Photosynthesis
Photosynthesis occurs within the chloroplasts of plant cells, mainly those of the leaf, and only in daylight hours. leaf cells, both in their structure and arrangement, show adaptations for maximising photosynthesis. Chloroplasts have structural adaptations enabling them to carry out efficiently the chemical reactions of photosynthesis.
Leaf adaptations
Leaves are the site of photosynthesis. The structure and location of leaves show adaptations for photosynthesis. Leaves:
Leaves in angiosperms have a characteristic layout of cell/tissues from top to bottom (dorsal to ventral)
In the stroma, the light-independent (Calvin cycle) photosynthetic reactions take place- the bonding of carbon dioxide with hydrogen (from water), and their rearrangement to form glucose, C6H12O6.
Chloroplasts are also found in the outer layer of cells (cortex) in the stems of non-woody plants. Light is able to penetrate to these cells, allowing photosynthesis to occur to supplement that from the leaves. Pores (lenticels) in the epidermis of the stem allow carbon dioxide and oxygen to diffuse into/out of the cells.
Leaves are the site of photosynthesis. The structure and location of leaves show adaptations for photosynthesis. Leaves:
- Are thin so that light penetrates to all the photosynthetic cells
- have a large surface area to capture as much light as possible
- are arranged in patterns around the stem and branches, to reduce overlap and so allow each leaf to capture as much light as possible
Leaves in angiosperms have a characteristic layout of cell/tissues from top to bottom (dorsal to ventral)
- Waxy cuticle- reduces water loss
- Upper epidermis (the "skin")- clear to let maximum sunlight through
- Palisade cell layer- cells are rich in chloroplasts for maximum photosynthesis, close to upper surface of the leaf. Chloroplasts near outer region of cell to intercept as much light as possible and reduce the distance carbon dioxide has to diffuse into the stroma. Cells are long, packed for maximum light absorption with large surface area.
- Spongy cell layer- involved in gas exchange, contain few chloroplasts, smaller than palisade cells. Cells are loosely packed and spaced out to give maximum surface area and air space to facilitate diffusion of carbon dioxide, oxygen and water vapor to/from cells. Close to lower epidermis allowing rapid diffusion of gases into and out of leaf.
- Vein- contains vascular tissue, (xylem- transports water and minerals, phloem- transports glucose)
- Lower epidermis- the "skin"
- Guard cells- open and close the stoma; closure prevents water loss
- Stoma- pore in the lower epidermis to allow entry/exit of gases
In the stroma, the light-independent (Calvin cycle) photosynthetic reactions take place- the bonding of carbon dioxide with hydrogen (from water), and their rearrangement to form glucose, C6H12O6.
Chloroplasts are also found in the outer layer of cells (cortex) in the stems of non-woody plants. Light is able to penetrate to these cells, allowing photosynthesis to occur to supplement that from the leaves. Pores (lenticels) in the epidermis of the stem allow carbon dioxide and oxygen to diffuse into/out of the cells.
There are two main chemical pathways in which the raw materials- water and carbon dioxide - are joined to produce the sugar glucose (with oxygen being a waste product). The source of energy is solar (from the sun) and its entrapment requires the presence of pigment molecules such as chlorophyll. All chemical pathways in photosynthesis are controlled by enzymes.
First chemical pathway - Light-dependent Reaction
The light-dependent reaction takes place on the thylakoid membranes of the grana of chloroplasts.
This step is catalysed by a series of enzymes. Chlorophyll in the thylakoid membrane absorbs solar energy (this gets the chlorophyll's electrons "excited" by solar energy hitting them), which splits water into hydrogen and oxygen. The high energy electrons are used to make the "energy molecule" ATP from ADP. Oxygen is a waste product and is removed form the cell. The hydrogen produced is picked up by NADPH and is carried into the dark/light independent stage.
The light-dependent reaction takes place on the thylakoid membranes of the grana of chloroplasts.
