2.7-2.79- Excretion

Question 1

What waste products or substances can be in excess in a plant?

Answer 1

Oxygen
Carbon dioxide
Water/water vapour
Other unwanted chemical substances

Question 2

What can cause oxygen to be in excess?

Answer 2

The amount or intensity of light affects the waste products within plants
During the day, when there is sufficient light:
The rate of photosynthesis is higher than the rate of respiration
More oxygen is released than used in respiration
Less carbon dioxide is released than used in photosynthesis
Net effect - oxygen is in excess and a waste product
Will diffuse out of the stomate via the leaf organ.

Question 3

What can cause Carbon dioxide to be in excess?

Answer 3

During the night, when there is insufficient light:
There is no photosynthesis, only respiration
Oxygen is used in respiration and carbon dioxide is produced
No photosynthesis means that no carbon dioxide is used
Net effect - carbon dioxide is in excess and a waste product
Whichever gas is in excess diffuses out of the plant via the leaf organ
The gases exit through the stomata

Question 4

How is water lost in a plant when is excess?

Answer 4

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

Question 5

How are chemical substances lost in a plant?

Answer 5

Plant cells can break down molecules into chemical substances no longer required by the plant
Some of these substances cannot be converted into another useful compound and so must be removed from the plant
Chemical waste materials such as this can be stored in the dying tissues of a plant
When the dying tissue falls off the plant the substances are removed

Question 6

What is excretion?

Answer 6

Removal of metabolic waste toxic material and substances in excess of requirements

Question 7

What metabolic waste can be in excess in a human body?

Answer 7

Carbon dioxide and water from aerobic respiration in cells
Urea produced by the breakdown of excess proteins (amino acids) in the liver

Question 8

What are the dangers of waste products?

Answer 8

If waste products are allowed to build up they can have a range of negative effects on the body:
Toxicity - waste products can have toxic effects if they are allowed to reach high concentrations
Carbon dioxide dissolves in water easily to form an acidic solution which can lower the pH of cells. This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions
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 their water potential and preventing them from carrying out essential reactions
Using up necessary storage - space within an organism is limited and is required for the storage of more useful molecules

Question 9

What are the organs of excretion?

Answer 9

The kidneys for the excretion of urea, water and mineral ions
The lungs for the excretion of carbon dioxide and water
The skin for the excretion of excess mineral ions (e.g. sodium) and water
The liver for the breakdown of proteins (amino acids)

Question 10

How are kidneys important for the removal of metabolic waste?

Answer 10

The kidneys are located in the back of the abdomen and have two important functions in the body:
They regulate the water content of the blood (vital for maintaining blood pressure)
They excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)

Question 11

What are the 2 key functions of the urinary system?

Answer 11

To filter waste products from the blood and expel it from the body as urine
To control the water levels of the body (osmoregulation)

Question 12

What organs are consisted in the urinary system?

Answer 12

The urinary system consists of two kidneys (found at the back of the abdomen) joined to the bladder by two tubes called the ureters
Another tube, the urethra, carries urine from the bladder to outside the body
Each kidney is also connected to:
The renal artery which comes from the aorta and delivers oxygenated blood to the kidney
The renal vein which delivers the deoxygenated blood from the kidney to the vena cava

Question 13

What are the main structures of the urinary system and their function?

Answer 13

Kidney- Two bean- shaped organs that filter the blood

Ureter- Tube connecting the Kidney to the bladder

Bladder- Organ that stores excess water urine salts and urea as it is produced by the kidney

Urethra- Tube that connects the bladder to the exterior where urine is released.

Question 14

What are the 3 regions of the Kidney?

Answer 14

Cortex - the outermost region
Medulla - the inner section of the kidney
Renal pelvis - the tube linking the kidney to the ureter

Question 15

What are nephrons?

Answer 15

Each kidney contains around a million tiny structures called nephrons, also known as kidney tubules or renal tubules
Nephrons start in the cortex of the kidney, loop down into the medulla and back up to the cortex
The contents of the nephrons drain into the renal pelvis and the urine collects there before it flows into the ureter to be carried to the bladder for storage

Question 16

What is the nephron made up of?

Answer 16

Bowman’s capsule
Proximal convoluted tubule
Loop of Henlé
Distal convoluted tubule
Collecting duct
Surrounding the tubule is a network of capillaries with a knotted section which sits inside the Bowman’s capsule

Question 17

Explain the process of ultrafiltration

Answer 17

Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule
The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them
This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate
This process is known as ultrafiltration
The substances forced out of the capillaries are glucose, water, urea, salts
Some of these are useful and will be reabsorbed back into the blood further down the nephron

Question 18

Where are different components reabsorbed at?

Answer 18

Water - loop of henile and collecting duct
Salts- loop of henile
Glucose - proximal convoluted
Urea - Not reabsorbed

Question 19

How is water reabsorbed?

Answer 19

As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion and active transport
As salts are reabsorbed back into the blood, water follows by osmosis
Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time

Question 20

How is Glucose selectively reabsorbed?

Answer 20

After the glomerular filtrate enters the Bowman’s Capsule, glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule
This takes place by active transport
The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules
Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule
In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood
People with diabetes cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule
As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in the urine

Question 21

What is Osmoregulation?

Answer 21

Process of maintaining water and salt concentration across membranes within the body (homeostasis)

Question 22

What is the importance of osmoregulation?

Answer 22

The cytoplasm of all cells is largely composed of water, as is the blood plasma
Maintaining water levels in the body 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 the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting)
Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death

Question 23

Why is water important in the human body?

Answer 23

There are two sources of water in the body:
Water produced as a result of aerobic respiration
Water in the diet
Water is lost from the body in the following ways:
Via the lungs during exhalation (breathing out)
Lost from the skin as sweat (along side mineral ions and urea)
Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys

Question 24

What is ADH?

Answer 24

The pituitary gland in the brain constantly releases a hormone called ADH
How much ADH is released depends on how much water the kidneys need to reabsorb from the filtrate
ADH affects the permeability of the tubules to water
If the water content of the blood is too high:
The pituitary gland releases less ADH which leads to less water being reabsorbed in the collecting ducts of the kidney by osmosis (the collecting ducts become less permeable to water)
As a result, the kidneys produce a large volume of dilute urine
If the water content of the blood is too low:
The pituitary gland releases more ADH which leads to more water being reabsorbed in the collecting ducts of the kidney by osmosis (the collecting ducts become more permeable to water)
As a result, the kidneys produce a small volume of concentrated urine

Question 25

What is the composition of urine?

Answer 25

Urea
Excess mineral ions
Excess water

Question 26

What can cause the colour and quantity of urine to change?

Answer 26

Large quantities of urine are usually pale yellow in colour because it contains a lot of water and so the urea is less concentrated
Small quantities of urine are usually darker yellow / orange in colour because it contains little water and so the urea is more concentrated

Question 27

What causes concentration of urine to change?

Answer 27

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