Explain the Functions, Definition of the Cell Flashcards

All animals and plants are made up of cells. Most cells have:

• A nucleus – controlling the activity of the cell. All cells have these at one point.
• Cytoplasm – where chemical reactions occur. Inside the cytoplasm are enzymes which speed up these reactions. Cytoplasm also contains mitochondria which is where energy is released.
• A cell membrane – to control the passage of substances in and out of the cell.

Plants also have

• Cell walls – to strengthen the cell.
• Chloroplasts – to absorb sunlight energy to make food by photosynthesis.
• Vacuole –full of cell sap.

Cells act together to form tissues. For instance, the cells on the surface of a leaf form "pallisade tissue". A group of cells with similar structures and a particular function are called a tissue. Tissues are grouped together to form an organ. A leaf is an organ. Organs are grouped together to form an organism such as a whole plant or animal. Cells ----> Tissue ----> Organ ----> Organism Tissues are usually formed from specialised cells. The cells in the pallisade tissue are specialised to perform photosynthesis and contain many chloroplasts. Some other specialised cells are:

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

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. 2 examples of diffusion are:

• Oxygen (required for respiration) passes through cell membranes and gas exchange surfaces (e.g. alveoli in lungs) by diffusion
• Carbon dioxide enters leaves and leaf cells by diffusion

Osmosis is the movement of water from an area of high water concentration (more dilute) to an area of a low water concentration (stronger) through a partially permeable membrane. A partially permeable membrane allows water molecules to pass through (as they are smaller) but not solute molecules (they are too big). It acts like a sieve. Visking tubing is a partially permeable membrane. It is used in dialysis machines.

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 Shareen Saqlain

Happens in cells

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 centriols 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.

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 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.

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 bisible in each daughter nucleus. Mitosis is complete

Plants are divided into flowers, stems, leaves and roots with root hairs. A generalised plant is shown in the illustration. The stem provides support for the leaves and flowers. It also allows water and food to travel both up and down the plant. The leaves make the food for the plant. Photosynthesis takes place in the leaves. The roots anchor the plant in the soil and take up water and salts (mineral ions) from the soil. The root hairs provide a large surface area for water and salt uptake. The flowers are reproductive organs. They attract insects that carry pollen from one plant to another. This process of transferring pollen from plant to plant is known as pollination.

Leaves produce the food for the plant. The structure of the leaf is shown in the illustration. The leaf has prominent veins that contain two types of tubes, the xylem tubes and the phloem tubes. The leaf has the following parts (from top to bottom):

• Waxy cuticle
• Upper Epidermis
• Pallisade layer
• Spongy layer
• Veins
• Lower epidermis
• Guard cells that form stomata

Leaves are green because they contain the green pigment called chlorophyll. Chlorophyll is used in photosynthesis.

Lowers are composed of:

• Sepals - these are arranged underneath the flower and are typically green.
• Petals - often brightly coloured to attract insects.
• Stamens - stalk-like filaments that have anthers at the top which produce pollen. Pollen contains the male gametes.
• Pistil - contains one or several carpels that contain the ovaries with ovules, the female gametes. Sometimes the carpels are merged. A stalk called the style leads upwards from each pistil and is topped by a sticky stigma that receives the pollen.

The pistil is the bottle shaped structure. A pistil can be composed of one or many carpels and a flower can have several pistils.

Plant growth requires glucose produced by photosynthesis and energy produced by respiration. It also requires minerals obtained from the soil. Plant growth is controlled by plant hormones called auxins. Auxins Minerals needed for plant growth There are three minerals that are essential for plant growth: phosphates, nitrates and potassium. Small quantities of iron and magnesium are also needed, especially for the production of chlorophyll. Phosphates: used in photosynthesis and respiration. Phosphate deficiency: purple leaves and small roots. Nitrates: used in the production of aminno acids. Amino acids are combined to make proteins.
Nitrate deficiency: yellowing of leaves and poor, stunted growth. Potassium: maintains electrical potentials and helps enzyme action. Potassium deficiency: leaves become yellow with spotty, brown, dead areas.

Like all living things, plants need food to live. This food is used for energy and to make new materials when plants grow. Plants are able to take two inorganic chemicals, carbon dioxide gas and water, to make an organic chemical, glucose. This simple food can be used as an energy source or converted into other useful organic molecules. The process requires an input of energy. Plants have found a way to capture the energy from sunlight using a pigment called chlorophyll. Once this light energy has been captured it can be used to create glucose, converting the light energy into chemical energy. Oxygen gas is released as a waste chemical. As light energy is used to create organic materials the process is named Photosynthesis. The formula for photosynthesis is: