Cellular Organization: Structure, Organelles, and Membrane Transport

Detailed Notes

3.1 The Cell Theory

The cell theory is one of the unifying principles of biology. It states three things:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells through cell division.

In humans, the body contains approximately 30 trillion cells organized into roughly 200 distinct cell types. Despite their diversity in shape and function, all human cells share a common set of structural features and perform the same basic life processes: obtaining nutrients, generating energy, eliminating wastes, and reproducing.

3.2 The Plasma Membrane

The plasma membrane (also called the cell membrane) forms the outer boundary of every cell. It is far more than a passive wrapper — it is a dynamic, selectively permeable barrier that controls what enters and leaves the cell, anchors the cell to its surroundings, and carries molecular markers that identify the cell to the immune system.

3.2.1 The Phospholipid Bilayer

The fundamental fabric of the membrane is the phospholipid bilayer. Each phospholipid molecule has:

• A hydrophilic (water-loving) head containing a negatively charged phosphate group.
• Two hydrophobic (water-fearing) fatty acid tails, which are long hydrocarbon chains.

Because phospholipids are amphipathic (possessing both hydrophilic and hydrophobic regions), they spontaneously arrange into a bilayer in an aqueous environment: the hydrophilic heads face outward toward the water-based extracellular and intracellular fluids, while the hydrophobic tails cluster together in the interior, shielded from water. This structure creates a barrier that blocks the free passage of water-soluble (polar or charged) molecules, while allowing small, nonpolar molecules (such as oxygen, carbon dioxide, and steroid hormones) to diffuse through the lipid core directly.

3.2.2 The Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which captures two essential features:

• Fluid: The phospholipids and many proteins drift laterally within the leaflet of the bilayer. The membrane has the consistency of olive oil. Cholesterol molecules, wedged between phospholipids, modulate this fluidity — stiffening the membrane at high temperatures and preventing it from becoming too rigid at low temperatures.
• Mosaic: The membrane is studded with a diverse array of proteins, glycoproteins, and glycolipids, creating a patchwork pattern.

3.2.3 Membrane Proteins

Membrane proteins account for roughly half the mass of the plasma membrane and perform most of its specialized functions. There are two broad categories:

Key functional classes of membrane proteins include:

• Channel proteins: Form aqueous pores that allow specific ions or small molecules to pass through (e.g., sodium channels, aquaporins).
• Carrier proteins (transporters): Bind a solute on one side, undergo a conformational change, and release it on the other side (e.g., glucose transporters, GLUTs).
• Receptor proteins: Bind extracellular signaling molecules (ligands) such as hormones or neurotransmitters, triggering intracellular responses.
• Cell recognition proteins (glycoproteins): Carbohydrate chains attached to proteins that serve as identification tags — important in immune recognition (e.g., MHC proteins).
• Enzymatic proteins: Catalyze reactions at the membrane surface (e.g., brush-border enzymes of the small intestine).
• Junction proteins: Form connections between adjacent cells (tight junctions, desmosomes, gap junctions).

3.2.4 The Glycocalyx

The outer surface of the plasma membrane is coated with a fuzzy layer of carbohydrate chains called the glycocalyx. These sugars are attached to membrane proteins (forming glycoproteins) or membrane lipids (forming glycolipids). The glycocalyx plays roles in cell-cell recognition, adhesion, and protection.

3.3 The Cytoplasm and Cytosol

The cytoplasm is everything between the plasma membrane and the nuclear envelope. It consists of:

• Cytosol: The gel-like, water-based intracellular fluid that fills the spaces between organelles. The cytosol contains dissolved ions (especially K+, Na+, Cl-, Ca2+), nutrients, proteins, RNA, and metabolic intermediates. It is the site of many metabolic reactions, including the initial steps of glucose breakdown (glycolysis).
• Organelles: The membrane-bound or non-membrane-bound structures that perform specialized functions.
• Inclusions: Non-living, temporary structures such as glycogen granules, lipid droplets, and pigment granules.

