topic 2 I) excretion

why do plants need excretion

• there are many metabolic reactions taking place that produce waste products

• these waste products can either be used up in other processes or some exit the plant via the leaf organ

• waste products in excess:

  • oxygen

  • CO2

  • water/water vapour

  • other unwanted chemical substances

waste products during the day v night

day net effect: oxygen in excess and waste product as photosynthesis>respiration

night net effect: carbon dioxide in excess and waste product as respiration>photosynthesis

excess gas diffuses out of stomata

how is water vapour lost in plants

• majority of water vapour lost is not a waste product of metabolism, but instead water that has been drawn up from roots in transpiration stream

how are chemical substances excreted in plants

• most molecules that are no longer needed by the plant can be broken down into chemical substances

• some substances cannot be converted into another useful compound so must be removed

• chemical waste materials can be stored in the dying tissues of a plant

• when the dying tissue falls off the plant the substances are removed

why do humans need excretion

• waste products produced from many metabolic reactions in cells

• excretion is the removal of waste substances of metabolic reactions, toxic materials and substances in excess of requirements

• metabolic wastes include:

  • CO2 and water from aerobic respiration

  • urea produced by breakdown of excess proteins (amino acids) in liver

  • other excesses substances e.g. medical drugs and dietary minerals

what happens when waste products are not excreted

• if they are allowed to build up:

  • toxicity - they have toxic effects if they are allowed to reach high concentrations e.g. CO2 dissolves into water easily to form acidic solution that can lower the pH of cells, reducing enzyme activity in body

  • osmotic effect - body fluids can become more concentrated due to higher amounts of waste products - concentrated body fluids can cause water to move out of cells, changing water potential and preventing them from carrying out essential reactions

  • using up necessary storage - space within organism is limited and is required for the storage of more useful molecules

excretory organs of the human body

• kidney for urea, water, excess salts (in urine, urea comes from liver)

• lungs for CO2 and water (during exhalation)

• skin for excess mineral ions and water as well as some urea (via sweating)

what is the kidney

• 2 located in back of abdomen

• regulate water content of blood (vital for maintaining blood pressure and osmoregulation)

• excrete toxic waste products of metabolism (e.g urea) and substances that are in excess e.g. salts

what is osmoregulation and why is it important

• process of maintaining water and salt concentrations across membranes within the body

• it is an example of homeostasis

importance:

• cytoplasm is largely composed of water, so is the blood plasma

• maintaining water levels is vital to prevent harmful changes occurring to cells of the body as a result of osmosis

• if body cells lose or gain too much water by osmosis they do not function efficiently:

  • too much water in blood causes cells swelling as water moves in - has diluting effect and can lead to cells bursting

  • too little water in blood means cells lose water by osmosis - has dehydrating effect and can lead to cell death

water content of the body

two sources of water:

• product of aerobic respiration

• water in diet

water loss through:

• lungs during exhalation

• skin as sweat

water loss through these cannot be controlled, but volume of water lost in urine can be controlled by kidneys

structure of urinary system

functions:

• to filter waste products from blood and expel it as urine

• to control water levels of body (osmoregulation)

structure:

• 2 kidneys joined to bladder by two tubes called ureters

• another tube carries urine from bladder (stores urine) to outside body called urethra

• each kidney is also connected to

  • renal artery - comes from aorta and delivers oxygenated blood to kidney

  • renal vein - delivers deoxygenated blood from kidney to vena cava

structure of kidneys

three regions:

• cortex - outermost region

• medulla - inner section

• renal pelvis - tube linking kidney to ureter

each kidney has around a million tiny structures called nephrons/kidney tubules/renal tubules

nephrons start at the cortex and loop down into the medulla and back to the cortex

contents of nephrons drain the renal pelvis and urine collects there before it flows into the ureter to be carried to the bladder for storage

structure of a nephron

• bowman’s capsule, surrounds glomerulus

• proximal convoluted tubule

• loop of henle

• distal convoluted tubule

• collecting duct

• network of capillaries surrounding tubule with knotted section that sits inside bowman’s capsule

function of a nephron

• kidneys filter the blood before reabsorbing useful substances prior to waste excretion

• process:

  • ultrafiltration

  • selective reabsorption of glucose

  • selective reabsorption of water and salts

how does ultrafiltration work

• arterioles branch off renal artery and lead to each nephron, where they form a bundle of capillaries (glomerulus) sitting inside the Bowman’s capsule

• capsule gets narrower as they get further into the glomerulus which increases blood pressure

• eventually causes smaller molecules in blood to be forced out of capillaries into the bowman’s capsule, where they form glomerular filtrate; substances in filtrate include:

  • glucose

  • water

  • urea

  • salts

• some components are useful and will be reabsorbed back into the blood further down the nephron

how does selective reabsorption work

• after glomerular filtrate enters BC, glucose is first substance to be reabsorbed at the proximal (first) convoluted tubule

  • takes place by active transport

  • nephron adaptation: many mitochondria to provide energy for active transport of glucose

• reabsorption of glucose cannot take place anywhere else in nephron as gates that facilitate active transport of glucose are only found in proximal convoluted tubule

• people with normal blood glucose levels have enough gates to remove all glucose from filtrate back into blood

• people with diabetes cannot control blood glucose levels (often too high) so not all glucose filtered out can be reabsorbed into blood in proximal convoluted tubule

  • singe there is nowhere else for glucose to be reabsorbed, continues in filtrate into urine

how does reabsorption of water work

• as filtrate drips through loop of henle, necessary salts are reabsorbed back into blood by diffusion and active transport

• as salts are reabsorbed back into blood, water follows by osmosis

• water is also reabsorbed from collecting duct in different amounts depending on how much water the body needs at the time

role of ADH

• water reabsorption happens along nephron tubules in kidneys - important for osmoregulation

• control of water reabsorption by tubules is example of negative feedback e.g. if water content of blood is too high, less water reabsorbed, if too low, more water reabsorbed - controlled by ADH

• changes in water level of blood detected by hypothalamus, sends signal to pituitary gland

• pituitary gland changes how much ADH released

• ADH affects permeability of tubules to water

• if water content of blood too high:

  • pituitary gland releases less ADH, so less water is reabsorbed in collecting ducts as they become less permeable to water

  • kidneys produce large volume of dilute urine

• if water content in blood too low:

  • pituitary gland releases more ADH which leads to more water being reabsorbed in ducts as ducts are more permeable to water

  • kidneys produce small volume of concentrated urine

composition of urine

• urea

• excess mineral ions

• excess water

colour and quantity changes quickly

• large quantities are usually pale yellow as urea is less concentrated due to high water levels

• small quantities are usually darker as they contain little water so urea is more concentrated

can change due to:

• water intake - the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced

• temperature - the higher the temperature the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced

• exercise - the greater the level of exercise, the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced