SG Biology Summary notes
Investigating cells
Sub-topic a: Investigating living cells
Cells are the basic units of living things (this means that all living things are made up of one or more cells).
Cells are very small and they cannot be seen easily without using a microscope, which magnifies them, making them look larger.
When using a microscope some parts of a cell are often difficult to see. To make cell parts easier to see they are usually stained, e.g. iodine stains the nucleus brown.
Cell Structure
All plant and animal cells have 3 parts in common:
Part of cell Function
nucleus Controls all the cells activities; contains genetic information cytoplasm The site of chemical reactions in the cell
cell membrane Controls the entry and exit of substances into/out of the cell
Plant and animal cells differ in some ways:
Plant cells have Function Animal cells
Chloroplasts Contains chlorophyll, which traps
light energy for photosynthesis no chloroplasts Large central vacuole Stores cell sap and water if present small vacuoles only Cell wall Supports the cell
(made of cellulose) no cell wall
Sub-topic b: Investigating Diffusion
Diffusion is the process by which molecules move from an area of (their) high
concentration to an area of (their) low concentration. The movement continues until the molecules are evenly spread out.
Living cells depend on the diffusion of substances to stay alive. These substances must be dissolved in water for diffusion to take place.
© Diffusion can occur from one cell to another. This is important to the whole organism, which can then obtain what it needs in all its cells. As all cells cannot be supplied directly from a transport system, diffusion of necessary substances occurs from cell to cell.
Substances that enter and leave cells by diffusion include:
dissolved food oxygen carbon dioxide water
The cell membrane controls the passage of substances into and out of the cell
Osmosis
The diffusion of water is called osmosis. Osmosis occurs in all living organisms.
Osmosis is the diffusion of water from an area of (its) high concentration to an area of (its) low concentration, through a selectively permeable membrane.
Osmosis will continue until the concentration of water is the same in all areas.
© Osmosis occurs across selectively permeable membranes.
These membranes allow some substances to pass through but prevent others.
In all living cells the cell membrane is a selectively permeable membrane.
© How osmosis effects plant and animal cells?
Plants: Osmosis is vital for plants, not just for them to uptake water, but also to give some rigidity to their structure.
When plant cells are placed in a solution of higher water concentration, water enters by osmosis and the cell swells. The cell wall stops the cell from bursting and helps it to keep their shape. The cells are said to be turgid.
When plant cells placed in a solution of lower water concentration, water leaves by osmosis and the cell membrane is pulled away from the cell wall. The cells are said to be plasmolysed.
Animals: Animal cells unlike plant cells, do not have a cell wall.
When animal cells are placed in a solution of higher water concentration, water enters by osmosis and the cell swells and will eventually burst.
When animal cells placed in a solution of lower water concentration, water leaves by osmosis and the cells will shrink.
Sub-topic c: Investigating enzymes Enzymes are present in all living cells
Enzymes are made of protein
Enzymes are biological catalysts (enzyme is the term used for catalysts that are found in living organisms)
Catalysts are substances that
1) speed up the rate of a chemical reaction (enzymes speed up the rate of chemical reactions in living things)
2) lower the activation energy (enzymes allow reactions to happen at lower temperatures)
3) remain unchanged by the reaction.
Without enzymes the rate of reaction of cell processes would not be fast enough to maintain the processes of life.
Enzymes are used by living things to break down or build up molecules.
Amylase is an enzyme that breaks down starch into maltose Amylase
Starch Maltose (sugar)
Building up a molecule is called synthesis.
Phosphorylase is an enzyme that builds up molecules (starch). It carries out synthesis using a sugar called glucose -1-phosphate.
Phosphorylase
Glucose-1-phosphate Starch
© The substance an enzyme acts on is called the substrate.
Enzymes and temperature
Rate of enzyme action is altered by changes in temperature. As temperatures fall towards 0˚C the rate of action (activity) decreases. The rate of action also falls as the temperature gets too hot.
The temperature at which the enzyme works best is known as its optimum temperature.
Effect of temperature on human enzyme activity
At freezing point an enzyme becomes inactive
At higher temperatures e.g. above 50˚C, enzymes become increasingly destroyed (denatured) as the protein becomes damaged.
© Optimum conditions for an enzyme are those in which it works best (is most active) e.g. optimum temperature for human enzymes is between 35°C and 40°C.
Enzymes and pH
The level of acidity of a substance is known as its pH.
pH 7 is neutral (neither acid nor alkaline, like tap water).
The optimum pH for the enzyme pepsin is pH 2.5.
The optimum pH for the enzyme catalase is pH 9.
Above and below the optimum pH the rate of enzyme action decreases.
Each enzyme has a number of pH’s it will work at, this is called the range, outside this range it will not work at all.
Effect of pH on activity of enzymes
© The range for pepsin is pH 0.5 to pH 4.5 with an optimum of pH 2.5 The range for catalase is pH 6 to pH 11 with an optimum of Ph 9
optimum for catalase
(pH 9) optimum for pepsin
(pH 2.5)
Sub-topic d: Investigating aerobic respiration Energy is needed by living things so they can:
-Grow -Synthesise
(e.g. produce new cells) (build up chemicals)
-Maintain themselves -Move
(e.g. replace worn out or damaged cells)
Energy is transformed (changed) from one kind to another in plants and animals, e.g. In green plants: light energy from the sun is changed into chemical energy
(food) during photosynthesis.
© In mammals: chemical energy in food is changed into heat energy.
The energy in food is chemical energy.
Fats (and oils) contain more chemical energy per gram than carbohydrate or protein.
Each gram of fat contains twice as much energy as carbohydrates and proteins.
Remember all living cells respire, (in both plants and animals).
Cells obtain the energy they need from food by carrying out respiration.
Respiration is the chemical breakdown of food to release energy.
When oxygen is used to release energy from food the respiration is called aerobic respiration. The word equation for respiration is.
GLUCOSE + OXYGEN CARBON DIOXIDE + WATER + ENERGY Aerobic respiration is sometimes referred to as tissue respiration.
(A tissue is a group of similar cells).
Carbon dioxide is given off from cells when food is broken down during tissue respiration.
During respiration some of the energy released may be in the form of heat.
© Energy released during respiration is used for the functioning of the cell
i.e. for cell metabolism. During metabolism chemical changes take place that require energy to make them happen.
The sum of all the chemical processes occurring in a cell (or organism) is called its metabolism.
Sub-topic e: Investigating cell division
The number of cells in organisms is increased by cell division. This cell division is called mitosis. In cell division one cell divides into two.
The stages of cell division (mitosis) take place in the following order.
1. The chromosomes appear as fine threads scattered through the nucleus.
2. The chromosomes shorten and thicken.
The chromosomes have already doubled into two exact copies, each
called a chromatid.
3. The nuclear membrane breaks down.
The chromatids are held together by a
centromere.
The nucleus controls all cell activities including cell division.
The chromosomes contain genetic information, which determine all inherited factors, for example hair colour and eye colour.
When chromosomes double up, before cell division, each chromosome makes an exact copy of itself.
The two parts are called chromatids and they are joined together (by a centromere) at the start of cell division.
When the chromosomes divide two identical copies are formed. During division each new cell will receive one copy of each chromosome, as each copy is identical, each new cell has the same genetic information.
© Each cell has the correct number of chromosomes to make it function properly.
It is important that each new cell has the full set of working instructions that is carried on the chromosomes. Too few or too many would upset the correct working of the cell.
When the correct number of chromosomes is not maintained abnormality occurs.