Cell Membranes - Module 2

The rate of diffusion depends on several factors...

- The concentration gradient - The thickness of the exchange surface - The surface area - The temperature

Peripheral proteins also known as _________

Extrinsic proteins and they are attached to surface of lipid bilayer.

Integral proteins also known as _________

Intrinsic proteins and they are embedded in the lipid bilayer.

Active Transport

The movement of particles against the concentration gradient which requires energy in the form of ATP.

the cell surface membrane also known as the __________ ___________

plasma membrane

What is the water potential of distilled water?

0

How wide is the cell surface membrane?

7nm

What is the Fluid Mosaic model?

A model that describes how the molecules of the membrane are arranged. The main features are: a bilayer of phospholipid molecules and various protein molecules floating in the phospholipid bilayer. These two components are arranged in a mosaic pattern. The molecules are not bonded together and they can move around and change positions giving the impression of a fluid. The membrane is stabilised by small cholesterol molecules.

Phospholipids are arranged in ________ and why?

Bilayer because the phosphate head is hydrophilic (water-loving) and the fatty acid tails are hydrophobic (water-hating). Therefore the phosphate heads are arranged so that they are next to the aqueous fluid outside of the cell and the aqueous cytoplasm inside the cell and the fatty acids are as far from it as possible. This structure is the most stable.

Glycoproteins and glycolipids

Glycoproteins are proteins that have a short carbohydrate chain attached. These chains protrude from the cell membrane and can form hydrogen bonds with water which stabilises the membrane structure. These are also important in cell signalling, acting as receptors for certain molecules and triggering specific changes in the cell when that molecule binds. Glycolipids are lipids that have a short carbohydrate chain attached. Glycoproteins and glycolipids form the surface antigens by which the immune system can identify the cell as belonging to the body or as being foreign.

What is the function of cholesterol in the membrane?

It has a hydrophilic and a hydrophobic portion which allows cholesterol to bind to the phospholipids which prevents the membrane from being too fluid. It also prevents adjacent fatty acids from coming together and crystallising which would make the membrane more rigid. Therefore cholesterol's main function is to maintain a suitable level of fluidity within the membrane.

What is cytolysis?

It is the process in which cells gain water in a hypotonic solution.

What is the function of the membrane?

The cell membrane on the surface of animal cells and just inside the cell wall of plant cells. It regulates the movement of materials in and out of the cell. It also has receptor molecules on it, which allow it to respond to chemicals like hormones.

Define cell signalling.

The communication by means of chemical messenger molecules which are produced in one cell and transported to others. Some cells are unaffected by the messenger. Cells that do have the correct cell membrane receptors detect the messenger molecule and changes occur in the cell as a result. This transfer of information between cells is called cell signalling.

What is the cell membrane composed of?

The membrane is composed of a phospholipid bilayer which is embedded with proteins. The phospholipids are polar molecules with hydrophilic phosphate heads and hydrophobic tails which causes the molecules to orient themselves in a bilayer with the hydrophilic heads pointing outward and the hydrophobic tails pointing inward.

Osmosis

The net movement of free water molecules from a high water potential to a lower water potential across a partially permeable membrane until equilibrium reached.

Facilitated Diffusion

The net movement of molecules (like glucose) down a concentration gradient from a high concentration to a low concentration through specific carrier or channel proteins.

Diffusion

The net movement of molecules down a concentration gradient from a high concentration of molecules to a low concentration of molecules until equilibrium is reached.

What happens to cell membranes at temperatures above 45°?

The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins denature (deform) so they can't control what enters or leaves the cell which increases the permeability.

What happens to cell membranes at temperatures below 0°?

The phospholipids don't have much energy , so they can't move very much. They are packed closely together and the membrane is rigid. But channel and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane making it highly permeable when it thaws.

What is plasmolysis?

The shrinking of cytoplasm away from the cell wall of a plant cell when water is lost due to osmosis, resulting in space between the cell wall and cell membrane.

Carrier proteins

These can change shape and in doing so move substances from one side of the membrane to the other. This could be down a concentration gradient by facilitated diffusion or against the concentration gradient by active transport.

Channel proteins

These form a passage through which water and polar substances can pass by diffusion down a concentration gradient .

Co-transport

When two molecules move through the protein together

Bulk Transport

is a means of carrying materials across plasma membrane. It involves the formation of vesicles.

What happens to a plant cell in a hypertonic solution?

it becomes flaccid

What happens to a plant cell in a hypotonic solution?

it becomes turgid

What happens to an animal cell in a hypotonic solution?

it bursts (lysis)

What happens to an animal cell in a hypertonic solution?

it shrivels and crenates

The plasma membrane is ____________ permeable

partially

What can't pass through the cell membrane by diffusion?

water-soluble molecules and ions these have to move through protein channels