TRANSPORT ACROSS CELL MEMBRANES
Chapter 4 of AQA's Oxford second edition textbook.
- Created by: charlotte.jakes7
- Created on: 19-05-18 12:27
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
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
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
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
CELL-SURFACE MEMBRANE - GLYCOPROTEINS
STRUCTURE.
- carbohydrate chains attached to extrinsic proteins
FUNCTION.
- recognition sites
- help cell adhesion to form tissues
- allow cell-cell recognition
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
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)
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
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
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
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
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)
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
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
Related discussions on The Student Room
- AQA A-Level Biology Paper 3 [21st June 2023] Exam Chat »
- AQA A Level Biology essay mark? »
- Biology essay - importance of proteins in living organisms »
- a level biology help »
- Grade Growth Chronicles | From C's to A's (23-24) »
- AQA A-level biology essay topics »
- What are the stalked particles on the inner membrane of the mitochondria ? »
- Biology transport across cell membrane »
- AQA Biology essay »
- AQA A Level Biology Paper 1 7402/1 - 9 Jun 2022 [Exam Chat] »
Comments
No comments have yet been made