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This lesson covers the structure and biological importance of water and inorganic ions as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand how the molecular structure of water relates to its properties and why these properties are essential for life. You also need to know the roles of key inorganic ions in biological systems.
A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. The bond angle between the two O–H bonds is approximately 104.5°, giving the molecule a V-shape.
Oxygen is more electronegative than hydrogen. This means that oxygen attracts the shared pair of electrons in each O–H covalent bond more strongly than hydrogen does. As a result, the oxygen atom carries a slight negative charge (δ⁻) and each hydrogen atom carries a slight positive charge (δ⁺).
This uneven distribution of charge makes water a polar molecule — it has a positive end and a negative end.
Key Definition: A polar molecule is one in which there is an uneven distribution of electrical charge, resulting in regions of partial positive and partial negative charge.
Because water is polar, the δ⁺ hydrogen atom of one water molecule is attracted to the δ⁻ oxygen atom of a neighbouring water molecule. This electrostatic attraction is called a hydrogen bond.
Individual hydrogen bonds are weak — roughly one-twentieth the strength of a covalent bond. However, water molecules form many hydrogen bonds simultaneously, and collectively these give water its remarkable properties.
| Property | Explanation | Biological Importance |
|---|---|---|
| High specific heat capacity | Many hydrogen bonds must be broken to raise the temperature of water | Aquatic environments remain thermally stable; body temperature is buffered against rapid changes |
| High latent heat of vaporisation | Considerable energy is needed to break hydrogen bonds and convert liquid water to vapour | Evaporative cooling (sweating, transpiration) is effective at removing heat from organisms |
| Cohesion and surface tension | Hydrogen bonds hold water molecules together | Allows water to be pulled through xylem vessels in transpiration; small insects can walk on water |
| Adhesion | Water molecules are attracted to other polar surfaces | Helps water move up narrow xylem vessels by capillary action |
| High polarity (excellent solvent) | Polar water molecules surround and separate ions and polar molecules | Most biological reactions occur in aqueous solution; transport of dissolved substances in blood and sap |
| Ice is less dense than liquid water | At 4 °C, hydrogen bonds form an open lattice structure in ice | Lakes freeze from the top down, insulating aquatic life beneath; aquatic ecosystems survive winter |
Exam Tip: When explaining why water is a good solvent, always link it to its polarity. State that the δ⁻ oxygen is attracted to positive ions (cations) and the δ⁺ hydrogen atoms are attracted to negative ions (anions), forming hydration shells around the ions.
Water dissolves more substances than any other common liquid, which is why it is sometimes called the universal solvent.
When an ionic compound such as sodium chloride (NaCl) is placed in water:
Polar molecules such as glucose and amino acids dissolve because they can form hydrogen bonds with water molecules. The polar –OH groups on glucose interact with the partial charges on water.
Non-polar molecules such as lipids do not dissolve in water. They are described as hydrophobic (water-hating). This is because water molecules are more strongly attracted to each other than to non-polar molecules, so the non-polar molecules are excluded.
Exam Tip: The terms hydrophilic (water-loving, dissolves in water) and hydrophobic (water-hating, does not dissolve in water) appear frequently in the specification. Be ready to use them when discussing membrane structure and protein folding.
Water is not merely a passive medium for reactions — it is itself a reactant and a product in many metabolic reactions.
| Reaction Type | Role of Water | Example |
|---|---|---|
| Hydrolysis | Water is a reactant — it is used to break covalent bonds | Hydrolysis of sucrose into glucose and fructose; digestion of proteins into amino acids |
| Condensation | Water is a product — it is released when monomers join together | Formation of peptide bonds between amino acids; formation of glycosidic bonds between monosaccharides |
| Photosynthesis | Water is a reactant — it is split (photolysis) in the light-dependent reactions | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
| Aerobic respiration | Water is a product of oxidative phosphorylation | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O |
Key Definition: Hydrolysis is the breaking of a covalent bond by the addition of water. Condensation is the formation of a covalent bond with the release of water.
The solvent properties of water make it ideal for transport in organisms:
Water's cohesive properties (due to hydrogen bonding) are critical for the transpiration stream in plants. Water molecules form a continuous column in the xylem, and as water evaporates from the leaf surface, the cohesion between molecules pulls the entire column upwards.
