Plant cells- structure and function
Algae are very simple aquatic organisms. They make their own food and have similar features to plant cells.
All plant and algal cells have:
- a cell wall made of cellulose that strengthenes the cell and gives it support
Many, but not all, plant cells also have:
- chloroplasts contaning chlorophyll. Chlorophyll absorbs light energy to make food by photosynthesis. Root cells do not have chloroplasts because they are underground and do not photosynthesise.
- a permanent vacuole is a space in the cytoplasm filled with cell sap. This is important in keeping the cells rigid to support the plant.
Osmosis in plants
Plants rely on osmosis to support their stems and leaves. Water moves into plant cells by osmosis. This causes the vacuole to swell uo and press the cytoplasm against the plant cell walls. The pressure builds up until no more water can physically enter the cell- this pressure is known as turgor. Turgor pressure makes the cells hard and rigid, which in turn keeps the leaves and stems of the plant rigid and firm.
Plants need the fluid surrounding the cells always to be hypotonic to the cytoplasm, with a lower concentration of solutes and a higher concentration of water than the plant cells themselves. If the solution surrounding the plant cells is hypertonic to the cell contents, water will leave the cells by osmosis. The cells will become flaccid as there is no pressure on the cell walls. At this point, the plant wilts as turgor no longer supports the plant tissues.
If more water is lost by osmosis, the vacuole and cytoplasm shirnk, and eventually the cell membrane pulls away from the cell wall. This is plasmolysis. Plasmolysed cells die quickly unless the osmotic balance is restored.
Within the body of a plant, tissues such as the palisade and spongy mesophyll, xylem and phloem are arranged to form organs. Each organ carries out its own particular function.
Plant organs include the leaves, stems and roots, each of which has a very specific job to do.
Anaerobic respiration in plants
When plant cells respire anaerobically they form ethanol and carbon dioxide.
The process of photosynthesis
Carbon dioxide + water (+ light energy) -> glucose + oxygen
6CO2 + 6H2O (+light energy) -> C6H12O6 + 6O2
The energy source for photosynthesis: light energy is absorbed by chlorophyll in the chloroplasts of the green parts of the plant
How plants absorb light: leaves are well adapted to allow the maximum amount of photosynthesis to take place.
- big surface area
- air spaces that allow carbon dioxide to get into the cells and oxyfgen to leave them by diffusion
- they have veins, which bring plenty of water in the xylem to the cells of the leavees and remove the products of photosynthesis in the phloem
You can show that a plant is photosynthesising by the oxygen it gives off as a by-product. Oxygen is a colourless gas. However, if you use water plants such as Cabomba or Elodea, you can see and collect the bubbles of gas they give off when they are photosynthesising. The gas will relight a glowing splint, showing that it is rich in oxygen
Testing for starch using iodine
Iodine solution is a yellow-brown liquid that turns blue-black when it reacts with starch. You can use this iodine test for starch to show that photosynthesis has taken place in a plant
Testing for starch
To show that light is vital for photosynthesis to take place:
Take a leaf from a plant kept in the light and a plant kept in the dark for at least 24 hours. Leaves have to be specially prepared so the iodine solution can reach the cells. Just adding iodine solution is not enough, because the waterproof cuticle keeps the iodine out so it can't react with the starch. Also, the green chlorophyll would mask any colour changes if the iodine did react with the starch. You therefore have to boil the leaves in ethanol, to destroy the waxy cuticle and then to remove the colour. The leaves are then rinsed in hot water to soften them. Then, *** iodine solution to them both. Iodine solution turns blue-black in the presence of starch. The iodine solution on the leaf kept in the dark remains orange-red.
Safety: take care when using ethanol. It is volatile, highly flammable and harmful. Always wear eye protection. No naked flames- use a hpt water bath to heat ethanol.
Which factors limit the rate of photosynthesis in plants?
The rate of photosynthesis may be limited by shortage of light, low temperature and shortage of carbon dioxide
How can we use what we know about limiting factors to grow more food?
We can artificially control the levels of light, temperature and carbon dioxide when growing crops in greenhouses to increase the rate of photosynthesis and so increase the yield of the crops
How plants use glucose
What do plants do with the glucose they make?
Plant and algal cells use the soluble glucose they produce during photosynthesis for respiration, to convert into insoluble starch for storage, to produce fats or oils for storage and to produce fats, proteins and cellulose of use in the cells and cell walls.
Starch is insoluble in water, so it will have no effect on the wate balance of the plant. This means that plants can store large amounts of starch in their cells.
- Starch is stored in the cells of the leaves. It provides an energy store for when it is dark or when light levels are low
- Starch is also kept in special storage areas of a plant.
