B2: Cells

cell structure

All living things are made of cells.

1) Compare the structure of a Plant Cell with an Animal Cell

All typical cells have:

Cell membrane: partially permeable to allow certain substances to enter and leave the cell.
Cytoplasm: where chemical reactions take place.
Nucleus: contains DNA and controls what happens inside the cell.
Mitochondria: where aerobic respiration happens.
Ribosome: makes protein and can be found floating within the cytoplasm.

Only found in Plant Cells:

Vacuole: contains a liquid called cell sap which keeps the cell firm.
Cell wall: made of tough substance called cellulose which supports the cell.
Chloroplasts: contain chlorophyll, which absorbs light energy for photosynthesis.

Cytoplasm contains ribosomes on rough endoplasmic reticulum and vesicles.
Almost all cells, except prokaryotes, have mitochondria and rough endoplasmic reticulum.
Mitochondria are responsible for producing energy from food substances through the process of aerobic respiration.
Cells with high rates of metabolism require large numbers of mitochondria to provide sufficient energy.

2) Specialised cells

Cells that have developed certain characteristics to perform a particular function.

Cell	Function	Adaptations
Red blood cell	Transport of oxygen	No nucleus. Has haemoglobin.
Muscle cell	Contracts to get structures closer together	Long. Many protein fibres in cytoplasm to shorten cell when energy available.
Ciliated cell	Move and push mucus	Tiny hair called cilia.
Root har cell	Absorb minerals and water	Elongated shape for more surface area.
Xylem vessel	Transport water and support plant	No cytoplasm so water passes freely. No cross wall so cells connect to form tubes. Lignin makes it strong and waterproof.
Palisade cell	Photosynthsizes	Many chloroplasts.

3) Calculate magnification and size of biological specimens using millimeters as units

\[magnification=\frac{size of image}{actual size of object}\]

4) Describe the importance of Diffusion of gases and solutes

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient, as a result of their random movement.
The energy for diffusion comes from the kinetic energy of random movement of molecules and ions.
Diffusion is important for gases and solutes. Without which, molecules (eg. glucose and oxygen) that are needed for life, would not be able to get to the places they are needed.
Substances moves into and out of the cells by diffusion through the cell membrane.

Factors affecting rate of diffusion:

Concentration gradient: as the concentration gradient increases, rate of diffusion increases.
Temperature: as temperature increases, the rate of diffusion increases. This is because the molecules gain kinetic energy and thus move faster.
Surface area: as the surface area increases, the rate of diffusion increases. This is because there is more space available for the substance to diffuse through.
Distance: a greater diffusion distance slows the rate of diffusion as molecules must travel further.

5) Describe how Osmosis affect tissues in solutions of different concentrations.

Osmosis is the movement of water molecules from a region of high water potential to a region of low water potential, through a partially permeable membrane.
Water moves in and out of cells by osmosis through the cell membrane.

In Animals Cells:

Animal cells do not have a cell wall.
They change size and shape when put into solution of different concentration to the cell contents.

In a more dilute solution: water enter the cell by osmosis, cell will swell and burst.
In a more concentrated solution, water leaves the cell by osmosis, cell will shrink, become crenated.

In Plants Cells:

Plant cells have a strong cellulose cell wall that supports the cell and stop it from bursting when it gains water by osmosis.

In a more dilute solution, water enter the cell by osmosis, cytoplasm pushes against the cell wall and cell becomes turgid.
In a more concentrated solution, water leaves the cell by osmosis, cytoplasm pulls away from the cell wall (plasmolysis) and the cell becomes flaccid and the plant wilts.

6) Explain the importance of Water Potential and Osmosis to Plants and Animals

In plants, when water moves into the cells, the vacuole gets bigger, pushing the cell membrane against the cell wall, making the cell rigid and firm. This provide support and strength for the plant, allowing the plant to stand upright with its leaves held out to catch sunlight. If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts.
In animals, concentration of body fluid eg. blood plasma, must be kept within strict limits. If cells gain or lose too much water, they will not function efficiently.

7) Discuss the importance of Active Transport as a process for movement across membranes

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using the energy from respiration.
It is needed when organism wants to optimize the amount of nutrients it can take up.

Examples:

uptake of glucose by epithelial cells in the villi of the small intestine and kidney tubules.
uptake of ions from soil water by root hair cells in plants.

8) Explain how Protein Molecules move particles across a membrane during Active Transport

Carrier proteins needed to pick up specific molecules and take them through the cell membrane against the concentration gradient.
Substance combines with carrier protein molecule.
Carrier transports substances across membrane using energy from respiration.
Substance released into cell.

Summary

You should know:

the structure of an animal cell and a plant cell as seen using a microscope, and able to compare them
the functions of the different parts of animal cells and plant cells
how to calculate magnification using millimeters (mm)
some examples of specialised cells
how diffusion results from the random movement of particles