Cell Membranes AQA A Level Biology

what are the functions of membranes?

• controls what enters and leaves the cell

• act as barriers between organelles and cytoplasm, allowing there to be different conditions within and without

• helps to maintain homeostasis by regulating internal contents of cell

what is the model of the structure of the cell membrane?

fluid mosaic model

why is the term ‘fluid-mosaic model’ appropriate to describe the cell membrane?

• fluid - phospholipids in bilayer can move laterally

• mosaic - membrane composed of many parts including proteins which are distributed through, making it a mosaic

• model - we don’t know exactly what it looks like, only a theory

what are the constituent parts of the cell membrane?

• phospholipid bilayer

• glycolipids

• glycoproteins

• channel proteins

• carrier proteins

• cholesterol

draw/describe the structure of the cell membrane

what is the function of cholesterol in the membrane?

• gives membrane stability

• binds to hydrophobic tails of phospholipids, causing them to pack more closely together, restricts movement, makes membrane less fluid

• prevents polar substances moving through membrane

what is a glycoprotein and what is it’s function?

• protein with short carbohydrate chain attached

• helps with cell recognition and acts as a receptor

• helps with immune response

• stabilises structure

what is a glycolipid and what is its function?

• lipid with short carbohydrate chain attached

• helps with cell recognition and acts as a receptor

• helps with immune response

• stabilises structure

what is the function of a protein receptor site?

allows other proteins to attach such as enzymes/antigens/hormones

what is the function of a channel protein?

• provides hydrophilic pathway for water and small polar ions to enter/leave cell

• key for facilitated diffusion

what is the function of a carrier protein?

• transport substances from one side of the membrane to the other

• can change shape to move molecules that are too large for facilitated diffusion

• specific to substances they transport

how do temperatures below freezing affect the cell membrane?

• phospholipids don’t have much energy

• packed closely together, membrane rigid

• channel and carrier proteins denature, increasing membrane permeability

• ice crystals may form and pierce membrane making it permeable when it thaws

how do temperatures between 0 and 45 C affect the membrane?

• phospholipids can move around and aren’t packed as tightly together

• membrane is partially permeable

• as the temperature increases, the phospholipids move more because they have more energy

• increases permeability of membrane

how do temperatures above 45C affect the membrane?

• phospholipid bilayer starts to melt/break down

• membrane becomes more permeable

• water inside the cell expands, putting pressure on the membrane

• channel and carrier proteins denature

• membrane permeability increases

how does surrounding cells in an increasing concentration of solvent affect membrane permeability?-

• increases membrane permeability

• solvent dissolves the lipids, causing it to lose its structure

define diffusion

movement of particles from an area of high concentration to an area of low concentration

what factors affect the rate of diffusion?

• concentration gradient - steeper gradient = faster rate of diffusion; as diffusion takes place, the difference decreases until it reaches equilibrium

• thickness of surface - shorter diffusion distance = faster rate

• surface area - larger surface area = faster diffusion rate

which molecules can diffuse directly through a cell membrane?

• lipid soluble molecules

• very small molecules such as oxygen and carbon dioxide

which molecules cannot diffuse directly through a cell membrane?

• polar molecules

• larger molecules

how do polar and large molecules diffuse through the cell membrane?

facilitated diffusion

describe the process of facilitated diffusion

same as normal diffusion, but substances diffuse through the membrane using transport proteins (channel and carrier)

describe how channel proteins function

• small pores/channels in the cell membrane

• hydrophilic

• when charged particles collide with them, and are the right kind of charged particle for the channel protein, they pass through it to the other side of the membrane

describe how carrier proteins function

• move larger molecules across the membrane

• large molecules attach to a specific carrier protein in the membrane

• protein changes shape

• releases molecule on the opposite side of the membrane

what does water pass through in the cell membrane?

aquaporins

which factors affect the rate of facilitated diffusion?

• concentration gradient

• number of channel/carrier proteins - once all the proteins in a membrane are in used, facilitated diffusion cannot happen any faster - causes rate of diffusion graphs to plateau

define active transport

movement of molecules or ions from a region of lower concentration to a region of higher concentration using energy and carrier molecules

what produces energy for active transport?

hydrolysis of ATP to form ADP and Pi

define osmosis

net movement of water molecules from a region of high water potential to one of low water potential across a partially permeable membrane

what is the water potential of water?

0

what sort of concentration does a solution with high water potential have?

dilute concentration

what sort of concentration does a solution with low water potential have?

strong concentration

what is an isotonic solution?

a solution with the same osmotic concentration as the cytoplasm of the cell

what is a hypertonic solution?

a solution with a higher osmotic concentration than the cytoplasm of the cell (low water potential)

what is a hypotonic solution?

a solution with a lower osmotic concentration than the cytoplasm of the cell (high water potential)

what happens to cells placed in an isotonic solution?

• no net movement of water

• plant: incipient (slight) plasmolysis

• animal: nothing

what happens to cells in a hypertonic solution?

• water moves out of the cell

• plant: full plasmolysis

• animal: shrivels

what happens to cells in a hypotonic solution?

• water moves into the cell

• plant: cells become turgid, structured

• animal: cells swell and burst

how would you use a graph of % change in mass of potatoes against sucrose concentration to find the water potential of the sucrose solution?

• find the point where the curve intersects the x acis

• find the concentration of sucrose solution at this point

• look this up to find the water potential of the solution

what does a more negative water potential show about the solution?

that it is more concentrated (low water potential)

what does a less negative water potential show about the solution?

that it is more dilute (high water potential)

what is the symbol for water potential?

Psi; trident shape

what is co-transport?

• when two substances are transported together using the same protein

• normally when one molecule is moving down its concentration gradient passively, and the other molecule can be brought with it

what type of proteins does active transport use?

carrier

give an example of where co transport takes place

small intestine

describe how co transport takes place in the small intestine

• glucose and amino acids need to move from the lumen of the small intestine, through the epithelial cells and out the other side into the blood stream

• sodium ions are actively transported out of the cell by the sodium-potassium pump in the membrane - a carrier protein

• lower conc of sodium ions inside cell compared to intestine lumen

• sodium ions diffuse into the epithelial cell down it’s concentration gradient through a different type of carrier protein

• the sodium ions bring the glucose with them