Transport in Cells - AQA GCSE Biology

Transport in Cells

Definition: Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
It occurs in solutions and gases due to the random movement of particles.
It is a passive process, meaning it doesn't require energy input from cells; it relies on the kinetic energy of particles.
Higher temperature increases kinetic energy, thus speeding up diffusion.

Cell membranes are partially permeable, allowing some substances to diffuse through while blocking others.
Examples of substances entering cells by diffusion:
- Oxygen for aerobic respiration.
- Carbon dioxide for photosynthesis in plant cells.
Examples of substances leaving cells by diffusion:
- Urea, a waste product from liver cells, for excretion.
- Carbon dioxide produced by aerobic respiration.
Movement is driven by the concentration gradient between the cell and its environment; changing this gradient alters the direction of movement.

Surface Area : Volume Ratio
- The surface area to volume ratio affects how easily substances can be exchanged between an organism and its environment.
- Most bacteria are single-celled organisms with a relatively large surface area compared to their volume.
- Substances do not have to travel very far to get where they are needed, so transport by diffusion, osmosis or active transport alone is sufficient for the cell to meet its needs

Larger organisms with smaller surface area: volume ratios need exchange surfaces and transport systems to ensure their cells obtain the materials they need

Multicellular organisms have a relatively small surface area compared to their volumes compared to single-celled organisms.
Larger organisms have exchange surfaces and transport systems because diffusion, osmosis, and active transport alone are insufficient to meet their needs.
Adaptations for Exchange in Animals:

Most absorption of digested food molecules into the bloodstream occurs across the wall of the small intestine.
It has a highly folded surface which is lined with specialised intestinal epithelial cells (which themselves have a highly folded cell membrane) to increases surface area.
Only one layer of epithelial cells covers the surface of each villus to decreases diffusion distance.
Each villus has a good blood supply to maintains a concentration gradient.

Gas exchange between air in the alveoli and the blood, to supply cells with oxygen for aerobic respiration and to remove carbon dioxide
Millions of alveoli which collectively provide a huge surface area to increases surface area
The wall of each alveolus is one cell thick, with a moist lining and excellent blood supply maintains a concentration gradient.

Gas exchange between water owing through the gills and the blood, to supply cells with oxygen for aerobic respiration and to remove the waste product carbon dioxide
Each gill is made from lots of smaller plates called laments, which themselves are covered in projections called lamellae to increases surface area
Dense capillary network ensures a good blood supply which ows in the opposite direction to water passing through the gills to maintains a concentration gradient
Adaptations for Exchange in Plants:

To absorb water and mineral ions (such as magnesium and nitrate ions) from the soil and anchor the plant
The root network is highly branched to increases surface area.
The surface of the roots are covered in root hair cells, which have a specialised structure with root hair projections to increases surface area.

The leaves contain most of a plant's photosynthetic cells.
Photosynthesis requires e cient gas exchange between air surrounding the leaf and the photosynthetic cells, whilst minimising water loss
Stomata are tiny openings that predominantly cover the lower side of the leaf, allowing air to circulate inside the leaf to decreases di usion distance for carbon dioxide and oxygen
The lower layer of the leaf is made from spongy mesophyll cells which allow air to circulate inside the leaf.

Multicellular organisms maximize exchange through:
- Large surface area.
- Thin barrier for short diffusion path.
- Extensive blood vessel network to reduce exchange distance and maintain concentration gradients.
- Well-ventilated gas exchange surfaces.

Definition: Osmosis is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.
Water moves from a more dilute solution to a more concentrated solution to equalize water concentration on both sides of the membrane.

Plant tissue gaining mass indicates water moving in from a more dilute surrounding solution.
Plant tissue losing mass indicates water moving out into a more concentrated surrounding solution.
No overall mass change indicates equal concentrations with no net water movement.

Aim: To investigate the range of concentrations of salt or sugar solutions on the mass of plant tissue
Prepare samples of potatoes and place in different concentrations of sugar or sodium chloride (salt) solution
Make measurements of mass and length of your samples before and after soaking them in solutions
Calculate the percentage change in mass of plant tissue
Plot, draw and interpret appropriate graphs measurements accurately
Independent variable: Concentration of salt or sucrose solution in mol \ dm^{-3}
Dependent variable: Mass and the length of each potato cylinder before and after it has been submerged in solution should be measured
Percentage change in mass and length will be calculated using these measurements
Important control variables are: type and volume of solute in solution temperature, time
Percentage change in mass = \frac{(final \ mass - initial \ mass)}{initial \ mass} \times 100

Where results lie above 0 on the X-axis, this means the potato has increased in mass and taken water in from the surrounding solution, via osmosis
Where results lie below 0 on the X-axis, this means the potato has decreased in mass and water has been lost from the potato, via osmosis
Where the line crosses the X-axis, this is the concentration of sugar/salt inside the potato and no water has been lost or gained at this point

Active transport moves substances against a concentration gradient, requiring energy from respiration.
Protein transport molecules in the cell membrane use energy to move substances.
It transports substances from a more dilute solution to a more concentrated solution.

Plants: Root hair cells use active transport to absorb minerals like magnesium ions from low concentrations in the soil to higher concentrations in the cell.
Animals: Glucose absorption in the small intestine, even when blood concentration is higher.

Sports drinks help replace water, salts, and glucose lost during exercise.
Maintaining the correct balance of ions and water is crucial to prevent cellular imbalances.
- Isotonic: Similar salt and sugar concentrations to the body; used for hydration.
- Hypertonic: Higher salt and sugar concentrations; provide glucose during intense exercise.
- Hypotonic: Lower salt and sugar concentrations; facilitate rapid rehydration through osmosis.