AP Biology (and all that fun stuff) by Kelly N.: 2.B.1-2.B.3: Cell Membranes and How They Work

AP Biology: 2.B.1 – 2.B.3 Cell Membranes

All cells have a cell membrane. These membranes are all semipermeable, which means only certain particles can pass through. A cell needs to be able to move things in and out of the membrane so that it can survive, as all living things need energy and nutrients. They serve the purpose of separating the inside of the cell from the outside as well.

These cell membranes are made of two layers of what is known as a phospholipid. These phospholipids are amphipathic because they contain a hydrophilic head, which likes water due to its polarity, and a fatty acid “tail” that’s nonpolar and detests water. The heads line both the outer facing part of the membrane and the inner facing membrane, with the tails facing one another.

Phospholipids also move around. Once a month the outer phospholipids switch places with the inner facing ones and are almost constantly moving around with other phospholipids around them laterally.

In order to move larger and polar substances into and out of the cell, protein channels are needed.

There are three types of protein channels: Integral membrane, trans-membrane and peripheral proteins.

Integral membrane proteins are proteins that are only in one half of the phospholipid bilayer.

Trans-membrane proteins go all the way through and are key to the transport of large substances from one side to the other. A good example of this is the sodium potassium pump.

Peripheral proteins are bound to the surface of the bilayer.

Membrane proteins, oddly enough, have hydrophilic and hydrophobic like phospholipids. Likely to stay in the membrane and perform their tasks to the best of their abilities. They function to transport things, produce and contain enzymatic activity, signal transduction, cell-to-cell identification, intercellular joining and forming attachments to the cytoskeleton & cellular matrix.

In addition to proteins, there are also other things attached to the phospholipid bilayer, such as membrane carbohydrates, glycolipids (which send and receive signals from other cells) and glycoproteins (also are important with interacting with other cells. They are very diverse).

Cholesterol, contrary to “common belief” are absolutely necessary to the functioning of cells (and bodily processes). These special molecules are integrated in the membrane and prevent it from becoming overly fluid or overly firm.

Transport proteins, which are often trans-membrane, serve the purpose of transporting substances in and out of the cell. They have a hydrophilic channel that goes through it vertically and connects the extracellular fluid and the cytoplasm. Certain molecules like sugars can use this.

Although all cells have a plasma membrane, not all have what is known as a cell wall. Only plants, fungi and bacteria have this rigid structure that in addition to providing protection, prevents the cell from absorbing too much water and bursting. Cell walls are made up of a substance called cellulose. In fact, one of a few ways scientists have discovered how to kill harmful bacteria is by destroying the cell.

Prokaryotes have a cell wall made up of a substance called peptidoglycan and fungi have cell walls made of chitin, which is also the substance that makes the exoskeleton of arthropods.

Osmosis is the process of water moving across a semipermeable membrane. As water is a polar molecule, it can’t freely pass through the phospholipid bilayer without assistance. A special protein called an aquaporin forms a protein channel in the membrane that allows water to diffuse (move from a point of high concentration to low concentration) into and out of the cell. This is an example of facilitated diffusion, which occurs when a protein is required to move a certain substance across a membrane.

When water inside a cell has a higher concentration of water inside it than outside, the cell is hypotonic. When that scenario is reversed, then it is hypertonic. When dynamic equilibrium is achieved and the concentration inside and out is equal to the point of there being no significant travel inside or outside by water, both the internal and external cell are isotonic.

There are two ways molecules move: through passive transport which requires no energy and active transport, which requires energy (ATP) to move molecules from areas of low concentration to high concentration. It is used whenever molecules need to be transported against their concentration gradient.

Sometimes groups of ions move through the plasma membrane through endocytosis and exocytosis in what is called bulk transport. In exocytosis it is often seen in organelles called vesicles that fuse with the lipid bilayer to remove wastes from the cell. Phagocytosis is when the vesicles take in large substances from outside the cell. Pinocytosis is when water and dissolved particles are taken in.

Like earlier stated, water prefers to move from areas of higher water potential to low lower water potential. Water potential is defined by the sum of the solute concentration and water pressure.

To understand this equation, one must first know the basics:

Yp = the pressure on the water. In an open container, it is 0. In a turgid plant cell, it is greater than zero.

Ys = - iCRT

i = ionization constant (for sucrose this is 1.0, for NaCl this is 2.0)

C = molar concentration

Molarity: Moles Solute/Volume of Solution. M = Moles/L

R = pressure constant (R = .0832 liter bars/mole K)

T = temperature in Kelvin (273 + degrees Celsius)

Also, in order for eukaryotic cells to function at best despite their large sizes, they have evolved to contain many internal membranes (called organelles) that carry out specific cell processes. They compartmentalize to efficiently carry out cellular processes that need different environments to work. The digestion that occurs in lysosomes requires an acidic environment, for example.

Internal membranes also increase the surface area, which allows more reactions to occur because there are more membrane bound organelles. The larger the surface area to volume ratio a cell has, the easier it is to transport things in and out, and to the organelles inside.

There are several membrane-bound organelles:

The Rough Endoplasmic Reticulum modifies secretory proteins that are then synthesized by ribosomes and moved to the lumen of the Rough E.R.

The smooth endoplasmic reticulum synthesizes lipids, phospholipids and steroids in addition to carrying out cell metabolism and detoxifying drugs like alcohol.

The Golgi Apparatus, which is known as the post office of the cell, packages and sorts proteins for secretion. These packaged proteins are then sent to the cisface and are modified as they move through the organelle’s cisternae (the inside of the Golgi’s main body). They are then sent in vesicles to the cell’s membrane.

The nuclear membrane is a bilayer with the same lumen as the E.R. It has many ribosomes imbedded into it and houses the nucleus, nucleolus and DNA of the cell.

The vacuoles in plant cells contain large amounts of water in addition to storing things. The vacuoles of animal cells contain food particles and can merge with lysosomes to digest it.

In Bacteria and Archaea lack inner membranes because they’re usually too small for that, but their cell processes and enzymes are confined to different parts of the cell.