Inorganic ions are atoms or groups of atoms that have gained or lost electrons, giving them an electrical charge. They are essential for many biological processes. The specification requires you to know the roles of several key ions.
| Ion | Symbol | Biological Role |
|---|---|---|
| Hydrogen ions | H⁺ | Determine pH; involved in chemiosmosis during oxidative phosphorylation and photophosphorylation; proton gradients drive ATP synthesis |
| Iron ions | Fe²⁺ / Fe³⁺ | Component of haemoglobin (binds reversibly to oxygen); part of cytochrome molecules in the electron transport chain |
| Sodium ions | Na⁺ | Generation of nerve impulses (influx of Na⁺ during depolarisation); co-transport of glucose and amino acids in the ileum and kidney |
| Potassium ions | K⁺ | Generation of the resting potential in neurones; opening of stomata in plants (K⁺ influx lowers water potential of guard cells) |
| Phosphate ions | PO₄³⁻ | Component of ATP, DNA and RNA (sugar-phosphate backbone); component of phospholipids in cell membranes |
| Calcium ions | Ca²⁺ | Needed for muscle contraction (binds to troponin); component of bones and teeth; involved in blood clotting; required for synaptic vesicle fusion |
| Magnesium ions | Mg²⁺ | Central atom in chlorophyll molecules; cofactor for many enzymes including ATPase and hexokinase |
| Nitrate ions | NO₃⁻ | Source of nitrogen for plants to synthesise amino acids and nucleotides |
| Chloride ions | Cl⁻ | Involved in the chloride shift in red blood cells during CO₂ transport; component of hydrochloric acid in gastric juice |
Exam Tip: The specification often tests inorganic ions in context — for example, asking about the role of iron in haemoglobin or magnesium in chlorophyll within a longer question about transport or photosynthesis. Always state the specific role rather than a vague answer.
The concentration of hydrogen ions (H⁺) in a solution determines its pH. pH is defined as:
pH = −log₁₀[H⁺]
| pH | H⁺ Concentration | Description |
|---|---|---|
| 1 | 0.1 mol dm⁻³ | Strongly acidic |
| 7 | 10⁻⁷ mol dm⁻³ | Neutral |
| 14 | 10⁻¹⁴ mol dm⁻³ | Strongly alkaline |
A change of one pH unit represents a tenfold change in H⁺ concentration. This is important because even small pH changes can alter the ionisation of amino acid R-groups in proteins, affecting enzyme activity and protein shape.
Buffers are solutions that resist changes in pH when small amounts of acid or alkali are added. Blood is buffered at approximately pH 7.4 by the carbonic acid–hydrogencarbonate system:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
If H⁺ concentration rises (pH falls), hydrogencarbonate ions (HCO₃⁻) combine with the excess H⁺ to form carbonic acid, restoring pH. If H⁺ concentration falls (pH rises), carbonic acid dissociates to release more H⁺.
Exam Tip: In 6-mark extended response questions on water, organise your answer around its properties (polarity, hydrogen bonding, high SHC, solvent ability) and link each property to a specific biological example. This shows the examiner you understand both the chemistry and the biology.
This lesson sits in Edexcel 9BI0 Topic 1 — Biological Molecules, specifically the sub-strand on the structure and properties of water and the roles of inorganic ions in living organisms. The relevant content statements paraphrase to: describe how the dipolar nature of water gives rise to hydrogen bonding; explain the importance of water's properties (specific heat capacity, latent heat of vaporisation, cohesion, density of ice, solvent action) in living organisms; and outline the roles of selected inorganic ions including calcium (Ca²⁺), sodium (Na⁺), potassium (K⁺), hydrogen (H⁺), ammonium (NH₄⁺), hydroxide (OH⁻), chloride (Cl⁻), hydrogencarbonate (HCO₃⁻), phosphate (PO₄³⁻), iron (Fe²⁺/Fe³⁺) and magnesium (Mg²⁺). This material is examined across all three Edexcel Biology B papers — Paper 1 (Advanced Biochemistry, Microbiology and Genetics) tests it directly; Paper 2 connects it to transport and homeostasis (Topic 7); Paper 3 uses it synoptically when assessing experimental and investigative skills (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).
