Cell Structure and Function

• The cell is the fundamental unit of life.

Cell Features

• All cells have a plasma membrane, cytosol, chromosomes (genes), and ribosomes.

Cell Types

• There are two types of cells: prokaryotic and eukaryotic.
• Prokaryotic cells lack a nucleus, have DNA in a nucleoid, lack membrane-bound organelles, and have cytoplasm bound by the plasma membrane.
• Eukaryotic cells have DNA in a nucleus with a membranous envelope, possess membrane-bound organelles, and have cytoplasm between the plasma membrane and nucleus.

Cell Size

• Cells are small to maintain a high surface area-to-volume ratio, facilitating nutrient absorption and waste removal.

Cell Compartmentalization

• Specific functions occur in specific compartments within the cell.

Cell Membranes

• The plasma membrane is a selective barrier controlling the entry of key elements and nutrients and the exit of waste.
• It includes a phospholipid bilayer composed of phospholipids, proteins, and cholesterol.

Cyloplasm

• Cytoplasm is the region between the membrane and nucleus, containing cytosol, organelles, and undergoing constant flow.
• Cytosol is the fluid portion of the cytoplasm.

Nucleus

• The nucleus houses DNA, organized into chromosomes, and is surrounded by a nuclear envelope (a double membrane).
• Nuclear pores regulate molecule entry and exit.
• The nucleolus synthesizes ribosomal RNA (rRNA) and produces ribosome subunits.
• Ribosomes, made of rRNA and proteins, carry out protein synthesis in the cytosol (free ribosomes) or on the rough ER (for export or membrane insertion).

Endomembrane System

• The endomembrane system includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

Endoplasmic Reticulum (ER)

• The ER is continuous with the nuclear envelope and has two regions: smooth ER and rough ER.
• Smooth ER lacks ribosomes and functions in lipid and steroid synthesis and calcium storage.
• Rough ER has ribosomes, produces glycoproteins, distributes products via transport vesicles, and makes membrane and lysosome components.

Golgi Apparatus

• The Golgi apparatus consists of flattened membranes called cisternae.
• It modifies ER products, makes polysaccharides, and sorts materials into transport vesicles.

Lysosomes

• Lysosomes (ONLY IN ANIMALS) are digestive compartments containing enzymes that break down macromolecules through phagocytosis (engulfing cells) and autophagy (digesting cell's own organelles).

Vacuoles

• Vacuoles are membrane sacs for storage.
• Food vacuoles are formed during phagocytosis.
• Contractile vacuoles, found in protists, pump out excess water.
• Central vacuoles (PLANTS ONLY) hold organic compounds, water, and waste.

Mitochondria

• Mitochondria are the sites of cellular respiration (ATP production) and are present in both animals and plants.
• They have a smooth outer membrane and a folded inner membrane (cristae) for ATP synthesis.

Chloroplast

• Chloroplasts (ONLY PLANTS) are the sites of photosynthesis, containing chlorophyll.
• They are plastids with three membrane layers: thylakoids (forming granum), inner and outer membranes, and stroma (fluid between inner membrane and thylakoid).

Cytoskeleton

• The cytoskeleton is a network of protein fibers that organizes cell structures and activities.
• It supports the cell, maintains shape, holds organelles in place, and enables cell movement.

Extracellular Components

• Animal cells lack a cell wall but are covered by an extracellular matrix (ECM) (ONLY ANIMALS) made of collagen.
• Integrins are receptor proteins in the plasma membrane that ECM proteins bind to, providing support, movement, and regulation.

Animal Cell Junctions

• Neighboring cells adhere and communicate through intercellular junctions.
• Tight junctions seal cells together, preventing extracellular fluid leakage with tightly packed cells.
• Desmosomes (anchoring junctions) fasten cells together.
• Gap junctions (communicating junctions) provide communication channels.

Plant Cell Walls

• Plant cells have cell walls, an extracellular structure providing support and structure.
• Plasmodesmata are channels through the cell wall, allowing passage of proteins, RNA, and water between cells.