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Deserts and Homeostasis in the Deset

Based on Mader, Sylvia S. 1996. Biology - 5th Ed. WCB

and

Cox, G.W. 1997. Conservation Biology - 2nd ed. WCB

and

Levine, J.S. and K.R. Miller. 1994. Biology: Discovering Life. D.C. Heath

Deserts

• less than 25 cm rainfall/year
• some less than 2cm rain/year
• hot and cold deserts
• 1/3 of all land area
• hot days cold nights because no water vapor to block radiation
• wide variety of plants

adaptations to dry conditions - plants

• short life cycles (annuals, rainy season)
• annuals spend much of year as seeds
• small leaves - thick cuticles
• photosynthetic stems (reduced leaf surface area)
• water storage
• spines or chemicals for defense

1. spines - cactus
2. creosote - creosote bush

• roots

1. deep to water source - mesquite tree
2. shallow to absorb infrequent rains - cactus
3. spacing of plants (chemical warfare underground) allelopathy

adaptations to dry conditions - animals

1. burrowing
2. nocturnal
3. short life cycles
4. living on water from respiration
5. efficient kidneys

Homeostasis

definition: the maintenance of internal conditions in cells or organisms

examples:

• temperature
• salinity
• sugar levels
• pH

Photosynthesis in Deserts

Three types of photosynthesis (Figure 8.10, page 130)

Normal - C₃

1. RuBP carboxylase grabs CO₂
2. inhibited at high oxygen concentrations because photorespiration occurs
3. 1. high oxygen under high light conditions
4. light reactions and carbon fixation proceed simultaneously in same cell

C₄ photosynthesis

1. PEP carboxylase grabs CO₂
2. _{carbon taken up in mesophyll cell and moved into bundle sheath cell as oxaloacetate}
3. no photorespiration
4. carbon fixation in mesophyll, Calvin cycle in bundle sheath cells
5. less efficient because of energy lost in pumping materials, but even though less efficient, still better than C₃ under hot, dry conditions

CAM photosynthesis Crassulacean-Acid metabolism

1. CO₂ taken up only at night
2. stored in vacuoles - build up of oxaloacetate (acidic)
3. partitioning of carbon fixation and Calvin Cycle over time
4. stomata only open at night
5. low production, but works in extreme environment

overview: C₃ best under moist conditions, C₄ under warm, sunny, dry conditions, CAM under desert conditions

1. crabgrass is C₄ plant that thrives in late summer

Water Balance

problem I: organism must maintain inside of body (cells) at constant salinity - balance of water and salt

definitions:

1. diffusion - the tendency of a substance to move from areas of high concentration to areas of low concentration
2. osmosis - diffusion across a semi-permeable membrane
3. osmotic concentration - the amount of material (solutes) dissolved in water
4. semi-permeable membrane: a membrane which allows certain materials (usually water) to pass, while retaining others (solutes)
5. note: water will move from areas of low osmotic concentration to areas of high osmotic concentration

problem revisited: an organism's body fluids (and cells) usually has a different osmotic concentration than the surrounding fluids

1. on land, organisms lose water by evaporation
2. in freshwater, organisms gain water and lose salts
3. in saltwater, many organisms lose water and gain salts
4. some saltwater organisms have the same osmotic concentration as seawater

solution 1: make body impermeable to water loss

waxy coatings

new problem: an organism must exchange some things with the environment to live:

food - and to use food the organism (animal) must:

1. obtain oxygen
2. release CO2

problem revisited: An organism must maintain some surfaces open for gas exchange, and water will be lost or gained over that surface. To maintain homeostasis, this water loss must be balanced.

one last problem - excretion

• proteins composed of amino acids
• amino acid composed of nitrogen + sugar (carbohydrate)
• when broken down, amino acids yield:

1. CO2
2. water
3. nitrogen (ammonia)

• ammonia is toxic and must be excreted

1. excretion - elimination of waste products from metabolism
2. defecation - elimination of undigested wastes

• excretion of ammonia requires water - will affect homeostasis

excretion: ammonia can be excreted in three forms:

ammonia - has one N

1. is highly toxic, requires much water to excrete
2. requires little energy to produce, easily moves into water
3. 2.
4. used by most aquatic organisms, moves across skin or gills

urea - has 2 N

1. less toxic, requires less water to excrete
2. requires some energy to produce
3. used by terrestrial amphibians, sharks, mammals

uric acid - has 4 N

1. least toxic, requires very little water to excrete
2. requires more energy to produce
3. used by reptiles, birds - related to life in egg

Excretion: four case studies:

freshwater fish

1. body fluids have greater osmotic concentration than freshwater
2. absorb water, lose ions (salts), chiefly across gills
3. fish must take up ions, remove water
4. produces copious dilute urine - undeveloped kidneys
5. pumps salt in across gills
6. does not drink
7.  

marine fish

1. body fluids have lesser osmotic concentration than saltwater
2. lose water, absorb ions (salts), chiefly across gills
3. fish must remove ions, drink water
4. produces isotonic urine - developed kidneys
5. pumps salt out across gills

human

1. loses water through evaporation, largely through lungs
2. water obtained from food, metabolism, drinking
3. concentrated urine produced by kidneys
   1. urine not as concentrated as seawater

kangaroo rat

1. loses water through evaporation, largely through lungs
2. stays in burrow during day
   1. burrow cooler, moister
3. water obtained from food, metabolism, - does not drink
4. concentrated urine produced by kidneys
   1. urine more concentrated than seawater
   2. porpoises, whales similar, can drink seawater

other forms of excretion:

nephridia - earthworms dilute urine, nutrients reclaimed

malpighian tubules - insects

1. branch of gut, trap uric acid, water
2. absorb water from uric acid

the mammalian kidney

urinary system

1. kidney forms urine
2. urine moves through ureter
3. urinary bladder hold urine
4. leaves body through urethra

kidney structure & function

made up of nephrons

1. Bowman's capsule
2. proximal tubule
3. loop of Henle
4. distal tubule

capillaries run through Bowman's capsule

1. blood pressure forces salts, water, wastes from capillary into capsule
2. blood cells and proteins stay in capillary

proximal tubule, loop of Henle

1. water, salts, glucose reabsorbed

distal tubule, loop of Henle

1. toxins pumped into urine
2. penicillin too!

process controlled by hormones

1. ADH - antidiuretic hormone
2. 1. causes more water to be reabsorbed
   2. alcohol reduces ADH formation, increases urine
   3. caffeine also

kidney dialysis

1. blood is pumped through tubing in a salt solution
2. wastes leave blood

roles of organs in maintaining homeostasis:

gut

1. responsible for maintaining nutrient levels in blood

lungs

1. responsible for maintaining O2 and CO2 levels in blood (also pH)

gills

1. responsible for maintaining O2 and CO2 levels in blood (also pH)
2. salt uptake or excretion
3. nitrogen excretion

kidney

1. responsible for water and salt balance, excretion

skin

1. also important in water and salt balance
2. may play role in gas exchange, excretion, temperature regulation

liver

1. maintains starches to regulate glucose levels
2. 1. low glucose level - starches broken down
   2. high glucose level - starch formed

blood

1. maintains constant environment for cells
2. moves nutrients, wastes, heat, gasses

brain

1. controls other functions

Desertification

soil becomes too dry to support plant life

causes

grazing

1. reduces plant life, which helps to hold down erosion, hold in moisture

physical disturbance

1. cultivation
2. mining
3. vehicles
4. all remove native plants

deforestation of tropical dry forests

1. building material
2. fuel

irrigation

1. cannot be sustained - salts build up in soils

changing rain patterns

global warming?

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