Plant Structure and Function

1
Plant Structure and Function

• Objectives of todays lecture
• Learn the basic structures that are common to all
  plants
• Learn what functions these structures perform
• Learn the fundamental chemical reactions that
  occur in photosynthesis and respiration

2
Reproduction is the goal of all organisms

• In order to reproduce a plant must
• Harvest energy
• From sunlight for plants
• From food for animals
• Obtain water
• Acquire mineral nutrients

3
Reproduction is the goal of all organisms

• Adapt to changes in the environment
• Pathogens
• Fungi, bacteria and viruses
• Environmental conditions
• Changes in temperature, light conditions,
  availability of water, etc.
• Plants must perform all these functions in order
  to grow and reproduce

4
How do plants perform these various functions?

• Plants harvest solar energy through
  photosynthesis
• primarily in LEAVES
• Plants acquire water and essential mineral
  nutrients from the soil
• function of ROOTS

5
How do plants perform these various functions?

• Water, nutrients and food are distributed
  throughout the plant via the vascular system
• Connects roots, stems, leaves, flowers and all
  other organs
• Reproduction occurs in FLOWERS
• other organs, such as roots and stems play a role
  in asexual reproduction

6
Why you need to learn about the structure of
plants

• Helps you to understand why plants have such a
  variety of forms
• There is a connection between form and function
  understanding the structure of plants helps to
  explain the functions that specific tissues
  perform
• Plant identification is based on morphological
  features

7
Why you need to learn about the structure of
plants

• Horticultural practices such as propagation
  require a knowledge of plant structures
• Knowledge of the anatomy and cellular structure
  of plants helps you understand how plants grow

8
The Plant Body

• Plants are composed of ORGANS such as roots,
  stems and leaves
• Organs are comprised of various TISSUES, for
  example mesophyll and xylem
• Tissues consist of different types of CELLS, the
  fundamental unit of all organisms
• Cells contain ORGANELLES, structures that are
  responsible for metabolic activity within a cell

9
The Plant Body

• The form of most plants is simple and follows the
  same pattern, regardless of size
• The stem supports leaves and flowers, as well as
  vegetative and flower buds
• The root supports the stem

10
Leaves

• Primary function of leaves is harvesting energy
  from the sun
• Light drives photosynthesis, the most important
  chemical reaction on this planet
• Photosynthesis is a complex series of more than
  50 reactions

11
Leaves and photosynthesis

• Photosynthesis can be summarized as follows
• 6 CO2 6 H2O C6H12O6 6 O2
• Need to know more than this summary reaction to
  understand how leaf structure facilitates
  photosynthesis

12
Photosynthesis can be divided into 2 parts

• The LIGHT REACTION
• Conversion of energy carried by light into
  chemical energy
• Absolutely dependent on light
• The CALVIN CYCLE
• Use of chemical energy captured in the light
  reaction to fix carbon dioxide in the
  atmosphere into organic compounds
• Does not require light

13
Photosynthesis can be divided into 2 parts

• The LIGHT REACTION
• Conversion of energy carried by light into
  chemical energy
• The CALVIN CYCLE
• Use of chemical energy captured in the light
  reaction to fix carbon dioxide in the
  atmosphere into sugars

14
The Light Reaction

• 1. Interception of solar radiation
• Leaves are the primary solar collectors
• 2. Absorption of light by plant pigments
• Chloroplasts within leaf cells contain
  chlorophyll, which absorbs energy from light

15
The Light Reaction

• 2. Absorption of light by plant pigments
• Chlorophyll absorbs red and blue light in the
  visible spectrum green light is transmitted by
  the leaves

16
The Light Reaction

• 3. Energy in light is used to
• Generate oxygen from water
• Produce compounds that capture this energy in
  chemical bonds

Light
H2O
O2
chlorophyll
ADP NADP
ATP NADPH
17
The Calvin Cycle

• Chemical energy captured in ATP and NADPH is used
  to fix carbon dioxide in the dark reaction
• 5C sugar CO2 ? 2 x 3C sugars
• CO2 is fixed into a 3 carbon sugar
• Referred to as C3 photosynthesis
• The enzyme that catalyzes this reaction is called
  ribulose bisphosphate carboxylase, or rubisco