This step is catalysed by a series of enzymes. Chlorophyll in the thylakoid membrane absorbs solar energy (this gets the chlorophyll's electrons "excited" by solar energy hitting them), which splits water into hydrogen and oxygen. The high energy electrons are used to make the "energy molecule" ATP from ADP. Oxygen is a waste product and is removed form the cell. The hydrogen produced is picked up by NADPH and is carried into the dark/light independent stage.
Second chemical pathway - Light-independent Reaction
The light-independent reaction is also known as the Calvin Cycle. This occurs in the liquid matrix of the chloroplast, the stroma.
This stage is likewise controlled by enzymes. The hydrogen from the first stage and carbon dioxide enters a biochemical cycle and react to produce glucose. Carbon dioxide and hydrogen are continuously fed into the cycle. ATP (produced in the light-dependent reaction) is used to run the cycle by supplying the energy needed to bond the carbon dioxide and water in forming glucose. The glucose is used to provide energy, and is changed into other chemicals (e.g cellulose) or is stored as starch.
The glucose produced is:
The light-independent reaction is also known as the Calvin Cycle. This occurs in the liquid matrix of the chloroplast, the stroma.
This stage is likewise controlled by enzymes. The hydrogen from the first stage and carbon dioxide enters a biochemical cycle and react to produce glucose. Carbon dioxide and hydrogen are continuously fed into the cycle. ATP (produced in the light-dependent reaction) is used to run the cycle by supplying the energy needed to bond the carbon dioxide and water in forming glucose. The glucose is used to provide energy, and is changed into other chemicals (e.g cellulose) or is stored as starch.
The glucose produced is:
- Stored as insoluble starch, typically in the cells of roots
- used in respiration.
- used to make other needed organic chemicals - e.g fats, amino acids (for proteins), cellulose
Photosynthesis word equation
Photosynthesis simple formula equation
Photosynthesis complex formula equation
As the (waste) oxygen produces comes from the water only, it is more accurate to write the balances equation as follows:
As the (waste) oxygen produces comes from the water only, it is more accurate to write the balances equation as follows:
Rate of photosynthesis
The rate of photosynthesis is determined by factors such as temperature, light intensity and carbon dioxide concentration.
Because these three factors combine to determine the rate of photosynthesis, maximum rates of photosynthesis can alter if one of the other factors changes.
The rate of photosynthesis is typically measured as the amount of oxygen produced or the amount of carbon dioxide consumed in a given time.
Other environmental factors that can influence photosynthesis are the following.
The rate of photosynthesis is determined by factors such as temperature, light intensity and carbon dioxide concentration.
- Temperature- increasing the temperature increases the rate of photosynthesis up to an optimum temperature. When temperatures rise too far above the optimum temperature, the enzymes controlling the reaction denature and can no-longer catalyse the reactions; photosynthesis ceases.
- Light intensity- increasing the light intensity increases the rate of photosynthesis up to a maximum. Above the maximum, further increases in light intensity have no further effect on the photosynthesis rate (because either the light-absorbing pigments are saturated and /or the concentration of carbon dioxide is limiting the Calvin cycle and/or the temperature is too low).
- Carbon dioxide concentration- increasing carbon dioxide concentration increase the rate of photosynthesis up to a maximum. Above the maximum carbon dioxide concentration, further increases in carbon dioxide concentration have no further effect on the photosynthesis rate (because either the light-absorbing pigments are saturated and /or the carbon dioxide which is saturating the chloroplasts is limiting the Calvin cycle and/or the temperature is too low).
Because these three factors combine to determine the rate of photosynthesis, maximum rates of photosynthesis can alter if one of the other factors changes.
The rate of photosynthesis is typically measured as the amount of oxygen produced or the amount of carbon dioxide consumed in a given time.
Other environmental factors that can influence photosynthesis are the following.
- Water- cells, even if dehydrated, contain quantities of water sufficient to allow photosynthesis; however, the closure of the stomata to prevent waterloss will also prevent entry of carbon dioxide which will limit photosynthesis.
