Transport of molecules

Question 1

What is Active Transport

Answer 1

Active transport refers to the movement of material through a plasma membrane against a concentration gradient with the requirement of metabolic energy.

Active transport is not reliant upon concentration gradient so it can import and export materials regardless of their intracellular and extracellular concentrations.

Question 2

endocytosis

Answer 2

Endocytosis is a form of transport in which a material is engulfed by the cell membrane, then enters the cell in a vesicle. Endocytosis requires the expenditure of energy.

Question 3

What is Passive Transport

Answer 3

Passive Transport refers to the movement of ions or molecules across a plasma membrane without any energy expenditure by the cell.

Question 4

Explain Facilitated Diffusion

Answer 4

• Certain molecules cannot cross the cell membrane due to the Size, Charge, Polarity
• Glucose, some amino acids and ions are transported passively (along concentration gradient) even though they cannot pass the phosphlipid bilayer. This is done through Facilitated Diffusion
• Facilitated Diffusion uses Carrier Proteins or or Channel protiens to bypass the phosolipid bilayer
• Carrier proteins change their shape allowing them to envolope and release transported substances sheilding them form the nonpolar regions of the membrane
• Channel proteins are lined with hydrophilic amino acids and filled with water molecules, they allow water soluble molecules to pass through the phosolipid bi layer

Question 5

Give an example of facilitated diffusion using channel proteins

Answer 5

Sodium ions moving into the cell
* Sodium is usaly more abundant outside of cells than inside the cell
* Sodium ions are charged and cannot pass through through the non polar lipid bilayer
* Membrane protiens form soduim channels so that Na+ ions can move down there concentration gradient from outside the cell inside the cell

Question 6

Give an example of facilitated diffusion using carrier proteins

Answer 6

Movement of Glucose into the cell
* Although glucose can be more concentrated outside of a cell, its both too large and polar, and therefore, repelled by the phospholipid membrane
* Carrier protien called a glucose transporter changes its shape allowing glucose to bind to it and transporting the glucose molecule into the cell to facilitate inwards diffusion

Question 7

Explain why the plasma membrane can be described as having a fluid mosaic structure

Answer 7

• It describes the arrangment of the molecules in the plasma membrane
• Phospholipid molecules form a continous bi layer which is fluid because they are constantly moving
• Different protiens are scattered throughout the bi layer (e.g. carrier, channel, receptor, glycoprotiens,) like tiles in a mosaic
• Cholestural helps to maintain the shape of the membrane

Question 8

What factors can affect the rate of diffusion

Answer 8

• Concentration gradient - the higher it is the faster the rate of diffusion (diffusion slows as the difference in concentration between the two sides decreases)
• Thickness of the exchange surface - the thinner the shorter distance the particles have to travel the faster rate of diffusion
• Surface area - the larger the surface area the more molecules can diffuse through it at one time

Question 9

What substance can move through the plasma membrane freely

Answer 9

lipid soluble substances e.g. gases, steroid hormones

Question 10

What are the channels which aid the diffusion of water across the plasma membrane

Answer 10

Aquaporins

Question 11

What is Filtration

Answer 11

A form of passive transport where water and dissolved substances pass through a semi permeable membrane down a pressure gradient from an area of high pressure to an area of lower pressure. A mechanical force is usually responsible for this pressure, with the membrane acting as a filter preventing larger substances from passing.

Rate of filtration dependant upon:
- force of the fluid pressure
- Size of the molecule relative to the size of the pores in the membrane

Question 12

Give an example of filtration in the body

Answer 12

Heart contractions increase the pressure in the capillaries (Blood pressure) above the pressure of the surrounding fluid. This gradient pushes water and electrolytes out of the capillary, but the capillary walls retain the larger proteins and blood cells.

Question 13

What is Diffusion

Answer 13

Diffusion is the net movement of particles down a concentration gradient through a semi permeable membrane from an area of high concentration to one of a lower concentration.

Diffusing substances spread throughout a solution until there is an equal concentration and equilibrium has been reached.

Rate of movement is affected by:
Temperature
Molecular weight
Concentration gradient
Membrane surface area
Membrane permeability
Polarity

Question 14

Give an example of Active transport

Answer 14

The Sodium potassium exchange pump is one form of active transport.

Sodium ion concentrations are higher outside of the cell than inside the cell and potassium ions concentrations higher inside of the cell than outside of the cell.

In order to maintain homeostasis the sodium potassium ion concentration gradients must remain.

The exchange pump maintains these gradients by ejecting sodium ions and recapturing potassium ions utilising the breakdown of ATP.

3 sodium ions bind to sodium potassium carrier protein in cell membrane

ATP molecule is broken-down by Sodium Potassium ATPase releasing energy which causes carrier protein to change shape.

Sodium ions are ejected out of the cell and 2 potassium ions bind to the carrier protein and it changes shape again depositing potassium into the cell

Question 15

What is Osmosis

Answer 15

Osmosis is the diffusion of water from high concentration of water to a low concentration of water through a partially permeable membrane along the concentration gradient
Or the movement of water from a low solute concentration to high solute concentration because that is where the concentration of water is lower.

Question 16

How does osmosis affect cells

Answer 16

because water can move easily in and out of most cells the extracellular fluid must have the same overall concentration of solutes as the cytoplasm (interstitial fluid) if this balance isn’t maintained water can move in and out rapidly through osmosis causing the cell volume to change.

Hypotonic solution (more dilute than the cell, water drawn in) = cell swells

HYPERtonic solution (more concentrated than the cell water drawn out) = cell shrinks

Question 17

How does the structure of the plasma membrane of a cell facilitate its function enclosing the cell components and the control of movement into and out of the cell

Answer 17

Fluid lipid layer which is relitavley impermiable barrier, contating phospholipids with hydrophobic tails and hydrophilic heads.

Contains cholestural which gives the cell shape as it stiffens the membrane

Contains proteins which can form channels and carriers in order to transport molecules and allow free passage to selective molecules and water

peripheral proteins on the cell surface act as receptors invoved in signalling systems

External coating (glycoclalyx) of membrane provides cell to cell recognition and adhesion

Question 18

The plasma membrane controls the movement of substances in and out of the cell in several ways. Diffusion, Osmosis and filtration are three ways in which cell attempt to maintain homeostasis. explain how these proccess occur

Answer 18

Purpose of diffusion is to acheive a uniform concentraion of molecules within a solution, with particles moving along there concentration gradient from a high concentration to low concentration. (gaseus exchange in the lungs)

Rate of movement affected by temprature, molecular weight, concentration gradient, number and size of openings, surface area

Osmosis is the movement of water from high concentrations of water to low concentrations of water through a partially permeable membrane
Or the movement of water from low solute concentration to high solute concentration because that is where the concentration of water is lower.

Rate of osmosis affected by osmotic pressure

Filtration is when materials are moved through a physical membrane by physical force

Question 19

What is Secondary active transport

Answer 19

. In secondary active transport the concentration gradient of one molecule provides the energy for the transport of one molecule against the concentration gradient (i.e. they use other forms of energy and not ATP).