Question (8 marks): Magnesium ions (Mg²⁺), iron ions (Fe²⁺) and phosphate ions (PO₄³⁻) all play essential roles in living organisms.
(a) Explain how the role of magnesium ions in plants depends on their chemical properties. (3)
(b) Describe how iron ions and phosphate ions are involved in cellular respiration. (5)
Solution with mark scheme:
(a) Step 1 — identify the role. Magnesium ions are required as a cofactor at the centre of the porphyrin (chlorin) ring of chlorophyll.
M1 (AO1) — naming chlorophyll as the molecule containing Mg²⁺. A common error is to claim Mg²⁺ "activates" chlorophyll without specifying that it is structurally bonded into the ring; that loses M1 because it conflates a cofactor role with an enzyme activator role.
Step 2 — link the property to the function. The 2+ charge of Mg²⁺ holds the chlorin ring planar and coordinates the four pyrrole nitrogens, stabilising the conjugated π-system that absorbs photons in the red and blue regions.
A1 (AO2) — linking the divalent cation to ring stabilisation / photon absorption. Many candidates lose marks here by stopping at "needed for chlorophyll" without connecting Mg²⁺ to either light absorption or photosynthetic electron transfer.
Step 3 — biological consequence. Without Mg²⁺ uptake from the soil, chlorophyll cannot be synthesised, photosynthesis is inhibited, and the leaves yellow (chlorosis).
A1 (AO2) — naming chlorosis or relating Mg²⁺ deficiency to reduced photosynthetic rate.
(b) Step 1 — locate iron in respiration. Fe²⁺/Fe³⁺ ions are bound in the haem prosthetic group of cytochromes in the inner mitochondrial membrane.
M1 (AO1) — naming cytochromes / electron transport chain.
Step 2 — describe the redox role. Iron alternates between Fe²⁺ and Fe³⁺ as it accepts and donates electrons along the chain, transferring electrons from reduced coenzymes (NADH, FADH₂) toward oxygen.
A1 (AO2) — explicit reference to the reversible Fe²⁺ ⇌ Fe³⁺ + e⁻ redox couple. A common pitfall is writing only "iron carries electrons" without naming the oxidation states — examiners look for the redox switching as the discriminator.
Step 3 — locate phosphate. Phosphate ions (PO₄³⁻) are added to ADP during oxidative phosphorylation to form ATP.
M1 (AO1) — naming ADP + Pᵢ → ATP.
Step 4 — link to chemiosmosis. The energy for this phosphorylation comes from the proton gradient driving ATP synthase; phosphate also forms the high-energy phosphoanhydride bonds whose hydrolysis releases energy in subsequent reactions.
A1 (AO2) — connecting phosphate to ATP synthase / phosphoanhydride bonds.
Total: 8 marks (M3 A2 in (a) — wait, M1 A1 A1; (b) M1 A1 M1 A1; the structure is M2 A2 in (a) reduced to 3 marks, M2 A2 in (b)). A clean candidate would secure all 8 with explicit naming of cofactor / prosthetic group / redox couple / phosphoanhydride bonds.
Question (6 marks): Many of water's properties arise from hydrogen bonding between water molecules. Explain how three named properties of water are essential to the survival of organisms.
Mark scheme decomposition by AO:
| Mark | AO | Awarded for |
|---|---|---|
| 1 | AO1 | Naming three correct properties (e.g. high specific heat capacity, high latent heat of vaporisation, cohesion, solvent action, lower density of ice). |
| 2 | AO1 | Linking each property to the underlying hydrogen bonding (energy needed to break H-bonds; H-bonds hold molecules together; polarity creates hydration shells). |
| 3 | AO2 | First biological application — e.g. high SHC stabilises aquatic habitat temperatures so enzyme-controlled reactions continue. |
| 4 | AO2 | Second biological application — e.g. cohesion plus adhesion produces the unbroken xylem water column underpinning transpiration in tall plants. |
| 5 | AO2 | Third biological application — e.g. solvent action permits dissolved transport of glucose and amino acids in blood plasma. |
| 6 | AO3 | Synthesis / evaluation — explicit comparison such as "without these combined properties, terrestrial life of macroscopic body size would be impossible because metabolic, transport and thermoregulatory functions all depend on aqueous chemistry." |
Total: 6 marks split AO1 = 2, AO2 = 3, AO3 = 1. This is a typical Edexcel "structured extended response" — the AO3 mark is reserved for the candidate who synthesises the points rather than listing them.
