Cytology: Nucleus and Cytoplasm

• Cytology is the study of cells.

Nucleus

• The nucleus is a large, spherical structure within the cell.
• It is surrounded by a nuclear envelope.
• The nuclear envelope is composed of two layers separated by 20-40 nm.
• The two layers merge in places to create nuclear pores.
• The genetic material (DNA occurs in the form of filaments.
• The functions of the nucleus are to contain genetic material inherited during cell division and to regulate the activity of the cell.

Cytoplasm

• Cytoplasm is the medium in which chemical reactions occur; it contains enzymes.
• It is a jelly-like cell material between the cell membrane and the nucleus.
• The cytoplasm consists of 80% water, 15% proteins and 5% fats, sugars, and mineral salts.
• It is comprised of a cytosol which is the liquid portion and organelles which are the 'rigid' portion.
• Cytosol is the area of cytoplasm that does not contain organelles.
• Without cytoplasm, the cell would be deflated, and materials would not be able to pass easily between organelles.
• Cytoplasm facilitates key cell activities and is the location for many.
• Cytoplasm is responsible for giving a cell its shape because it helps to fill out the cell and keeps organelles in their place.

Mitochondria

• Mitochondria are among the largest cell organelles.
• They are rod-shaped or spherical organelles.
• Mitochondria are surrounded by double membranes, with the inner membrane bending inward to form folds called cristae.
• The cavity of the mitochondria is filled with a thick liquid called the matrix.
• The matrix contains enzymes involved in chemical processes like cellular respiration (Krebs cycle & electron transport chain).
• The greater the need for power in a cell, the greater the number and size of mitochondria.
• Functions of the mitochondria include energy production (ATP) and self-cleavage using mitochondrial DNA.

Endoplasmic Reticulum (ER)

• The ER is a multifold membranous structure within animal and plant cells.
• There are two types of ER: rough (RER) and smooth (SER).
• The two types are separate entities and not joined together.
• The ER plays a role in the synthesis of complex molecules.
• Cells producing proteins have more RER and ribosomes.
• Cells producing fat and steroids have more SER.
• RER has many ribosomes, giving it a rough appearance.
• Smooth ER is where the synthesis of lipids with phospholipids and cholesterol occurs.
• Rough ER is mainly for the synthesis and modification of proteins.

Ribosomes

• Ribosomes are non-membranous organelles.
• Ribosomes consist of 2 units: one large and one small.
• The two units don't unite only when synthesis of protein is occurring.
• The function of ribosomes is translation of mRNA into amino acids.
• The function of Ribosomes is the synthesis of proteins.
• Ribosomes are the place where amino acids are created.

Peroxisomes

• Peroxisomes are vesicles containing enzymes.
• These enzymes break down toxic organic compounds, such as hydrogen peroxide, into water and oxygen.

Golgi Apparatus

• The Golgi apparatus is composed of flat sacs called cisternae.
• It also contains small hollow spheres of membrane called vesicles.
• The function of the Golgi apparatus is the synthesis of vesicles such as lysosomes.
• It is responsible for manufacturing, warehousing, and shipping cellular products produced by the endoplasmic reticulum (ER).
• The number of Golgi apparatus can vary depending on the cell type
• Cells that specialize in secreting various substances typically have a high number of Golgi apparatus.

Lysosomes

• Lysosomes are small vesicles formed in the Golgi apparatus.
• They contain enzymes that analyze/break down carbohydrates, proteins, and lipids.
• Lysosomes are formed by the inclusion of digestive enzymes such as proteases and lipases
• Lysosomes occur in cells which are phagocytic.
• Lysosomes breakdown materials from the cell.
• Enzymes within lysosomes are isolated from the rest of the cell by the lysosome membrane.
• Self digestion of the cell can occur in instances of disease such as rheumatoid arthritis, where the cartilage of the joint is attacked by lysosome enzymes.

Cytoskeleton

• The cytoskeleton has three different types of protein elements.
• From narrowest to widest these are: microfilaments (actin filament), intermediate filaments (keratin filament), and microtubules (tubulin).

Centrioles

• Centrioles are small, hollow, cylindrical, non-membranous organelles.
• They are present in pairs.
• Each centriole is made up of nine triplets of microtubules (9+0).
• During cell division, the centrioles replicate themselves to the opposite poles of the cell.
• Centrioles have a role in the formation of spindle fibers.
• Have a role in the formation of microtubules.

Cilia and Flagella

• Cilia and flagella are organelles, that are thin cytoplasmic threads projecting from the surface of the cell.
• They also containing microtubules.
• Both have similar structure (9+2) arrangement.
• Flagella are longer and fewer in number, and move the whole cell or organism.
• Flagella sites: prokaryotes and sperm cells with single flagellum.
• Cilia are shorter and higher in number. Additionally, they beat backwards & forward in one direction and line the trachae.
• Cilia and flagella both contain nine outer pairs & two central ones, known as a (9+2) arrangement.

Microvilli

• Microvilli are finger-like projections present on the outer surface of cells, especially in the small intestine.
• They increase the surface area of cells, enhance absorption of materials, and facilitate faster uptake of the products of digestion.

The Endomembrane System

• The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together.
• Its functions are to modify, package, and transport lipids and proteins.
• Included components are: nuclear envelope, lysosome, vesicles, endoplasmic reticulum, golgi, and cell membrane.
• This system does not include the membranes of either mitochondria or chloroplasts.

Plasma (Cell) Membrane

• All membranes have similar structures.
• Some organelles can have single membrane but most organelles have a double membrane (i.e. nucleus mitochondria).
• Phospholipids molecules naturally consist of a bilayer.
• Membranes are composed of phospholipids interspersed with proteins.
• The plasma membrane is selectively permeable: it allows some materials to freely enter or leave the cell, while others materials cannot move freely.
• Phospholipids are molecules with are a phosphate head and two fatty acid tails.
• In cell membrane, phosphate heads are attracted to water (hydrophilic), while fatty acid tails are repelled by water (hydrophobic).
• The main purpose of hydrophobic center of the membrane is to prevent the free diffusion of water & polar molecules through the membrane.
• Its act as barrier for free movement.
• If the membrane is disrupted by molecules passing through it, its springs back into original position with phosphate on the outside & fatty acids on the inside.

Proteins on Cell Membrane

• There are proteins floating in the cell membrane between phospholipids.
• The main functions of proteins are to provide support & stability in the fluid structural, and transport molecules across the membrane.

Types of Proteins in Cell Membrane

• Include: transport proteins, channel proteins, receptors, carrier proteins, enzymatic recognition proteins.

Glycoproteins and Glycolipids

• Polysaccharide compounds.
• They are always on outside of the cell surface membrane.
• Lipid + polysaccharide = glycolipid
• Protein + polysaccharide = glycoprotein
• Functions of both are: to form hydrogen bonds with water molecules outside the cell helping to stabilize the membrane, to act as receptor sites, and to get involved in cell to cell recognition.
• The varying chains emerging from cell surface determine blood type A, B, AB and O.