Unit 2: Structure and function of cell organelles

Cell Wall and Cell Membrane

Cell Wall

The cell wall is a rigid, protective outer layer found in plant cells, bacteria, fungi, and algae. Animal cells do not have a cell wall.

• Function: Provides structural support and protection to the cell, prevents over-expansion (lysis) when the cell takes in too much water.
• Composition in Plants:
  • Primary Wall: Thin, flexible layer in young, growing cells. Composed mainly of cellulose.
  • Secondary Wall: Forms inside the primary wall in mature cells (e.g., wood). It's thick, rigid, and often contains lignin for extra strength.
  • Middle Lamella: A layer rich in pectins that glues adjacent cells together.

Cell Membrane (Plasma Membrane)

The cell membrane is a flexible, selectively permeable barrier that encloses the cytoplasm of every cell. It controls what enters and leaves the cell.

Its structure is described by the Fluid Mosaic Model.

Models of Cell Membrane

While earlier models (like the Davson-Danielli "sandwich" model) existed, the currently accepted model is the Fluid Mosaic Model, proposed by Singer and Nicolson in 1972.

The Fluid Mosaic Model:

This model describes the cell membrane as a fluid, two-dimensional "sea" of phospholipids, with a mosaic (patchwork) of different proteins, lipids, and carbohydrates embedded or attached to it. These components are not static and can move laterally.

Components of the Fluid Mosaic Model

• Lipids (Phospholipid Bilayer):
  • Forms the basic structure.
  • Each molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.
  • They arrange in a bilayer with tails facing inward, away from the water.
• Proteins (Role of membrane proteins):
  • Integral Proteins: Span the entire membrane (transmembrane). Function as channels, pumps, or receptors.
  • Peripheral Proteins: Attached to the inner or outer surface. Function as enzymes or in cell signaling.
• Carbohydrates (Role of membrane carbohydrates):
  • Attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the outer surface.
  • They form the glycocalyx, which is crucial for cell-to-cell recognition, adhesion, and signaling.

Role of Channels and Pumps in Cellular Transport and Signaling

The cell membrane controls passage of substances using transport proteins.

Cellular Transport

• Passive Transport (No ATP energy): Movement down a concentration gradient (high to low).
  • Facilitated Diffusion: Uses protein channels (e.g., ion channels, aquaporins) to help polar molecules or ions cross the membrane.
• Active Transport (Requires ATP energy): Movement against a concentration gradient (low to high).
  • This process requires carrier proteins called pumps.
  • Example: The Sodium-Potassium (Na+/K+) pump, which is vital for nerve cell function.

Cellular Signaling

Many membrane proteins act as receptors. A signaling molecule (like a hormone) binds to the receptor on the outside, causing a shape change in the protein that triggers a response inside the cell.

Cytoskeleton and Cytosol

Cytosol

The cytosol is the semi-fluid, jelly-like substance that fills the cell and surrounds the organelles. (Note: Cytoplasm = Cytosol + Organelles).

Cytoskeleton

The cytoskeleton is an intricate network of protein fibers that extends throughout the cytoplasm. It is not static but dynamic, constantly breaking down and reforming.

Functions: Provides structural support, maintains cell shape, anchors organelles, and enables cell movement (locomotion) and intracellular transport.

Components of the Cytoskeleton:

1. Microtubules: Thick, hollow tubes made of tubulin protein.
   • Functions: Maintain cell shape, form tracks for organelle movement, make up cilia, flagella, and the mitotic spindle (used in cell division).
2. Microfilaments (Actin Filaments): Thin, solid rods made of actin protein.
   • Functions: Muscle contraction, amoeboid movement (pseudopods), cytoplasmic streaming (in plant cells), and formation of the cleavage furrow during animal cell division.
3. Intermediate Filaments: Fibers with a diameter in between the other two.
   • Functions: Purely structural. They are very stable, maintain cell shape, and anchor the nucleus and other organelles. (Ex: Keratin in skin cells).

The Endomembrane System & Other Organelles

These are the functional compartments (organelles) of the cell.

Organelles of the Endomembrane System:

(A group of organelles that work together to synthesize, modify, package, and transport lipids and proteins).

• Endoplasmic Reticulum (ER): A network of interconnected membranes (cisternae).
  • Rough ER (RER): Surface is studded with ribosomes. Function: Synthesizes and modifies proteins destined for secretion or insertion into membranes.
  • Smooth ER (SER): Lacks ribosomes. Function: Lipid synthesis (steroids), detoxification of drugs and poisons, and calcium storage.
• Golgi-bodies (Apparatus/Complex): The "post office" of the cell. A stack of flattened sacs (cisternae).
  • Function: Receives proteins and lipids from the ER, modifies them, sorts them, and packages them into vesicles for transport to other destinations.
• Lysosomes: The "recycling center" or "suicide bags."
  • Function: Membranous sacs containing hydrolytic enzymes. They digest food particles (phagocytosis), old organelles (autophagy), and can trigger cell death.

Other Key Organelles:

• Ribosomes:
  • Structure: Not membrane-bound. Made of rRNA and protein. Composed of two subunits (large and small).
  • Function: The site of protein synthesis (translation). Found free in the cytosol or bound to the RER.
• Peroxisomes:
  • Function: Small, specialized metabolic compartments. They contain enzymes (like oxidase) that break down fatty acids and detoxify harmful substances. A byproduct is hydrogen peroxide (H2O2), which the peroxisome then neutralizes with the enzyme catalase.

Endosymbiotic Theory: Mitochondria and Chloroplast

The Endosymbiotic Theory:

Proposed by Lynn Margulis, this theory states that mitochondria and chloroplasts were once free-living prokaryotic cells that were engulfed (phagocytosed) by a larger anaerobic host cell. Instead of being digested, they formed a symbiotic (mutually beneficial) relationship.

The host cell provided protection and nutrients, while the endosymbionts provided a huge metabolic advantage:

• Mitochondria: (Originally an aerobic bacterium) Provided the ability to use oxygen for cellular respiration, generating large amounts of ATP ("powerhouse" of the cell).
• Chloroplast: (Originally a photosynthetic bacterium, like cyanobacteria) Provided the ability to convert sunlight into food (photosynthesis).