Animal and Plant cell

Organelle function and features

Organelle	Function	Features
Plasma membrane	Holds cell contents together. Semi-permeable to control entry and exit of materials	Made of a phospholipid bilayer. Very long and highly folded to absorb material
Cell Wall	Present only in plants. Provide structure and support	Made of cellulose
Centriole	Only in animal cells. Forms spindle fibre for cell division	Has 9 groups of microtubules
Chloroplast	Only in Plants. Has chlorophyll, carries out photosynthesis	Grana catch light to split water. Glucose synthesis occurs in liquid stroma
Chromosome	Contains information in the genetic code to give instructions to all life processes	Made up of tightly coiled DNA and protein
Cytoplasm	Holds dissolved food and gases	Watery liquid
Endoplasmic reticulum (ER)	Surface area for reactions and transport system	Rough ER covered in ribosomes, smooth ER is not
Golgi bodies	Package proteins into usable form and transport them out of cell	More found in cells that produce a lot of protein (mucus cells)
Lysosomes	Collect cell wastes	Has digestive enzymes to break down waste
Mitochondria	Site of respiration. Internal folds provide a large surface area	More in cells that need a lot of energy (muscle cells, cells that transport material)
Nucleus	Holds chromosome, controls cell activity	Surrounded by double layered nuclear membrane
Ribosomes	Site of protein synthesis	Found along ER, more in cells that make a lot of proteins (enzymes)
Vacuoles	Storage of materials, water/starch, in plants	Contain water to give cells strength

Compare and Contrast Smooth and Rough ER

The main similarity between smooth and rough ER is that they are both networks of folded membranes.
The main difference is that rough ER has ribosomes attached which make proteins. The proteins enter the ER to be transported where they are needed.
Smooth ER does not have ribosomes attached. The main functions are making lipids and steroids,metabolism of carbohydrates, and storage of calcium ions.
Muscle cells need both types of ER. Rough ER to produce the proteins for muscle contraction and smooth ER to store calcium ions for muscle contractions.

Compare and Contrast Mitochondria and Chloroplasts

• The mitochondrion (plural: mitochondria) carries out respiration, a process that breaks down glucose and produces ATP that the cell uses as an energy source for active transport and chemical reactions.

• The chloroplasts carries out photosynthesis, a series of reactions that require carbon dioxide and water, from which glucose is produced.

• Both structures are bound by membranes and both have internal surfaces that hold enzymes important in the chemical reactions they do.
• Chloroplasts are oval shaped  and located around the surface of cells and cells at the top of the leaf, photosynthesis occurs quickly as diffusion distances for CO2 are short and light easily reaches the chloroplasts.
• The stacks of thylakoids (grana) in chloroplasts contain chlorophyll which traps energy from light and passes it to energy carries that go to the stroma to take part in the light independent phase of photosynthesis.
• Mitochondria are oval-shaped with highly folded internal membrane that give a large surface area for reactions.
• The second stage of respiration in mitochondria occur in the internal matrix but the third stage occurs on the cristae where the chemicals for the electron transport chain are held and which produces the large amount of ATP for cell life processes.
• Cells that require lots of energy have many mitochondria eg muscle cells. Sperm cells also have a lot to provide ATP to swim to the ovum. Chloroplasts are in cells exposed to most light eg palisade cells in leaf.
• The guard cells in the leaf epidermis have a large number of chloroplasts and mitochondria as they need to produce much glucose and from it ATP to open and close stomata.

Diffusion, Osmosis and Active Transport

Diffusion is the movement of material from an area of high concentration to an area of low concentration without requiring energy from ATP. Example- oxygen diffusing into cells. Oxygen is used for respiratation so concentration remains low in cells and oxygen diffuses in.

Facilitated diffusion - movement of materials such as glucose, amino acids through protein channels and carrier proteins in the cell membrane. Movement is from a high concentration to low concentration so no energy is needed.

Osmosis is the movement of only water  from an area of high water potential/concentration to an area of low water potential/concentration through a semi-permeable membrane. No energy is needed.  Example - root hair cells absorb water from soil. The high solute concentration  inside the root hair cells lowers water concentration so water from the cell moves in.

Active transport - Number of different processes that require energy from ATP to transport material against the concentration gradient (low concentration to high concentration). Example - Ion pump requires energy to pump sodium ions out of the cell and potassium ions into the cell against concentration gradient.

Another type of active transport is cytosis, which is the movement of large amount of substances into/out of cell by the folding of membranes.
Endocytosis is the taking in of substances by folding the membrane. Fluids are taken in by pinocytosis where membrane makes small infoldings which pinch off liquid. Large particles are taken in by phagocytosis where the cell membrane flows around particle and close off (white blood cell consuming bacteria).

The similarity between Diffusion, Osmosis and Active transport is that all three processes transport material across cell membranes.
The need for energy from ATP during active transport is the main difference between active transport compared to diffusion and osmosis. Another important difference is that during diffusion and osmosis material are moved with the concentration gradient, but in active transport material are moved against the concentration gradient.

Enzymes

• In a living cell, thousands of chemical reactions are taking place. These processes are called metabolism. Most of the reactions of metabolism cannot occur without globular proteins called enzymes.
• Enzymes are biological catalysts. They speed up chemical reactions without being used up. They do this by lowering the activation energy needed for a reaction to start. This allows reactions like photosynthesis and respiration to occur at a suitable rate.
• Enzymes control different reactions. Anabolic reactions build up molecules, eg proteins. Catabolic reactions break down molecules, eg digestion.
• Enzymes are different from other catalysts because they work thousands of times faster, they are specific (one enzyme can only work on one type of substance) and they are sensitive to conditions (temperature, pH, chemicals).

