Cell Structure and Types

• Cells are the structural and functional units of living organisms
• The major parts of a cell are the nucleus and the cytoplasm

Types of Living Cell

• Eukaryotes have a membrane enclosed nucleus containing DNA; animals, plants, and fungi belong to this group
• Eukaryotic cells are generally larger and more complex than prokaryotes
• Eukaryotes contain membrane-enclosed organelles such as mitochondria, lysosomes, the endoplasmic reticulum, and Golgi complexes in their cytoplasm.
• Prokaryotes lack a typical nucleus; instead, they have a nucleoid where their genetic material (DNA) is stored
• Bacteria and blue-green algae are prokaryotes
• Prokaryotes lack membrane-enclosed organelles in their cytoplasm, are relatively small, and are invariably unicellular.
• Viruses are not considered living organisms, as they cannot replicate outside a host cell and have limited biochemical activity
• They possess genomes and can enter living cells, using the host's machinery to replicate, causing disease

Structure and Functions of a Cell

• A cell has three major components: the plasma membrane (cell membrane), the cytoplasm with its organelles, and the nucleus

Plasma Membrane

• The cell is enveloped by a thin membrane called the cell or plasma membrane
• Plasma membranes mainly consist of lipids, proteins, and a small proportion of carbohydrates

Membrane Lipids

• The major classes of membrane lipids are phospholipids, glycolipids, and cholesterol
• These lipids are amphipathic molecules with both hydrophobic and hydrophilic ends
• Membrane lipids spontaneously form a bilayer in aqueous medium, burying their hydrophobic tails and exposing their hydrophilic ends to water

Membrane Proteins

• Membrane proteins are classified into two major categories: integral (intrinsic or transmembrane) proteins and peripheral (extrinsic) proteins
• Integral proteins are either partially or totally immersed in the lipid bilayer
• Transmembrane proteins span the lipid bilayer and act as enzymes and transport carriers for ions and water-soluble substances
• Peripheral proteins are attached to the surface of the lipid bilayer and function as enzymes and receptors

Membrane Carbohydrates

• Membrane carbohydrates are combined with proteins or lipids in the form of glycoproteins or glycolipids
• The carbohydrate portion of these molecules protrudes to the outside of the cell and can act as receptors for hormones, or in antibody processing

Functions of Cell Membrane

• The plasma membrane maintains the physical integrity of the cell, regulates the entry and exit of substances, protects the cytoplasm and organelles, and helps maintain the cell’s shape and size

The Fluid Mosaic Model

• The fluid mosaic model of membrane structure, proposed by Singer and Nicolson in 1972, is widely accepted
• The plasma membrane is composed of a lipid bilayer made of phospholipids, with interspersed proteins

Cytoplasm and its Organelles

• Cytoplasm is the internal volume bounded by the plasma membrane, and the clear fluid portion in which particles are suspended is called cytosol
• Six important organelles suspended in the cytoplasm are: endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, and the nucleus

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is an interconnected network of tubular and flat vesicular structures in the cytoplasm
• It forms a link between the nucleus and cell membrane
• Rough ER has ribosomes attached to its surface and functions in protein biosynthesis
• Smooth ER lacks ribosomes and functions in the synthesis of steroid hormones and cholesterol, and in the metabolism of certain drugs, toxic compounds, and carcinogens

Golgi Apparatus

• The Golgi apparatus is present in all cells except red blood cells, situated near the nucleus, and prominent in secretory cells
• It functions in association with the endoplasmic reticulum
• Proteins synthesized in the ER are transported to the Golgi apparatus, where they are processed by chemical modifications
• The Golgi apparatus is involved in the synthesis of intracellular organelles like lysosomes and peroxisomes.

Lysosomes

• Lysosomes are vesicular organelles formed from the Golgi apparatus and dispersed throughout the cytoplasm
• They have a thick covering membrane to prevent hydrolytic enzymes from contacting other substances in the cell
• Lysosomes contain more than 40 different hydrolases (lysozymes), collectively called lysozymes
• Lysosomes digest proteins, carbohydrates, lipids, and nucleic acids, and are responsible for destruction of bacteria, removal of excessive secretory products, and removal of unwanted cells in the embryo.

