Section 2 4 Transport across cell membranes

The role of phospholipids in cell membrane:

The hydrophilic heads are attracted towards the water, so face on the outside of the membrane. The hydrophobic tails are repelled by water so face inside the membrane.

• Allow lipid soluble substances to enter and leave the cells (small, lipid, uncharged e.g. oxygen)

• Prevent water-soluble substances entering and leaving the cell

• Membrane flexible and self-healing

The role of cholesterol in cell membrane:

Cholesterol binds to the phospholipid tail, they are hydrophobic and pull together the fatty acid tails.

• restricts the fluidity of cell membrane, preventing ions and water from leaking out of the cell

• Membrane less fluid at higher temperatures

The role of glycolipids in the cell membrane

They are a carbohydrate attached to a lipid.

• Act as recognition sites

• helps cells attach to one another to form tissues

• Maintain stability of membrane

The role of glycoproteins in the cell membrane

They are carbohydrates attached to the protein.

• They act as recognition sites for other cells, and for specific substances such as hormones

The role of proteins in the cell membrane

Surface of the membrane:

• Provide mechanical support

• Act as cell receptors for example hormones

Span the cell membrane:

• Transport proteins that span the cell membrane, where protein channels are water filled tubes to allow water soluble ions to diffuse. Carrier proteins that bind to ions change shape and enable them to cross the membrane.

Functions of proteins:

• Structural support

• act as receptors to identify hormones

• act as cell-surface receptors identifying cells

• Attach to other cells

• Allow active transport

• Allow water soluble substances across

Fluid-mosaic model

• Fluid: the phospholipids can move around relative to one another

• Mosaic: the proteins are embedded around the membrane

Simple diffusion

The net movement of particles from an area of high concentration to an area of low concentration.

Small, lipid, uncharged (O2, CO2) => diffuse directly across the cell membrane.

Smaller, polar molecules (Na+ and H+) => facilitated diffusion using channel proteins.

Larger, lipid-insoluble molecules (amino acid, glucose) => facilitated diffusion using carrier proteins.

Channel proteins VS Carrier proteins

• Channel proteins: Water-filled hydrophilic tube, allow specific water-soluble ions. [Na+ // H+]

• Carrier proteins: allow specific large and lipid soluble [amino acid, glucose], binds with protein, the protein changes shape and is released inside. (active transport only)

Osmosis

The net movement of water particles from a region of high water potential to a region of low water potential, through partially permeable membrane.

Active transport

The net movement of molecules from a region of low concentration to a region of high concentration using ATP and carrier proteins.

• The specific molecule binds to the carrier protein on the side with high concentration, on the other side ATP molecule binds.

• The ATP gets hydrolysed into ADP and Pi, which releases energy

• Changes the transport proteins’ shape.

• Allowing the molecule to be tranported to the area of high conc

• The ADP is released and protein reverts back to normal.

Absorption of glucose in the ileum

• Sodium ions are actively pumped out of the epithelial cells, via the sodium potassium pump. Increasing the concentration of Na+ ions in the blood.

• Na+ ions are transported down the concentration gradient along with a glucose or amino acid, via co-transport.

• No ATP required as Na+ ions transported down the concentration gradient, with the glucose/ amino acids against their concentration gradients.

• The glucose or amino acid is transported down the concentration gradient via facilitated diffusion into blood capillary