How enzymes work

There are two models on how enzymes work. The out-dated model is the lock and key model which said that substrate fits into the active site like a key fits into a lock. When the substrate fits, the chemical reaction occurs and products are released.

This model has been replaced by induced-fit model.

The accepted model of how enzymes work is the induced-fit model. In this model, each type of enzyme has a specific area called the active site which is shaped so that the substrate can fit into it. When the substrate joins the active site, it causes the enzyme to change shape slightly, causing the chemical reaction the enzyme is catalysing to occur. The products are released and the enzyme returns to its normal shape ready for another reaction.

Factors that affect the activity of enzymes-Temperature

Temperature

Chemical reactions go faster as the temperature increases because molecules move faster and collide more often and more violently.
At low temperature, enzyme activity is slow due to the slower movement of particles so there is less collisions between the substrate and the enzyme active site. As the temperature increases, the rate of activity increases as molecules move faster and collide more often until a optimum temperature is reached. At the optimum temperature, the activity of the enzyme is the highest. At temperatures above the optimum, the rate of enzyme activity slows because the enzyme denatures as there is enough energy to break bonds holding the enzyme and its active site in its special shape. At very high temperatures, the enzyme is completely denatured so activity stops completely.

Factors that affect enzyme activity - pH

pH

Most enzymes work inside cells that make them. Since the pH inside cells is normally between 7.2 - 7.4, these are the best conditions for most enzymes. However enzymes in the gut (eg protease) work in acidic conditions so loose their shape and do not function in neutral or basic conditions.
For example, in the diagram below, the enzyme pepsin works in a pH range of around 1-3 If it is put in a environment of pH 5 or above it will be denatured. The enzyme amylase works in a pH range of around 5-9. If it is put in an acidic environment of pH 3 or below it will be denatured.

Factors that affect enzyme activity - substrate conditions

Substrate concentration

The higher the substrate concentration, the more likely it is to collide with the enzyme, so the faster the reaction. But the active site of enzyme can only deal with one substrate at a time. At very high substrate concentrations, they will collide with occupied active sites so enzymes cannot work any faster (saturated).

Factors that affect enzyme activity - Co-factors

Co-factors

Some enzymes will only function when another chemical (co-factor or co-enzyme is present). The co-factors may be (small) inorganic ions like cobalt, selenium and magnesium. The co-enzymes are (large) organic molecules such as some vitamins. As the concentration of cofactors increases, rate of reaction increases until a maximum where all enzymes have co-factors.

Factors that affect enzyme activity - Inhibitors

Inhibitors

Many enzymes are poisoned by heavy metals (lead, mercury) or cyanide. cyanide stops the enzymes of respiration. lead and mercury can stop enzymes in cells of the nervous system functioning. These inhibitors change the shape of enzymes or take over and block the active site to stop substrate from binding. The more inhibitor there is, the lower the activity rate of enzymes.

Photosynthesis

• Photosynthesis is a series of enzyme controlled reactions that occur in chloroplasts in some cells of green plants.
• Water and carbon dioxide are needed and glucose and oxygen are produced.

• Glucose can be used in respiration to make ATP, be stored as starch for later use, or changed into one of many chemicals such as cellulose, fat and amino acids the plant needs to survive.

• The reactions of photosynthesis are divided into two phases:

1) light-dependent phase - occurs in the thylakoids of chloroplast. The walls of the thylakoids have lipid bilayers that contain the light absorbing pigment chlorophyll.

The chlorophyll traps energy from light which is used to split water. The hydrogen from water is transferred to the carrier molecule NADP+ to produce NADPH and the enzyme ATP synthase to produce ATP. Oxygen from water is released as a waste product.

The membranes in the thylakoids provide a large surface area to hold the enzymes and chlorophyll that are needed.


2) Light-independent-phase. This takes place in the stroma of chloroplasts where the NADPH and ATP from the light phase are used to convert CO2 to glucose.


Chloroplasts are found in their greatest number in palisade cells near the upper surface of the leaf. Here as much light energy as possible can be absorbed by chlorophyll. Chloroplasts are pushed up close to the inside of the cell membrane which reduces the diffusion distance for CO2 and water so photosynthesis can occur at a higher rate.

• Photosynthesis is important as it changes solar energy to chemical energy as organic molecules (eg carbohydrates). Plants are producers of food, they are the starting point of food chains.

Factors that affect the rate of photosynthesis

• The rate of photosynthesis is usually measured as the amount of oxygen produced or carbon dioxide used up in a given time.
• Temperature - The rate of photosynthesis increase with increase in temperature  up to the optimum. Above optimum, heat energy denatures enzyme so the rate falls until all enzymes are denatured and the activity stops.

• Carbon dioxide concentration - Rate of photosynthesis increase with increase C02 concentration to a point where rate levels off as some other factor becomes limiting.

• Light intensity - Rate of photosynthesis increase with increase in light intensity up to a point where some other factor becomes limiting or energy in light damages the leaf.

• Wavelength of light - Wavelength in violet, indigo and red are absorbed in greater amounts so rate increases. wavelength in green yellow and orange are not absorbed as much result in a lower rate

• Leaf adaptations - Leaves that are adapted to photosynthesise best in low light intensity are called 'shade leaves'. Shade leaves are thinner have a greater surface area and their cells contain more chlorophyll than 'sun leaves' Rate of photosynthesis in shade leaves is higher at low light intensities whereas leaves adapted to high light intensity have a higher rate of photosynthesis in higher light intensities.

•  water content - lack of water wilts plants and close stomata that prevents diffusion of CO2.
• lack of magnesium ions reduce production of chloropyll.
• Lack of enzymes and enery carriers (ATP/NADPH) can also limit rate.
• Because temperature, light intensity and carbon dioxide combine to determine rate of photosynthesis, the maximum rate can alter if one of the other factors change.