Eukaryotic Cell

Unlike prokaryotic cells, eukaryotic cells have an organized nucleus with a nuclear envelope and many membrane-bound organelles. Their genetic material is also organized into chromosomes. Examples of eukaryotes include plants, animals, protists and fungi.

Parts of Eukaryotic Cells

Cytoplasmic Membrane

Description: It is also called plasma membrane or cell membrane. The plasma membrane is a semi-permeable membrane that separates the inside of a cell from the outside.

Structure and Composition: In eukaryotic cells, the plasma membrane consists of proteins, carbohydrates and two layers of phospholipids (i.e. lipid with a phosphate group). These phospholipids are arranged as follows:

• The polar, hydrophilic (water-loving) heads face the outside and inside of the cell. These heads interact with the aqueous environment outside and within a cell.
• The non-polar, hydrophobic (water-repelling) tails are sandwiched between the heads and are protected from the aqueous environments.

Scientists Singer and Nicolson described the structure of the phospholipid bilayer as the ‘Fluid Mosaic Model’. The reason is that the bi-layer looks like a mosaic and has a semi-fluid nature that allows lateral movement of proteins within the bilayer.

Image: Fluid mosaic model. Orange circles – Hydrophilic heads; Lines below – Hydrophobic tails.

Functions

• The plasma membrane is selectively permeable i.e. it allows only selected substances to pass through.
• It protects the cells from shock and injuries.
• The fluid nature of the membrane allows interaction of molecules within the membrane. It is also important for secretion, cell growth and division etc.
• It allows transport of molecules across the membrane. This transport can be of two types:
  • Active transport – This transport occurs against the concentration gradient and therefore, requires energy. It also needs carrier proteins and is a highly selective process.
  • Passive transport – This transport occurs along the concentration gradient and therefore, does not require energy. Thus, it does not need carrier proteins and is not selective.

Cell Wall

Description: The cell wall is a non-living, rigid structure outside the plasma membrane in plant cells and fungi. It is absent in Eukaryotic cells of animals

Structure and composition: It is made of different components in different Eukaryotes:

• Cellulose, hemicellulose, proteins and pectin – in plants.
• Cellulose, galactans, mannans and calcium carbonate – in fungi.

The cell wall is divided into the following three layers:

• Middle lamella – It is the outermost layer and is made of calcium pectates. It holds adjoining cells together.
• Primary wall – It is the middle layer and is made of cellulose and hemicellulose. It is present in young, growing cells and is capable of growth.
• Secondary wall – It is the innermost layer and similar in composition to the primary wall.

Functions

• Provides shape to the cell.
• Helps in cell-cell interaction.
• Protects the cell from injury, undesirable molecules and pathogens.

Endoplasmic reticulum (ER)

Description: It is a network of small, tubular structures. It divides the space inside of Eukaryotic cells into two parts – luminal (inside ER) and extra-luminal (cytoplasm).

Structure: ER can be of two types –

Smooth Endoplasmic Reticulum (SER)	Rough Endoplasmic Reticulum (RER)
Smooth due to lack of ribosomes	Rough due to presence of ribosomes
The main site of lipid synthesis	Site of protein synthesis.

Functions

• SER is involved in lipid synthesis and RER is involved in protein synthesis.
• RER helps in folding proteins and transports it to the Golgi apparatus in vesicles.

Golgi Apparatus

Description: It is named after the scientist who discovered it, Camillo Golgi. Golgi is made of many flat, disc-shaped structures called cisternae. It is present in all eukaryotic cells except human red blood cells and sieve cells of plants.

Structure: The cisternae are arranged in parallel and concentrically near the nucleus as follows:

• Cis face (forming face) – It faces the plasma membrane and receives secreted material in vesicles.
• Trans face (maturing face) – It faces the nucleus and releases the received material into the cell.

Functions

• An important site for packaging material within the cell.
• Proteins are modified in the Golgi.
• An important site for the formation of glycolipids (i.e. lipids with carbohydrate) and glycoproteins (i.e. proteins with carbohydrates).