Connects to:
Water and ion questions on 9BI0 typically split AO marks toward AO1 and AO2:
| AO | Typical share | Earned by |
|---|---|---|
| AO1 (knowledge) | 40–50% | Naming the property / ion correctly; stating the role (e.g. "Fe²⁺ in haemoglobin"); recalling that water is dipolar |
| AO2 (application) | 35–45% | Linking the molecular cause (hydrogen bonding, polarity, charge) to the biological effect; using the data in a stem to justify the ion's role |
| AO3 (analysis / evaluation) | 10–20% | Synthesising across properties; evaluating limits (e.g. "evaporative cooling is only effective when relative humidity is low"); designing or critiquing investigations |
Examiner-rewarded phrasing: "the δ⁻ oxygen of water is attracted to the positively charged sodium ion, forming a hydration shell"; "the divalent Mg²⁺ ion coordinates the four pyrrole nitrogens"; "iron alternates between Fe²⁺ and Fe³⁺, transferring electrons along the chain"; "phosphate forms phosphoanhydride bonds, hydrolysis of which releases energy". Phrases that lose marks: "water is wet" (does not describe a property); "ions help with stuff" (no specificity); "iron carries oxygen" (omits the redox/binding mechanism); "polar means it has poles like a magnet" (confuses polarity with magnetism).
A specific Edexcel pattern: questions phrased "explain how the structure of water relates to its function" demand a structure → property → function chain. Many candidates lose marks here by giving structure and function without the intermediate property. Always include the middle step.
Question: Explain why water has a high specific heat capacity and how this is important for living organisms.
Grade C response (~200 words):
Water has a high specific heat capacity because it has hydrogen bonds between the molecules. These bonds need a lot of energy to break, so it takes a lot of heat to raise the temperature of water. This is important for organisms because their bodies are mostly water, so their temperature does not change quickly. It is also important for fish living in lakes because the water stays at a steady temperature even when the air gets cold or hot. This means enzymes can keep working because they have an optimum temperature.
Examiner commentary: Awarded 2/3. The candidate correctly attributes high SHC to hydrogen bonding and gives a relevant biological consequence (thermal stability of habitat / body). The middle step — that the energy goes into breaking H-bonds rather than increasing kinetic energy — is missing, so the explanation feels mechanically unfinished. The reference to enzymes is on-topic but underdeveloped. A common pitfall is to mention hydrogen bonds without explaining what energy does to them.
Grade A response (~225 words):*
Water has a high specific heat capacity because raising its temperature requires energy not only to increase the kinetic energy of the molecules but also to break a substantial fraction of the hydrogen bonds holding them together. Because each water molecule forms up to four hydrogen bonds with neighbouring molecules, a relatively large input of thermal energy produces only a small temperature rise (4.18 J g⁻¹ K⁻¹).
This property is biologically essential. First, large bodies of water — oceans, lakes, the cytosol — resist rapid temperature change, providing thermally stable habitats and intracellular environments in which temperature-sensitive enzymes operate close to their optimum. Second, organisms whose bodies are predominantly water (~70% by mass in mammals) experience buffered core temperatures despite fluctuations in ambient conditions, which is critical because most enzymes denature beyond ~40 °C. Third, the property scales — small unicellular organisms living in transient water films are also protected, giving the cytosolic environment its homeostatic stability.
Examiner commentary: Full marks (3/3). The candidate explicitly distinguishes kinetic-energy increase from H-bond breaking, links to the enzyme optimum / denaturation argument, and uses the quantitative SHC value as an optional flourish. The "scales" point shows the synthetic thinking that distinguishes A from A*. Examiner-rewarded phrasing throughout.
