Biology unit 2

Most human and animal cells have the following parts:

• a nucleus, which controls the activities of the cell
• cytoplasm, in which most of the chemical reactions take place
• a cell membrane, which controls the passage of substances into and out of the cell
• mitochondria, which is where energy is released in aerobic respiration
• ribosomes, which is where protein synthesis occurs

 Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell. Plant cells often have:

• chloroplasts, which absorb light energy to make food
• a permanent vacuole filled with cell sap.

 A bacterial cell consists of cytoplasm and a cell membrane surrounded by a cell wall; the genes are not in a distinct nucleus.

Yeast is a single-celled organism. Yeast cells have a nucleus, cytoplasm and a cell membrane surrounded by a cell wall.

• A fat cell consists of fat storage, cytoplasm with nucleus and mitochondria surrounded by a cell membrane.
• They have a small amount of cytoplasm and large amounts of fat.
• As the cell needs little energy, the cell has few mitochondria.
• The cell can expand as it fills with fat. 

• The cone cell consists of mitochondria and a nucleus in the middle section and an outer segment which contains visual pigment. It also contains a synapse.
• They are found in the retina in the eye.
• The visual pigment in the outer segment changes chemically in coloured light. Enery is needed to change it back to its original form.
• The energy released by mitochondria is needed to reform the visual pigment, which lets you see continually in colour.
• The synapse connects nerve cells to optic nerve. When coloured light makes your visual pigment change, an impulse is triggered. The impulse crosses the synapse and travels along the optic nerve to your brain.

• The root hair cell consists of a nucleus, permanent vacuole, root hair and a cell membrane surrounded by a cell wall.
• The root hair cell increase the surface area of the root so that it can absorb water and mineral ions effectively.
• The root hair has a large surface area and a large permanent vacuole, which contains cell sap.
• The large permanent vacuole speeds up the movement of water (by osmosis) from the soil across the root hair cell.

• A sperm cell consists of acrosome, nucleus, mitochondria and a tail.
• The middle section contains mitochondria to release energy for movement.
• The long tail helps move the sperm towards the egg.
• The acrosome stores digestive enzymes for breaking down the outer layers of the egg.
• A large nucleus contains genetic information to be passed on.

• The ciliated cell consists of cilia and nuclei.
• Cilia sweeps dust or mucus, which keeps your lungs clean.
• A red blood cell contains a cytoplasm and cell membrane surrounded by a cell wall with no nucleus.
• The cell contains haemoglobin to carry oxygen to the cells.

• A muscle cell contains many nuclei, which produces a lot of protein to repair damaged muscles.
• A xylem vessel contains a tough wall with a woody material called lignin and no cytoplasm.
• No cytoplasm means that there is a continuous flow of water and lignin is a strong and waterproof material that helps support the plant.
• Xylem vessels consist of dead cells and it transports water in the plant.

• Dissolved substances can move into and out of cells by diffusion.
• Diffusion is the net movement of molecules in a fluid from a region of high concerntration to a region of low concerntration until they reach equilibrium.
• The net movement = particles moving in - particles moving out
• A difference in concertration between two areas is called a concerntration gradient.
• The greater the difference in concerntration, the faster the rate of diffusion.
• The bigger the difference in concerntration, the steeper the concerntration gradient. The steeper the concerntration gradient, the faster diffusion will take place. 
• Oxygen required for respiration passes through cell membranes by diffusion.
• Factors that affect diffusion: temperature, increasing the speed of the molecule, increase concerntration gradient, increase the surface area and add more substance.

• Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.
• Organelle - cell - tissue - organ - organ system

A tissue is a group of cells with similar structure and function. Examples of tissues include:

• muscular tissue, which can contract to bring about movement
• glandular tissue, which can produce substances such as enzymes and hormones
• epithelial tissue, which covers some parts of the body.

Organs are made of tissues. One organ may contain several tissues. The stomach is an organ that contains:

• muscular tissue, to churn the contents
• glandular tissue, to produce digestive juices
• epithelial tissue, to cover the outside and the inside of the stomach.

