Respiration and Photosynthesis

Learning Objective

In this lesson we will learn about two of the most important chemical processes in living things – cellular respiration and photosynthesis.

Learning Outcomes

By the end of this lesson you will be able to:

  • Write chemical equations for respiration and photosynthesis.

  • Describe the role of respiration and photosynthesis in living things and explain how energy transfers are involved in these processes.

  • List types of organisms where respiration and photosynthesis take place, as well as the cell structures where these chemical processes occur.

  • Compare aerobic and anaerobic respiration.

  • Compare respiration and photosynthesis.

 

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Introduction

  • All living things require a continual supply of energy to function.
    Energy is essential for numerous processes, such as growth, repair, movement, reproduction, maintaining homeostasis and immunity.
  • All living things get their energy from a chemical process known as cellular respiration.
    The main type of cellular respiration is aerobic respiration, which involves the breakdown of glucose molecules in the presence of oxygen.
    Animals and plants both get the oxygen required for respiration from their surrounding environment, but they obtain their glucose through very different processes – digestion and photosynthesis.

 
respiration photosynthesis digestion

Photosynthesis and respiration are two of the most important chemical processes in biology.

(Image: loilamtan, Pixabay)

 

Respiration

  • Respiration is a chemical process that converts the energy stored in glucose (a type of sugar) to a form of energy that cells can readily utilise.
  • Respiration takes place in the cells of all living things and involves the breakdown of glucose molecules.
  • Although respiration produces chemical products, such as carbon dioxide and water, its primary purpose is the conversion of energy (to a form known as ATP).

 

Aerobic and Anaerobic Respiration

  • There are two types of respiration, depending on whether oxygen is involved or not.
    1. Aerobic respiration – the breakdown of glucose in the presence of oxygen.
    2. Anaerobic respiration – the breakdown of glucose in the absence of oxygen.

 
aerobic anaerobic respiration

Aerobic respiration requires oxygen, whereas anaerobic respiration does not.

(Image: pxhere)

 

Aerobic Respiration

  • The word equation for aerobic respiration is:

 
aerobic respiration word equation

 

  • The formula equation for aerobic respiration is:

 
aerobic respiration formula equation

 

  • Aerobic respiration is a much more efficient way of producing energy.
    It produces almost 20 times more energy than anaerobic respiration.
  • Multicellular organisms such as animals and plants use aerobic respiration most of the time.

 aerobic respiration oxygen carbon dioxide test

Aerobic respiration provides energy for cells by breaking down glucose in the presence of oxygen, producing carbon dioxide and water as by-products.

(Image: Rachel Spencer, US Air Force)

 

Anaerobic Respiration

  • There are several types of anaerobic respiration.
    Two of the main types are alcoholic fermentation and lactic acid fermentation.
  • Alcoholic fermentation produces energy by converting glucose into ethanol and carbon dioxide, as shown in the following equation:

 
alcoholic fermentation equation

 

    Plant root cells can carry out alcoholic fermentation when they are in waterlogged soils.
    Yeast cells carry out alcoholic fermentation during the production of wine and beer.

 
wine grape yeast fermentation anaerobic respiration

During the production of wine, yeast cells convert the glucose in grapes to alcohol and carbon dioxide gas – an example of anaerobic respiration.

(Images: Agne27, Wikimedia Commons)

 

  • Lactic acid fermentation produces energy by converting glucose into lactic acid, as shown in the following equation:

 
lactic acid fermentation equation

 

    Muscle cells in animals carry out lactic acid fermentation during strenuous exercise, causing muscles to ‘burn’.
    Bacteria carry out lactic acid fermentation during the production of yoghurt.

 
yoghurt anaerobic respiration lactic acid fermentation

During the production of yoghurt, bacteria convert the glucose in milk to lactic acid – an example of anaerobic respiration.

(Images: moritz320, Pixabay)

 

Aerobic Respiration in Animals and Plants

 

Mitochondria

  • Aerobic respiration takes place in cell structures called mitochondria.
    There are many mitochondria in each cell; cells that need more energy, such as muscle cells, will have more mitochondria than cells that need less energy, such as skin cells.
  • The reactants for respiration – glucose and oxygen – are transported into the mitochondria of cells.
    The products of respiration – carbon dioxide and water – are transported away from the mitochondria of cells.

 
mitochondrion structure

Structure of a mitochondrion.

