Lecture 2d

Energy Conversion from One Form to Another
- Mitochondria
- Primary sites of cellular respiration.
- Chloroplasts
- Found only in plants and algae.
- The sites of photosynthesis.
- Mitochondria and chloroplasts are not part of the endomembrane system.
- Peroxisomes
- Specialized oxidative organelles.

Chemical Energy Conversion
- Mitochondria are present in nearly all eukaryotic cells.

Components of Mitochondria
- Intermembrane space
- Outer membrane
- Inner membrane
- Folded into cristae.
- Matrix
- Mitochondrial DNA
- Free ribosomes present in the mitochondrial matrix.
Membrane Structure
- The outer membrane is smooth.
- The inner membrane is folded into cristae, increasing surface area for ATP synthesis.
Compartmentalization
- The inner membrane creates two distinct compartments:
- Intermembrane space.
- Mitochondrial matrix.
- Localizes some metabolic steps of cellular respiration within the mitochondrial matrix.

Capture of Light Energy
- Chloroplasts belong to a family of organelles known as plastids.

Components of Chloroplasts
- Chloroplast DNA
- Ribosomes
- Stroma: the internal fluid.
- Inner and outer membranes.
- Granum: A stack of thylakoids.
- Thylakoids: Membranous sacs within the chloroplast.
- The chloroplasts contain the pigment chlorophyll, along with enzymes and other molecules critical for photosynthesis.
- Primarily located in leaves and other green organs of plants and in algae.

Peroxisome Structure
- Specialized metabolic compartments bounded by a single membrane.
Functionality
- Peroxisomes carry out oxidative reactions.
- Produce hydrogen peroxide (H2O2) and convert it into water.

Overview of Cytoskeleton
- A network of fibers that organizes structures and activities within the cell.
- Extends throughout the cytoplasm, organizing the cell's structures and anchoring many organelles.
Composition of Cytoskeleton
- Composed of three types of molecular structures:
- Microtubules: The thickest component.
- Microfilaments (Actin filaments): The thinnest components.
- Intermediate filaments: Fibers with intermediate diameters.

Functions of the Cytoskeleton
- Provides support to the cell and helps maintain its shape.
- Interacts with motor proteins to produce motility.
- May help regulate biochemical activities.
Vesicular Transport
- Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton.

Microtubule Composition
- Composed of tubulin polymers, structured as hollow tubes.
- Wall consists of 13 columns of tubulin molecules.
- Diameter: 25 nm with a 15-nm lumen.
Protein Subunits
- Main subunits include α-tubulin and β-tubulin.
Key Functions
- Maintenance of cell shape (compression-resisting “girders”).
- Cell motility (e.g., as in cilia or flagella).
- Chromosome movements during cell division.
- Movement of organelles within the cell.

Microfilaments Composition
- Comprised of two intertwined strands of actin, each strand being a polymer of actin subunits.
- Diameter: 7 nm.
Protein Subunits
- Dominant protein subunit is actin.
Main Functions
- Maintenance of cell shape (tension-bearing components).
- Changes in cell shape.
- Facilitation of muscle contraction.
- Enable cytoplasmic streaming.
- Motility (e.g., as in pseudopodia).
- Cell division (formation of cleavage furrow).

Intermediate Filaments Composition
- Composed of fibrous proteins coiled into thicker cables.
- Diameter: 8-12 nm.
Protein Subunits
- Comprise various proteins from the keratin family, varying by cell type.
Main Functions
- Maintenance of cell shape (tension-bearing components).
- Anchorage of the nucleus and certain organelles.
- Formation of the nuclear lamina.

Microtubule Organization
- Microtubules emanate from a centrosome located near the nucleus in many cells.
- The centrosome is a microtubule-organizing center.
- In animal cells, the centrosome contains a pair of centrioles arranged in a ring, with nine triplets of microtubules.

Cilia and Flagella
- Cilia
- Membrane-bound organelles shaped like slender projections.
- Function in cell movement; found in various eukaryotic cells.
- Flagella
- Longer than cilia; enable swimming through liquids.
- Similar structure to cilia but different undulating motion.
- Microtubules control the beating of cilia and flagella, which are motile cellular appendages.
- They assist in cell movement and the movement of surrounding substances.
Common Ultrastructure of Cilia and Flagella
- Composed of a core of microtubules sheathed by the plasma membrane.
- Contains a basal body that anchors the cilium or flagellum.
- Contains a motor protein named dynein, which drives the bending movements.

Microfilaments Functionality
- Microfilaments function for cellular motility with the protein myosin along with actin.
Muscle Contraction Mechanism
- Arranged in parallel in muscle cells, with thicker myosin filaments interdigitating with thinner actin fibers.
- Muscle contractions arise from the interaction between myosin and actin filaments, causing movement relative to one another.

Cytoplasmic Streaming
- Defined as the circular flow of cytoplasm within cells.
- Facilitates effective distribution of materials within a cell.
- In plant cells, driven by actin-myosin interactions and sol-gel transformations, leading to dynamic flow between gel and sol states of the cytoplasm.