- Mineral ions- certain mineral ions are essential for photosynthesis to occur (e.g Fe is needed for the synthesis of chlorophyll, Mg is and essential part of the structure of the chlorophyll molecule (a deficiency of Mg causes yellowing leaves)).
- Light wavelength- the light-absorbing pigments are most active when absorbing light in the blue and red wavelengths of the electomagnetic spectrum; wavelengths of green are not absorbed, but reflected (hence the green colour of plants).
Importance of photosynthesis
The importance of photosynthesis is that it converts solar energy into chemical energy as organic molecules (carbohydrates, proteins, fats).
Plants are autotrophs (self-feeders) since they are the producers (of food)- the starting point of food chains. Organisms that feed on other organisms are called heterotophs. The wastes and dead bodies of autotrophs and heterotrophs are broken doen by decomposers (bacteria and fungi), and the chemicals locked in the wastes and bodies are released into the soil. These enter the roots of plants, allowing their growth and new supplies to food chains.
Photosynthesis by plants accounts for the high level (approximately 20%) of oxygen in the atmosphere. This gas is essential for the process of areobic respiratioin.
Without photosynthesis, animal life could not exist.
The importance of photosynthesis is that it converts solar energy into chemical energy as organic molecules (carbohydrates, proteins, fats).
Plants are autotrophs (self-feeders) since they are the producers (of food)- the starting point of food chains. Organisms that feed on other organisms are called heterotophs. The wastes and dead bodies of autotrophs and heterotrophs are broken doen by decomposers (bacteria and fungi), and the chemicals locked in the wastes and bodies are released into the soil. These enter the roots of plants, allowing their growth and new supplies to food chains.
Photosynthesis by plants accounts for the high level (approximately 20%) of oxygen in the atmosphere. This gas is essential for the process of areobic respiratioin.
Without photosynthesis, animal life could not exist.
Respiration
Respiration occurs in the mitochondria of the cells in all organisms all the time (both day and night). Respiration is the process by which the cell breaks down glucose to produce ATP - adenosine triphosphate; heat energy is a by-product. ATP is the so-called "energy molecule" in all organisms and is used to fuel all the chemical reactions of the cell, including the following.
The breakdown of glucose may be aerobic or anaerobic; aerobic respiration produces much larger amounts of ATP per glucose molecule than anaerobic respiration does.
ATP is constantly made in cells from ADP - adenosine diphosphate. ATP is made up of a molecule called ADP (adenosine diphosphate) bonded with a phosphate ion. Energy is needed to for the bond and is released when the bond is broken. The energy from glucose metabolism adds a high-energy phosphate bond to ADP to make ATP.
When a cell needs energy, the high-energy phosphate bond is broken and ATP returns back to ADP (ADP could be thought of as a "flat battery" and ATP as a "charged battery"). The process is very rapid and on-going.
ATP receives energy from the breakdown of glucose, and then releases energy from the breakdown of ATP.
The heat produced as a by-product of respiration (aerobic respiration is approximately 45% efficient) is used in maintaining body temperature in homeotherms ("warm blooded animals - birds and mammals).
- Active transport of substances across membranes.
- Synthesis of molecules -e,g proteins from amino acids.
- Movement- e.g phagocytosis, action of cilia and flagella, action of actin and myosin in muscle contraction.
- Bioluminescence (light production) in cells of such animals as glow-worms and fireflies.
- Chemical reactions in cells to produce molecules ee proteins, enzymes
The breakdown of glucose may be aerobic or anaerobic; aerobic respiration produces much larger amounts of ATP per glucose molecule than anaerobic respiration does.
ATP is constantly made in cells from ADP - adenosine diphosphate. ATP is made up of a molecule called ADP (adenosine diphosphate) bonded with a phosphate ion. Energy is needed to for the bond and is released when the bond is broken. The energy from glucose metabolism adds a high-energy phosphate bond to ADP to make ATP.
