Cell Structure: Prokaryotes and Eukaryotes

• The cell is a small room or chamber and is the structural and functional unit of living organisms
• The two major components of a cell are the nucleus and cytoplasm

Types of Living Cells

• Based on the presence or absence of a true nucleus, cells are classified as prokaryotes and eukaryotes, first distinguished using electron microscopes
• Eukaryotes have a true membrane enclosed nucleus containing DNA; animals, plants and fungi are eukaryotes
• Eukaryotic cells are more complex and larger than prokaryotes; they can be multicellular or unicellular
• Eukaryotes possess membrane-enclosed organelles like mitochondria, lysosomes, endoplasmic reticulum, and Golgi complexes
• Prokaryotes have no true nucleus; the nucleoid contains DNA and associated proteins, not separated by membranes, found in bacteria and archaea
• Prokaryotes lack membrane-enclosed organelles and have smaller structures; all bacteria are prokaryotes
• Viruses are not living organisms, they cannot replicate outside of a host cell and have limited biochemical activity despite possessing genomes and the ability to cause host cells to replicate them

Structure and Functions of a Cell and its Subcellular Components

• The three major components of a cell are the plasma membrane (cell membrane), cytoplasm with its organelles, and the nucleus

Plasma Membrane

• The cell is enveloped by a thin membrane known as the plasma membrane or cell membrane
• Plasma membranes consist mainly of lipids, proteins, and a smaller proportion of carbohydrates that are linked to lipids and proteins
• Plasma membranes are composed of a lipid bilayer primarily of phospholipids and penetrated protein molecules, forming a mosaic-like pattern

Membrane Lipids

• The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol
• All membrane lipids are amphipathic molecules, meaning they have both hydrophobic and hydrophilic ends
• Membrane lipids spontaneously form bilayers in aqueous environments, burying hydrophobic tails and exposing hydrophilic ends to water

Membrane Proteins

• Membrane proteins are classified as integral (intrinsic or transmembrane) or peripheral (extrinsic)
• Integral proteins are immersed in the lipid bilayer, some spanning the bilayer (transmembrane proteins) which act as enzymes and transport carriers
• Peripheral proteins are attached to the lipid bilayer surface via electrostatic and hydrogen bonds, functioning as enzymes and receptors

Membrane Carbohydrates

• Membrane carbohydrates are bound to proteins (glycoproteins) or lipids (glycolipids)
• Carbohydrates protrude outside the cell and act as receptors for hormones or function in antibody processing

Functions of Cell Membrane

• The plasma membrane maintains the cell's physical integrity, controls what enters and exits the cell, protects the cytoplasm and organelles, and maintains cell shape and size

The Fluid Mosaic Model of Cell Membrane

• Proposed in 1972 by Singer and Nicolson; the plasma membrane is composed of different parts

Cytoplasm and its Organelles

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

Endoplasmic Reticulum (ER)

• Endoplasmic reticulum (ER) is an interconnected network of tubular and flat vesicular structures in the cytoplasm
• The ER links the nucleus and cell membrane
• Rough ER has ribosomes attached to the outer surface and is involved in protein synthesis
• Smooth ER lacks ribosomes and functions in steroid hormone and cholesterol synthesis
• Smooth ER metabolizes certain drugs, toxic compounds, and carcinogens

Golgi Apparatus

• The Golgi apparatus is found in all cells except red blood cells, close to and related to, the endoplasmic reticulum
• Consisting of four or more membranous sacs, the Golgi apparatus is prominent in secretory cells
• The Golgi apparatus works with the endoplasmic reticulum and are involved in synthesizing intracellular organelles

Functions of Golgi Apparatus

• Proteins made in the ER are transferred to the Golgi apparatus, and the chemical modifications are necessary for protein transport
• The Golgi apparatus is involved in synthesizing intracellular organelles (lysosomes and peroxisomes)

Lysosomes

• Lysosomes are vesicle organelles formed from the Golgi apparatus and dispersed that prevent hydrolytic enzymes from digesting action
• The granules contain more than 40 different hydrolases (hydrolytic enzymes) called lysozymes
• Lysozymes digest proteins, carbohydrates, lipids and nucleic acids; apart from digestion, enzymes removes products and unwanted cells

Peroxisomes

• Peroxisomes resemble lysosomes but differ in function and synthesis; peroxisomes do not arise from Golgi membranes
• Peroxisomes function to metabolize peroxide (detoxification), and carry out β-oxidation of fatty acids

Mitochondria (Powerhouse of Cell)

• Mitochondria convert energy to ATP and are double-membrane organelles; the outer is freely permeable for most metabolites and the inner forms cristae for oxidative phosphorylation
• The mitochondrial matrix contains enzymes the citric acid cycle, the ẞ-oxidation of fatty acid, and some degrative cells
• The mitochondrial matrix is the site for reactions of the citric acid cycle and fatty acid oxidation, oxidative phosphorylation takes place in the inner mitochondrial membrane
• Mitochondria contains DNA (mtDNA) and encodes a few polypeptides involved in oxidative phosphorylation

Nucleus

• Cells with a nucleus are eukaryotes and cells without a nucleus are prokaryotes
• Only one nucleus exists in the majority of cells except for skeletal muscles, and red blood cells that contain no nucleus
• A spherical shape composes the nucleus, it is situated near the center of the cell, and the nuclear envelope surround it
• The nucleolus is present in the nucleoplasm an organized structure of DNA, RNA and protein, that is involved in the synthesis of ribosomal RNA
• Nuclear DNA dispersed throughout the nucleoplasm in the form of chromatin fibers, chromosomes

Functions of Nucleus

• Replicates DNA
• Transcribes: Ribosomal RNA (rRNA), Messenger RNA (mRNA), and Transfer RNA (tRNA).

Cytoskeleton

• The cytoplasm of most eukaryotic cells contains a network of protein filaments with each other and with the component of the plasma membrane
• There is an extensive intracellular network of protein called the cytoskeleton, this is not rigid but an evolving structure
• Microfilaments consist of protein actin, they help muscles contract and allows for cellular movement; diameter of ~5nm
• Microtubules consist of protein tubulin and is necessary for the formation and provides stability with a diameter of 20-25 nm
• Intermediate are fibrous proteins and play a role in cell-to-cell attachment; with a diameter of 10 nm
• The cytoskeleton shapes a cell, provides attachment points, and internal movement for organelles: The cytoskeleton acts as a "track"

Membrane Transport

• The plasma membrane functions to regulate the passage of small molecules
• Biological membranes contain molecules that freely diffuse and restrict movement based upon size, charge and solubility
• The two types of transport are passive transport or passive diffusion and active transport

Passive Transport or Passive Diffusion

• Molecule movement across membranes occurs occurs without energy (ATD)
• Direction always occurs from a higher concentration to a lower concentration

Simple Diffusion

• Lipid soluble pass through the cell membrane

Facilitated Diffusion

• Movement of water soluble molecules and ions requires a specific transport system (carrier proteins); glucose

Active Transport

• Molecule movement occurs with the use of an external energy source

Primary Active Transport

• Energy is derived from the hydrolysis of ATP
• Transports sodium, potassium, calcium, hydrogen and chloride ions

Secondary Active Transport

• Transports substances and contains two sub types: cotransport (symport) and counter transport (antiport)

Transport of Macromolecules Across the Plasma Membrane

• Endocytosis is the process of uptake and exocytosis is the releasing of large molecules and contains two endocytosis subtypes: Pinocytosis (cellular drinking) and Phagocytosis (cellular eating)
• The macromolecular contents are digested and reused