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GCSE Biology Revision
Transcript of GCSE Biology Revision
Needed for Photosynthesis LSA: Volume Ratio Network of Veins Thin Sturcture Diffusion of Gasess is easier and quicker 6CO2 + 6H2O --> C6H12O6 + 6O2 Stomata On the Underside of the leaf Upper Epidermis Palisade Layer Veins Lower Epidermis Stomata
(Gap) Guard Cell Spongy Mesophyll
Layer Waxy Cuticle Restricts Water loss
due to Evaporation Transparent to allow Light to get through
to the Palisade Layer Vacuole
Nucleus Cholorplasts containing Chlorophyll Cytoplasm Cell Wall Cell Wall Cell Membrane These rotate and there are more of them than in a typical plant cell Contain Xylem and Phloem
to transport water and sugars Contains Stomata Allows the diffusion of Gasses
in and out of the leaf Control the size of the Stomata to restrict water loss Contains lots of Air Spaces to
allow for optimum exchange of
Gasses Water, Water Everywhere Osmosis
The Diffusion of water for a dilute solution to a more concentrated solution,
through a partially permeable membrane.
From a high concentration to a low concentration of water.
When predicting the direction of water movement, only the water concentration matters
as solute molecules cannot pass through the cell membrane.
Osmosis In Plants:
Water moves out of a leaf by osmosis in the Spongy Mesophyll layer (where it passes out of the leaf and into
Cell Walls prevent plant cells from bursting due to too high water pressure.
A lack of water can cause Plants to wilt, as low pressure stops them being rigid.
If there is pleanty of water, then the stomata open to allow transpiration and diffusion of gasses.
When there is not enough water, the stomata close to try to prevent water loss (transpiration), and the diffusion of gases. Therefore Photosynthesis must stop.
Osmosis In Animal:
Animal cells do not have a cell wall, therefore if they have too high water pressure, then they will burst.
Red Blood cells have the same water concentration as cytoplasm, so keep their biconcave shape.
When Red Blood cells are put into pure water, they gain lots of water via osmosis. Eventually they burst, and this is called LYSIS.
When Red Blood cells become shivelled up (ie when put into a concentrated solution), it is said that they become CRENATED.
Turgor Pressure The Plant becoming rigid due to pressure on the cell walls is called
When cells are fully turgid, the plant is rigid and upright.
When cells beign to lose water through osmosis, cells become
Plasmolysis is when the cells contract like a deflating balloon due
to losing high volumes of water. Water Loss from Leaves Traspiration and water loss are unavoidable due to photosynthesis.
To reduce water loss, the turgidity of guard cells changes in relation to light intensity and availability of water.
When exposed to sunlight, a plant photosynthesises and produces glucose, this creates a concentration gradient so that water enters the leaves via osmosis. This makes guard cells turgid and the stomata fully open allowing the exchange of gases and water loss. When there is insufficient water, the guard cells become flaccid and the stomata close. This does prevent water loss, but also the exchange of gases so photosynthesis cannot occur:
High light intensity causes the stomata to open which increases the rate of evaporation of water from the leaf.
High temperatures increase the movement of water molecules so increase the rate of transpiration.
Wind blows water molecules away from the stomata, which increases transpiration
High humidity decreases the concentration gradient and so slows down transpiration. Transport in Plants Stems allow the plant to transport substences from the roots
to the leaves. This is done by the Xylem and Phloem.
Xylem transports water and soluble mineral salts from the roots
to the leaves
Phloem allows the movement of food substances around the plant
Structure wise, there are many Xylems inside one Phloem. Transpiration is the evaporation and diffusion of water from inside leaves.
1. Water evaporates from the internal leaf fells through the stomata
2. Water passes by osmosis from the xylem vessel to leaf cells, which pulls the entire thread of water in that vessel upwards by a small amount.
3. Water enters the xylem from root tissue to replace water which has moved upwards.
4. Water enters root hair cells by osmosis to replace water which has entered the xylem. Xylem vessels are hollow tubes made from dead plant cells. The cellulose cell walls are thickened and strengthened with a waterproof substance.
Phloem cells are long colums of living cells able to translocate sugars both up and down the plant. Factors Affecting
Tanspiration Light Wind Heat Humidity More light increases the rate of photosynthesis
and therefore increases the rate of
transpiration Wind increasing causes Transpiration
to increase Photosynthesis and therefore
transpiration increases with heat increasing decreases the rate of transpiration