Cell Membrane And Transport.

The cell surface membrane has several structures; Phospholipids, Proteins, Cholesterol, glycolipids and glycoproteins.

Structure and Functions of:

Phospholipids-

  • Structure

    • Has a hydrophilic head (outwards) and hydrophobic tail (inwards)

  • Functions

    • Partially permeable

    • Allows lipid soluble molecules to pass through.

Cholesterol

  • Structure

    • Has a hydrophobic region that binds with the tail of phospholipids.

  • Functions

    • Stabilise the membrane at different temperatures.

    • When temperature is too high

      • cholesterol pack phospholipids more closely

      • reduces fluidity

    • When temperature is low

      • cholesterol prevent phospholipids from packing too closely.

      • Increase the fluidity

Proteins

  • Structure

    • Intrinsic protein found across the bilayer

      • Has a channel and carrier protein

    • Extrinsic protein found on one side of the bilayer

  • Functions

    • Transport large molecules or ion across the membrane

    • Help for structural support and cell signalling.

Glycoproteins and glycolipids

  • Structure

    • Carbohydrate attached to proteins

    • Carbohydrate attached to lipids

  • Functions

    • Help in cell adhesion [keeping cells together]

    • Cell recognition [identify each other]

    • Cell signalling [A way for cells to communicate with each other.]

  

Cell Signalling

  1. A stimulus causes the cell to secrete a substance called the ligand.

  2. The ligand targets cells and binds to them by the receptors which have complementary shape to the ligand.

  3. The receptors changes shape which activates the molecules in the cell the transmit the message and bring about a response.

Diffusion

The movement of particles from a region of high concentration to a region of low concentration.

Simple diffusion

  • Small & non-polar molecules that can pass between the phospholipids.

Facilitated diffusion

  • Molecules that are too large to pass between the phospholipids.

  • They use proteins to by pass the bilayer

  • The channels has pores that allow the molecule to pass across the membrane easily.

  • The carrier protein grabs on to the molecule and flips over to release it.

Factors affecting the rate of diffusion:

  • Temperature

    • More kinetic energy → more molecules to pass through

  • gradient concentration

  • Surface area

    • Increase surface area → increase the rate of diffusion

  • The number of carrier proteins or channels

Osmosis

The movement of water molecules from a region of high water potential to a region of flow water potential.

Active Transport

the movement of particles from a region of low concentration to a region of high concentration

Active transport makes use of carrier proteins to pass across the membranes.

Factors affecting the rate of active transports:

  • Higher rate of respiration → more ATP → more energy

  • More carriers proteins there is, the more molecules that is transferred.

  • The higher the temperature, the more kinetic energy and the more molecule can move through the membrane.

  • If temperature is too high, the carrier proteins can denature and reduce the rate of active transport.

How active transport works: