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, containing various subcellular components that perform specialized functions. Understanding the structure and function of these components is essential for studying cellular processes.

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

• Ribosomes are responsible for protein synthesis. There are free ribosomes (in cytosol) and bound ribosomes (attached to the endoplasmic reticulum).

• Endoplasmic Reticulum (ER) is divided into rough ER (with ribosomes, involved in protein synthesis) and smooth ER (involved in lipid synthesis and detoxification).

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

• Mitochondria are the site of cellular respiration and ATP production. They have an outer and inner membrane, with the inner membrane containing enzymes for the electron transport chain.

• Lysosomes contain hydrolytic enzymes for intracellular digestion.

• Chloroplasts (in plant cells) are the site of photosynthesis, containing thylakoids and stroma.

• Vesicles transport materials within the cell.

Compartmentalization in eukaryotic cells allows for specialized functions and increased efficiency.

• Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells possess them.

• Compartmentalization enables separation of incompatible chemical reactions.

Example: The mitochondrion's double membrane creates distinct environments for the electron transport chain and ATP synthesis.

Additional info:

• Light-dependent and light-independent reactions in chloroplasts occur in different compartments.

Topic 2: Cell Size

Surface Area to Volume Ratio

The size and shape of cells are limited by the surface area to volume ratio, which affects the efficiency of material exchange.

• As cells increase in size, their volume grows faster than their surface area.

• High surface area to volume ratio allows for efficient exchange of materials with the environment.

• Cells may be elongated, flattened, or have projections to increase surface area.

Formula:

• For a sphere: $\text{Surface Area} = 4\pi r^2$

• For a sphere: $\text{Volume} = \frac{4}{3}\pi r^3$

• Surface area to volume ratio: $\frac{\text{Surface Area}}{\text{Volume}}$

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

Additional info:

• Specialized cell shapes (e.g., red blood cells, neurons) optimize function by maximizing surface area.

Topic 3: Cell Membranes

Structure and Function of Cell Membranes

Cell membranes are composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. They regulate the movement of substances in and out of cells and organelles.

• Phospholipid bilayer forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins serve as channels, carriers, receptors, and enzymes.

• Cholesterol modulates membrane fluidity.

• Carbohydrates are involved in cell recognition and signaling.

• Fluid mosaic model describes the dynamic nature of the membrane.

Selective permeability allows only certain molecules to cross the membrane.

• Small, nonpolar molecules pass easily; large or charged molecules require transport proteins.

Example: Aquaporins facilitate water transport across cell membranes.

Additional info:

• Membrane proteins include integral and peripheral types, each with specific functions.

Topic 4: Membrane Transport

Mechanisms of Transport Across Membranes

Cells use various mechanisms to move substances across membranes, maintaining internal environments distinct from the external surroundings.

• Passive transport includes diffusion and osmosis, requiring no energy input.

• Facilitated diffusion uses transport proteins to move substances down their concentration gradient.

• Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Endocytosis and exocytosis are bulk transport mechanisms for large molecules.

Example: The Na+/K+ ATPase pump is an example of active transport.

Table: Comparison of Membrane Transport Mechanisms

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

Additional info:

• Osmosis is the diffusion of water across a selectively permeable membrane.

Topic 5: Osmoregulation

Maintaining Water and Solute Balance

Osmoregulation is the process by which organisms regulate water and solute concentrations to maintain homeostasis.

• Organisms in different environments face challenges in maintaining water balance.

• Specialized structures (e.g., contractile vacuoles in protists, kidneys in vertebrates) help regulate osmotic pressure.

• Communication mechanisms coordinate osmoregulatory responses.

Example: Marine fish excrete excess salt through gills, while freshwater fish actively uptake ions.

Additional info:

• Osmoregulation is essential for cell function and overall organismal health.

Topic 6: Neurons as an Example of Membrane Function

Structure and Function of Neurons

Neurons are specialized cells that transmit electrical and chemical signals, relying on membrane properties for function.

• Neuronal membranes contain ion channels and pumps that generate action potentials.

• Membrane potential is maintained by the differential distribution of ions across the membrane.

• Action potentials are rapid changes in membrane potential that propagate signals along the neuron.

Example: The Na+/K+ pump restores resting potential after an action potential.

Additional info:

• Neurons communicate via synapses, where neurotransmitters cross the synaptic cleft.