When a cell needs energy, the high-energy phosphate bond is broken and ATP returns back to ADP (ADP could be thought of as a "flat battery" and ATP as a "charged battery"). The process is very rapid and on-going.
ATP receives energy from the breakdown of glucose, and then releases energy from the breakdown of ATP.
The heat produced as a by-product of respiration (aerobic respiration is approximately 45% efficient) is used in maintaining body temperature in homeotherms ("warm blooded animals - birds and mammals).
Aerobic respiration
Aerobic respiration needs oxygen for the complete breakdown of glucose into carbon dioxide and water; energy is released in the form of ATP and heat. There are three main enzyme-controlled chemical pathways in aerobic respiration - glycolysis, Krebs cycle and the electron transfer chain.
Glycolysis ("glucose splitting"), the first chemical pathway, occurs in the cytoplasm of the cell.
The Krebs cycle (also known as the Citric acid cycle), is the second chemical pathway of aerobic respiration, and it occurs in the matrix of the mitochondria.
The electron transfer chain (also known as the Hydrogen transfer chain or the Respiratory chain) is the final chemical pathway of aerobic respiration, and it occurs on the cristae of the mitochondria.
The third stage of aerobic respiration produces the most ATP. One molecule of glucose that enters glycolysis yields 38 molecules of ATP by the end of the electron transfer chain.
Aerobic respiration needs oxygen for the complete breakdown of glucose into carbon dioxide and water; energy is released in the form of ATP and heat. There are three main enzyme-controlled chemical pathways in aerobic respiration - glycolysis, Krebs cycle and the electron transfer chain.
Glycolysis ("glucose splitting"), the first chemical pathway, occurs in the cytoplasm of the cell.
- Each glucose molecule is broken down into two pyruvate molecules.
- Two molecules (small amount) of ATP are produced in the breakdown, as well as hydrogen which moves to the cristae.
- This stage does not require oxygen.
The Krebs cycle (also known as the Citric acid cycle), is the second chemical pathway of aerobic respiration, and it occurs in the matrix of the mitochondria.
- Products from glycolysis are modified. Pyruvate enters matrix of mitochondria and broken down. Carbon dioxide (waste product) is removed, producing acetyl co-enzyme A and more hydrogen which moves to the cristae. (A series of reactions occur producing carbon dioxide, hydrogen ions and 2 ATP)
- Hydrogen ions are picked up by a carrier molecule (NAD, a co-enzyme) and taken to the third chemical pathway of aerobic respiration.
The electron transfer chain (also known as the Hydrogen transfer chain or the Respiratory chain) is the final chemical pathway of aerobic respiration, and it occurs on the cristae of the mitochondria.
- Hydrogen ions and their high-energy electrons are passed along the electron transfer chain, releasing energy which is captured as 34 ATP. Oxygen is required at this stage and Water is produced. Each glucose molecule produces 38 ATP during aerobic respiration. At the end of the electron transfer chain, the electrons are returned to the hydrogen ions, which become atoms again and combine with oxygen to form water.
The third stage of aerobic respiration produces the most ATP. One molecule of glucose that enters glycolysis yields 38 molecules of ATP by the end of the electron transfer chain.
Aerobic respiration word equation
+ ATP + heat energy
+ ATP + heat energy
Aerobic respiration formula equation
Anaerobic respiration
Anaerobic respiration occurs in the absence of oxygen. Only glycolysis takes place, therefore only 2 molecules of ATP are produced from each glucose molecule. This is what happens if the Krebs cycle or electron-transfer chain get backed up because there is no/not enough oxygen available.
Anaerobic respiration in animals
Anaerobic respiration in animals occurs when oxygen is short in supply; typically during prolonged exercise (only animals such as endoparasites respire anaerobically as a way of life). The pyyruvate formed in glycolysis is broken down into lactic acid. The build up pf lactic acid causes muscle fatigue. The muscles need to stop working to allow replenished supplies of oxygen to continue the breakdown of the lactic acid into water and carbon dioxide.
