Cell Structure and Function

Discusses the structure and functions of eukaryotic cells with particular attention to organelles and their roles.

Definition: A protective, permeable layer found in many eukaryotic cells, primarily plant cells, alongside the plasma membrane.
Types:
- Primary Cell Wall:
- Composed mainly of cellulose.
- Forms fibrils that aid in structural strength by lying at right angles to one another.
- Secondary Cell Wall (if present):
- Forms inside the primary cell wall.
- Contains lignin, enhancing strength beyond that of primary cell walls.
Fungi:
- Composed of cellulose and chitin (from exoskeletons of insects).
Algae:
- Contain cell walls made of cellulose.

Definition of Organelle: Initially referred only to membranous structures; now includes any well-defined subcellular structure performing specific functions.
Analogy: Eukaryotic cell compared to a factory, where:
- Raw materials enter and are processed by organelles.
- Waste management is also part of cellular functions.

Both contain mitochondria, but:
- Chloroplasts: Primarily in plant cells.
- Centrioles: Present in animal cells.

Size: Approx. 5 μm in diameter.
Function:
- Stores genetic material (DNA).
- Acts as the head office in the cell factory analogy, controlling cell characteristics and metabolic functions.
Contents:
- Chromatin: DNA with associated proteins; displays as dark structures in micrographs.
- Transforms to chromosomes during cell division (46 in human cells).
Nucleoplasm:
- Semifluid medium surrounding chromatin; distinct pH compared to cytoplasm.
Nuclear Envelope:
- Structure: Double membrane; continuous with the endoplasmic reticulum (ER).
- Nuclear Pores: Size ~100 nm; facilitate bidirectional protein transport.
Nucleolus:
- Production site for ribosomal RNA (rRNA).
- Site where rRNA combines with proteins for ribosomal subunit formation.

Role: Synthesize proteins using messenger RNA templates.
Structure: Composed of large and small subunits made of rRNA and proteins.
Location:
- Free in the cytoplasm or in polyribosomes.
- Attachable to the rough endoplasmic reticulum (ER).
Function: Proteins synthesized vary in destination based on ribosome location (attached vs. free).

Structure: Complicated system of membranes and sacs, continuous with the nuclear envelope.
Types:
- Rough ER: Studded with ribosomes; processes proteins for membrane insertion or secretion.
- Smooth ER: Lacks ribosomes; synthesizes phospholipids and performs additional functions like detoxification and calcium storage.

Structure: Stacked membranous sacs, curved, resembling pancakes.
Function: Acts as cell's shipping center, collecting, sorting, packaging, and distributing materials (proteins and lipids).
- Modifies and repackages proteins and lipids received from the ER;
- Involves in lysosome formation (digestive enzymes).

Structure: Membrane-bound vesicles containing hydrolytic digestive enzymes.
Function:
- Digests macromolecules by fusing with vesicles.
- Involved in intracellular digestion and the process of autodigestion.
- Disorders like Tay-Sachs result from enzyme deficiency leading to accumulation of undigested substrates.

Definition: Large membranous sacs, larger than vesicles.
Function: Support plant cell structure through turgor pressure, store various substances, including water, sugars, and toxins.
Contractile Vacuoles: Special adaptations in some organisms for water expulsion.

Structure: Membrane-bound vesicles containing enzymes synthesized by cytoplasmic ribosomes.
Function: Enzymatic breakdown of fatty acids and detoxification; contain catalase to break down hydrogen peroxide.

Function:
- Convert solar energy into chemical energy through photosynthesis.
Structure:
- Double membrane enclosing stroma and thylakoids (where chlorophyll resides).

Function: Utilize chemical energy in carbohydrates to produce ATP.
Structure: Double membrane with inner membrane folds (cristae) increasing surface area.

Function: Maintains cell shape and facilitates movement of cell and organelles.
Key Components:
- Actin Filaments: 7 nm, form dense web under the plasma membrane, involved in cell motility.
- Intermediate Filaments: 8-11 nm, provide structural support; vary by cell type (e.g., keratin in skin).
- Microtubules: 25 nm, hollow, aid in cell shape and organelle movement; can form spindle apparatus during cell division.

Definition: Structures found in centrosomes of animal cells, assist with microtubule assembly.
Composition: Structure characterized by a 9+0 pattern of microtubule triplets.

Fluid-Mosaic Model: Describes the structure of membranes as a flexible bilayer with various proteins embedded; regulates entry and exit of molecules.
Types of Membrane Proteins:
- Channel Proteins: Form pores for flux of substances.
- Carrier Proteins: Bind substances to assist crossing.
- Receptor Proteins: Allow signaling molecules to bind, causing responses.
- Glycoproteins: Involved in cell recognition.

Passive Transport: Movement across membranes without energy (e.g., diffusion and osmosis).
Active Transport: Movement requiring energy to transport molecules against gradients (e.g., sodium-potassium pump).
Bulk Transport: Vesicle-mediated transport for large macromolecules in and out of cells (e.g., phagocytosis, pinocytosis).

Isotonic Solutions: Equal solute concentration inside and outside; no net water movement.
Hypertonic Solutions: Higher solute concentration outside; water leaves cell, leading to shrinkage.
Hypotonic Solutions: Lower solute concentration outside; water enters cell, can lead to bursting in animal cells, while plants experience turgor pressure.