Question: Sodium ions (Na⁺), potassium ions (K⁺) and hydrogencarbonate ions (HCO₃⁻) all play important roles in mammalian physiology. Explain the role of each ion, naming the physiological process involved.
Grade B response (~265 words):
Sodium ions are involved in the action potential in nerve cells. When the nerve is stimulated, sodium channels open and Na⁺ flows into the cell, making the inside positive. This is called depolarisation. Sodium is also pumped out of cells by the sodium-potassium pump.
Potassium ions are also involved in nerve impulses. They flow out of the cell during repolarisation, restoring the negative resting potential. Potassium is the main positive ion inside cells.
Hydrogencarbonate ions are involved in keeping blood pH constant. They act as a buffer. If H⁺ ions increase, hydrogencarbonate combines with them to make carbonic acid, which lowers the H⁺ concentration. This stops the pH from changing.
Examiner commentary: Awarded 4/6. The candidate names all three ions and the correct physiological process for each (action potential, repolarisation, blood pH buffering). Mark-loss patterns: (i) the Na⁺/K⁺ pump is mentioned but not linked back to the resting potential; (ii) the buffer description is correct but does not name the equation H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂ or explain the role of carbonic anhydrase / lung exhalation in resetting the buffer; (iii) no linkage between the three ions in any synthetic statement (which would target the AO3 mark).
Grade A response (~295 words):*
Sodium ions and potassium ions together establish and reset the resting and action potentials of excitable cells. At rest, the Na⁺/K⁺ ATPase pump expels three Na⁺ for every two K⁺ imported, generating a net negative interior of approximately −70 mV; selective K⁺ leakage via leak channels reinforces this. On stimulation, voltage-gated Na⁺ channels open and Na⁺ flows down its electrochemical gradient, depolarising the membrane to roughly +40 mV (the action potential). Voltage-gated K⁺ channels then open, K⁺ flows out, and the membrane repolarises. The pump restores the original ion distribution. Without precise Na⁺ and K⁺ concentration gradients, no action potential can propagate, no synaptic transmission occurs, and no muscle contracts.
Hydrogencarbonate ions are the dominant blood buffer. The reversible reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, catalysed in red blood cells by carbonic anhydrase, allows blood plasma to absorb additions of H⁺ (which combine with HCO₃⁻ to form H₂CO₃, eventually exhaled as CO₂) and additions of OH⁻ (which titrate H⁺, shifting the equilibrium to release more from H₂CO₃). The system holds arterial pH within 7.35–7.45.
Synoptically, all three ions illustrate how concentration gradients and charge convert chemistry into physiology: Na⁺ and K⁺ deliver electrical signals; HCO₃⁻ delivers chemical homeostasis; together they show that life depends on ions held out of equilibrium by metabolic energy.
Examiner commentary: Full marks (6/6). The candidate names each ion, names the process, gives the mechanism with quantitative anchors (−70 mV, +40 mV, pH 7.35–7.45), references the relevant enzyme (carbonic anhydrase) and ATPase, and closes with an AO3-targeted synoptic paragraph. The "out of equilibrium" framing is precisely the kind of synthetic insight examiners reward at A*.
Question: Water is sometimes described as "the most important biological molecule." With reference to its molecular structure and named biological examples, evaluate this claim.
Grade A response (~385 words):*
Water's biological importance derives from its dipolar molecular structure. The high electronegativity of oxygen relative to hydrogen, combined with the bent (104.5°) geometry of H₂O, creates a permanent dipole — δ⁻ on oxygen, δ⁺ on each hydrogen. This dipole permits hydrogen bonding between adjacent molecules, and the cooperative network of these weak bonds is the cause of every biologically significant property of water.
Solvent action. The polar molecules orient around dissolved ions and polar solutes, forming hydration shells that separate and stabilise charged species. This permits aqueous transport — glucose, amino acids, urea, ions and respiratory gases all travel dissolved in blood plasma — and provides the medium in which essentially all enzyme-catalysed reactions occur.
Thermal stability. A high specific heat capacity (4.18 J g⁻¹ K⁻¹) and a high latent heat of vaporisation arise because thermal energy must break hydrogen bonds before raising kinetic energy. Aquatic habitats and the cytosol resist temperature swings; sweating and transpiration exploit the latent heat to provide effective evaporative cooling, sustaining homeotherms in heat stress.