Minerals, proteins and carnivorous plants
The extra materials that plant cells need to produce protein:
Plant and algal cells also need nitrate ions, which are absorbed from the soil, to make amino acids which make up proteins. This uses energy from respiration.
How carnivorous plants get the nitrates they need:
Carnivorous plants such as the Venus Fly Trap are adapted to live in nutrient-poor soil by taking minerals from the animals they catch and digest
Gas exchange in plants
In flowering plants carbon dioxide enters leaves by diffusion through the stomata. Plants have stomata to obtain carbon dioxide from the atmosphere and to remove oxgyen produced in photosynthesis as a bi-product.
How the leaves of a plant are adapted for gaseous exchange:
- Plant leaves have stomata that allow the plant to obtain carbon dioxide from the atmosphere.
- Carbon dioxide enters the leaf by diffusion. Leaves have a flat, thin shape and internal air spaces to increase the surface area available for diffusion
- Plants mainly lose water vapour from their leaves, and most of this loss takes place through the stomata.
Uptake of water and mineral ions in plants
The roots themselves are thin, divided tibes with a large surface area. The cells on the outside of the toots near the growing tips are called the root hair cells.
These root hair cells have tiny projections from the cells which push out between the soil particles.
Water moves through the root hair cells by osmosis across the partially permeable root cell membrane. It then has only a short distance to move across the root to the xylem, where it is moved up and around the plant.
Plant roots are also adapted to take in minerals using active transport. They have plenty of mitochondria to supply the energy they need for this process. They also have all the advantages of a large surface area and the short pathways needed for the movement of water.
What is transpiration?
The loss of water vapour from the surface of the plant leaves. Most of the loss of water vapour takes place through the stomata.
As water evaporates from the surface of the leaves, more water is pulled up through the xylem to take its place. This constant movement of water molecules through the xylem from the roots to the leaves is known as the transpiration stream.
Water is lost through the stomata, which open to let carbon dioxide for photosynthesis.
The factors that affect how quickly plants transpi
Transpiration is more rapid in hot, dry, windy and bright conditions.
Anything that affects the rate of evaporation will affect transpiration.
Anything that increases the rate of photosynthesis will increase the rate of transpiration. This occurs because more stomata are opened up to let carbon dioxide in. In turn, more water is lost by evaporation and then diffusion through the open stomata.
Water will diffuse more rapidly into dry air than humid air, and windy conditions both increase the rate of evaporation and also maintain a steep concentration gradient from the inside of the leaf to the outside by removing water vapour as it diffuses out.
If plants lose more water than is replaced by the roots, the stomata can close to prevent wilting.
The size of stomata is controlled by the guard cells, which surround them.
Controlling water loss
Mos leaves have a waxy, waterproof layer (the cuticle) to prevent uncontrolled water loss. In very hot environments, the cuticle may be very thick and shiny. Most of the stomata are found on the underside of leaves. This protects them from the direct light and energy of the Sun, and reduces the time they are open.
If a plant begins to lose water faster than it is replaced by the roots:
- The plant may wilt. Wilting is a protection mechanism against further water loss. The leaves all collapse and hang down. This greatly reduces the surface area available for water loss by evaporation.
- The stomata close, which stops photosynthesis and risks overheating. However, it prevents water loss and any further heating.
The plant will remain wilted until the temperature drops, the sun goes in, or it rains.
The phloem moves food.
Phloem tissue transports dissolved sugars from the leaves to the rest of the plant, including the growing regions and storage organs.
The stems and the roots where dissolved sugars are needed for making new plant cells.
The movement of dissolved sugars from the leaves to the rest of the plant is called translocation.
Xylem moves water and mineral ions
Xylem tissue transports water and mineral ions from the roots to the stems and leaves.
Mature xylem cells are dead whereas phloem cells are alive
The mineral ions are needed for the production of proteins and other molecules within the cells.
The movement of water from the roots through the xylem and out of the leaves is called the transpiration stream.
Plants are sensitive to:
- Phototropism: response to light
- Gravitropism: response to gravity
- Hydrotropism: response to moisture/ water
Plant responses to light and gravity are brought about by the plant hormone auxin.
The responses of the roots and shoots to stimuli of loght and gravity are the results of unequal distribution of auxin
Shoots grow towards light and against the force of gravity.
Roots grow towards moisture and in the direction of the force of gravity.
Gravitropism in shoots and roots
The responses of plant roots and shoort to light, gravity and moisture are the result of unequal distribution of auxin, causing unequal growth rates.
1. The normal young bean plant is laid on its side in the dark. Auxin is equally spread through the tissues.
2. In the root, more auxin gathers on the lower side.
In the shoot, more auxin gathers on the lower side.
3. The root grows more on the side with the least auxin, making it bend and grow down towards the force of gravity. When it has grown down, the auxin becomes evenly spread again.
