CAPE Bio (P1/2) - Photosynthesis and ATP Synthesis

• photosynthesis

the process in which organisms use sunlight to convert inorganic materials into organic substances for energy

• Describe how the lamina and veins contribute to photosynthesis

The large surface area and thinness of the lamina allows it to absorb large amounts of light. Its thinness maximizes the diffusion pathway for gaseous exchange, optimizing it. In addition, each plant having an array of leaves arranged in the leaf mosaic helps to maximize the amount of light absorbed. The veins in the leaf help to support its large surface area with each containing xylem which bring in the water necessary for photosynthesis and for cell turgor, and phloem, which takes the products of photosynthesis to other parts of the plant

• Describe how epidermal cells contribute to photosynthesis

The upper epidermis is made of thin, flat and transparent cells which allow light through to the cells of the mesophyll. Epidermal cells secrete a waxy cuticle which helps to prevent loss other than through the stoma. Both the cuticle and epidermis form a protective layer against microorganisms and some insects. The epidermis, especially the lower parts, has stomata cells which open to facilitate gaseous exchange between the intercellular spaces and the external environment. This opening is regulated by guard cells in response to environmental conditions and is dependent on changes in the osmotic concentration of the vacuoles of the guard cells

• Describe how palisade mesophyll cells contribute to photosynthesis

Palisade mesophyll cells are compact and typically found right beneath the upper epidermis. As per their name, these cells are shaped like palisades or fences and stack next to each other to form a column while orienting themselves at a right angle to the epidermis — directing less sunlight towards the cell walls which can scatter light unproductively if arranged otherwise. These cells also have a large amount of chloroplasts which tend to be pushed to the edge of the cell via their large vacuole, maximizing light absorption on multiple fronts. In addition, proteins in their cytoplasm keep chloroplasts mobile to help with light absorption

• Describe how spongy mesophyll cells contribute to photosynthesis

Spongy mesophyll cells are irregularly shaped and this allows large air cavities to exist among the cells — providing paths for gases to diffuse through. Spongy mesophyll cells create a moist environment within these air cavities that gases can dissolve in — making diffusion easier. The concentration gradient guides gases to and from the palisade cells. Spongy mesophyll cells also temporarily store products of photosynthesis and provide the leaf with structural support — allowing it to bend

• Describe how chloroplasts contribute to photosynthesis

Chloroplasts contain the pigment chlorophyll which absorbs light for the process. They have their own DNA and hence can synthesize specialized proteins for photosynthesis and multiply independently based on need. Chloroplasts contain thylakoid membranes which host photosynthetic pigments, electron carries, proton pumps and ATP synthetase which are used to generate energy. These membranes stack to form grana which trap light efficiently

• pigment

compounds that absorb wavelengths of light and reflect others

• Differentiate between the two types of chlorophylls involved in photosynthesis

Chlorophyll a occurs in all photosynthetic organisms while chlorophyll b is an accessory pigment that has a different absorption spectrum

• grana

a mass of stacked thylakoid membranes

• stroma

a fluid-filled space inside the inner membrane of the chloroplast that surrounds the grana, contains DNA, ribosomes and enzyme molecules

• absorption spectrum

the light absorption pattern of a pigment

• action spectrum

the relative effectiveness of different wavelengths of light for the light-requiring process of photosynthesis

• Explain why photosynthesis depends on the relationship between the absorption spectrum of chlorophyll and action spectrum of photosynthesis

When chlorophyll absorbs solar energy, electrons are temporarily excited to a higher energy level and when they return to their original energy level, energy is released that is used for photosynthesis. Hence, the action spectrum of photosynthesis depends on the rate at which the absorption spectrum of chlorophyll allows it to excite electrons

• photosystems

units within photosynthetic organisms which are responsible for capturing solar energy for photosynthesis that pass electrons from one molecule to another, with a portion of this electron flow being allocated towards ATP synthesis.

• State and differentiate between the two photosystems involved in photosynthesis

PSI and PSII both have a group of packed pigment molecules of chlorophyll a and chlorophyll b but the reaction center, chlorophyll a, of PSII is called P680 because it absorbs light with a wavelength of 680nm the best while the reaction center, chlorophyll b, of PSI is called P700 because it absorbs light with a wavelength of 700nm the best

• antenna complex

group of pigment molecules that absorbs light and concentrates its energy into one chlorophyll a molecule at the reaction center where they become excited and move to an electron acceptor molecule

• Briefly state how ATP is used as the energy currency for all organisms

ATP stores energy in cells through its high-energy phosphate bonds which when broken via hydrolysis, releases energy to power cellular activities

• photophosphorylation

an electron-dependent photosynthetic process in which light energy is used to create an electron transport chain that creates a proton gradient which is used by ATP synthase to produce ATP

• State the two types of electron transport in chloroplasts and differentiate between them

The two types of electron transport are cyclic and non-cyclic. Cyclic electron transport uses only PSI to generate ATP and is known as cyclic photophosphorylation. Non-cyclic electron transport uses both PSI and PSII to generate both ATP and NADPH and is known as non-cyclic phosphorylation

• Describe the cyclic electron pathway

It starts when the PSI antenna complex absorbs solar energy and the accessory pigment funnels it to the chlorophyll a reaction centre molecules. The excited electrons leave the reaction center molecule and enter the electron transport system and are eventually returned to PSI due to the low amount of light energy absorbed being unsustainable for their continuous movement along the electron transport chain. Electrons pass from one carrier molecule to the next in a series and releases energy as they go along. This energy release causes protons to be actively pumped into the thylakoid from the stroma across the thylakoid membrane. This creates a proton gradient in which the concentration of protons is higher inside the thylakoid space than in the stroma. Protons diffuse from the thylakoid membrane back into the stroma via chemiosmotic channels against their electrochemical gradient, generating energy to drive the enzymatic conversion of ADP and P into ATP.

