Cell Structure, Function, and Membrane Transport: Study Notes for General Biology

Cell Types and General Characteristics

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, capable of performing all essential activities. There are two main types of cells: prokaryotic and eukaryotic. Understanding their differences is crucial for studying cell biology.

• Prokaryotic Cells: Found in Bacteria and Archaea. Typically 1-10 μm in diameter. DNA is circular and located in the cytoplasm. Lack membrane-bound organelles.

• Eukaryotic Cells: Found in Plants, Animals, Fungi, and Protists. Typically 10-100 μm in diameter. DNA is coiled in linear strands within a nucleus. Contain membrane-bound organelles.

All living things are composed of cells, and cells arise from pre-existing cells. The cell is the smallest unit capable of performing life activities.

Limitations to Cell Size

Cells are generally small due to the surface-to-volume ratio. As cells grow larger, their volume increases faster than their surface area, limiting the efficiency of material exchange.

• Smaller cells: More efficient exchange of materials.

• Larger organisms: Composed of more cells, not larger cells.

Formula:

$\text{Surface-to-Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Structure Reflects Function

The anatomy and physiology of a cell are closely related to its function. Specialized cells have unique structures to perform specific roles.

• Red blood cells: Few organelles, contain proteins for oxygen transport.

• Sperm cells: Tail for mobility, packed with nutrients for fertilization.

• Egg cells: Large, immobile, nutrient-rich.

Cellular Components and Organelles

Plasma Membrane

The plasma membrane separates the cell's interior from its external environment, providing structural support and regulating the movement of substances.

• Phospholipid bilayer: Fluid mosaic model; proteins and carbohydrates are embedded or attached.

• Functions: Recognition, communication, tissue formation, selective permeability.

Cytoplasm

The cytoplasm is the thick, transparent fluid between the plasma membrane and the nucleus, containing organelles with specialized functions.

Nucleus

The nucleus is the control center of the cell, containing genetic information (DNA) organized into chromosomes.

• Chromatin: Extended form of chromosomes during interphase.

• Chromosomes: Condensed during cell division; humans have 46 chromosomes (23 pairs).

• Nuclear envelope: Double membrane with pores for communication with cytoplasm.

Gene: A section of DNA that codes for a unique protein, determining phenotype (physical characteristics).

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; produces proteins and membranes, distributes via transport vesicles.

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, stores calcium, detoxifies poisons.

Golgi Complex

The Golgi complex consists of flattened membrane sacs that process, sort, and modify proteins for export or use within the cell.

• Receives materials from ER, packages them into transport vesicles.

Lysosome

Lysosomes are membrane-bound organelles responsible for cellular digestion and recycling.

• Contain over 40 enzymes to break down proteins, DNA, RNA, and lipids.

• Acidic interior (pH ~5).

Vesicles & Vacuoles

Vesicles and vacuoles are membrane-enclosed sacs for storage and transport. Vacuoles are typically larger and found in plant cells.

• Produced by ER and Golgi apparatus.

Mitochondria

Mitochondria are the "powerhouses" of the cell, responsible for cellular respiration and energy (ATP) production.

• Contain inner foldings (cristae) to increase surface area for respiration.

• Site of glucose breakdown and ATP synthesis.

Membrane Transport Mechanisms

Overview of Transport Across the Plasma Membrane

The plasma membrane controls the movement of substances into and out of the cell, maintaining homeostasis. Transport mechanisms are classified as passive or active.

Mechanism	Description
Simple diffusion	Random movement from region of higher concentration to lower concentration.
Facilitated diffusion	Movement from higher to lower concentration with the aid of a carrier or channel protein.
Osmosis	Movement of water from region of higher water concentration (lower solute) to lower water concentration (higher solute).
Active transport	Movement from lower to higher concentration with the aid of a carrier protein and energy (usually from ATP).
Endocytosis	Process by which materials are engulfed by plasma membrane and drawn into the cell in a vesicle.
Exocytosis	Process by which a membrane-bound vesicle fuses with the plasma membrane and releases contents outside the cell.

Passive Transport

• Simple diffusion: No energy required; movement of small, nonpolar molecules (e.g., O2, CO2).

• Facilitated diffusion: No energy required; movement of larger or polar molecules via proteins.

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

Active Transport

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

• Endocytosis: Engulfing of large molecules, bacteria, or liquids by the cell membrane to form vesicles.

• Exocytosis: Release of large molecules (e.g., neurotransmitters, hormones) from the cell via vesicles.

Types of Endocytosis

• Phagocytosis (cell eating): Engulfment of large particles or bacteria; forms a "food vacuole".

• Pinocytosis (cell drinking): Absorption of extracellular fluids and dissolved solutes.

Summary of Membranous Organelles

• Cell membrane

• Nucleus

• Endoplasmic reticulum (ER)

• Golgi complex

• Vesicles

• Lysosomes

• Mitochondria

Each organelle plays a specific role in the cell's structure, function, and interactions.

Additional info:

• Some content inferred from context and standard biology curriculum (e.g., details on organelle functions, examples of membrane transport).

• Scientific names and terms italicized where appropriate.