Biology of Plants

Plant Gas Exchange

Occurs by direct diffusion with the surroundings in the leaves, stems and roots for plants lack organs for this process.

1.      Roots: Between cells at the exterior of the root hairs and air spaces in the soil

2.      Stems: Openings between cells called lenticels that permit movement of air around them = surface for the exchange of gases

3.      Leaves: Through small pores called stomata which are found on many plant surfaces but mainly on the underside of leaves. (Away from the drying effects of the sun) The size of the stoma (pore) is controlled by 2 crescent shaped guard cells which have a thin and elastic outer wall and thick and rigid inner wall. The opening and closing of the stomata is influenced by the amount of light, H₂O and CO₂

i) An increase in the degree of light = opening of the stomata so CO₂ can be utilized for photosynthesis

ii) A low level of CO₂ will cause the stomata to open

iii) If water is abundant the stomata will open to result in an increased transpiration flow

iv) If water is scarce the stomata will close to reduce water loss = prevents loss of turgor pressure and wilting

Opening/closing of stomata:

a) Water moves into the guard cell that causes the outer surface to bulge and the inner surface to buckle – the elasticity of the outside wall and the rigidity of the inner wall causes the inner wall to buckle inwards = opens the pore

b) An osmotic gradient between the guard cells and its surrounds causes water to move into them

c) This osmotic gradient is caused by an increase in sugars in the guard cell by it performing either photosynthesis or by enzyme action.

d) The opening and closing of the stomata is one of the ways plants control the rate of transpiration

Leaf

Tissue is adapted to receive light for photosynthesis, permit the movement and uptake of water and allow for gas exchange. The external structure of a leaf:

1.      Petiole: Narrow stalk that attaches the leaf blade to the stem

2.      Lamina (blade) is the photosynthetic portion of the leaf that is generally flat and thin to decrease surface area for the absorption of light

3.      Midrib and veins enter the blade via the petiole and consist of vascular tissue which is subdivided into small vessels so that all the cells are in close contact with the plant circulatory system.

4.      Surfaces: Abaxial = upper Adaxial = lower Isobilateral = same surface

3 main internal tissue regions in a leaf:

1.      Epidermis which consists of:

a) Cuticle: Waxy layer covering the leaf surface that prevents excessive water and gas loss. Varies in thickness depending on leaf and habitat type

b) Epidermal cells: Protect upper and lower leaf surfaces, containing stomata which regulate transpiration and gas exchange

2.      Mesophyll which consists of:

a) Palisade mesophyll: Cells that contain chlorophyll – the active sites of photosynthesis and lie directly under the upper epidermal cells
b) Spongy mesophyll: Irregularly shaped and loosely arranged cells that have few chloroplasts and are surrounded by air spaces for stomata in the lower epidermis.

3. The vascular tissues: xylem (strong thick pipes) and phloem (thinner tubes) that make up the plant transport system

Root Structure:

1.      Epidermis: Single cell layer with long extensions = root hairs which increase surface area

2.      Cortex: Thick layer of packing cells often containing stored starch

3.      Endodermis: Single layer of tightly packed cells containing a waterproof layer = casparian strip that prevents the movement of water between cells

4.      Pericycle: Layer of undifferentiated meristematic (growing) cells

5.      Vascular tissue: Contains xylem and phloem cells which are continuos with the stem vascular bundles. The arrangement is different and the xylem usually forms a stat shape with 2-6 arms

Root Tip Structure:

1.      Zone of elongation: Area behind the meristem where new cells increase in length

2.      Meristematic zone: Behind the root cap – zone of cell division

3.      Root cap: Protective layer of cells covering the delicate area of dividing cells that aids in soil penetration

Root Hair:

Single celled extensions of the epidermal cells that are located on the maturing area of the root close behind the root tip. They provide increased surface area and thus more efficient absorption of water and minerals from the soil.

