Membranes and membrane transport

A **phospholipid bilayer** — two rows of phospholipids — with proteins, glycoproteins and cholesterol embedded.

Having **both** a hydrophilic (water-loving) part and a hydrophobic (water-hating) part in the same molecule.

The **phosphate head** is hydrophilic (water-loving); the **two fatty-acid tails** are hydrophobic (water-hating).

They are **attracted to the water** present on both the outer and inner surfaces of the membrane.

They are **repelled by water**, so they are pushed into the centre, away from the water, forming the core.

Because phospholipids are amphipathic and there is **water on both sides**: heads go to the water, tails away from it, giving two rows.

It makes the membrane **selectively permeable** — small non-polar molecules pass, but large/polar molecules cannot cross freely.

Acts in **cell recognition** and cell signalling — its carbohydrate chain on the outer surface is an 'identity tag'.

Wedges between the phospholipids and **stabilises fluidity**, reducing leakiness to small molecules.

**Fluid** because phospholipids and many proteins drift sideways; **mosaic** because different molecules are dotted through the bilayer like tiles.

Davson–Danielli put **two continuous protein layers** coating the bilayer; the fluid mosaic model **scatters proteins through** it.

Electron-microscopy images showing **proteins embedded within** the bilayer, not just coating its surfaces.

**Integral** proteins are embedded right through the bilayer (e.g. channels, carriers); **peripheral** proteins rest on one surface.

Movement across a membrane that needs **no energy (ATP)** — particles move **down a gradient** on their own.

The **net movement of small or non-polar particles** down their concentration gradient, **straight through the phospholipid bilayer**.

The net movement of **water** across a **partially permeable membrane**, from **higher water potential (dilute) to lower (concentrated)**.

**Small, non-polar** molecules — e.g. **O₂, CO₂** — and **lipid-soluble** molecules such as **steroid hormones**.

The bilayer's core is **non-polar / hydrophobic**, so non-polar molecules are **not repelled** — they dissolve in and pass through.

They are **repelled by the hydrophobic core** (or too large), so they need a **protein** to cross.

From the **more dilute** solution to the **more concentrated** one (high → low **water potential**).

A measure of how free the water is to move. **Pure water is highest**; adding solute lowers it. Water moves from **high to low** water potential.

A **channel protein** that lets water cross **quickly**, speeding up osmosis. It uses **no ATP** and doesn't change the direction.

**Passive** — it uses no ATP, even when aquaporins speed it up.

A **steeper** gradient gives a **faster** rate of net diffusion; a shallower one gives a slower rate.

Water is **entering** the cell (net water movement in), so the outside solution is **more dilute / hypotonic**.

Water is **leaving** the cell (net water movement out), so the outside solution is **more concentrated / hypertonic**.

A **higher temperature** and a **larger membrane surface area**; a **thicker** membrane slows it down.

The **passive** movement of **ions and large polar molecules** across a membrane through a **channel or carrier protein**, **down the concentration gradient** (no ATP).

They are **large/polar or charged**, so they are repelled by the **hydrophobic (water-hating) core** of the bilayer — they need a transport protein.

A membrane protein with an **open water-filled pore** that lets specific ions or polar molecules pass **straight through**.

A membrane protein that **binds** a specific molecule and **changes shape** to move it across the membrane.

A channel is an **open pore** (fast; ions, water); a carrier **binds and changes shape** (slower; glucose, fructose).

**Down the concentration gradient** — from **high** to **low** concentration.

**No** — it is **passive**, because particles move down their concentration gradient.

The **route**: simple diffusion goes **straight through the bilayer**; facilitated diffusion goes **through a protein**. Both are passive and down the gradient.

A **channel protein** (an open pore).

A **carrier protein** (it binds and changes shape).

The transport proteins become **saturated** — every channel/carrier is occupied, so the rate reaches a **maximum** and cannot rise further.

Aquaporins are **channel proteins** for **water** — water crosses through them by facilitated diffusion (a fast, passive route).

The movement of a substance **against** its concentration gradient, using energy from **ATP**.

Active transport moves particles **against** the gradient and **uses ATP**; diffusion is passive — **down** the gradient with **no ATP**.

A **pump protein** (the protein responsible for active transport).

**'Against' the gradient** and **'uses ATP'** — either one signals active transport.

Pumps **3 sodium ions (Na⁺) out** of the cell and **2 potassium ions (K⁺) in**, both against their gradients.

**Sodium (Na⁺) OUT**, **potassium (K⁺) IN**.

Because ions constantly **leak back** down their gradients; the pump keeps replacing them to **maintain** the gradients.

By **active transport** — the Na⁺/K⁺ pump uses **ATP** to keep moving ions against their gradients.

The pump stops, ions keep leaking back, and the gradients gradually **even out**.

It **needs energy** — it always uses **ATP**.

**Simple diffusion**, **facilitated diffusion** (both passive) and **active transport** (active, uses ATP).

Uptake of **mineral ions by plant root cells** against their concentration gradient.

The membrane lets **some substances cross but blocks others**, mainly depending on their **size** and whether they are **polar**.

**Small, non-polar** molecules such as **oxygen** and **carbon dioxide** (water crosses too, helped by aquaporins).

**Large** molecules such as **starch** and **proteins** — they are too big to pass through.

As a **model** of a partially permeable membrane: its pores let **small** molecules through but hold back **large** ones.

**Glucose passes out** through the pores (it is small); **starch stays inside** (it is too large).

Its molecules are **too large** to fit through the pores of the partially permeable tubing.

Moving **large amounts of material**, or particles too big to cross the bilayer, **in vesicles** — it **uses ATP**.

Bulk transport that brings material **INTO** the cell: the membrane **folds inwards** and pinches off a vesicle around the material.

Bulk transport that releases material **OUT** of the cell: a vesicle **fuses** with the plasma membrane and empties its contents.

**Yes** — both endocytosis and exocytosis **use ATP**, so bulk transport is **active**.

Taking in **large food particles** or engulfing a **pathogen** (e.g. a white blood cell engulfing a bacterium).

**Secreting** proteins, **enzymes** or **hormones** (e.g. a gland cell releasing a digestive enzyme).

**Endo** = **into** the cell ('enter'); **exo** = **exit** the cell.