(Images: LadyofHats, Wikimedia Commons)

 

Obtaining Glucose

  • Animals get their glucose by breaking down the food they eat, by a process known as digestion.
    Digestion takes place in the stomach and intestines.
    Glucose then enters the bloodstream, where it is transported around the body to all cells.
  • Plants synthesise their own glucose from gases in the atmosphere, by a process known as photosynthesis.
    Photosynthesis takes place in leaves.
    Glucose then enters tubes called phloem, where it is transported around the plant to all cells.
  • Glucose that is not immediately used for respiration can be converted into storage molecules for later use.
    In animal, glucose is converted to glycogen.
    In plants, glucose is converted to starch.

 

animal digestion glucose respiration    plant leaf sunlight photosynthesis glucose

Animals obtain glucose by digesting food that they eat.
Plant synthesis their own glucose by photosynthesis.

(Images: Couleur, Pixabay; llmicro, Pixabay)

 

Obtaining Oxygen

  • Animals such as mammals, reptiles and birds obtain oxygen from surrounding air by inhaling it into their lungs. Fish obtain oxygen by extracting it from water using their gills.
    Oxygen is then transported to cells via the bloodstream.
  • Plants obtain oxygen from surrounding air by absorbing it though holes in their leaves known as stomates.
    Oxygen then diffuses into cells.

 

lungs oxygen respiration    leaf stomate oxygen carbon dioxide

Humans absorb oxygen from the surrounding air via their mouth/nose and lungs.
Plants absorb oxygen from the surrounding air through pores in their leaves called stomates.

(Images: Theresa knott, Wikimedia Commons; Photohound, Wikimedia Commons)

 

Carbon Dioxide and Water

  • For animals, the carbon dioxide produced by respiration is toxic.
    It is therefore transported to the lungs/gills via the bloodstream where it is released from the body.
    Water produced by respiration is used by the body.
  • For plants, the carbon dioxide produced by respiration can be used for photosynthesis.
    Water produced by respiration is also used by the plant.

 

Photosynthesis

  • Plants do not consume other organisms like animals do; instead, they produce their own food by a process called photosynthesis.
    Photosynthesis is a process that uses the energy of sunlight to synthesise glucose (and oxygen) from carbon dioxide and water.
    In addition to plants, green algae and some bacteria can carry out photosynthesis.
  • The word equation for photosynthesis is:

 
photosynthesis word equation

 

  • The formula equation for photosynthesis is:

 
photosynthesis chemical formula equation

 
 
photosynthesis diagram

The process of photosynthesis.

(Image: blueringmedia, Adobe Stock)

 

Chloroplasts

  • Photosynthesis takes place in cell structures called chloroplasts.
    These structures mainly occur in leaf cells, but also in other green tissues such as young stems.
  • Within chloroplasts are chlorophyll molecules.
    These molecules are able to absorb the energy from sunlight that enables the photosynthesis reaction to proceed.
    They also reflect green light, which is the reason for the green colour of plants.
  • The highest concentration of chloroplasts occurs in cells in the upper surface of leaves.
    This maximises the harnessing of sunlight energy by plants.
  • Similar to mitochondria and respiration, the reactants for photosynthesis – carbon dioxide and water – are transported into the chloroplasts of photosynthetic cells, and the products – glucose and oxygen – are transported away.

 
chloroplast structure

Structure of a chloroplast.

(Image: Kelvinsong, Wikimedia Commons)

 

Carbon Dioxide and Water

  • Carbon dioxide gas enters plant cells in the same way that oxygen does – it is absorbed via the stomates, then diffuses into leaf cells.
  • Water is absorbed by the roots of plants and then transported to all cells via tubes called xylem.
    Water is not only necessary for photosynthesis, but a variety of other purposes, including being a solvent for chemical reactions and providing turgidity for leaves and stems.

