Cell branes and ort

describe the structure of the cell membrane

• composed of a phospholipid bilayer

• partially permeable membrane

• fluid-mosaic model due to the mixture and movement of the phospholipids, proteins, glycoproteins, and glycolipids

why do phospholipids align as a bilayer

• hydrophilic heads being attracted to and interacting with water

• hydrophobic tails being repelled by water

function of extrinisic proteins

• provide mechanical support

• connected to proteins or lipids to make glycolipids or glycoproteins → function of which is cell recognition

function of intrinsic proteins

• protein carriers or channel proteins

  • involved in transport of molecules acroos membrane

difference between protein channels and carrier proteins

• Protein channels

  • form tubes which fill with water to allow water-soluble ions to diffuse

• carrier proteins

  • bind with other ions and larger molecules (e.g. glucose and amino acids) and change shape to transport them to the other side of the membrane

function of cholesterol

• present in some membranes

• resticts lateral movement of other molecules in the membrane

• useful as it makes membrane less fluid at high temperatures and perevents water and dissolved ions from leaking out of the cell

what is the purpose of cell signalling

• signalling pathways coordinate cellular activities

  • pathways can be electrical (nervous system)

  • pathways can be chemical (hormone system)

  • involve various molecules such as hormones and neurotransmitters

main stages in chenical signalling pathways

1. secretion of specific chemicals (ligands)

   • cells secrete specific chemical messengers known as ligands in response to stimuli - ligands are released into the extracellular space

2. transport of ligands to target cells

   • ligands transported to target cells often through bloostream in the case of hormones - signalling molecules are typically small which facilitates their transport

3. binding of ligands to cell surface receptors

   • ligands bind to cell surface receptors on target cells, initiating the signalling cascade

   • cell surface receptors are protein molecules located in the cell membrane

   • ligand-receptor binding triggers conformational changes in the receptor, allowing the message to be transmitted into the cell

   • this transduction often involves G proteins and the production of second messengers, which amplify the signal

   • second messengers relay the message by activating enzymes and other molecules within the cell, ultimately bringing about required changes in cell metabolism

what do receptors do

• can alter cell acitvity

  • by opening ion channels

  • acting as membrane-bounds enzymes

  • serving as intracellular receptors

what do hydrophobic signalling receptors do

• e.g. steroid hormones

• can diffuse directly across the cell membrane and bind to receptors in the cytoplasm or nucleus

6 types of transport into and out of cells

1. simple diffusion

2. facilitated diffusion

3. osmosis

4. active transport

5. endocytosis

6. exocytosis

describe simple diffusion

• net movement of molecules from and area of high concentration to an area of low concentration until equilibrium is reached

• passive process → doesn’t require energy from ATP

• happens due to kinetic energy

• molecules must be soluble and small

• e.g. oxygen and carbon dioxide

descibe facilitated diffusion

• passive process

• ions and polar molecules - which cannot simply diffuse - are transported across the membrane by facilitated diffusion using carrier proteins and protein channels

• protein channels form tubes filled with water which enables water-soluble ions to pass through the membrane → still selective as the channel proteins only open in the presence of certain ions when the bind to the protein

• carrier proteins will bind with a molecule, such as glucose, which causes a change in shape of the protein → this enables the molecule to be released to the other side of the membrane

define osmosis

• movement of water from an area of high water potential to and area of low water potential across a partially permeable membrane

water potential

• the pressure created by water molecules

• measure in kPa

• pure water has water potential of 0 → so when solutes are dissolved in water the water potential will become negative

• the more negative the water potential, the more solute must be dissolved in it

isotonic, hypotonic, and hypertonic solutions

• isotonic: when the water potential is the same in the solution and in the cell within the solution

• hypotonic: when the water potential of a solution is more positive(closer to 0) than the cell

• hypertonic: when the water potential of a solution is more negative than the cell

what happens to animal and plant cells when placed in a hypotonic solution

• Animal

  • water moves into cell by osmosis

  • pressure causes cell to burst because of lack of cell wall

• plant

  • water moves into cell by osmosis

  • cell swells but does not bust due to cell wall

describe active transport

• movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins

• carrier proteins act as pumps to move substances across the membrane

• very selective as only crtain molecules can bind to the carrier proteins to be pumped

• certain molecules bind to the receptor site on carrier proteins

• ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi → causes protein to change shape and open towards inside of membrane → Pi molecule released from protein so protein reverts to original shape

describe endocytosis

• type of active transport

• bulk transport of molecules into a cell

• cell surface membrane bends inwards around molecules surrounding it to form a vesicle

• the vesicle inches off and moves within the cytoplasm

• phagocytosis - solid particle taken in

• pinocytosis - liquid taken in

• process requires energy from ATP for the cell to engulf and chanfe shape around the material

describe exocytosis

• bulk transport of molecules out of a cell

• vesicles move towards the cell-surface membrane, fuse with the membrane and the content of the vesicle is released outside of the cell

• process requires energy because ATP is needed to move the vesicle along the cytoskeleton

describe procedure to investigate diffusion and osmosis

1. place plant tissue or agar blocks of different sizes in solutions of varying concentrations

2. measure changes in mass, volume, or turgidity over time

3. record observations to determine the effects of diffusion and osmosis on the samples

describe procedure to investigate the effect of surface area to volume ratio on diffusion

1. place the agar blocks in a solution containing a diffusible substance e.g. sucrose solution

2. measure the rate of diffusion of the substance into the agar blocks over time

3. analyse the results to observe how changes in surface area to volume ration affect the rate of diffusion using a calibration curve