Chapter 3 section 2 The Cell in Its Environment
The Cell in Its Environment Cells have structures that protect their contents from the world outside. All cells are surrounded by a cell membrane that separates the cell from the outside environment. The cell membrane is selectively permeable, which means that some substances can pass through the membrane while others cannot. Cells must let in needed materials, such as oxygen and food molecules. In contrast, waste materials must move out of cells.
Diffusion Substances that can move into and out of a cell do so by one of three methods: diffusion, osmosis, or active transport. Diffusion is the main method by which small molecules move across the cell membrane. Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. The concentration of a substance is the amount of the substance in a given volume.
Diffusion Molecules are always moving. As they move, the molecules bump into one another. The more molecules there are in an area, the more collisions there will be. Collisions causes molecules to push away from one another. Over time, the molecules of a substance will continue to spread out. Eventually, they will be spread evenly throughout the area. Some organisms obtain the oxygen they need from the water they are in. There is more oxygen molecules in the water then in the cell. If you remember, the cell membrane is permeable to oxygen molecules. The oxygen molecules diffuse from the area of higher concentration through the cell membrane to the area of lower concentration.
Osmosis Osmosis is the diffusion of water molecules through a selectively permeable membrane. Because cells cannot function properly without adequate water, many cellular processes depend on osmosis. In osmosis, water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.
Osmosis For a cell to be healthy there needs to be the same concentration of water in the cell as there is on the outside of the cell. When there is low water concentration outside the cell then water will move out of the cell during osmosis. This reaction tends to happen when there is a large amount of salt; salt takes up space in the water. When there is high water concentration outside the cell then during osmosis, water will move into the cell. This means there is more water outside the cell then inside, so when the water enters the cell, it begins to swell since there is too much water entering.
Active Transport When a cell moves material through the cell membrane by diffusion and osmosis it does not use its own energy. This movement is called passive transport because dissolved materials are moving through the cell membrane without using cellular energy. Active transport is the movement of materials through a cell membrane using cellular energy. Active transport requires the cell to use its own energy, while passive transport does not.
Active Transport Cells can move materials several different ways by active transport. One method involves transport proteins in the cell membrane will “pick up” molecules outside the cell and carry them in, using energy. Transport proteins will also carry molecules out of cells in a similar way. Some of these substances are calcium, potassium, and sodium. Another method of active transport is when the cell membrane surrounds and engulfs or encloses a particle. Once the particle is engulfed, the cell membrane wraps around the particle and forms a vacuole within the cell.
Active Transport One reason cells are so small is because of how materials move into and out of cells. As a cell’s size increases, more of its cytoplasm is located farther from the cell membrane. Once a molecule enters a cell, it is carried to its destination by a stream of moving cytoplasm. In large cells, the streams of cytoplasm must travel farther to bring materials to all parts of the cell. It would take much longer to reach the center of a very large cell than it would in a small cell. So, if a cell grew too large, it could not function well to survive.
Sources of Energy The process by which a cell captures energy in sunlight and uses it to make food is called photosynthesis. Nearly all living things obtain energy either directly or indirectly from the energy or sunlight captured during photosynthesis. When an animal eats grass, it obtains its energy indirectly from the sun. Grass obtains energy directly from the sun because it makes its own food during photosynthesis.
Sources of Energy Plants manufacture their own food through the process of photosynthesis. An organism that makes its own food is called an autotroph. An organism that cannot make its own food, such as animals and humans, are called heterotrophs. Many heterotrophs obtain food by eating other organisms.
The Two Stages of Photosynthesis During photosynthesis, plants and some other organisms use energy from the sun to convert carbon dioxide and water into oxygen and sugars. In the first stage of photosynthesis energy is captured from sunlight. Pigments are colored chemical compounds that absorb light. The main photosynthetic pigments in chloroplasts is chlorophyll.
The Two Stages of Photosynthesis In the second stage of photosynthesis the cell uses the captured energy to produce sugars. The cells needs two raw materials for this stage: water and carbon dioxide. Carbon dioxide enters the plant through small openings on the undersides of the leaves called stomata. Once water and carbon dioxide enter the chloroplasts they undergo a series of chemical reactions. The reactions are powered by the energy captured in the first stage. Almost all of the oxygen in Earth’s atmosphere is produced by living things through the process of photosynthesis.
The Two Stages of Photosynthesis 6 CO2 + 6 H20 C6H12O6 + 6 O2 Carbon Water A sugar Oxygen Dioxide