 

The Products of Photosynthesis

  • Plants use the glucose produced by photosynthesis to generate energy via respiration.
    In fact, the chemical equation for photosynthesis is the reverse of the equation for aerobic respiration.
  • Plants also use glucose for other cellular processes and can store excess glucose in the form of starch.

 
glucose starch molecules

Glucose that is not immediately used by a plant can be converted to starch for storage.

 

  • Most of the oxygen produced by photosynthesis is not used by plants, and is subsequently released into the atmosphere as a by-product.
    For most other lifeforms however, this oxygen is essential for survival – photosynthesis is the sole source of atmospheric oxygen which they require for respiration.

 
photosynthesis oxygen atmosphere

There would be no oxygen in Earth’s atmosphere if it were not for photosynthesis.

(Image: pxhere)

 

Comparing Respiration and Photosynthesis

  • Respiration and photosynthesis are two essential life processes, with many similarities:
    • They are both complex chemical processes, involving numerous steps and enzymes – molecules that enable each step to take place.
    • They both occur in the cells of living things.
    • They involve common molecules – glucose, oxygen, carbon dioxide and water.
    • The both involve energy conversions.
  • Some of the key difference between respiration and photosynthesis are:
    • Respiration occurs in all organisms, whereas photosynthesis only occurs in plants, green algae and some bacteria.
    • Respiration occurs mitochondria, whereas photosynthesis occurs chloroplasts.
    • Respiration consumes glucose and oxygen, whereas photosynthesis produces them.
    • Respiration produces carbon dioxide and water, whereas photosynthesis consumes them.
    • Respiration is an exothermic process that releases energy (from glucose), whereas photosynthesis is an endothermic process that absorbs energy (from sunlight).
    • Photosynthesis requires sunlight, whereas respiration does not.

 

animal plant cell mitochondria chloroplast respiration photosynthesis

Plant cells contain both mitochondria and chloroplasts – they can perform respiration and photosynthesis.
Animal cells contain mitochondria but not chloroplasts – they can perform respiration but not photosynthesis.

(Images: achiichiii, Adobe Stock)

 

Summary

  • Living organisms obtain energy by breaking down glucose in their cells – a process known as respiration.
  • Respiration can be summarised as:
    glucose  +  oxygen    carbon dioxide  +  water  +  energy
  • Animals obtain glucose for respiration by digesting food that they eat.
    Plants obtain glucose for respiration by producing it through a process known as photosynthesis.
  • Photosynthesis uses the energy of sunlight to convert carbon dioxide and water into glucose and oxygen.
  • Photosynthesis can be summarised as:
    carbon dioxide  +  water  +  sunlight energy    glucose  +  oxygen
  • Glucose that is not immediately used by plants can be converted to starch for storage.
    Glucose that is not immediately used by animals can be converted to glycogen for storage.
  • Respiration mostly occurs in the presence of oxygen, where it is called aerobic respiration.
    It can also occur in the absence of oxygen, where it is called anaerobic respiration.
    Examples of anaerobic respiration are alcoholic fermentation and lactic acid fermentation.
  • Aerobic respiration produces significantly more energy than anaerobic respiration.
  • Respiration takes place in cell structures called mitochondria.
    Mitochondria occur in all cells; therefore respiration takes place in all organisms.
  • Photosynthesis takes place in cell structures called chloroplasts.
    Chloroplasts occur in the cells of plants, green algae and some bacteria; therefore photosynthesis only takes place in these organisms.
  • Respiration and photosynthesis both involve the exchange of gases between organisms and their surrounding environment.
    In humans, this involves the nose or mouth, lungs and bloodstream.
    In plants this involves pores in leaves known as stomates.

 

    Respiration Respiration and Photosynthesis Photosynthesis
    Occurs in all organisms Occur in living things Occurs mainly in plants
    Occurs in mitochondria Occur in cells Occurs in chloroplasts
    Consumes glucose Involve glucose Produces glucose
    Consumes oxygen Involve oxygen Produces oxygen
    Produces carbon dioxide Involve carbon dioxide Consumes carbon dioxide
    Produces water Involve water Consumes water
    Releases energy Involve energy conversion Absorbs energy
    Does not require sunlight Are complex chemical process Requires sunlight

Comparing respiration and photosynthesis.

 

(Header image: Zbysek.nemec, Wikimedia Commons)

 

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