GCSE Science/Cells

Cell organelles

Diagram showing the parts of animal and plants cells. Plant-only organelles are shown in green.

The organelles of a yeast cell

All animals and plants are made up of cells. Parts of cells are called organelles. Organelles common to all cells are:

• A nucleus – It controls the actions of the cell and contains DNA or RNA.
• Cytoplasm – Where chemical reactions occur. Inside the cytoplasm are enzymes which speed up these reactions.
• A cell membrane – It controls the passage of substances in and out of the cell
• Mitochondria - Where respiration takes place

Plants cells have:

• A cellulose cell wall – to keep the structure of the cell
• Chloroplasts – to absorb sunlight energy to make food by photosynthesis
• A vacuole – It stores nutrients and food in the form of cell sap

Yeast cells have:

• A chitin cell wall - to keep the structure of the cell
• A vacuole - It stores nutrients and food

Cells, tissues and organs

A group of cells with similar structures and a particular function are called a tissue. For instance, the cells on the surface of a leaf form "palisade tissue". Tissues are grouped together to form an organ. A leaf is an organ. Organs are grouped together in organ systems. These form an organism, for example, a whole plant.

Organelle > Cells > Tissue > Organ > Organ system > Organism

Tissues are usually formed from specialised cells. The cells in the palisade tissue are specialised to perform photosynthesis and so contain many chloroplasts. Some other specialised cells are:

• Guard cells
• Egg cells
• Sperm cells
• Red blood cells

Diffusion

Diffusion occurs in gas exchange; the oxygen in the air diffuses into the blood, and the carbon dioxide diffuses out.

Diffusion is the net movement of particles from an area of high concentration to an area of a lower concentration. The steeper the concentration gradient, the more rapid the rate of diffusion.

Examples of diffusion are:

• Oxygen (required for respiration) enters the blood through capillaries next to air sacks in the lungs (alveoli) by diffusion
• Carbon dioxide entering leaves for photosynthesis

If you spray a can of air freshener in a room in one corner and have another person sit somewhere else in the room, how come he/she can smell it after a few seconds? The air freshener diffuses throughout the room- from an area of high concentration (the area where you sprayed the deodorant) to an area of low concentration (where the other person is).

Osmosis

A semi-permeable membrane doesn't allow things that are too big to pass through. Here it shows the red blood cells and the blue particles are too big to fit through the holes

Osmosis is the movement of water from an area of high water concentration (more dilute) to an area of a low water concentration (less dilute) through a partially permeable membrane.

A partially permeable membrane allows water molecules to pass through but not solute molecules, because they are too big. It's like a sieve. Visking tubing is a partially permeable membrane. It is used in dialysis machines.

Turgor

Plant cells need to be turgid (i.e. rigid) to support plant tissues.

If the water potential outside the cell is higher than outside (hypotonic), water moves into the cell, the vacuole expands and the plant cell becomes turgid.

If the water potential outside the cell is lower than outside (hypertonic), water moves out of the cell, the vacuole shrinks and the cell loses its shape and becomes flaccid. A cell which is flaccid (due to water loss from osmosis) is called plasmolysed. A plant that contains plasmolysed cells is said to wilt and the leaves droop, collapsing and hanging down from the stem.

An isotonic solution is when the water potential is equal on the inside and outside of the cell.

• 
  Before Plasmolysis...
• 
  ...and after.

Real microscopic image of stomata on the underside of a leaf.

Turgor is used to control the opening and closing of stomata, the pores on the underside of leaves. The stomata are surrounded by guard cells that can become turgid or flaccid. Stomata allow carbon dioxide into the leaf and water to escape through evaporation. They can control the rate of water loss from the plant. They also control transpiration where water loss from the leaves due to evaporation causes the flow of water and fresh minerals up the plant.

open - turgid

closed - flaccid

If an animal cell becomes turgid it can be in danger of bursting because it does not have a cell wall to protect it.

Active Transport

Diffusion and osmosis can only work if the concentration gradient is right. Sometimes an organism needs to transport something against a concentration gradient. The only way this can be done is through active transport, using energy produced by respiration. In Active transport, the particles move across a cell membrane from a lower to a higher concentration.

Examples of active transport

In plants: Plants need mineral salts (e.g. nitrates) for making proteins and growth. Nitrates are at a higher concentration inside the root cells than they are when dissolved in the water around the soil particles. If the plant relied on diffusion alone, the vital nitrate salts would drain out of the cells into the soil. So energy is deployed by the cells to actively transport nitrates across the cell membrane into the root cells, against the concentration gradient.

In humans: Active transport takes place during digestion of food in the small intestine. After food has been absorbed by the villi for some time, the concentration of food molecules inside the villi increases, making it impossible for more food to diffuse into the villi. So simple sugars, amino acids, minerals and vitamins are actively absorbed into the villi, from an area of lower to an area of higher concentration

The Cell Cycle

At one time, biologists described the life of a cell division after another "Interphase". During the Cell Cycle, a cell grows, prepares for division, and divides to create two daughter cells, each of which begin the cell cycle. During the normal cell cycle, interphase can be quite long, whereas the actual division of the cell takes place quickly. Interphase is divided into three phases: G1, S, and G2. The G1 period is a period of activity in which cells do most of their growing. During this phase, cells increase in size and synthesize new proteins and organelles. G1 is followed by the S phase, in which chromosomes are replicated and the synthesis of DNA molecules takes place. Key proteins associated with the chromosomes are synthesized during the S phase. Usually, once a cell enters the S phase and begins the replication of its chromosomes, it completes the rest of the cell cycle, including mitosis. When DNA replication is completed, the cell enters the G2 phase. G2 is usually the shortest of the three stages of interphase. During the G2 phase, many of the organelles and molecules required for cell division are produced. When the events of the G2 phase is complete, the cell is ready to enter the M phase and begin the process of cell division.

Prophase

The first and longest phase of mitosis, prophase, can take as much as 50-60 percent of the total time to complete mitosis. During prophase, the chromosomes become visible. The centrioles, two tiny structures located in the cytoplasm near the nuclear envelope, separate and take up positions on opposite sides of the nucleus. The centrioles lie in a region called the centrosome that helps to organise the spindle, a fanliek microtubule structure that help separate the chromosomes. During prophase, the condensed chromosomes become attached to fibers in the spindle at a point near the centromere of each chromatid.

Metaphase

The second phase of mitosis, metaphase, often only lasts a few minutes. During metaphase, the chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the poles of the spindle.

Anaphase

Anaphase is the third phase of mitosis. During anaphase, the centromeres that join the sister chromatids separate, allowing the sister chromatids to separate and become individual chromosomes. The chromosomes continue to move until they have separated into two groups near the poles of the spindle. Anaphase ends when the chromosomes stop moving.

Telophase

Following anaphase is telophase, the fourth and final phase of mitosis. In telophase, the chromosomes, which were distinct and condensed, begin to disperse into a tangle of dense material. A nuclear envelope re-forms around each cluster of chromosomes. The spindle begins to break apart, and a nucleolus becomes visible le in each daughter nucleus. Mitosis is complete. Back to GCSE Science