OCR AS Biology: Biological Membranes

Roles of membranes

Partially permeable barriers between the cell and its environment, partially permeable barriers between organelles and the cytoplasm, partially permeable membranes within organelles, sites of chemical reactions, sites of cell signalling

Other term for cell communication

Cell signalling

Fluid Mosaic Model of Membrane Structure

Phospholipids are free to move within the phospholipid bilayer relative to each other, making them fluid and giving the membrane flexibility. Proteins are embedded in the bilayer and they vary in shape, size and position, like tiles do in a mosaic.

Components of cell membrane structure

Phospholipids, cholesterol, glycolipids, extrinsic proteins, glycoproteins, intrinsic proteins

Role of phospholipids in a cell membrane

To form the lipid bilayer which is the basic structure of the cell membrane.

Role of cholesterol in a cell membrane

Provides stability and flexibility

Role of glycoplipids in a cell membrane

Recognition site (Antigens)

Role of intrinsic proteins

Channel and carrier proteins which are important for passive and active transport into and out of cells

Role of glycoproteins

Recognition site, cell adhesion

Channel proteins

Hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes,

Uses of carrier proteins

Passive transport, active transport

Role of membrane-bound receptors

Sites where hormones and drugs can bind

Factors affecting membrane structure and permeability

Temperature, solvents

Effect of temperature on membrane structure and permeability

Increased temperature will increase the kinetic energy of phospholipids so they will move more, making the membrane more fluid and causing it to lose its structure, increasing the permeability of a cell membrane

Effect of solvents on membrane structure and permeability

Organic solvents dissolve cell membranes, non-polar solvent molecules get between the phospholipids and disrupt the membrane so it is more fluid and permeable

Passive methods of movement across membranes

Diffusion, facilitated diffusion

Diffusion

Net movement of particles from a region of higher concentration to a region of lower concentration

Facilitated Diffusion

Net movement of particles from a region of higher concentration to a region of lower concentration through protein channels

Methods of movement across membranes

Diffusion, facilitated diffusion, active transport, endocytosis, exocytosis

Methods of movement across membranes that require ATP

Active transport, endocytosis, exocytosis

Factors that can affect diffusion rates

Surface area, thickness of membrane

Practical investigation into how surface area affects diffusion rates

Use blocks of different surface areas containing phenolphthalein which turns pink when in the presence of an alkali. Immerse them in a sodium hydroxide solution for ten minutes. Remove the blocks and use a ruler to see the distance that the sodium hydroxide has diffused

How to make a model cell

Tying a piece of dialysis tubing at one end, filling it with a solution and then tying the other end before placing the 'cell' into a solution

How to use model cells to estimate diffusion rates

Changes in concentration of solute molecules inside and outside the model cell can be measured over time. Rates of diffusion can then be calculated from this.

How to use model cells to see how concentration can change diffusion rates

Place the model cells in solutions with different solute concentrations.

Osmosis

The net movement of water molecules from a region of a higher water potential down a water potential region to a region of lower water potential through a partially permeable membrane.

Effect on a plant cell of being in a more concentrated solution

Water will leave the cell, causing it to plasmolyse and the contents to shrink.

Effect on a plant cell of being in a less concentrated solution

Water will enter the cell, causing it to swell and become turgid.

Effect on an animal cell of being in a more concentrated solution

Water will leave the cell, causing it to shrink and shrivel and be crenated

How active transport works

Molecule binds to receptors in the channel of the carrier protein on the outside of the cell, ATP binds to the carrier protein on the inside of the cell and is hydrolysed into ADP and phosphate, binding of phosphate molecule to the carrier protein causes the protein to change shape which opens up the inside of the cell, molecule is released to inside of the cell, phosphate molecule is released from the carrier protein and recombines with ADP to form ATP, carrier protein returns to original shape

Types of bulk transport

Endocytosis, exocytosis

Endocytosis

Bulk transport of material into cells

Types of endocytosis

Phagocytosis, pinocytosis

Phagocytosis

Bulk transport of solid materials into cells

Pinocytosis

Bulk transport of liquid materials into cells

How endocytosis works

Cell surface membrane invaginates when it comes into contact with the material to be transported. The membrane enfolds the material until the membrane fuses to form a vesicle. Vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.