• chemiosmotic channels

the specific membrane protein complexes that allow ions to diffuse across a membrane down their electrochemical gradient

• Describe the non-cyclic electron pathway

Light is used to oxidize water to form oxygen, protons and electrons. These electrons are used to replace those lost from the clorophyll amolecules in the reaction center when they are exicted. PSI uses light energy to reduce NADP to NADPH while PSII uses light energy to oxidize water molecules. Electrons move from water to PSII and then to PSI and then to NADP. The chlrophyll a molecules in the antenna complex of PSII absorb light energy and the excited electrons leave the reaction centre, immediately after which PSII takes replacement electrons from a water molecule. Two oxygen atoms then combine to form a molecule of oxygen while the protons stay inside the thylakoid space to form the proton gradient used to generate ATP. Excited electrons from PSII are taken to an electron transport system where a series of molecules carry electrons and release energy which is stored in the form of a proton gradient. Eventually, protons inside the thylakoid flow against their electrochemical gradient for the purpoes of creating ATP. Once the excited electrons travel along the transport system, they lose their energy and enter PSI. Excited electrons in PSI leave the reaction centre and are taken up by an electron acceptor which passes them to NADP where they bind with protons and NADP to form NADPH

• Differentiate between the light-independent and light-dependent stages of photosynthesis

The light-independent stage focuses on converting solar energy into chemical energy, while the light-independent stage utilizes the product of the light-dependent stage to create organic substances

• The Calvin cycle

a series of reactions that reduce carbon dioxide and convert it into organic substances during photosynthesis in which ATP is used as an energy source and NADPH supplies electrons to make sugar

• State the three basic steps of the Calvin cycle

fixation of carbon dioxide, reduction of carbon dioxide, regeneration of ribulose biphosphate

• Describe the fixation of carbon dioxide in the Calvin cycle

Carbon dioxide is attached to ribulose biphosphate carboxylase together with ribulose phosphate. A carboxylation reaction occurs in which a carbon-carbon is formed between carbon dioxide and one of the carbons in RuBP, forming an unstable 6-carbon which immediately forms into two molecules of phosphogyleric acid

• Describe the reduction of carbon dioxide in the Calvin cycle

Each of the two PGA molecules is reduced to another three-carbon molecule called glyceraldehyde 3- phosphate (PGAL), using the energy supplied by ATP — converting it into ADP in the process. Electrons for the reduction are supplied by NADPH which is changed to NADP in the process.

• Describe the regeneration of ribulose biphosphate

Using the energy supplied by ATP, for every three turns of the Calvin cycle, five molecules of PGAL are used to regenerate three molecules of RUBP and the net gain is one PGAL molecule which is used to generate other organic compounds that the plant needs. PGAL can be converted to glucose phosphate which is the starting molecule for the synthesis of starch and cellulose. Glucose phosphate and fructose are combined to form sucrose, where phosphate is removed from the gluocse during the reaction and transported around in the form of sucrose. After which, it is hydrolsed when a plant cells needs glucose and the excess is turned into starch.

• State the limiting factors of photosynthesis

Light intensity, availability of carbon dioxide, water supply and temperature

• Explain how light intensity is a limiting factor of photosynthesis

All reactions in photosynthesis are dependent on solar energy and, hence, when light intensity is low, the rate of photosynthesis decreases

• Explain how the availability of carbon dioxide is a limiting factor of photosynthesis

Carbon dioxide is the primary “ingredient” of all the organic products of photosynthesis, thus, the less there is available, the lower the rate of photosynthesis, especially since it is used rapidly

• Explain how temperature is a limiting factor of photosynthesis

Temperature is important for maintaining optimal conditions for the enzymes involved during photosynthesis

• Explain how water supply is a limiting factor of photosynthesis

The loss or absence of water causes the plant cells to plasmolyse and wilt. This is particularly detrimental as guard cells close when they begin to plasmolyse which limits the diffusion of carbon dioxide into the leaf. In addition, water is necessary for the light-dependent stage during non-cyclic photophosphorylation as it supplies electrons to PSII for the generation of ATP

• hydroponics

the growth of plants in aqueous solutions containing essential materials

• Explain how knowledge of limiting factors have been used to improve crop production by farmers

The use of hydroponics takes advantage of the role water plays in photosynthesis and its ability to carry minerals. Greenhouses are designed specifically to specific stable conditions curated to the plants and their growth by controlling light intensity, water, temperature, carbon dioxide concentration and supply of minerals. These are used to produce crops in places with adverse, changing or unsuitable climatic conditions which is useful especially for plants that are sensitive to fluctuating abiotic factors

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Example of optimum temperature for enzymes

Image the relationship between the concentration of carbon dioxide/light intensity with photosynthesis rate

Image the Calvin cycle

Image non-cyclic photophosphorylation

Image cyclic phosphorylation

Image the structure of chloroplast

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Spongy mesophyll cells

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Palisade mesophyll cells

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Structure of the stomata

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The internal structure of the dicotyledonous leaf

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Simple structure of the leaf