Plant biology

Plant cells features

• Cuticle ; Outer waxy layer on leaves which reduces water loss

• Cell wall ; Multi-layered structure. Protects cells

• Middle lamella ; Separates primary and secondary cell wall

• Plasmodesmata ; Cytoplasmic connections between cells

• Cellulose ; Glucose molecules forming a long chain

Plant cell wall structure

• Primary wall contains cellulose

• Secondary cell has hemi-cellulose and lignin, which provide cell wall strength and thickening

• Secondary growth seen in trees is due to the thickening of the secondary cell walls

• Between individual cells is the middle lamella that holds cells together. It contains pectin and calcium

Cell to cell communication

• Has connections between them called plasmodesmata

• Allows communications and signals to occur between cells and transport of materials

Whole plant structure

• Have shoots that grow above ground

• Roots grow below ground

• Shoots consist of a stem, leaves, bear flowers and seeds

• Roots function as storage organs and are most important for absorption of water and nutrients

• Roots create large surface area

Whole plant diagram

1. Apical bud

2. Node

3. Internode

4. Apical bud

5. Vegetative shoot

6. a) Blade leaf

   b) Petiole

7. Stem

8. Taproot

9. Lateral roots

10. Axillary bud

11. Shoot system

Function of leaves

• Primary site of photosynthesis

• Simple or compound in structure

• Upper epidermis and a lower epidermis, in between are mesophyll cells - palisade mesophyll and spongy mesophyll

• Cuticle on the surface that contain waxes that reduces water loss

Diagram of a leaf

1. Cuticle

2. Sclerenchyma

3. Stoma

4. Upper epidermis

5. Palisade mesophyll

6. Spongy mesophyll

7. Lower epidermis

8. Cuticle

9. Vein

10. Guard cells

11. Phloem

12. Xylem

13. Bundle-sheath

Functions of stems

• Provides physical support to the plant and are also involved in movement of water and nutrients up the plant through the vascular system

• Allows for continued growth through the apical meristem

• Contain axillary buds that give rise to side shoots

• Used for storage of food and water

• Allow for lateral growth to increase width of the stem

Diagram of shoot

1. Apical bud

2. Bud scale

3. Axillary buds

4. Leaf scar

5. Node

6. Bud scar

7. Internode

8. Leaf scar

9. Stem

10. Bud scar

11. Leaf scar

Meristem

• Actively growing regions found a the tips of shoots and roots of plants allow for continued growth

• Growth of side shoots from stems is also to axillary bud meristems

• Expansion of width of stems is also due to the actively of lateral meristems, especially cambium

Functions of roots

• Anchor the plant/tree in the soil

• Absorb water and nutrients form the soil

• Have a large surface area due to root hairs

• Continued growth occurs through the root meristems

• Used for storage of nutrients

Tissue types in plants

• Dermal ; found on the outside layer of plant tissues provides protection to the plant

• Meristematic ; found a the growing tips

• Ground ; there are 3 types = parenchyma, collenchyma and sclerenchyma

• Vascular ; there are two types = xylem and phloem

Parenchyma cells

• Found in leaves, tubers

• Constitute living cells

• Involved in producing sugar during photosynthesis and they store food

• Thin walls and large vacuoles

Collenchyma

• Do not store food

• Structural support to plants

• Have thick walls

Sclerenchyma

• Cell are non-living

• Thick walled

• Function in mainly providing support and rigidity to plants

Xylem tissues

• Moves water and nutrients up the lant from roots through stems to leaves

• Consists of cells called tracheid’s and vessel elements which have pores in them

• Non living

Phloem

• Sugar and water solution from leaves to other parts of the plant

• Consists of sieve tube elements and companion cells

• Are living

Vascular cambium

• Ring of actively dividing cells found separating the xylem and phloem

• Cell divisions result in the formation of secondary xylem and secondary phloem

• Continuous division over the years caused formation of secondary xylem in larger trees

