1.3-the cell membrane

Question 1

What does the cell membrane appear as under the electron microscope?

Answer 1

A double line

This characteristic appearance is crucial for identifying cell membranes in microscopy.

Question 2

What is the width of the cell membrane?

Answer 2

7-8 nm

This measurement is consistent across different organisms.

Question 3

what are the functions of the cell membrane?

Answer 3

-the cell surface membrane is the boundary that separates the living cell from its non living surroundings
-the membrane controls which substances pass into and out of the cell

Question 4

what is the cell membrane made of?

Answer 4

made up of almost entirely phospholipids and proteins

Question 5

what are phospholipid?

Answer 5

-they can form a bilayer
-the phosphate head is a polar molecule (hydrophilic)
-the 2 fatty acid tails are non polar (hydrophobic)
-phospholipid bilayers are the basis of a membrane structure

Question 6

what are membrane proteins?

Answer 6

proteins within the membrane that are arranged randomly

Question 7

what are extrinsic proteins?

Answer 7

-occur on the surface of the bilayer/partly embedded into it
-they provide structural support
-form recognition sites by identifying cells

Question 8

what are intrinsic proteins?

Answer 8

proteins that go through the phospholipid bilayer
-some act as channels or carries to facilitate the diffusion of polar molecules (ions) across the membrane
-other proteins form pumps and carry out active transport against a concentration gradient

Question 9

what did Singer and Nicholson propose?

Answer 9

The Fluid Mosaic model

Question 10

why is the model called fluid mosaic?

Answer 10

-the phospholipid bilayer is capable of movement
-components of the membrane are free to move with respect to each other
-the proteins are dotted throughout the bilayer in a mosaic arrangement

Question 11

describ the function of choleseral

Answer 11

-found in animal cells
-fits between phospholipid molecules
-increases the rigidity and stability of the membrane

Question 12

describe the function of glycolipids

Answer 12

-lipids which have combined with polysaccharides
-found in the outer layer of the membrane and are involved in cell to cell recognition

Question 13

describe glycoproteins

Answer 13

-proteins combined with polysaccharides
-stick out of some membranes

Question 14

how do non-polar molecules pass through the membrane?

Answer 14

small uncharged molecules like oxygen or carbon dioxide freely pass through the membrane by simple diffusion
-lipid soluble molecules such as glycerol can also pass through the membrane

Question 15

how do polar molecules pass through the phospholipid bilayer?

Answer 15

-the hydrophobic core of the membrane prevents the transport of ions and polar molecules
-charged particles (ions) and relatively large charge molecules (glucose) cannot diffuse through the non polar center
-intrinsic proteins allow these particles to cross the membrane

Question 16

how do channel proteins facilitate diffusion?

Answer 16

-consist pores lined with polar groups (hydrophilic) which allow charged particles to pass through
-each channel protein is specific for one type of ion
-they can open and close depending on the needs of the cell (gated channels)

Question 17

how do carrier proteins facilitate diffusion?

Answer 17

-allow facilitated diffusion across the membrane of larger polar molecules (sugars/amino acids)
-a particle attaches to a carrier protein at its binding site causing the carrier protein to change shape/rotate within the membrane. this realeases he molecule on the other side of the membrane

Question 18

what is co-transport?

Answer 18

-a type of facilitated diffusion that brings molecules and ions into cells on the same carrier protein
-e,g, sodium-glucose co-transport is significant in absorbing glucose and sodium ions across the cell membranes and into te blood in the ileum/kidney nephron

Question 19

what is active transport?

Answer 19

-process where ATP is required (affected by a respiratory inhibitor)
-ions and molecules are moved across the membrane against the concentration gradient

Question 20

how are molecules actively transported across the membrane?

Answer 20

-molecules/ions combine with a specific intrinsic protein called a pump
-ATP transfers a phosphate group to the pump causing the pump to change shape and transport the molecules to the other side

Question 21

what processes involve active transport?

Answer 21

protein synthesis
muscle contraction
nerve impulse transmission

Question 22

describe the graph of concentration difference across the membrane across the rate of uptake of active transport

Answer 22

-there is an initial increased rate of uptake as the conc gradient increases (due to pumps actively pumping ions across the membrane)
-rate of active transports levels off at higher conc differences (due to pumps being occupied -limits the rate of active transport)
-if a respiratory inhibitor is added the rate is also affects as ATP production is stopped

Question 23

what is osmosis?

Answer 23

the passage of water from a region of high water potentials to a region of low water potentials through a selectively permeable membrane

Question 24

what does water potential mean?

Answer 24

tendency of water molecules to move from a high to low concentration

Question 25

what is the water potential of pure water?

Answer 25

0

Question 26

what happens to the water potential when a solute like sugar dissolves in water?

Answer 26

becomes more negative

Question 27

what is the equation to work out the cells water potential?

Answer 27

Ψcell=Ψsolute+Ψpressure

Question 28

how is the potential of the cell affected by different factors?

Answer 28

-the presence of solutes in the vacuole lowers the cells water potential -the concentration of dissolved substances inside the cell vacuole is called the solute potential (always negative value) -when water enter the vacuole by osmosis a hydro-static pressure is set up and pushes outwards on the cell wall -as the outward pressure builds up the cell wall develops an opposing force called the pressure potential (usually positive)

Question 29

how is hydro-static pressure generated?

Answer 29

cell contents push against the cell wall due to water entering the cell -this is called pressure potential

Question 30

what is a hypo-tonic external medium?

Answer 30

id the water potential of the external solution is hypo-tonic water will move into the cell -cells will swell -animal cells may burst (lysis) -plant cells will become turgid as the cell contents are pushed against the cell wall

Question 31

what is a hyper-tonic external medium?

Answer 31

if the water potential of the external solution is hyper-tonic then water will move out of the cell -animal cells will shrink -plant cells the vacuole and cytoplasm shrink and the membrane pulls away from the wall. the cell is now plasmolysed and flaccid. the plant will wilt

Question 32

what is plasmolysis?

Answer 32

when the water leaves the plant cell causing the vacuole and cytoplasm to shrink and the membrane to pull away from the wall

Question 33

what is an isotonic external medium?

Answer 33

an isotonic solution has the same water potential as the cell contents so there will be no net movement of water -in animal cells there is a natural isotonic medium (homeostasis maintains this) -in plant cells the pressure potential and solute potential are equal. this is the point of incipient plasmolysis. cells become flaccid

Question 34

describe the potato cylinders at different sucrose concentrations (0.2)

Answer 34

-cells are fully turgid ( -an increase in mass is caused by water entering the potato cells by osmosis from a high water potential in the external medium to a lower water potential in the potato cells down a water potential gradient across the partially permeable membrane

Question 35

describe the potato cylinders at different sucrose concentrations (0.3M)0

Answer 35

-incipient plasmolysis -no change in mass at this point. water potential on either side of the membrance must be equal. no net movement of water the solute potential of the external medium is equivalent to the solute potential of the cell. point of incipient plasmolysis

Question 36

describe the potato cylinders at different sucrose concentrations (1.0M)

Answer 36

-potato cells are fully plasmolysed -a decrease in mass is caused by water leaving the potato cells by osmosis from a high water potential in the cells to a lower water potential in the external medium down the water potential gradient across a partially permeable membrane

Question 37

what are the two type os endocytosis?

Answer 37

phagocytosis (cell eating) and pinocytosis (entry of liquid into the cell)