Biology 1001 - Unit 3 Terms & Key Notes

Plasma Membrane

The outer living layer of each cell, it separates the internal cell contents from the external environment, acting as a the interface between inside and outside of the cell. A gatekeeper, if you will. The term "Fluid Mosaic" is used to describe this. They are fluid mosaics of lipids and proteins.

Amphipathic Molecule

A molecule that has a water-hating (hydrophobic) region as well as a water-loving (hydrophilic) region, the hydrophobic tail is excluded by water, ending up in the center of the bilayer, while the hydrophilic head points outwards. The dual property of ___________ molecules is key to plasma membrane structure and function. This was briefly covered within the last unit.

Membrane Proteins

Embedded in the membrane with their hydrophobic sections in the bilayer and their hydrophilic sections sticking out. ________ also perform specific functions for the cell membrane. They are also amphipathic.

Rapid Lateral Movement

Movement exhibited by phospholipids and proteins in the cell membrane within the lipid bilayer. A quick side-to-side motion that occurs on a molecular level. If phospholipids and proteins were unable to move, the cell would die.

Phospholipid Bilayer

A double layer of phospholipids that makes up plasma and organelle membranes. Things can move in and out of it.

Membrane Fluidity

The property by which most of the plasma membrane lipids and proteins easily rotate and move side ways in their own half of the lipid bilayer. This property allows the membrane to self-seal if torn; proteins seldom flip-flop from one half of the bilayer to to the other.

How Membrane Fluidity can be Affected

1. Temperature - The lower the temperature, the more chance of the membrane losing fluidity and solidifying. (Think of how molecules move faster under higher temperatures)

2. The nature of phospholipid hydrocarbon tails - Whether they are bendy or straight affects fluidity at different temperatures

Answer: How Membrane Fluidity can be Affected

Seasonal Acclimation

Shift from winter to summer relation of metabolic rate to temperature. Organisms that live in environments where temperatures vary can show this. During cold periods, they increase the proportion of unsaturated fatty acids in their cell membranes.

Cholesterol "Temperature Buffer"

At low temperatures, ___________ molecules keep membranes fluid by disrupting the regular packing of phospholipid tails. At high temperatures, ___________ can stabilize membrane fluidity by restraining phospholipid movement. Note that there is a limit to temperature range that can be tolerated.

Membrane Proteins (Ex.)

These proteins have many jobs; transport, enzyme activity, signal transduction, cell to cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Cell-Cell Recognition

Cells _________ other cells by binding to carbohydrate ID tags attached to membrane proteins. Diversity of the carbohydrates and their locations on the surface allow them to function as markers that distinguish one cell from another.

Intercellular Joining

Long-lasting binding of cells. Ex. tight junctions and gap junctions of animal cells.

Attachment to the Cytoskeleton and Extracellular Matrix

These attachments combine to give animal cells a stronger framework. Don't forget, the cytoskeleton is all protein. Proteins hold things together.

The Movement of Molecules Across Membranes

Can occur at any one time in any cell, a steady traffic of small molecules pass into and out of the cell. (Ex. H2O, )2, and sugars going in, CO2, and metabolic waste products going out). Additionally, ions; charged atoms/molecules (Ex. Na+, K+, Cl-, and Ca++) bass back and forth. These processes are NOT random, but are tightly controlled by the plasma membrane.

The Two Ways to Get Through the Membrane

1. Molecules can get through by dissolving through the lipid bilayer; small hydrophobic (non-polar) molecules can easily move through the lipid bilayer, but hydrophilic (polar, charged, ionized) molecules cannot cross the hydrophobic core.

2. Polar and charged (hydrophilic) molecules are assisted through the membrane by transmembrane transport proteins.

Channel Proteins

These proteins have a hydrophilic channel running through the middle so that polar-charged molecules can pass through. For example, a certain type of these proteins, Aquaporins, facilitate water passage through the plasma membrane.

Aquaporins

Special water channels in cell membranes. They allow 3 billion water molecules to pass through within just a second in a single file line, with 10 at a time in the channel.

Carrier Proteins

These transport proteins take in the molecules they are designed to transport, followed by changing their own shape in a way that boosts their "cargo" (cells) across the membrane, releasing it on the other side. These transport proteins are also very target specific. For example, Glucose transporters will reject fructose.

Diffusion

Refers to the movement of solute and or gas molecules. This is a spontaneous process, driven by inherent thermal energy within the molecules; no additional energy input is needed, and the warmer it is, the quicker the equilibration. Molecules move through their concentration gradient, until the concentration is the same throughout the available space and a dynamic equilibrium is reached. For example, think of food coloring droplet being put into water, and having the food coloring spread around in the water until the water is the same color as the coloring that was used.

Net Movement

The most movement in one direction. If the concentrations of solute molecules are not equal the ___ movement is away from the area with a higher concentration of the solute. The solute then moves to the area with lower concentration until eventually the concentration is qual on both sides.

Osmosis

The diffusion of free water across a selectively permeable membrane. For there to be a free water concentration gradient, there must be a solute dissolved in the water. Otherwise, it's just water. This process can be described as water moving from the less concentrated (lower solute concentration with more free water) solution to the more concentrated (higher solute concentration with less free water) solution.

Tonicity

This gives information on the relative concentration of solutes on either side of a semi-permeable membrane. This is also the ability of a solution surrounding a cell to cause that cell to gain or lose water.

Hypotonic

When comparing two solutions, the solution with the lesser concentration of solutes. For example, "x is less salty/sweet compared to y".

Hypertonic

When comparing two solutions, the solution with the greater concentration of solutes. For example, "x is saltier/sweeter than y".

Isotonic

Describes a solution whose solute concentration is equal to the solute concentration inside a cell. For example, "the x solution is as salty/sweet as the y cell".

How Does Tonicity Affect Living Cells Without Cell Walls?

1. Concentration of solutes outside the cell is equal to the concentration inside of the cell. Both are isotonic.

2. Higher concentration of solutes outside of the cell compared to the cell itself. The solution would be hypertonic to the cell, and the cell is hypotonic to the environment around it. As a result, the cell shrivels up, and most likely dies.

3. Higher concentration of solutes inside of the cell compared to the external environment. Cell is hypertonic to the environment, and the solution is hypotonic to the cell. As a result, the cell bursts and dies.

Answer: How does tonicity affect living cells without cell walls?

How Does Tonicity Affect Living Cells With Walls?

The rigid cell walls prevent cells from bursting. In a hypotonic environment, water enters by osmosis and becomes turgid. In an isotonic environment, there is no net tendency for water movement, and the cells become flaccid. Finally, for a hypertonic environment, the cell wall cannot prevent net outward water movement. The cell shrinks and the plasma membrane pulls away from the cell wall through a process called plasmolysis. This process is typically lethal.

Answer: How Does Tonicity Affect Living Cells With Walls?

Passive Transport

Here's a recap of this process, in case you forgot it:

1. Non-polar, non-ionized molecules diffuse across the membrane through lipid bilayers. The direction is always downwards.

2. Polar molecules diffuse down their gradients as well with the assistance of channel proteins and carrier proteins, which facilitate their diffusion across the plasma membrane, hence the term 'facilitated diffusion'.

Active Transport

The movement of ions or molecules from a lower concentration gradient across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy (ATP). Carrier proteins are the only transport proteins that can facilitate this form of transport. This process is VERY important.

Electrochemical Gradient

All cells maintain a voltage across their membranes by the active transmembrane transport of ions. The inside of a cell is generally electronegative relative to the outside. Additionally, the membrane potential within cells ranges from -50 to -200mV. This _______________ gradient acts like a charged battery, providing power to drive certain transmembrane reactions, such as nerve conduction and muscle contraction.

Na+/K+ Pump

A type of carrier protein present in animals. An active transporter that moves three Na+ out of a cell and two K+ into the cell against their respective concentration gradients. This establishes an electrical and a chemical gradient (an electrochemical gradient) across the cell membrane.

Proton Pumps

These pumps move hydrogen ions (H+) out of the cell. Building up these unequal distributions of ions across cell membranes is very important to the functioning of living cells. Plants and fungi use this kind of electrogenic pump.

How Do Water and Smaller Molecules Enter and Leave the Cell?

1. Diffusing through the lipid bilayer down their concentration gradient.

2. Facilitated diffusion down their concentration gradient via either channel or carrier proteins

3. Active transport against their concentration gradient via carrier proteins powered by ATP.

Answer: How Do Water and Smaller Molecules Enter and Leave the Cell?

Exocytosis

Process by which a cell releases large amounts of material. Large molecules being transported out of the cell.

Endocytosis

Process by which a cell consumes large amounts of material. Large molecules being transported into the cell. There are three variations of this process in particular; pinocytosis, receptor-mediated endocytosis, and phagocytosis.