A2 Biology: KIDENYS

The kidney functions:

• blood filtration
• selective reabsorption via active transport and passive absorption

• human kidneys are about 12 cm long 7 cm wide
• They are covered by a layer of fat and are part of the urinary system

• Blood enters a kidney through the renal artery and leaves through the renal vein.
• Excretory products are removed from the blood and are collected in the form of urine.
• Urine collects in the central part of the kidney called the pelvis.
• Urine passes from each kidney to the bladder along the ureter tube.
• The outer darker region is called the cortex and the inner and lighter region is called the medulla.

The Nephron

• The nephron is the functional unit of the kidney
• It makes up the bulk of its structure
• There are about 1 million in each kidney
• At one end of each nephron is a cup shaped Bowman's capsule (in the cortex)
• This encloses a dense network of capillaries called the glomerulus
• This capsule leads into a tubule, first part coiled aka the proximal convoluted tubule
• Then it leads to a U shape loop of Henle
• This leads to another coiled section called the distal convoluted tubule
• These join to forma  collecting duct and many of these lead though the medulla and coverage of the renal pelvis where they empty into the ureter which takes urine to the bladder

Function of the nephron

• The kidney works by the processes of ultrafiltration and reabsorption
• The fluid parts of the blood are filtered into the capsular space and the resulting fluid flows along the tubules
• As it does so useful substance are reabsorbed back into the bloodstream

Ultrafiltration 

• Blood is brought to each glomerulus by an afferent arteriole and it leaves via the efferent arteriole. The afferent is wider in diameter than the efferent which results in a relatively high hydrostatic pressure of blood in the glomerular capillaries. This pressure tends to force the fluid part of blood into the Bowman's capsule lumen.

Barrier

• the barrier between the blood in the capillar and the lumen of the Bowman's capsule consists of three layers:

• Endothelium of the capillary there are pored between the calls hat plasma and dissolved  molecules can pass through.

•  Basement membrane; this is a fine mesh of collagen fibres and glycoproteins. These act as a filter preventing any molecule with a mass greater than 69 000 from passing through. This mean most plasma proteins and blood cells are held in the capillaries. 

•  Podocytes - have specialised shape. They have finger like projections called major and minor processes. These ensure gaps between the cell that fluid can pass through into the Bowman's capsule

Blood contains: Digested food, white blood cells, urea, platelets, hormones, plasma proteins, carbon dioxide, oxygen and red blood cells.
Plasma contains; Carbon dioxide, glucose, amino acids, proteins, minerals, etc.

Selective reabsorption

• As the filtrate flow along the tubules, its composition is altered.
• Reabsorption occurs in both the proximal and distal convoluted tubules.
• Water is also reabsorbed form the collecting ducts.
• Most reabsorption occurs in the proximal convoluted tubule. (85%)

Adaptions for efficient reabsorption

• Epithelial cells of the proximal convoluted tubule have a large surface area due tot he presence of the microvilli on both inner and outer surfaces.
• They also have many mitochondria which can supply energy for active absorption.
• The inner membrane contains special co-transporter proteins that transport glucose or amino acids in association with sodium ions, from the tubule into the cell. This is facilitated diffusion.
• The outer membrane contains sodium-potassium pumps that pump sodium ions out of the cell and potassium ions into the cell.

1. This sodium potassium pumps remove sodium ions from the cells lining in the proximal convoluted tubule.
2. This reduces the concentration of sodium ions in the cell cytoplasm.
3. Sodium ions enter the cells long with glucose or amino acids by facilitated diffusion.
4. As the concentration of glucose an amino acids rise inside the cell, they diffuse out of the cell into the tissue fluid
5. from the tissue fluid they diffuse into the blood and are transported away
6. the reabsorption of sodium, glucose and amino acids reduced the water potential of the cells and increase the water potential of the filtrate
7. this means water will enter the cells and be reabsorbed into the blood by osmosis.

The loop of Henle

• Role of loop of Henle is to produce a low water potential in the tissue of the medulla.
• This will ensure that even more water can be reabsorbed from the fluid in the collecting duct

Ascending Limb

• at base of ascending limb, sodium and chloride ions diffuse out into the tissue fluid. 
• Further up, the ascending limb of the loop of Henle pumps out sodium and chloride ions by active transport.
• This movement makes the tissue fluid surround the Loop of Henle more concentrated 
• Water does not move out of these ions because the wall of the ascending limb is quite thick and is impermeable to water.

The descending limb

• The descending limb is permeable to water and solutes. As the filtrate passes down the descending limb water moved out by osmosis.
• Sodium and chloride ions move in by diffusion.
• The fluid within the descending limb therefore becomes more and more concentrated as it flows towards the bottom of the loop,

AS Biology: Plant cells and water

Water potential

• Pure water has a water potential of 0 however cells have negative water potential when there are more solute particles than water molecules therefore cells have higher water potential when there are less solute particles and more water molecules
• The plant cells becomes TURGID when water enters by osmosis, vacuole sweels and pushes against the cell wall. Plant cell is FLACCID when the water is lost from the cell and the vacuole shrinks hence the cell loses its shape

Water can travel via three different pathways:

1. Apoplast pathway - water moves through the cell wall

• The cellulose cell wall has many water filled spaces between the cellulose molecule, water can move through these spaces and between cells. The water does not pass through any plasma membranes.

2. Symplast pathway - water moves through the plasma membrane into the cytoplasm

• Water enters the cytoplasm via the plasma membrane. The plasmodesmata allows the movement of water from one cell to the next.

3. Vacuolar pathway - water moves through the vacuole

• This is similar to symplast pathway but the water can now enter and pas through the vacuoles as well.

Water uptake from soil

• The roots have root hair cells, to increase the surface area to absorb minerals by active transport.
• This lowers the water potential in the roots so water moves into the root hair cells by osmosis. 
• Water enter root hair cell by osmosis - minerals are actively transported into xylem. (water moves into xylem my osmosis.) 

Solute can enter the xylem by going through cell membrances - the Casparian strip blocks apoplastic route (outside cells) - water cannot pass between the cell or through cell walls - it must pass into cytoplasm or into symplast pathway

Water moves up the stem due to root pressure, transpirational pull, capillary action

• Transpiration pull - as water molecules are removed from the xylem, more water molecule are pulled to replaced them aka transpirational pull.

Mass flow of water also relies on the properties of water

• Cohesion - the water molecules tend to stick together
• Adhesion - the water molecules also stick the the inside of the xylem vessel
• the drawing of continuous column of water up to xylem vessel is aka cohesion-tension theory