Roles and Structurers of membranes (Phospholipid bilayer)

A level Biology

Basic structure of a cell membrane in a eukaryotic cell

Phospholipid bilayer

Proteins

What is the phospholipid role in the cell membrane?

because it is mainly made out of non-polar, hydrophobic phospholipid tails, it only allows non-polar, hydrophobic, uncharged substances to enter and exit the cell

therefore it controls the movement of substances across the membrane

Lipids in a cell membrane

Phospholipids

Cholesterol

Glycolipids

Proteins in a cell membrane

Surface proteins

glycoproteins

transport proteins

Role of Cholesterol in a cell membrane

• it is a lipid that binds to the fatty acid tails of the phospholipids

• it regulates membrane fluidity

• makes cell membranes less fluid by restricting the movement of phospholipids and proteins —> which prevents water/ions from leaking out of the cell

Role of Glycolipids in a cell membrane

• they are phospholipids that are attached to a carbohydrate

• they act as recognition sites for other cells—> allowing cells to attach to one another to form tissues

Where can proteins in a cell membrane be?

Either:

on the surface of the cell membrane

or

span the entire membrane

Which proteins are found on the surface?

• Surface proteins

• Glycoproteins

Which protein is found spanning the entire membrane?

Transport proteins

Role of surface proteins in cell membrane?

• Surface proteins are found on inner or outer surface of phospholipid bilayer

• provide mechanical support for the membrane

Role of Glycoproteins in a cell membrane

• have a carbohydrate attached to them

• either act as a recognition site for foreign cells or for specific chemicals

• so specific hormones outside the cell can trigger a response inside the cell

Role of transport proteins (brief summary)

• help transport substances across membrane 

Meaning behind Fluid mosaic model

Fluid- describes how phospholipids and some proteins move in their layer of the membrane

Mosaic- describes the random scattering of proteins

What is the function of the phospholipid bilayer? (uplearn)

The phospholipid bilayer allows the diffusion of lipid-soluble substances across the membrane.

 

It also prevents the entry and exit of polar substances.

What is a glycolipid

a phospholipid that is attached to a carbohydrate

What is a glycoprotein

a protein with a carbohydrate chain attached

Describe and explain how the structure of the cell membrane determines the movement of substances in and out of the cell (6 marks)

The phospholipid bilayer has a nonpolar core made of hydrophobic fatty acid tails.

This permits small, non-polar, uncharged and hydrophobic particles to cross the membrane by simple diffusion, but blocks the diffusion of large, polar, charged and hydrophilic particles. (max 2 marks)

Transport proteins allow large, polar, charged and hydrophilic particles to cross the membrane by facilitated diffusion.

Transport proteins control which of these particles diffuse across the membrane by opening and closing in response to signals, such as chemical messengers. (max 2 marks)

The rate of passive transport depends on the surface area to volume ratio, the length of the diffusion pathway, how steep the concentration gradient is, and the number of transport proteins in a given area (facilitated diffusion only). (max 2 marks)

Transport proteins can actively transport particles across the membrane against their concentration gradient, using energy from ATP. (1 mark)

Some transport proteins can co-transport two particles at the same time, using energy from ATP or the passive transport of one particle. (1 mark)

Only particles with a shape that is complementary to the structure of the binding site will be transported by the protein. (1 mark)