Cell Structure and Function

Cell Organelles and Their Functions

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding these organelles is fundamental to cell biology.

• Nucleus: Contains the cell's genetic material (DNA) and controls gene expression and cell division.

• Ribosomes: Sites of protein synthesis; can be free in the cytoplasm or bound to the endoplasmic reticulum.

• Mitochondria: The "powerhouse" of the cell; site of aerobic respiration and ATP production.

• Chloroplasts: Found in plant cells; site of photosynthesis, converting light energy into chemical energy.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; involved in protein synthesis and modification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Example: The mitochondria generate ATP through cellular respiration, providing energy for cellular processes.

Cell Membranes

Membrane Composition

The plasma membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Form the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve various functions (transport, signaling, structure).

• Cholesterol: Maintains membrane fluidity and stability.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface; involved in cell recognition.

Membrane Permeability

The plasma membrane's permeability depends on the size, polarity, and charge of molecules:

• Permeable: Small, nonpolar molecules (e.g., O2, CO2)

• Slightly Permeable: Small, uncharged polar molecules (e.g., H2O)

• Impermeable: Ions and large polar molecules (e.g., glucose, Na+)

Types and Functions of Membrane Proteins

Membrane proteins are essential for various cellular processes:

• Transport Proteins: Facilitate movement of substances across the membrane (channels, carriers).

• Receptor Proteins: Receive and transmit signals from the environment.

• Enzymatic Proteins: Catalyze chemical reactions at the membrane surface.

• Anchor Proteins: Attach the membrane to the cytoskeleton and extracellular matrix.

• Cell Recognition Proteins: Allow cells to identify each other (often glycoproteins).

Membrane Transporters and Types of Transport

Substances cross the membrane via different mechanisms:

• Passive Transport: Does not require energy; moves substances down their concentration gradient.

  • Simple Diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer.

  • Facilitated Diffusion: Movement of larger or polar molecules via transport proteins (channels or carriers).

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient.

  • Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from the movement of another substance down its gradient.

Example: The Na+/K+ pump moves sodium out of and potassium into the cell using ATP.

Hypertonic, Hypotonic, and Isotonic Solutions

These terms describe the relative concentration of solutes in solutions separated by a membrane:

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, causing the cell to shrink.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, causing the cell to swell or burst.

• Isotonic Solution: Equal solute concentration; no net movement of water.

Example: Red blood cells placed in a hypotonic solution will swell and may burst (hemolysis).

Cell Cycle and Division

Interphase Stages

Interphase is the period of the cell cycle between divisions, consisting of three main stages:

• G1 Phase (First Gap): Cell grows and carries out normal functions.

• S Phase (Synthesis): DNA is replicated.

• G2 Phase (Second Gap): Cell prepares for division; organelles duplicate, and error checking occurs.

Mitosis Stages

Mitosis is the process by which a cell divides its nucleus and contents. It consists of several stages:

• Prophase: Chromatin condenses into visible chromosomes; spindle fibers form; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform around the two sets of chromosomes; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Example: Skin cells divide by mitosis to replace damaged tissue.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell cycle progresses correctly:

• G1 Checkpoint: Checks for cell size, nutrients, growth factors, and DNA damage.

• G2 Checkpoint: Ensures DNA replication is complete and checks for DNA damage.

• M Checkpoint (Spindle Checkpoint): Ensures all chromosomes are attached to the spindle before anaphase begins.

Example: If DNA damage is detected at the G1 checkpoint, the cell may enter a resting state (G0) or initiate repair mechanisms.

Additional info: Academic context and examples have been added to expand upon the brief points in the original study guide, ensuring the notes are comprehensive and self-contained for exam preparation.