cell membranes

Question 1

extrinsic proteins

Answer 1

these are found on either surface of the membrane

Question 2

intrinsic proteins

Answer 2

these are found within the membrane

Question 3

How is a bilayer formed

Answer 3

• due to the hydrophillic polar head and hydrophobic fatty acid tails phospholipids form a bi layer in water
• polar heads face outwards interacting with water outside the cell and inewards interacting with water in the cytoplasm

Question 4

What does the cell membrane consist of and give descriptions of the what type of protein it is and what they do

Answer 4

• phospholipids arrange themselves ito a bilayer with proteins scattered through
• some proteins are extrinsic and found on the surface of the bilayer
• acting as receptors for hormons and recognition sites
• other are instrinsic and extend across both layers acting as channels and carrier proteins for the transport of molecules
• som proteins are enzymes eg ATP synthetase in the cristae or digestive enzymes in the epithelium of the villi

Question 5

What is the fluid mosaic model

Answer 5

membrane structure proposed by singer and nicolson

Question 6

What do membrane in animal cells contain which make them different

Answer 6

cholesterol which stabilises it
glycoproteins acts as antigens meanwhile glycolipids act as receptor sites for molecules such as hormones

Question 7

Describe briefly transport across membranes

Answer 7

• properties of molecules passing across the mebrane will directly affect how they cross it
• non polar molecules and small molecules such as oxygen can dissolve in the fatty acid tails and diffuse across the membrane
• polar molecules ef glucose have to pass via a transport protein because they cannot dissolve in fatty acid tails

Question 8

What is simple diffusion

Answer 8

• example of passive transport
• molecules move rom a high concentration to a low concentration until they are equally distributed

Question 9

What will increase the rate of diffusion

Answer 9

• molecules ae constantly moving due to their kinetic energy any factor that increases this energy or decreases the distance they they have to diffuse will increase the rate of diffusion

Question 10

What is the rate of diffusion affected by

Answer 10

• concentration gradient - the greater the difference in concentration of molecules in two areas the more molecules that can diffuse in a given time so collisions with the membrane are more likely
• diffusion distance - it takes less time for molecules to diffuse a shorter distance
• the surface area of the membrane - larger the area the more molecules that can diffuse in a given time
• the thickness of the exchange surface - takes less time for molecules to diffuse a shorter distance
• an increase in temperature - molecules possess more kinetic energy so they move faster and collide with the membrane more frequently

Question 11

What is the equation of diffusion

Answer 11

(surface area x difference in concentration / length of the diffusion path

Question 12

Why can’t polar molecules pass through the cell membrane

Answer 12

relativley insoluble in lipids

Question 13

What is facilitated diffusion

Answer 13

• is like simple diffusion
• passive porocess requires no ATP
• relying upon kinetic energy of molecules involved
• relies on transport proteins found within the membrane to assist the movement of polar molecules across the membrane
• therfore affected by the same factors as diffusion with one addition - ultimatley the rate determined by the number and availability of the transport proteins involved

Question 14

What are the two types of transport proteins and describe what each one does

Answer 14

• channel proteins - consist of pores with a hydrophillic lining allowing charged ions and polar molecules to pass through they are specific and can be opened and closed to regulate the movement of particular molecules
• carrier proteins - allow diffusion across the mebrane of larger molecules such as sugars and amino acids

Question 15

What is active transport and waht affects it

Answer 15

• requires energy in the form of ATP
• therefore able to transport molecules against the concentration gradient
• anything that affects the respiratory process will affect active transport eg cyanide which is a repiratory inhibitor which will prevent aerobic respiration and the procution of ATP
• active transport cannot occur in the absence of ATP
• active transport utilises carrier proteins that span the membrane therfore maximum rate limited by number and availability of these transport proteins

Question 16

Describe the relationship between active transport and respiratory inhibitors

Answer 16

• maximumm rate of transport can still be reached when the carrier proteins are saturated
• the arate of uptake is reduced with the addiction of a respiratory inhibitor - active transport must be taking place ass the process requires ATP

Question 17

What is co transport

Answer 17

• involves transporting two different molecules together eg glucose and sodium ions and is the mechanism by which glucose is absorbed in the ileum of mammals

Question 18

Describe the mechanism which glucose is absorbed in the ileum of mammals

Answer 18

• sodium ions are actively transported out of the epithelial cells lining the ileum into the blood creating a low concentration of sodium ions within cells
• the high concentration of sodium ions in the lumen of the gut compared to epitherlial cells causes sodium ions to diffuse into the epithelial cells via co transport protein
• and they do so they couple with glucose molecules carrying them with them
• finally glucose molecules pass via facilitated diffusion into blood capillaries and sodium ions by active transport

Question 19

solute

Answer 19

any substance that is dissolved in a solvent

Question 20

water potential

Answer 20

tendency for water to move into or out of a system and is the pressure created by water molecules

