Week 5 Flashcards
Two main types of glucose anabolism
Glycogenesis - formation of glycogen- polysaccharides
Gluconeogenesis -synthesis of new glucose molecules
Process of glycogenesis
Glucose Uptake:
Glucose enters cells through transport proteins in the cell membrane, such as GLUT4 in muscle cells and GLUT2 in liver cells.
Phosphorylation of Glucose:
Inside the cell, glucose is phosphorylated to glucose-6-phosphate (G6P) by the enzyme hexokinase (in muscle cells) or glucokinase (in liver cells). This step consumes one ATP molecule and traps glucose within the cell, as G6P cannot easily cross the cell membrane.
Conversion to Glucose-1-Phosphate:
G6P is then converted to glucose-1-phosphate (G1P) by the enzyme phosphoglucomutase.
Activation of Glucose:
G1P is activated to UDP-glucose (uridine diphosphate glucose) by the enzyme UDP-glucose pyrophosphorylase. This step involves the coupling of G1P with uridine triphosphate (UTP), producing UDP-glucose and pyrophosphate (PPi).
Glycogen Synthesis:
UDP-glucose is then used by the enzyme glycogen synthase to add glucose units to a growing glycogen chain. Glycogen synthase catalyzes the addition of glucose from UDP-glucose to the non-reducing end of a glycogen molecule, forming α-1,4-glycosidic bonds.
Branching of Glycogen:
To create the highly branched structure of glycogen, the enzyme branching enzyme (or amylo-(1,4→1,6)-transglycosylase) introduces α-1,6-glycosidic branches into the glycogen molecule. This branching increases the solubility and allows for more rapid synthesis and mobilization of glycogen.
Glycogen Storage:
The resulting glycogen is stored in the liver and muscle tissues. In the liver, glycogen helps regulate blood glucose levels, while in muscle tissue, it serves as a local energy reserve for muscle contraction.
Glyconeogenesis
Lactic acid +amino acids (alanine and glutamine) + pyruvic acid = glyceraldehyde 3 phosphate
GA3P + glycerol = glucose 6 phosphate
glucose-6-phosphate is converted to glucose by the enzyme glucose-6-phosphatase which then releases the new glucose molecule into the bloodstream
What hormones initiate gluconeogenesis
Cortisol and glucagon
Why are lipids the primary energy storage molecules?
Over twice as energy dense per gram compared to carbs or proteins
Hydrophobic- cells do not exert osmotic pressure
What is the purpose of lipolysis
lipolysis is crucial for converting stored fat into usable energy, maintaining blood glucose levels, and adapting to changes in energy requirements.
What happens in the process of lipolysis
The fatty acids from triglycerides can be oxidised and used to produce ATP. First, the fatty acids must be removed from the glycerol molecule.
What is the process of Beta oxidation
A catabolic process occurring in the mitochondrial matrix. It removes two carbon atoms at a time from a fatty acid (long carbon chain. The carbons are then attached to coenzyme A to create acetyl coenzyme A and therefore put into the Krebs cycle to produce ATP
Fatty acid classifications
Saturated or unsaturated
Length of hydrocarbon chain
Short chain fatty acids
<5 carbons
Medium fatty acids
6-12 carbons
Long fatty acids
13-21
Very long fatty acids
22>
Lipogenesis
Takes place in liver and adipocytes
Initiated by insulin
Requires +ve energy balance
Carbs, proteins, fats all converted into triglycerides to be stored
Lipid transportation lipoproteins
Chylomicrons - Formed in intestines, highly dense, for dietary lipid transportation
Very low density and low density lipoproteins - liver (no good for you, they drop the triglycerides)
High density lipoproteins - intestines and liver (very good for you, they pick up waste)
Why does Protein catabolism occur?
So that the proteins can be broken down into amino acids so they can be:
- converted into different amino acids
- Used to construct new proteins
- Converted to fatty acids, ketone bodies or glucose
- Oxidised to make ATP
Why does deamination occur?
Involves the removal of amino group from the amino acid in the liver or kidney to maintain an internal environment (ie. nitrogen is toxic in excess or the amino acids need to have amino group removed to be processed into ATP)
Involves NADH, deaminase, and water
What is transamination
It transfers an amino group into a keto group. It recycles nitrogen to produce non-essential amino acids and prevent ammonia production and excretion of nitrogen from kidneys.
What 6 processes is the digestive system broken down into
Ingestion
Secretion
Mixing and propulsion
Digestion
Absorption
Defacation
What four layers is the GI tract made of
Mucosa
Submucosa
Muscularis
Serosa
Mucosa
Innermost layer
Divided into surface epithelium and lamina propria
Submucosa
Connective tissue
Extensive vascularisation, nervous supply and lymphatics
Muscularis
Muscle layer divided into circular and longitudinal tissue
Moves food and fluid through tract by peristalsis
Serosa
Outermost layer
Thin connective tissue surrounds tract and extends to form mesentery that anchors tract to abdominal wall.
