Biology Revision


Photosynthesis #1

Photosynthesis takes place largely in the the leaf (chloroplasts). Leaves are adapted to bring together the three raw materials of photosynthesis (water, co2 and light) and remove its products (oxygen and glucose) Adaptations: * A large S.A. that collects as much sunlight as possible * An arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another. * Thin, as most light is absorbed in the first few millimetres of the leaf, and the diffusion distance is this kept short. * A transparent cuticle and epidermis tha let ligh through to the photosynthetic mesophyll cells beneath. * Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight. * Numerous stomata for gaseous exchange . * Stomata that open and close in response to changes in light intensity. * Many air spaces in the lower mesophyll layer to allow diffusion of carbon dioxide and oxygen. * A network of xylem that brings water to the leaf cells, the phloem that carries away the sugars produced in photosynthesis.

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Photosynthesis #2

The overall equations for photosynthesis is: (6CO2) + (6H20) ----> (C6H12O6) + (6O2) The equation is highly simplified. Photosynthesis is a complex metabolic pathway involving many intermediate reactions. It is a process of energy transformation in which light energy is firstly changed into electrical energy and then into chemical energy. There are three main stages to photosynthesis: 1) Capturing of light energy- by chloroplast pigments such as chlorophyll. 2) Light-dependent reaction- in which light energy is converted into chemical energy. During the process an electron flow is created by the effect of light on chlorophyll and this causes water to split (photolysis) into protons, electrons and oxygen. The products are reduced NADP, ATP and oxygen. 3) The light-independent reaction- In which these protons (hydrogen ions) aer used to reduce carbon dioxide to produce sugars and other organic molecules.

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Structure and Role of chloroplasts in photosynthes

Photosynthesis takes place within cell organelles called chloroplasts. These vary in shape and size but are typically disc shaped, 2-10 micrometres long and 1 micrometer in diameter. They ae surrounded by a double membrane. Inside the chloroplast membranes are two distinct regions: -The grana are stacks of up to 100 disc-like structures calld thylakoids where the light-dependent stage of photosynthesis takes place. Within the thylakoids is the photosynthetic pigment called chlorophyll. Some thykaloids have tubular extensions that join up with thylakoids in adjacent grana. These are called inter-granal lamellae. -The stroma is a fluid-filled matrix where the light independent stage of photosynthesis takes place. Within the stroma are a number of other structures such as starch grains.

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