18
Importance of the Calvin Cycle

• Removes 200 billion tons of CO2 from the air
  every year
• Rubisco is the most abundant protein on earth, 20
  kgs for every human
• When forests are cut down and cleared, some of
  this capacity to remove CO2 from the atmosphere
  is lost, contributing to global warming and the
  greenhouse effect

19
Alternative Carbon Fixation Reactions

• There are some alternative methods for carbon
  fixation, e.g. C4 photosynthesis
• 3C sugar CO2 ? 4C sugar
• CO2 is fixed into a 4 carbon sugar molecule
• Prevalent in tropical grasses such as sugar cane
  and maize
• C4 photosynthesis is typically found in plants
  that grow at higher temperatures and under high
  light intensity

20
The two reactions of photosynthesis

• The LIGHT REACTION
• Conversion of energy carried by light into
  chemical energy
• The CALVIN CYCLE
• Use of chemical energy captured in the light
  reaction to fix carbon dioxide in the
  atmosphere into sugars

21
How can we study photosynthesis?

• Plant is enclosed in a sealed container
• Light is switched on and concentrations of CO2
  and O2 are measured

• The concentration of CO2 falls as CO2 is consumed
  by photosynthesis
• As O2 is evolved during this reaction, the
  concentration of O2 increases

CO2 ? O2 ?
22
How can we study photosynthesis?

• As light intensity increases, the rate of
  photosynthesis increases to a maximum
• Photosynthesis is also affected by other
  environmental conditions, e.g. temperature

Photosynthesis CO2 fixation
Light
23
Respiration

• What happens in the dark?
• No photosynthesis occurs, but we still see
  changes in CO2 and O2
• CO2 increases while O2 decreases in the dark, due
  to RESPIRATION
• Respiration is the metabolism of organic
  compounds to produce energy

Lights off
CO2 ?, O2 ?
24
Respiration

• The chemical reactions in respiration can be
  summarized as
• C6H12O6 6 O2 ? 6 CO2 6 H2O
• Respiration can be considered as the reverse of
  photosynthesis
• All organs of a plant are continually respiring

25
Carbon Gain

• Growth of plants depends on how much carbon is
  retained, or gained
• Amount of carbon gained is the difference between
  photosynthesis and respiration
• Carbon Gain Photosynthesis - Respiration
• Carbon gain is not constant but is influenced by
  light levels, stage of development and other
  factors

26
Carbon Gain

• Some organs are net suppliers of fixed carbon,
  termed sources
• Primarily leaves
• Other organs are net users of fixed carbon,
  termed sinks
• Fruits, flowers, roots
• Photosynthate (fixed carbon) must be transported
  to these sinks

27
Carbon Gain

• A high rate of photosynthesis will generate more
  fixed carbon for plant growth
• A low rate of photosynthesis, e.g. because of a
  low level of light, will reduce growth
• When photosynthesis provides just enough
  carbohydrates for respiration, a plant will not
  grow
• What happens if photosynthesis doesnt fix enough
  carbohydrates for respiration?

28
Summary

• Knowledge of plant structure and function is
  important in horticulture
• Plant structure is not complex
• Primary function of leaves is to harvest solar
  energy and fix carbon dioxide through
  photosynthesis
• Respiration also takes place in all plant cells
  and tissues

29
Carbon dioxide

• In an enclosed greenhouse at night, plants are
  respiring, producing CO2
• The concentration of CO2 increases in the
  greenhouse at night

Photosynthesis depletes CO2
400
CO2 ppm
300
200
Ventilation
Dark
Light
sunrise
30
Carbon dioxide

• In the light, CO2 is consumed
• CO2 can fall below 300 ppm (depends on
  temperature, light level, leaf area)
• Ventilation restores CO2 level to ambient

Photosynthesis depletes CO2
400
CO2 ppm
300
200
Ventilation
Dark
Light
sunrise
31
Carbon dioxide enrichment

• Carbon dioxide level can limit photosynthesis,
  especially under high light conditions
• Supplying additional CO2 can lead to increased
  growth under some conditions (high light, optimal
  temperature)
• CO2 enrichment may be used in some controlled
  environments, such as for growing plants in space