UNIT 2 CELL STRUCTURE AND FUNCTION

Prokaryotic Cells:
- No nucleus.
- No membrane-bound organelles.
- Generally smaller in size.
- Examples: Bacteria and Archaea.
- DNA is located in the nucleoid region of the cell.
- Ribosomes are present for protein synthesis.
Eukaryotic Cells:
- Have a nucleus that contains DNA.
- Contain membrane-bound organelles.
- Generally larger than prokaryotic cells.
- Examples: Animals, Plants, Fungi, Protists.

Phospholipid Bilayer:
- Hydrophilic heads face the aqueous environment (outside and inside the cell).
- Hydrophobic tails face each other, creating the interior of the bilayer.
Fluid Mosaic Model:
- Membrane is described as fluid; phospholipids can move laterally within the layer.
- Proteins are embedded throughout, providing a mosaic appearance and various functions.
Cholesterol:
- Functions to regulate fluidity of the membrane.
- Keeps the membrane stable at high temperatures and maintains fluidity at low temperatures.
Proteins:
- Functions include transport of materials, acting as receptors for signaling, serving as enzymes, and facilitating cell recognition.
Carbohydrates:
- Attached to proteins (glycoproteins) or lipids (glycolipids), playing a key role in cell recognition.

Nucleus:
- Contains the cell's DNA and controls cell activities.
Ribosomes:
- Responsible for protein synthesis.
- Can be found free in the cytoplasm or attached to the endoplasmic reticulum (ER).
Endoplasmic Reticulum (ER):
- Rough ER:
- Has ribosomes on its surface, primarily synthesizes proteins.
- Smooth ER:
- Lacks ribosomes, functions in lipid synthesis and detoxification.
Golgi Apparatus:
- Modifies, packages, and ships proteins and lipids to their appropriate destinations within or outside the cell.
Mitochondria:
- Organelles where cellular respiration occurs, producing ATP (Adenosine Triphosphate), which serves as the cell's energy currency, often referred to as the powerhouse of the cell.
Lysosomes:
- Involved in digestion and waste removal, containing various enzymes to breakdown cellular waste.
Vacuoles:
- Mainly serves as storage compartments.
- Notably, plants have a large central vacuole that maintains turgor pressure by storing water.
Chloroplasts (Plants Only):
- Organelles responsible for the process of photosynthesis, converting carbon dioxide and water into glucose.
Cell Wall (Plants, Fungi, Bacteria):
- Provides structure and support to the cell; located outside the plasma membrane.
Cytoskeleton:
- Provides structural support and facilitates movement within the cell.
- Composed of three types of filaments: microfilaments, intermediate filaments, and microtubules.

Passive Transport (No Energy Required):
- Simple Diffusion:
- Movement of small nonpolar molecules down their concentration gradient through the membrane.
- Facilitated Diffusion:
- Large or polar molecules move across the membrane through protein channels or carriers, still down their concentration gradient (no energy needed).
- Osmosis:
- The diffusion of WATER across a semipermeable membrane from an area of high water concentration to an area of low water concentration.
Active Transport (Requires ATP):
- Movement of molecules against their concentration gradient (from low to high concentration).
- Utilizes protein pumps, such as the sodium-potassium pump which maintains the essential concentrations of sodium and potassium ions in cells.
Bulk Transport:
- Endocytosis:
- The process by which cells take in large molecules by engulfing them.
  - Phagocytosis: Engulfs solid particles.
  - Pinocytosis: Engulfs liquid substances.
- Exocytosis:
- The process by which a cell releases large molecules by fusing vesicles with the plasma membrane.

Hypertonic Solution:
- Higher solute concentration outside the cell compared to inside, resulting in the movement of water out of the cell, leading to cell shrinkage.
- In animal cells, this is referred to as crenation.
- In plant cells, this leads to plasmolysis.
Hypotonic Solution:
- Lower solute concentration outside the cell compared to inside, resulting in water entering the cell, causing the cell to swell.
- In animal cells, this is known as lysis when the cell bursts.
- In plant cells, the turgor pressure increases (positive pressure against the cell wall).
Isotonic Solution:
- The concentration of solute is equal inside and outside the cell, resulting in no net water movement; the cell remains the same size.

Water Potential:
- Concept used to predict the direction of water movement through osmosis.
- Water moves from areas of High Water Potential to areas of Low Water Potential.
- Water Potential Formula:
- $ext{Water Potential} = ext{Pressure Potential} + ext{Solute Potential}$
- Increasing solute concentration results in a lower (more negative) water potential.

Smaller cells have a higher Surface Area to Volume (SA:V) ratio, which allows for more efficient exchange of materials across the cell membrane.
As cells increase in size, their volume grows at a faster rate than their surface area, leading to inefficiency in material exchange.
This growth characteristic underscores why cells remain small and undergo division rather than enlarging significantly.

Compartmentalization in Eukaryotic Cells:
- Eukaryotic cells utilize membrane-bound organelles to isolate and carry out specific functions within the cell.
- This separation creates optimal conditions (e.g., differing pH levels, specific enzymes) for reactions to occur without interference.
- Example: Lysosomes keep their digestive enzymes separate to prevent cellular damage.

Endosymbiotic Theory:
- A hypothesis explaining the evolution of mitochondria and chloroplasts within eukaryotic cells.
- Proposal states that an ancestral eukaryotic cell engulfed prokaryotic cells (specifically, bacteria).
- Origin of Organelles:
- Mitochondria are derived from aerobic bacteria.
- Chloroplasts are derived from photosynthetic bacteria.
- Evidence Supporting this Theory:
- Both mitochondria and chloroplasts have their own circular DNA, which is similar to bacterial DNA.
- They possess double membranes.
- They reproduce independently of the host cell.
- Their size is comparable to that of bacteria and they contain ribosomes that resemble prokaryotic ribosomes.