3.4 Cellular Organelles

Organelles are the "little organs" of the cell. Each has a distinct structure that enables a specific set of functions. The following table summarizes the major organelles:

3.4.1 The Nucleus

The nucleus is the largest organelle and the control center of the cell. It is enclosed by the nuclear envelope, a double membrane perforated by nuclear pores that regulate the passage of molecules between the nucleus and cytoplasm. Inside the nucleus:

• Chromatin: DNA complexed with histone proteins. During cell division, chromatin condenses into visible chromosomes.
• Nucleolus: A dense, non-membrane-bound region where ribosomal RNA (rRNA) is synthesized and ribosomal subunits are assembled.

The nucleus houses the cell's genetic blueprint. Transcription — the synthesis of messenger RNA (mRNA) from a DNA template — occurs here.

3.4.2 Ribosomes

Ribosomes are tiny, non-membrane-bound structures composed of ribosomal RNA (rRNA) and proteins. Each ribosome has two subunits (large and small) that clamp around an mRNA molecule during translation — the process of decoding the mRNA sequence into a polypeptide chain. Ribosomes can be:

• Free ribosomes: Floating in the cytosol; they synthesize proteins that function within the cytosol.
• Bound ribosomes: Attached to the rough ER; they synthesize proteins destined for secretion, membrane insertion, or lysosomal delivery.

3.4.3 The Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is a network of interconnected membranous tubules and flattened sacs continuous with the nuclear envelope.

• Rough ER (RER): Studded with ribosomes on its cytoplasmic surface. The RER is the site where newly synthesized proteins enter the ER lumen for folding, quality control, and initial glycosylation (the addition of sugar groups). The RER is especially abundant in cells that secrete large quantities of protein, such as plasma cells (antibodies) and pancreatic acinar cells (digestive enzymes).
• Smooth ER (SER): Lacks ribosomes. Its functions include:
  • Synthesis of phospholipids and steroid hormones (e.g., testosterone, estrogen).
  • Calcium ion storage — the SER in muscle cells (called the sarcoplasmic reticulum) stores and releases Ca2+ to trigger contraction.
  • Detoxification of drugs, alcohol, and metabolic waste products, particularly in liver cells.

3.4.4 The Golgi Apparatus

The Golgi apparatus is a stack of flattened, membrane-enclosed sacs called cisternae. It has a distinct polarity:

• The cis face receives transport vesicles from the rough ER.
• The medial cisternae modify the cargo — trimming sugar groups, adding new ones, or attaching phosphate or sulfate tags that act as address labels.
• The trans face sorts the finished products into vesicles destined for the plasma membrane (secretion), lysosomes, or other organelles.

Think of the Golgi as the cell's post office: it receives, modifies, sorts, and ships proteins and lipids to their final destinations.

3.4.5 Mitochondria

Mitochondria (singular: mitochondrion) are the "powerhouses" of the cell. They are oval-shaped organelles with:

• An outer membrane that is smooth and permeable to small molecules.
• A highly folded inner membrane whose folds, called cristae, dramatically increase the surface area for ATP production.
• The matrix, the innermost compartment, which contains mitochondrial DNA (mtDNA), ribosomes, and enzymes for the citric acid cycle (Krebs cycle) and beta-oxidation of fatty acids.

Mitochondria generate ATP (adenosine triphosphate) — the cell's main energy currency — through aerobic cellular respiration, which couples the citric acid cycle with the electron transport chain and oxidative phosphorylation embedded in the inner membrane. Cells with high energy demands (e.g., muscle cells, neurons, sperm) contain thousands of mitochondria. Mitochondria also play roles in calcium homeostasis and programmed cell death (apoptosis).

