6: Membrane Boundaries and Capturing Energy
- Page ID
- 3886
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 6.0: Introduction
- This page covers the significance of water in biological systems, focusing on its impact on lipid-based membrane formation, which is essential for energy capture and macromolecule synthesis. It also discusses coupled reactions that enhance these biological processes, emphasizing their interconnected nature. Acknowledgments are made for contributions from scholars and editorial help.
- 6.1: Defining the Cell’s Boundary
- This page discusses the significance of lipid membranes in cell function and origin, detailing their composition, structure, and interaction of lipids and proteins. It emphasizes how variations in fatty acids affect membrane fluidity and cellular activity, and highlights adaptive mechanisms for maintaining membrane integrity under varying conditions.
- 6.2: The origin of biological membranes
- This page covers the composition and evolution of cell membranes, focusing on the roles of various lipids and their chain lengths in forming micelles and bilayers. It addresses the difficulties faced by lipids at bilayer edges and the evolution of impermeable membranes. The importance of co-evolution in creating selective material passage mechanisms necessary for life is highlighted, alongside questions that encourage further investigation into these topics.
- 6.3: Transport across membranes
- This page explains that living cells function as non-equilibrium systems that require energy and matter exchange through their plasma membranes. Internally, cells contain a protein and nucleic acid-rich cytoplasm, while the external environment is mainly aqueous. The plasma membrane acts as a selective barrier, with processes like endocytosis and exocytosis allowing molecular transport via vesicles, upheld by molecular machines, maintaining cell structure and function.
- 6.4: Transport to and across the membrane
- This page discusses the membrane's role in regulating molecule movement through passive diffusion, carrier-assisted transport, and active transport using energy-dependent pumps. It highlights the influence of protein carriers and channels on permeability, noting that hydrophobic membranes hinder hydrophilic molecules. While small non-polar molecules and water can traverse membranes easily, the movement of substances like O2 is determined by concentration gradients.
- 6.5: Channels and carriers
- This page discusses early 20th-century scientific advancements in understanding cell membranes, highlighting their role beyond simple hydrophobic barriers. Researchers like Collander revealed that membranes act as selective molecular sieves with integral proteins that aid in transporting hydrophilic molecules. These proteins, functioning as channels, carriers, and pores, lower activation energy and utilize thermal motion to facilitate efficient transport without requiring extra energy sources.
- 6.6: Generating gradients: using coupled reactions and pumps
- This page explains molecular transport across membranes, emphasizing the role of carrier proteins and channels in moving molecules against concentration gradients via energy-dependent pumps. It details light-driven and chemical-reaction driven pumps, with ATP synthesis via the H+ gradient as an example, highlighting the coupling of unfavorable and favorable processes to sustain essential concentration gradients for cellular function.
- 6.7: Simple Phototrophs
- This page discusses how phototrophs transform light energy into chemical energy using pigments to absorb light. It showcases Halobacterium halobium, which employs bacteriorhodopsin to capture light, creating a proton gradient that drives ATP synthesis. This process generates both chemical and electrical gradients through electron movement, enabling the organism to utilize energy for cellular activities even without light.
- 6.8: Chemo-osmosis (an overview)
- This page covers Peter Mitchell's chemiosmotic hypothesis, explaining how H+ gradients are universally utilized in ATP generation by both phototrophs and chemotrophs. It details the role of electron transport chains in creating these gradients through the movement of electrons, which powers ATP synthesis via ATP synthase. The page notes the evolutionary relationship between these processes, emphasizing the differences in how high-energy electrons enter the transport chain.
- 6.9: Oxygenic Photosynthesis
- This page explores the photosynthetic systems of organisms like cyanobacteria, green algae, and higher plants, tracing their common ancestry. It details the oxygenic, non-cyclic photosynthesis process, emphasizing chlorophyll's role in capturing light and generating ATP through an H+ gradient. The page explains reduction-oxidation reactions in which water molecules supply electrons, enabling photosynthesis while producing oxygen, significantly impacting Earth's atmosphere.
- 6.10: Chemotrophs
- This page discusses chemotrophs, which are organisms that obtain energy by converting unstable molecules into stable ones without light, divided into organotrophs and lithotrophs. Methanogens generate methane from H2 and CO2 anaerobically. The presence of oxygen enhances energy capture efficiency; aerobic organisms completely break down carbohydrates, while anaerobic processes yield energy-rich byproducts. Energy capture is vital for synthesizing complex molecules essential for life.
- 6.11: Using the energy stored in membrane gradients
- This page discusses how organisms harness energy from ATP synthesis for various functions, including coupled transport mechanisms. ATP-driven transporters can move molecules against their concentration gradients, creating stored potential energy. Membrane proteins, such as symporters and antiporters, aid in this process by enabling the simultaneous transport of different molecules based on their gradients.
- 6.12: Osmosis and living with and without a cell wall
- This page discusses how cells maintain osmotic balance through high concentrations of solutes that attract water, leading to osmosis. It highlights the role of semi-permeable membranes and aquaporins in water transport, and how rigid cell walls prevent cells from bursting due to excessive water intake. This concept is essential for understanding various biological functions, including plant stability and the action of antibiotics that affect bacterial cell walls.
- 6.13: An evolutionary scenario for the origin of eukaryotic cells
- This page covers the origins of early life, detailing primitive organisms' characteristics, the role of genetic data, cell structure, and ecological interactions. It highlights the flexibility of animal cells lacking rigid walls, facilitating phagocytosis, compared to bacteria and archaea. The evolution of cell walls is discussed as a potential protective adaptation.
- 6.14: Making a complete eukaryote
- This page explores the key distinctions between prokaryotes and eukaryotes, emphasizing their structures and functions. It introduces the endosymbiotic hypothesis, which explains the evolution of eukaryotic cells, particularly the origins of mitochondria and chloroplasts from engulfed bacteria.
Contributors and Attributions
Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.