Process of peristalsis
Circular muscles contract behind bolus and relax ahead
Longitudinal muscles ahead of bolus contract to shorten segment
This sequence pushes bolus through tract
Four key functions of the stomach
Accomodation of ingested food
Secretion of gastric juice
Mixing food, saliva, and gastric juice using peristalsis waves to form chyme
Secretion of the hormone gastrin
Four sections of the stomach
Fundus
Cardia
Body
Pylorus
Three segments of small intestine
Duodenum
Jejunum
Ileum
Three main functions of the small intestine
Mix chyme with pancreatic juice and bile
Complete chemical digestion of nutrients
Absorb nutrients and water
Duodenum
25cm long, c shaped from pyloric sphincter
Receives chyme together with digestive secretions from liver and pancreas
Jejunum
1m long
Site of most chemical digestion and nutrient absorption
Ileum
2m long
Absorbs bile salts, Vit B12, and remaining nutrients
Ends with ileocecal valve (controls flow into large intestine)
Internal structure of small intestine
Intestinal villi cover mucosa. Each villus is covered with epithelial cells with their own microvilli, increasing surface area. This is known as the brush border.
Four regions of large intestine
Caecum
Colon
Rectum
Anus
Four divisions of colon
Ascending
Descending
Transverse
Sigmoid
Large intestine mucosa contents
Absorptive cells primarily for water absorption
Goblet cells produce mucus for lubrication
(Both are contained in tubular glands called Crypts of Lieberkuhn
Saliva is secreted from which 3 major salivary glands
Parotid
Sublingual
Submandibular
Components of saliva
Water
Amylase
Lipase
IgA
Lyzosyme
Ions
Urea and uric acid
Antioxidants
Where is gastric juice secreted from?
Gastric glands in the epithelial lining of the stomach
What are the 3 gastric glands and what do they release?
Parietal cells - HCl and intrinsic factor
Chief cells - pepsinogen and gastric lipase
G cells - produce and release gastrin
HCl in gastric juice
Kills microrganisms
Denatures proteins and breaks down plant cell walls
Activates pepsinogen to form pepsin
Promotes the flow of bile and pancreatic juice
Intrinsic factor in gastric juice
Required for vit B12 absorption
Pepsinogen in gastric juice
Converts to the protease pepsin inside stomach, which helps to break down proteins for digestion
Gastric lipase in gastric juice
Breaks down triglycerides into fatty acids and glycerol for energy production
Gastrin in gastric juice
Stimulates HCl secretion
Stimulate chief cells
Causes lower oesophageal sphincter to contract
Increases gastric motility
Causes pyloric sphincter to relax
What is Bile
Bile is both a digestive aid and excretory product
It consists mainly of:
- water
- Bile salts that aid emulsification of lipids
- cholesterol
- lecithin
- bilirubin
What is the exocrine function of the pancreas?
Produce pancreatic juice which contains digestive enzymes and buffers such as amylase, lipase, nucleases, proteases. It is responsible for chemical digestion and raises pH of chyme
Enterocrine cell hormones
secretin
Gastric inhibitory peptide (GIP)
Cholecystokinin (CKK)
Function of secretin
Reduces secretion of HCl
Increases release of bicarbonate ions in pancreatic juice
Neutralises gastric acid in small intestine
Gastric Inhibitory Polypeptide
Reduces secretion of stomach acid
Induces insulin secretion
Cholecystokinin (CKK)
Inhibits stomach emptying
Increases the release of pancreatic digestive enzymes and bile
Stimulates gut motility
Suppresses Hunger
How is intestinal absorption achieved
Through the nutrients passing into enterocytes (epithelial cells) of intestinal villi. They are then transferred to blood and lymph for distribution
How are carbohydrates absorbed?
Monosaccharides
How are proteins absorbed?
Absorbed into enterocytes as either amino acids, dipeptides or tripeptides. Transferred to blood only as amino acids (broken down in enterocytes)
How are lipids absorbed?
Pass into enterocytes
Large numbers of triglycerides are packed into chylomicrons and pass into lacteals (lymphatic capillaries) in villi and into blood via lymphatic system
Haustral Churning
Haustra are bubble shaped segmented pouches of large intestine. Chyme enters and causes a haustrum to distend, this then squeezes its contents along into the next haustrum.
Absorption in large intestine
Water is absorbed from chyme to make it faeces.
Vit K is absorbed (required from blood clotting)
B vitamins 1,2,5,7,9,12 are absorbed
Na and Cl ions absorbed
Myenteric plexus
Found between longitudinal and circular smooth layers of muscularis
Primarily responsible for gut motility via activation of smooth muscle
Submucosal plexus
Found within mucosa
Control secretory cells of organs
Autonomic nervous system and the digestive system
Vagus nerves provide parasympathetic connection to GI tract
Can directly activate smooth muscle and glandular activity in GI tract
Sympathetic nerves connect with ENS
- Causes a decrease in secretory action and motility of GI tract
- Coupled with reduction in blood flow
