Biology Unit 2: Cell Structures and Functions

Objectives of Unit

• Understand definitions of cytology and organelles.
• Identify cell parts from diagrams and models, recognizing their functions.
• Study organelles like endoplasmic reticulum, chloroplasts, and mitochondria.
• Learn about the nucleus and its components, including nuclear envelope and nucleolus.

Introduction to Cells

• Eukaryotic cells possess a membrane-bound nucleus, organelles, and multiple rod-shaped chromosomes.
• Cells are classified as the smallest units of life, capable of self-replication and vital to bodily structure.
• Key cellular regions include the cytoplasm (fluid matrix) and nucleus (genetic material).
• Organelles perform specific functions to maintain cellular homeostasis, including energy production and protein synthesis.

Organelles Classification

• Membranous organelles have their own membranes and separate lumen (e.g., endoplasmic reticulum).
• Non-membranous organelles, such as ribosomes and structures within the cytoskeleton, lack surrounding membranes.

Cytoplasm and Nucleoplasm

• Cytoplasm refers to the intracellular fluid, while nucleoplasm is within the nucleus.
• Organelles are categorized to understand their structures and functions in detail.

Nucleus Overview

• The nucleus is the most prominent organelle, varying from uninucleate to multinucleate conditions.
• Contains DNA genome, RNA synthesis machinery, and a fibrous matrix, encased by two membranes.

Nuclear Structure

• Nuclear envelope consists of inner and outer membranes, continuous with the rough endoplasmic reticulum.
• Nuclear pores, formed by nucleoproteins, regulate material exchange between nucleus and cytoplasm.

Chromosome Composition

• Chromosomes are DNA strands organized with proteins (histones), forming chromatin.
• Chromatin is classified into euchromatin and heterochromatin, depending on gene availability for expression.

Nucleolus Function

• The nucleolus synthesizes rRNA and assembles ribosomes; not membrane-bound, situated within the nucleus.
• Comprises three distinct regions: fibrillar center, dense fibrillar component, and cortical granule component.

Functions of the Nucleus

• Controls hereditary traits and regulates cell activities through gene expression.
• Stores genetic material and proteins and is the site of mRNA transcription.
• Facilitates selective transport of molecules via nuclear pores.

Animal Cell Nucleus

• Surrounded by a double membrane; communicates with cytoplasm through nuclear pores.
• Maintains genetic material essential for protein synthesis and differentiation.

Plant Cell Nucleus

• Similar structure to animal cells; controls cell activities and is vital for growth and division.
• Continuous with the endoplasmic reticulum, ensuring efficient communication.

Bacterial Cell Structure

• Bacteria lack a proper nucleus; their circular chromosome is located in the cytoplasm.
• Considered the earliest form of life, with no membrane-bound organelle.

Ribosomes Overview

• Essential for synthesizing proteins from amino acids; composed of RNA and protein.
• Found freely in cytoplasm or attached to the rough endoplasmic reticulum, with an average cell containing around 10 million ribosomes.

Ribosome Functions

• Active ribosomes can form polysomes, where multiple ribosomes translate a single mRNA.
• Amino acids are joined at a rate of approximately 200 per minute, enabling efficient protein synthesis.

Eukaryotic vs. Prokaryotic Ribosomes

• Ribosomal function differs between prokaryotes and eukaryotes, impacting how proteins are produced.### Ribosomes

• Ribosomes vary significantly among bacteria, archaea, and eukaryotes in both structure and RNA sequences.

• Antibiotics exploit these differences; they inhibit bacterial ribosome activity while leaving human ribosomes unaffected.

• Eukaryotic ribosomes share similarities with bacterial ribosomes, suggesting a common evolutionary origin.

• Composed of ribonucleic acid (RNA) and proteins, ribosomes are crucial for protein synthesis, linking amino acids based on messenger RNA (mRNA) instructions.

Structure

• Ribosomes consist of two subunits: a small subunit that reads mRNA and a large subunit that forms polypeptides.
• Each ribosomal subunit is made of rRNA molecules and proteins, contributing to the translational apparatus.
• Two main types of ribosomes are classified by sedimentation coefficients: 70S (prokaryotic) and 80S (eukaryotic).
• Ribosomes found in mitochondria and chloroplasts are 70S, resembling prokaryotic structures.

Origin

• Ribosomes may have originated in an RNA world, evolving as self-replicating complexes prior to protein synthesis capability.
• Ancient ribosomes likely had multifunctional roles, coding for tRNAs and facilitating ribosomal self-replication.

Function

• Ribosomes synthesize nearly all proteins needed by cells; they are present in cytoplasm and on the rough endoplasmic reticulum (RER).
• They link amino acids to form polypeptides, operating at a rate of 200 amino acids per minute.
• Ribosomes' catalytic activity is primarily governed by rRNA, while proteins stabilize their structure.

Nucleoplasm

• Nucleoplasm is a gelatinous substance within the nucleus, analogous to cytoplasm, supporting nuclear structures like chromatin and nucleoli.
• It contains water, dissolved ions, nucleotides, and enzymes crucial for DNA replication and RNA synthesis.