Cohesion, adhesion, surface tension. Hydrogen bonds hold water molecules to each other and to polar surfaces. The unbroken xylem water column in tall trees — ascended by transpiration pull — depends on cohesion; pond skaters exploit surface tension; mammalian alveoli depend on surfactant precisely because surface tension would otherwise collapse them.
Density anomaly. Ice's open hydrogen-bonded lattice makes it less dense than 4 °C water, so lakes freeze top-down and aquatic ecosystems persist through winter beneath an insulating ice layer.
Metabolic role. Water is itself a reactant — in hydrolysis of polysaccharides, proteins, lipids and nucleic acids — and a product, in condensation reactions, respiration and dehydration synthesis. Photolysis of water in PSII supplies the electrons that ultimately reduce CO₂ in photosynthesis: the global oxygen cycle begins with H₂O.
The claim is justified. No other small molecule combines polarity, hydrogen-bonding capacity, thermal mass, density anomaly and metabolic centrality. Where alternative solvents exist (e.g. liquid methane on Titan), they support neither the temperature buffering nor the dissolution of charged biomolecules that terrestrial biochemistry requires.
Examiner commentary: Full marks (9/9). The candidate moves from molecular structure → named property → named biological consequence at every step, names quantitative anchors, includes a synoptic photosynthesis link, and closes with an evaluative comparison to non-aqueous chemistries. The "where alternative solvents exist" sentence is the AO3 evaluation that earns the top band.
The errors that distinguish A from A* on water and ion questions:
Oxbridge-style interview prompt: "Why is liquid water — rather than, say, liquid ammonia or liquid methane — the universal solvent of life on Earth? What would biology look like if it weren't?"
This lesson underpins Edexcel 9BI0 Core Practical 1 (use of qualitative reagents to identify biological molecules) at the most fundamental level: water as the solvent that makes every biochemical test possible. The biuret reagent (NaOH + dilute CuSO₄) and Benedict's reagent (alkaline Cu²⁺ citrate complex) only function because Cu²⁺ ions are fully solvated by water, free to coordinate with peptide bonds (biuret) or be reduced by aldehyde-bearing reducing sugars to brick-red Cu₂O (Benedict's). Take the water away and these reagents collapse to insoluble solids that cannot react.
The same principle extends to inorganic-ion detection: flame tests rely on dissolving the salt to deliver mobile ions to the flame, and precipitation reactions (e.g. Cl⁻ + Ag⁺ → AgCl(s); SO₄²⁻ + Ba²⁺ → BaSO₄(s)) work only because both reactants must first exist as dissociated, hydrated ions in aqueous solution. A required-practical-literate candidate links water's polarity → ion solvation → reagent–biomolecule reaction → observable colour change as a single causal chain, rather than memorising reagents in isolation.
This content is aligned with the Pearson Edexcel GCE A Level Biology B (9BI0) specification, Paper 1 — Lifestyle, Transport, Genes and Health, Topic 1: Biological Molecules. For the most accurate and up-to-date information, please refer to the official Pearson Edexcel specification document.
graph TD
A["Water molecule<br/>H₂O, bent geometry<br/>104.5° bond angle"] --> B["Polar O–H bonds<br/>δ⁻ on O, δ⁺ on H"]
B --> C["Hydrogen bonding<br/>between molecules"]
C --> D["High specific<br/>heat capacity"]
C --> E["High latent heat<br/>of vaporisation"]
C --> F["Cohesion and<br/>adhesion"]
C --> G["Solvent action<br/>(hydration shells)"]
C --> H["Lower density<br/>of ice"]
D --> I["Thermal stability<br/>of habitats / cytosol"]
E --> J["Evaporative cooling<br/>(sweating, transpiration)"]
F --> K["Xylem water column<br/>in transpiration"]
G --> L["Transport in blood;<br/>medium for metabolism"]
H --> M["Lakes freeze<br/>top-down; aquatic<br/>life survives winter"]
style B fill:#27ae60,color:#fff
style C fill:#3498db,color:#fff
style L fill:#e67e22,color:#fff