• result in the formation of growth rings seen in large trees

Plant reproduction

• Increase their numbers for the next generation

• Reproduction may be asexual or sexual

• In asexual reproduction the offspring are all genetically identical as this is clonal reproduction

• In sexual reproduction there is the advantage of increasing genetic diversity

Sexual reproduction

• Requires the fusion of male and female gametes to produce a zygote followed by seed production

• Male gamete = pollen grains

• Female gamete = ovules

• Gametes are found in the flowers

• For reproduction it requires the transfer of the male gamete to the ovules to initiate fertilization

• Male gametes are produced in anthers

• Female gametes are found in the ovary

Pollination and fertilization

• Process of transfer of the male pollen to female stigma

• Can be transferred by wind, this requires production of very large amount each spring

• Can also be transferred by insects, are attractive and produce nectar to reward the pollinators

Steps in fertilization

• Pollen grain has 2 sperm nuclei

• Pollen grain germinates on the stigma of the same plant specie and produces a germ tube

• Germ tube grown down the style

• When it reaches the ovary it seeks out the opening in the ovule called a micropyle

• One sperm nucleus fuses with the eff nucleus inside the ovule to form the zygote

• Second sperm nucleus fuses with the 2 polar nuclei to form the endosperm

• Called double fertilization

Development of zygote

• Divides several times to form an embryo

• Differentiates to form a root and shoot apex

• Ovary expands and the wall become the seed coat

• Each fertilized ovule forms one seed

• Ovary wall can be fleshy

• Each seed has one or cotyledons depending on it it is a monocot or dicot plant species

• Seeds will germinate to form a root and shoot and the plant has now been propagated sexually

Self pollination

• Where pollen from the same flower fertilizes ovules from the same flower

• More efficient as the pollen and ovules are found in the same flower

Cross-pollination

• Where pollen from one flower fertilizes ovules from a different flower

• Less efficient but evolutionary more advantageous as more genetic diversity is created

• Plants have developed mechanisms to try and increase cross-pollination

How can plants increase cross-pollination

1. Involve more than one method of pollen transfer

2. Make the male and female flowers separate on the same plant. All use wind pollination and are called monoecious

3. Make the male flowers mature earlier than female flowers

4. Produce male and female flowers on different plants

5. Make the male pollen incompatible with stigmas of the same plant

How to spread seeds far and wide

• For survival plants must be able to produce lot’s of seeds and spread them as far as possible

• This allows opportunity for survival in a different

Evolved different methods to spread seeds

1. Make lots of seeds that spread by wind

2. Make seeds float on water

3. Make seed spiny so that catch to animals and humans

4. Make seeds aerodynamic

5. Make Fruits attractive so seeds inside them are spread

6. Make fruits fleshy and attractive for animals toe at

Why do some plants not produce seeds

• Most plants are diploid like in humans allowing normal meiosis to occurs and gamete production

• Some plants are polyploid with greater than 2 sets of chromosomes

• These plants cannot pair chromosomes in meiosis causing no gametes to form therefore no seeds are produced

• Bana is a triploid

• Potato is a tetraploid

• Strawberry is an octoploid

Roots take up water

• Roots take up water through osmosis ; movement of water from a concentration of low solute to one of higher concentration of solute

• The root hairs provide greatly increased surface area for absorption of water and nutrients

• Root hairs provide greatly increased surface area for absorption of water and nutrients

• Once water enters the root it can move in between cells (apoplast) or through the cells themselves (symplast)

• Once it reaches a layer of cells called the endodermis the water is redirected to move via the symplast

• Allows for control of water uptake

• Endodermis is the casparian strip which is how water movement is regulated

Water movement

• Once inside the xylem tracheids and vessel elements, water molecules adhere to each other by hydrogen bonding as well as to the walls of the xylem vessels

• Creates a column of water internally

• Water travels upwards to the leaves

• Rate of travel can be 15 metres per hour in a large tree

Water flow up the plant (tree)

• Loss of water due to transpiration creates a “water deficit” or negative water protentional inside the leaf

