كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

B2.1Theme B — Form and function

Membranes and membrane transport — IB Biology SL Revision Notes & Topical Questions

5 min read 12 parts Lesson 4 of 13 2 quizzes

📚 IB Biology SL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

B2.1 Membranes and membrane transport — IB BIOLOGY SL notes: Describe membrane structure and the processes by which substances cross membranes.Phospholipid — a glycerol, two fatty acid tails and a phosphate headThe head is polar and hydrophilic and the tails are non-polar and hyd…

Membranes everywhere
Integral proteins
Peripheral proteins
Protein content
Transport proteins
Selectivity

Part 1 of 10Phospholipid bilayer as a barrier

Phospholipid

Phospholipid a glycerol, two fatty acid tails and a phosphate head
The head is polar and hydrophilic and the tails are non-polar and hydrophobic.
This makes the molecule amphipathic.

Monolayer

Monolayer on a water surface the heads sit in the water and the tails point into the air

Bilayer

Bilayer with enough phospholipid, two layers form spontaneously with the tails meeting in the middle
The membrane is about 7-10 nm thick.

Barrier

Barrier the tightly packed hydrophobic core has a low permeability
Large molecules, polar molecules and ions are attracted to water and do not mix with the tails.
Small non-polar molecules such as O₂ and CO₂ cross freely.
Small polar molecules such as urea diffuse slowly.

Membranes everywhere

Membranes everywhere the plasma membrane and organelle membranes compartmentalise the cell and communicate between cells

Part 2 of 10Membrane proteins

Integral proteins

Integral proteins amphipathic and embedded in one or both layers
Their hydrophobic parts meet the lipid tails.
Examples are channel proteins.

Peripheral proteins

Peripheral proteins hydrophilic and attached to the surface
They bind to integral proteins or to a lipid anchor and can lie inside or outside the cell.

Protein content

Protein content it varies with function
Mitochondrial and chloroplast membranes are the most protein-rich because of their electron carriers.

Part 3 of 10Functions of membrane proteins

Function	What it does	Example
Transport	Channels and carriers move ions and polar molecules	Aquaporins and glucose carriers
Receptor	Binds a signal and triggers events inside	Hormone and neurotransmitter receptors
Immobilised enzyme	Catalyses reactions at the membrane	Digestive enzymes on gut cells
Cell adhesion	Holds neighbouring cells together	Cells joined within a tissue
Recognition	Marks a cell as self	ABO antigens

Functions of membrane proteins

Transport proteins

Transport proteins each is specific to one ion or molecule, so the cell controls what enters or leaves

Receptors

Receptors bind peptide hormones such as insulin, neurotransmitters or antibodies

Part 4 of 10Simple diffusion

Diffusion

Diffusion the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, caused by their random motion

Particles move down the concentration gradient until they reach equilibrium.
At equilibrium net movement stops although random motion continues.

Examples

Examples oxygen diffuses into respiring cells and carbon dioxide diffuses out

Rate of diffusion

A steeper concentration gradient gives a faster rate.
A higher temperature gives more kinetic energy and faster movement.
A larger surface area allows more crossing at once.
Small, uncharged, non-polar particles diffuse fastest.

Selectivity

Selectivity simple diffusion is selective only by size and polarity

Part 5 of 10Facilitated diffusion

Facilitated diffusion

Facilitated diffusion passive movement of large molecules, polar molecules and ions through transport proteins

It needs no energy and goes down the concentration gradient.
Each protein is highly specific for one substance.

Channel proteins

Channel proteins pores with a fixed shape that allow specific charged substances to pass
Gated channels open only with a signal such as voltage or a neurotransmitter.

Carrier proteins

Carrier proteins switch between two shapes

Glucose is more concentrated outside the cell.

Glucose binds to the open site of the carrier.

The carrier switches to its other shape.

The site now faces the cytoplasm.

Glucose is released inside the cell.

The empty carrier switches back.

Part 6 of 10Osmosis

Osmosis

Osmosis the net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane

It is a type of diffusion down the water concentration gradient.
It can also be described as movement from higher to lower water potential.

Aquaporins

Aquaporins channel proteins that let water cross more freely
Water is unusual for a polar molecule in passing directly between phospholipids.

Potato cylinder practical

Cut potato cylinders of the same size and place them in a range of sucrose solutions.

Keep the time and temperature the same.

Blot the cylinders dry and record the mass change.

Calculate percentage change = (final - initial) / initial × 100.

Find the isotonic point where the graph crosses zero change.

Part 7 of 10Active transport and selective permeability

Active transport

Active transport movement of molecules or ions across a membrane from lower to higher concentration using energy from respiration

It needs carrier proteins, called pumps.
ATP is hydrolysed to release energy that changes the shape of the pump.
Root cells take up mineral ions from dilute soil water in this way.

Selective permeability

Selective permeability membranes let some substances through and block others
Facilitated diffusion and active transport give the control.
Large or polar molecules cannot cross without transport proteins.

Part 8 of 10Transport methods compared

Feature	Simple diffusion	Facilitated diffusion	Osmosis	Active transport
Energy	None	None	None	ATP required
Direction	Down the gradient	Down the gradient	To higher solute concentration	Against the gradient
Protein needed	No	Channel or carrier	Aquaporins speed it up	Pump
Substances	O₂, CO₂, small non-polar	Ions and glucose	Water	Na⁺, K⁺, minerals
Example	Gas exchange in alveoli	K⁺ channels in neurons	Water into root hair cells	Sodium-potassium pump

Part 9 of 10Glycoproteins and glycolipids

Glycoprotein

Glycoprotein a membrane protein with a carbohydrate chain on the extracellular side

Glycolipid

Glycolipid a lipid with a carbohydrate chain, also on the outer surface

Functions

Receptors for hormones and neurotransmitters that trigger signalling.
Receptors for endocytosis.
Adhesion that holds cells together in tissues.
Markers (antigens) that let the immune system tell self from pathogen.

Part 10 of 10The fluid mosaic model

The fluid mosaic model

Fluid mosaic model

Fluid mosaic model proposed by Singer and Nicolson in 1972
Fluid, because phospholipids and proteins move within their own layers.
Mosaic, because the scattered proteins look like a mosaic from above.

Components

Components phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins

Fluidity

Fluidity unsaturated fatty acid tails increase membrane fluidity

Drawing it

Draw two rows of phospholipid heads facing outwards.

Add paired tails meeting in the middle.

Insert an integral protein spanning both layers.

Place a peripheral protein on one surface.

Attach a carbohydrate chain on the outer side to make a glycoprotein.

Add cholesterol between the tails with its OH group near the heads and label every part.

Key terms

Term	Meaning
Amphipathic	Having both a hydrophilic region and a hydrophobic region, as phospholipids do.
Integral protein	A membrane protein embedded in the bilayer.
Peripheral protein	A hydrophilic membrane protein attached to the surface.
Facilitated diffusion	Passive movement down a concentration gradient through a transport protein.
Osmosis	The net movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
Active transport	Movement of substances against a concentration gradient using ATP and a carrier protein.
Fluid mosaic model	The model of the membrane as a fluid phospholipid bilayer with scattered proteins.
Selective permeability	The property of letting some substances cross a membrane while blocking others.

Avoid these mistakes

Saying the membrane is a single layer — the phospholipids form a bilayer

Saying facilitated diffusion needs ATP — it is passive and goes down the gradient

Saying water moves to the more dilute side in osmosis — it moves to the more concentrated solution

Placing carbohydrate chains on the inner surface — they are on the outer surface

Last updated: 26 October 2026