6.1 Digestion Flashcards
What are the 2 major groups of organs which comprise the human digestive system?
alimentary canal and accessory organs
What does the alimentary canal consist of?
The alimentary canal consists of organs through which food actually passes (oesophagus, stomach, small & large intestine)
What are the accessory organs?
salivary glands, pancreas, liver, gall bladder
What is the difference between alimentary canal an accessory organs?
Food passes through alimentary canal, but accessory organs aid in digestion but do not actually transfer food
What is the structure of the oesophagus?
A hollow tube connecting the oral cavity to the stomach (separated from the trachea by the epiglottis)
What is the role of the oesophagus?
Food is mixed with saliva and then is moved in a bolus via the action of peristalsis
What is the structure of the stomach?
A temporary storage tank where food is mixed by churning and protein digestion begins
What is the stomach lined with?
It is lined by gastric pits that release digestive juices, which create an acidic environment (pH ~2)
What is the structure and role of the small intestine?
A long, highly folded tube where usable food substances (nutrients) are absorbed
What are the 3 sections of the small intestine?
Consists of three sections – the duodenum, jejunum and ileum
What is the structure of the large intestine?
Consists of the ascending / transverse / descending / sigmoidal colon, as well as the rectum
What is the role of the large intestine?
The final section of the alimentary canal, where water and dissolved minerals (i.e. ions) are absorbed
What is the structure of the salivary glands?
Salivary glands include the parotid gland, submandibular gland and sublingual gland
What is the role of the salivary glands?
Release saliva to moisten food and contains enzymes (e.g. amylase) to initiate starch breakdown
What is the role of the pancreas?
Produces a broad spectrum of enzymes that are released into the small intestine via the duodenum
What, apart from enzymes, does the pancreas secrete?
Also secretes certain hormones (insulin, glucagon), which regulate blood sugar concentrations
What is the overall role of the liver?
Takes the raw materials absorbed by the small intestine and uses them to make key chemicals
What are the 5 main functions of the liver?
Its role includes detoxification, storage, metabolism, bile production and haemoglobin breakdown
What is the role of the gall bladder?
The gall bladder stores the bile produced by the liver (bile salts are used to emulsify fats)
Where is bile stored and where is it released into?
Bile stored in the gall bladder is released into the small intestine via the common bile duct
Learn how to draw the digestive system
How is food digested? (methods)
Food can be digested by a combination of two methods – mechanical digestion and chemical digestion
What is mechanical digestion?
In mechanical digestion, food is physically broken down into smaller fragments via the acts of chewing (mouth), churning (stomach) and segmentation (small intestine)
Where is food initially broken down mechanically?
Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
How does the bolus move to the stomach?
The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
How does the bolus go down the oesophagus and not the trachea?
The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity
What is churning?
The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices (‘churning’)
What happens to the food after prolonged churning?
Food is digested within the stomach for several hours and is turned into a creamy paste called chyme
Where does the chyme go?
Eventually the chyme enters the small intestine (duodenum) where absorption will occur
What is peristalsis?
Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
How does peristalsis work?
Continuous segments of longitudinal smooth-muscle rhythmically contract and relax
How (general) does food move?
Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
What does segmentation involve? (how does it work)
Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
What is the purpose of segmentation?
Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices
Is segmentation effective?
While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement
What is chemical digestion?
In chemical digestion, food is broken down by the action of chemical agents (such as enzymes, acids and bile)
What does the stomach contain?
The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
Why does the stomach have an acidic environment?
The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
What lines the stomach and what is its purpose?
The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
What pH compounds does the pancreas secrete and why?
The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine
Where is bile produced and stored?
The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
What does bile contain and what is its role?
Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
What does the emulsification of fats cause?
The emulsification of fats increases the total surface area available for enzyme activity (lipase)
What are enzymes?
Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
Why are enzymes needed for digestion?
Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
Why is it important that enzymes are specific?
Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations
What secretes digestive enzymes?
Digestive enzymes are secreted predominantly by the pancreas, although other organs also contribute (salivary gland, stomach)
What does the type of enzyme secreted and location depend on?
the type of enzyme secreted and location of secretion depends on the specific macromolecule required for hydrolysis
Where does carbohydrate digestion first occur?
Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
Where is amylase secreted from (apart from salivary glands)?
Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
Where are enzymes used for disaccharide hydrolysis usually located?
Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
Can humans digest cellulose? why?
NO
Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested
Where does protein digestion begin?
Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
What type of proteins enters the small intestine and what breaks them down?
Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
What is the optimum for endopeptidases?
These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine
Where does lipid breakdown occur?
Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
What breaks down lipids after emulsification?
The smaller fat droplets are then digested by lipases released from the pancreas
What does the pancreas secrete, apart from digestive enzymes?
The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides
What is the role of human intestines?
The human intestines function to absorb the products of digestion and have specialised structures to fulfil this function
What is the small intestine composed of? (4)
serosa
muscle layer
submucosa
mucosa
What is the serosa?
a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
What is the muscle layer?
outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
What is the submucosa?
composed of connective tissue separating the muscle layer from the innermost mucosa
What is the mucosa?
a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen
How is the inner epithelial intestine arranged?
The inner epithelial lining of the intestine is highly folded into finger-like projections called villi (singular: villus)
What is the role of the villi?
Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption
What features do villi have to facilitate the absorption of digestive products (monomers, ions and vitamins)?
Microvilli Rich blood supply Single-layer epithelium Lacteals Intestinal glands Membrane proteins
Mr Slim
How do microvilli facilitate absorption of digestive products?
Ruffling of epithelial membrane further increases surface area
How does a rich blood supply facilitate absorption of digestive products?
Dense capillary network rapidly transports absorbed products
How does a single layer epithelium facilitate absorption of digestive products?
inimises diffusion distance between lumen and blood
How do intestinal glands facilitate absorption of digestive products?
Exocrine pits (crypts of Lieberkuhn) release digestive juices
How do lacteals facilitate absorption of digestive products?
Absorbs lipids from the intestine into the lymphatic system
How do membrane proteins facilitate absorption of digestive products?
Facilitates transport of digested materials into epithelial cells
What 4 features does the epithelial lining of villi to optimise capacity to absorb digested materials?
tight junctions
microvilli
mitochondria
pinocytosis vesicles
What are tight junctions?
Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
What is the role of tight junctions?
They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement
What is the role of microvilli?
Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
What may be embedded upon microvilli?
The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake
What type of organelle do epithelial cells of intestinal villi have a lot of?
Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
Why do intestinal cells have a lot of mitochondria?
ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis
What is pinocytosis?
Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
How will liquids be absorbed by pinocytosis?
These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle
During absorption, where must digested food molecules go?
During absorption, digested food monomers must pass from the lumen into the epithelial lining of the small intestine
What is the only option of where the digested food monomers may go and why?
Tight junctions between epithelial cells occlude any gaps between cells – all monomers must cross the membrane
Do all monomers travel the same way during absorption?
NO
Different monomers undertake different methods for crossing the apical and basolateral membranes
What 4 ways may digested food monomers move across a membrane?
secondary active transport
facilitated diffusion
osmosis
simple diffusion
How does secondary active transport work?
A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)
What is transported via secondary active transport?
Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na+)
What is the role of facilitated diffusion?
Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
Where are channel proteins usually located?
Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)
What is transported by facilitated diffusion?
Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion
What is absorbed via osmosis?
Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)
Where does the absorption of water and dissolved ions occur?
The absorption of water and dissolved ions occurs in both the small and large intestine
How does simple diffusion function? What travels via simple diffusion?
Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane
How are lipids transported once absorbed?
Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood
What does endocytosis involve?
method of bulk transport
Endocytosis involves the invagination of the plasma membrane to create an internal vesicle containing extracellular material
What does vesicle formation during endocytosis require?
Vesicle formation requires the breaking and reforming of the phospholipid bilayer and hence is an energy-dependent process
In the intestines where does endocytosis usually occur?
In the intestines, vesicles commonly form around fluid containing dissolved materials (pinocytosis – cell ‘drinking’)
What is the purpose of pinocytosis?
Pinocytosis allows materials to be ingested en masse and hence takes less time than shuttling via membrane proteins
In what forms can starch be found?
Starch can exist in one of two forms – linear chains (amylose) or branched chains (amylopectin)
What initiates starch digestion? What continues the digestive process?
The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
Why does starch digestion by amylase not occur in the stomach?
Starch digestion by amylase does not occur in the stomach as the pH is unsuitable for amylase activity (optimal pH ~ 7)
What does amylase digest and what does it convert it into?
Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins
What are maltose and dextrin digested by?
Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
What does the hydrolysis of maltose form?
The hydrolysis of maltose / dextrin results in the formation of glucose monomers
What is the purpose of the hydrolysis of glucose?
Glucose can be hydrolysed to produce ATP (cell respiration) or stored in animals as the polysaccharide glycogen
Is the breakdown of maltose the only source of glucose?
