Plasma Membrane

Allows some substances to cross it more easily than others.
- Small, nonpolar molecules (e.g., O2 + CO2) can pass freely;
  - O2 is needed for cellular respiration, while CO2 is a byproduct released as a toxin.
- Polar molecules such as sugars and amino acids do not pass through the membrane easily.

The membrane is a fluid structure composed of varied and scattered proteins embedded within it.
Hydration cells form around phospholipids.
Freeze-fracture is a method used to study the plasma membrane by freezing a cell, which supports the fluid mosaic model by allowing the membrane to split along its middle.

Some lipids and proteins drift laterally across the membrane, with some flip-flopping rarely.
Phospholipids can move within the bilayer, as shown by the hybrid cell formed from a mouse and human cell. This indicates mixed proteins in hybrid cells, implying membrane fluidity.

Membrane fluidity changes with temperature; as temperatures cool, membranes transition from a fluid state to a solid state.
Membranes rich in unsaturated fatty acids (fa) help maintain fluidity.
- Plants and warm-blooded animals have higher proportions of unsaturated and saturated fatty acids, respectively.

Cholesterol has different effects on membrane fluidity at different temperatures.
- It restrains the movement of phospholipids at cooler temperatures, maintaining fluidity by preventing tight packing.
- Cholesterol is only found in animal cells.

The membrane contains a collage of different proteins embedded in the fluid matrix of the lipid bilayer.
These proteins determine most of the membrane's specific functions:
- Peripheral Proteins: Bound to the membrane surface; some attach to the cytoskeleton.
- Integral Proteins: Penetrate the hydrophobic core (amphipathic) and include transmembrane proteins that span the membrane.
- Glycoprotein: A protein that is covalently bonded to carbohydrate (sugar) chains, playing key roles in cell-cell recognition and signaling.
- Glycolipid: A phospholipid that is covalently bonded to carbohydrate chains; they function similarly to glycoproteins and are involved in cellular recognition processes. Both glycoproteins and glycolipids contribute to the membrane's ability to mediate cell interactions and signaling.

Made in the rough endoplasmic reticulum (ER), every protein serves a specific purpose; their functions are dictated by the sequence of their amino acids.
- For example, channel proteins have nonpolar entries and a polar passageway, allowing them to facilitate specific transport across the membrane.

Diffusion refers to the tendency for molecules to spread evenly into available space until equilibrium is reached.
- Movement occurs from areas of high concentration to low concentration, driven by random molecular movements.
- Passive transport enables this process without energy expenditure, maintaining dynamic equilibrium where molecules can move in and out to balance concentrations.

Substances diffuse down their concentration gradient, making use of passive transport due to lack of energy requirement.

Osmosis is the diffusion of water across a selectively permeable membrane from high to low concentration, wherein cohesion plays a role in pulling it across as well as solute concentration.
- Water is pulled to create hydration shells around different solute concentrations; low solute equals high water concentration and vice versa.
- Water moves toward the area of higher solute concentration due to the concentration’s effect on ionic bond aspirations (e.g., mimicking a cell's needs).

Tonicity is the ability of a solution to affect cell water gain or loss:
- Isotonic: Solute concentration is the same inside and outside of the cell; no net movement, favored in animal cells.
- Hypertonic: Solute concentration outside is greater than that inside, causing cell water loss (e.g., flaccid plant cells due to plasmolysis).
- Hypotonic: Solute concentration is less outside than inside, leading to cell water gain; favored in plant cells where it results in turgidity.
Osmoregulation describes the control of water balance; for example, a paramecium in a hypertonic environment utilizes a contractile vacuole to pump excess water out.