2.12 Excretion (in Plants)

REVISION NOTES

IGCSE Edexcel Biology

2.12 Excretion (in Plants)

2.12.1 Understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf

Excretion:

Metabolic reactions required within cells produce waste products
Excretion removes:
- Waste products of metabolic reactions
- Toxic substances
- Substances in excess

Waste products of metabolism in a plant:

Waste products of photosynthesis are either O2 or CO2 depending on light intensity
In the presence of sufficient light:
- Rate of photosynthesis is higher than rate of respiration
- Increased O2 production and decreased excess CO2
- Net increase in O2 levels making it the waste product
In the absence of sufficient light
- The rate of respiration is higher than the rate of photosynthesis
- Increased CO2 levels and no production of O2
- Net increase in excess CO2 levels making it the waste product
Metabolic waste products diffuse out of the plant through the stomata

2.12.2 Know the excretory products of the lungs, kidneys and skin (organs of excretion)

Excretion in humans:

Kidneys excrete urea, water and mineral ions through urine
Lungs excrete carbon dioxide and water through exhalation

Skin excretes excess mineral ions and water through sweat

2.12.3B Understand how the kidney carries out its roles of excretion and Osmoregulation
Osmoregulation

Process of maintaining water and salt concentration within the body
Prevent harmful changes to cells due to osmosis
Transport of too much water into cells causes swelling
- Diluting effect and can lead to cell lysis
Transport of too little water into cells causes shrinking

dehydrating effect and cell death

2.12.4B Describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

Urinary system:

Responsible for filtering waste products from the blood
Osmoregulate the body
Consists of:
- Two kidneys
- A bladder
- Ureters
The kidneys are connected to the bladder by ureter
The urethra carries urine from the bladder to outside the body
Each kidney is connected to:
- Renal artery where oxygenated blood enters the kidney
- Renal vein through which deoxygenated blood leaves the kidney

Structure of urinary system:

Structure of kidney:

Consists of 3 regions:
- Cortex
- Medulla
- Renal pelvis

Each kidney has millions of nephrons that start in the cortex, loop down to the medulla and come back up to the cortex
Nephrons drain into renal pelvis

Urine is collected before being transported to the bladder by the ureter for storage

2.12.5B Describe the structure of a nephron, including:

The Bowman’s capsule
Glomerulus
Convoluted tubules
Loop of Henle
Collecting duct

Structure of nephron:

Nephron is made up of kidney tubule with various sections:
- Bowman’s capsule
- Proximal and distal convoluted tubules
- Loop of Henle
- Collecting duct
- Glomerulus

2.12.6B Describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate

Ultrafiltration:

Capillaries branched off from the renal artery lead into a nephron
- They form a knot and sit inside the Bowman’s capsule
Capillaries get narrower as they lead towards the glomerulus
- Increases blood pressure

Small molecules in the blood get forced out of the capillaries
- They enter the Bowman’s capsule due to the high blood pressure

This process is known as ultrafiltration

The small molecules entering the bowman’s capsule is known as the filtrate which consists of
- Glucose
- Water
- Urea
- Salt
Useful substances get reabsorbed back into the blood down the nephron
- Water is reabsorbed at the loop of henle and collecting duct
- Salts are reabsorbed at the loop of henle
- Glucose is reabsorbed at the proximal convoluted tubule
- Urea is not reabsorbed

2.12.7B Understand how water is reabsorbed into the blood from the collecting duct

Water reabsorption:

Filtrate drips through loop of henle
- Salts are reabsorbed by diffusion
Water is then reabsorbed through osmosis
Water is also reabsorbed from the collecting duct
- Based on the water demand of the body at that time

2.12.8B Understand why selective reabsorption of glucose occurs at the proximal convoluted tubule

Glucose reabsorption:

- The glomerular filtrate enters the bowman’s capsule
- Glucose is reabsorbed at the proximal convoluted tubule

Transport proteins for glucose absorption are only found in the proximal convoluted tubule

Reabsorption of glucose requires active transport
- Nephrons contain an abundance of mitochondria
At normal blood glucose level
- All glucose is reabsorbed from the filtrate into the blood
- There are enough transport gates in the proximal convoluted tubule to Remove all the glucose
For people with diabetes or very high blood glucose levels
- Not all glucose can be reabsorbed
- Glucose ends up in the urine

2.12.9B Describe the role of ADH in regulating the water content of the blood

Role of ADH:

Water reabsorption occurs throughout the nephron
- It is a negative feedback loop
If water concentration in the blood is too high, less water is reabsorbed
If the water concentration in the blood is too low, more water is reabsorbed
- Controlled by hormone ADH
ADH is a hormone released by the pituitary gland
- Dependent on how much water needs to be reabsorbed from the filtrate in the kidneys
ADH increases the permeability of the tubules to water
High water concentration in blood leads to a decrease in release of ADH
- Permeability of tubules to water decreases
- Less water is reabsorbed
- Urine becomes very dilute
Low water concentration in blood leads to increase in release of ADH
- Permeability of tubules to water increases
- More water is reabsorbed
- Urine becomes very concentrated

2.12.10B Understand that urine contains water, urea and ions

Urine content:

Urea
Excess mineral ions
Excess water
Excess glucose in the case of diabetics

Changes in concentration of urine:

Urine concentration changes due as:
- The higher the water intake the more water will be excreted
- The higher the temperature, the more water is lost in sweat so more concentrated urine will be produced

The more sweat lost in exercise, the more water is lost overall so more concentrated urine will be produced