09 photosynthesis & plant structures

Leaf Structure• Most photosynthesis occurs in the palisade layer.• Gas exchange of CO2 and O2 occurs at openings called

stomata surrounded by guard cells on the lower leaf surface.

Palisade

Spongy

Chloroplast Structure• Inner membrane

called the thylakoid membrane.

• Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

Overview of photosynthesis Overview of photosynthesis and respirationand respiration

PHOTOSYNTHESISPHOTOSYNTHESIS CELLCELLACTIVITIESACTIVITIES

RESPIRATIONRESPIRATION

SUNSUN

RADIANT RADIANT ENERGYENERGY

GLUCOSEGLUCOSE ATP(ENERGY)ATP(ENERGY)

EQUATIONEQUATION FORFOR PHOTOSYNTHESISPHOTOSYNTHESIS

6CO6CO22 + + 6H6H22OO + +ENERGYENERGY CC66HH1212OO66 + + 6O6O22

CARBON CARBON DIOXIDEDIOXIDE

WATERWATER

GLUCOSEGLUCOSE

OXYGENOXYGEN

EQUATION FOREQUATION FOR RESPIRATIONRESPIRATION

CC66HH1212OO66 + +GLUCOSEGLUCOSE

6O6O22

OXYGENOXYGEN

6CO6CO22 ++

CARBON CARBON DIOXIDEDIOXIDE

6H6H22OO + +ENERGYENERGY

WATERWATER

ATPATP

Photosynthesis in Overview

• Process by which plants and other autotrophs store the energy of sunlight into sugars.

• Requires sunlight, water, and carbon dioxide.• Overall equation:

6 CO2 + 6 H20 C6H12O6 + 6 O2

• Occurs in the leaves of plants in organelles called chloroplasts.

Photosynthesis: The Chemical Process

• Occurs in two main phases.– Light reactions– Dark reactions

• Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments.

• Dark reactions are the “synthesis” part of photosynthesis. Trapped energy from the sun is converted to the chemical energy of sugars.

Light Reactions

• Light-dependent reactions occur on the thylakoid membranes.– Light and water are required for this

process.– Energy storage molecules are formed.

(ATP and NADPH)– Oxygen gas is made as a waste product.

Dark Reactions

• Dark reactions (light-independent) occur in the stroma.– Carbon dioxide is “fixed” into the sugar

glucose.– ATP and NADPH molecules created during

the light reactions power the production of this glucose.

The photosynthetic reaction

• Water + ADP – sunlight → oxygen + hydrogen ions + ATP

• Carbon dioxide + hydrogen ions + ATP → glucose + ADP

H2O + ADP + CO2 + H+ + ATP --sunlight→ O2 + H+ + ATP + C6H12O6 + ADP

6H2O + 6CO2 --sunlight→ 6O2 + C6H12O6

General leaf adaptations

• Flat shape .

• Many stomatal pores• Very thin with internal

air chambers• Extensive network of

vascular tissue – Xylem– Phloem

• Large photosynthetic surface

• Gaseous exchange

• Easy diffusion of CO2 to photosynthetic cells

• Delivery of essential nutrients– Water & minerals– Sucrose & hormones

General stem adaptations

• Xylem vessels running entire length

• Rigid structure of xylem

• Phloem vessels running entire length

• Branching of stem . .

• Transport water and minerals from roots

• Helps plant to remain upright

• Transports sucrose from leaves

• Provides broader, multileveled photosynthetic area

General adaptations of roots

• Broad and deep spread of roots .

• Root hairs

• Taps a large area of soil for water and nutrients

• Provide larger surface area for absorption of water and minerals

Xylem• Water and mineral are transported from

the roots to the photosynthetic areas of the plant.

• Composed of dead, hollow cells

• Provides extra internal support for plant

• Water moves up the plant through the process of a transpiration vacuum.

Transpiration moves water and minerals up the stem of the plant

Movement of water through the xylem

• Water transpires from stomatal pores• …which is replaced by water from the air

spaces in the mesophyll• …which is replaced by H2O in leaf cells• …which is replaced by H2O in leaf xylem• …which is replaced by H2O in stem xylem• …which is replaced by H2O in root xylem• …which is replaced by drawing H2O from

soil

Movement of water through the xylem

Phloem• Glucose is produced through the process of

photosynthesis in the leaves and is then packaged in to sucrose

• Transported through living sieve cells of the phloem, along with hormones to the non-photosynthetic parts of the plant known as ‘sinks’.

• This movement is known as ‘translocation’• Sugars are either stored as starch or broken

down in to monosaccarides and metabolised for growth

• Transport of sugars is performed via active transport & osmosis.

• ATP is provided by the companion cells• High sugar content in sap in leaf phloem

causes intake of water via osmosis• As sugar is transported out of phloem at

sinks, water returns to xylem via osmosis.• Therefore there is a region of high

pressure at the source and low pressure at the sinks, facilitating movement of sap

Movement of sugars through the phloem

Ringbarking

I would never do that!

For what purposes is gaseous exchange required?

• Photosynthesis– Intake of carbon dioxide– Disposal of excess oxygen

• Respiration– Intake of oxygen– Disposal of excess carbon dioxide

Mechanisms for gaseous exchange

• # 1 Stomata

• Stomata comprise of an opening, the stoma and two guard cells either side.

• When the guard cells are turgid (full of water) the stomata is open

• When the guard cells are flaccid (empty) the stomata is closed

Structure of a guard cell

Cellulose fibrilsThick cell wall

Thin cell wallWhen the guard cell swells with water, it bows in to the shape of a jelly bean.

Effect of Chloroplasts on Stomata function

K+

K+

K+

ATP

H2O

Opening of stomata

• Sun shines on plant

• Chloroplasts carry out photosynthesis 1 (light dependant)

• ATP from photosynth powers NA/K pump

• K+ ions pumped in to guard cells, thus creating a concentration gradient

• Water enters cell via osmosis

• Guard cells swell, stomata opens

Mechanisms for gaseous exchange

• # 2 Lenticels

• The woody part of plants is still growing, and therefore performing cellular respiration. (O2 in, CO2 out)

• Lenticels are a protrusion of living cells in to the dead cambium layer

Lenticels

Lenticel (dead) cork cambium

(living) parenchyma

cells

Altered photosynthetic cycle in desert plants

• Desert plants would lose too much water if they opened their stomata during the day.

• Instead they only perform photosynthesis 1 during the day.

• Then at night they release the stored waste O2 and take in CO2 to complete the photosynthetic process.