NO
Glucose monomers can also be generated from the breakdown of other disaccharides (such as lactose and sucrose)
What are the 2 functions of the pancreas in relation to the digestion of starch? (brief)
produces pancreatic amylase
produces hormones for glucose regulation
What is the pancreas’ role in producing pancreatic amylase?
It produces the enzyme amylase which is released from exocrine glands (acinar cells) into the intestinal tract
What is the pancreas’ role in producing hormones involved in glucose regulation?
It produces the hormones insulin and glucagon which are released from endocrine glands (islets of Langerhans) into the blood
What are the hormones involved in glucose regulation and where do they act?
The hormones insulin and glucagon regulate the concentration of glucose in the bloodstream (controls availability to cells)
What is the role of insulin?
Insulin lowers blood glucose levels by increasing glycogen synthesis and storage in the liver and adipose tissues
What is the role of glucagon?
Glucagon increases blood glucose levels by limiting the synthesis and storage of glycogen by the liver and adipose tissues
What form is most food in?
Most food is solid and in the form of large complex molecules which are insolube and chemically inert (not readily usable)
What two key functions does the process of digestion carry out?
It breaks down insoluble molecules into smaller subunits which can be readily absorbed into body tissues
It breaks down inert molecules into usable subunits which can be assimilated by cells and reassembled into new products
Why most large molecules be broken down into smaller ones?
Cell membranes are impermeable to large molecules (polypeptides, polysaccharides) unless transport is facilitated by proteins
What is used to model absorption?
The size-specific permeability of cell membranes can be modelled using dialysis tubing (Visking tubing)
What pores does dialysis tubing have?
Dialysis tubing contains pores typically ranging from 1 - 10 nm in diameter and is semi-permeable according to molecular size
What can cross dialysis tubing?
Large molecules such as starch cannot pass through the tubing, however smaller molecules (such as maltose) can cross
How does dialysis tubing differ from the membranes of living organisms?
Unlike the membranes of living cells, dialysis tubing is not selectively permeable based on charge (ions can freely cross)
What is peristalsis?
Peristalsis is series of muscle contractions in the walls of the oesophagus or small intestine that pass like a wave along the alimentary canal
What is the purpose of peristalsis?
This wave forces the bolus of food along the alimentary canal
What controls peristalsis?
These contractions are controlled unconsciously by the autonomic nervous system
What two muscles are involved in peristalsis?
Peristalsis is controlled by circular and longitudinal muscles
What do circular muscles do in peristalsis?
These muscles are smooth muscle (not striated)
Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine
What is the purpose of circular muscles in peristalsis?
This prevents the food moving backwards towards the mouth
What do longitudinal muscles do in peristalsis?
Longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine
What is the purpose of longitudinal muscles in peristalsis?
This forces the food forwards through the alimentary canal
What happens when the bolus is moved via peristalsis? Where does it end up?
Once the bolus has reached the stomach, it is churned into a less solid form, called chyme, which continues on to the small intestine
What helps the movement of the bolus in peristalsis?
Mucus is produced to continually lubricate the food mass and reduce friction
Where is peristalsis faster?
In the small intestine peristalsis is slow compared to the peristalsis that occurs in the oesophagus.
What is the purpose of peristalsis in the small intestine?
. It also aids digestion by churning up the food with enzymes as it pushes it along the gut
What two types of tissue is the pancreas made up of?
The pancreas is a gland made up of two types of tissue
The first type of tissue secretes the hormones insulin and glucagon into the blood
The second type of tissue synthesises and secretes digestive enzymes into the lumen of the small intestine
How are enzymes produced in the pancreas?
Enzymes are synthesised on the ribosomes of the rough endoplasmic reticulum. They are then processed within the Golgi apparatus before being secreted by exocytosis into the lumen of the small intestine
What stimulates the secretion of pancreatic enzymes?
Secretion of pancreatic enzymes is stimulated by the release of hormones into the stomach and intestines in response to ingestion of food
What is secretion of pancreatic enzymes controlled by? i.e what system
This is an automatic response of the autonomic nervous system
What enzymes are found in pancreatic juice?
The enzymes found in pancreatic juice include amylase, lipase, phospholipase, and protease enzymes.
Where are digestive enzymes produced?
Digestive enzymes are produced in specialised gland cells which are known as acinar cells
How are acinar cells arranged?
These cells are located in clusters around the ends of tubes called ducts
What do the ducts join together to make?
Ducts join together to form larger ducts and eventually, one pancreatic duct
What is the role of the pancreatic duct?
This is where the pancreatic juices, containing enzymes, are secreted into the duodenum of the small intestine
What is the role of enzymes in digestion?
