4.3
糖酵解：准备阶段
在细胞代谢（完全分解葡萄糖以提取能量）的过程中，糖酵解是第一步。糖酵解通常会发生在原核细胞和真核细胞的细胞质中。葡萄糖能够通过两种方式进入到异养细胞中。一种方法是通过次级主动运输，其中的运输过程会逆葡萄糖的浓度梯度进行。另一种机制使用一组称为 GLUT 蛋白的整合蛋白，也称为葡萄糖转运蛋白。 这些转…
葡萄糖是人体大多数细胞的主要能量来源。在细胞质中，葡萄糖通过一系列酶促反应被分解为两分子丙酮酸，该过程称为糖酵解。
根据ATP是被分解还是被合成，糖酵解分为两个阶段。
在糖酵解的第一阶段，己糖激酶将葡萄糖磷酸化生成葡萄糖-6-磷酸，并消耗一分子ATP。
接着，磷酸葡萄糖异构酶将葡萄糖-6-磷酸转化为果糖-6-磷酸，后者再由磷酸果糖激酶催化，消耗另一分子ATP，进一步磷酸化为果糖-1,6-二磷酸。
随后，醛缩酶将果糖-1,6-二磷酸裂解，生成两个三碳分子：3-磷酸甘油醛（G3P）和二羟基丙酮磷酸（DHAP）。
最后，磷酸丙糖异构酶通过可逆反应将二羟丙酮磷酸（DHAP）转化为甘油醛-3-磷酸（G3P）。因此，在第一阶段，葡萄糖通过消耗两个ATP分子转化为两个G3P分子。
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Q1: Why does glucose need to be phosphorylated in the first step of glycolysis?
Hexokinase phosphorylates glucose to produce glucose-6-phosphate, making glucose more reactive and trapping it in the cell. The negatively charged phosphate group prevents the molecule from crossing the plasma membrane, ensuring glucose remains available for energy extraction rather than leaving the cell through GLUT proteins.
Q2: What is the role of isomerases in the preparatory phase of glycolysis?
Isomerases convert glucose-6-phosphate into fructose-6-phosphate and later transform dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. These conversions rearrange the molecular structure, allowing the six-carbon glucose to eventually split into two three-carbon molecules. This structural rearrangement is essential for the subsequent cleavage and energy extraction steps in glycolysis.
Q3: How does phosphofructokinase regulate glycolysis based on cellular energy status?
Phosphofructokinase is a rate-limiting enzyme that responds to ATP and ADP levels through end product inhibition. When ATP concentration is high, the enzyme slows down, reducing glucose breakdown. When ADP levels are high, the enzyme accelerates, increasing energy production to meet cellular demands.
Q4: What happens when aldolase cleaves fructose-1,6-bisphosphate?
Aldolase splits fructose-1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This cleavage is essential for converting the single six-carbon glucose molecule into two three-carbon molecules. Both products continue through the remainder of glycolysis to eventually yield pyruvate and energy.
Q5: Why is the preparatory phase of glycolysis considered an energy investment?
The preparatory phase consumes two ATP molecules to phosphorylate glucose and fructose-6-phosphate. Although this requires energy expenditure, it destabilizes the glucose molecule and enables its cleavage into two three-carbon units. This investment sets up the energy-yielding reactions of the glycolysis pay-off phase.
Q6: How do glucose transporter proteins facilitate glucose entry into cells?
GLUT proteins, also called glucose transporter proteins, are integral membrane proteins that facilitate the diffusion of glucose across the plasma membrane. This transport mechanism allows glucose to enter heterotrophic cells efficiently, making it available for glycolysis and energy production in the cytoplasm.
Q7: What is the net outcome of the preparatory phase in terms of molecular structure?
The preparatory phase converts one six-carbon glucose molecule into two three-carbon glyceraldehyde-3-phosphate molecules through phosphorylation and cleavage reactions. This transformation requires two ATP molecules and produces molecules that serve as substrates for the energy-extracting reactions. The net result is a structured, activated form ready for the next phase.