transport across membranes

all membranes around and within cells…..

have the same basic structure and are known as plasma membranes

what do cell-surface membranes do?

• Allows a separation of conditions inside and outside of a cell

• Controls movement of substances in and out of cell

• Cells adapt for rapid transport across both internal and external membranes by increase of surface area of/by an increase in number of protein channels and carrier proteins in membrane

structure of phospholipid bilayer and functions

• Hydrophilic heads of both layers point to the outside of cell-surface membrane attracted by the water on both sides

• Hydrophobic tails of both phospholipid layers point to the centre of the membrane repelled by water

• Lipid-soluble substances move through the membrane via this portion

• Functions:

  • Allow lipid-soluble substances to enter and leave cell

  • Prevent water-soluble substances entering and leaving cell

  • Make membrane flexible and self-sealing

structures and functions of proteins in membrane?

• Some occur in the surface of the bilayer and don't extend across it - give mechanical support or (with the glycolipids) they are cell receptors for molecules like hormones

• Other proteins span across the layer - protein channels form water-filled tubes to allow water-soluble ions to diffuse across whilst carrier proteins bind to ions or molecules and change shape to move the molecules across the membrane

• Functions

  • Provide structural support

  • Act as channels transporting water-soluble substances across the membrane

  • Allow active transport across membrane through carrier proteins

  • Form cell-surface receptors for identifying cells

  • Help cells adhere together

role of cholesterol?

• add to stability of membranes

• Hydrophobic and play role in preventing loss of water and dissolved ions from the cell

• Pull together fatty acid tails limiting their movement and that of other molecules without making it too rigid

• Functions

  • Reduce lateral movement of phospholipid fatty acid tails

  • Make membrane less fluid at higher temps

  • Prevent leakage of water and dissolved ions from the cell

role of glycolipids

• Made of carbohydrate covalently bonded with lipid

• Carbohydrate part extends from phospholipid bilayer into watery environment outside and acts as receptor for some chemicals

• Functions

  • Recognition sites

  • Cell signalling

  • Maintain stability of membrane

  • Help cells to attach to one another - form tissues

role of glycoproteins

• Carbohydrate chains attached to extrinsic proteins on outer surface of cell membrane

• Act as cell-surface receptors for hormones and neurotransmitters

• Functions

  • Recognition sites

  • Receptor sites

  • Antigens

  • Cell signalling

  • Help cells attach to one another to form tissues

  • Allows cells to recognise one another (lymphocytes can recognise body cells vs foreign cells/virus)

permeability of cell-surface membrane?

• Controls movement of substances into and out of cell

• Small, nonpolar and lipid-soluble molecules can pass, some small uncharged polar molecules can pass

• Ions and large uncharged polar molecules cannot

• Many molecules do not freely diffuse across it because:

  • Not soluble in lipids and cannot pass through phospholipid bilayer

  • Too large to pass through channel proteins

  • Same charge as charge on protein channels so repelled

  • Electrically charged so have difficulty passing through non-polar hydrophobic tails

explain the fluid-mosaic model

• Fluid - individual phospholipid molecules can move and this gives membrane a flexible structure - constant change in shape

• Mosaic - proteins are embedded in phospholipid bilayer and vary in shape, size and pattern

function of plasma membranes around organelles?

• Control entry and exit of materials in discrete organelles such as mitochondria and chloroplasts

• Separate organelles from cytoplasm - specific metabolic reactions can take place within them

• Provide internal transport system

• Isolate enzymes that may damage the cell

• Provide surfaces for reactions to occur on

permeability can be affected by..

• Temperature - denature proteins in cell membrane, phospholipids have high KE and move apart

• pH - denatures proteins in cell membranes by changing tertiary structure

• Solvent - may dissolve the membrane (alcohol does this)

diffusion definition

net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed (reaches a dynamic equilibrium)

passive process - no metabolic energy required

how does diffusion work?

• All particles are constantly in motion due to kinetic energy that they possess

• Motion is random - no set pattern

• Particles are constantly bouncing off/colliding with one another as well as other objects

small and non-polar molecules can be transported like this

facilitated diffusion?

charged ions and polar molecules cannot travel via diffusion

use passive process of facilitated diffusion - down conc gradient

can only occur where channel/carrier proteins are present

how do channel proteins work?

• Form water-filled hydrophilic channels across the membrane

• Allows water-soluble ions to pass through but are selective and only allow one specific ion through

• Channel is closed if that ion is not present - control over entry and exit of ions

• Ions bind with the protein causing it to change shape so one side is open at a time

how do carrier proteins work?