Enzymes are required to carry out the hydrolysis reactions required to digest large insoluble macromolecules into small, soluble, monomers
What is the role of amylase?
for the partial digestion of starch into maltose
What is the role of lipase?
for digestion of triglycerides into fatty acids and glycerol/monoglycerides
What is the role of phospholipase?
for digestion of phospholipids into fatty acids, glycerol and phosphate
What is the role of protease?
- for the partial digestion of proteins and polypeptides into shorter peptides
Where can enzymes be produced apart from the pancreas?
As well as those enzymes found in pancreatic juices, enzymes are also produced in the walls of the small intestine
What is the role of the enzymes secreted by the small intestine?
These enzymes break the products of pancreatic enzyme digestion down into monomers, e.g.
What are 5 enzymes secreted by the small intestine?
Nucleases Lactase Sucrase Maltase Dipeptidase
What is the role of nuclease?
break down nucleic acids
What is the role of lactase?
digests lactose
What is the role of sucrase?
digests sucrose
What is role of maltase?
digests maltose
What is the role of dipeptidase?
digests dipeptides
In what two ways can enzymes be located in the small intestine?
Some enzymes are secreted from the epithelial cells into the intestinal lumen with partially digested food
Other enzymes e.g. maltase, are immobilised and are attached to the membrane of the epithelial cells where they digest substrate molecules as the food is forced through the small intestine
What are enzymes attached to the membrane of epithelial cells categorised as?
These enzymes are examples of integral proteins
What substances may remain undigested?
Some substances that we consume, such as cellulose, may remain undigested as humans are unable to produce the enzymes required
- What two types of enzymes accosiated with protein digestion do pancreatic juices contain?
Pancreatic juice contains endopeptidases and exopeptidases
What is the role of endopeptidases?
eptidases
Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides
What is the role of exopeptidases?
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides
- What enzyme involved in protein digestion is found in the small intestine? What is its role?
Lastly, there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine.
These enzymes hydrolyse dipeptides into amino acids which are released into the cytoplasm of the cell
- What happens when fatty liquid arrives in the small intestine?
When fatty liquid arrives in the small intestine bile (containing bile salts), which has been made in the liver and stored in the gallbladder, is secreted
- How does bile interact with the fatty liquid?
The bile salts bind to the fatty liquid and break the fatty droplets into smaller ones via emulsification
- What is the role of emulsification?
Emulsification helps to increase the surface area of the fatty droplets for action of digestive enzymes
- Where does the digestion of lipids take place?
The digestion of lipids takes place solely in the lumen of the small intestine
- What are lipids broken down into?
Lipase enzymes break down lipids to glycerol and fatty acids
- What can lipids be alternatively broken down into?
Lipids can also be broken down into monoglycerides and fatty acids
- Where are lipase enzymes produced?
Lipase enzymes are produced in the pancreas and secreted into the small intestine
- What is starch made of?
Starch is a macromolecule made up of many α-glucose molecules bonded together in condensation reactions
- What are the 2 main forms of starch?
Amylose
Amylopectin
What bonds does amylose contain?
an unbranched molecule containing 1-4 glycosidic bonds only
What bonds does amylopectin contain?
a branched molecule with 1,4 and 1,6 glycosidic bonds
- Where does the digestion of starch begin?
The digestion of starch begins in the mouth and the small intestine with the enzyme amylase
- WHat is amylase?
Amylase is a carbohydrase that is made in the salivary glands, the pancreas and the small intestine
- What is the role of amylase?
It hydrolyses the 1,4 glycosidic bonds found in both amylose and amylopectin
- What are the end products of amylase hydrolysis?
Amylase action breaks starch down into maltose
- What can amylase not break down, what does this form?
Amylase is unable to digest the 1,6 bonds found in amylopectin; as a result, short strands of amylopectin (containing these bonds) are produced. These short strands are called dextrins.
- Once starch has been digested in the small intestine lumen, what happens next, where is it digested?
The next stage of starch digestion involves enzymes immobilised in the membranes of the microvilli
- What two enzymes are immobilised in the membrane of microvilli?
maltase and dextrinase
- What is the role of maltase and dextrinase?
Maltase - a disaccharidase which hydrolyses maltose into glucose
Dextrinase - digests the 1,6 glycosidic bonds found in dextrins
- What can be done to the products of starch digestion?
After digestion, the monosaccharides can be absorbed into epithelial cells of the small intestine which pass them into the blood stream
- How is glucose transported?
Glucose is absorbed by co-transport with sodium ions into the epithelium cells
It then moves by facilitated diffusion into the spaces between villus cells, before entering the villus capillaries
How do microvilli help with the transport of materials into the bloodstream?
Note that the lining of the small intestine is folded and there are microvilli present. This increases the surface area for proteins such as membrane-bound disaccharidases and co-transporters
- What do nucleases break … into?
Nucleases are enzymes which break down DNA and RNA into nucleotides
- WHat do nucleases break?
They break the phosphodiester bonds between the nucleotide bases
- What is done with the products of nucleic acid digestion?
These can then be absorbed into the blood
Where are the products of digestion transported?
The products of digestion travel via the hepatic portal vein into the liver
Where is excess glucose stored?
The liver absorbs excess glucose and stores it as glycogen
Glycogen has a similar branched structure to amylopectin but is more branched due to having a higher proportion of 1,6 glycosidic bonds
What is absoprtion?
Absorption takes place in the ileum
Absorption is the movement of digested food molecules, vitamins and mineral ions from the digestive system into the blood and lymph
What substances are absorbed? 6
This includes the following products of digestion:
Simple monosaccharides e.g. glucose, fructose, galactose
Amino acids
Fatty acids, monoglycerides and glycerol
Nucleotide bases
And the following additional substances:
Mineral ions e.g. calcium, potassium, sodium
Vitamins e.g. vitamin C
Where is water absorbed?
Water is absorbed in both the small intestine and the colon, but most absorption of water occurs in the small intestine
What does the epithelium provide in terms of harmful substances?
The epithelium provides a barrier to prevent absorption of some harmful substances
What unwanted substances can pass into the blood? 3
However, some unwanted substances can still pass into the blood. This includes:
Some harmless chemicals found in food colourings and flavours; these are removed by the kidney and lost in the urine
Small numbers of bacteria - these are engulfed and digested by phagocytes in the blood
Some other harmful substances - these are removed from the blood and broken down by the liver
How are amino acids and monosaccharides transported?
Amino acids and monosaccharides both use facilitated diffusion, active transport and co-transport proteins in order to move across the epithelial membrane
How are lipids transported?
Lipids are absorbed in a different way using simple diffusion, facilitated diffusion and exocytosis
- What is the first step of amino acid transportation, what transports them?
Specific amino acid co-transport proteins (a type of carrier protein) are found within the cell-surface membrane of the epithelial cells lining the ileum
- When can amino acids be transported?
They transport amino acids only when there are sodium ions present
- How do the sodium ions diffuse into the cell and how many amino acids accompany them?
For every sodium ion that is transported into the cell, an amino acid is also transported in
This occurs via facilitated diffusion, which requires the movement of molecules down their concentration gradient
- How do amino acids pass into capillaries?
Amino acids diffuse across the epithelial cell and then pass into the capillaries via facilitated diffusion
- What maintains the sodium ion concentration gradient?
The concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell
- Why can glucose (monosaccharide) not move via diffusion?
Glucose is polar so cannot pass into the blood by diffusion
- WHat aids the transport of glucose? What is it similar to?
The glucose carrier proteins in the cell-surface membrane of the small intestine work in a similar way to the amino acid carrier proteins
- What moves alongside glucose into the epithelial cells? What process?
Sodium ions and glucose molecules are co-transported into the epithelial cells via facilitated diffusion
- Is the transport of glucose passive or active?
This is a passive process but depends on the concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell
- What makes the passive transport of glucose active?
The gradient is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell
- How does glucose enter the capillary?
The glucose molecules diffuse across the epithelial cell and enter the capillary by facilitated diffusion through a glucose channel protein
- What are the products fo lipid digestion?
The products of lipid digestion are fatty acids, monoglycerides, and glycerol
- Are lipids absorbed in the same way as amino acids/monosaccharides?
NO
Absorption of these products is different from the absorption of monosaccharides and amino acids
- How can fatty acids and monoglycerides enter epithelial cells?
Fatty acids and monoglycerides can enter the epithelial cell by simple diffusion
- WHy do fatty acids use simple diffusion?
They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell surface membrane
- Apart from simple diffusion, how can fatty acids be transported?
Fatty acids also move by facilitated diffusion through fatty acid transport proteins
- How do fatty acids not diffuse back into the lumen?
Inside the epithelial cell, fatty acid chains recombine with monoglycerides or glycerol to form triglycerides, which are unable to diffuse back into the lumen
- What is done with the triglycerides? How are they now transported?
The triglycerides are packaged up with cholesterol and encased in phospholipids and proteins to form lipoproteins
- How do lipoproteins entre capillaries/lacteal?
These droplets then enter the lacteal or capillaries via exocytosis