Stem Structure:

1.      Epidermis: One cell thick that secretes a waterproof cuticle in younger plants and may be replaced in older plants by bark

2.      Cortex: Composed of various “packaging cells” that are the source of plant fibres such as sisal and hemp and give young plants strength and flexibility

3.      Vascular tissue: Contains phloem and xylem tissue which grow out from the cambium. In dicot (broad leaved) plants the vascular tissue is arranged in vascular bundles with phloem on the outside and xylem on the inside. In older plants the xylem bundles fuse together to form the bulk of the stem

4.      Pith: Central region of a stem used for food storage in young plants but may be absent in older plants (they’re hollow)

Xylem Tissue

Composed of dead cells joined together to form stiff long empty tubes that are impermeable and strengthened with fibres. Different kinds of cells form wide and narrow tubes and the end cells walls are either full of holes or are absent completely. Before death the cells form thick cell walls containing lignin which is often laid down in rings or helices and make the xylem vessels very strong so they don’t collapse under pressure also making the woody stems strong.

Phloem Tissue:

Composed of sieve tube cells which form long columns with holes in their end walls called sieve plates. These cells are alive with walls made of cellulose and are permeable but lose their nuclei and other organelles, and their cytoplasm is reduced to strands around the edge of the cells. These strands pass through the holes in the sieve plates = continuos filaments (perforated end walls) and the centre of these tubes is empty. Each sieve tube cell is associated with one or more companion cells with nuclei and organelles and are connected by plasmodesmata, (cytoplasm) that provide them with proteins, ATP and other nutrients.

Description	Xylem	Phloem
Cell Wall Thickness	Thick	Thin
Cell Wall Material	Linin	Cellulose
Permeability	Impermeable	Permeable
End Walls	Absent	Seive plates
Function	Transport H2O/minerals	Transport food
Carried to	Leaves	All parts of plant
Direction of Flow	Upward	Up and down
Also Has	Fibres for stength	Companion cell for control

Water Flow in Plants:

1.      Continuos stream entering via the roots where it flows upwards in the xylem to evaporate from the surface of the leaf via the stomata

2.      Evaporation of water from the lead surface is the force by which water is drawn upwards via the roots to the leaves. Transpiration = Drawing of water upwards through the plant by evaporation

3.      Transpiration is important because:

a) Evaporation causes cooling = prevents damage by sunlight

b) Water is transported to the sights of photosynthesis

c) Cells in the leaf are kept moist which is necessary for gas exchange (gases must be in solution)

4.      Force of transpiration is strong enough to lift water up tall trees

5.      An osmotic gradient generates the movement of water from the xylem in the leaf to the stomata. Water concentration is higher in the porous xylem than in the surrounding cells = water flows in this direction and the osmotic gradient is maintained by evaporation.

6.      A pressure difference assists movement of water through the xylem because a low water pressure created by evaporation causes water to be drawn up through the xylem in the root.

7.      Root hair actively transports inorganic nutrients and salts which has the effect of increasing the concentration inside root cells. Therefore water flows osmotically as there is a lower concentration of water in the root cells. The build up of water in the roots is called root pressure and forces water up the xylem vessels which can result in small droplets of water being squeezed out at the top of the leaf. This is called guttation

8.      Root pressure can push water up about 10cm = isn’t as effective as transportation

9.      3 other physical factors of water assist in the process of respiration:

a) Capillarity: Forces of attraction between walls of the tube and the liquid in it = the smaller the diameter the greater the force of attraction

b) Adhesion: Force of attraction between unlike molecules that assists the capillarity and supports the water column all the way up to the leaves = There’s a force of attraction between the xylem walls and the water

c) Cohesion: Force of attraction between like molecules which aids the passage of water by maintaining the upward pull. As water is evaporating the osmotic movement of water molecules attracts others to them.

