2.1.5- biological membranes

Question 1

what are the roles of the cell surface membrane?

Answer 1

To isolate the cell from it’s surroundings (compartmentalisation).
To control the passage of substances into and out of a cell.
To have receptors for hormones or neurotransmitters.
Cell recognition.
So lipid-soluble substances can diffuse across

Question 2

what are the functions of the organelle membrane?

Answer 2

To control the entry and exit of substances into organelles like the mitochondria.
To isolate organelles so specific metabolic reactions can occur within them.
To isolate enzymes that might damage other regions.
To provide surfaces for reactions to occur- protein synthesis on the ER

Question 3

explain the structure of a phospholipid

Answer 3

a hydrophilic phosphate head with two hydrophobic fatty acid tails.

Question 4

how do phospholipids arrange themselves in a bilayer?

Answer 4

hydrophilic phosphate heads on outside, hydrophobic fatty acid tails on the inside in a bilayer.

Question 5

describe the fluid mosaic model

Answer 5

proteins can move within this phospholipid bilayer, they fit together like a mosaic.

Question 6

what is the impact of too much cholesterol in a bilayer?

Answer 6

reduced fluidity as phospholipids group together too closely and crystallise

Question 7

what are intrinsic proteins?

Answer 7

transmembrane proteins embedded through both sides of the bilayer- they have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane.

Question 8

what is the function and structure of a channel protein?

Answer 8

they provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a conc. gradient through membranes. The structure is essentially like a pore

Question 9

what is the structure and function of a carrier protein?

Answer 9

an important role in both active and passive transport, the movement involves changing the shape of the protein as it binds to the substance. the structure has an indent in it to carry the molecule depending on it’s shape

Question 10

what are glycoproteins, what is their function?

Answer 10

proteins with attached carb groups embedded in the membrane, attached to intrinsic proteins,
function: cell adhesion, receptors for chemical signals, cell signalling (when a chemical binds to a receptor to elicit a cellular response)

Question 11

what is the structure and function of extrinsic proteins?

Answer 11

peripheral proteins present on one side of the bilayer, they have hydrophobic R-groups on their outer surfaces and interact with the polar heads of phospholipids.

Question 12

what are the functions of glycolipids?

Answer 12

cell markers or antigens which can be recognised by the cells of the immune system (cellular recognition), stability

Question 13

what is cholesterol? function and structure

Answer 13

a lipid in the bilayer, has a hydrophilic end and a hydrophobic head. Plays a role in the regulation of membrane fluidity- molecules are positioned between phospholipids. the hydrophilic end interacts with the heads and hydrophobic end interacts with the tails. therefore it adds stability

Question 14

describe a triglyceride

Answer 14

contains C H O, is insoluble in water, contains glycerol, contains ester bonds, contains fatty acids

Question 15

describe a phospholipid

Answer 15

insoluble in water, contains glycerol, contains ester bonds, important in membrane structure, contains fatty acids

Question 16

describe cholesterol

Answer 16

contains C H O, insoluble in water, important in membrane structure.

Question 17

define diffusion

Answer 17

the net movement of particles from a region of high concentration to a region of low concentration, down a concentration gradient. a net movement until equilibrium is reached.

Question 18

why does diffusion occur?

Answer 18

because of the natural energy possessed by molecules or ions which makes them move randomly- the process is passive.

Question 19

what is simple diffusion?

Answer 19

membranes are freely permeable to the respiratory gases, water, uncharged molecules, and lipid-soluble molecules. Passive transport across the membrane without the use of transmembrane proteins.

Question 20

what is facilitated diffusion?

Answer 20

the membrane is selectively permeable, carrier proteins which change shape to bind to the molecule or ion carry the molecule across. Carries molecules that are large, ions, or polar.

Question 21

factors affecting diffusion

Answer 21

Steepness of gradient.
Temperature.
Surface area.
Diffusion distance.

Question 22

how does the rate of diffusion change with steepness of gradient?

Answer 22

the steeper the gradient, the faster the rate of diffusion as the imbalance of equilibrium is greater

Question 23

how is the rate of diffusion affected by temperature?

Answer 23

the molecules have more kinetic energy so they diffuse across the membrane faster

Question 24

how is the rate of diffusion affected by surface area?

Answer 24

if there is a greater area for molecules to diffuse across , i.e. more channel proteins to aid diffusion, the rate will increase

Question 25

how is the rate of diffusion impacted by diffusion distance?

Answer 25

the longer the distance, the slower the rate of diffusion as the distance is larger.

Question 26

define active transport

Answer 26

the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration against a concentration gradient. the process requires ATP and carrier proteins.

Question 27

how does active transport work?

Answer 27

the molecule or ion binds to receptors in the channel of the carrier protein on the outside of the cell. inside the cell, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate. the phosphate binds to the carrier protein forcing it to change shape, this opens up the inside of the cell.

Question 28

what is exocytosis?

Answer 28

active process by which bulk materials are removed from, or transported out of cells. The substances released are packaged into secretory vesicles from the golgi body. Vesicles travel to the cell surface membrane. They fuse to it and release their contents outside of the cell. e.g. the secretion of digestive enzymes from pancreatic cells.

Question 29

what is endocytosis?

Answer 29

active process of the transport of bulk materials into cells. The two forms are phagocytosis for solids and pinocytosis for liquids. The cell surface membrane invaginates, the membrane enfolds the material until the sides fuse forming a vesicle which is transported into the cell. for example vesicles containing bacteria are transported to lysosomes.

Question 30

define osmosis

Answer 30

the net movement of water molecules from an area of high water potential to an area of lower water potential through a partially permeable membrane.

Question 31

what is the water potential of pure water?

Answer 31

0

Question 32

what happens to an animal cell with a higher water potential inside the cell, than outside the cell?

Answer 32

water will move out by osmosis, forcing the cell to crenate

Question 33

what happens to an animal cell with a lower water potential inside the cell than outside the cell?

Answer 33

water will move in by osmosis, forcing the cell to swell and become haemolysed (lysis)

Question 34

what happens to a plant cell with a higher water potential inside the cell than outside the cell?

Answer 34

water will move out by osmosis. the cell membrane (and protoplast) will shrink away from the cell wall, becoming flaccid then plasmolysed

Question 35

what happens to a plant cell which has a lower water potential inside the cell than outside the cell?

Answer 35

water will move in by osmosis, making the cell turgid, the cell wall prevents lysis