Cells and Plants

A synopsis of two basic chapters of AQA Additional Science - Biology guide

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Animal and plant cells

Nucleus - Controls cell activities and contains instructions for reproduction.

Cytoplasm - A liquid gel where most chemical reactions take place, including respiration.

Cell membrane - Controls what passes in and out of the cell.

Mitochondia - structures in the cytoplasm where oxygen and energy is released in respiration.

Ribosomes - protein synthesis.

Call Wall - added support in plant - made of cellulose.

Chloroplasts - containing green chloropjyll which absorbs light energy for photosynthesis.

Perminant Vacuole - Filled with cell sap for a rigid plant structure.

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Enzymes in the cell

Cell chemistry is controlled by enzymes.

They controle the rate of reactions, and keep reactions seperated.

We find the most enzymes in mitochondria and chloroplasts.

A few of the reactions they're involved in are:

  • Respiration.
  • Photosynthesis
  • Protein Synthesis
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Specialised cells - fat cells and cone cells

A fat cell has delevoped to suit its job in the following ways:

  • Little normal cytomplasm to leave room for large amounts of fat.
  • Few mitochondria as they use very little energy.
  • Room for expantion of up to 1000 times its origional size.

A cone cell from the human eye has adapted to suit its job by:

  • Having an outer segment filled with the chemical visual pigment, which changes colour according to the colour of the light.
  • Having lots of mitochondria in its middle segment to produce lots of energy, needed for returning the visual pigment to its origional form so the eye can still see colour.
  • Having a synapse or specialised nerve ending which connects to the optic nerve, carrying impulses from the pigment change to the brain.
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Specialised cells - root hair and sperm

Root hair cells have adapted by:

  • Increasing their surface area for more water to move into the cell (the root hairs.)
  • Having a large perminant vacuole effecting the movement of water from soil across the root hair cell.
  • Positioning themselves close to the xylem tissue.

Sperm cells have adapted by:

  • Having long tails made of muscle like protiens allowing them to swim.
  • Packing their middle section with mitochondria to give lots of swimming energy.
  • Using an acrosome full of digestive enzymes to break down the outer layers of the egg.
  • Having a large nuclues with all the genetic information needed to pass on and make a baby.
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Organised cells

Specialised cells are ogten grouped together to form a tissue, which joins things in the body together.

Cells -->

Tissues -->

Organs -->

Organ Systems -->

Whole Body!

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Diffusion = the net movement of particles from an area of high concentration to an area of lower concentration.

The overall (net) movement = particles moving in - parlicles moving out

Bigger the difference in concentration, the faster rate of diffusion. This differece is called the concentration gradient. The bigger the difference, the steeper the gradient.

Concentration and an increase in temperature will make the rate faster. Particles have more energy to move when they are warmer.

Individual cells are adapted to make diffision faster and easier by increasing the surface area of the cell membrane (where diffusion occurs.) This involves folding up membranes.

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A partially permiable membrane will only allow some types of particles to diffuse across it.

In a dilute solution, the water is the solvent and the sugar is the solute. The water is of high concentration and the sugar is lower concentration. This swaps if the solution is described as a sugar solution.

Osmosis is a special type of diffusion wherby only water moves across a partially permeable membrane.

When concentration of body fluids = that inside red blood cells then the amount of water entering or leaving the cell is in equalibriam and cell maintains its shape.

When concentration of solution of body fluids > those in cell, water will leave cell making it shrink and inable to function properly.

When concentration of solution of body fluids < those in cell, water will enter cell making it expand and eventually burst!

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Osmosis in plants and active transport

Plants rely on well-regulated osmosis to support their stems and leaves.

Water moves into roots by osmosis, making the cytoplasm swell and press against plant cell walls. This pressure builds until no more water can fit and this the cell is hard and rigid.

So for plants it is important that the fluid surrounding the plants always has a higher concentration of water for osmosis to work in the right direction.

If plant and animal cells need to move substances like glucose against a concentration gradient then they use active transport-needing energy from respiration.

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Plants make their own food by photosynthesis.

During photosynthesis, light energy is absorbes by chlorophyll in chloroplasts and used to convert carbon dioxide and water into the simple sugar glucose and oxygen.

Some of the produced, soluble glucose is used by the cells in the plant, but a lot is converted into the insoluble startch.

Iodine solution is yellow-brown which turns dark blue when it reacts with startch. This is how we test for photosynthesis in a plant.

Leaves are well adapted because:

  • Most have a large surface area.
  • They contain chloroplasts.
  • They have stomata allowing for a regular flow of gasses.
  • They have veins called Phloem and Xylem tubes whichh transport substances.
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Structure of a leaf

Waxy Cutical - waterproof to prevent water loss.

Upper Epidermis

Palisade layer - full of palicade cells near the top of the leaf, packed tightly with lots of chloroplasts.

Spongy layer - cells not tightly packed but with a large surface area and air spaces between cells for gass exchange. A few chloroplasts, but not many.

Lower epidermis - Also featuring guard cells which open and close the stomata to control waterloss. Stomata allow gasses to move in and out of the leaf.

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Limiting factors

When carbon dioxide, light or heat are in short supply they are known as limiting factors. They cap the amount of photosynthesis that can happen in a plant.

The brighter the light, the more photosynthesis.

The higher the temperature, the faster the photosynthesis until about 40 degrees, when enzymes in the photosynthesis reaction become denatured.

On a sunny day, carbon dioxide is often the limiting factor as lots of plants want the gas, but at night there is plenty-just no light!

In a laboratory or greenhous everything can be monitored. The higher the supply of CO2, the faster the photosynthesis.

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How plants use glucose

Plants respire to provide energy for their cells to grow (building bigger molecules from smaller ones.)

Some glucose is built up into more complex carbohydrates like cellulose for new cell plant walls.

Some glucose is combined with other nutrients (mineral ions) to make amino acids. These amino acids then build into proteins for use in the cell.

Energy from resiration builds up fats and oils for storage in the seeds.

Phloem tubes are made of living tissue and transports sugars from the leaves around the entire plant, including growing regions or storage organs (tubers, bulbs etc) for survival over the winter.

Xylem tubes are not living but have thick, strong walls to carry mineral ions and water around the plant.

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Plants and minerals

The problem with the products of photosynthesis is that they are all carbohydrates.

Why Do Plants Need Nitrates?

Proteins are made up of amino acids, made by carbon, hydrogen, oxygen and nitrogen! Proteins make up enzymes which are vital for cellular reactions. These nitrates are taken up from the soil by the plant roots, and are returned to the soil when the plant dies. A plant with nitrate deficiencies won't grow properly, it will be small and stunted.

Why Do Plants Need Magnesium?

Plants need mineral ions and magnesium to make chlorophyll, and thus they need them to be able to photosynthesise. Otherwise the plant can't make food and it will die! Plants only need tiny amounts, however if they don't get this then they have pale, yellowing leaves.

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Hydroponics is planting crops in soil with artificial additions of the needed minerals, instead of planting them in soil.

They use massive greenhouses where all other factors can be monitored too.

The final crops are very clean and can be grown fast, and out of season.

However, they are very expensive to farm and use a lot of resources, but farmers can charge more for their produce!

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