Biological Membranes

Question 1

Plasma membrane, functions

Answer 1

It separates the interior of the cell from the extracellular environment (Env), receives information about changes in the Env, regulates the passage of materials into and out of the cell and communicates with other cells.
It also forms compartments within the cell for a variety of separated functions.

Question 2

The fluid mosaic model

Answer 2

According to this model, membranes consist of a fluid phospholipid bilayer in which a variety of proteins are embedded. The phospholipid molecules are amphipathic (hydro-). The hydrophilic heads of the phospho- are at the two surfaces and de hydrophobic fatty acid chains are in the interior.

Question 3

Properties of the lipid bilayer

Answer 3

The lipids of the bilayer are in fluid or liquid-crystalline state, which allos the lipid molecules to move rapidly in the plane of the membrane and within.
It is also flexible and sel-sealing and can fuse with other membranes, allowing the transport of materials from one region of the cell to another.

Question 4

Integral membrane proteines.

Answer 4

They are embedded in the bilayer with their hydrophilic surfaces exposed to the aqueous environment. They can cross the bilayer or not, when they cross are called transmembrane proteins.

Question 5

Peripheral membrane proteins

Answer 5

They are proteins associated with the surface of the bilayer, usually bound to exposed regions of integral proteins.

Question 6

Membrane protein functions

Answer 6

They anchor cells, transport materials, act as enzymes or receptors, recognize cells and communicate with them, and structurally link cells.

Question 7

Selectively permeable membranes in the cell…

Answer 7

They allow the passage of some substances but not others. This way they control internal composition of ions and molecules.

Question 8

Carrier proteins

Answer 8

They are transport proteins that undergo a series of conformational changes as they bind and transport a specific solute. E.g., ABC transporters which use ATP.

Question 9

Channel proteins

Answer 9

They are transport proteins that form passageways through which water and certain ions travel through the membrane. E.g., porins.

Question 10

Diffusion

Answer 10

It is a type of passive transport. Is the net movement of a substance down its concentration gradient from a region of greater concentration to one of lower. It does not require directly metabolic energy.

Question 11

Simple difussion

Answer 11

It is a type of passive transport. In this, solute molecules or ions move directly through the membrane down their concentration gradient.

Question 12

Facilitated diffusion

Answer 12

It is a type of passive transport. It uses a specific transport proteins to move solutes across a membrane dow its concentration gradient.

Question 13

Osmosis

Answer 13

It is a kind of diffusion in which molecules of water pass through a selectively permeable membrane from a region where water has a higher effective concentration to a region where its effective concentration is lower.

Question 14

Osmotic pressure

Answer 14

The concentration of dissolved substances or solutes in a solution determines this value.

Question 15

Isotonic solution

Answer 15

It has equal solute concentration compared with that of another fluid.

Question 16

Hypertonic solution

Answer 16

It has a greater solute concentration than that of another solution. When cells are placen in a solution like this, they lose their water and shrink. In the case od plant cells, they undergo plasmolysis, where plasma membrane separates from the cell wall.

Question 17

Hypotonic solution

Answer 17

It has a lower solute concentration than the solute concentration of another solution. In this case, cells swell as water enters the cell.

Question 18

Turgor pressure

Answer 18

When the plant cell swells due to water intake collected by their central vacuole.

Question 19

Cotransport

Answer 19

It is a type of active transport. Two solutes are transported at the same time. An ATP-powered pump maintains a concentration gradient, then a carrier protein cotransport two solutes, one down its gradient and the other against its gradient.

Question 20

Exocytosis

Answer 20

It is a type of active transport. The cell ejects waste products or secretes substances such as mucus by fusion of vesicles with the plasma membrane. It also increases the surface area of the plasma membrane.

Question 21

Endocytosis

Answer 21

It is a type of active transport. Materials such as food particles are moved into the cell. A portion of the plasma membrane envelops the material, enclosing it in a vesicle or vacuole that is then released inside the cell. This process decreases the surface area of the plasma membrane.

Question 22

There are 3 types of endocytosis: phagocytosis, pinocytosis and receptor-mediated endocytosis, explain.

Answer 22

In phagocytosis the plasma membrane encloses a large particle such as a bacterium, forms a vacuole around it, and moves it into the cell.
Pynocytosis, the cell takes in dissolved materials by forming tiny vesicles around droplets of fluid trapped by folds of the plasma membrane.
Receptor-mediated endocytosis, specific receptors in coated pits along the plasma membrane bind ligand molecules. These pits, coated by the protein clathrin, form coated vesicles by endocytosis, then they fuse with lysosomes and are digested.

Question 23

Anchoring junctions

Answer 23

They include desmosomes and adhering junctions; they are found between cells that form a sheet of tissue. Desmosomes spot-weld adjacent animal cells together. Adhering junctions are formed by cadherins, transmembrane proteins that cement cells together.

Question 24

Tight junctions

Answer 24

They seal membranes of adjacent animal cells together, preventing substances from moving through the spaces between the cells.

Question 25

Gab junctions

Answer 25

Composed of the protein connexin, form channels that allow communication between the cytoplasm of adjacent animal cells.

Question 26

Plasmodesmata

Answer 26

They are channels connecting adjacent plant cells. Openings in the cell walls allow the plasma membranes and cytosol to be continuous; certain molecules and ions can pass from cell to cell.