Cytoplasm

Prokaryotic vs Eukaryotic cells

Prokaryotic cells lack a true nucleus and membrane-bound organelles; eukaryotic cells contain a nucleus and membrane-bound organelles.

Basic structure of a eukaryotic cell

Composed of plasma membrane, cytoplasm (organelles + cytosol), and nucleus.

Cytoplasm

Includes cytosol (fluid matrix) and organelles; supports biochemical reactions and intracellular transport.

Plasma membrane structure

Phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates; described by the Fluid Mosaic Model.

Plasma membrane composition

~50% lipid, 50% protein by mass; lipids include phospholipids, cholesterol, and glycolipids.

Phospholipid bilayer

Amphipathic molecules with hydrophilic heads and hydrophobic tails; forms a selective barrier controlling movement in/out of the cell.

Fluid Mosaic Model

Membrane components (lipids and proteins) move laterally; membrane is flexible and dynamic.

Functions of the plasma membrane

Selective permeability, intercellular communication, cell recognition, adhesion, and maintaining electrochemical gradients.

Integral membrane proteins

Span the lipid bilayer; function as channels, transporters, or receptors.

Peripheral membrane proteins

Bound to membrane surfaces; function in signaling, cytoskeletal attachment, or enzymatic activity.

Cholesterol in the membrane

Modulates membrane fluidity and stability.

Carbohydrates in the membrane

Present as glycolipids or glycoproteins; function in cell recognition and adhesion.

Passive transport

Movement of molecules down their concentration gradient without energy (e.g., diffusion, facilitated diffusion, osmosis).

Active transport

Movement of molecules against a concentration gradient using ATP and transport proteins (e.g., Na⁺/K⁺ pump).

Endocytosis

Process by which cells internalize materials by forming vesicles from the plasma membrane; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Phagocytosis

‘Cell eating’; uptake of large particles or microorganisms into phagosomes; performed by macrophages and neutrophils.

Pinocytosis

‘Cell drinking’; non-specific uptake of extracellular fluid and small molecules.

Receptor-mediated endocytosis

Specific uptake of molecules via ligand binding to membrane receptors (e.g., LDL, transferrin).

Exocytosis

Process where vesicles fuse with the plasma membrane to secrete materials outside the cell (e.g., neurotransmitter release).

Signal transduction

Process by which a cell converts an external signal into a functional response via receptors and intracellular pathways.

Types of signaling

Autocrine (self), paracrine (nearby), endocrine (distant via bloodstream), synaptic (neurotransmitters).

Hydrophilic signaling molecules

Cannot cross the membrane; bind to surface receptors triggering second messenger cascades (e.g., peptide hormones).

Hydrophobic signaling molecules

Diffuse through the membrane and bind to intracellular receptors (e.g., steroid hormones).

Second messengers

Intracellular molecules like cAMP, Ca²⁺, and IP₃ that amplify the signal within the cell.

Mitochondria structure

Double-membrane organelle with inner membrane folds (cristae) enclosing the matrix; contains its own DNA and ribosomes.

Mitochondria function

Generates ATP through oxidative phosphorylation; involved in apoptosis and calcium storage.

Rough Endoplasmic Reticulum (RER)

Network of membranes with ribosomes; site of protein synthesis and initial post-translational modification.

Smooth Endoplasmic Reticulum (SER)

Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Golgi apparatus

Stack of flattened cisternae; modifies, sorts, and packages proteins and lipids for secretion or delivery to organelles.

Secretory vesicles

Membrane-bound vesicles that store and release materials (e.g., hormones, enzymes) via exocytosis.

Lysosomes

Membrane-bound organelles containing hydrolytic enzymes for intracellular digestion of macromolecules and worn-out organelles.

Proteasomes

Cylindrical protein complexes that degrade ubiquitin-tagged proteins in an ATP-dependent process.

Peroxisomes

Small organelles containing oxidases and catalase; detoxify hydrogen peroxide and break down fatty acids.

Cytoplasmic inclusions

Non-membrane-bound storage materials such as glycogen granules, lipid droplets, pigments (melanin, lipofuscin), and crystals.

Glycogen granules

Energy storage form of glucose; appear as dense granules in hepatocytes and muscle cells.

Lipid droplets

Energy reserves found in adipocytes and steroid-secreting cells; composed of neutral lipids.

Pigments in cytoplasm

Include melanin (skin, retina) and lipofuscin (wear-and-tear pigment accumulating with age).