Question 21

osmosis

Answer 21

net passive diffusion of water molecules across a selectivley permeable membrane from a region of a higher water potential to a region of lower water potential

Question 22

pressure potential

Answer 22

• represents the pressure exeerted by the cell contents on the cell wall it can be 0KPa or higher

Question 23

turgid

Answer 23

means that the plant cell can hold no more water as the cell wall cannot expand further

Question 24

hypertonic solution

Answer 24

• lower water potential relative to the solution inseide the cell due to the prescence of solutes

Question 25

hypotonic solution

Answer 25

higher water otential relative to the solution inside the cell due to the absence of solutes

Question 26

isotonic solution

Answer 26

has the same water potential relative to teh solution inside the cell

Question 27

How is the water potential loweres

Answer 27

• most cell mebranes are permeable to water and certain solutes only
• when there is a high concenteation of water molecules htey can move about freely when a solute such as glucose is dissolved there are proportionally fewer water molecules so they can move about less freely
• adding solute lowers the potential for water molecules to move - it lowers the water potential making it more negative

Question 28

give examples of water potential

Answer 28

• pure water -0kPa
• typical cell -200kPa
• strong glucose solution -1000kPa

Question 29

Describe what happens when water enters a plant cell via osmosis

Answer 29

• vacoule expands pushing the cytoplasm against the cellulose cell wall
• the cell wall can only expand a little so this creates a resistance to more water entering the plant cell by osmosis
• which is known as pressure potential
• the cell is said to be turgid

Question 30

what is the equation for water potential

Answer 30

• solute potential + pressure potential

Question 31

What happnes is a cell is placed into a solution hypotonic to the cell

Answer 31

• water flows into the cell
• solution has a higher water potential

Question 32

Wbat happens if a cell placed into a solution hypertonic to the cell

Answer 32

• water flows out of the cell
• solution has a lower water potential than the cell

Question 33

What happens if the cell has the same water potential as the surrounding solution

Answer 33

• external solutions and internal solutions are isotonic
• no net water movement in or out of the cell

Question 34

When is a cell flaccid

Answer 34

• when a plant cell placed into a hypertonic solution it loses water by osmosis
• vacoule shrinks and cytoplasm will draw away from the cell wall
• process is plasmolysis and cell is said to be flaccid when complete

Question 35

What is incipient plasmolysis

Answer 35

• the point at which the cell membrane just begins to move away from the cell wall
• the water potenital is now equal to pressure and solutie potentials

Question 36

What happens when the cell is turgid

Answer 36

• cell cannot take in any more water
• cytoplasm pushed against cell wall

Question 37

Why is turgor important in plants

Answer 37

• especially young seedlings
• provides support
• maintains their shape
• holds them upright

Question 38

How can you measure incipient plasmolysis

Answer 38

• estimated by placing plant cells into solutions with varying solute potentials and then looking at the cells under a microscope
• percentage of plasmolysed cells calculated using formula
• (number of plasmolysed cells/total number of cells observed) x100
• when percentage plasmolysis is equal to 50% incipient plasmolyis has been reached the external sucroe concentration must be equal to internal solute concentration of onion tissue - no net movement of water overall

Question 39

Describe how osmosis affects animal cells

Answer 39

• animal cells lack a cell wall therfore cannot sustain pressure potential
• no pressure potential can exist
• water potential = solut potential

Question 40

What happens when red blood cells placed into a hypotonic solution

Answer 40

• water enters by osmosis and they bust
• haemolysis

Question 41

What happens when red blood cell placed into a hypertonic solution

Answer 41

• water passes out of the cells
• crenated

Question 42

How can you investigate permeability of membranes

Answer 42

• using beetroot which has vacoules containing a red pigment called betacyanin
• rate that betacyanin diffuses out of the vacoule through its membrane affected by a number of factors including temperature and prescence of organic solvents
• beetroot disc of equal size and volumes cut using a borer
• then washed and blooted dry
• discs placed into water and the quantity of betwacyanin that leaked through the membrane measured using a colorimenter
• experiment epeated at different temperatures

Question 43

What is bulk transport

Answer 43

• cell transports materials in bulk into the cell (endocytosis) or out of the cell (exocytosis)
• endocytosis involved the engulfing of material by infolding of the plasm mebrane bringing it into the cell enclosed within a vesicle

Question 44

What is exocytosis

Answer 44

• refers to substances leaving the cell after being transported through the cytoplasm in a vesicle
• digestive enzymes often secreted this way

Question 45

What are the two type of endocytosis

Answer 45

• phagocytois
• pinocytosis

Question 46

What is phagocytosis

Answer 46

proces by which the cell can obtain solid materials that are too large to be taken in by other methods
eg phagocytes (white blood cells) destroy bacteria and remove cell debris by phagocytosis and this is how amoeba feed

Question 47

What is pinocytosis

Answer 47

• proces by which the cell can obtain liquid material similar to phagocytosis but vesicles produced are smaller