Anaerobic respiration in yeast and bacteria
In yeast and bacteria, anaerobic respiration is a way of life. In yeast, the pyruvate is broken down into ethanol, C2H5OH, and carbon dioxide. This process is also known as fermentation. The small energy yield of 2 ATP molecules is sufficient to meet the lifestyle needs of these organism.
Anaerobic respiration occurs in the absence of oxygen. Only glycolysis takes place, therefore only 2 molecules of ATP are produced from each glucose molecule. This is what happens if the Krebs cycle or electron-transfer chain get backed up because there is no/not enough oxygen available.
Anaerobic respiration in animals
Anaerobic respiration in animals occurs when oxygen is short in supply; typically during prolonged exercise (only animals such as endoparasites respire anaerobically as a way of life). The pyyruvate formed in glycolysis is broken down into lactic acid. The build up pf lactic acid causes muscle fatigue. The muscles need to stop working to allow replenished supplies of oxygen to continue the breakdown of the lactic acid into water and carbon dioxide.
Anaerobic respiration in yeast and bacteria
In yeast and bacteria, anaerobic respiration is a way of life. In yeast, the pyruvate is broken down into ethanol, C2H5OH, and carbon dioxide. This process is also known as fermentation. The small energy yield of 2 ATP molecules is sufficient to meet the lifestyle needs of these organism.
Anaerobic respiration word equation
+ATP + heat
+ATP + heat
Anaerobic respiration formula equation
Fatty acids and glycerol (from digestion of fats), and amino acids (from digestion of proteins), can also be used in respiration (entering Krebs cycle after prior processing) to generate ATP.
Rate of respiration
The rate of respiration is determined by factors such as temperature and energy demands.
The rate of respiration is typically measured by the amount of oxygen consumed or the amount of carbon dioxide produced in a given time.
The full name for respiration is cellular respiration- because it occurs in the cells to release energy, a chemical process. It is not the same process as breathing (inhaling and exhaling of gases into and out of the body, a physical process) or gas exchange (exchange of oxygen and carbon dioxide across a membrane by diffusion, a passive process).
The rate of respiration is determined by factors such as temperature and energy demands.
- Temperature- increasing temperature increases the rate of respiration up to an optimum temperature. As temperatures rise too high above the optimum temperature, the enzymes controlling the reaction denature and can no longer catalyse reaction; respiration ceases (and the organism dies). In homeotherms, the optimum temperature is core body temperature (e,g 37 degrees Celsius in humans),
- Body'e energy demands- the rate of respiration will increase up to a maximum as the demand from the cells of tissues increases (e.g muscle cells of legs when a human sprints). The amount of oxygen needed and the amount of carbon dioxide produced increase, and breathing rate increases to compensate. The build up of carbon dioxide will slow down the rate of respiration.
The rate of respiration is typically measured by the amount of oxygen consumed or the amount of carbon dioxide produced in a given time.
The full name for respiration is cellular respiration- because it occurs in the cells to release energy, a chemical process. It is not the same process as breathing (inhaling and exhaling of gases into and out of the body, a physical process) or gas exchange (exchange of oxygen and carbon dioxide across a membrane by diffusion, a passive process).
Comparing photosynthesis and respiration
Photosynthesis
- Converts solar energy into chemical energy in the form of glucose (and starch)
- Uses carbon dioxide and water as raw materials to make glucose and release oxygen as a waste product
- Occurs in plant cells (chloroplasts) only during daylight hours
Respiration
- Converts chemical energy in the form of glucose into chemical energy in the form of ATP and heat energy
- Aerobic respiration uses glucose and oxygen as the raw materials and releases water and carbon dioxide (carbon dioxide is a waste product)
- Occurs in the cells (mitochondria) of all organisms all the time
In plants, the effect of respiration in the daytime is typically masked by photosynthesis, with the net result being the consumption of carbon dioxide and the production of oxygen. Therefore, the effects of respiration are typically seen only at night in plants.