4.2 - Cell transport mechanisms

What is the function of the cell membrane?

To control what substances can enter & leave the cell

Why is the cell membrane called a fluid mosaic model?

Due to the mixture & sideways movement of the phospholipids, proteins, glycoproteins & glycolipids it is made of

How do the phospholipids arranged themselves?

In a phospholipid bilayer:

• the hydrophilic, polar phosphate heads face outwards towards the water (aqueous environment)

• the hydrophobic, non-polar fatty acid tails face inwards towards each other

What is the difference between integral & peripheral proteins?

• Integral proteins: span from one side of the phospholipid bilayer to the other

• Peripheral proteins: sit on one of the surfaces of the phospholipid bilayer

What is the function of integral & peripheral proteins?

Integral proteins:

• protein carriers/channel proteins involved in the transport of molecules across the membrane

Peripheral proteins:

• provide mechanical support, or are connected to proteins/lipids to make glycoproteins/glycolipids

• cell recognition, as receptors

What is the function of cholesterol in the cell membrane?

• Restricts the sideways movement of other molecules in the membrane

• Makes the membrane less fluid at higher temperatures & prevents water/dissolved ions leaking out of the cell

What are enzyme proteins?

Catalyse reactions in the cytoplasm or outside the cell (e.g. maltase in the small intestine)

What is the difference between glycoproteins & glycolipids?

• Glycoproteins: proteins with carbohydrates attached

• Glycolipids: phospholipids with carbohydrates attached

What are the four key types of transport?

• Simple diffusion (i.e. lipid diffusion)

• Facilitated diffusion

• Active transport

• Osmosis

What is simple diffusion?

Net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached (does not require ATP)

What is a requirement of molecules in simple diffusion?

They must be lipid-soluble, small & non-polar

What is facilitated diffusion?

Protein channels or carrier proteins are used to transport ions/polar molecules across the cell membrane (does not require ATP)

What is the difference between protein channels & carrier proteins?

Proteins channels:

• form tubes filled with water, enabling water-soluble ions to pass through the membrane

• selective process → only open in the presence of certain ions

Carrier proteins:

• bind with a molecule (e.g. glucose) which causes it to change shape, enabling the molecule to be released to the other side

What is osmosis?

Movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane

What is water potential (Ψ) ?

A measure of the tendency of water to move from one area to another by osmosis (measured in kPa)

What is the water potential of pure water?

Zero:

• when solutes are dissolved in water, the water potential becomes negative

• the more negative the water potential, the more solutes dissolved in it

What are the two ways water can diffuse across a partially permeable membrane?

• Phospholipid bilayer (simple diffusion)

• Aquaporin protein channels (facilitated diffusion)

Despite the concentration of water being very high in cells, why is much of this water not free to diffuse?

It is bound to solvent molecules as hydration shells:

• the greater the concentration of a solution, the greater the solute molecules → more water molecules bound in hydration shells, so fewer free water molecules

What is the equation to calculate water potential?

Water potential = turgor pressure + osmotic pressure/potential

• turgor pressure: inward pressure exerted by cell wall

• osmotic pressure/potential: ability of water to move across a partially permeable membrane

What is a hypotonic solution & what happens to the cell?

Water potential of the cell is lower than the solution around it → water moves into the cell

• animals cells: burst

• plant cells: become turgid

  • how to remember → think ‘hippotonic’

What is a hypertonic solution & what happens to the cell?

Water potential of the cell is higher than the solution around it:

• water moves out of cell, causing it to shrink or become flaccid

  • how to remember → when someone is ‘hyper’, they burn calories, so become skinny

What is an isotonic solution & what happens to the cell?

Water potential is the same in the cell & solution around it:

• water moves in & out of cell equally, so cell remains same size

  • how to remember → think isoperfect, like ‘i’m so perfect’, so no change

What is active transport?

Movement of molecules/ions from an area of lower concentration to an area of higher concentration using ATP & carrier proteins

What is co-transport?

Some carrier proteins have two binding sites & so transport two molecules across the cell membrane at once

How is ATP used in active transport?

• ATP binds to the protein on the inside of the membrane & is hydrolysed into ADP & Pi

• This causes the molecule to be released on the other side of the membrane

• The phosphate molecule is then released from the protein & this results in the protein reverting back to its original shape

How might solubility, size & charge affect how a molecule is transported?

• Solubility: lipid-soluble molecules pass through membranes more easily

• Size: smaller molecules diffuse faster

• Charge: charged molecules cannot diffuse by simple diffusion

What is bulk transport & what are the two types?

The transport of larger molecules into & out of cells using vesicles:

• endocytosis: bulk transport into cells

• exocytosis: bulk transport out of cells

Describe how mineral (inorganic) ions are taken up by active transport

Moved across cell membrane into root hair cell by carrier proteins, against a concentration gradient, using ATP

What is the structure of a phospholipid?

Glycerol attached to two fatty acids by ester bonds & one phosphate attached to a glycerol