• This causes a “pull” of water into the leaf from the xylem. This in turn pulls water up the xylem from the roots

• The “transpiration pull” is the main force that causes water to move up the plant from roots to leaves

The functions of stomata

• Found in the epidermal layer of leaf cells

• More on the underside of leaves compared to the upper surface as they as not exposed to direct light and it is cooler

• Guard cells regulate the stomatal opening size

• The larger opening the more water will be lost

• Open when there is lots of water when there is sunlight and when potassium ion levels are high inside guard cells

• Will close when there id not enough water when it is dark and when potassium levels are low

How do plants adapt in desert environments

• Have reduced leaf size and fleshy leaves to store water

• Some close the stomata during the day and open them at night when its cooler

• May be located deeper inside the leaf than in the epidermis

• Leaves have thick waxy cuticles to retain water

• Causes CO2 levels in leaf cells to decline and oxygen builds up

• Calvin cycle slows down as there is less CO2

• Plants undergo “photorespiration” where the presence of oxygen the phosphoglycerate molecule is oxidized to release CO2

• This causes up to 50% of the carbon to be lost

• to avoid this loss plants have evolved a C4 pathway to fix the carbon in hot climates

• Are called xerophytes

Function of the phloem

• During photosynthesis sugar is produced in the leaves that must be transported to other parts to the plant

• This requires movement from the source to where it is needed for growth

• Sucrose is “loaded” into the phloem cells by sucrose transporters this causes a high osmotic pressure which draws in water from the xylem

• Causes pressure to build up which forces the flow of sugar down the plant

• Turgor pressure causes the sugar solution to move via “bulk flow” to reach cells that need it

Plants response to physical injury

• Injury causes cells to break and release contents

• Enzymes are produced to heal the the injury

• Oxidative reactions cause browning that helps heal the wound

• Other chemical reactions occur to start the process of cell division

Plant stresses cause chemical changes

• Environmental or biological stresses will cause plants to express genes to produce chemical produces to deal with the stress

• Response may occur in a matter of minutes chemical signals are produced

• Which signal other parts of the plant that something is going on

• This in turn can cause other changes to help the plant recover from the stress

• Plants do respond to their environment

Gravitropism

• Plants respond to gravity

• Positive ; roots grow down

• Negative ; stems grow against gravity

Thightropism

• Thigma means touch in greek

• If you touch certain plants this changes the plants behavior

Photoperiod

• Plant responses to different lengths of time of exposure to light

Phytochrome

Protein sense light

Photosynthesis

• Production of sugar in plants using carbon dioxide and water in the presence of light

• Solar energy is used to produced chemical energy which is used to produce organic molecules

• 6CO2 + 12H2O => C6H12O6 + 6H20 + 6O2

• Takes place in plants and trees

Chlorophyll and pigments

• Only respond to light in the visible spectrum

• Two forms of chlorophyll - ‘a’ and ‘b’

• Other pigments that are orange and yellow called carotenoids (absorb light around 480-500 nm)

• Chlorophyll ‘a’ absorbs light around 440nm and also at 680nm-700nm

• Carotenoid pigments show up when chlorophyll is broken down in the fall season to produce vivid colors of leaves

• They are anti-oxidants that reduce oxidative damage due to sunlight and UV rays

Features of chloroplasts

• Chlorophyll is contained within

• Found in the mesophyll cells of leaves

• They are surrounded by a double layer of cell membranes inside the chloroplasts are stacks of granum that are surrounded by the thylakoid membrane

• Light is absorbed by the granum except for green wavelength which is transmitted

• Light energy is packaged into photons which strike the chlorophyll and causes it to emit higher energy level electrons

Photosynthetic reactions

• During light reactions ADP and NADP are combined with P to form ATP and NADPH this provides energy for the next step of photosyntheis

• Oxygen is released from water

• Next step is the calvin cycle which is light independent in which energy form the light reactions is used to drive the formation of carbon molecules from CO2

