transportation across cell membranes

What’s the function of a cell membrane

• to control what substances can enter and exit the cell → do this by creating an enclosed space separating internal environment from external environment

What is the fluid mosaic membrane model

Fluid → phospholipids form a bilayer where phospholipid molecules are constantly moving

Mosaic → proteins of different shapes and sizes are embedded in true phospholipid bilayer

What is a phospholipid

Each phospholipid is made up of

• hydrophilic head that points outwards

→ head contains a phosphate group and a phosphoester bond

• hydrophobic tail that points inwards

→ tail contains glycerols and an ester bond

What’s a phospholipid bilayer

When phospholipids arrange themselves so their heads point outwards (towards the water) and the tails point inwards (away from the water). This arrangement creates a hydrophobic centre in a bilayer so water soluble substances cannot pass through

What substances can and cannot pass through phospholipid bilayer

• Can: lipid-soluble substances

• Cannot: water-soluble substances

What are the components of cell membranes

• phosophjolipid bilayer → made up of phospholipids with hydrophilic head and hydrophilic tails

• Cholesterol → adds stability

• Proteins → may be intrinsic or extrinsic proteins

• Glycoproteins → thee proteins are attached to a carbohydrate

• Glycolipids → these are lipids attached to a carbohydrate

Cholesterol in cell membrane

• cholesterol molecules consist of hydrophilic and a hydrophobic region

• The hydrophobic regions bind to phospholipid fatty acid tails, causing them to pack more closely together

• This reduces the fluidity of cell membrane

Proteins in cell membrane

• can be either intrinsic or extrinsic

• Intrinsic proteins → embedded through both sides of the phospholipid bilayer. These proteins include channel and carrier proteins which target large molecules and ions across the membrane

• Extrinsic proteins → present on only one side of phospholipid bilayer. These proteins provide support to the membrane or may be involved in cell signalling

Glycoproteins in cell membrane

• consists of intrinsic proteins attached to carbohydrates

Glycolipids in cell membrane

• consists of lipids attached to carbohydrates

What 3 things are both glycolipids and glycoproteins involved in

1. Cell adhesion

• attachment of cells to one another

2. Cell recognition

• this allows cells to recognise one another

3. Cell signalling

• this is communication between cells

What are the two types of cell membranes

1. Cell surface membranes

• these surround cells to act as a barrier between the cell and its environment, controlling what substances enter and leave cell

2. Membranes around organelles

• these surround organs to act as a barrier between organelles and cytoplasm, dividing cell into different compartments (compartmentation)

Both types are partially permeable

What is diffusion

Movement of particles from an area of higher concentration to an area of lower concentration

• particles diffuse down a concentration gradient and eventually reach equilibrium when particles are evenly distributed (but still moving)

• Does not require energy (passive process)

Simple diffusion

• only some molecules can diffuse by simple diffusion across cell membranes, e.g. carbon dioxide and oxygen

• This is because they are

  • small - means they can mass though spaces between phospholipids

  • Non polar - means they can dissolve in hydrophobic core of the cell membrane

Facilitated diffusion

Passive process that allows large or polar molecules to move from an area of higher concentration to an area of lower concentration across a partially permeable membrane with the help of carrier or channel proteins

• each carrier or channel protein is highly specific, allowing only one or two types of molecules to pass through

Carrier proteins

• mainly transport large molecules across cell membranes

• Steps:

1. Large molecule attaches to a carrier protein

2. Causes carrier protein to change shape

3. Carrier protein releases molecule on opposite side of membrane

Channel proteins

• mainly transport ions across cell membrane

• These proteins form pores in cell membrane which ions can travel through

What factors effect rate of diffusion

1. Temperature → higher temps particles have more kinetic energy so can diffuse faster

2. Concentration gradient → steeper the CG the faster the rate of diffusion

3. Thickness of membrane → particles travel shorted distances through thin exchange surfaces, so diffuse faster due to shorter diffusion pathway

4. Surface area - larger surface areas mean more particles can cross membrane at once, increasing rate of diffusion

5. Number of channel or carrier proteins → the more of these proteins the faster the rate of facilitated diffusion

What are solutions

Mixtures made up of water and a solute dissolved in a solvent

What’s water potential (Ψ)

• the pressure exerted by water molecules on the membrane surrounding a solution

• measured in kiloPascals (kPa)

