Cell Structure and Function

Functions of cells:

• Provide structure to living organisms
• Carry out metabolism
• Produce energy
• Synthesize proteins
• Respond to signals
• Divide and reproduce

Prokaryotic Cells Prokaryotic cells are simple cells that lack a nucleus and membrane-bound organelles. Their DNA is located within a region called the nucleoid.

• No nucleus
• No membrane-bound organelles
• Smaller and simpler
• Usually unicellular
• DNA is circular
• Divide by binary fission

Examples:

• Bacteria
• Archaea

Eukaryotic Cells Eukaryotic cells are more complex cells with a nucleus and membrane-bound organelles. The nucleus contains most of the cell's DNA, while organelles perform specialized cellular functions.

• Membrane-bound nucleus
• Membrane-bound organelles
• Larger and more complex
• Can be unicellular or multicellular
• DNA is organized into linear chromosomes
• Divide by mitosis or meiosis

Examples:

• Animal cells
• Plant cells
• Fungal cells
• Protists

Plasma Membrane (Cell Membrane) The cell membrane is a fine, flexible barrier that surrounds the cell. It is composed of a phospholipid bilayer. Each phospholipid contains:

• A hydrophilic (water-loving) head that faces the watery environments inside and outside the cell
• A hydrophobic (water-fearing) tail that points inward, away from water

Functions of the plasma membrane:

• Protects the cell
• Controls movement of substances into and out of the cell
• Maintains homeostasis
• Enables cell communication
• Anchors membrane proteins

• Unsaturated fatty acids increase fluidity as their double bonds create bends in the membrane that prevent phospholipids from packing tightly together.
• Saturated fatty acids decrease fluidity because their straight tail structure allows the phospholipids to pack tightly together, thereby increasing the membrane's rigidity.
• Higher temperatures increase fluidity, while lower temperatures reduce it.
• Cholesterol serves as a fluidity buffer by preventing the membrane from becoming too fluid at higher temperatures and from being too rigid at lower temperatures.

Membrane Transport Substances cross the plasma membrane by several mechanisms:

• Simple (passive) diffusion — small, nonpolar, lipid-soluble molecules move directly through the phospholipid bilayer down their concentration gradient. This is how O₂, CO₂, and other small lipid-soluble molecules cross membranes; no energy or transporter required.
• Facilitated diffusion — polar or charged molecules move down their gradient through channel or carrier proteins (e.g., glucose via GLUT transporters).
• Active transport — moves substances against their electrochemical gradient and requires energy. Primary active transport directly uses ATP (e.g., Na⁺/K⁺-ATPase), while secondary active transport uses energy stored in another ion gradient.
• Endocytosis and exocytosis — bulk transport via vesicles.

Cytoplasm The cytoplasm is everything inside the cell membrane but outside the nucleus. It suspends organelles, allows movement of materials within the cell, and consists of:

• Cytosol — the fluid portion of the cytoplasm
• Organelles
• Cytoskeleton
• Other cellular components

Nucleus The nucleus is the control center of the cell and stores most of the cell's DNA.

• Stores DNA
• Controls gene expression
• Directs protein synthesis
• Regulates cell growth and division

Types:

• Free ribosomes — float in the cytoplasm and produce proteins used inside the cell
• Bound ribosomes — attached to the rough ER and produce proteins for secretion, membranes, or lysosomes

Functions:

• Protein synthesis
• Protein folding
• Initial protein modification
• Protein transport

Functions:

• Lipid synthesis
• Steroid hormone synthesis
• Detoxification of foreign substances
• Calcium storage

Functions:

• Modifies proteins
• Forms glycoproteins
• Sorts proteins
• Packages proteins into vesicles
• Directs proteins to their final destination

Functions:

• ATP production
• Cellular respiration
• Energy metabolism
• Regulation of apoptosis

Functions:

• Digest damaged organelles
• Destroy pathogens
• Break down cellular waste
• Recycle cellular components

Protein Synthesis Pathway The pathway from gene to functional protein follows this route:

• DNA (nucleus) → transcription → messenger RNA (mRNA)
• mRNA exits the nucleus → ribosome → translation → protein formed
• Free ribosomes → proteins used primarily in the cytosol or targeted to certain intracellular locations
• Rough ER-bound ribosomes → proteins destined for secretion, membranes, or the endomembrane system
• Final destination: cell membrane, another organelle, or secretion outside the cell

Tonicity and Osmosis Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to higher solute concentration. The tonicity of the surrounding solution determines how a cell behaves:

• Isotonic — approximately equal effective concentrations of nonpenetrating solutes inside and outside the cell; no net water movement occurs, so cell volume remains stable.
• Hypotonic — lower effective concentration of nonpenetrating solutes outside the cell; water moves into the cell.
• Hypertonic — higher effective concentration of nonpenetrating solutes outside the cell; water moves out.