Why Wilted Palak Turns Crisp Again in a Bowl of Water

A cell's outer membrane chooses what may pass, plant cells add a tough wall outside it, and inside, a system of membranes works like a factory and postal service combined.

This part covers the plasma membrane and transport, the plant cell wall, the endoplasmic reticulum and Golgi apparatus, and lysosomes and vacuoles.

The fluid mosaic model describes the plasma membrane as a double layer of lipids in which proteins are embedded or attached, with the lipids and proteins free to move sideways; materials cross it passively down a concentration gradient without energy, or actively against the gradient using energy.

Structure:

- Lipid bilayer — phospholipids with water-loving heads facing outwards and water-hating tails inside
- Integral proteins — partly or wholly buried in the membrane
- Peripheral proteins — lie on the surface
- Fluidity — lets proteins move within the layer, which is important for cell growth, secretion, endocytosis and cell division

Transport:

- Simple diffusion — neutral molecules move from higher to lower concentration, no energy
- Osmosis — water moves across a membrane from where water is more concentrated to where it is less concentrated
- Facilitated transport — polar molecules cross with the help of carrier proteins, still down the gradient
- Active transport — movement against the gradient using ATP, as in the sodium-potassium pump

An everyday example. Raisins soaked overnight swell up as water enters their cells by osmosis.

The substance. Polar molecules cannot slip easily through the lipid core, so they need carrier proteins.

The plant cell wall is a non-living covering outside the plasma membrane, made of cellulose, hemicellulose, pectins and proteins; it has a primary wall in young cells, a secondary wall added on its inner side, a middle lamella that cements neighbouring cells, and plasmodesmata that connect their cytoplasm.

Layers:

- Middle lamella — a layer mainly of calcium pectate that glues neighbouring cells together
- Primary wall — thin and flexible, able to grow; found in young cells
- Secondary wall — added on the inner side, towards the membrane, as the cell matures
- Plasmodesmata — channels of cytoplasm passing through the walls, linking neighbouring cells

Functions:

- Gives the cell shape
- Protects from mechanical damage and infection
- Helps cell-to-cell interaction
- Acts as a barrier to unwanted large molecules

An everyday example. An overripe banana turns soft and mushy as enzymes break down the pectin that holds its cells together.

The substance. Fungal cell walls are made of chitin, not cellulose — cell wall chemistry differs between kingdoms.

The endoplasmic reticulum is a network of membranes where rough ER, studded with ribosomes, makes proteins and smooth ER makes lipids; vesicles carry these products to the Golgi apparatus, which receives them at its cis face, modifies and packages them, and releases them from its trans face to their destinations.

The endomembrane system includes the ER, Golgi apparatus, lysosomes and vacuoles, whose functions are coordinated; mitochondria, chloroplasts and peroxisomes are not part of it.

Endoplasmic reticulum:

- Rough ER — ribosomes on its surface; synthesises and secretes proteins; continuous with the outer nuclear membrane
- Smooth ER — no ribosomes; main site of lipid synthesis, including steroid hormones in animal cells

Golgi apparatus:

- Stacks of flat, disc-shaped sacs called cisternae, near the nucleus
- Cis face (forming face) — receives vesicles from the ER
- Trans face (maturing face) — releases packaged materials
- Adds sugars to form glycoproteins and glycolipids

Worked example — trace a digestive enzyme. It is made on rough ER, carried in a vesicle to the Golgi cis face, modified as it moves through the cisternae, and sent out from the trans face in a secretory vesicle.

An everyday example. A courier network is a good picture: rough ER is the factory, vesicles are parcels, and the Golgi is the packing and labelling centre that dispatches them.

The substance. Cells that make steroid hormones have abundant smooth ER, matching its role in lipid synthesis.

Lysosomes are membrane-bound vesicles made by the Golgi apparatus and filled with hydrolytic enzymes that digest carbohydrates, proteins, lipids and nucleic acids; vacuoles are membrane-bound spaces whose membrane, the tonoplast, pumps ions and other materials in, keeping the sap concentrated and the plant cell turgid.

Lysosomes:

- Formed by packaging in the Golgi apparatus
- Contain hydrolases — lipases, proteases, carbohydrases
- Enzymes work best at acidic pH

Vacuoles:

- Contain water, sap, excretory products and other materials
- Bounded by a single membrane, the tonoplast
- In plant cells they can fill most of the cell's volume
- The tonoplast moves ions against their concentration gradient into the vacuole, so sap is more concentrated than the cytoplasm

Turgor. Water drawn into the concentrated vacuole by osmosis presses the cytoplasm against the cell wall, keeping the cell firm.

Other vacuoles. In Amoeba, a contractile vacuole removes excess water; many protists form food vacuoles around swallowed food.

Worked example — predict the change. If a plant loses water, its vacuoles shrink, turgor falls, and leaves droop — wilting.

An everyday example. Wilted palak leaves dipped in cold water turn crisp again as their vacuoles refill and turgor returns.

The substance. Lysosomal enzymes are far less active at the neutral pH of the cytoplasm, which gives the cell some protection if a lysosome leaks.