Cell Structure and Membranes

• All cells share common features, but specialized cells possess unique characteristics.
• Cells are classified as either prokaryotic or eukaryotic.
• Eukaryotic cells can be further distinguished as plant or animal cells based on specific features.
• It is important to be able to identify cell types and the function of their structures and organelles based on diagrams or pictures.
• Plant cells as seen under a microscope have a specific appearance.
• Biological rules have exceptions, such as atypical animal cells.

Cell Membrane

• The cell membrane is composed of a phospholipid bilayer.
• Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails.
• Phospholipids' amphipathic nature causes them to spontaneously form a bilayer in water.
• The hydrophobic tails face inward, and the hydrophilic heads face the watery inside and outside of the cell.
• The cell membrane separates the cell's interior from the external environment.
• It regulates the transport of materials into and out of the cell.
• The bilayer makes the membrane semi-permeable, allowing certain molecules to pass through.

Membrane Permeability

• The membrane is permeable to small and hydrophobic molecules (e.g., O2 and CO2).
• The membrane is impermeable to large, charged molecules (ions), and hydrophilic molecules.

Diffusion

• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, moving "down" a concentration gradient.
• Facilitated diffusion is the passive movement of molecules across the plasma membrane via transport proteins.

Active Trasport

• Active transport is the movement of molecules across the membrane against the concentration gradient, requiring ATP energy.

Osmosis

• Osmosis is the net movement of water molecules across a semipermeable membrane from a region of high water concentration to low.
• Water concentration is affected by the amount of water compared to other molecules like salt.
• In a hypotonic solution, the water concentration outside the cell is higher than inside, causing water to enter the cell.
• In a hypertonic solution, the water concentration outside the cell is lower than inside, causing water to exit the cell.
• In an isotonic solution, the water concentration inside and outside the cell is the same, and the amount of water in the cell remains constant.
• Animal cells thrive in isotonic environments, while plant cells thrive in hypotonic environments.
• Osmotic pressure can build up inside a cell when water moves in by osmosis.
• A cell wall helps maintain water balance, making osmotic pressure the main source of support in plants.
• A plant cell in a hypotonic environment with osmotic entry of water exerts turgor pressure against the cell wall, making the cell turgid.
• The action of osmosis can be harmful to organisms without cell walls.
• Saltwater fish in freshwater will experience water influx, cell lysis, and death.
• Using table salt kills slugs and snails due to harmful osmotic effects.

Atypical Cell Structures

• Individual striated muscle cells fuse to form long, striated muscle fibers.
• These fibers are surrounded by a continuous plasma membrane and have multiple nuclei.
• Striated muscle fibers challenge the idea that all living things are comprised of discrete cell units.
• Mature red blood cells lack a nucleus or mitochondria, maximizing space for hemoglobin storage.
• Lacking genetic material, red blood cells cannot independently replicate.
• New red blood cells are continually produced in the bone marrow.