2.4 membranes

2.4 Membranes

Topic 2 Cells

Membranes2.4.1 Draw and label a diagram to show the structure of membranes.

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.

2.4.3 List the functions of membrane proteins.Include the following: hormone binding sites, immobilized enzymes, cell adhesion, cell-to-cell communication, channels for passive transport, and pumps for active transport.

Membranes2.4.4 Define diffusion and osmosis.

Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.

Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Membranes2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion.

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes.

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.

The Fluid Mosaic Model

In 1972, Singer and Nicolson proposed a model for the membrane structure in which a mosaic of protein molecules float in a fluid lipid layer.This is know as the Fluid Mosaic Model of Membranes.The model shows membranes to consist of a Phospholipid Bilayer, with proteins molecules within the fluid bilayer.Parts of the membrane model include:– Phospholipid bilayer– Protein molecules– Cholesterol molecules– Glycoprotein molecules


Ref: Advanced Biology Roberts etal.


The phospholipid bi-layer consists of a double layer of phospholipid molecules.The phospholipid molecules have a hydrophilic end (water loving) and a hydrophobic end (water hating).Phospholipids are amphipatic.

Hydrophilic Phosphate head Hydrophobic fatty acid tail

Fluidity of Membranes

Phospholipids in membranes are in a fluid state.This allows membranes to change shape in a way that would be impossible if they were solid.The fluidity also allows vesicles to be pinched off from membranes or fuse with them.

Membrane Proteins

Membrane proteins can be:– Integral Proteins

• Protein molecules embedded in the fluid phospholipid bilayer.

– Peripheral Proteins• Protein molecules spread over the surface of the membrane.

Membrane Proteins

Ref: IB Biology OSC

Membrane Proteins

Membrane proteins can have many different functions:– Hormone binding sites– Enzymes– Electron carriers– Channels for passive transport– Pumps for active transport

Membrane Proteins

Ref: Allott

Diffusion

Diffusion is the passive movement of particles from a region of higher concentration to a region of lower concentration, as a

result of random motion of particles.

Diffusion is a passive process (no energy is required).A membrane is not essential.There are a number of types of diffusion.

Diffusion

Ref: Allott

Diffusion

Some substances can diffuse through membranes.

Small uncharged particles pass between the lipid molecules.– eg: oxygen, carbon dioxide


Diffusion Through Channel ProteinsSome substances pass through channel proteins.

Channel proteins are concerned with transporting ions through membranes.– Eg: water soluble substances, water, ions

Movement is passive (no energy is required).Ref: Advanced Biology Roberts etal.

Facilitated Diffusion Through Channel Proteins

Some substances are larger and require carrier proteins to move them through the membrane.

Carrier proteins carry molecules which are polar and larger.– Eg: glucose, amino acids

Carrier proteins are specific for certain substances.Movement is passive (no energy is required). Ref: Advanced Biology Roberts etal.

Osmosis

Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region higher solute

concentration, across a partially permeable membrane.

Simply put, Osmosis is the diffusion of water.Osmosis is a passive process (no energy is required).A membrane is essential.

Active Transport

Active transport is the movement of substances across a membrane using energy from ATP.Active transport can move substances against the concentration gradient:– From a region of lower concentration to an area of higher concentration

Protein pumps in the membrane are used for active transport.Each pump only transports particular substances, so cells can control what is absorbed.

Active Transport

Active transport allows cells to take up nutrients when their concentrations outside the cell are low.Also allows cells to get rid of wastes when the outside concentration is higher.


Exocytosis and Endocytosis

Sometimes substances need to be moved into or out of a cell that can’t be done by diffusion or active transport.These substances are usually larger.These methods rely on the fluidity of the plasma membrane.Examples include:– Exocytosis:

• Moving substances out of the cell– Endocytosis:

• Moving substances into the cell

Exocytosis

Sometimes large molecules need to be expelled from the cell.This is done by a process called:– Exocytosis

The vesicle containing the material to be expelled moves to the plasma membrane.The vesicle fuses with the plasma membrane.The vesicle then opens to the exterior of the cell, releasing its contents out of the cell.

Exocytosis

Ref: Biology Key Ideas

Endocytosis

Small molecules can enter and leave cells through the plasma membrane by diffusion or active transport.Sometimes larger molecules need to be taken into a cell.This is done by a process called:– Endocytosis

Two types of Endocytosis are:– Phagocytosis

• Large solid particles– Pinocytosis (sometimes referred to as cell drinking)

• Liquids

Endocytosis

Ref: Biology Key Ideas

Transportation Using Vesicles


Phagocytic Intake and Digestion

1. Small particles are taken in by phagocytosis to form a phagocytic vesicle.2. A lysosome fuses with the vesicle and discharges its contents into it.3. The lysosome enzymes digest the particles and the products of digestion are absorbed into the surrounding cytoplasm.4. The vesicle membrane fuses with the plasma membrane and any indigestible matter is expelled.


IBO guide:2.4.1 Draw and label a diagram to show the structure of membranes.The diagram should show the phospholipid bilayer, cholesterol, glycoproteins, and integral and peripheral proteins. Use the term plasma membrane, not cell surface membrane, for the membrane surrounding the cytoplasm. Integral proteins are embedded in the phospholipid of the membrane, whereas peripheral proteins are attached to its surface. Variations in composition related to the type of membrane are not required.

Aim 7: Data logging to measure the changes in membrane permeability using colorimeter probes can be used.

IBO guide:2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.

2.4.3 List the functions of membrane proteins.Include the following: hormone binding sites, immobilized enzymes, cell adhesion, cell-to-cell communication, channels for passive transport, and pumps for active transport.

2.4.4 Define diffusion and osmosis. Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.

IBO guide:Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion.

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes.

IBO guide:2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.

Technology

2.4 membranes