Ribosomes

Description: These structures are not bound by a membrane. Ribosomes are also called ‘Protein factories’ since they are the main site of protein synthesis.

Structure: They are made of ribonucleic acids and proteins. Eukaryotic ribosomes are of the 80S type, with 60S (large subunit) and 40S (small subunit).

Functions: A Major site for synthesis of proteins and polypeptides (chain of amino acids).

Mitochondria

Description: They are membrane-bound organelles, also known as ‘powerhouses of the cell’.

Structure: It has two membranes – outer and inner. The outer membrane forms a continuous boundary around the mitochondria. The inner membrane is semi-permeable and divided into folds called ‘cristae’. The membranes divide the lumen of the mitochondria into an inner and outer compartment. The inner compartment is called matrix and outer compartment forms the intermembrane space.

Functions

• They produce energy (ATP) and therefore are called the ‘powerhouse of the cell’.
• Helps in regulating cell metabolism.
• Mitochondria possess their own DNA, RNA and components required for protein synthesis.

Lysosomes

Description: They are membrane-bound vesicles formed in the Golgi apparatus. Lysosomes are also called ‘suicidal bags’ since they are rich in hydrolytic enzymes such as lipases, proteases, carbohydrates etc. These enzymes are optimally active at acidic pH (less than 7).

Function: The main function of lysosomes is to digest lipids, proteins, carbohydrates and nucleic acids.

Nucleus

Description: Nucleus is the main organelle of a cell. It is a double membrane structure with all the genetic information. Therefore, it is also called the ‘brain’ of a cell. The nucleus is found in all eukaryotic cells except human RBCs and sieve cells of plants.

Structure: A nucleus has the following parts:

• Nuclear envelope – It is a double membrane structure that surrounds the nucleus. The outer membrane is continuous with the endoplasmic reticulum. The inner membrane has small pores called ‘nuclear pores’.
• Nucleoplasm – It is the fluid material in the nucleus that contains the nucleolus and chromatin.
• Nucleolus – Nucleoli are not membrane-bound and are active sites for ribosomal RNA synthesis.
• Chromatin – It consists of DNA and proteins called ‘histones’. The DNA is organised into chromosomes. Chromosomes have certain constriction sites called ‘centromeres’. Based on the position of the centromere, they can be divided as follows:
  • Metacentric – With centromere in the centre and having equal chromosome arms.
  • Sub-metacentric – Centromere is slightly off-centre creating one short and one long arm.
  • Acrocentric – Centromere is extremely off-centre with one very long and one very short chromosome arm.
  • Telocentric – Centromere is placed at one end of the chromosome. Humans do not possess telocentric chromosomes.

Functions

• It stores genetic information (in the form of DNA) necessary for development and reproduction.
• It contains all information necessary for protein synthesis and cellular functions.

Cytoskeleton

Description: It is the filamentous network present in the cytoplasm of a cell.

Function: It provides mechanical support, maintains the shape of the cell and helps in motility.

Cilia and Flagella

They are both responsible for the movement of a cell.

Cilia	Flagella
Short, hair-like structures	Long structures
There are many cilia per cell	There are fewer flagella per cell
Cover the entire surface of a cell	Are present at one end of a cell
Rowing movement	Up and down movement

Plastids

Description: They are double membrane organelles found in plant cells. They contain pigments and are of three types:

• Chloroplasts – They contain chlorophyll and are involved in photosynthesis, where light energy is converted to chemical energy. Chloroplasts contain compartments called stroma and grana. Grana contains structures called thylakoids that contain chlorophyll. Stroma contains enzymes needed for carbohydrate and protein synthesis.
• Chromoplasts – These give plants yellow, red or orange colours because they contain pigments like carotene.
• Leucoplasts – These are colourless plastids that store either carbohydrates (Amyloplasts), oils and fats (Elaioplasts) or proteins (Aleuroplasts).