General Biology Study Notes: Cell Structure, Membrane Transport, and Osmoregulation

Topic 1: Cell Structure, Subcellular Components

Overview of Cell Structure

Cells are the fundamental units of life, and their internal organization is essential for biological function. Eukaryotic cells contain various subcellular components, each with specialized roles.

• Subcellular components include organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and ribosomes.

• Ribosomes are responsible for protein synthesis. They can be free in the cytosol or bound to the rough endoplasmic reticulum (ER).

• Endoplasmic Reticulum (ER) comes in two forms: rough (with ribosomes, involved in protein synthesis) and smooth (involved in lipid synthesis and detoxification).

• Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria are the site of cellular respiration and ATP production. Their structure includes an outer membrane, inner membrane, and matrix.

• Lysosomes contain digestive enzymes for breaking down macromolecules.

• Chloroplasts (in plants) are the site of photosynthesis.

• Compartmentalization in eukaryotic cells allows for specialized environments and functions within the cell.

Example: The mitochondrion's double membrane and internal matrix allow for efficient ATP synthesis via the electron transport chain.

Additional info: Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells possess them, allowing for greater complexity and specialization.

Topic 2: Cell Size

Limits and Implications of Cell Size

Cell size is constrained by the need to efficiently exchange materials with the environment. Surface area-to-volume ratio is a key factor in determining cell size and function.

• As cells increase in size, their surface area-to-volume ratio decreases, limiting the rate of material exchange.

• Cells may adopt specialized shapes (e.g., elongated, flattened) to maximize surface area for exchange.

• Small cell size allows for efficient diffusion of nutrients and waste products.

Example: Microvilli in intestinal cells increase surface area for nutrient absorption.

Additional info: The formula for surface area-to-volume ratio for a sphere is $\frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$.

Topic 3: Membrane Transport

Mechanisms of Transport Across Cell Membranes

Cell membranes regulate the movement of substances into and out of cells, maintaining internal environments distinct from the external surroundings.

• Passive transport includes diffusion and osmosis, where substances move down their concentration gradients without energy input.

• Facilitated diffusion uses membrane proteins to help substances cross the membrane.

• Active transport requires energy (often ATP) to move substances against their concentration gradients.

• Endocytosis and exocytosis are processes for bulk transport of materials into and out of the cell.

Example: The Na+/K+ ATPase pump uses ATP to transport sodium and potassium ions across the plasma membrane.

Additional info: The fluid mosaic model describes the membrane as a dynamic structure with proteins embedded in a phospholipid bilayer.

Table: Comparison of Membrane Transport Mechanisms

Transport Type	Energy Required?	Direction Relative to Gradient	Example
Simple Diffusion	No	Down gradient	O2 crossing membrane
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Endocytosis/Exocytosis	Yes	Bulk transport	Phagocytosis, neurotransmitter release

Topic 4: Osmoregulation

Maintaining Water and Solute Balance

Osmoregulation is the process by which organisms control the balance of water and solutes in their bodies to maintain homeostasis.

• Organisms in different environments (freshwater, marine, terrestrial) face unique osmoregulatory challenges.

• Specialized structures (e.g., contractile vacuoles in protists, kidneys in vertebrates) help regulate water and solute levels.

• Communication mechanisms, such as hormones, coordinate osmoregulatory responses.

Example: The antidiuretic hormone (ADH) regulates water reabsorption in the kidneys of mammals.

Additional info: Osmosis is the diffusion of water across a selectively permeable membrane from low solute concentration to high solute concentration.

Topic 5: Neurons as an Example of Membrane Function

Structure and Function of Neurons

Neurons are specialized cells that transmit electrical and chemical signals in the nervous system. Their function relies on membrane properties and transport mechanisms.

• Neuronal membranes contain ion channels and pumps that establish and maintain membrane potential.

• Action potentials are generated by the rapid movement of ions across the membrane.

• Neurotransmitter release involves exocytosis at synaptic terminals.

Example: The sodium-potassium pump maintains the resting membrane potential in neurons.

Additional info: The equation for membrane potential (Nernst equation) is $E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$.