Biology Class: Plasma Membrane Overview

Study Notes

The Plasma Membrane

The plasma membrane's function is homeostasis, maintaining balance in an organism's internal environment. This is crucial for cell survival.
The plasma membrane is a thin, flexible boundary separating the cell from its environment.
It allows nutrients into the cell and waste products out.
Prokaryotic and eukaryotic cells contain a plasma membrane which separates them from their surroundings.
Selective permeability means a membrane allows certain substances to pass while keeping others out.

Plasma Membrane Structure

The polar heads of phospholipid molecules are attracted to water.
The nonpolar tails repel water.
Phospholipid molecules arrange themselves into a double layer (bilayer), with their hydrophilic heads facing the watery environments inside and outside the cell, and their hydrophobic tails facing each other in the membrane interior.
Lipids (like glycerol and fatty acids) make up most of a cell's plasma membrane.
If a phosphate group replaces a fatty acid, a phospholipid forms.
Phospholipids arrange to keep the polar heads closest to water and nonpolar tails farthest away.
This arrangement creates a barrier that is polar at the surface and nonpolar in the middle.

Phospholipid Bilayer

Two layers of phospholipids form the plasma membrane, arranged tail-to-tail.
This structure allows the plasma membrane to exist in watery environments
A phospholipid has a glycerol backbone, two fatty acid chains and a phosphate containing group.
Other components of the plasma membrane are found moving with and among the phospholipids.
- cholesterol
- proteins
- carbohydrates
Proteins act as receptors, transmitting signals to the cell's interior. Proteins anchor the plasma membrane to the internal cell support structures, giving the cell shape. Other proteins create tunnels for substances to enter and exit the cell.

Transport Proteins

Transport proteins move needed substances or waste materials across the plasma membrane.
This aids in the selective permeability of the membrane.
Nonpolar cholesterol is embedded among the phospholipids.
Cholesterol helps prevent fatty acid tails from sticking together, contributing to membrane fluidity.
Cholesterol is an important substance for maintaining homeostasis in a cell
Carbohydrates are attached to proteins, sticking out from the plasma membrane.
Carbohydrate structures define a cell's characteristics and help cells recognize chemical signals.

Cytoplasm and Cytoskeleton

Cytoplasm is the environment inside the plasma membrane.
The fluid mosaic model describes the dynamic nature of the plasma membrane's components.
Components of the membrane slide past one another.

Other Cellular Structures

Ribosomes are responsible for protein production.
Ribosomes are made of RNA and protein and lack a membrane, unlike other organelles.
Some ribosomes float freely in the cytoplasm, while others are attached to the endoplasmic reticulum.
The endoplasmic reticulum is a membrane structure composed of folded sacs and channels.
The rough endoplasmic reticulum (ER) has ribosomes attached.
The smooth ER doesn't have ribosomes.
The Golgi apparatus modifies, sorts, and packages proteins for distribution.
Vacuoles are membrane-bound vesicles used for temporary material storage.
- Food, enzymes, and substances are stored
- Waste materials are stored
Lysosomes contain digestive enzymes.
They break down excess/worn-out materials and bacteria/viruses.
Centrioles are composed of microtubules, and active in cell division.

Mitochondria and Chloroplasts

Mitochondria generate energy. Transform fuel particles into usable energy.
Chloroplasts harness light energy to produce chemical energy (photosynthesis).
Thylakoid membranes are where sunlight energy is captured by chlorophyll.

Cell Walls

Cell walls are a sturdy, rigid structure found in plant cells.
Cell walls provide support and protection. They're made primarily of cellulose.

Cilia and Flagella

Cilia are numerous, short hair-like projections that aid in movement and feeding.
Flagella are longer, fewer projections that aid in movement. These structures have microtubules arranged in a 9+2 configuration.

Cellular Transport (Diffusion)

Diffusion is the movement of particles from a high concentration to a low concentration region, due to random motion (Brownian motion).
Additional energy is not required.
Factors that affect diffusion rate are pressure, temperature, and concentration.

Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane. • Water moves from a high concentration to a low concentration. • Important for maintaining homeostasis. • Three types of solutions cells can be found in relative to their surroundings are isotonic, hypotonic, and hypertonic. - Isotonic: Water enters and leaves the cell at the same rate. - Hypotonic: Water enters the cell faster than it leaves, causing swelling. - Hypertonic: Water leaves the cell faster than it enters, causing shrinking.

Active Transport

Active transport occurs when substances move against their concentration gradient (from lower to higher concentration).
Energy (ATP) is required.
Common examples include the sodium-potassium pump (Na+/K+ ATPase pump).

Transport of Large Particles

Endocytosis takes material into the cell by engulfing it with a portion of the plasma membrane
Exocytosis releases material outside the cell by fusion of a vesicle with the plasma membrane.