Transport Across Membranes and Digestion

Question 1

Fluid mosaic model of cell membrane

Answer 1

Dynamic structure consisting of different molecules constantly moving in relation to each other

Question 2

Reasons for membranes in cells

Answer 2

• control entry and exit of materials (barrier)
• control cell signalling
• site of chemical reactions
• isolate enzymes/ separate organelles from cytoplasm so reactions can occur

Question 3

Describe phospholipid layer

Answer 3

• form a bilayer
• hydrophilic heads are attracted to water so face outwards
• hydrophobic heads repel water so face inwards

Question 4

Role of glycoproteins/lipids in cell membrane

Answer 4

• cell recognition sites
• provide stability to membrane
• helps cells attach to form tissues

Question 5

Intrinsic Protein

Answer 5

• span complete membrane
• protein channels allow diffusion of water soluble ions
• carrier proteins bind to ions and molecules (e.g. glucose) and change their shape to allow their passage

Question 6

Extrinsic Protein

Answer 6

• found on either side of cell membrane
• mechanical support
• cell receptors

Question 7

Role of cholesterol in cell membrane

Answer 7

• strength
• ensures membrane in less fluid at high temperatures
• membrane less soluble so prevents leakage of water/dissolved ions from cell

Question 8

Reasons molecules do not diffuse freely through cell membrane

Answer 8

• not lipid soluble or are polar

- too large

Question 9

Simple Diffusion

Answer 9

• net movement of molecules from high to low concentration (until equilibrium reached)
• passive (no ATP)
• no carrier proteins / protein channels
• very small, lipid soluble substances e.g. gases

Question 10

Facilitated Diffusion

Answer 10

• net movement of molecules from high to low concentration (until equilibrium reached)
• passive (no ATP)
• carrier proteins/protein channels
• large, polar molecules/charged ions
• binds to carrier proteins by complementarity of shape or pass through selective channels

Question 11

Osmosis

Answer 11

• net movement of water molecules from a high to low water potential across a partially permeable membrane
• passive (no ATP)

Question 12

Active Transport

Answer 12

Movement of molecules from a low to high concentration

• using carrier proteins
• requires (hydrolysis of) ATP (active process)
• large, polar molecules (e.g. glucose) /charged ions

Question 13

Hypertonic

Answer 13

High solute concentration and high osmotic pressure

Question 14

Hypotonic

Answer 14

Low solute concentration and low osmotic pressure

Question 15

Describe active transport across cell membrane

Answer 15

• molecule binds to carrier protein
• ATP attaches to carrier protein and donates phosphate group
• addition of phosphate group changes tertiary structure of carrier protein
• molecule released moving against concentration gradient
• phosphate group leaves and protein returns to original shape

Question 16

Adaptations of villi to increase absorption

Answer 16

• microvilli to increase surface area
• thin walls for short diffusion path
• rich blood supply to maintain concentration gradient

Question 17

Explain reason for active transport of glucose and amino acids across ileum

Answer 17

• maximum absorption of glucose and amino acids

- since not all glucose and amino acids absorbed by facilitated diffusion since equilibrium is reached

Question 18

Describe co-transport of glucose into ileum

Answer 18

• Na+ actively transported out of cells into capillary by sodium-potassium pump
• low Na+ concentration in cell means it diffuses by facilitated diffusion back into cells from lumen of ileum carrying either glucose or amino acids with it (co-transport=passive)
• glucose diffuses into capillary due to high concentration in cell

Question 19

Digestion

Answer 19

• large molecules are hydrolysed by enzymes

- forming smaller soluble molecules that can be absorbed into the blood through the ileum

Question 20

Describe physical digestion

Answer 20

Teeth break down food to increase surface for chemical digestion and stomach muscles churn food

Question 21

Endopeptidase

Answer 21

Enzymes found in stomach that hydrolyse INTERNAL peptide bonds between amino acids in a polypeptide / from shorter chains

Question 22

Exopeptidase

Answer 22

Enzymes produced in pancreas and used in duodenum that hydrolyse peptide bonds on terminal amino acids of a polypeptide

Question 23

Dipeptidase

Answer 23

Membrane bound enzymes found ileum that hydrolyses bond between amino acids in a dipeptide

Question 24

Purpose of endopeptidase

Answer 24

Speeds up digestion because it provides more ends for action of exopeptidase

Question 25

Amylase

Answer 25

Enzyme produced in salivary glands/pancreas and found in mouth and stomach that digests starch into maltose

Question 26

Maltase

Answer 26

Membrane-bound found in the small intestine that digests maltose into glucose

Question 27

Lipase

Answer 27

Enzyme produced in pancreas and used in duodenum that digests triglycerides into fatty acids and glycerol

Question 28

Bile

Answer 28

- produced in liver and stored in gall bladder - emulsifies fats thus increasing surface area for action of lipase - neutralises stomach acid

Question 29

Describe digestion of triglycerides

Answer 29

- lipid globules are emulsified by bile to form smaller emulsion droplets - triglycerides are hydrolysed by pancreatic lipase into monoglycerides and fatty acids - these associate with bile salt and phospholipids to form even smaller micelles - lipid soluble and small enough to pass between microvilli - monoglycerides and fatty acids move out of micelles and enter cells by SIMPLE diffusion

Question 30

Suggest why rehydration therapy hydrates the body better than water alone

Answer 30

Contains glucose which increases uptake of Na+ ions so more water enters cells by osmosis than it would otherwise

Question 31

Suggest why cholesterol and many other lipids are incorporated into the cell membranes of cells

Answer 31

- provides stability to cell membrane (less fluid) | - accelerates diffusion of non polar substances, e.g. O2 and CO2

Question 32

Describe the role of micelles in lipid absorption

Answer 32

- transport poorly lipid soluble monoglycerides and fatty acids to SURFACE of epithelial cell - small enough to pass between microvilli

Question 33

Describe action of membrane-bound dipeptidases and explain their importance

Answer 33

- hydrolyse peptide bond in dipeptide to release amino acids - amino acids can cross cell-surface membrane but dipeptides cannot - maintain concentration gradient of amino acids/ maximum break down of dipeptides

Question 34

Explain the difference between facilitated diffusion with carrier proteins and channel proteins

Answer 34

- substrate binds to carrier protein by complementarity of shape and changes shape so substrate transported to other side of membrane - selective channel protein allows passage of mostly ions based on size and charge

Question 35

Compare and contrast carrier proteins and protein channels

Answer 35

- both intrinsic proteins - both involved in facilitated diffusion - both transport large, polar molecules and charged ions - only carrier proteins involved in active transport - molecule/ion must bind to carrier protein which changes shape to transport substrate - molecule/ion passes through water filled pathway in protein channels - substrate must be complementary to carrier protein where molecule/ion must be certain size or charge to pass through channel protein

Question 36

Describe how sodium-potassium pump works

Answer 36

- active transport - carrier protein which binds to three Na+ ions and molecule of ATP - ATP hydrolysed which induces change in shape of carrier protein so Na+ released across membrane - two potassium ions bind to newly shaped carrier protein on opposite side - release of phosphate reverts carrier protein to original shape so potassium ions are released across membrane

Question 37

Co-transport

Answer 37

movement of one molecule with its concentration gradient and another against its concentration gradient

Question 38

Explain why higher temperatures result in more leakage out of cells

Answer 38

- damage cell-surface membrane / denature proteins | - increase fluidity