Mitochondria are unique in having their own circular DNA and ribosomes, and they replicate independently of the cell by binary fission — evidence supporting the endosymbiotic theory, which proposes that mitochondria evolved from ancient aerobic bacteria engulfed by ancestral eukaryotic cells.

3.4.6 Lysosomes

Lysosomes are membrane-bound sacs filled with powerful hydrolytic enzymes (acid hydrolases) that digest:

• Worn-out organelles (a process called autophagy).
• Material brought into the cell by endocytosis (e.g., bacteria engulfed by white blood cells).
• Extracellular debris in certain specialized cells (e.g., osteoclasts breaking down bone matrix).

Lysosomal enzymes function optimally at an acidic pH (~5.0), which is maintained inside the lysosome by proton pumps in the membrane. This compartmentalization protects the rest of the cell from self-digestion.

A clinically important example is Tay-Sachs disease, a lysosomal storage disorder in which a missing enzyme causes toxic accumulation of glycolipids in neurons, leading to progressive neurological deterioration.

3.4.7 Peroxisomes

Peroxisomes are small, membrane-bound organelles that contain oxidative enzymes (oxidases and catalase). Their functions include:

• Detoxification of harmful substances such as alcohol and formaldehyde (abundant in liver and kidney cells).
• Beta-oxidation of very-long-chain fatty acids.
• Breakdown of hydrogen peroxide (H2O2): Oxidases produce H2O2 as a byproduct, and the enzyme catalase immediately converts it to water and oxygen, preventing oxidative damage.

3.4.8 The Endomembrane System

The endomembrane system is a functionally integrated group of organelles that work together to synthesize, modify, package, and transport proteins and lipids. It includes the nuclear envelope, rough and smooth ER, Golgi apparatus, lysosomes, transport vesicles, and the plasma membrane. The components are connected either by direct physical continuity (e.g., the ER membrane is continuous with the outer nuclear envelope) or by the transfer of membrane segments via vesicles.

3.5 The Cytoskeleton

The cytoskeleton is an internal network of protein filaments that provides structural support, enables cell movement, anchors organelles, and facilitates intracellular transport. It consists of three types of filaments:

Microfilaments and microtubules are dynamic — they can rapidly polymerize and depolymerize — while intermediate filaments are more permanent and provide enduring mechanical reinforcement.

3.6 Membrane Transport

The plasma membrane's phospholipid bilayer is selectively permeable: it allows some substances to cross freely while restricting others. Transport mechanisms are classified by whether they require cellular energy (ATP).

3.6.1 Passive Transport

Passive transport moves substances down their concentration gradient (from high to low concentration) without expending cellular energy. It includes:

Simple Diffusion

In simple diffusion, small, nonpolar molecules (O2, CO2, steroid hormones, fatty acids) and very small uncharged polar molecules (water, urea, though water movement is now understood to be primarily channel-mediated via aquaporins) pass directly through the phospholipid bilayer. The rate of diffusion depends on the concentration gradient, temperature, molecular size, and lipid solubility.

Facilitated Diffusion

Facilitated diffusion moves larger, polar, or charged molecules (such as glucose, amino acids, and ions) down their concentration gradient with the help of membrane proteins — but still without ATP. Two types:

• Channel-mediated: Aqueous protein channels allow specific ions (Na+, K+, Ca2+, Cl-) or water (aquaporins) to pass. Many channels are gated — they open or close in response to signals:
  • Ligand-gated channels: Open when a chemical messenger (e.g., neurotransmitter) binds.
  • Voltage-gated channels: Open in response to changes in membrane potential.
  • Mechanically-gated channels: Open in response to physical deformation (e.g., stretch, pressure).
• Carrier-mediated: A carrier protein binds the solute, undergoes a conformational change, and releases it on the other side. Glucose enters most cells via GLUT carrier proteins. Carrier-mediated transport exhibits saturation kinetics — once all carriers are occupied, the transport rate plateaus.