Structure

• Made up of a viscous mix, nucleoplasm includes chromosomes and structures for RNA synthesis.
• Within the nucleoplasm, nucleoli serve as sites for RNA precursor synthesis.

Function

• Acts as a suspension medium for nuclear components and plays a role in maintaining the nucleus's structure.
• Houses nucleotides and enzymes necessary for various nuclear processes.

Mitochondria

• Mitochondria are double-membraned organelles known as the "powerhouses of the cell," essential for energy production via oxidation.
• First described in the 1890s, they generate ATP through a series of enzymatic reactions, particularly in the Krebs cycle.

Structure

• Feature an outer membrane, inner membrane, intermembrane space, cristae space, and matrix, each with distinct functions:
  • Outer Membrane: Smooth, composed of phospholipids and porins allowing nutrient molecules to pass freely.
  • Inner Membrane: Highly folded (cristae) to increase surface area for ATP production.
  • Matrix: Contains enzymes, mitochondrial DNA, and ribosomes.

Function

• Responsible for producing ATP via oxidative phosphorylation from simpler nutrient molecules.
• Helps regulate calcium ion concentration and play roles in hormone production and ammonia detoxification in liver cells.
• Involved in programmed cell death (apoptosis), highlighting their significance in cellular health and function.### Abnormal Cell Death
• Dysfunctional mitochondria can lead to abnormal cell death and impact organ function.

Chloroplasts

• The term "chloroplast" originates from the Greek words "chloros" (green) and "plastes" (form).
• Organelles found in plant and algal cells, credited to Julius von Sachs, a pioneer in plant physiology.
• Chloroplasts are crucial for photosynthesis, converting light energy into sugars while producing oxygen as a byproduct.
• Animal cells lack chloroplasts, limiting their capability for photosynthesis.
• Typically, a plant cell contains around 50 chloroplasts, distributed homogeneously in the cytoplasm.

Origin of Chloroplasts

• Chloroplasts likely originated from endosymbiotic bacteria.
• They develop from precursors called Proplastids and are semi-autonomous, possessing their own protein synthesis machinery.
• Chloroplast division is observable in algae during cell division, though more challenging in higher plants.

Structure of Chloroplasts

• Generally biconvex or planoconvex in shape with variations like spheroid or discoid based on plant species.
• Comprised of a double membrane surrounding the stroma, which contains chloroplast DNA, ribosomes, and various proteins.
• The thylakoid system within the stroma facilitates photosynthesis, where chlorophyll is stored and light reactions occur.

Components of Chloroplasts

• Outer Membrane: Semi-porous, allows small molecules and ions to pass but restricts larger proteins.
• Inner Membrane: Regulates material passage and synthesizes fatty acids and carotenoids.
• Stroma: Alkaline, protein-rich fluid where thylakoids and starch granules float.
• Thylakoids: Membranous sacks arranged in stacks (grana) where light reactions occur, and chlorophyll captures light energy.

Functions of Chloroplasts

• Chloroplasts facilitate the plant immune response by working with the nucleus and cell structures.
• Their primary function is converting light energy into biological energy through photosynthesis, producing sugars and oxygen.
• Light reactions take place in the thylakoid membranes, resulting in NADPH2 production and oxygen release.
• Dark reactions, or Calvin cycle, occur in the stroma, where carbon dioxide is fixed into sugars.

Types of Plastids

• Plastids are double-membraned organelles exclusive to plant cells, with an average size of 4-6 µm.
• Divided into grana (membrane-bound sacs with chlorophyll) and stroma (homogeneous matrix).
• Three main types: Leucoplasts (colorless, food storage), chloroplasts (green, photosynthetic), and chromoplasts (colored, found in flowers/fruits).

Functions of Plastids

• Green plastids manufacture food via photosynthesis.
• Chromoplasts contribute color to flowers and fruits.
• Leucoplasts store starch, proteins, and oils, with subtypes including amyloplasts (starch storage) and elaioplasts (oil storage).

Golgi Complex

• The Golgi apparatus functions as the processing, packaging, and secretion center of the cell, identified by Camillo Golgi in the late 19th century.
• Composed of flattened sacs (cisternae) and vesicles, found in all eukaryotic cells except mammalian erythrocytes and certain specialized cells.
• Plays a critical role in modifying proteins from the rough endoplasmic reticulum before secretion.

Structure of Golgi Complex

• Varies in shape and size based on cell type; consists of cisternae, tubules, and vesicles.
• Contains 40-100 stacks of cisternae, which are disc-shaped and enzyme-rich for protein modification.
• The Golgi complex is polarized, facilitating the direction of proteins and other molecules.

Origin of Golgi Bodies

• Debate exists on Golgi body origins, with a consensus that they form from vesicles dispatched from the endoplasmic reticulum (ER).
• Transition vesicles contribute to the growth of Golgi bodies, allowing cellular processes and trafficking.

Summary of Functions

• The Golgi complex acts as a "post office" for the cell, packaging and directing proteins to various cellular locations or outside the cell.

Explore the essential structures and functions of cell organelles in this comprehensive quiz on Biology Unit 2. You'll delve into the nucleus, ribosomes, and nucleoplasm, focusing on their origins, structures, and functions. Test your understanding and reinforce your knowledge of cellular components.