Organ systems are groups of organs that perform a particular function. The digestive system is one example of a system in which humans and other mammals exchange substances with the environment.
The digestive system includes:
■ glands, such as the pancreas and salivary glands, which produce digestive juices
■ the stomach and small intestine, where digestion occurs
■ the liver, which produces bile
■ the small intestine, where the absorption of soluble food occurs
■ the large intestine, where water is absorbed from the undigested food, producing faeces.

• Food is taken from the mouth.
• The salivary glands consists of enzymes to break down the food and release digestive juices, which contain enzymes.
• The oesophagus carries the insoluble food molecules to the stomach.
• The stomach contains enzyes that break down large insoluble molecules into small soluble ones.
• The small intestine is where soluble food molecules are absorbed in the blood. The molecules are then transported into the bloodstream around your body.
• The undigested food is passed on to the large intestine, where water is absorbed form the undigested food into the blood
• The material left forms the faeces, which is stored through the rectum.
• The faeces pass out of your body through the anus.

 Plant organs include stems, roots and leaves.

• The root system anchors the plant and takes in water and nutrients (N,P,K).
• The shoot system transports food and water throughout the plants.
• Leaves are the main site of photosynthesis in a plant.
• Stems support the plant and assist in the transport of water and nutrients between the roots and the leaves.
• Roots absorb water and mineral ions for the plant and hold the plant in place.

• Epidermal tissue creates a barrier protecting the plant from the outside world. It protects against water loss and the environment. It also contains stomata for gas exchange. 
• In the palisade mesophyll tissue, cells are tightly packed with lots of chloroplasts. Cells near the surface of the leaf capture maximum light intensity for photosynthesis.
• In the spongy mesophyll tissue, cells have fewer chloroplasts. Cells are spaced out to allow gas exchange. Less photosynthesis happens here as light intensity is lower.
• Xylem tissue transports water. The cells have thickened cell walls but the cells are dead. The water only moves in one direction form the root to the leaves (upwards).
• Phloem tissue transports cell sap around the plant. Sap can flow in both directions. For exmple, down to the tubers or roots for storage. The cells are alive.

 Examples of plant tissues include:

• epidermal tissues, which cover the plant
• mesophyll, which carries out photosynthesis
• xylem and phloem, which transport substances around the plant.

 Photosynthesis is summarised by the equation:

                                       light energy

carbon dioxide  + water                 glucose + oxygen

                                        chlorophyll

                       light energy

6CO2 + 6H20                  C6H12O6 + 6O2

                             chlorophyll

 During photosynthesis:

• light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae
• this energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose)
• oxygen is released as a by-product.

 The rate of photosynthesis may be limited by:

• shortage of light
• low temperature
• shortage of carbon dioxide.

Light intensity affects the rate of photosynthesis because light provides the energy for the process of photosynthesis.

Temperature affects the rate of photosynthesis because as the temperature rises so does the rate of photosynthesis but if the temperature is too high enzymes do not work effectively.

Carbon dioxide affects the rate of photosynthesis because it's needed to make glucose.

The glucose produced in photosynthesis may be converted into insoluble starch for storage. Plant cells use some of the glucose produced during photosynthesis for respiration.

 Some glucose in plants and algae is used:

• to produce fat or oil for storage
• to produce cellulose, which strengthens the cell wall
• to produce proteins.

To produce proteins, plants also use nitrate ions that are absorbed from the soil.

 Physical factors that may affect organisms are:

• temperature
• availability of nutrients
• amount of light
• availability of water
• availability of oxygen and carbon dioxide.

Quantitative data on the distribution of organisms can be obtained by:

• random sampling with quadrats
• sampling along a transect

Protein molecules are made up of long chains of amino acids. These long chains are folded to produce a specific shape that enables other molecules to fit into the protein. Proteins act as:

• structural components of tissues such as muscles
• hormones
• antibodies
• catalysts

Catalysts increase the rate of chemical reactions. Biological catalysts are called enzymes. Enzymes are proteins.