Cell membranes

What are the functions of membranes?

surround cells to form a barrier between them and the environment

control entry/exit of substances

partially permeable

divide cell into compartments

What are membranes composed of?

lipids (mainly phospholipids), proteins and carbohydrates

What is the fluid filled mosaic model?

describes the arrangement of molecules in the membrane

in the model, phospholipids form a bilayer that is filled with fluid

What types of proteins are in membranes?

integral and peripheral

What are the types of integral proteins and what do they do?

channel proteins and carrier proteins that allow large molecules/ions to pass through the membrane.

What do peripheral proteins do?

on the cell-surface membrane let the cell detect chemicals released by other cells which signal the cell to respond.

What do phospholipids do in membranes?

form a barrier (bilayer) to dissolve substances

What is the difference between phospholipd head and tail?

head is hydrophilic and tail is hydrophobic.

How do the phospholipids ensure that water-soluble substances dont diffuse through the membrane?

the head faces outwards and the center is hydrophobic

What substances can diffuse through the membrane?

small, non-polar

What does cholesterol do in the membrane?

gives it stability by binding to the hydrophobic tails - makes the membrane less fluid and more rigid

it has hydrophobic regions to create another barrier against polar substances

Why is beetroot used in the practical?

has a clear pigment

What are the steps for testing the permeability of cell membranes?

1.    Use cork borer to cut 5 pieces of beetroot and make them equal sized with a scalpel.

2.    Rins beetroot to allow any excess pigment to leave.

3.    Put each piece in different test tube, that have water placed in with pipette.

4.    Put each test tube in beaker containing different temps of water.

5.    Start a stopwatch for 15 minutes.

6.    Remove the beetroot, leaving the coloured water.

7.    Use colorimeter. To use this it needs to be first calibrated with distilled water in a cuvette. Only touch the side of the cuvette with lines.

8.    Use a pipette to move a sample of the coloured water into the cuvette and read the colorimeter.

9.    Do this for all temps of water.

How do you find the water potential of the potato tissue (potato experiment)?

Plot a graph with concentration on x-axis and percentage change in mass on y-axis.

Find concentration were percentage change is zero

Use another source to find potential sucrose concentration

What is diffusion?

the net movement of particles from an area of higher concentration to an area of lower concentration across a partially permeable membrane

What type of process is diffusion?

passive

What is simple diffusion?

when molecules diffuse directly through a membrane.

How does the concentration gradient affect the rate of diffusion?

the higher it is, the faster the rate as during diffusion

How does the distance affect diffusion?

the less distance the particles must go, the faster the rate

How does surface area affect diffusion?

larger the surface area, faster rate as the particles have more space to collide

How does the size of the molecules affect the rate of diffusion?

smaller molecules such as oxygen will diffuse quicker than large molecules like proteins

How does temperature affect the rate of diffusion?

the higher the temp, the higher the kinetic energy of the molecules so more collisions

What is Fick’s law of diffusion?

The rate of diffusion is proportional to: (surface area x difference in conc)/ length of diffusion path

What is facilitated diffusion?

large and charged molecules diffusing into the bilayer with carrier/channel proteins

What type of process is facilitated diffusion?

passive

How do carrier proteins help facilitate diffusion?

1.  large molecule attaches in the membrane.

2.  changes shape

3.  release molecule on other side

How do channel proteins help facilitate diffusion?

Forms pores in the membrane for charged particles to diffuse through, can be gated

How does the concentration gradient affect the rate of facilitated diffusion?

higher conc gradient, faster rate. As equilibrium is reached, the rate will level off

How does the number of channel/carrier proteins affect the rate of facilitated diffusion?

once all the proteins are in a membrane are in use, facilitated diffusion cannot happen any faster, even if you increase the conc gradient

What is osmosis?

Osmosis is the net movement of water molecules from a higher water concentration to a lower water concentration across a partially permeable membrane.

Water molecules always move from a region of higher water potential to a region of lower (negative) water potential.

What is water potential?

Water potential is the tendency of water molecules in a system to move, measured in kPa

What is the water potential of pure water?

0kPa

What is the water potential of any solution other than pure water?

negative value

What is osmotic potential/pressure?

the tendency of water to move onto a solution, used when talking about animal cells.

What does isotonic mean?

a solution that has the same solute concentration inside and outside the cell, there is no movement of molecules

What does hypertonic mean?

more water in cell, water moves out

What does hypotonic mean?

more water outside cell, water moves in

What is the term used when the water potential in animal cells is higher than the surroundings?

lysed

What is the term used when water potential in animal cells are equal to the surroundings?

normal

What is the term used when water potential is lower in animal cells than their surroundings?

shrivelled

What is the term used when the water potential in a plant cell is higher than its surroundings?

turgid (normal)

What is the term used when the water potential in the plant cell is equal to that of the surroundings?

flaccid

What is the term used when the water potential in the plant cell is lower than that of the surroundings?

plasmolyzed

What are the factors affecting the rate of osmosis?

Water potential gradient

The thickness of the exchange surfaces- the thinner the exchange surface, faster osmosis.

Surface area

What types of process is active transport?

active as it uses energy

Why does active transport need energy?

the molecules move against the conc gradient

Why do epithelial cells have a large surface area?

they contain microvilli which are finger-like projections of the cell-surface membrane

What are some examples of active transport?

o   Uptake of glucose and amino acids into the small intestine.

o   Exchange of sodium and potassium ions in neurons and muscle cells.

How are carrier proteins used in active transport?

1.    A molecule attaches to carrier proteins.

2.    ATP molecule provides energy to go against the concentration gradient.

3.    ATP -> ADP + Pi

4.    Carrier changes shape and molecule is released on the other side (the side with higher concentration).

5.    The phosphate molecule is released from the protein which causes the protein to go back to it’s original shape and the phosphate joins with the ADP to form ATP again during respiration.

What are co-transporters?

They are a type of carrier protein. They bind to two molecules at a time and the conc gradient of one is used to move the other against its conc gradient

What is the sodium-potassium pump?

sodium ions are actively removed from the cell/organelle, whilst potassium ions are being taken in and are essential for different processes e.g., nerve impulses.

How does glucose get absorbed into the bloodstream?

1.    Sodium ions actively transported out epithelial cells, into the blood, by the pump. This makes the conc gradient, so there is a higher conc of sodium ions in the lumen of the ileum than inside the cell.

2.    This causes sodium ions to diffuse from the ileum (lumen) into the epithelial via proteins. This carries the glucose into the cell with sodium so the conc of glucose inside the cell increases.

3.    Glucose diffuses out the cell, into the blood, down its conc gradient through a channel protein

What is bulk transport?

Used when extremely large substances need to be moved across a cell membrane.

What are the two types of bulk transport?

endocytosis and exocytosis

What is endocytosis?

bulk transport of material into the cell, can be split into three processes: phagocytosis, pinocytosis and receptor-mediated endocytosis.

What is exocytosis?

bulk transport of material out the cell.

What factors affect the rate of active transport?

1.    Speed of each carrier protein

2.    Number of carrier proteins

3.    Rate of respiration/availability of ATP.