transport across membranes

what is the model for cell membranes?

FLUID MOSAIC MODEL

• Mosaic - cell membranes are composed of a phospholipid bilayer, proteins, glycoproteins & glycolipids which are arranged scattered, like a mosaic

• Fluid – The phospholipid bilayer is flexible because phospholipids can move within the layer. This allows the membrane to change shape and enables the movement of substances across it.

simple diffusion

• the net movement of particles from an area of high concentration to low concentration down a concentration gradient(passive : ni energy needed)

• for molecules to diffuse simply, they must be small and lipid soluble (non polar)

what factors effect the rate of simple diffusion?

Factors affecting rate:

• Concentration gradient (steeper = faster)

• Surface area (larger = faster)

• Thickness of exchange surface (thinner = faster)

what is facilitated diffusion?

— The net movement of molecules or ions from a region of higher to lower concentration, down a concentration gradient, via a transport protein (passive – no energy required).

• • Molecules involved: Large, polar, or charged molecules (e.g., glucose, amino acids, ions) that cannot pass through the phospholipid bilayer.

• • Channel proteins – Form pores for specific ions.

  • Carrier proteins – Bind to specific molecules, change shape, and transport them across.

• the more carrier/channel proteins, the greater the rate of diffusion (unless the proteins are all saturate, this means increasing concentration has no effect)

what is osmosis?

The net movement of water molecules from a region of higher water potential to lower water potential, across a selectively permeable membrane, without energy (passive transport)

what is water potential? what are the different types of solutions?

water potential (kPa) is the pressure created by water molecules

• the more negative the wp, the more solute in the solution

different solutions:

• isotonic : the wp is the same inside the cell & the solution

• hypertonic : the wp is higher in the cells than the solution so the net movement of water is out of cells

• hypotonic : cells have lower wp than the solution so the net movement is inside of cells

what are the effects of the different types of solution on animal and plant cells?

Animal Cells:

• Isotonic solution (same water potential as the cell) → No net water movement, cell stays the same.

• Hypertonic solution (lower water potential outside) → Water moves out, cell shrinks (crenation).

• Hypotonic solution (higher water potential outside) → Water moves in, cell swells and bursts (lysis).

Plant Cells:

• Isotonic solution → No net water movement, cell is flaccid.

• Hypertonic solution → Water moves out, cell becomes plasmolysed (cytoplasm pulls away from the wall).

• Hypotonic solution → Water moves in, cell becomes turgid (cell wall prevents bursting).

what is active transport?

The movement of molecules or ions from a region of lower to higher concentration, against the concentration gradient, using ATP and carrier proteins. (active process - energy required in the form of atp)

• 1. Molecule/ion binds to a specific carrier protein at the binding site of the membrane

  2. ATP binds to the carrier protein, causing atp to be hydrolysed to release ADP & inorganic phosphate

  3. Carrier protein changes shape, and open to the opposite side of the membrane, moving the molecule across the membrane.

  4. Inorganic phosphate is released on the other side, and the carrier returns to its original shape.

co transport ( a type of active transport )

Example: Sodium-Glucose Co-Transport (in the small intestine)

1. Sodium ions (Na⁺) actively transported out of epithelial cells into the blood using the sodium-potassium pump.

2. This creates a low Na⁺ concentration inside the epithelial cell.

3. Na⁺ moves back into the cell from the lumen of the intestine down its concentration gradient, via a co-transporter protein.

4. Glucose is carried into the cell with Na⁺ against its concentration gradient.

5. Glucose then diffuses into the blood via facilitated diffusion.

how are cells adapted for rapid transport?

Cells may be adapted for rapid transport across their internal or external membranes by an

increase in surface area of, or by an increase in the number of protein channels and carrier

molecules in, their membranes

name 2 adaptation of specialised cells for transport

Adaptations of Specialised Cells for Transport:

• Epithelial cells in the small intestine (microvilli) → Increase surface area for faster absorption of nutrients via diffusion, facilitated diffusion, and active transport.

• Root hair cells → Large surface area and many carrier proteins for active transport of minerals.

• Red blood cells → No nucleus to increase space for oxygen diffusion.

• Neurons → Sodium-potassium pumps maintain ion gradients for rapid nerve impulses.

factors affecting rate of transport

Factors Affecting Rate of Transport:

1. Surface Area → More membrane area means faster diffusion, osmosis, and active transport.

2. Number of Channel/Carrier Proteins → More proteins allow faster facilitated diffusion and active transport.

3. Concentration Gradient →

• Steeper gradient = faster diffusion/facilitated diffusion.

• Active transport works best when a large difference exists.

4. Water Potential Gradient → A greater difference = faster osmosis