(2) cells: transport across cell membrane

Question 1

what are the functions of a cell membrane?

Answer 1

keep all cellular components inside the cell
allows selected molecules to move in and out the cell
isolates organelles from the rest of the cytoplasm, allowing cellular processes to occur separately
site for biochemical reactions
allows a cell to change shape

Question 2

what is the structure of a phospholipid and how are they structured in water?

Answer 2

two hydrophobic “tails,” which are fatty acid chains, and one hydrophilic “head,” which is phosphate group
bilayer / Micelle (circle)
hydrophilic heads face H2O and hydrophobic tails point inwards

Question 3

what is the role of phospholipids in a cell membrane?
how does size and polarity affect the diffusion rate?

Answer 3

to act as a barrier to most substances helping control what enters/exists the cells

the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane

Question 4

how do
a) small, non polar molecules
b) small polar molecules
c) charged particles (ions)
diffuse across a cell membrane

Answer 4

a) e.g O2 n CO2 rapidly diffuse across a membrane
b) e.g water n urea diffuse but much more slowly than small non polar molecules
c) unlikely to diffuse across a membrane

Question 5

what are the structural components in a cell membrane?

Answer 5

glycolipid
glycoprotein
cholesterol
internal protein
peripheral protein
phospholipid
pore
filaments of cytoskeleton

Question 6

what is the role of cholesterol in the cell membrane?

Answer 6

reduces lateral movement of other molecules including phospholipids (pulls together the fatty acid tails of the phospholipids)
controls membrane fluidity (the more C, the less fluid)
keeps membranes stable at normal body temperature (wo it the cell would burst open)

Question 7

where are the integral and peripheral proteins located?

Answer 7

integral (intrinsic) p spans the whole width of the membrane
peripheral (extrinsic) confined to inner/outer surface of the membrane

Question 8

what are the functions of integral protein

Answer 8

carrier molecules/channels
help transport substances such as ions sugars and amino acids, that cannot across the membrane
receptors for hormones and neurotransmitters
receptors for enzymes for catalysing reactions

Question 9

what is the role peripheral proteins?

Answer 9

free on the membrane surface or bound to integral protein
extracelluar side: acts as receptors for hormones or neurotransmitters or are involved in cell recognition

cytosolic side: involved in cell signalling or chemical reactions, can dissociate from the membrane and move into the cytoplasm

provide mechanical support
connected to carbs to make glycoproteins or bind to lipids to make glycolipids

Question 10

what is the role of glycolipids?

Answer 10

made from a carb covalently bonding w a lipid
carb portion extends from bilayer into environment outside of cell acting as a cell surface receptors for specific chemicals

acts as recognition site
help maintain the stability of the membrane
hells cells to attach to one another and so form tissues

Question 11

what is the role of glycoproteins?

Answer 11

carb chains attached to extrinsic proteins, these glycoproteins also act as cell surface receptors for hormones and neurotransmitters

act as recognition sites
helps cells to attach to one another and so form tissues
allows cells to recognise one another

Question 12

why is membrane fluidity important?

Answer 12

need to maintain fluidity or else the cell would not be able to function
needed for:

Diffusion of substances across the membrane
membranes to fuse (e.g vesicles fluidity w the cell memory during exocytosis)
cells to move and change shape

Question 13

what factors affect membrane fluidity?

Answer 13

temperature
cholesterol
fatty acids

Question 14

what happens when there is an increase in temp?

Answer 14

lipids have more kinetic energy and are packed together more loosely, increasing fluidity

Question 15

what happens if there is a decrease in temp?

Answer 15

lipids have less kinetic energy and are packed together more tightly, decreasing fluidity

Question 16

what happens when there is an increase in cholesterol?

Answer 16

at body temp, cholesterol interacts with the 2 tails of the phospholipids, decreasing fluidity

Question 17

what happens when there is a decrease in cholesterol?

Answer 17

a low level of cholesterol con make the membrane too fluid, increasing the risk of the cell bursting

Question 18

what happens when the fatty acid chains are long and saturated?

Answer 18

lipids w long and saturated tails have a high melting point
the straight unlinked fatty acid chains mean the lipid molecules can pack together more tightly, which decreases fluidity

Question 19

what happens when the fatty acid chains are short and unsaturated?

Answer 19

lipids w short unsaturated tails have a low melting point
the double bonds in the chain creates kinks meaning the lipid molecules are more loosely lacked, increasing fluidity

Question 20

what is the definition of diffusion?

Answer 20

the net movement of particles down a concentration gradient from a region of high to low concentration

is a passive transport, meaning it does not require the use of ATP

only small non-polar molecules e.g O2 CO2

Question 21

what factors affect the rate of diffusion?

Answer 21

temperature
membrane thickness/diffusion difference
surface area
concentration

Question 22

how does temperature affect the R of diffusion?

Answer 22

increase: particles gain kinetic energy and increase the movement of the molecules

Question 23

how does membrane thickness affect the R of diffusion?

