4: Transport across cell membranes Flashcards
What is plasma membrane?
All membranes around and within all cells
All have the same basic structure
What does the cell-surface membrane do?
Allows different conditions in and out of the cell
Controls the movement of substances
Provides structural integrity
How are phospholipids essential for cell-surface membranes?
Hydrophillic heads point towards outside of the cell as attracted by water on both sides
Hydrophobic tails point to the center of the cell membrane, repelled by water
What does amphiphatic mean?
Contains both hydrophobic and hydrophillic components
Why is a membrane said to be a fluid mosaic?
Fluid: flows, moves, changes shape
Mosaic: different size, patterns, shapes
Why does the cell-surface membrane have pores?
Selectively permeable
What types of proteins exist in the cell-surface membrane?
Intrinsic/integral/transmembrane
Extrinsic/peripheral
What is an intrinsic protein?
Span the full bilayer from one side to the other
They interact with the lipophillic core and it embeds them there
Protein channels / Carrier proteins
What is a protein channel?
Water filled tubes to allow water-soluble ions to diffuse across the membrane
What is a carrier protein?
Binds to ions or molecules (glucose/amino acids)
Causes a change in conformational shape
Moves the molecule across the membrane
What are the function of proteins in the bilayer?
Structural support
Acts as channels transferring water-soluble substances
Allows active transport using carrier proteins
Cell-surface receptors for identifying cells
Helps cells adhere (join) together
Acts as receptors (hormones)
What is an extrinsic protein?
Surface of bilayer, never extends beyond
What are the functions of extrinsic proteins?
Mechanical support
Cell receptors with glycolipids
Cell receptors for hormones
Where is cholestrol found?
Within the bilayer
What does cholestrol do in the bilayer?
Provides strength to membrane
Reduce lateral movement of molecules
Make the membrane less fluid at high temp
Prevent leakage of water and dissolved ions
What is a glycolipid?
Carbohydrate covalently bonded to a lipid
Carb. extends outside the cell
What is the function of glycolipids?
Recognition sites (ABO Blood) Maintain stability of membrane Helps cells to attach to each other
What is a glycoprotein?
Carbohydrate chains bonded to extrinsic protein
What are the functions of glycoproteins?
Recognition sites
Help cells attach to each other
Recognition (e.g lymphocytes recognise own cells)
When is transport across the bilayer not possible?
Molecules not lipophillic
Too large to pass through channels
Same charge as proteins
Polar (electrically charged) therefore cannot move through the non-polar hydrophobic tails
What are the functions of intracellular membranes?
Divides cytoplasm from organelle contents Entry and exit Internal transport mechanisms Enzyme isolation Interface for reactions
How can a rate of entry graph differ with facilitated diffusion and diffusion?
Facilitated diffusion graph plateaus as the channel protein becomes saturated
What are similarities between how H2O and O2 diffuse?
Move down conc. gradient
Diffuses through phospholipid bilayer
Why does the rate of uptake increase in the presence of oxygen?
Active transport - using O2 creates ATP energy to cause the carrier proteins to change shape
Why does uptake of a substance increases as conc. does?
Diffusion is proportional to conc. gradient
What are the things that hold chromatids together?
Centromere
How many chromosomes found in nuclei in telophase and a sperm cell if there are 26 in prophase?
Prophase: 26
Telophase: 26
Sperm cell: 13
How much mass of DNA found in nuclei in telophase and a sperm cell if there is 60 in prophase?
Mass of DNA in Prophase: 60
Telophase: 30
Sperm cell: 15
How does faulty DNA cause cancer?
Protein is faulty / not made
Cells with faulty DNA divide
Uncontrolled division
A cancer drug that stops spindle fibers shortening would stop what stage?
Anaphase
Explain diffusion
Net movement of molecules or ions from a region of higher to lower concentration until even distribution
How does kinetic energy affect particles?
All particles are constantly in motion due to kinetic energy
Motion is random
Particles constantly bounce off each other as well as other objects
What is required for facilitated diffusion?
Trans-membrane channels and carriers
Is facilitated diffusion require energy?
No, it is passive
How are protein channels selective?
Each opens in the presence of a specific ion
Ion binds with protein causing it to change shape and opens one side and close the other
What causes the particles to move in facilitated diffusion?
Kinetic energy of the molecules
Define osmosis
Passage of water from region of high to low water potential through a selectively permeable membrane
What is water potential (Ψ)?
Pressure created by water molecules (kPa)
What is water potential of pure water under standard conditions?
0
What are “standard conditions” for Ψ?
25C and 100 kPa of pressure
How does adding solute affect Ψ?
Lowers water potential
Therefore any solutions Ψ must be negative
How is water potential affected as a solution becomes more concentrated?
As concentration increases the Ψ becomes more negative
How could you find the Ψ of cells?
Place them in a series of solutions with different Ψ
Where there’s no net change of water, the Ψ must be equal to the solution
Does osmosis require energy?
Only requires kinetic energy of the water molecules
Why does a partially permeable membrane not allow solute molecules?
They are too big to fit through the pores
What is an isotonic solution?
A solution where the Ψ is the same as the cell
What is a hypertonic solution?
A solution where the Ψ is lower than that in the cell
Hyper - high SOLUTE
What is a hypotonic solution?
A solution where the Ψ is higher than that in a cell
Hypo - low SOLUTE
What are the principles of exchange to increase exchange?
High SA:V ratio
Thin layer
Selectively permeable
Movement of the external & internal environment
How does a high SA:V ratio affect exchange?
Increases rate of exchange - as increased frequency of collision with exchange surface
How does a thin layer affect exchange?
Short diffusion pathway length
Quicker transport through membranes
What is villi?
1mm in length which is folded to increase SA
What is microvilli?
600nm in length
Found on enterocyte border
What is an enterocyte?
Cell of the intestinal lining
What is the purpose of the circularity systems?
Maintains concentration gradients
What occurs when cells are placed in a hypertonic solution?
Plant cells tissue flaccid & plasmolysis
Animal cells shrink (RBC crenation)
What occurs when cells are placed in a hypotonic solution?
Animal cells burst (cytolysis) & RBC undergo haemolysis
Plant cells become turgid
Define active transport
Movement of molecules/ions from low to high conc. using ATP and carrier proteins
How does active transport affect net movement?
No equilibrium reached
How selective is active transport?
Highly selective
Explain the process of active transport
Molecule/ion binds to carrier protein
ATP attaches to protein and phosphorylation occurs and causes conformational change
Opens to opposite side of the membrane and molecule/ion released
What is the phosphorylation?
ATP ADP + Phosphorus
Explain the process of a sodium-potassium pump?
3 Na+ binds to binding site
Phosphorylation occurs (ATP -> ADP + P)
Conformational change in shape and Na+ released
2 K+ ions bind to protein and dephosphorylation causes change (P dissociates)
2 K+ ions released and process repeats
What is direct/primary active transport?
Ions/molecules pumped against conc. gradient
What is indirect/secondary active transport?
Conc. gradient generated by ion pump, then co-transport of ion with chemical
What is co-transport?
A molecule is transported against conc. gradient with another substance down the conc. gradient
Describe the co-transport in the enterocyte?
Direct active transport causes Na+ to go to blood and K+ ions to diffuse into cell (Na-K pump)
Indirect active transport (co-transport) glucose binds to Na+ which moves down conc. gradient
Facilitated diffusion of glucose from enterocyte to bloodstream
What is the effect of an isotonic solution?
No affect on animal cells
Incipient plasmolysis in plant cells
What is incipient plasmolysis?
When the protoplast is just about to pull away from cell wall
