5.7
Osmosis is the movement of water across a semipermeable membrane, such as a cell membrane, from an area of lower dissolved solute concentration to an area of higher dissolved solute concentration.
The dissolved solutes cannot cross this semipermeable membrane on their own and remain separated, which establishes a concentration gradient across the membrane.
Imagine one liter of a solution. The total number of dissolved particles in it is its osmolarity. As the number of solute particles increases, osmolarity also increases. The same idea applies to biological cells. Osmolarity determines osmotic pressure, the minimum pressure required to prevent net water movement across a semipermeable membrane.
When the solute concentration inside the cell is higher than the concentration outside, an osmotic gradient forms, with relatively more water outside the cell.
As a result, water slowly moves from the region of lower osmolarity to the region of higher osmolarity. Small channel proteins called aquaporins help water move across the membrane much faster. This movement of water continues until osmotic equilibrium is reached.
生体の重量の約60%〜95%は水です。したがって、細胞内の水分バランスを適切に保つことは最も重要です。浸透とは、細胞膜などの半透性の膜を水が通過することです。生体内では、水は他の分子を溶かす溶媒として重要な役割を果たしています。
拡散も浸透も、エネルギーを必要としない受動的な細胞内輸送です。拡散とは、液体(溶媒)に溶けている物質(溶質)が、濃度の高いところから低いところへ移動することです。膜が溶質に対して透過性であれば、膜を介して拡散が起こることもあります。特定の溶質の通過を妨げる膜は半透膜です。半透膜が溶質の流れを止めている間、溶媒は自由に移動し、これは浸透と呼ばれるプロセスにあたります。
浸透は、半透膜の一方の側に、もう一方の側よりも多くの溶質がある場合に生じます。水と溶質の比率は浸透圧と呼ばれます。浸透の際、水は浸透圧の低い側(溶質に対し水の量が多い側)から浸透圧の高い側(溶質に対し水の量が少ない側)へと、両側の浸透圧がほぼ等しくなるまで流れます。例えば、外部に比べ内部の溶質濃度が高い場合、半透膜で囲まれた細胞には水が流れ込みます。
浸透圧のバランスが崩れると、細胞は膨張したり収縮したりします。この浸透圧不均衡や、その結果の危険な状況を防ぐメカニズムは、次のセクションで論じます。
Osmosis is the movement of water across a semipermeable membrane, such as a cell membrane, from an area of lower dissolved solute concentration to an area of higher dissolved solute concentration.
The dissolved solutes cannot cross this semipermeable membrane on their own and remain separated, which establishes a concentration gradient across the membrane.
Imagine one liter of a solution. The total number of dissolved particles in it is its osmolarity. As the number of solute particles increases, osmolarity also increases. The same idea applies to biological cells. Osmolarity determines osmotic pressure, the minimum pressure required to prevent net water movement across a semipermeable membrane.
When the solute concentration inside the cell is higher than the concentration outside, an osmotic gradient forms, with relatively more water outside the cell.
As a result, water slowly moves from the region of lower osmolarity to the region of higher osmolarity. Small channel proteins called aquaporins help water move across the membrane much faster. This movement of water continues until osmotic equilibrium is reached.
From Chapter 5:
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