B4

Net movement of substances in and out of cell membranes, from high to low concentration.

Rate of diffusion is increased when:

greater surface area of cell membrane,

greater difference between concentrations (steeper concentration gradient),

particles have a shorter distance to travel.

CO2 and O2 move in and out of plants via their leaves.

DURING THE DAY:

CO2 is used in photosynthesis. The concentration inside the leaves is therefore lower than the concentration outside the leaf.

CO2 diffuses into the plant through the stomata (on the bottom of the leaves).

O2 (a product of photosynthesis) diffuses from the plant to the atmosphere.

AT NIGHT photosynthesis stops. O2 diffuses into the leaf cells and CO2 diffuses out.

The stomata on the underside of the leaf adapt to:

open - to increase the rate of diffusion

close - to prevent excess water loss

Osmosis - the diffusion of water from a high concentration to a low concentration through a partially-permeable membrane. - a net movement.

Water diffuses in and out of animal cells - animal cells don't have a cell wall - too much water entering the cell could cause it to burst.

EXAMPLE:

When red blood cells are in solutions with the same concentration as their cytoplasm, they retain their shape.

In a weaker solution, they absorb water, swell, and may burst due to their lack of cell wall - lysis

When in a more concentrated solution, they lose water, (without a cell wall to prevent water loss)  shrivel up becoming crenated.

Plant cells have inelastic walls. - preventing cells from bursting - contributing to rigidity

Pressure of water pushing against the cell walls is called turgor pressure.

Lack of water causes plants to wilt - reduced turgor pressure -> less rigid.

Water moves into plant cells by osmosis -> pressure inside cell increases -> inelastic wall withstands pressure -> cell becomes turgid -> once all cells are fully turgid plant is firm and upright.

Water in short supply -> cells lose water by osmosis -> cells lose turgor pressure -> cells become flaccid -> plant wilts.

When cells lose a lot of water, the inside of the cell contracts. This is called plasmolysis.

Xylem and phloem form a continuous system of tubes from roots to leaves, called vascular bundles.

Xylem transports water and soluble mineral salts from roots to leaves (transpiration). Dead plant cells - hollow lumen - cellulose cell walls are thickened with waterproof substance.

Phloem moves food substances (sugars) around the plant (via transolcation), up and down stems growing and storing tissues. - Long columns of living cells.

Root hairs - huge surface area for absorbing water, increasing plant's ability to take up water.

The diffusion and evaporation of water from inside a leaf. Causing water to be moved up xylem vessels providing plants with water (cooling, photosynthesis, support) and brings minerals to the plants.

The transpiration stream is powered by the evaporation of water from the leaf:

1. Water evaporates from the internal leaf cells through the stomata,

2. Water passes by osmosis from the xylem vessels to leaf cells, which pull the thread of water in that vessel upwards by a very small amount.

3. Water enters the xylem from root tissues, to replace water which has moved up.

4. Water enters root hair cells by osmosis to replace water  which has entered the xylem.

Rate of transpiration can be affected by:

light /heat - more light /heat increases rate of photosynthesis and transpiration.

Low humidity /more air movement (wind)  increases the rate of transpiration

Plants need to balance the amount of water they take and lose:

1. Water is absorbed by the plant by the root hair cells - large surface area to take in water

2. The water diffuses through the plant to leaves

3. When it reaches the leaves it can be lost by transpiration (evaporation).

Leaf adaptations:

A waxy cuticle on the surface of the leaf

The majority of the stomata are on the underside of the leaf.

Transpiration and water loss are unavoidable consequences of photosynthesis. During photosynthesis, stomata allow water molecules to pass out of the leaf. The leaf is adapted to reduce water loss:

The number, position, size, and distribution of stomata vary between plants depending upon their environment. The turgidity of guard cells changes in relation to light intensity and avaliability of water, to alter the size of stomatal openings.

During photosynthesis guard cells are turgid and the stomata are fully open. If there is a lack of water, the guard cells become flaccid and the stomata close to prevent water loss and photosynthesis. Transpiration is affected by:

High light intensity - stomata open - increasing rate of water evaporation

High temperatures - increased water molecule movement - speeds up transpiration

Wind - blows water molecules away from stomata - increases transpiration

High humidity - decreases concentration gradient - slows transpiration

Plants absorb dissolved minerals in the soil through their roots.

Minerals are naturally present in the soil, in low concentrations - so fetilisers are used. Each mineral is needed for a different purpose:

Nitrates (N) make amino acids that form proteins for cell growth - poor growth - yellow leaves

Potassium compounds (K) helps enzymes in respiration and photosynthesis - poor flower/fruit growth/discoloured leaves

Phosphates (P) make DNA and cell membranes/respiration/cell growth - poor root growth/ discoloured leaves

Magnesium -makes chlorophyll for photosynthesis - yellow leaves

If one or more are missing, the growth of the plant will be affected.

Key factors in the process of decay are: microbes, temperature, oxygen and moisture.

The rate of decay is affected by several factors: Temperature - microorganisms work slow at low temperatures, denatured at 40⁰c - decay stops

Amount of oxygen - increased -> increases microorganisms' rate of respiration -> they produce more energy -> enables to grow and reproduce faster

Amount of water - microorganisms grow quickest in moist condions, too much/little will slow decay.

Detrivores quicken decay - breaks down detritus to small particles with large surface area - easier for decomposers (bacteria/fungi) to feed. - earthworms/ woodlice/ maggots - feed on: dead organisms, decaying material produced by organisms.

Microorganisms are used to break down: human waste in sewage treatment works/ plant waste -  Materials which decay can be recycled - release minerals back to soil.

Fungi are saprophytes - feed on dead organic material by secreting enzymes - then absorbing the digested products. Saprophytes are essential for decay.

Remove oxygen/ warmth/ moisture - what microorganisms need to grow + survive. Food can be:

sealed inside sterile cans - prevents entry of decomposers

kept at low temperatures in a fridge/freezer - slows reproduction of microorganisms growth

pickled in vinegar - acid kills decomposers

preserved in sugar/salt - removes water from decomposers by osmosis - killing them

dried - reduces water

Aim to produce maximum yield. Methods include: Herbicides: used to kill weeds which compete with crops for nutrients

Pesticides: used to kill pests - organisms that may damage crop/farm animal - Insecticides (type of pesticide) kills insect pests - Fungicides (pesticide) kills fungi. - can harm other organisms, can accumulate in food chains, some stay in food chains for years.

Some other methods are:

battery farming

glasshhouses

hydroponics

fish farming

Keeping animals warm and penned up inside (battery farming) to prevent loss of energy. - Cruel, suffer health problems - ethical delimmas - some find it morally unacceptable.

Aim to produce food without chemicals - minimising impact on environment

Methods:

natural fertilisers (compost/manure) -growing nitrogen fixing crops (peas/clover) - rotating crops to maintain soil fertility - weeding - varying seed planting times to discourage pests.

Advantages:

no contamination - soil erosion limited and fertility is maintained - biodiversity is promoted - livestock has space to roam.

Disadvantages:

less efficient - organic fertilisers take time to rot and don't supply a specific balance of minerals - expensive - more space required.