The shoot grows more on the side with the most auxin, making it bend and grow up away from the force of gravity. When it has grown up, the auxin becomes evenly spread again.
Practical: You can investigate the effect of one-sided light on the growth of seedlings using a simple box with a hole cut in it and cress seedlings growing in a Petri dish.
Plant growth hormones are widely used in agriculture and horticulture as weed killers and as rooting hormones to increase the success of cuttings.
Horticulture: growing plants for food and pleasure in gardens
Plant growth hormone used is auxin
Plant growth hormones are widely used in agriculture as weed killers, selectively killing broad-leaved weed plants
If you spray auxin solution onto the leaves of plants, the hormone is absorbed. This extra auxin can send the plants into rapid, uncontrolled growth, which kills them. Becayse these chemicals affect one type of plant and not another, they are known as selective herbicides.
The structure of a flower
Sexual reproduction in plants is all about the production of gametes in flowers, pollination, fertilisation, and seed and fruit formation.
Sexual reproduction in flowering plants involves the production of male and female gametes.
- The anther produces the male gametes (pollen)
- The ovary produces the female gametes (ovules)
- Sepal: small, green, leaf-like structures that protect the flower when it is in bud
- Stamens: the male parts of the flower, made up of the anther; which produces pollen grains and the filament; which holds the anther.
- Carpel: the female part of the flower which hols the stigma; where pollen lands, the style; which transports the male sex cell to the ovary (with style), and the ovary; which produces ovules (female gametes).
- The ovary also often forms the fruit when the ovules are fertilised.
The transfer of the male gametes to the female gametes is called pollination
The wind or insects can carry the polen to the female sex organs during pollination
Flowers have distinctive adaptations to increase the likelihood of succesful pollination taking place
- Petals: large, brightly coloured; many are patterned to guide insects in
- Often scented to atract insects
- Anthers hang outside the petals so the pollen is blown away by the wind
- Many, small and light to float easily in the wind
1. Anther produces the male gametes in pollen grains.
2. The pollen grains attach to the stigma on top of a carpel, in which the female gametes (pvules) are located.
3. A pollen tube grows down the style to the ovary.
4. Nuclei pass from the pollen grain along the pollen tube and fuse with the egg cell nucleus and the endosperm nuclei to fertilise it
5. Th resulting zygote develops into an embryo, which forms into a seed with a food store and a tough outer coat. The endosperm and the female tissues of the ovule give it rise to seed.
6. The ovary grows into a fruit, which surrounds the seed.
In Asexual reproduction there is no joining of special sex cells (gametes) and there is no variety in offspring- just clones. Their genetic material is identical to their parents and to each other
Asexual reproduction is very common in small plants and large plants such as daffodils, strawberries, and brambles.
Taking cuttings is a form of artificial asexual reproduction or cloning that has been carried out for hundreds of years.
In recent years, scientists have come up with a more modern way of cloning plants called tissue culture. It is more expensive but it allows you to make thousands of new plants from one tiny piece of plant tissue.
1. Use a mixture of plant hormones to make a small group of cells from the plant you want to clone and produce a big mass of identical plant cells called a callus
2. Using a different mixture of hormones and conditions, you can stimulate each of these cells to form a tiny new plant.
This type of cloning guarantees that you produce thousands of offspring with the characteristics you want from one individual plant.
Genetically modified crops
Crops that have had their genes modified are known as genetically modified (GM) crops.
GM crops often have improved resistance to insect attack because they have been modified to make their own pesticide
GM plants that are more resistant than usual to herbicides allow farmers to spray and kill weeds more effectively without damaging their crops and this generally produce a higher yield.
Adapt and survive
Epiphytes are found in rainforests. They have adaptations which allow them to live high above the ground attached to other plants. They collect water and nutrients from the air in their specially adapted leaves.
The South African sausage tree is one of a relatively small number of plants that rely on bats to pollinate their flowers. The flowers open at night, have a strong perfume and produce lots of nectar. They hang down below the branches and leaves which makes it as easy as possible for bats to approach and feed from them- and at the same time transfer pollen from one flower to another on their fur.
Adaptations in plants
Plants lose water vapour from the surface of their leaves
Plant adaptations for surviving in dry conditons include reducing the surface area of the leaves, having water-storage tissues and growing extensive root systems.
Competition in plants
Plants often compete with each other for light, space, water and for nutrients (minerals) from the soil
Plants have many adaptations that make them good competitors:
- Small plants found in woodlands grow and flower very early in the year so they get plenty of light through the bare branches of the trees
- By having different types of roots
- Leguminous plants have nitrate-creating-bacteria in their roots
- Having thorns so animals dont eat them
- Spreading their seeds to avoid competition with its own seedlings