Cell Structure and Function

Lesson 1: The Cell

• The cell is the fundamental morphological and functional unit of all living things
• It has the capacity to perform all life functions and can exist independently under favorable conditions
• A human being begins as a fertilized ovum (zygote) resulting from the union of male (sperm) and female (ovum) gametes
• Cell multiplication (mitosis) and differentiation produce complex organisms, with an estimated 100 trillion cells in adults

Cell Membrane (Plasma Membrane/Plasmalemma)

• The cell membrane acts as a barrier, delimiting the cell from its surroundings
• It regulates the movement of substances into and out of the cell
• It provides attachment for the cell's cytoskeleton
• It receives and sends out stimuli
• It contains binding sites and receptors for enzymes and other substances
• It allows cell-to-cell recognition
• The membrane is approximately 8-10 nm thick
• It is trilaminar in electron microscopy, appearing as two electron-dense sheets sandwiching a thin, electron-lucent layer
• Primarily composed of phospholipids, cholesterol, and polysaccharides (glycolipids and glycoproteins)

Phospholipid Molecules in the Cell Membrane

• Phospholipid molecules have a hydrophilic head and two hydrophobic tails
• The head is globular, polar, and hydrophilic
• The tails are slender, non-polar, and hydrophobic
• The tails consist of saturated and unsaturated fatty acids
• The head is attached to the tails through covalent bonds to glycerol
• Phospholipid molecules form a bilayer in the membrane
• The hydrophobic tails face inward, and the hydrophilic heads face outward

Protein Molecules in the Cell Membrane

• Proteins comprise roughly half the membrane's mass
• Integral proteins span the entire membrane thickness; transmembrane proteins extend through both surfaces
• Peripheral proteins are either loosely bound to the inner or outer membrane surface
• Membrane proteins perform functions such as substance transport across the membrane

Cholesterol Molecules in the Cell Membrane

• Cholesterol molecules are found in the irregular spaces between phospholipid molecules
• They stiffen and strengthen the membrane
• They reduce membrane permeability to water-soluble substances

Glycocalyx

• Glycolipids and glycoproteins project from the cell membrane's exterior, forming a coating called the glycocalyx
• The glycocalyx is a thin layer (2-20 nm) of electron-dense material
• It plays a role in cell-to-cell recognition, cell-to-cell adhesion, and immunological responses

Specialized Junctions Formed by the Cell Membrane

• Specialized junctions allow cells to adhere to each other and the extracellular matrix, and facilitate communication
• Tight junctions (zonula occludens) seal cells together
• Adherens junctions (zonula adherens) attach cells together, forming bands
• Desmosomes (macula adherens) act as spot adhesions to reinforce cell-to-cell connections
• Hemidesmosomes are similar to desmosomes but link cells to the extracellular matrix
• Gap junctions (nexus) allow communication between cells via channels

Cytoplasm

• Cytoplasm is the contents within the cell membrane and excluding the nucleus
• Cytoplasm consists of the Cytosol (homogenous substance), Organelles (permanent structures with specific functions), Inclusions (temporary or inert structures), and Fibrillar Elements (the cytoskeleton)

Organelles

• Organelles are permanent structures within cells performing specific functions
• Examples: mitochondria, ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and centrosomes
• All organelles, except ribosomes and centrosomes, are enclosed by membranes

Mitochondria

• Mitochondria are organelles present in almost all cells
• They are typically rod-shaped, but can be filamentous, spherical, etc
• The mitochondria are 0.5-1.0 μm in diameter and can reach up to 10 μm in length
• Mitochondria are enclosed by two membranes, outer and inner, with a matrix inside the inner membrane containing cristae
• They replicate using a mechanism similar to bacterial binary fission.

Ribosomes

• Ribosomes are minute organelles (15-30 nm)
• They are seen as small electron-dense granules in an electron microscope, sometimes occurring singly or as clusters called polyribosomes or polysomes
• Ribosomes are involved in protein synthesis

Endoplasmic Reticulum (ER)

• The ER is an extensive network of interconnected tubes, vesicles, and flattened sacs (cisternae) within the cytoplasm
• The rough ER (rER) has ribosomes on its surface and is involved in protein processing and transport
• The smooth ER (sER) lacks ribosomes and plays a role in lipid synthesis, carbohydrate metabolism, and detoxification

Golgi Complex (Golgi Body/Apparatus)

• Composed of membrane-bound flattened sacs (cisternae) stacked in a semicircular manner
• Processes, concentrates, sorts, and packages proteins received from the rER for secretion or use within the cell
• Forms secretory vesicles for transport of proteins
• The Golgi complex is dynamic, continually adding membrane from transfer vesicles from the rER and losing material through pinched-off secretory vesicles

Lysosomes

• Lysosomes are chemical-containing pouches containing hydrolytic enzymes (hydrolases)
• They assist in digestion of unneeded or senescent cellular components.
• They have diverse shapes (spherical, ovoid) and diameters ranging from 0.05-0.08 µm
• Two main types of intracellular processes, heterophagy and autophagy involve lysosomes

Peroxisomes

• Membrane-bound, spherical bodies containing enzymes (oxidases and catalases)
• Peroxisomes are primarily involved in the detoxification of substances within the cytoplasm of active cells like hepatocytes
• Important in catalyzing oxidation reactions for lipid catabolism, and detoxification

Centrosome (Microtubule Organizing Center)

• Located near the nucleus, a major hub for microtubule organization
• Contains a pair of centrioles, which are short, cylindrical structures
• Surrounded by a granular structures called centriolar satellites
• The two centrioles lie perpendicular to each other
• The centrosome is involved in mitosis activities, generating microtubules and directing their positioning and function

Inclusions

• Inclusions are temporary or inert structures within cells, sometimes membrane-bound
• Some examples include fat droplets, glycogen, melanin, hemosiderin

Nucleus

• The Nucleus is the largest organelle in the cell
• It controls cell activities by directing protein synthesis
• It contains DNA, chromosomes, and chromatin during interphase
• The nucleus is enclosed by the nuclear envelope, a double-membrane with pores

Nuclear Envelope (Nuclear Membrane)

• The perinuclear space is continuous with the rough endoplasmic reticulum (rER)
• The nuclear envelope is perforated by nuclear pores, providing channels for the passage of substances between the cytoplasm and the nucleus
• Nuclear pores are surrounded by nuclear pore complexes

Chromatin

• Chromatin is the genetic material within the nucleus during interphase
• Contains DNA that is wound around proteins called histones
• Two major types of proteins, histones and nonhistones, are associated with the DNA

Nuclear Matrix

• A network of filamentous proteins within the nucleus that is thought to provide structural support.
• May anchor DNA loops and play a role in regulation of DNA replication

Nucleolus

• Spherical, deeply staining structure within the nucleus
• Involved in ribosomal RNA (rRNA) synthesis and the assembly of ribosomes

Movement of Materials Across the Cell Membrane

• Phagocytosis is the uptake of solid materials, such as bacteria
• Pinocytosis is the intake of liquid materials
• Exocytosis is the release of materials from the cell
• Constitutive secretion is a continuous release of materials from cells
• Regulated secretion utilizes specific signals to trigger release of products

Explore the intricate world of cell biology. Questions cover cell junctions, cytoplasm components (inclusions, cytosol, fibrillar elements), membrane structure, and organelle functions. Test your knowledge of how cells are organized and how they maintain their internal environment.