Answer 23

increase in thickness means there is a larger diffusion distance so the particles require more time to travel, therefore there is a decrease in the rate

Question 24

how does surface area affect the rate of diffusion?

Answer 24

a larger surface area means more particles can cross the membrane at the same time as there is more space

Question 25

how does concentration gradient affect the rate of reaction

Answer 25

the higher the concentration gradient, the more particles can change sides

Question 26

what is facilitated diffusion?

Answer 26

the transport of polar molecules and ions into and out of the cell using carrier and channel proteins
(polar molecules cannot diffuse over the membrane because of the hydrophobic barrier)

Question 27

How are channel proteins used in facilitated diffusion?

Answer 27

they form water-filled hydrophilic channels across the membrane and allow specific water-soluble ions to pass through
selective to specific ions: the ions bind with the protein causing it to change shape that closes one side of the membrane and opens to the other side

Question 28

how are carrier proteins used in faciilitated diffusion?

Answer 28

when a molecule specific to the protein is present, it binds to the protein, causing it to change shape in a way that the molecule is released to the inside of the membrane and the molecules move from a region of higher concentration to a lower one, using their own kinetic energy

Question 29

what is osmosis?

Answer 29

the movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane

Question 30

what is water potential (ψ)?

Answer 30

the tendency of water molecules in a system to move
measured in kilopascals (kPa)

Question 31

how does the amount of solute affect the water potential?

Answer 31

the greater the amount of solute, the lower the water potential
water molecules bind to the solute molecules, reducing the amount of water molecules that are free to diffuse

Question 32

how to calculate water potential?

Answer 32

ψ = ψs + ψp

water potential = solute potential ( negative value) + pressure potential (positive value)

Question 33

plant cell: how does a hypertonic solution affect the ψ and the cell overall?

Answer 33

the ψ is LOWER outside than the inside so water will move out of the cell
the cytoplasm will pull away from the cell wall and the cell is plasmolysed

Question 34

plant cell: how does a hypotonic cell affect the ψ and the cell overall?

Answer 34

the ψ outside the cell is HIGHER than inside the cell therefore water will enter the cell
the cytoplasm will push against the cell wall and the cell will be turgid

Question 35

animal cell: how does a hypertonic solution affect the ψ and the cell overall?

Answer 35

the ψ outside of the cell is LOWER than the cell cytoplasm so water will move out of the cell
the cell becomes crenated as the cytoplasm pulls away from the cell membrane

Question 36

animal cell: how does a hypotonic solution affect the ψ and the cell overall?

Answer 36

the ψ outside is HIGHER than the inside of the cell therefore the water moves into the cell
as the cell has no protective cell wall, lysis takes place (cell internally bursts)

Question 37

what is active transport?

Answer 37

movement of molecules across a biological membrane from an area of low concentration to an area of high concentration against the concentration gradient

uses carrier proteins to pump the substances across the membrane
requires energy in the form of ATP ( produced by respiration and mitochondria)

Question 38

what are examples of active transport?

Answer 38

• uptake of glucose and amino acids in the small intestine
• absorption of mineral ions by plant roots
• excretion of hydrogen ions and urea by the kidney
• exchange of sodium and potassium ions in muscle cells

Question 39

how does active transport work?

Answer 39

the phosphate group from the ATP attaches to the carrier protein, causing it to change shape and as it does, it pumps the molecule/ion into the cell

Question 40

what is bulk transport?
and what are the 2 types?

Answer 40

when extremely large substances need to be moved across a cell membrane

endocytosis: bulk transport of material into the cell

exocytosis: bulk transport of material out of the cell

Question 41

what are the 3 processes of endocytosis?

Answer 41

phagocytosis
pinocytosis
receptor-mediated endocytosis

Question 42

what is the process of phagocytosis?

Answer 42

1. cell membrane forms pseudopods which surround the material to be taken into the cell
2. pseudopods fuse together around the material, fully enclosing it in a vesicle
3. the vesicle is then transported within the cell

Question 43

what is the process of pinocytosis?

Answer 43

smaller-sized particles are taken in
vesicles are formed by the cell membrane without the creation of pseudopods

Question 44

what is the process of receptor-mediated endocytosis?

Answer 44

the molecules to be transported into the cell bind to the receptors on the surface of the membrane
one all the receptors are bound, the membrane folds inwards, forming a protein coated pit

Question 45

how is glucose absorbed into the mammalian ileum?

Answer 45

• sodium ions are actively transported out of the ileum epithelial cells into the blood by the – sodium potassium pump (creates a concentration gradient - higher conc of NA ions in the lumen of ileum than inside the cell)
• causes Na ions to diffuse from lumen into epithelial cell via sodium-glucose co transporter proteins
• co-transporters carry glucose into the cell with sodium -> conc of glucose in cell increases
• glucose diffuses out of the cell into blood down the conc gradient by facilitated diffusion