CAM plants: photosynthesizing in the desert

Although C₄ plants are well adapted to occasional periods of drought, they cannot cope well with desert conditions. A group of plants including cacti and pineapples have evolved a third type of carbon dioxide fixation which enables them to survive in very dry climates. These plants are called CAM plants. CAM is an abbreviation for crassulacean acid metabolism, a type of metabolism first observed in the family of plants called Crassulaceae (which includes the stonecrops, fleshy-leaved plants that will grow on rocks and walls).

CAM plants conserve water by only opening their stomata at night. During the night, they fix carbon dioxide into oxaloacetate which is converted into malate. This acts as a carbon dioxide storage compound. During the day, malate releases carbon dioxide into the Calvin cycle. This allows photosynthesis to take place on hot, dry, sunny days, even though the stomata are closed.

CAM plants conserve water very well and are able to survive in extremely dry conditions, but CAM plants do not photosynthesise very efficiently. Most are very slow growing. Where there is plenty of water, CAM plants cannot compete well with C₃ and C₄ plants.

CAM plants and C₄ plants have a similar metabolism: carbon dioxide is first fixed into a four-carbon intermediate before it enters the Calvin cycle. However, in CAM plants the initial fixation and the Calvin cycle occur at separate times, whereas in C₄ plants the initial fixation and the Calvin cycle are separated structurally but both occur during the day. C₄ plants live in hot, very sunny, and periodically dry environments but where lack of water is rarely a limiting factor (partly because the plants can reduce water losses due to their C₄ metabolism) and annual rainfall is high (typically, tropical rainforest-type climates); CAM plants are desert plants that live in areas of very low annual rainfall. Note that C₃, C₄, and CAM plants all eventually use the Calvin cycle to make glucose from carbon dioxide.

■ Glossary of essential terms for you to know

№	English term	Russian equivalent
	to thrive	разрастаться
	to depend on	зависит от
	to adapt	приспособиться
	to survive	выживать
	to release	выпускать
	to breed	размножаться
	to cope with	справиться с
	photorespiration	световое дыхание
	to fix	закрепить, фиксировать
	compound	смесь
	to relate to	относиться к
	efficiently	продуктивно, эффективно
	temperate	умеренный
	to suffer	страдать
	sufficient	достаточный, подходящий
	to escape	убежать
	to cease	прекращать
	leaf	листок
	to wilt	ослабевать
	to decrease	уменьшаться
	to combine	объединять
	loss	потеря
	to reduce	уменьшать
	to evolve	развивать
	to carry out	выполнять
	bundle sheath	обкладка сосудистых пучков
	vein	вена, жилка
	to surround	окружать
	cell	клетка
	initial	первоначальный
	pathway	тропа, путь
	carboxylase	карбоксилаза
	to increase	увеличивать
	to enable	позволять
	to compete	конкурировать, соперничать
	maize	кукуруза
	sugar cane	сахарный тростник
	millet	просо
	drought	засуха
	desert	пустыня
	rock	горная порода, скала
	to conserve	сохранять
	to enter	входить
	to occur	происходить
	lack of	недостаток
	annual	ежегодный
	rainfall	осадки
	rainforest	тропический лес

■ Your Essential Assignments

I. Quick check

1. Name two C₃ plants.

2. Why is sugar cane called a C₄plant?

3. When do CAM plants fix carbon dioxide?

4. Suggest which type of carbon dioxide fixation (C₃, C₄, or CAM) is most efficient:

a. If it is not hot and sunny and the carbon dioxide level is low, but water is freely available

b. In hot, dry and sunny climates where stomata are closed

c. In bright light and temperate regions where there is an ample water supply.

II. Fill in the missing words:

Term (verb)	Noun	Adjective
adapt	.......	.......
fix	.......	.......
conserve	.......	.......
separate	.......	.......
dry	.......	.......

III. Use monolingual English dictionary and write down what could the words given below mean:

adapt, hot, dry, condition, growth, compound.

IV. Match these words with their definitions:

	adaptation	A.	happening at the beginning, first
	metabolism	B.	a large amount of something in one place or area
	cycle	C.	the gas produced when animals breathe out, when carbon is burned in air or when anima or vegetable substances decay
	initial	D.	a plant such as wheat, rice or fruit that is grown by farmers, especially in order to be eaten
	sufficient	E.	the state of not having something, or not having enough of it
	wilt	F.	chemical activity in your body that uses food to produce the energy you need to work and grow
7.	concentration	G.	To continue to exist in spite of many difficulties and dangers
8.	desert	H.	To succeed in dealing with a difficult problem or situation
9.	carbon dioxide	I.	the smallest part of a living thing that can exist independently
	survive	J.	As much as is needed for a particular purpose; enough
	crop plant	K.	To stop doing something or happening
12.	cope with	L.	a number of events happening in a regularly repeated order
13.	cell	M.	if a plant wilt, it bends over because it is too dry or old
14.	cease	N.	the process by which something changes or is changed so that it can be used in a different way or in different conditions
15.	lack of	O.	a large area of sand where it is always very hot and dry

V. Find English equivalents to the following word combinations:

№	Russian term	English equivalent
1.	жаркий и сухой экваториальный регион
2.	замерзающий полярный регион
3.	пшеница, соевые бобы и рис
4.	умеренные условия
5.	маленькие поры в листьях
6.	уменьшает световое дыхание
7.	приспособлены, чтобы выполнять
8.	уменьшает потери воды
9.	высокая концентрация углекислого газа
10.	справляться с условиями пустыни
11.	выжить в очень сухом климате

VI. Give Russian equivalents to the following English terms:

№	English term	Russian equivalent
	thrive in environments
	cope with the demands
	ways of fixing carbon dioxide
	common and widely distributed
	hot, dry environments
	obtain sufficient carbon dioxide
	allow water to escape
	cease photosynthesising
	plant growth
	the loss of
	a special metabolic adaptation
	leaf veins
	the metabolic pathway
	the bundle sheath
	photosynthetic efficiency
	survive in extremely dry conditions
	occur at separate times

VII. Find synonyms among the pool of words:

Pool of words	Synonyms
1)1.thrive/2.lack/3.grow/4.absence/5.cycle/6.rotation
2)1.damage/2.harm/3.cease/4.condition/5.situation/6.finish
3)1.efficient /2.sufficient /3.ample /4.productive
4)1.annual /2.drought /3.lack of rain /4.once a year