolic enzymes which help in the synthesis of phospholipids, cholesterol, and triglycerides.
5. SER found in the liver helps in the synthesis of glycogen and lipoproteins, and also in the lysis of macromolecules.

Ribosomes

Ribosomes are cell organelles where protein synthesis takes place. They are abundant in living cells. They are either found as free particles (as in prokaryotic and eukaryotic cells) or as particles attached to ER membranes (as in eukaryotic cells).

Functions of Ribosomes

The primary function of ribosomes is protein synthesis. The information carried in the genetic code is converted into protein molecules within the ribosomes.

Nucleus

Nucleus (plural nuclei) is a spherical organelle that consists of a doublelayered membrane build-up of phospholipids and proteins. In eukaryotic cells, it is enclosed within the membrane and consists mostly of genetic material of the cell. The genetic material includes the long linear DNA molecules, which intertwine around the huge variety of proteins known as histones for forming the chromosomes.

Structure of Nucleus

1. Nuclear Envelope: It contains large number of small circular or octagonal nuclear pores at specific points. These nuclear pores function as a channel for the regulated movement of macromolecules from the nucleus to the cytoplasm and vice-versa.
2. Nucleoplasm: It is a specialised form of protoplasm that fills the nuclear cavity of the nucleus.
3. Chromatin Material: This material, when saturated with DNA, transmits the coded structural and functional information between the cell and organism, respectively.
4. Nucleolus: The nucleus of the cell has a nucleolus, also known as nucleoli. It is a non-membrane bound structure containing proteins and nucleic acids.
5. Chromosome: The word chromosome has been derived from the Greek word ‘chroma’ means colour and ‘soma’ means body, as it has the strong ability to be stained with vital and supravital dyes.

Functions of the Nucleus

1. Chromatin stores the genetic material and genes in the long and thin form of DNA strands.
2. It is responsible for the storage of proteins and RNA.
3. It also acts as a transcription site where messenger RNA is produced for protein synthesis.
4. It is responsible for the exchange of genetic molecules, like RNA and DNA between the nucleus and the rest of the cell.
5. It is responsible for selective transport of regulatory factors and energy molecules by nuclear pores.
6. Cell division involves the arrangement of chromatins into chromosomes within the nucleus.

CELL CYCLE

A cell undergoes cell cycle in order to multiply. A complete cell cycle usually comprises of two broad phases, i.e., interphase, and dividing phase. The period of a cell cycle during which the cell is not in a state to divide is known as interphase.

Types of Cell Division

1. Mitosis: A type of cell division in which the chromosomes present in the nucleus of a parent cell divides into two identical daughter nuclei is termed as mitosis. It is a continuous process comprising of four phases: Prophase, Metaphase, Anaphase, Telophase, followed by Cytokinesis.
2. Meiosis: A type of cell division characterised by the formation of germ cells (eggs and sperm) is known as meiosis. During meiosis, the quantity of genetic material in each cell is decreased. In meiosis, two cell divisions produce four daughter cells, each containing half the genetic material of the parent cell.