Cellular Transport, Metabolism, Photosynthesis & Respiration
Cellular Energy and Material Exchange Processes
Cellular Transport
Cellular transport refers to the exchange of substances between the cell interior and the exterior environment through the plasma membrane, or the movement of molecules within the cell.
Metabolism: Catabolism and Anabolism
Catabolism
Catabolism is the part of metabolism involving the transformation of complex biomolecules into simpler molecules. This process releases chemical energy, which is stored in the phosphate bonds of ATP molecules. It achieves this by breaking down molecules with high-energy covalent bonds through exothermic chemical reactions.
Anabolism
Anabolism is one of the two parts of metabolism. It is responsible for the synthesis (or bioformation) of more complex organic molecules (biomolecules) from simpler ones or nutrients, requiring energy input.
Photosynthesis: Capturing Light Energy
Chlorophyll: The Light-Absorbing Pigment
Chlorophyll is the pigment that gives plants their green color and is responsible for absorbing the light needed for photosynthesis. Chlorophyll primarily absorbs red, violet, and blue light and reflects green light.
Chlorophyll Types
- Chlorophyll a: C55H72O5N4Mg
- Chlorophyll b: C55H70O6N4Mg
Photolysis: Splitting Molecules with Light
Photolysis is the breaking of chemical bonds caused by radiant energy. It refers to the dissociation of complex molecules due to the effect of light. Photosynthesis relies on this process.
Example: Photolysis of Water
During photosynthesis, water molecules are split by light energy.
Photosynthesis Process and Stages
Photosynthesis is the process that transforms light energy into chemical energy. Its objective is to produce glucose for the plant’s nourishment. It takes place in chloroplasts and consists of a series of reactions requiring light energy.
Glucose is made up of carbon, hydrogen, and oxygen. This process occurs in two main stages:
Light-Dependent Stage
Plants absorb water from the soil through their roots, which is transported to the chloroplasts. Light energy splits water molecules (photolysis), releasing oxygen into the atmosphere and storing energy in molecules like ATP and NADPH.
Light-Independent Stage (Calvin Cycle)
The plant incorporates carbon dioxide from the air. Using the energy stored during the light-dependent stage, it combines carbon dioxide and hydrogen (derived from water) to form glucose (C6H12O6).
Cellular Respiration: Releasing Chemical Energy
Cellular respiration is a process by which cells break down food molecules (like glucose) to obtain energy, primarily in the form of ATP. It occurs partly in the cytoplasm and partly in the mitochondria.
Comparing Photosynthesis and Respiration
- Photosynthesis: Builds glucose, storing energy from light.
- Cellular Respiration: Breaks down glucose, releasing stored chemical energy for cellular work.
Types of Cellular Respiration
Cellular respiration can occur in two ways: with oxygen (aerobic) or without oxygen (anaerobic). Aerobic respiration yields significantly more energy (ATP) than anaerobic respiration.
Aerobic Respiration (With Oxygen)
This process consists of three main steps:
Glycolysis
Occurs in the cytoplasm. This initial metabolic pathway oxidizes one molecule of glucose into two molecules of pyruvate, generating a small amount of ATP and NADH.
Krebs Cycle (Citric Acid Cycle)
Occurs in the mitochondrial matrix. Pyruvate is further processed, and its derivatives enter the cycle, which fully oxidizes the carbon atoms to CO2, generating ATP, NADH, and FADH2.
Electron Transport Chain (Oxidative Phosphorylation)
Occurs on the inner mitochondrial membrane. Electrons from NADH and FADH2 are passed along a series of protein complexes, releasing energy used to pump protons and create a gradient. Oxygen acts as the final electron acceptor, forming water. The proton gradient drives ATP synthase to produce a large amount of ATP.
Anaerobic Respiration (Without Oxygen)
This process consists of two main steps:
Glycolysis
Occurs in the cytoplasm, identical to the first step of aerobic respiration, breaking down glucose into pyruvate.
Fermentation
Occurs in the cytoplasm. In the absence of oxygen, pyruvate is converted into other molecules (like ethanol and CO2 in alcoholic fermentation, or lactic acid in lactic acid fermentation). This step regenerates NAD+ from NADH, allowing glycolysis to continue producing small amounts of ATP.
Glycolysis: The Common First Step
Both aerobic and anaerobic respiration share the initial step: Glycolysis. In this process, occurring in the cytoplasm, one glucose molecule is broken down into two pyruvate molecules.
After glycolysis, the pathway diverges based on oxygen availability. If oxygen is present, aerobic respiration proceeds in the mitochondria. If oxygen is absent, anaerobic fermentation occurs in the cytoplasm.