• The chlorophyll and pigment molecules are arranged in a light-harvesting complex called a photosystem, inside is a primary electron acceptor called pheophytin

• the photosystem I and photosystem II are embedded in the thylakoid membrane

The photosystems

• Photosystem I absorbs light in the range of 700 nm

• Photosystem II absorbs light in the range of 680 nm

• Electrons released from the splitting of water by light photons reach P680 first and the chlorophyll energizes electrons to the primary electron acceptor

• The electrons are transferred down an “electron transport chain” to Plastoquinone (Pq), then to Cytochrome complex (Cc) and then to Plastocyanine (Pc)

• At this point of reaching Cc, energy from the electrons is used to generate ATP

Electron transport chain

• Light energy then strikes PS I (P700) and electrons are transferred to the primary electron acceptor

• Electrons continue down the electron transport chain to the next molecule which is Ferredoxin (Fd)

• The energy from the electrons creates the formation of NADPH reductase

• End result of electrons being released from water is the production of ATP and NADPH and the production of oxygen

Cyclic electron flow

• Cyclic electron flow occurs when electrons from ferredoxin are transferred back to cytochrome complex instead of moving on to NADPH reductase

• This results in no NADPH being formed but ATP is still produced

• The cells would undergo cyclic electron flow if there was sufficient NADPH present of it the cells needed more ATP to be produced

• ATP production occurs via ATP synthase which is driven by a flow of protons through the thylakoid membrane and into the stroma of the chloroplast, this is called chemiosmosis

Calvin cycle

• Also called the C3 cycle

• results in the conversion of carbon dioxide to sugar using energy from the light reactions

• Also called “carbon fixation”, about 160×10^12 kg/yr is fixed by plants

• Occurs in the stroma of chloroplasts

Steps of the calvin cycle

1. Reaction starts with rulose-1,5-biphosphate adding CO2 molecule to form (2X) 3-phosphoglycerate

2. Enzyme involved here is called Rubisco = ribulose-1,5-biphosphate carboxylase

3. The next step is a requirement for STP to produce 1,3-biphosphoglycerate (3C)

4. NADPH is required to form glyceraldehyde-3-phosphate this goes on to form sugar

5. RuBP is reformed to continue the cycle

6. Energy in the form of ATP is required here

7. Total energy required for producing a 6C sugar molecule is 18 ATP molecules and 12 NADPH

Hormones respond ot environmental changes

• Auxin - stimulates cell elongation and regulates branching and organ bending

• Cytokinins - stimulate plant cell division and promote later bud growth

• Gibberellins - Promote stem elongation, helps seed break dormancy and used stored reserves

• Brassinosteroids - Chemically similar to the sex hormones of animals, induce cell elongation and division

• Abscisic acid - produces stomatal closure in response to drought, promotes seed dormancy

• Strigolactones - regulate apical dominance, seed germination and mycorrhizal associations

• Ethylene - Mediates fruit ripening

The C4 pathway in plants

• Molecule of malate (C4) instead of phosphoglycerate (C3)

• This is produced in mesophyll cells by adding CO2 to a molecule of phospho-enol-pyruvate (PEP) to form oxaloacetate which is converted to malate

• Enzyme involved is PEP carboxylase which has a higher affinity for CO2 than Rubisco and so can capture lower concentration of OC2 in hot climates

• These plants also have specialized cells called “bundle sheath cells” These cells break down malate to release CO2 and form pyruvate

• This CO2 molecule is used in the Calvin cycle to form sugar this allows plants to grow in hot climates

CAM plants

• These are Crassulacean Acid Metabolism plants

• Occur in plants such as cactus and pineapple

• During the day stomata are closed

• CO2 is taken up at night to produce organic acids

• This is stored in mesophyll cells at night

• Day time, light reactions continue and ATP and NADPH are produced

• Then crassulacen acid is broken down to release CO2

• Used in the Calvin cycle which can operate while the stomata are closed during the day