• Molecule is specific to the carrier protein and binds with the protein at an attachment site

• This causes a change in shape so the molecule is released to the other side

• Change in shape includes flipping or rotating of carrier protein

what factors affect diffusion rate?

temp - higher = more kinetic energy = more movement and collisions = more net movement

conc grad - high on one side = more random movement and collisions

surface area - larger area for molecules to cross = microvilli or folded membranes

distance - shorter distance = faster rate as less travel

size of molecules - smaller molecules are lighter and move quicker

number of channel/carrier proteins - more = more availability = faster rate

stirring/movement - molecules move more

osmosis definition

passage of water molecules from a region of high water potential to a region of low water potential through a partially permeable membrane to reach dynamic equilibrium

passive process

how does water potential work?

pure water = 0kPa

any solution (added solute) will have a lower wp = more negative is more concentrated solution

water potential = free molecules that are not attracted to solute

• Water molecules move from area of higher water potential (nearer to 0) to lower water potential (further from 0)

• To test water potential of cells put them in solutions of different water potentials and when you find one that has no net gain or loss of water - that means both internal and external environment have same water potential

isotonic, hypotonic, hypertonic?

• Isotonic - same concentration of solutes - incipient plasmolysis

• Hypertonic - higher concentration of solutes in solution (lower water potential) than in cytoplasm/vacuole - water moves into solution from cell, shrivelling up of animal cell (plant cell its called plasmolysis, bacteria cell this will cause ceasing of metabolic reactions)

• Hypotonic - lower concentration of solutes in solution (higher water potential) than in cytoplasm/vacuole - water moves into cell from solution, animal cell swells and can burst due to osmotic lysis (plant cell/cells that have walls becomes turgid)

example of osmosis and RBCs

• Red blood cells have solutes in their cytoplasm and so if water entered, the cell membranes would break, as they do not stretch, and the cell would burst and release its contents

• To prevent this, blood plasma must have the same water potential as red blood cells to prevent haemolysis

• On the other hand, if water potential of blood plasma is lower then red blood cell, water leaves the cell and it shrinks and shrivels

active transport definition

• Movement of molecules or ions into or out of cells from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

• active process - requires metabolic energy

• extremely selective process - specific carrier proteins carry only certain molecules

• Sometimes more than one molecule/ion can be moved in the same direction at the same time with active transport

• Sometimes one molecule/ion enters a cell/organelle whilst another is exiting

  • Sodium-potassium pump, sodium ions are actively removed from the cell/organelle whilst potassium ions are taken in

process of active transport

• Molecule/ion binds to receptor sites on carrier proteins

• On the inside of the cell/organelle, ATP binds to the carrier protein causing a split to form ADP and a phosphate molecule which releases energy - this causes the protein molecule to change shape and open towards the inside

• The molecule or ion is released into the other side of the membrane

• Phosphate molecule is released from carrier protein so it reverts back to its original shape and the phosphate molecule rebinds with ADP to form ATP in respiration process

how is the ileum adapted to support transport of membranes

• Epithelial cells lining the ileum possess microvilli which are finger-like projections of the cell-surface membrane - provide more surface area for carrier proteins through which facilitated diffusion, active transport and normal diffusion can occur

• Increased number of protein channels and carrier proteins in any given area of membrane also help to increase rate of movement

role of diffusion in absorption in ileum

• Net movement of molecules or ions from a region where they are highly concentrated to a region where their concentration is lower

• Greater concentration of glucose and amino acids within the ileum than the blood

• Concentration gradient down which glucose moves via facilitated diffusion from ileum lumen into the blood

• Blood is constantly circulated by the heart and so glucose which is absorbed will be removed by the cells via respiration

• Maintains concentration gradient so rate of movement across epithelial cell-surface membranes increases

role of active transport in absorption in ileum

• Diffusion only results in concentrations on either side becoming equal so not all glucose and amino acids can be absorbed this way which is a waste

• Glucose and amino acids will be actively transported so all of it can be absorbed into the blood

• This is done by co-transport in the intestine - glucose or amino acids is drawn into cells along with sodium ions that have been actively transported out via sodium-potassium pump

co-transport process

1. Sodium ions are actively transported out of epithelial cells via sodium-potassium pump and into blood (takes place in one type of protein-carrier molecule found in cell-surface membrane of epithelial cells)

2. Maintains higher concentration of sodium ions in the lumen of intestine than in the epithelial cells

3. Sodium ions diffuse into epithelial cells down the concentration gradient through a co-transport protein in cell surface membrane - as they do this they carry amino acid molecules/glucose molecules with them

4. Glucose/amino acids pass into blood plasma by facilitated diffusion by using another carrier protein

• Both sodium ions and glucose/amino acids move into cell but sodium ions move down concentration gradient whilst the glucose molecules move against theirs

  • Sodium ion concentration gradient rather than ATP directly that powers the movement so it is an indirect form of active transport