2.3.: Transport Across Membranes

Flashcards made from this slideshow: https://docs.google.com/presentation/d/1iH8N7JguBMza9oPIu4MahkWu2Rosq7wLRoh-j1AWu2o/edit#slide=id.g30956a8a0d6_0_2850

What is the structure of a phospholipid bilayer?

• Hydrophillic, polar heads point outwards

• Hydrophobic, non-polar tails point inwards

What is the function of a phospholipid bilayer?

• Allows lipid-soluble substances to move in and out of the cell (by simple diffusion)

• Prevent (charged/polar) water-soluble substances entering and leaving the cell

• Makes membranes flexible and self-sealing

What is the structure of a channel protein?

• Span the membrane (transmembrane proteins)

• “Water filled tubes”

• Have a hydrophillic channel through the centre

  • The size and charge of this chennel varies

What is the function of a channel protein?

• Allows specific water soluble ions to diffuse across the membrane

(Aquaphorins are cannel proteins that allow water molecules to move across the membrane)

What is the structure of a carrier protein?

• Span the membrane (transmembrane proteins)

• Have a “binding site” that is specific to a particular molecule or ion

What is the fucntion of a carrier protein?

• Bind to specific ions or molecules like glucose or amino acids

• Change shape (bonding causes a change in the tertiary structure of a protein)

• To move molecules across the membrane

(If this is up a concentration gradient and requires ATP, it is active transport. If it is down a concentration gradient and doesn’t require ATP, it is facilitated diffusion)

What is the structure of cholesterol?

A very hydrophobic tail

What is the function of cholesterol?

Stabilises the membrane:

• Reduces lateral movement of membrane components, e.g phospholipids

• Makes membranes less fluid at higher temperatures

• Reduces leakage of water and ions from the cell

What is the structure of a receptor protein?

Often glycoprotein

What is the function of a receptor protein?

When signalling a molecule (e.g a hormone) binds to a receptor it stimulates the cell to do something

What is the structure of glycoproteins?

Protein with a carbohydrate group attached

Wha si the function of glycoproteins?

• Cell surface receptors for hormes and neurotransmitters

• Act as recognition sites (for cell recognition)

• Help cells to attach to each other and form tissues (cell to cell adhesion - animal cells only)

What is the structure of a glycolipid?

A phospholipid with a carbohydrate group attached

What is the fucntion of a glycolipid?

• Help maintain the stability of the membrane (make it less fluid at high temperatures)

• Cell surface receptors for hormones and neurotransmitters - not common

• Act as recognition sites (cell recognition)

• Help cells to attach to each toher and form tissues (cell to cell adhesion - animal cells only)

What si a transmembrane protein?

Intrinsic proteins that cross the phospholipid bilayer. All transport proteins are transmembrane proteins

What are receptors/receptor proteins?

Transmembrane proteins that transmit signals from signalling molecules (eg hormones) across the membrane

What are transport proteins?

A general term for any protein involved in transporting molecules across the cell membrane (includes carrier and channel proteins)

What are cotransporters?

Proteins that transport two substances across the membrane at once. These could be symports (in the same direction) or antiports (in opposite directions)

What are extrinsic/peripheral proteins?

proteins that only associate with one side of the membrane. Usually just associate with the hydrophillic heads of phospholipids in the bilayer

What are intrinsic/integral proteins?

Proteins that embed into the hydrophobic region of the phospholipid bilayer. These have a hydrophobic region (with hydrophobic R groups on the outside of the protein)

What is the Fluid Moasaic Model?

A model that describes the structure of cell membranes as a mosaic of various proteins that float in or on the fluid lipid bilayer, allowing for flexibility and movement.

Define active transport

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Describe the process of active transport (4 marks)

A molecule binds to receptor sites on a carrier protein and ATP binds to the arrier protein on the outside of the cell/organelle.

ATP is hydrolysed into ADP which causes the carrier protein to change shape (because eneergy is released)

The carrier protein is now open on the other side of the membrane, and the molecule is released

The molecule causes the carrier protein to return to the original shape

(Carrier proteins span the plasma membrane)

How do hydrogen ion pumps move hyrogen ions into a cell? (4 marks)

A hydrogen ion binds to receptor sites on a carrier protein

ATP binds to a separate site on the carrier protein and is hydrolysed into ADP + P

This releases energy which causes the carrier protein to change shape, so there is an opening on the inside of the cell

The hydrogen ion is released and this causes the carrier protein to revert back to the original shape