cell membranes

What is the definition of simple diffusion?

Simple diffusion is the net movement of particles from an area of high concentration to an area of lower concentration.

Why is diffusion a passive process?

No energy is needed for it to occur.

What are 5 factors that affect the rate of diffusion?

• The concentration gradient

• Thickness of the membrane

• Surface area

• Temperature

• Number of channel/carrier proteins

How does the concentration gradient affect the rate of diffusion?

The steeper the concentration gradient, the quicker the rate of diffusion.

How does the thickness of the membrane affect the rate of diffusion?

The thicker the membrane the slower the rate of diffusion will be due to a longer distance particles have to travel.

How does surface area affect the rate of diffusion?

The larger the SA, the greater the rate of diffusion as more particles can diffuse across the membrane at one time.

How does temperature affect the rate of diffusion?

As the temperature increases, the faster the rate of diffusion, as the particles have more kinetic energy so they can move across the membrane faster.

How does the number of channel/carrier proteins affect the rate of diffusion?

The more channel/carrier proteins there are on the membrane, the faster the rate of diffusion as more particles can move across the membrane at one time.

What is the definition of osmosis?

The diffusion of water molecules across a partially permeable membrane down a water potential gradient. (From an area of high water potential to an area of lower water potential).

What is meant by the term water potential?

The likelihood of water molecules to diffuse into or out of a solution.

What does pure water have a water potential value of?

0

why is the water potential of any solution always negative?

• because adding solutes to a pure water solution lowers the water potential.

• the more negative the water potential, the higher the concentration of solutes in that solution.

What would happen to cells in an isotonic solution?

Cells in an isotonic solution would not gain or lose any water as there is no net movement of water molecules as the water potential is the same inside and outside of the cell.

What would happen to cells in a hypotonic solution?

Hypotonic solutions have a higher water potential so if a cell was placed in a hypotonic solution then water would move into the cell by osmosis. Animal cells in the solution would swell and could eventually burst however plant cells in the solution would become turgid as the vacuole and cytoplasm would swell and push against the cell wall.

What would happen to cells in a hypertonic solution?

• Hypertonic solutions have a lower water potential so if a cell was placed inside of one, water would move out of the cell by osmosis.

• If an animal cell was placed in a hypertonic solution then the cell would shrink.

• If a plant cell was placed in a hypertonic solution then it would become flaccid and the cell membrane will eventually pull away from the cell wall (plasmolysis).

What are the two types of membrane protein involved in facilitated diffusion?

Channel proteins and Carrier proteins

What is the role of carrier proteins during facilitated diffusion?

move large molecules (like ions and polar molecules) into or out of the cell.

How do carrier proteins work?

• First, a large molecule attaches to a carrier protein in the membrane.

• Then, the protein changes shape.

• This, then releases the molecule on the opposite side of the membrane.

What is the role of channel proteins during facilitated diffusion?

Form pores in the membrane for smaller ions to diffuse through, down their concentration gradient.

What are the two types of active transport?

Endocytosis and Exocytosis

What is the process of Endocytosis?

• The cell membrane invaginates to make a fluid filled pocket.

• The membrane then folds inwards to produce a vesicle.

• The vesicle then moves into the cell, separating it from the membrane.

What is the process of Exocytosis?

• Vesicle buds off from the Golgi apparatus

• Vesicle then moves towards the plasma membrane

• Vesicle fuses with the plasma membrane

• Contents of the vesicle are released outside of the cell.

What are the two factors that can affect membrane permeability?

solvents and temperature

how do solvents affect membrane permeability?

• as you increase the solvent concentration, the permeability of the membrane also increases.

• this is because some solvents dissolve the lipids in the cell membrane, so the membrane loses it’s structure.

What is an example of a solvent that may dissolve lipids in the CM?

Ethanol

How does a temperature below 0 degrees affect membrane permeability?

The phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid. Ice chards may form in the membrane, and could pierce the membrane, this would lead to a rapid increase in membrane permeability.

How does a temperature between 0 degrees and 45 degrees affect membrane permeability?

The phospholipids can move around and aren’t as tightly packed together - so the membrane is partially permeable. As the phospholipids gain more kinetic energy they can move more which would increase the permeability of the membrane.

How does a temperature above 45 degrees affect membrane permeability?

The phospholipid bilayer starts to melt and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel and carrier proteins inside the membrane denature so they can’t control what enters and leaves the cell - this increases the permeability of the membrane.

Function of the plasma membrane?

The plasma membrane regulates the movement of substances in and out of the cell and maintains homeostasis and provides protection and structural support.

Function of membrane surrounding organelles within cells?

Divides the cell into different compartments to perform specific functions. Also, acts as a barrier between the organelles and its environment.

What makes up the phospholipid bilayer?

Phospholipids, cholesterol and proteins.

What is the structure of a phospholipid?

A phospholipid consists of a hydrophilic head (attracts water) and 2 fatty acid tails which are hydrophobic (repels water). The hydrophilic head is made of a phosphate group whereas the hydrophobic fatty acids tails are made up of a chain of hydrocarbons.

What is the function of the phospholipids?

• They form the phospholipid bilayer

• They allow for selective permeability

• They form the double layer structure that creates a barrier for the movement of substances in and out of the cell.

What is the function of cholesterol?

• Help maintain membrane fluidity by regulating how tightly packed the phospholipids are in the plasma membrane.

• It stabilizes the structure across varying temperatures;

  • when there’s a low temperature cholesterol helps prevent the membrane from becoming to rigid

  • however at higher temperatures, cholesterol prevents the membrane from becoming to fluid.

What are the types of protein in the plasma membrane?

Channel proteins and carrier proteins.

Function of the channel proteins in the plasma membrane?

They facilitate the passage of ions and molecules across the plasma membrane. This enables selective transport.

function of the carrier proteins in the plasma membrane?

• they transport specific molecules across the membrane by changing shape in response to the binding of the molecule.

• then they release the molecule out of the opposite end of the membrane.

What do carrier proteins carry across the membrane?

Large molecules and charged particles.

What do channel proteins carry across the cell membrane?

Small, charged particles.