Cell Structure and Function Overview

Cell Structure and Function Overview

• Cells are the fundamental units of life in all animals, including humans
• Cells are the building blocks of tissues
• When cells die, tissues perish and stop functioning

Cell Theory

• All organisms are composed of cells
• All cells come from pre-existing cells
• Cells contain the genetic information necessary for their functions and reproduction
• Metabolism encompasses all chemical reactions occurring within cells, including anabolism and catabolism

Types of Cells

• Two types of cells exist: prokaryotic and eukaryotic
• Prokaryotic cells are simpler and smaller than eukaryotic cells. They lack membrane-bound organelles and a nucleus.
• Eukaryotic cells are more complex and larger than prokaryotic cells. They possess membrane-bound organelles and a nucleus.

Prokaryotic Cells Characteristics

• Minimal internal organization and smaller in size
• Lack membrane-bound organelles
• Genetic material (DNA) is not enclosed by a nuclear membrane
• DNA is not complexed with histones, a protein found in eukaryotic DNA
• Respiratory system closely associated with the plasma membrane
• Sexual reproduction does not involve mitosis or meiosis.

Eukaryotic Cells Characteristics

• A high degree of internal organization with many internal membrane-bound organelles
• The nucleus contains the genetic material (DNA) enclosed by a double membrane
• The genetic material is complexed with proteins called histones
• Have a specific/well-defined nucleus, mitochondria
• Sexual reproduction involves both mitosis and meiosis
• Respiratory site includes the mitochondria

Cell Organelles: Nucleus

• The nucleus contains most of a cell's DNA
• The control center of the eukaryotic cell
• Nuclear envelope: double membrane structure separating the nucleus from the cytoplasm
• Nuclear pore complexes: embedded in the nuclear envelope controlling the movement of proteins and nucleic acids (RNAs) across the nuclear envelope
• Chromatin: DNA in the nucleus coiled into a dense mass
• Nucleolus: A second dense mass closely associated with the inner nuclear envelope responsible for rRNA synthesis and ribosome subunit assembly
• Nucleoplasm: The nucleoplasm of the nucleus contains enzymes such as DNA polymerases and RNA polymerases for mRNA and tRNA synthesis

Cell Organelles: Mitochondria

• Mitochondria are the "powerhouses" of the cell
• Numerous in some cells, few in others
• Vary in shape and size
• Found in virtually all eukaryotic cells
• The number of mitochondria in a cell varies dramatically; some contain only one, while others can contain thousands
• Mitochondrial membranes: Outer Mitochondrial membrane: mostly phospholipids and cholesterol, containing the protein Porin. Inner Mitochondrial membrane: highly folded (cristae)
• Intermembrane space: located between the inner and outer membrane
• Mitochondrial Matrix: The region enclosed by the inner membrane containing enzymes for the citric acid cycle, fatty acid oxidation, and synthesis of certain proteins
• DNA in the matrix is partly independent (semi-autonomous)
• Mitochondria are involved in Parkinson's disease, a neurodegenerative disorder, where they do not function correctly

Cell Organelles: Endoplasmic Reticulum (ER)

• Extends from the cell membrane, coats the nucleus, and surrounds the mitochondria
• Involved in protein synthesis, transport, modification, storage, and secretion
• Rough ER: covered in ribosomes, involved in protein synthesis
• Smooth ER: lacks ribosomes, involved in lipid synthesis and detoxification

Cell Organelles: Golgi Complex (Golgi Apparatus)

• These are unique stacks of smooth compartments in eukaryotic cells, often associated with ER
• Has a Proximal compartment, Medial compartment, and a Distal compartment
• Golgi complexes sort and modify proteins for transport within the cell or secretion outside the cell
• proteins without signaling or transit peptides are rejected

Cell Organelles: Lysosomes

• Contain packets of enzymes
• Involved in cellular digestion (break-down of molecules)
• Important in autophagy (cell's self-digestion) and in the death (autolysis) of cells
• Lysosomal enzymes have an optimal pH of around 5

Cell Organelles: Peroxisomes

• Small organelles also called Microbodies, present in eukaryotic cells
• No energy-coupled electron transport systems
• Involved in oxidation reactions involving hydrogen peroxide; destroyed by catalase enzyme
• Oxidizing long-chain fatty acids located in liver peroxisomes

Cell Organelles: Cytoplasm (Cytosol)

• Simplest structure of the cell
• Contains organelles-free sap (cytosol)
• Many metabolic reactions occur in cytosol
• Contains proteins, ribosomal RNA (rRNA), and free ribosomes (polysome form)
• Contains necessary substrates and cofactors that interact with various enzymes

Biological Membranes Structure and Function

• Plasma membrane, a prototype cell membrane, extensively studied
• Separates cell contents from the outer environment acting as a barrier and a regulator
• Has selective permeability, controlling molecule and ion flow
• Contains surface molecules for cell recognition and communication
• Eukaryotic cells have internal membrane systems surrounding organelles, assisting their function

Biological Membranes Chemical Composition

• Membranes consist of lipids, proteins, and carbohydrates
• Lipids (mostly phospholipids and cholesterol) form the basis of membranes, while proteins are embedded in or associated with them
• Proteins and carbohydrates are covalently attached to the membrane lipids; lipids are about 40% of the dry weight, proteins 60%, and carbohydrates 1-10%
• Composition of different membranes can vary significantly