Peroxisomes

• Peroxisomes resemble lysosomes in appearance but differ in function and synthesis
• They contain peroxidases and catalase, enzymes involved in peroxide metabolism and detoxification
• Peroxisomes are also capable of carrying out β-oxidation of fatty acids

Mitochondria

• Mitochondria, called the "Power Plant" of the cell, convert energy to form ATP
• A mitochondrion is a double-membrane organelle with a smooth outer membrane and a folded inner membrane forming cristae
• The inner membrane is the site of oxidative phosphorylation, and the mitochondrial matrix contains enzymes for the citric acid cycle and β-oxidation of fatty acids

Nucleus

• The nucleus is present in eukaryotic cells, while prokaryotes lack a nucleus
• Most cells have one nucleus, but skeletal muscle cells have many, and mature red blood cells have none

Structure of Nucleus

• The nucleus is spherical, situated near the center of the cell, and surrounded by the nuclear envelope
• The space enclosed by the nuclear envelope is called nucleoplasm, which contains the nucleolus
• The nucleolus consists of DNA, RNA, and protein, and synthesizes ribosomal RNA
• Remaining nuclear DNA exists as chromatin fibers, which condense into chromosomes during mitosis

Functions of Nucleus

• The nucleus is responsible for replication (DNA synthesis) and transcription (RNA synthesis)
• Three major types of RNA synthesized are ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA (tRNA)

Cytoskeleton

• The cytoplasm of eukaryotic cells contains a network of protein filaments called the cytoskeleton
• The cytoskeleton provides structural support, anchors the plasma membrane, and facilitates cell movement and communication
• It consists of three primary protein filaments: microfilaments, microtubules, and intermediate filaments

Microfilaments

• Microfilaments are made of protein actin, form a meshwork underlying the plasma membrane, and help with muscle contraction, cell shape maintenance, and cellular movement

Microtubules

• Microtubules are made of protein tubulin, are necessary for the formation of the mitotic spindle, provide stability to the cell, and prevent tubules of the ER from collapsing

Intermediate Filaments

• Intermediate filaments are formed from fibrous protein, vary with tissue type, and play a role in cell-to-cell attachment and stabilization of the epithelium

Functions of Cytoskeleton

• The cytoskeleton gives cells their characteristic shape, provides attachment points for organelles, and makes communication possible between parts of the cell
• It is responsible for the separation of chromosomes during cell division and internal movement of organelles

Membrane transport

• One of the functions of the plasma membrane is to regulate the passage of a variety of small molecules across it
• Biological membranes are semipermeable, allowing certain molecules to diffuse freely while restricting others based on size, charge, or solubility

Passive transport

• Passive transport is the process by which molecules move across a membrane without energy (ATP)
• The direction of passive transport is always from a region of higher concentration to one of lower concentration
• Two types of passive transport are simple diffusion and facilitated diffusion

Simple Diffusion

• Lipid-soluble or lipophilic molecules can pass through the cell membrane without interacting with carrier proteins, moving along the concentration gradient

Facilitated Diffusion

• The movement of water-soluble molecules and ions across the membrane requires specific transport system
• They pass through specific carrier proteins- also known as uniport transport processes

Active Transport

• If a molecule moves against a concentration gradient, an external energy source is required

Primary Active Transport

• In primary active transport, the energy is derived directly from the hydrolysis of ATP
• Sodium, potassium, calcium, hydrogen and chloride ions are transported by primary active transport

Secondary Active Transport

• Secondary active transport uses an energy generated by an electrochemical gradient
• Two types- Cotransport or symport, and counter transport or antiport

Transport of Macromolecules Across the Plasma Membrane

• Endocytosis is the process of taking up large molecule
• Two types of endocytosis- Pinocytosis and phagocytosis

Pinocytosis

• Pinocytosis is the cellular uptake of fluid and fluid contents
• Molecules first attach to specific receptors on the surface of the membrane that receptors are generally concentrated in small pits on the outer surface of the cell membrane

Phagocytosis

• Phagocytosis involves ingestion of large particles

Exocytosis

• In exocytosis, the cytoplasmic exocytic vesicles fuse with the internal surface of the plasma membrane

Cell Fractionation

• Cell fractionation is the process used to separate cellular components for study, involving tissue homogenization and differential centrifugation
• Differential centrifugation separates organelles based on size and specific gravity, with denser components like nuclei sedimenting first at lower speeds

Marker Enzymes

• Marker enzymes are unique to specific subcellular fractions and used to assess the purity of that fraction
• By measuring the activity of marker enzymes, contamination from other organelles can be determined.