Biology: B2 Additional

Sections 1.1-6.4 of AQA Science Addtional Revision Guide

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1.1 - Animal & Plant Cells

Animal and plant cells have a few similarities:

  • A nucleus to control the cell and it's activities
  • Cytoplasm; this is where chemical reactions take place
  • A cell membrane which controls the movement of materials in and out of the cell
  • Mitochondria where energy is realeased during aerobic respiration
  • Ribosomes where proteins are made 

An animal cell has an irregular shape with all of the above inside surrounded by the cell membrane.

Plant cells are different because they have all of the above as well as:

  • A cell wall - offers rigid support to every cell. Without this the plant would collapse.
  • Chloroplasts which contain chlorophyll for photosynthesis
  • A permanent vocuole containing the sap

KEY WORDS: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, cell wall, chloroplasts, vacuole

A plant cell although has similar features to a animal cell it also has its own unique charateristics.

GET IT RIGHT: Make sure you can label and animal cell and a plant cell and know th function of each part

Exam hint! Remember not all plant cells have choloroplasts.. Don't confuse them with chlorophyll.

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1.2 - Specialised Cells

When an egg is fertilised it begins to grow and develop. At first, it is a small cluster of cells. Then, as the organism grow, some of the cells become specialised. There are many different type of specialised cell:

  • Some cells in a plant may develop into xylem or root hair cells
  • Some cells in animals will develop into nerve or sperm cells.

KEY WORDS: specialised, differentiation

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1.3 - How substances get in and out of cells?

Molecules move randomly because of the energy they have. Diffusion is the random movement of molecules from an area of high concentration to an area of lower concentration.

The larger the difference in concentration, the faster the rate of diffusion. Examples are:

  • The diffusion of oxygen in the cells of the body from the blood stream as the cells are respiring
  • The diffusion of CO2 in actively photosynthesising plant cells
  • the diffusion of simple sugars and amino acids from the gut through the cell membranes

KEY WORDS: random, diffusion, concentration

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1.4 - Osmosis

Osmosis is just the movement of water.

Just like in diffusion, the movement of the molecules is random and doesn't require any energy from the cell.

Osmosis is the diffusion of water across the partially permeable membrane from a dilute solution to a more concentrated solution. No solute molecule can move across the membrane. The cell membrant is also partially permeable.

Water is needed to support cells and because chemical reactions take place in solution.

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2.1 - Photosynthesis

The equation for photosynthesis is:

carbon dioxide + water (+ light energy) → glucose + oxygen

The CO2 is taken in by the leaves and water in through the roots.

In the chlorophyll traps the energy needed for photosynthesis. In photosynthesis the sugar glucose is made. This then produces oxygen as waste.

The structure of a leaf -

Upper epidermis - A waxy cuticle. This layer is waterproof which stops water loss

Palisade layer - Palisade cells at the top of the leaf, they are close to the light, tightly packed together and full of chloroplasts.

Spongy layer - The cells aren't tightly packed together, they have a larger surface area available for gas exchange and some chloroplasts.

Lower epidermis - Guard cells open and close to control water loss.

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2.2 - Limiting Factors

A lack of light would slow down the rate of photosynthesis as light provides the energy for the process. Chlorophyll traps the light. Even on sunny days, light may be limited on the forest floor where the plant may be growing.

If it's cold, then enzymes do not work effectively and this slows down the rate of the reaction.

Also, if there is too little CO2 that will also slow down the rate of the reaction.. The carbon dioxide may be limited because of an enclosed space e.g. in a greenhouse on a hot day, in a rapidly photosynthesising rain forest.

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