Osmosis

Osmosis is the net diffusion of water across a selectively permeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). Clinically, it is more practical to describe osmosis in terms of osmotic pressure — the pressure required to stop the net movement of water.

The effect of extracellular solute concentration on cell volume is described by tonicity:

This is why intravenous fluids (e.g., 0.9% saline, 5% dextrose) must be isotonic to prevent damage to red blood cells.

3.6.2 Active Transport

Active transport moves substances against their concentration gradient (from low to high), requiring energy — usually from ATP hydrolysis. It is carried out by pumps.

Primary Active Transport

In primary active transport, ATP is directly hydrolyzed to power the pump. The classic example is the sodium-potassium pump (Na+/K+ ATPase):

• For each ATP consumed, the pump moves 3 Na+ ions out of the cell and 2 K+ ions in, both against their concentration gradients.
• This pump maintains the steep Na+ and K+ gradients that are essential for:
  • Resting membrane potential (inside of cell is negative relative to outside).
  • Secondary active transport (see below).
  • Electrical excitability of neurons and muscle cells.
  • Regulation of cell volume.

Other examples of primary active transport pumps include the calcium pump (Ca2+ ATPase) in the sarcoplasmic reticulum and the proton pump (H+ ATPase) in the stomach and lysosomes.

Secondary Active Transport

Secondary active transport (also called coupled transport) does not directly use ATP. Instead, it harnesses the potential energy stored in an ion gradient (usually Na+) that was established by a primary active transport pump. As Na+ moves back down its gradient through a co-transporter protein, it drags another substance with it:

• Symport (cotransport): The transported solute moves in the same direction as Na+. Example: the sodium-glucose symporter (SGLT) in the small intestine and kidney tubules moves glucose into cells along with Na+.
• Antiport (countertransport): The transported solute moves in the opposite direction to Na+. Example: the sodium-calcium exchanger (NCX) in cardiac muscle cells, which pumps Ca2+ out of the cell as Na+ enters.

The table below compares passive and active transport:

3.6.3 Vesicular (Bulk) Transport

Large particles, macromolecules, and fluids are transported across the plasma membrane via vesicles — membrane-bound sacs that bud off from or fuse with the membrane. This process requires ATP.

Endocytosis

In endocytosis, the plasma membrane invaginates and pinches off to form a vesicle that brings material into the cell. Three types:

• Phagocytosis ("cell eating"): The cell extends pseudopods to engulf large particles such as bacteria, cell debris, or foreign material. This occurs primarily in specialized cells like macrophages and neutrophils. The resulting vesicle, called a phagosome, fuses with a lysosome for digestion.
• Pinocytosis ("cell drinking"): The cell takes in droplets of extracellular fluid containing dissolved solutes. This is a non-specific process carried out by most cells.
• Receptor-mediated endocytosis: Highly specific. Ligands (e.g., low-density lipoproteins, transferrin, certain hormones) bind to receptor proteins clustered in coated pits on the cell surface. The pit invaginates and pinches off to form a coated vesicle. This mechanism allows cells to selectively concentrate and internalize specific molecules even when they are present at low concentrations in the extracellular fluid. A classic clinical example involves LDL cholesterol: defective LDL receptors prevent cholesterol uptake, leading to familial hypercholesterolemia and premature atherosclerosis.

Exocytosis

In exocytosis, a vesicle inside the cell fuses with the plasma membrane, releasing its contents outside the cell. This is the mechanism by which cells secrete:

• Proteins (e.g., hormones, enzymes, antibodies).
• Neurotransmitters from neurons.
• Waste products.

In constitutive secretion, substances are continuously released. In regulated secretion, substances are stored in secretory vesicles and released only in response to a specific signal (e.g., insulin release triggered by rising blood glucose).

Transcytosis

A combination of endocytosis and exocytosis: material is taken in on one side of a cell, transported across the cytoplasm in vesicles, and released on the opposite side. This is important in the endothelial cells lining blood vessels.