Cell Structure and Function

Fundamental Units of Life

• Prokaryotic cells (Bacteria and Archaea) lack membrane-enclosed internal compartments.
• Eukaryotic cells (Eukarya) possess membrane-enclosed organelles where different functions occur.

Prokaryotic Cell Features

• Defined by being enclosed by a cell membrane
• DNA is located in a region named the nucleoid
• The cytoplasm is the rest of the cell contents
• Ribosomes are the site of protein synthesis
• Prokaryotes use flagella composed of flagellin to swim
• Pili, hairlike structures, project from the cell surface to help bacteria adhere to other cells.
• Fimbriae are shorter than pili and facilitate adhesion to surfaces such as animal cells.

Ribosomes

• Site of protein synthesis
• Exhibit a similar structure across both prokaryotes and eukaryotes
• Made up of two subunits: ribosomal RNA (rRNA) and over 50 different protein molecules

Nucleus

• The largest organelle, typically
• Contains the majority of the cell's DNA
• Functions as the DNA replication site
• Region where gene transcription is regulated
• Ribosome assembly starts in the nucleolus
• Surrounded by a double-membrane nuclear envelope
• Nuclear pores in the envelope regulate the movement of molecules
• The outer membrane connects to the endoplasmic reticulum

Endomembrane System

• An interconnected network of membrane-enclosed compartments
• Vesicles transport substances between components
• Membranes and contents are constantly in motion

Endoplasmic Reticulum Function

• The endoplasmic reticulum (ER) is a network of membranes within the cytoplasm that provides a large surface area.
• The rough endoplasmic reticulum (RER) has ribosomes attached is the site of protein synthesis
• Newly synthesized proteins enter the RER lumen where folding, modification, and transportation to other regions occur, using vesicles that pinch off from the RER.

Cellular Energy

• Mitochondria transform energy in fuel molecules like glucose into ATP through cellular respiration
• High-energy-demanding cells contain lots of mitochondria that reproduce and divide independently of the nucleus.

Cytoskeleton

• Supports & maintains cell shape
• Maintains organelle positioning
• Facilitates organelle movement
• Is involved in cytoplasmic streaming
• Interacts with extracellular structures to hold cell in place
• Acts as a composite of 3 filament types

Fluid Mosaic Model

• Describes the general structure of biological membranes
• Phospholipids form a bilayer where proteins "float"

Biological Membranes

• Contain lipids and proteins
• Phospholipids feature polar, hydrophilic heads facing outward and hydrophobic fatty acid tails facing inward
• Phospholipids exhibit variability in chain length, degree of unsaturation, and phosphate groups
• Cholesterol maintains membrane integrity and regulates fluidity
• Membrane protein counts depend on membrane function
• Integral, peripheral, and anchored

Cell Junctions

• Specialize to hold cells together
• Tight junctions ensure directional material movement
• Desmosomes function as "spot welds."
• Gap junctions facilitate communication

Osmosis

• Water diffusion depending on water molecule concentrations
• Isotonic: equal solute concentrations
• Hypertonic: higher solute concentration
• Hypotonic: lower solute concentration
• Water diffuses from high (hypotonic) to low (hypertonic) concentration if separated by a water-permeable membrane.
• Animal cells may burst in hypotonic solutions or shrink in hypertonic solutions
• Plant cells build internal turgor pressure thanks to cell walls
• Turgor pressure limits water entry

Membrane Transport

• Polar/charged molecules use facilitated diffusion down concentration gradients via protein channels/carriers.
• Channel proteins: Form tunnels
• Carrier proteins: Bind substances to speed diffusion through the bilayer.
• Active transport moves items against concentration/electrical gradients & needs energy, often ATP.
• Active transport moves in one direction and utilizes:
  • Uniporters to shift one substance in one direction
  • Symporters to move two substances together in the same direction
  • Antiporters move two substances oppositely
• Primary active transport needs ATP hydrolysis directly
• Secondary Active Transport gets power from ion gradients, made by Primary Transport.

Phagocytosis

• Involves engulfing molecules or entire cells
• Protists feed this way
• White blood cells engulf foreign substances this way
• A phagosome forms and fuses with a lysosome where the material is digested

Chemical Signaling Systems

• All cells process environmental info through chemical/physical stimulus (e.g., light)
• Signals sourced internally/externally
• Signal transduction pathway - A series of events leading to a cellular response
• Chemicals bind receptors

Cell Communication

• Juxtacrine signals: Affect adjacent cells dependent on contact
• Paracrine signals: Affect nearby cells through local regulators
• Endocrine signals: Travel through distance via hormones
• Agonists: Chemicals that have the same effect as ligands
• Antagonists: Chemicals (inhibitors) binding and blocking natural ligands from binding/activating signal transduction

Overview of cell structures, including prokaryotic and eukaryotic cells. Covers cell membranes, DNA location, cytoplasm, ribosomes, flagella, and pili. Focuses on the structure and function of ribosomes and the nucleus.