Energy in Cells

All cells require energy. Energy is obtained in physical or chemical form from the cell's environment: physical (inorganic) forms include sunlight and heat, while chemical (organic) forms include carbohydrates and lipids. Once obtained, the energy undergoes transformations before it is used or stored. Cellular processes that require energy include:

1. DNA replication and cell division.
2. Synthesis of proteins, carbohydrates and lipids.
3. Active transport of substances across the cell membrane.
4. Maintenance and repair of the internal environment.
5. Certain enzyme-dependent reactions, such as the contraction of muscle cells.

AUTOTROPHS AND HETEROTROPHS

Organisms are classified by how they obtain chemical energy. Autotrophs make their own food; heterotrophs must consume it. Each group is further divided.

Photo-autotrophs (plants, algae, phytoplankton) capture light energy — ultimately derived from the Sun — and transform it into chemical energy by photosynthesis, using the green pigment chlorophyll (non-green organisms use other pigments). The glucose produced supplies their own metabolism: building nucleic acids, proteins, polysaccharides and lipids.

Chemo-autotrophs make food from carbon dioxide using oxidation reactions on simple inorganic molecules — hydrogen sulfide, ferrous iron or ammonia. They are usually bacteria living in harsh environments (nitrifying bacteria, sulfur-oxidising bacteria), where they are the primary producers of their food webs.

Heterotrophs occupy every other trophic level:

• Consumers - eat autotrophs (primary consumers) or other heterotrophs (secondary consumers) to obtain glucose, protein and lipids.
• Decomposers - feed on once-living organisms; they are pivotal in returning matter to the environment.

The wastes of this chemistry must be removed before they become toxic. Most animals use specialised excretory systems; most unicellular organisms excrete by diffusion; plants store wastes in leaf cells and bark, which are shed when the leaves drop or the bark is replaced.

PHOTOSYNTHESIS

Photosynthesis is the process by which some cells, particularly plant cells, transform light energy into chemical energy. It occurs on the thylakoid membranes of the chloroplast — their stacked arrangement increases the surface area available for the reactions. Photosynthesis underpins every food chain; respiration is then required (by autotrophs and heterotrophs alike) to release the energy stored in glucose.

$$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$

AEROBIC RESPIRATION

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$$

Respiration begins with glycolysis ("glucose splitting") in the cytoplasm: glucose is converted to pyruvate. Glycolysis consumes 2 ATP and produces 4 ATP — a net gain of 2 ATP. Every step of respiration is catalysed by a specific enzyme, keeping the process controlled and efficient.

The remaining stages depend on the cell type: in prokaryotes they run in the cytosol; in eukaryotes they run in the mitochondria, where glucose is completely broken down to carbon dioxide and water.

FERMENTATION

Without oxygen, only partial breakdown of glucose is possible. Glycolysis still runs, but the pyruvate remains in the cytoplasm and is converted into either ethanol or lactic acid — and the cell gains only glycolysis's 2 ATP, compared with 36–38 from aerobic respiration.

Every pathway begins with glycolysis (net 2 ATP). With oxygen, the mitochondria yield 36–38 ATP; without it, pyruvate becomes ethanol + CO₂ or lactic acid for only 2 ATP.

Alcoholic fermentation (in fungi — yeast is the standard example — and plants) partially breaks glucose down to ethanol and carbon dioxide. Ethanol is toxic: above about 15% concentration, cells are inhibited or destroyed. Commercially, this pathway is used to produce beer, wine, spirits and bread.

Lactic acid fermentation (in animals and bacteria) is a short-term measure for when oxygen is unavailable — or is not arriving fast enough, as in sprinting muscle. When oxygen returns, the lactic acid is broken down into carbon dioxide and water (the same end products as aerobic respiration). Commercially, bacterial cultures use this pathway in the production of cheese and yoghurt.

ADENOSINE TRIPHOSPHATE (ATP)

One of the most energy-dense molecules in the cell is adenosine triphosphate (ATP); the terms ATP and energy are often used interchangeably. ATP is constantly synthesised and used, but not stored — long-term energy storage is left to molecules such as glycogen, starch and lipids. ATP powers:

1. Synthesis of new molecules.
2. Membrane transport — every mechanism except simple and facilitated diffusion.
3. Growth.
4. Maintenance.

ATP is made up of the nitrogenous base adenine, a ribose sugar, and three phosphate groups. The bond holding the third phosphate group is unstable (often called a high-energy bond).

ATP STRUCTURE — adenine, a ribose sugar and three phosphate groups. The bond holding the third phosphate is unstable, and its hydrolysis gives a net release of energy.

CHEMICAL BONDS AND THE ATP CYCLE

Chemical bonds hold atoms together to form molecules, and the direction of energy flow follows one rule: energy is required to break bonds, and energy is released when new bonds form. Energy is never created or destroyed (the law of conservation of energy); in a reaction, atoms are rearranged, and any energy not stored in the products' bonds is released as heat.

Why, then, does "breaking down" ATP supply energy? When ATP is hydrolysed (split in the presence of water) into ADP and inorganic phosphate (Pi), the new bonds formed in the products store less energy than the reactants' bonds held — so the reaction releases energy overall for the cell to use. When respiration supplies energy, ADP and Pi are re-bonded into ATP, a reaction catalysed by the enzyme ATP synthase. This loop is the ATP cycle.

ATP CYCLE — hydrolysis of ATP to ADP + Pi releases energy overall for cellular work; energy from respiration drives ATP synthase to re-form ATP.

In summary: light energy → photosynthesis → chemical energy in glucose → respiration → ATP → cellular work.