• symbol → Ψ

What’s a high water potential

When the solution has a high water concentration (not very much solute dissolved in it)

What’s a low water potential

When the solution has a low water concentration (lots of solute dissolved in it)

What’s the highest water potential

0 kPa → pure water

• as more solute os added the value becomes more negative

What is osmosis

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

• water molecules small and diffuse directly through cell membrane whereas larger solute molecules cannot

• Water molecules diffuse down a water potential gradient until the water potential is equal on both sides of the membrane (equilibrium)

What’s a hypotonic solution

• has higher water potential than cell

• Water molecules move into cell

• Cell swells and bursts

What’s an isotonic solution

• has same water potential as cell

• No net movement of water in or out of cell

• Cell stays same size

What’s hypertonic solution

• lower water potential than cell

• Water molecules move out of cell

• Cell shrinks

Osmosis in plant cells

• plant cells do not burst due to cell wall

• In hypotonic solutions the cell swells and becomes turgid

• In isotonic solutions cell stays same size

• In hypertonic solutions cell shrinks and becomes plasmolysed

What’s are the factors that affecting the rate of osmosis

1. Temperature → higher temperatures, water molecules have more kinetic energy and diffuse faster

2. Water potential gradient → the steeper gradient, faster the rate of osmosis

3. Thickness of membrane → water molecules travel shorter distances through thin exchange surfaces, so diffuse faster

4. Surface area - larger surface areas mean more water molecules can cross the membrane at once, making osmosis faster

What is active transport

The movement of particles from an area of lower concentration to an area of higher concentration through a partially permiable membrane. It requires energy from respiration in the form ATP

• energy is needed for particles move against conc gradient so is an active process

How do carrier proteins use active transport

• active transport involved carrier proteins to transport molecules our ions across membranes

• Steps:

1. Molecule or ion ind to carrier protein

2. ATP binds to carrier protein

3. Hydrolysis of ATP and ADP and phosphate causes carrier protein to change shape. This releases the molecule ion on opposition side of membrane

4. Phosphate is released from Carrie protein, causing carrier protein to return to original shape, ready to be reused

What factors effect rate of active transport

1. Temperature → higher temps, particles have more kinetic energy so move faster. Respiration also increases with temperature. Very high temperatures denature carrier proteins, which decreases rate of active transport

2. Thickness of membrane → particles travel short distances through thin exchange surfaces, so travel faster as shorter diffusion pathway

3. Number of Carrie proteins → more proteins, the faster the rate of active transport

4. Rate of respiration → the more respiration, the more ATP available for active transport

What’s co transport

• Some carrier proteins can bind to two molecules at once, these proteins are known as co-transporters. They use the concentration gradient of one molecule to move the other molecule against its own concentration gradient

How is glucose absorbed

• during digestion carbohydrates are broken down into glucose

• Glucose absorbed from lumen of small intestine into bloodstream.

• Diffusion only result in equal glucose concentrations, so not all glucose will be absorbed

→ to overcome this glucose absorbed from intestines into bloodstream using co transport

• as food passes through ileum, glucose passes from lumen into epithelial cells and then into blood

What proteins does the co transport of glucose involve

• sodium- potassium pump → these actively transport sodium and potassium ion

• Sodium - glucose co transporter protein → these use facilitated diffuse ton transport Na+ ions and glucose molecule molecules

• Glucose protein channels → use facilitated diffusion to transport glucose molecules

How is glucose absorbed into the blood (co transport of sodium and glucose)

1. Sodium actively transported out of epithelial cells into blood by sodium potassium pump. Pump transports 3 sodium out for every 2 potassium in. This creates a conc gradient as now higher conc of sodium in lumen than epithelial cells

2. Sodium diffuses from high conc in lumen to low conc in epithelial cells. It is transported via sodium-glucose co transporter proteins, which also carry glucose molecules. Causes conc of glucose in epithelial cells to increase

3. Now higher glucose conc in epithelial cells than blood, so glucose diffuses out of epithelial cells into blood via facilitated diffusion

Why do glucose molecules use co transport rather than active transport

• glucose molecules move against their conc gradient, rather than by using ATP

• Same process is sued to absorb amino acids in small intestine

• ATP is used to maintain conc gradient of sodium ions between lumen and epithelial cells of ileum