Cells in Animals & Plants (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Flashcards

No structures are exclusive to animal cells. Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes — all of which plant cells also have.

False.

In prokaryotic cells, the DNA is not enclosed in a nucleus. Instead, it exists as a single loop of DNA in the cytoplasm.

Complete the table to show which structures are present in animal and plant cells.

The function of mitochondria is to carry out aerobic respiration, releasing energy for the cell to use in its activities.

A cell wall is a rigid outer layer found in plant and algal cells, made of cellulose. It strengthens the cell and helps it maintain its shape.

True.

The permanent vacuole is a large fluid-filled space found in plant cells. It is filled with cell sap and helps to keep the cell rigid.

Prokaryotic cells have their DNA in the cytoplasm as a single loop, and may also contain smaller circular pieces of DNA called plasmids.

Only some plant cells contain chloroplasts because only cells exposed to light need to carry out photosynthesis. Cells in roots, for example, are not exposed to light and do not require chloroplasts.

A temporary mount is a slide prepared for short-term viewing under a microscope. A thin specimen is placed on a glass slide with a drop of water and covered with a coverslip.

A stain is used when preparing a microscope slide to make cells or cell structures more visible. Many cell components are colourless, so stains such as iodine or methylene blue add contrast.

False.

You should always start with the lowest power objective lens. This makes it easier to locate the specimen before switching to higher magnification for more detail.

When making a temporary mount, the specimen is placed on a glass slide, a drop of water is added, and it is covered with a coverslip.

The total magnification of a microscope is calculated by multiplying the magnification of the eyepiece lens by the magnification of the objective lens. For example, ×10 eyepiece × ×40 objective = ×400 total.

True.

Onion skin cells are largely colourless, so staining with iodine solution colours the cells and makes the nucleus and other structures visible under the microscope.

Complete the table showing steps for preparing a microscope slide.

It is important to lower the objective lens carefully when first focusing to avoid cracking the slide or damaging the lens. The lens should be lowered while looking from the side, then raised slowly while looking through the eyepiece.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, due to the random movement of particles in a solution or gas.

True.

Diffusion does not require energy from the cell. The particles move using their own kinetic energy, so diffusion is entirely passive.

The three factors that affect the rate of diffusion are the concentration gradient, the temperature and the surface area.

At higher temperatures, particles have more kinetic energy. This means they move faster, so the net movement from high to low concentration happens more quickly.

Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.

False.

Active transport moves substances against a concentration gradient, from a more dilute solution to a more concentrated solution. This requires energy from respiration.

A partially permeable membrane is a membrane that allows small molecules such as water to pass through, but does not allow larger molecules through.

In plants, root hair cells use active transport to absorb mineral ions from the dilute solution in the soil.

1. Root hair cells absorb mineral ions from dilute solutions in the soil into the cell

2. Sugar molecules are absorbed from the gut into the blood when the blood already has a higher sugar concentration

Transport processes across cell membranes:

Osmosis is the diffusion of water molecules through a partially permeable membrane, from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).

The independent variable is the concentration of the salt or sugar solution (measured in mol/dm³). Different concentrations are tested to see how they affect the potato tissue.

False.

In a concentrated solution, the water concentration outside the potato is lower than inside, so water moves out of the potato by osmosis, causing it to lose mass.

Percentage change in mass = ((final mass − initial mass) ÷ initial mass) × 100

When the line crosses the x-axis, the percentage change in mass is zero. This means the concentration of the solution matches the concentration inside the potato, so no net movement of water occurs by osmosis.

True.

Temperature must be kept constant to ensure it is a fair test. Temperature affects the rate of osmosis, so any change would affect the results.

A partially permeable membrane is a membrane that allows small molecules such as water to pass through, but blocks larger molecules. Cell membranes act as partially permeable membranes.

Complete the variables table for the osmosis required practical.

Percentage change in mass is used rather than just the change in mass because it accounts for differences in the starting mass of each potato cylinder. This allows for fair comparison between samples.

Chromosomes are thread-like structures found in the nucleus of cells, made from coiled DNA. In human body cells, chromosomes are found in 23 pairs (46 in total).

A typical human body cell contains 46 chromosomes, arranged in 23 pairs. One chromosome in each pair is inherited from each parent.

True.

Mitosis produces two daughter cells that are genetically identical to each other and to the original parent cell. The DNA is copied exactly before the cell divides.

During the cell cycle, the genetic material is copied and the cell then divides to form two new cells that are genetically identical.

Before a cell can divide by mitosis, it must:

1. Grow larger

2. Make copies of all its organelles (e.g. mitochondria, ribosomes)

3. Replicate all chromosomes in the nucleus

False.

Human body cells contain 46 chromosomes (23 pairs). Only gametes (sex cells) contain 23 chromosomes — one from each pair.

Mitosis is a type of cell division that produces two genetically identical daughter cells. It is used for growth and repair of tissues in multicellular organisms.

Complete the table describing what happens during mitosis.

Mitosis is important in multicellular organisms because it enables growth during development and the repair of damaged tissues. It produces new cells that are genetically identical to the original.

Gametes are sex cells produced in the reproductive organs. In humans, they are the egg (female) and sperm (male) cells. Each gamete contains only one set of chromosomes (23 in humans).

Meiosis produces four daughter cells. Each contains a single set of chromosomes (23 in humans), which is half the number found in a normal body cell.

False.

Cells produced by meiosis are all genetically different from each other. This genetic variation is important for natural selection and the survival of species.

When a cell divides by meiosis, it first copies its genetic information, then divides twice to produce four gametes.

The chromosome number must be halved so that when two gametes fuse at fertilisation, the resulting zygote has the correct number of chromosomes. Without halving, the chromosome number would double with each generation.

True.

After fertilisation, the zygote divides repeatedly by mitosis to form an embryo, and eventually all the tissues and organs of the offspring.

A zygote is a fertilised egg cell, formed when a male and female gamete fuse. It contains the full number of chromosomes (46 in humans), half from each parent.

Complete the comparison table for mitosis and meiosis.

Meiosis takes place in the reproductive organs. In females this is the ovaries, and in males this is the testes.

Cell differentiation is the process by which cells become specialised to carry out a particular function. During development, cells develop different structures that allow them to perform specific roles.

A stem cell is an undifferentiated cell that can divide and develop into many different types of specialised cells. Stem cells are found in embryos and in some adult tissues.

False.

Once a cell has become specialised, it cannot change into a different type of cell. Only stem cells retain the ability to divide and differentiate into other cell types.

In an early embryo, cells are called stem cells and can develop into any of the cell types found in the human body.

Cell differentiation is essential in multicellular organisms because it produces a variety of specialised cell types with different functions. Without differentiation, all cells would be identical and unable to carry out the specific roles needed for an organism to survive.

True.

Embryonic stem cells are described as pluripotent — they can develop into any cell type. This is why they have potential uses in treating diseases caused by damaged or missing cells.

Adult stem cells are undifferentiated cells found in specific tissues of the body. They divide to replace damaged cells or produce new cells for growth, but can only form a limited range of cell types.

Complete the table comparing embryonic and adult stem cells.

Specialised cells differ from stem cells in that they have a fixed structure adapted to carry out a specific function and cannot change into other cell types. Stem cells remain undifferentiated and can still divide to form a range of cell types.