Transport Across Membranes and Digestion Flashcards
Fluid mosaic model of cell membrane
Dynamic structure consisting of different molecules constantly moving in relation to each other
Reasons for membranes in cells
- control entry and exit of materials (barrier)
- control cell signalling
- site of chemical reactions
- isolate enzymes/ separate organelles from cytoplasm so reactions can occur
Describe phospholipid layer
- form a bilayer
- hydrophilic heads are attracted to water so face outwards
- hydrophobic heads repel water so face inwards
Role of glycoproteins/lipids in cell membrane
- cell recognition sites
- provide stability to membrane
- helps cells attach to form tissues
Intrinsic Protein
- 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
Extrinsic Protein
- found on either side of cell membrane
- mechanical support
- cell receptors
Role of cholesterol in cell membrane
- strength
- ensures membrane in less fluid at high temperatures
- membrane less soluble so prevents leakage of water/dissolved ions from cell
Reasons molecules do not diffuse freely through cell membrane
- not lipid soluble or are polar
- too large
Simple Diffusion
- 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
Facilitated Diffusion
- 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
Osmosis
- net movement of water molecules from a high to low water potential across a partially permeable membrane
- passive (no ATP)
Active Transport
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
Hypertonic
High solute concentration and high osmotic pressure
Hypotonic
Low solute concentration and low osmotic pressure
Describe active transport across cell membrane
- 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
Adaptations of villi to increase absorption
- microvilli to increase surface area
- thin walls for short diffusion path
- rich blood supply to maintain concentration gradient
Explain reason for active transport of glucose and amino acids across ileum
- maximum absorption of glucose and amino acids
- since not all glucose and amino acids absorbed by facilitated diffusion since equilibrium is reached
Describe co-transport of glucose into ileum
- 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
Digestion
- large molecules are hydrolysed by enzymes
- forming smaller soluble molecules that can be absorbed into the blood through the ileum
Describe physical digestion
Teeth break down food to increase surface for chemical digestion and stomach muscles churn food
Endopeptidase
Enzymes found in stomach that hydrolyse INTERNAL peptide bonds between amino acids in a polypeptide / from shorter chains
Exopeptidase
Enzymes produced in pancreas and used in duodenum that hydrolyse peptide bonds on terminal amino acids of a polypeptide
Dipeptidase
Membrane bound enzymes found ileum that hydrolyses bond between amino acids in a dipeptide
Purpose of endopeptidase
Speeds up digestion because it provides more ends for action of exopeptidase
Amylase
Enzyme produced in salivary glands/pancreas and found in mouth and stomach that digests starch into maltose
Maltase
Membrane-bound found in the small intestine that digests maltose into glucose
Lipase
Enzyme produced in pancreas and used in duodenum that digests triglycerides into fatty acids and glycerol
Bile
- produced in liver and stored in gall bladder
- emulsifies fats thus increasing surface area for action of lipase
- neutralises stomach acid
Describe digestion of triglycerides
- 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
Suggest why rehydration therapy hydrates the body better than water alone
Contains glucose which increases uptake of Na+ ions so more water enters cells by osmosis than it would otherwise
Suggest why cholesterol and many other lipids are incorporated into the cell membranes of cells
- provides stability to cell membrane (less fluid)
- accelerates diffusion of non polar substances, e.g. O2 and CO2
Describe the role of micelles in lipid absorption
- transport poorly lipid soluble monoglycerides and fatty acids to SURFACE of epithelial cell
- small enough to pass between microvilli
Describe action of membrane-bound dipeptidases and explain their importance
- 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
Explain the difference between facilitated diffusion with carrier proteins and channel proteins
- 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
Compare and contrast carrier proteins and protein channels
- 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
Describe how sodium-potassium pump works
- 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
Co-transport
movement of one molecule with its concentration gradient and another against its concentration gradient
Explain why higher temperatures result in more leakage out of cells
- damage cell-surface membrane / denature proteins
- increase fluidity
