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Introduction

The production of energy is an essential component of life. For QCE, the general focus is on the production of the ATP (Adenosine Triphosphate) from the energy-rich compound glucose. When ATP is produced by cells, it can be converted into ADP (Adenosine Diphosphate) which is the same as ATP but has two phosphates instead of three. When it is converted, a chemical bond between the second and third phosphate is broken, releasing the chemical energy stored.

Key Components

Before we begin, here are some terms which you should familiarise yourself with.

• Endosymbiosis: a symbiotic relationship where one organism takes up a permanent residence inside other living organisms
• Metabolism: all the chemical reactions that are occurring in a cell
• Autotrophs: organisms that are able to produce their own source of energy e.g. plants can use photosynthesis to produce glucose
• Heterotrophs: organisms that obtain their source of energy from other living organisms e.g. carnivores eating other animals

Photosynthesis

Photosynthesis is an anabolic process where light energy is converted into chemical energy. This chemical energy is then stored within large organic compounds such as glucose and is derived from low energy inorganic compounds such as CO₂, H₂O. Pigments, such as chlorophyll, which are present within phototrophic autotrophs facilitate the absorption of light energy. Although prokaryotic cells lack membrane bound organelles and therefore lack chloroplasts, they can still undergo photosynthesis due to the presence of certain photosynthetic pigments. When there is a surplus of glucose, plants store it as starch and it is transported around the plant in the form of sucrose.

6CO₂ + 12H₂O ---^{Light/Chlorophyll}---> C₆H₁₂O₆ + 6O₂ + 6H₂O

Chloroplast

Chloroplasts are the descendants of free-living photosynthetic cyanobacteria that were engulfed by and entered a symbiotic relationship with a eukaryotic cell. This is supported by the following evidence: they contain their own circular DNA, divide by binary fission and have their own ribosomes just like bacterial cells. Moreover, they are contained within a plasma membrane suggesting that they entered the cell via endocytosis.

• The thylakoid is a membrane-bound compartment. A stack of these is called a granum or grana (plural), both of which contain chlorophyll, the light trapping pigment and other enzymes
• The stroma is the fluid between the thylakoid membranes that contain enzymes, ribosomes and chloroplast DNA
• The chlorophyll is located within the thylakoids and is a green pigment that absorbs wavelengths of light mostly from the red/blue light part of the light spectrum

Chloroplast

Light Dependent Stage

The light dependent stage requires sunlight and occurs within the thylakoid membranes of the grana in the chloroplast. During this stage, light energy is captured by the chlorophyll and is used to split water molecules into protons and oxygen molecules which diffuse out as a by-product. The electron transport chain then moves protons from the stroma into the thylakoid membrane to create a proton imbalance and thereby facilitating the production of ATP through an enzyme called ATP synthase. Electrons and protons are also collected by NADP to form NADPH.

Inputs	Outputs
Light	Oxygen
Water	NADPH
NADP	ATP
ADP + Pi

Light Independent Stage or Calvin Cycle

Although the light independent stage does not directly require the input of sunlight, it does rely on the outputs of the light dependent stage. This process occurs at the stroma of the chloroplast and its overall objective is to convert carbon dioxide into glucose. Just remember, you do not need to know the specific biochemical pathways for this process so don’t panic if you don’t fully understand how it works!

Inputs	Outputs
ATP	Glucose
NADPH	NADP
CO2	ADP + Pi
	Water

Factors Affecting Photosynthesis

When either light intensity or carbon dioxide concentration is increased, the rate of photosynthesis increases to a certain point. At this point, some other factor is limiting the rate of photosynthesis. Furthermore, because enzymes catalyse many of the reaction in photosynthesis, the rate of photosynthesis is also dependent on factors affecting enzymatic activity including temperature, the concentration of the substrate or the inputs and pH.

Cellular Respiration

Cellular respiration is a series of catabolic reactions that break down organic compounds such as glucose releasing energy in the form of ATP (Adenosine Triphosphate).

6O₂ + C₆H₁₂O₆ + 36ADP + 36Pi → 6CO₂ + 6H₂O + 36ATP

Mitochondria

Similarly, the mitochondria are also believed to be free living microbial cells that were engulfed and entered a symbiotic relationship with a eukaryotic cell. This is also supported by the evidence of their own circular DNA, are contained within a double plasma membrane, divide by binary fission and have their own ribosomes.

It is important to note, more active cells, that is, cells that require more energy e.g. skeletal cells, have not only more mitochondria, but can have mitochondria with a more highly folded cristae to facilitate a greater ATP production.

Mitochondria

Glycolysis

Glycolysis is the first stage of cellular respiration occurring in both anerobic and aerobic respiration in the cytosol of the cell. It is the initial breakdown of glucose into two pyruvate molecules, without requiring any oxygen.

Inputs	Outputs
Glucose	Pyruvates
NAD	NADH
ADP + Pi	2 ATP

Transition Stage

The pyruvate produced enters the mitochondrion via active transport and is converted into Acetyl CoA and a carbon dioxide molecule is produced

Inputs	Outputs
Pyruvates	Acetyl CoA
NAD	NADH
	CO2

Aerobic Respiration – Krebs Cycle

The Krebs Cycle is the second stage of aerobic respiration and occurs when oxygen is present in the matrix of the mitochondria.

Inputs	Outputs
Acetyl CoA	CO2
ADP + Pi	2 ATP
FAD	FADH2
NAD	NADH

Electron Transport Stage

The electron transport chain is composed of a system of electron carriers called cytochromes located in the cristae of the mitochondria. During this process, electrons are passed along the cytochromes and oxygen molecules act as the terminal electron acceptor thereby producing water. ATP is synthesised by the enzyme ATP synthase.

Inputs	Outputs
NADH	NAD
FADH2	FAD
32/34 ADP + Pi	32/34 ATP
O2	H2O

Anerobic Respiration

Anerobic respiration does not involve the mitochondria as no oxygen is involved and therefore only occurs in the cytosol. For each glucose molecule, two ATP are produced along with a by-product. In yeast, ethanol and CO₂ are produced while in animals, lactic acid is produced. The purpose of anerobic respiration or fermentation in yeasts is to regenerate NAD so that glycolysis can proceed

Factors Affecting Cellular Respiration

Likewise, with photosynthesis, the concentration of the inputs, e.g. glucose availability and oxygen concentration and the factors affecting enzymatic activity affect the rate of cellular respiration. When oxygen and glucose concentrations are increased, the rate of respiration will initially increase until it plateaus. At this point, another factor is acting as a limiting factor of cellular respiration