Transport across cell membranes - A-Level Biology notes PDF

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Chapter 4: Transport across cell membranes 4.1 Structure of cell-surface membranes Plasma membranes - Membranes around and within all cells - It forms a boundary between the cytoplasm and the environment - It controls what exit and enters the cell Phospholipids - Hydrophilic heads point to the outside of the cell surface membranes as they attract water - Hydrophobic tails point to the centre as they repel water - They allow lipid-soluble substances to exit and enter the cell - They prevent water soluble substances entering and leaving the cell - They make membranes flexible and self-sealing Proteins - There are embedded in the phospholipid bilayer in 2 ways: o Some are on the surface of the membrane which act as mechanical support or work with glycolipids o Some go completely across the membrane and become either protein channels or carrier proteins. Protein channels are water-filled tubes for water soluble ions to diffuse across. Carrier proteins bind to ions and change shape to transport the molecules across by active transport - They provide structural support Cholesterol - They form cell-surface receptors to identify cells - Add strength - Homophobic meaning it prevents water loss and loss Permeability of cell-surface membrane of dissolved ions - Controls movement of molecules in and out of cell - It pulls together fatty acid tails of the bilayer which - Most molecules can diffuse easily because: limits the movement of phospholipids without o They’re too big making it rigid o Not soluble in lipids therefore can’t pass through the bilayer Glycolipids (carbohydrate + lipid) o They have the same charge as the protein - Act as a recognition site channels therefore they will be repelled - Help maintain stability of membrane o They’re a polar molecule as the hydrophobic tails - Help cells attached to one another to form tissues are nonpolar Fluid-mosaic model of the cell-surface membrane - Fluid: the individual phospholipids can move therefore the structure is flexible - Mosaic: because proteins in the bilayer vary in shape, size and pattern

Glycoproteins (carbohydrate + protein) - Act as a recognition site - Help cells attached to one another to form tissues - Allows cell recognition ex. lymphocytes can recognise an organism’s own cell

4.2 Diffusion: the net movement of molecules or ions from high to low concentration until evenly distributed Facilitated diffusion: - Charged ions, polar molecules and large ones diffused by facilitated diffusion - Passive process - Down a concentration gradient - Two types: o Protein channels:  Water soluble molecules only  Closes and opens to control what enters and leaves

Simple diffusion: o Passive diffusion: natural energy o Active diffusion: added energy like ATP - Particles are always in random motion - Particles are always bouncing off things - Down a concentration gradient - Let small, non-polar molecules through

Chapter 4: Transport across cell membranes

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 Ions bind with the protein so it changes shape and opens to the other side  Selective Carrier proteins:  One molecule which is specific to the protein is present, it binds with the protein and changes its shape so that it opens to the other side  Used for larger molecules

4.3 Osmosis: the movement of water from a high to low region of water potential through a selectively permeable membrane - Down a concentration gradient Solutions and water potential - When an equilibrium is established there is no net - A solute is any substance that is dissolved in a solvent movement of water ex. water - The highest value of water potential zero the rest are - Solute + solvent = solution negative so the more negative the value is the lower the water potential - Water potential, measured in kPa, is the pressure created by water molecules. Pure water has 0 kPa. Animal cells - If red blood cells are placed in pure water it will - A way of finding the water potential of cells and tissues absorb water by osmosis. These cells are flexible and placed them in a series of different water potentials can stretch to an extent. The red blood cell will and if there is no gain or loss of water from the cells therefore burst. If the cells are placed in a solution and tissue then that’s its water potential. with a lower water potential the cell will shrink.

4.4 Active transport: the movement of molecules or ions into or out of the cell from low to high concentration using ATP and carrier proteins An active process: - Uses ATP Direct active transport - Substances go against the 1. Carrier proteins bind to the molecule / ion to be transported concentration gradient 2. Molecule / ion binds to receptor sites on the carrier protein - Carrier proteins act as pumps 3. On the inside of the cell ATP binds to the protein (ADP + phosphate) - Selective 4. The protein changes shape and opens to the other side of the membrane 5. Ion / molecule is released to the other side 6. Phosphate molecule is released which makes the protein return to its original shape. First favourite combines with ADP to form ATP during respiration. Sodium-potassium pump - This is used when an ion needs to enter the cell while another needs to leave at the same time - During this process, sodium ions are removed well potassium ions enter

4.5 Co-transport, absorption of glucose in the ileum To increase the rate of movement across membranes: - Epithelial cells lining the ileum and they have microvilli. These provide a large surface area for carrier proteins, resulting in an increase of transport Role of diffusion in absorption: - There is usually a higher concentration of glucose and amino acids in the ileum than the blood. Due to blood always circulating glucose is always being used up by cells for respiration. This helps maintain the concentration gradient between the ileum and the blood meaning the rate of movement by facilitated diffusion and is increased. Role of active transport in absorption:

Chapter 4: Transport across cell membranes -

Simple diffusion results in an equilibrium between the ileum and the blood so the glucose and amino acids that should be in the blood aren’t being absorbed. Through co-transport glucose and amino acids diffuse into the blood with sodium ions that have been transported by the sodium potassium pump.

Process of co-transport: 1. Sodium ions are transported out of epithelial cells by the sodium potassium pump into the blood 2. This maintains a higher concentration of sodium ions in the intestine then inside epithelial cells 3. Sodium ions diffused into the epithelial cells down the concentration gradient. The second time the sodium ions diffuse they carry either in amino acid or glucose molecule into the cell with them. 4. Amino acids and glucose pass into the blood through facilitated diffusion while sodium ions move by the sodium potassium pump -

Because sodium carries amino acid and glucose molecules against a concentration gradient it’s an indirect form of active transport...