TRANSPORT ACROSS CELL MEMBRANES

Chapter 4 of AQA's Oxford second edition textbook.

?

CELL-SURFACE MEMBRANE - PHOSPHOLIPIDS

STRUCTURE.

  • hydrophilic heads point to outside of membrane, attracted by water
  • hydrophobic tails point to inside of membrane, repelled by water

FUNCTION.

  • allow lipid-soluble substances to cross the membrane
  • prevent water-soluble substances from crossing the membrane
  • make the membrane flexible + self sealing
1 of 14

CELL-SURFACE MEMBRANE - PROTEINS

EXTRINSIC PROTEINS.

  • occur on surface of bilayer 
  • give mechanical support
  • act as cell receptors (e.g. for hormones) in conjunction with glycolipids

INTRINSIC PROTEINS.

  • span the bilayer from side to side
  • PROTEIN CHANNELS: form water filled tubes to allow water-soluble ions to diffuse across
  • CARRIER PROTEINS: bind to ions/other molecules and change shape to allow them to cross the membrane

FUNCTIONS SUMMARY.

  • structural support + cell adhesion
  • channels to support facilitated diffusion, allow active transport
  • form cell-surface receptors to identify cells / act as receptors for hormones etc
2 of 14

CELL-SURFACE MEMBRANE - CHOLESTEROL

STRUCTURE.

  • hydrophobic - prevent loss of water + soluble ions
  • pull together fatty acid tails of phospholipids

FUNCTION.

  • reduce lateral movement of other molecules (e.g. phospholipids)
  • make membrane less fluid at high temperatures
  • prevent leakage of water + dissolved ions from cell
3 of 14

CELL-SURFACE MEMBRANE - GLYCOLIPIDS

STRUCTURE.

  • carbohydrate covalently bonded with a lipid
  • carbohydrate extends from bilayer into external environment around cell

FUNCTION.

  • recognition sites for specific chemicals
  • maintain membrane stability
  • help cell adhesion to form tissues
4 of 14

CELL-SURFACE MEMBRANE - GLYCOPROTEINS

STRUCTURE.

  • carbohydrate chains attached to extrinsic proteins

FUNCTION.

  • recognition sites
  • help cell adhesion to form tissues
  • allow cell-cell recognition
5 of 14

CELL-SURFACE MEMBRANE

SUBSTANCES THAT DO NOT FREELY CROSS THE MEMBRANE...

  • not lipid-soluble = can't cross the bilayer
  • too large to pass through channels
  • of same charge as channels = repulsion
  • electrically charged (polar) so cannot pass through non-polar tails in bilayer

FLUID MOSAIC MODEL.

FLUID: individual phospholipids can move relative to one another, allowing a flexible structure that is constantly changing shape

MOSAIC: proteins embedded in bilayer vary in shape and size, like the tiles of a mosaic

6 of 14

FUNCTIONS OF MEMBRANES

CELL-SURFACE MEMBRANE.

  • plasma membrane that surrounds cells and forms boundary between cytoplasm + environment
  • allows different conditions to be established in and outside cels
  • controls movement of substances in and out of cells

MEMBRANES WITHIN CELLS.

  • control entry and exit of substances in and out of organelles
  • separate organelles from cytplasm to allow metabolic processes to occur within them
  • provide internal transport system (e.g. endoplasmic reticiulum)
  • isolate enzymes that might damage the cell (e.g. lysosomes)
  • provide surfaces on which reactions can occur (e.g. protein synthesis on ribosomes of RER)
7 of 14

SIMPLE DIFFUSION

the net movement of molecuels or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

  • all particles constantly in motion due to kinetic energy they posess
  • motion is random - no set pattern to the movement
  • particles constantly bouncing off one another as well as other objects
  • therefore, particles concentrated together in a closed vesel will distribute themselves evenly throughout that vessel
8 of 14

FACILITATED DIFFUSION

  • allows movement of polar molecules across the membrane
  • occurs down the concentration gradient - relies on natural kinetic energy of particles

PROTEIN CHANNELS.

  • water filled hydrophilic channels
  • selective - allow specific water-soluble ions through
  • ion binds to channel, allowing it to open and change shape to form a channel

CARRIER PROTEINS.

  • molecule binds to protein
  • protein changes shape and allows molecule to other side of membrane
9 of 14

OSMOSIS

the passage of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.

WATER POTENTIAL.

  • pressure created by water molecules
  • 0 under standard conditions - 25 degrees and 100kPa
  • addition of solute to pure water lowers its water potential
  • the more solute, the lower (more negative) the water potential

PROCESS.

  • low concentration of solute vs high concentration of solute
  • both solute and water molecules are in motion due to kinetic energy
  • selectively permeable membrane only allows water molecules across
  • water molecules diffuse from higher water potential to lower water potential until equilibrium is reached
10 of 14

OSMOSIS + CELLS

ANIMAL CELLS.

  • cell has higher water potential = water exits cell, cell shrinks
  • cell has same water potential = no movement
  • cell has lower water potential = water enters cell, cell swells + bursts

PLANT CELLS.

  • cell has higher water potential = water exits cell, protoplast pulls away from cell wall (plasmolysis)
  • cell has same water potential = protoplast beginning to pull away from cell wall (incipient plasmolysis)
  • cell has lower water potential = water enters cell, protoplast swells and pushes against cell wall causing turgidity
11 of 14

ACTIVE TRANSPORT

the movement of ions or molecules from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

PROCESS.

  • molecule binds to receptor sites on carrier protein
  • ATP binds to protein and splits into ADP and Pi causing the protein to change shape
  • molecule is released on the other side of the membrane
  • phosphate molecule is released from the protein, causing it to revert back to its shape
  • phosphate recombines with ADP to form ATP in respiration
  • multiple molecules may be moved - e.g. sodium potassium pump (Na+ removed, K+ taken up)
12 of 14

ABSORPTION OF GLUCOSE IN THE ILEUM

RATE OF MOVEMENT.

  • epithelial cells of villi posess microvilli - projections
  • provide more surface area for insertion of carrier proteins for diffusion, facilitated diffusion + active transport to take place
  • can also increse protein density

DIFFUSION + ABSORPTION.

  • carbohydrates + proteins constnatly digested - higher concentration within ileum than blood
  • concentration gradient down which molecules move by facilitated diffusion
  • blood constantly circulated by the heart = glucose constantly removed by cells for respiration
  • therefore concentration gradient is maintained and rate of facilitated diffusion increased

ACTIVE TRANSPORT + ABSORPTION.

  • diffusion only ever results in equilibrium, therefore not all available molecules absorbed
  • active transport allows movement from low conc to high conc
13 of 14

CO-TRANSPORT

  • glucose/amino acids drawn into cells with sodium ions that have been actively transported out by sodium-potassium pump

PROCESS.

  • sodium ions actively transported out of epithelial cells into blood bye a protein-carrier molecule
  • maintains higher conc of sodium ions in the lumen of the intestine than the cells
  • sodium ions diffuse into epithelial cells down concentration grdient via co-transport protein
  • carry amino acids/glucose molecules with the sodium ions against their concentration gradient
  • concentration gradient of sodium ions provides energy required
  • glucose/amino acids pass into blood by facilitated diffusion
14 of 14

Comments

No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Cellular processes and structure resources »