Transpiration Rate Factors:

1.      Humidity: Increase in the levels of atmospheric water between the plant and the air surrounding it will reduce the transpiration rate

2.      Wind: Air movement naturally increases evaporation = increased rate of transpiration

3.      Temperature: Increase will increase the amount of evaporation = increase in the rate of transpiration

4.      Soil moisture: A plant requires water in the soil to replenish the water it loses through evaporation = effects the rate of transpiration

Translocation:

Involves the active dumping of materials which requires energy as is why phloem cells must be living.

1.      Movement of materials within the plant = specifically the movement of the products of photosynthesis to areas of the plant in which they are needed. Eg. Supply energy for active transport of inorganic materials and ions by the root hairs

2.      Different to transpiration = movement of substances up and down the through plant tissues

Chloroplasts

The site of photosynthesis – composed of stacks of thylakoid discs (with chlorophyll in each disc) called grana suspended in a fluid matrix called stroma while the organelle is surrounded by a double layered membrane.

Photosynthesis:

6CO₂ + 12H₂O + Solar Energy = C₆H₁₂O₆ + 6O₂ + 6H₂O occurs in the chloroplasts. There are 2 phrases of photosynthesis – the light and dark reactions:

Light Reaction:

Changing of light energy to chemical energy which is stored in ATP and NADPH but only occurs in the presence of light. Occurs in the thylakoid membrane.

1.      Light strikes a de-energised chlorophyll molecule which energises its electrons which move to the other boundaries of the chlorophyll molecule

2.      Chlorophyll is now classified as an energy carrier

3.      Some of the energy from these energised chlorophyll electrons is used to synthesise ATP from ADP + P

4.      The rest of the energy is used to split into H + O : H₂O = 2H + 2e- + O

5.      The oxygen is released into the atmosphere

6.      The 2 protons and 2 electrons are trapped by NADP to carry H = NADPH

7.      The NADPH and ATP are used in the dark reaction

Dark Reaction:

Doesn’t require light as the hydrogen carried in the NADPH is combined with CO₂ to become glucose – the energy to perform this is provided by the light reaction.

1.      Takes place in stoma within the chloroplast

2.      DADPH and ATP move from the thylakoid to the stroma

3.      This process fixes CO₂ to become glucose in the Calvin Cycle.

_4.        The CO₂ is fixed by ribosome diphosphate (RDP) RDP is a 5-carbon sugar which becomes an unstable 6-carbon sugar once it fixes the _CO2

5.      6-carbon sugar breaks down into 2 3-carbon molecules of phosphoglyceric acid (PGA)

6.      At this stage energy from ATP made in the light reaction combine PGA with 2H supplied by NADPH from the light reaction

7.      This forms PGAL which can be used as a nutrient or converted into either:

a)      RDP which can combine with CO₂ (and repeat the process)

b)      Glucose

Pigment

Substances which absorb light energy so it can be used by living organisms. Chlorophyll is the pigment used in photosynthesis that absorbs violet, blue and red light and reflects green light.

Photosynthesis and Respiration Balance:

1.      Plants respire and photosynthesise

2.      Certain cells in plant tissues constantly carry out the process of cellular respiration that is unlike the process of photosynthesis which only occurs at certain wave lengths

3.      Plants that are photosynthesising at a greater rate than they are respiring have a net production of oxygen and a net utilization of CO₂

4.      Plants that are respiring at a greater rate than they are photosynthesising have a net production of CO₂ and a net utilization of oxygen

5.      Plants do not reach a point where there is equal balance between the processes of photosynthesis and respiration. This is the compensation point = situation where the amount of CO₂ produced by cellular respiration is equal to that utilized by the process of photosynthesis.

Photosynthesis	Respiration
Food is accumulated	Food is broken down
Energy from the sun is stored in glucose	Energy from glucose broken down by oxidization
Oxygen is given off	Oxygen absorbed
Produces glucose	Produces CO2 and H2O
Goes on only in the light	Goes on day and night