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: Movement of Water

The Casparian strip

The Casparian strip blocks the apoplast pathway to ensure that water and dissolved nitrate ions have to cross the cell membrane which is done by the transporter proteins Nitrate cane be actively transported into the xylem which lowers the water potential of the xylem and water follows by osmosis.

• The endodermis around the xylem is aka starch sheath which contains starch and uses it as its source of energy.
• The endodermis consists of special cells that have waterproof strip in their walls. - The Casparian strip.
• This strip block the apoplast pathway and ergo water is forced into the symplast pathway.
• The endodermal cells moves minerals by active transport from the cortex to the xylem. - decreases water potential in the xylem by osmosis.
• This reduces the water potential in the cells just outside the epidermis.
• This sets up a water potential gradient across the whole cortex.
• Ergo, water is moved along the symplast pathway from the root hair cells across the cortex and into the xylem - at the same time water an move through the apoplast pathway across the cortex

Movement up the stem

The force that pulls water up the stem of a plant is the evaporation of water from leaves - a process called transpiration. Water molecule evaporates from the leaves, hough the tiny openings called stomata on the surface of a leaf.

• As water move into the xylem by osmosis this pushes the water already present up the xylem. Root pressure can push water a few metres up a stem, but cannot account for movements over great distances.
• Capillary action - the same forces that hold water molecules together also attract the molecules of the side of the xylem vessel - adhesion. These forces can pull up the sides of a vessel.
• Transpiration pull -  water evaporates from leaves as a result of transpiration. Water molecules form hydrogen bonds between one another so they stick together aka cohesion.. Water forms a continuous, unbroken pathway across the mesophyll cells in the leaf and down the xylem. As water evaporates from the mesophyll cells into he leaf  into the air spaces beneath the stomata, more molecules of water draws up as a result of cohesion. Water is then pulled up the xylem as a result of transipation pull. This puts xylem under tension (cohesion tension theory) The lignified xylem vessels prevent collapse under pressure.